PROCESS OF PAD PRINTING

Pad printing is an indirect photogravure process. An image is etched into a flat printing plate and ink is flooded and doctored across the surface leaving ink only in the etch. A silicone rubber pad then presses down onto the etched plate and picks up the ink, which due to solvent evaporation has become tacky.

The image is now on the surface of the silicone pad. Silicone rubber does not allow ink to penetrate its surface and therefore when the pad presses down onto the product to be printed it releases the ink as a clean film. The range of inks and solvents used enable the inks to adhere to the surface after releasing the ink the pad is then clean and free to repeat the process.

Because such a wide range of inks are available almost any material can be printed and due to the flexibility of the silicone rubber pads uneven, curved and awkward shaped surfaces can be printed. As the ink is quick drying, pad printing is capable of ‘wet on wet’ printing and high quality multicolour designs can easily be achieved including four colour process.

The more recent development of closed cup machines has made the process ‘cleaner’ and enabled the process to become more user friendly and automated.


APPLICATION OF PAD PRINTING

Still today, pad printing is not well known as a printing process, but we see and touch pad printed items every day. For example:

  • Electronic micro components, such as cables, connectors, IC chips, relays, etc.
  • Industrial Buttons and Keys as found on calculators, telephones and computer keyboards
  • Electrical Household Appliances such as iron, VCR, TV, coffee pots, telephones, etc.
  • Large Industrial Appliance panels such as on dishwashers, washing machines and dryers
  • Toys such as figurines, dolls, cars, infant bottles, pacifiers, rattles and night lights.
  • Advertising Specialty Items like pens, lighters, key chains, clocks, watch faces
  • Sporting Goods such as golf balls, baseballs, tennis rackets, footballs

INK TRANSFER IN THEORY

Pad printing is an indirect offset (or a gravure) printing process, where an image is transferred via a soft silicone pad onto the surface to be printed. Today there are two primary doctoring methods used. One being the old traditional open inkwell system (illustrated below) and two being the newer more widely accepted sealed cup system. For the sake of starting with the basics, we have illustrated the old “standard” method below.

To do this, a cliché plate is used that has the artwork chemically etched into the lapped smooth steel surface (photo polymer clichés can also be used). Typically the etch depth is around .001″ to .0012″ or 24 to 28 microns.

Using a “flood bar” the etched image is flooded (coated) with ink and then a doctor blade (steel ink blade) removes the ink from the flat printing plate, leaving a deposit of ink in the etched area only. The silicone pad or tampon then lifts the ink from the etched image and transfers the image directly onto the surface to be printed. By virtue of the shape of the silicone pad, the ink releases onto the product to be decorated and the silicone pad moves back to its “home” position, ready for another print cycle.

Today in pad printing, transfer pads made from silicone rubber are used almost exclusively since the silicone materials are the best known release agents for this printing process. Also there has been some pad printing done using Polyurethane pads, for specialized applications


1: MANUAL PAD PRINTING MACHINE

MANUAL PAD PRINTING MACHINE

The first machines were manually operated. Very high quality watch and instrument dials have been printed over the years using hand powered machines. Now the only reason for buying them is the initial cost. They are not really entry level machines except where printing is carried out on a small scale. Print rates are very low and maintaining quality over a continuous run is extremely difficult. A well engineered unit can be useful for proofing plates, pads, inks and designs as an alternative to taking a production machine off line.


2: OPEN INKWELL PAD PRINTING MACHINE

OPEN INKWELL PAD PRINTING MACHINE

There are many machines of this type out in the field as it was the original system designed in the late 1960′s early 1970′s that has been used as the basis of many clones. Original machines were mechanically operated giving little opportunity for adjustment of individual actions as the inking mechanism and pad horizontal action were tied together on the same drive. Adjusting the speed of the main drive motor controls the overall speed of the machine. Later more sophisticated mechanically driven machines gave greater flexibility of adjustment. This is a very robust system that is unaffected by fluctuations in air line pressure. It gives a smooth action and has been used extensively with larger machines working on long runs.

The concept has developed and a host of machines have been produced using electro pneumatics and even servo motor drives. PLC’s (Programmable Logic Controllers) can provide a wide range of sequencing options.
Whatever the degree of sophistication the control of ink conditions is problematic due to the large surface area exposed to atmosphere. This can be ameliorated in different ways. Some manufacturers help reduce evaporation by shrouding the inking mechanism and ink well and others by using ink pumps and solvent addition systems.
Changeover of ink and plate can be carried out relatively quickly by removing the ink tray containing the plate and replacing it by another that has already been prepared. Consideration regarding the cost of spare ink trays must be given, as they can be very expensive. The ink tray can be designed to catch ink that spills over the front of the plate, thus keeping the machine cleaner


3: ROTARY PAD PRINTING MACHINE

ROTARY PAD PRINTING MACHINE

The development of rotary systems stems directly from gravure printing. It is very suitable for cylindrical parts and also for continuous flat printing.
A rotary drum type silicone pad is used in conjunction with a steel cylinder plate. The cylindrical plate has the design etched onto it, the ink flows onto the plate from an ink trough or ink ducts. A doctor blade removes the excess ink and as the cylinder rotates the silicone rubber printing roller picks the ink out of the etching and transfers it onto the item to be printed.
One of the advantages of this system is high speed printing. With small components such as caps and closures 120,000 parts per hour can be achieved. Printing a single item for example a 25 mm diameter cylinder around the periphery would run at 3000 per hour. Another advantage is the very fine detail that is printable. Ink deposits tend to be slightly less than in conventional pad printing particularly at the higher speeds that are attainable. The ink has to be run with high levels of solvent. Ink manufacturers recommended 20% solvent but I was using 30% on a multicolour application. Each colour may require a separate mix that must be determined at the start of a job and maintained throughout and on subsequent runs. Control of the solvent balance is essential. This is not a short run process unless you are using a single print head with manual loading. I would recommend the use of ink pumps to maintain the ink conditions. These will normally have solvent feeds that have to be carefully set to the correct feed rate. Some pumps have viscosity measurement devices but I have not found them particularly successful.
Remember that changing colour means purging the system and often changing the ink reservoir. This is why I do not recommend the system for short runs.
There are alternatives to the steel printing cylinder. Steel foil plates or even photopolymers can be mounted onto a modified cylinder. Their effectiveness is debatable as the pressure on the doctor blade causes them to wear fairly quickly. Although the steel cylinders are by no means cheap the investment is paid back by reduced down time.
The capital cost of the equipment is relatively high, as a feed system is nearly always necessary to make full use of the technique. Output levels keep the cost per print extremely competitive. The quality should be as good, if not better than conventional pad printing.
Although most of the major suppliers produce some variation on this equipment, availability is limited. Very careful selection is critical.


4: CLOSED CUP PAD PRINTING MACHINE

CLOSED CUP PAD PRINTING MACHINE

This system is considered by some to be the panacea to all Pad Printing ills, would that it was so. What is surprising is that this “New development” has been around for many years. A Swiss manufacturer of Pad Printing machines for the watch dial industry had a system 35 years ago! Although pretty basic it performed very well indeed.

The concept is to contain the ink in a cup that is turned upside down and the ink sealed in by pressing the cup down onto the plate. The cup not only holds the ink but when traversed over the plate acts as a doctor blade and leaves ink just in the etched areas on the plate.
The obvious advantage of the system is that solvent evaporation is reduced to a minimum. This allows much closer control of ink conditions resulting in the opportunity for reduced down time and partial unmanned operation.
But, you say; “The machinery salesman told me it will run 24 hours 7 days a week 52 weeks a year.” Did he say what the print quality would be, how often do you have to top up with ink, how much will the plate wear, how long does the cup last, does the fact that certain ink corrode the plate matter, using two component inks is acceptable but don’t leave them for an extended period in the cup, with some systems it is better to keep the plates and cups as matched pairs. A question that must be asked is. “How much does a replacement cup cost”. The answer can come as a surprise. £500 is not at all unusual. Fortunately if the damage to the cup is very slight it is possible to carefully hone the contact surface on a fine carborundum stone. Some cups can be re-machined at a quarter of the replacement costs. This all means the cups must be handled very carefully. The plate has to be twice the size of a conventional plate for a given image.
The issue of ink condition is crucial. With closed cup systems it is easy to have the attitude out of sight out of mind. Ink mixing is just as important as when you use an open ink well system. The mechanism of pad printing remains the same, evaporation of solvents being the governing characteristic. Solvents must be weighed into the ink and the mix is dependent on the image being printed. For example when fine detail is being printed the mix of solvent will contain a higher percentage of retarder; otherwise the ink will dry in the etching on the plate.

The most common problem is brought on by the impression that inks have an indefinite pot life when held in the closed cup. This is simply not so. Time and again users who are unhappy with print quality contact me. The solution often is to mix a new batch of ink and take out the old and replace it with new. If print quality is critical it is recommended that the ink is replaced once a day. When high volumes of ink hungry images are being printed regular topping up of the ink will be necessary. As long as correctly mixed inks are added ink condition will be maintained. Addition of solvents whilst in production is fraught with problems as the ink volumes are very low and one squirt can completely upset the solvent balance. Old ink that you remove from the cup should be disposed of and not mixed back in to the new ink. If print quality is not important then ink condition can have a much greater latitude.
Ambient conditions can still effect the performance of an ink but not to the same extent as machines with open ink wells. Differing ambient conditions will mean you will have to adjust the initial mix of ink to control solvent evaporation on the pad and in the etched plate.
Another problem that can occur is that some ink systems when used form a crust of dried ink around the edge of the cup. This crust will drag lines of ink across the image and ruin the print. Ink manufacturers have developed inks that overcome this problem so if your current supplier cannot solve it consider another supplier.
With closed cup systems plates generally have to be twice the size of open ink well systems. If you have a lot of plates this can be a substantial cost increase. Ideally the surface finish of the plate needs to be better than when it is cleared with a conventional doctor blade. The doctoring characteristics of the cup depend on the contact surface, machined from solid, spring steel or ceramic. This is allied to the bearing and clamping mechanism. There is no such thing as a cheap system. The cheaper it is the more expensive it is to run.
At times like these I would love to be able to give you my recommendations as to which manufacturer had the best combination but I have to remain impartial. By the very nature of these cups plate wear can be a problem. With steel harder flatter plates will be more effective.
Photopolymer plates are best used with cups that have ceramic rings. These rings are very flat and run well on the photopolymer material. Different photopolymer materials will perform better or worse and you will need to experiment. Broadly the harder materials are better not just from a wear point of view but also as regards to print quality.
Multicolour machines are common place. The quality of engineering will determine the effectiveness of the system. Some manufacturers can offer either sealed ink cup or open inking mechanisms on the same basic machine. Later in this article I show the use of sealed ink cups in sophisticated CNC controlled equipment for multicolour printing.
As mentioned above ink cups have different constructions. The simplest is machined from solid metal. Normally hardened steel. The second type has a ribbon steel contact surface that can be replaced when worn. The third is a metal cup with a ceramic contact surface attached to the metal. This gives a very good life but is more expensive than the conventional cup.
To achieve maximum life out of any system minimum pressure must be applied and the cup should be mounted on a stable bearing. Any tendency for the cup to rock will cause uneven wear and a consequential film of ink on the plate, which is transferred by the pad. Some manufacturers rotate the cup during the machine cycle can overcome this. Some systems use internal magnets to hold the cup onto the plate this appears to work very well. Be careful that the pigments in the ink are not effected by magnetism, if so they collect around the magnets and they are virtually impossible to use.
There is no doubt that the system is being refined and the use of larger cups is increasing the print area. Four colour machines with ink cup diameters of 210 mm are now available along with six colour machines with 150 mm cups.
The use of the sealed ink system is becoming more important as the effects of legislation require a much reduced level of solvents in the working environment and expelled into the atmosphere.
Machines will either have cups that move backwards and forwards and the plate remains stationary or stationary cups and moving plates. The first system is normally used for larger images at slower speeds whereas the second is for higher speed printing of smaller images. There are exceptions.
Whether a closed cup or open ink trough is used the same systems can apply. The moving plate with a single plane pad movement provides a cost saving for the machine supplier because there is one less actuation on the machine. From the users point of view as the pad only has to move in the vertical plane there is less vibration on the pad and the machine can cycle faster. This system is used with high speed coding machines. Here a small plate is used with a sealed ink cup.
Larger machines with plate areas of 200 mm by 450 mm have been produced using this method but they need very substantial bearings to support the plate when it is in the pick up position as the loading exerted by a large pad is very considerable. Manufacturers claim as the pad strokes below the base of the machine there is a larger work area. The down side to this is that the machine has to be well guarded because of the back and forwards movement of the plate. Automatic loading can take advantage of the shorter cycle times.

5: TOTAL COLOR TRANSFER SYSTEM

TOTAL COLOR TRANSDER PAD PRINTING

This technique was developed for printing onto ceramic items it uses a mix of Pad Printing technology with Screen Printing methods.
As a general description, the system does not use etched plates as those normally associated with conventional Pad printing. Instead, the ink or ceramic colour is screen printed onto a flat silicone coated plate. The ink does not adhere to this plate, but rests on top.
The screen printed image is then picked up by the pad and transferred by pad onto the item.
When this process was first developed in the 1950′s the colour was cold. This did not allow total transfer of the ink, to meet this need thermoplastic colour was used. This colour is a wax like solid at room temperature. The screen has a metal mesh, which is heated by passing an electric current through it. This raises the temperature of the ink to 75oC causing it to melt and flow with the action of the squeegee like a conventional ink onto the flat silicone surface. This surface is controlled at some 35-40oC lower than the printing ink. The pad lifts the colour from the smooth silicone plate and transfers it to the ceramic item. The item chills the ink and cases a complete transfer.
Although this process is used almost exclusively in the ceramics industry it demonstrates the happy marriage of the two processes. Some of the advantages are as follows:
Patterns can be offset printed on areas and surfaces that cannot be printed by direct Screen Printing. e.g. Double curved surfaces.
Fine Lines and heavy bands can be applied at the same time as the principal pattern in one machine pass.
Heavy deposits of colour can be applied to complex shapes. This was previously only possible using Screen printing onto its associated limit of printable shapes.
The Screen process determines the quantity of colour applied. This ensures consistent high quality prints, using a full range of effects from solid lines to half-tones.
Many users manufacture screens in house

6: CAROUSEL CLOSED CUP PAD PRINTING MACHINE

ROTARY PAD PRINTING MACHINE

This novel machine is a multicolour pad printing system designed around rotary tables. The system is fully mechanical, operated by cams, which enables up to 750 cycles per hour. Printing 4 colours at 250 per hour is achievable.
The machine is ideal for short runs and medium length runs. The design of the system is such that set up is easy and quick and different pad shapes can be accommodated on the carousel, to suit the design.
The system operates by a combination of two rotating tables and a rotating pad carousel. On one table, the part to be printed is mounted. On the second table, the plates containing the image separations are mounted. These plates can be adjusted in both X, Y and rotary axis allowing accurate positioning of the image.
Above the plates there is a system of sealed ink cups, which ink the plate by passing over the image area and returning to the rest position. The plates are inked just prior to the plate being offered to the pad.
The pad picks up the image. The pad carousel and the plate-carrying table then rotate so the pad carrying the image is over the part to be printed, whilst a fresh pad is positioned over the freshly inked second plate. The pad then descends and prints the part. Simultaneously, the fresh pad descends and picks up the image from the second plate.
This sequence continues until the multicolour image has been printed. When the image is complete, the rotary table is indexed. This positions a new part under the pad, while the printed part is removed and replaced with a new, unprinted part.
The system allows for the use of photopolymer plates but as with conventional machines steel plates give a longer life.
The Carousel Printer is a very useful tool particularly for sub-contract printing companies who are often faced with shorter runs of multicolour images. For longer runs conventional linear, rotary and oval systems are more economic because of their higher output.

7: VERTICAL PAD PRINTING MACHINE

VERTICAL PAD PRINTING MACHINE

This technique was developed for printing onto ceramic items it uses a mix of Pad Printing technology with Screen Printing methods.
As a general description, the system does not use etched plates as those normally associated with conventional Pad printing. Instead, the ink or ceramic colour is screen printed onto a flat silicone coated plate. The ink does not adhere to this plate, but rests on top.
The screen printed image is then picked up by the pad and transferred by pad onto the item.
When this process was first developed in the 1950′s the colour was cold. This did not allow total transfer of the ink, to meet this need thermoplastic colour was used. This colour is a wax like solid at room temperature. The screen has a metal mesh, which is heated by passing an electric current through it. This raises the temperature of the ink to 75oC causing it to melt and flow with the action of the squeegee like a conventional ink onto the flat silicone surface. This surface is controlled at some 35-40oC lower than the printing ink. The pad lifts the colour from the smooth silicone plate and transfers it to the ceramic item. The item chills the ink and cases a complete transfer.
Although this process is used almost exclusively in the ceramics industry it demonstrates the happy marriage of the two processes. Some of the advantages are as follows:
Patterns can be offset printed on areas and surfaces that cannot be printed by direct Screen Printing. e.g. Double curved surfaces.
Fine Lines and heavy bands can be applied at the same time as the principal pattern in one machine pass.
Heavy deposits of colour can be applied to complex shapes. This was previously only possible using Screen printing onto its associated limit of printable shapes.
The Screen process determines the quantity of colour applied. This ensures consistent high quality prints, using a full range of effects from solid lines to half-tones.
Many users manufacture screens in house

8: CNC PAD PRINTING MACHINE

CNC PAD PRINTING MACHINE

The use of servo motor drives to all the actuators gives a very smooth, highly controllable print action. Stroke lengths are infinitely variable within their extremities. Linked into a Programmable Logic Controller this system allows simple setting procedures and enables several set ups to be programmed into the machine. Manufacturers claim considerable energy savings over pneumatically actuated machines and they are not effected by fluctuations in airline pressure. This system of control and actuation is built into standard machines.
Modular machine assemblies can be produced to suit almost any application. An example of such a configuration is shown below. The degree of complexity is only governed by the imagination of the designer and the number of modules he wishes to use.
All the elements of component manipulation can be combined with multiple closed cups, pad cleaning, varying pad stroke lengths, alternative pad shapes etc. This system is often used where items have to be printed in line with other assembly processes or more complex multiple prints on different surfaces. The conventional multicolour feed system is inappropriate for this type of work.
This flexibility comes close to producing the ideal machine for a particular application but it is substantially more expensive than a conventional machine. If the workload can justify the increased investment it is well worth considering. Don’t be carried away by the control technology it still must be a capable printer. Print rates will not necessarily be higher but down time and set up should be much reduced. The ability to store and retrieve setting details for the next time a job is run must give more consistent quality.


1: PAD PRINT CLEANING SYSTEM

Automatic pad cleaning is considered to be a useful addition. If the components are clean, ambient conditions are controlled and the ink is mixed correctly, automatic pad cleaning could be considered a luxury rather than a necessity.
Adhesive tape is dispensed onto a platform, which at programmed intervals is placed under the pad. When the pad comes down onto the tape any dried ink or debris on the pad surface is picked off by the tape. The tape platform then retracts for the printing to continue. This system is used mainly on fully automatic machines. It is sometimes fitted to operator fed machines here the operator initiates its use when necessary


2: PAD PRINTING AIR BLOWER

PAD PRINTING PAD BLOWER

Assisting the solvents to evaporate from the ink when in the etching, on the pad or on the item can be achieved by directing air from either a compressed air line or a hot air blower. These can be mounted on the machine but need to be used with care and only when ambient conditions demand them. It compressed air is used it is vital to ensure that the supply is dry and oil free. Components sprayed with an aerosol of oil and water are useless. The simplest form of blower is a hair dryer on a stand. Whatever method you use you are creating a micro climate in which your inks will perform at their best.

Fixed hot air blowing device mounted internally to the pad printing machines.

  • Hot air blower can be oriented directly between the pad and the pieces that have to be printed in order to have better release of the ink on the substrate.
  • For use when working area is cold
  • When it is necessary to print large area format wet on wet
  • Equipped with a regulator to adjust intensity of the air
  • Able to increase or decrease heating power with a thermal controller.

3: SOLVENT DISPENSING UNIT

This is a simpler alternative to ink pumps where the volume of ink used is low. The system will dispense a specific amount of solvent at a set interval. The amount and time interval can be adjusted to suit the ink and printing conditions. Care must be taken in selection of the system to ensure that the component parts will withstand the solvents. There is also a small modification necessary to the spatula to ensure the solvent is mixed in immediately. Other systems have a slowly rotating cylinder in the ink trough that keeps the ink agitated and the solvent well mixed. In principle this regular addition of solvent is an excellent concept and as long as the design is sound will go a long way to reducing one of the major variables in the process. This technique is also used with some closed cup systems, great care must be taken in regulating the flow as the volume of ink is much less and it is possible to over thin the ink.
There are modifications of both ink pumps and solvent dispensing systems that can be used with closed cup assemblies.


4: IMAGE RECOGNITION SYSTEMS

With more companies using Pad Printers on automatic lines without operators in attendance there is an increasing need for image recognition. It is possible for these systems to see minute changes in the print. They can also sense colour change. The system can be used to alert operators, reject components or stop the machine.
Costs are continuing to fall to much less than the annual wage bill for an operator. The CD manufacturing industry is heavily involved with these systems and demonstrating its advantages.


5: FEED SYSTEMS

his topic is an article in itself. The decision you make has to be based on the hourly out put you need from the machine and the changeover time that is tolerable, if you have to changeover from one item to another. Whether you have a static jig, linear feed, rotary table, carousel, indexing jig, fixed jig and indexing pads etc.
The key aspects are accuracy and repeatability of index and the quality of the jig or jigs. Ease of loading and unloading and stability during the print cycle is crucial. Expenditure put into good quality jigging will be repaid many times over by increased productivity.


6: STATIC ELIMINATORS

ANTI STATIC EMILINATOR

By its very nature Pad Printing generates considerable amounts of static electricity. Silicone rubber is an excellent insulator and the continuous mechanical working of the pad can generate many thousands of volts of static charge. If this effect is added to the static charge that is inherent in moulded plastic, especially when the plastic is stored in a plastic bag immediately after moulding and removed from the bag just prior to printing you have a static cocktail that could cause real problems to the print. Feathering is a common effect of static.

This is where the ink tries to jump back onto the pad as the pad is lifting away. Before static is diagnosed make sure the etch depth on the plate is not too deep or the ink too thick. If it is definitely static this has to be conducted away to earth. Blowing a drift of ionised air over the pad and or the object to be printed can do this. This ionised air conducts the static electricity away from the print area. It is produced by a Static Eliminator which is effectively a series of electrodes in the air stream that are charged with a very high voltage 20-40,000 volts. These electrodes give off charged ions that give the air its static elimination properties. Static is to a large extent effected by ambient conditions. Very low humidity is the greatest problem. In some print shops humidifiers are used to ameliorate the effect. The Static Eliminator is a more targeted method of dealing with this hindrance to good quality printing.

There are three common static-related problems which we often meet in
pad printing:
1. Dust on the product before printing.
2 Ink fly /feathering or cobweb patterning on the product.
3. Static build-up on the pad attracting contaminants.

Clean Products

Plastic moldings need to be clean and static free before being printed, otherwise the dust will transfer to the pad and cause hickies. The type of equipment needed depends on the process. Ionised Airguns are often used if the product is manually handled. Ionised Air nozzle systems are used for more automated feeding. If the dust has been on the product for days, the attraction can be very stubborn and a cloth with a solvent may be the only solution.

Ink Fly, Feathering

This is where the impact of the pad onto the moulding causes a strong electric field which make the ink create patterns around the printed area. The solution is to position one or two (depending on the size of the product) 1250-Slot Bars as shown below. The ionised air will absorb the static charge as soon as it is created.

Dust and Static on the Pad

If static builds up on the pad during a print run it will attract dust, which impairs the printing. The solution is to position a 1250-Slot Bar to neutralise the pad as it travels between the product and the plates.

This Technical Guide will help you choose Pad Printing Plate which will be best suitable for you.
it will also provides Tips and Tutorials on usage, maintenance of Pad Printing Plate.

pad-printing-steel-plate


The cliche is the carrier of the print motif.

Varying types of cliches are used depending on the desired print quality and quantity. The motif is recessed, etched into or washed out of the cliche. In order to assure smooth fitting and printing, the motif should be at a distance of at least 25 mm (1 inch) from the outer edge, and centered on the plate. In the case of long or fine lines, the motif must be at an angle of approx. 5° to 15° to the blade track in order for the doctor blade not to dip too low and scoop out too much ink. Numerous trials have shown that an etching depth of 25 μm is optimal for pad printing. Since the pad can only transfer a limited quantity of ink, a greater depth is pointless. At an etching depth of 25 μm, the pad only picks up an ink film of 12 μm. The rest remains in the depressions of the cliche. Because the ink film consists of approx. 40 % to 60 % thinner, which evaporates during the transfer and drying, the actual thickness of the ink film left on the substrate comes to around 5 μm – 8 μm, in some cases, as little as 2 μm. As an exception to the above statement, very fine fonts are etched at approx. 15 μm, and very large motifs, at approx. 30 μm. For very large surfaces, the dip of the doctor blade can be avoided by additionally copying a fine screen into the motif.

Steel clichés have a very long life expectancy, typically around 700,000 to 1,000,000 or more imprints can be expected. While nylon clichés have a much shorter life cycle, typically around 10,000 to 40,000 imprints depending upon various production factors. Obviously, steel plates are more expensive than nylon plates, but many customers choose steel over nylon for ease of production.

The proper etch depth for the printing plates will vary, between .0006″ to .0015″. The most common etch depths range from .001″ to .0012″ (24 to 28 microns). The theoretical ink lay down will vary according to the plate type, etch depth, shape & hardness of the silicone pad. The theoretical lay down ranges from .00057″ to .00075″ (12 to 16 microns), depending upon etch depth. Also, temperature, humidity and other factors in your environment may influence the variations of ink lay down.



1: THIN STEEL PAD PRINTING PLATE

THIN STEEL STRIP PLATE

This is a strip of spring steel, 0.5 mm (0.02 in.) in thickness, with a very fine surface, and a hardness ranging between approx. 48 and 54 Rockwell. In the press, the steel strip is held in the ink well by a metal plate. The advantage of steel strip cliches over photopolymer ones is that within one cliche, sections of a motif can be screened while other sections are not. As well, etching can be done in stages, i.e., certain portions of a motif can be etched more or less deeply than others. The quantity of prints which can be made depends on the type of press used and the settings, as a steel strip cliche is somewhat softer than a doctor blade. In practice, the serviceability allows between 20,000 and 50,000 printing runs.

Thin Plates are Suitable for use on both open and sealed cup pad-printing machines.

Advantages of using thin steel pad printing plates

  1. Less dependent on a clean air environment, considerably more durable and less easily damaged than softer photopolymer plates.
  2. Have accurately controlled etch depths adjusted to suit all requirements
  3. Hard wearing and more resistant to scratching than photopolymer plates
  4.  Can be effectively screened
  5.  Can be punched with holes to accommodate pin registration requirements
  6. Are capable of producing much finer detail than photopolymer plates
  7. Are ideal for use on medium volume applications up to 200,000 impressions
  8. thin cliché plates are priced to be a sound economic alternative to both the softer photopolymer pad printing plates and 10mm thick cliché dies

2: THICK STEEL PAD PRINTING PLATE

pad printing thick steel plate

This classic cliche is produced from special high-grade, abrasion-proof steel. Its area of application is found primarily in the industrial sector. Steel cliches are especially suited to high quantity printing runs and closed ink systems. The quality of these cliches is unequaled. This is evident from their absolute sharpness of contour, multi-stage etching, utilization of screens of all types, very fine surface, low susceptibility to physical damage and consistency in high quantity printing runs. The number of prints which can be achieved using this cliche comes to at least 2,00,000 doctoring operations.

Material
Steel plates are made from a high-quality, abrasion-proof special steel.The processing is very complicated and expensive machines are needed. The raw material from the steel manufacturer is cut according to the required dimensions and all 6 sides are ground, hardened (up to 65 HRC), and lapped. The surface roughness is approx. 3 µ. For a very fine finish, the lapped side can be additionally polished

Thick Plates are Suitable for use on both open and sealed cup pad-printing machines.

Advantages of using Thick steel pad printing plates are

  1. Are the most durable and resilient pad printing cliché plates
  2. Have accurately controlled etch depths adjusted to suit all requirements
  3. Can be effectively screened
  4. Are capable of producing fine detail
  5. Can be re-lapped and re-used
  6. Are perfect for high volume applications up to 1,000,000+ impressions
  7. For superior quality images, particularly when printing fine detail, choose steel cliché pad printing plates

Thick Steel plates are mainly used in the industrial sector as they are most
suitable due to their long service life and are especially suitable for closed ink cup systems. The quality achieved with these plates cannot be reached by other means, i.e.:
• Absolute definition
• Various etch depths on one die plate
• Use of process of all types
• Very fine surface finish
• No need for additional supporting plate
• Low risk of mechanical damage
• Extreme durability (high print run)
This way steel plates can be used for all print jobs. The number of prints
possible with steel plates is mostly given by manufacturers with the magical number “1 million”. Since the closed ink cup system has been on the market, the steel plate has won back its great significance, as its extreme surface durability is very important in this application.


3: PHOTO POLYMER PAD PRINTING PLATE

THIN STEEL STRIP PLATE

Photopolymer cliches consist of a UV light sensitive layer of plastic which, if necessary, is bonded to a metal carrier by an adhesive coating. Single as well as double-layer materials are used for photopolymer cliches. With the latter, the upper layer of approx. 25 μm is generally carried off during the developing process. These cliches are unsuited to screened motifs. The thickness of the single-layer material is up to 400 μm, and the etching depth is controlled by the exposure and the use of a screen. The number of prints which can be obtained using this cliches lies in the range of some 10.000 doctoring operations. Under optimal conditions runs up to 100.000 prints are attainable. Areas of use for this type of cliche include (progressive) proofs, pre-series runs and smaller jobs.


4: PAD PRINTING ROLLER / DATE CODE

pad printing thick steel plate

Two further types of cliche are used for pad printing. For rotary presses, steel rollers are necessary. Since the printing may sometimes be done in a scope of
360°, the film montage and production of the rollers is very expensive. Special etching equipment is also required.
In many areas of industrial production, transitional codes are to be printed, such as the date of production, batch number, model number, etc. In order to avoid the constant production and changeover of new cliches, it is advisable to work with encoding cliches or encoding rods. These cliches are the same thickness as steel cliches, but are polished on the two long sides. Thereby, the rods lie very close together and can be pushed against each other in the ink well. This shift permits any combination of digits and letters to be printed without the necessity of interrupting the production process.

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PAD PRINTING TAMPON BUY GUIDE

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PAD PRINTING INK BUY GUIDE

Check out buying guide for Pad Printing Ink



pad-printing-silicon-rubber-pad

This Technical Guide will help you choose Pad Printing Silicone Rubber Pad or Tampon which will be best suitable for you. Our aim is to help you achieve best possible printing results from your Pad Printing Machines and Supplies.

The pad receives the motif from the cliche, transfers the ink film to the substrate and deposits it there. The pad must be constructed so that it is pliable, but guarantees transfer of the motif without smudges or blurring..Mr. Atul Jain

Various shaped silicone pads along with a variety of hardness’ (durometers) are commercially available. Most pad print equipment suppliers offer several hundred pad shapes and sometimes a custom pad shape will need to be designed specifically for your application. A final recommendation for the pad shape and hardness can be given according to your specific requirements and printing conditions. In other words, when choosing a silicone pad there may be several shaped pads and/or a variety of durometers which will imprint your product in varying degrees of acceptable quality. You cannot easily know beforehand if a specific pad will work… you actually may need to test several pad shapes and durometers to find the one that works the best! Furthermore, the quality of the printing is often directly related to the quality of the silicone pad. The surface of the pad is highly sensitive and may be destroyed by volatile cleaners, solvents or sharp edges on your part. Therefore we recommend you handle and clean the pads with care.

CRITERIA FOR SELECTING PAD PRINTING TAMPONS


1

SHAPE

In pad printing there are only a few shapes that are considered “standard” shapes. At Careprint we have categorized our pad shapes into five style categories – and four of which are considered “standard” shapes:
• Square or rectangular pads
• Round pads
• Loaf (like a loaf of bread) pads
• “V” shaped or bar pads
• Custom (specially designed pads for specific applications).

SILICON PAD GOOD ANGLE BAD ANGLE

It is best to use a pad that has a high angle of attack (see illustration above) and you should avoid using flat-surfaced (or low-profile) pads whenever possible, as they will trap air when they compress against the cliché, and the ink will not be lifted out of the cliché but rather it will “smoosh” outward and create a poor quality print. By having a high angled pad, the greater outward rolling action that is achieved, will yield a better quality print every time.

The square and round shaped pads are considered the most popular pads on the market and these two shapes can often times be interchanged and print the same products. As a general rule, I prefer round shaped pads whenever possible because they provide concentric compression that is not distorted in one direction or the other. A square shaped pad also has these same concentric compression characteristics, and sometimes a square shaped pad (with near 90 degree side walls) is all that will fit into the dimensions of a particular pad-printing machine. This is especially true in small compact sized printers.

A loaf shaped pad is a modified rectangle pad that is designed to allow for linear type or straight-lined graphics. A classic use of a loaf pad is pad printing on pen barrels. A V shaped pad is a pad that is a long bar like pad that is typically molded to have a sharp V shaped bevel. With most V shaped pads you want to print on one side or the other side of the apex of the pad. With V shaped pads you can get double the life from that pad by using both sides of the pad. When the first side of the pad wears out, simply turn it around and use the opposite side for printing the same graphic.

In general, most pad printing production facilities will have a half a dozen “favorite” standard pads that will cover nearly all of their printing needs. But standard shapes don’t always meet the needs for every application and that is where custom pads come into play. A special pad is typically a hybrid design and it may encompass any one or more of the other four basic shapes in its design. One classic example is creating a pad that has two round shapes molded in a side-by-side manner. That way there is no need for any special set up when printing the particular project. Another example of a custom pad is one to print onto a control knob. The custom pad has a machined recess or hole in it to accommodate the raised portion of the knob.


2

SIZE

In order to guarantee sharp, smudge free print, the pad selected should be as large as possible. The less the pad is deformed, the sharper the printed motif. The pad must be larger than the actual motif to be printed, particularly with “problem prints” , where corners are to be reproduced at an exact angle. The disadvantage of the large pad volume is that a very large pad requires a large press, and, such a large pad is more subject to the vibration caused by the movement of the press than is one of smaller mass. Besides this, the price is considerably higher, as the main factor in pricing pads is the weight of the material.

Even though we recommend using a large sized pad, we also recommend that you use the minimum amount of pad stroke pressure to pick up and print the image. By using a small amount of force you create less wear on the pad and you have less chance of distorting the image by “over-driving” the pad. An easy way to determine how little force is enough. When you are printing the image satisfactorily simply back off on the pad force until you stop printing the entire image and then work your way back up in pad force so that you are making a full transfer every cycle.


3

DUROMETER OR HARDNESS

shore-a-hardness

shore-a-hardness

Pads are generally available in varying grades of hardness ranging from 2 to 18 Shore A. However, special grades of hardness from 0 Shore A to over 40 Shore A can be utilized. Here, the rule is: the higher the number, the harder the pad. The hardness has a major influence on the quality of the printed motif and the life expectancy. A hard pad can reproduce print well, and has a greater life expectancy due to its physical stability. In many cases, this hardness cannot be exploited, as the pad would damage the material to be printed. In the same way, softer pads must be used for very curved surfaces, as they can adapt to such surfaces better than very hard pads. Selection of the grade of hardness does, of course, depend on the force of the press which is utilized. Many presses are, accordingly, pushed to their limits by large, hard pads.


4

QUALITY

imported-silicon

imported-silicon

In the case of silicone rubber, there are basically two different systems: crosslinking by polycondensation and crosslinking by polyaddition. Physical properties, such as tear resistance or resistance to swelling in contact with solvents, are better in polyaddition crosslinking materials than in polycondensation systems. Obtaining the raw material is more expensive, however. The smoothness of the surface is a decisive factor in the quality of the print. The smallest impurities or air bubbles caused by defects in manufacturing result in unclear print. In the beginning, new pads tend not to pick the ink up from the cliche as well. This problem can be solved by making a few prints on paper or by a short cleaning operation using alcohol. If cleaned with aggressive agents, such as thinner, the pad immediately takes the ink from the cliche, but does not transfer it to the piece to be printed quite as well. Once a pad is “broken in”, it is recommendable to dab its surface with adhesive tape to remove any dust particles.

Regarding the pad itself, virtually all pads today are made of silicone rubber. At CAREPRINT we use only the best quality silicone rubber materials available. In the past, the first printing pads were made of gelatine and in these early days there was a limited range of pad shapes available due to the poor mechanical properties of gelatine, and these pads were designed much flatter than modern day silicone pads, because of gelatines lack of elasticity.

At CAREPRINT we provide both wood and aluminum bases – and ALL of our bases are mounted accurately and all wooden bases
We do not recommend having the operator use wood screws to mount the pad to his pad holder, as this old-school method makes it very difficult to get repeatable pad positioning and it results in lengthy set-up times. Furthermore if you use wood screws to mount your pads, after you have taken the screws in and out several times, the wood is quickly stripped out and will no longer firmly hold the pad to the mounting bracket. On many occasions, I have seen pads literally fall off the machine during production because of this problem.

Similarly, if a setup requires multiple pads (such as found on a keyboard matrix), aluminum bases are preferable because they will make pad positioning easier and more repeatable. Another advantage to using aluminum bases is that they can be recycled with CAREPRINT and we can remount these bases for you onto new silicone rubber.

GUIDELINES FOR SELECTING PROPER SHAPE FOR PARTICULAR JOB

• First, select a print pad that is sized large enough to compress by hand over the product to be printed. As you are squeezing it down, watch it roll outward and down onto the part, completely covering the printable area. Usually such a pad will suit your needs. Do this with a few pads that you can later mount onto the pad printing press.
• Next, try printing with each of the pads that you have hand selected. A little trial and error is the best method and actually printing with the pad(s) is most often the only way to really determine if that pad is going to deliver the required results. When sample printing, make sure that you are accurately transferring the artwork onto the part without distortion or pinholes.
• If the pad shape you have chosen prints only part of the image area properly, look for similar shaped pad that is larger in the direction that the image is not printing. Having a distorted image around edges is almost always a sign of having a pad that is too small for the job.
• Don’t be afraid to try pads that might seem too large or have too steep of an angle… strange things can happen and remember the #1 rule… BIGGER IS BETTER. An unusual shaped pad just might solve your printing problem.
• Poor quality or irregular ink pickup during the test printing usually means that air is trapped between the pad and the cliché… not enough rolling action! To prove this theory watch carefully as the pad is being compressed onto the cliché to pick up the image – be certain that you see a good rolling out action from that pad. No rolling = no quality printing.
• Whenever it’s possible, try to set up the pad so that the apex (the point) of the pad does not come into contact with the image area on the cliché. Air tends to get trapped at the apex and the ink deposit is not always consistent at the contact point.
• Distortion will occur if the pad is “overdriven” because it is really too small for the image to be printed or if the transferred image is too close to the edges of the pad. Remember to ALWAYS USE AS LITTLE PAD FORCE AS POSSIBLE, both on ink pick up and on ink transfer.

GUIDELINES FOR SELECTING PROPER HARDNESS FOR PARTICULAR JOB

• Hard pads are most suitable for heavy textured surfaces you can also use them when you need to print an image in a recessed area next to a raised surface and a hard pad will roll over this “step”.
• You can also use hard pads in a pad adapter or matrix, when you must fit a single machine with numerous pads that are spaced with small gaps between them (for example, when pad printing computer keyboards or calculator keys).
• Use softer pads when printing onto heavily contoured surfaces and when printing on fragile items.
• You must use a softer pad if the power of your machine can’t compress the pad sufficiently to achieve a satisfactory rolling action – or use a dual durometer pad.
• Avoid using pads of radically different hardness’s for the same part / application, or else the thickness of the ink deposit may vary. This is particularly true when dealing with a pad matrix.

SPECIAL PAD DESIGN FOR PARTICULAR JOB

In some applications a large graphic image must be printed and your machine does not have the power to compress such a large pad in a smooth motion. Three solutions to this problem are available;

  1. Use a pad with a hollow interior that provides the same surface hardness. This hollow area will allow the machine to compress this pad further because there is no extra silicone material to provide resistance. This molding technique also reduces the cost of silicone rubber for such a large pad.
  2. Use what is called a “dual-durometer” pad. A dual-durometer pad is one where the core of the pad is made of a softer durometer material (easier to compress) and the outer layer is of a harder rubber (yielding quality printing results). Both of these methods can help, but the second produces a more stable pad.
  3. Use a pad of the same shape but of a taller design. This taller shape will allow for more compression with less machine force. And yet a fourth option is to look at a different printing method altogether such as screen-printing. Remember, pad printing was not originally designed for printing very large images… it was first developed for printing the fine graphic details found on Swiss watch dials.

QUALITY CONTROL AND CLEANING SILICON PADS

We all know that poor-quality consumables products can ruin your chances for getting quality printing results on press. All CAREPRINT pads are made in a near clean room environment. Prior to shipping, all CAREPRINT pads are 100% hand inspected. Below we outline what our quality control people look for when inspecting. We also recommend that when you receive pads (from us or anybody else), you also check them for the following:
• Blemishes on the print surface
• Foreign particles in the print surface, such as wood splinters or other defects
• Firm attachment of the pad to the backing plate (The pad should be secure, with no air bubbles that will cause the rubber to come away from the base.)
• Hardness within +- 5 Shore (using your 00 durometer gauge)
• Positioning on the backing plate (It should be concentric, with its vertical center line at a 90o angle to the backing.)
• Uniform height (This is particularly important in multiple-pad applications).

When you have a brand new Silicone pad, we recommend that you first “break them in”. To do so, we suggest that you wipe the pad surface with a clean rag soaked with Acetone, Alcohol or thinning solvent. This will remove the build-up of silicone oils that are present on the new pads surface and will help break down the high glossy sheen you find on most new pads.

After the pad has been “broken in” with 3 to 6 wiping (between each wipe, print on paper approx. 10 to 20 times) we suggest that you use clear adhesive tape. We recommend that you use a good quality tape. It is best to use 1″ or 2″ wide 3M Magic brand tape (as this has been de-ionized and has a very low static count). There are some advanced machines on the market  that have built in tape cleaning systems on the machine as a standard feature. These advanced machines make the break in period of a printing pad very short.

If you continually use solvents on your printing pads, you will cause them to prematurely wear out (making pad manufacturers very happy).

IMPORTANT NOTE: When handling any chemical, always remember to wear proper personal protection as outlined by the manufacturer, including rubber gloves.

FOLLOW THESE GUIDELINES AND GET MOST FROM SILICON PADS

Although no hard and fast guidelines regarding pad life are available, you can take a number of steps to get the most life from all your pads:

  • Use a strong solvent only for the initial removal of the excess silicone oils on the surface.
  • Use a mild solvent such as alcohol, or preferably adhesive tape, if the pad must be cleaned during production.
  • Always use tape to remove debris and dried ink before starting a production run.
  • Don’t use too much pad pressure.
  •  Never print onto an empty nesting fixture, sharp edges can cut your pad.
  • Ensure that the substrate is free of debris, particularly sharp particles, before printing.
  • With wood backed pads (those without a threaded insert), don’t let the wood screws penetrate into the rubber.
  • Whenever possible, avoid printing near sharp substrate edges.
  • Use as large a pad as is reasonable for the job at hand.
  • Never store a pad on top of another one.
  • If pads are shipped with a vacuum formed plastic protective covering… remove them from the plastic immediately. Do not store the pads with the covering on them.
  • Handle and store the pads very carefully. Keep them in a storage cabinet at room temperature.

Certain inks have aggressive solvents that will be absorbed by the pad during printing, much like in screen printing, when a squeegee will absorb solvents during a long print run. This solvent absorption will cause the image area to “swell” on the pad, to the point where it will eventually affect the print quality. At this point, stop the machine and replace this swollen pad. This isn’t a permanent condition, and if you allow the pad to stand unused, the solvents that have penetrated the rubber surface will evaporate and the swelling will go down to the original size

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PAD PRINTING INK SELECTION

PAD PRINTING INK SELECTION

This Technical Guide will help you choose Pad Printing Ink which will be best suitable for you. Our aim is to help you achieve best possible printing results from your Pad Printing Machines and Supplies.

To achieve optimal print quality, special pad printing inks must be used. These inks display very highly concentrated pigmentation, as only very small quantities of ink are transferred in the pad printing process.Mr. Atul Jain

BEFORE CHOSING THE RIGHT TYPE OF INK WE MUST UNDERSTAND COMPONENTS OF INK


1

RESINS

This is the component of the ink, which is primarily responsible for the formation of the finished ink film, and the carrier for the colouring material used in ink formulation. Typical examples of resins are PVC, Alkyd, Polyester and Epoxy. Inks may be designed to produce systems, which dry by solvent evaporation, polymerisation or other methods depending on the precise resin structure.


2

PIGMENTS

The pigment imparts the colour and opacity to the finished print and is usually supplied in powder form. It has to be incorporated into the ink by the ink manufacturer using a mechanical dispersion process, (e.g. triple roll mill, ball mill). A commonly encountered pigment is titanium dioxide, obtained from a naturally occurring mineral. This pigment is intensely white and has the particular property of imparting great opacity to any system into which it is incorporated. Titanium Dioxide is often used to create pastel shades by intermixing with coloured pigments.
Several factors must be taken into consideration when the ink producer selects an appropriate pigment; these include light fastness, wash resistance, cost and toxicity.


3

DYES

Unlike pigments, dyes are materials, which are capable of dissolving into the ink medium, and as a consequence of this dissolution such inks have good transparency. Dyes are normally more expensive and have lower resistance properties than pigments. These are the main reasons why dyes are limited to specific applications.
One area of printing where dyes are used in preference to pigments is in textile printing. Dyes may be selected which chemically combine with the material being printed, eliminating the low resistance disadvantage, and thus giving the finished product good resistance properties.


4

SOLVENTS

The function of the solvent is to enable the resin-pigment mixture to be transferred to the substrate via the printing process. As discussed in past articles the evaporation of solvents is critical to the pad printing process. Unlike screen printing where the speed of evaporation of solvents needs to be relatively slow so the ink does not dry in the screen. Pad printing requires fast evaporating solvents to enable the ink to transfer. The advantage of this is that multi colour printing is possible on the same machine.
The exact choice and amount of solvents will depend on the resins and pigment used in the ink, and in a number of cases, on the nature of the substrate.For most inks it is impossible to find a single solvent which will fulfil all the demands made on it, normally a mixture of solvents is used, selected on the basis of their individual properties. Inks supplied may by design not contain the amount of solvent or the type of solvent required. This is to enable the printer to “fine tune” the system to suit the ambient conditions.


5

MINOR ADDITIVES & FLOW AIDS

The properties of ink produced by mixing only resin, pigment and solvents is unlikely to produce a system, which is satisfactory in use. The major defects in such a system are likely to be brittleness, poor film strength, pigment separation and unsatisfactory flow. To overcome such problems one or more minor items are added to the ink system at levels of between 0.001% and 10%.Additives such as plasticisers and surfactants are used to improve flexibility, flow, pigment stability and are thus essential for the adequate performance of the ink. Other additives such as thixotrops are used to tailor the ink for specific purposes where, for example, a property such as flow must remain within certain limits, (e.g. tri-chromatic system). Additives such as barium sulphate or calcium carbonate can be added as fillers to reduce the cost of the ink while still maintaining its essential properties.


PAD PRINTING INK COMPONENTS

PAD PRINTING INK COMPONENTS

While good ink flow and consistent mesh opening are important factors in a screen printing operation, pad printing inks must above all, have excellent release characteristics from the silicone pad. It is also extremely important, that the ink film on the pad becomes tacky during the transfer process, by way of solvent evaporation.

Such a tackiness is necessary, since the adhesive tendency to the substrate must be stronger than the adhesion of the ink to the silicone pad. It is therefore very important to select the proper ink solvent for your particular needs. If the solvent is evaporating too fast, the ink will dry in the etch and a proper pick-up by the silicone pad would be inconsistent. Conversely, if the solvent used dries too slowly and does not become tacky in the plate and on the silicone pad, the transfer will also be very poor and inconsistent (the first few prints may look acceptable and the subsequent prints will be missing portions of the printing).

Because of this need to adjust your inks drying characteristics, some manufacturers offer a variety of evaporation rates and solvents. By having a variety of thinners, you to compensate for a variety of printing environments. Also, by blowing cold or hot air on the pad surface, the evaporation of solvents are accelerated and the transfer may be improved along with faster production rates.

The ink viscosity has to be determined individually by adding the appropriate thinner in a set amount according to the nature of the image to be printed and the substrate. It is very important that the proper ink viscosity is maintained during the printing process by adding thinner throughout the day. In many production environments, thinner is usually added approx. every 45 to 60 minutes, since the solvents are continually evaporating out of the ink mixture. However, if the ink is diluted too much problems with the substrate showing thru the inks surface/ film might show up, since the opacity of the ink is decreased by adding too much thinner. If the ink viscosity is too high (i.e. the dilution is not sufficient) static problems might also occur.

With the newer sealed cup systems that dominate the market today, the fast solvent evaporation that was once common with open inkwell systems is minimized, but not completely eliminated. Most sealed cup pad printing systems that are on the market today cannot completely seal out the air, but they do aid in making day to day production much easier.

There is a unique system from microPrint that features a viscosity control system inside their sealed cup. The viscomatic is an optional system for microPrint machines which has a Teflon coated propeller that is mounted inside the sealed cup. This propeller is directly driven by a motor that feeds back torque information to the on-board PLC. As the torque increases (as the ink thickens from loss of solvents) the PLC senses this increased drag on the motor and signals an integrated micro pump to open its valve and pump a programmable amount of thinner into the cup. So the inks viscosity is constantly being checked, monitored and properly maintained through-out the production run. The viscomatic is able to run single component inks for many successive days and two component inks pot life can be greatly extended. The viscomatic is the only system on the market that offers this care free approach to the pad printing process.Mr. Atul Jain

DIFFERENT TYPES OF PAD PRINTING INKS AVAILABLE IN THE MARKET


1

SOLVENT EVAPORATING OR SINGLE COMPONENT INK

These inks make up the major part of pad printing ink production. They dry only by solvent evaporation; that is to say when the solvent has evaporated from the deposited ink film. Care must be taken when printing non-absorbent materials with this system, as absorbency is required to give best adhesion. Solvent evaporating systems are available in gloss and matte finish and dry very rapidly. Solvent ink systems perform particularly well with many thermoplastics. A guide to their suitability for a particular plastic is to rub the solvent used to thin the ink onto the plastic. If it melts the surface of the plastic the chances are the ink will adhere very well. Generally, one component inks are used to print on thermoplastic substrates, such as Polystyrene (ABS and SAN), Polycarbonate, soft and rigid PVC, Polyethylene and Polypropylene. Since one component inks do not typically have a pot life, pad printing with these type of inks is rather easy in comparison to Two component ink systems , since one component inks can be utilized for a longer period of time.


2

OXIDATION DRYING INKS

In these systems the resin in the ink absorbs oxygen from the atmosphere and undergoes a polymerization process producing a very tough, flexible weather resistant ink film. Because of their slow drying speed, inks of this type are usually allowed to dry in the air. They are limited in their application for pad printing due to their slow drying.


3

TWO PACK OR REACTIVE (CATALYST CURING INKS)

These inks also contain resins capable of polymerisation. However, the required catalyst is normally either blended into the ink or else supplied in a separate pack, which may be mixed with the ink when required. If blended in the inks have a restricted shelf life. They may be cured rapidly by heating. Two pack inks are used extensively in pad printing
They are generally applied to difficult substrates for example, metals, thermoset plastics, glass, etc. Where good chemical and abrasion resistance is required these inks are very popular.
Particular care must be taken when mixing the base ink with the catalyst. The ratio specified by the ink manufacturer is by weight. Therefore these inks must always be weighed during mixing. There are no exceptions. Inaccurate mixing can give differing ink adhesion and product resistance characteristics.

Another important factor to be aware of with these ink systems is that after printing and prior to complete curing the temperature of the printed ink film must not drop below 15 Degrees Centigrade. Should this occur the ink will cease curing and cannot be restarted. This may not be a problem if the ink is force dried with heat, as the curing may nearly be complete. However if curing takes place in storage over a period of time the ink film is vulnerable. An ink film is not necessarily cured when it is dry. It takes time or temperature or a combination of both to effect a complete cure. Typically 5 Days at 20 degrees Centigrade or 10 minutes at 100 Degrees Centigrade. This information is available on the ink Technical Data Sheet, which, unfortunately most users do not bother to read.


4

STOVING INKS

Stoving inks can be influenced regarding the elasticity of the ink film. They need a certain minimum temperature and time to cure within the minimum and maximum temperature of curing. The cure time varies which means that the higher the temperature, the shorter the drying time. Equally important is that if the ink film is more elastic, the lower the temperature.


5

SUBLIMATION

This is a process where a solid turns into a gas when heated. Sublimation inks make use of this characteristic. When applied to an appropriate surface and the temperature of the ink and the surface is raised to approximately 200o C dyes in the ink sublime and the surface of the material becomes porous and the dyes pass into the material. This actually changes the colour of the base material.
Polyester, polythene, acetyl and some nylons are most suitable for this, as they need to be able to withstand the high temperatures.
Once the material has cooled the ink is sealed into the surface. The wear characteristic of the ink is equivalent to that of the base material.
Care must be taken in the selection of colours, which are limited, as certain inks are sensitive to UV light and fade very quickly.
Another limitation is that the background material colour must be lighter than that of the ink. The range of colours is very limited and matching to Pantone colours is almost impossible.
Pad printing is a suitable process for this system, as ink deposit must be kept to a minimum other wise the colours will bleed.


6

WATER SOLUBLE

With the awareness of the dangers of exposure to harmful solvents and the consequent introduction of regulations, e.g. COSHH and the EPA industry is attempting to move away from solvent based systems. It is unlikely that they will ever be suitable for pad printing, as the speed of evaporation is very slow.


7

CERAMIC AND GLASS THERMOPLASTIC INKS

The use of these systems has proved to be very successful for pad printing onto glass and ceramic. It uses the system developed for screen printing onto glass, where at ambient conditions the ink is in a solid form (rather like candle wax). When the ink is raised to 80 Degrees Centigrade it is fluid. Unlike conventional pad printing ink the pick up and print is not by evaporation of the solvents but by the cooling effect of the pad when it comes into contact with the ink in the etching on the plate. Similarly the ink transfers from the pad to the substrate because the ink surface exposed to the air is tackier than the surface on the pad. A complete transfer is achieved by the cooling effect of the substrate, normally glass or ceramic. This process enables fairly heavy deposits of ink to be printed. The etch depth is greater when using this ink system. 30 to 50 microns. Once the item has been printed the ink has to be fired onto the surface at approximately 580 Degrees Centigrade for glass and 1200 Degrees Centigrade for ceramic. In glass printing the firing chamber is known as a lehr and in ceramics printing a kiln. This area is a topic all of its own. A certain amount of success has been achieved with solvent systems carrying ceramic pigments but the thermoplastic system is the most popular. These inks are dishwasher safe. If this characteristic is not necessary Stoving and Reactive Curing inks are very effective for printing onto glass and ceramic.


8

ULTRA VIOLET (UV) LIGHT-CURABLE (SOLVENTLESS) INKS

This is a system widely used in screen process inks. It offers the printer faster curing speeds, ease of printing and fewer of the environmental problems normally associated with solvent-based systems. Extraction is necessary to remove the Ozone, produced by the light sources, out of the building to atmosphere, although these emissions are now restricted due to the Environmental Protection Act.
Care must still be taken when handling these systems and the normal precautions regarding direct contact with skin, eyes, and ingestion apply. In addition as the ink does not dry without intense UV Radiation, it will transfer upon contact.
Most of the major pad printing ink suppliers are carrying out developments to produce a UV ink suitable for the pad printing process. The initial target market is the Compact Disc Industry. This rapidly expanding market (in excess of 20% year on year) is constantly searching for more effective ways of printing the discs. Audio CD and CD ROM are becoming even more colourful this need is being addressed with seven colour screen printers and offset printing machines. The use Pad printing in the CD industry has, in recent years, been declining. This development could bring new life into the use of Pad printing onto CD’s.

Machinery suppliers have been working with ink manufacturers to develop a UV Curing Pad Printing Ink.
There are applications where UV Curing inks have solvents added to them to enable them to perform as conventional pad printing inks in the transfers from plate to pad and pad to substrate. This system is used when a hard over-lacquer is required to protect an ink from abrasion. Keyboards are a typical use of this technique.
Precise details regarding ink technology and application are still somewhat sketchy however, information received indicates that the process operates as follows:
As the inks cannot contain solvents the normal tackiness achieved with conventional inks is not possible. However the pad still has to pick the UV curing ink out of the plate this is done by modifying the rheology of the inks to give the necessary tackiness. When the pad comes down onto the plate and its surface comes into contact with the tacky ink surface the ink sticks to the pad and when the pad lifts it causes a percentage of the ink in the etching to shear away from the remainder of the ink.
This will only be approximately 6 microns of ink. When the ink comes into contact with the substrate it releases from the pad because of its silicone content. The deposited ink is then cured with high intensity UV light and as there are no solvents in the ink the dried film is the same as the wet film. The depth of the etching on the plate would be about 20 microns. It is important to ensure that the thickness of the ink film is controlled otherwise curing will be very difficult.
This is still in the experimental stage and different manufacturers will specify different pad materials, hardnesses and etch depth of plate. It is unlikely that the range of substrates printable will be as extensive as conventional solvent based inks.
As with conventional UV curing inks ink opacity is compromised. It is likely that a white background will be necessary to give the intensity of colour required. Considerable testing is essential before putting the technique into production. The big advantage of this system is that it is not affected by small changes in ambient conditions and will give much greater stability to the process.


GENERAL RULES TO FOLLOW WHILE USING INK

  1. Read the Technical Data Sheets.
  2. Read and understand the Health and Safety Data Sheets.
  3. Always wear protective clothing, gloves, goggles, when mixing inks.
  4. Ensure there is adequate ventilation at all times.
  5. Stir ink and hardeners before use.
  6. Use only specified solvents.
  7. Weigh the ink, solvent, hardener, components and mixture.
  8. Do not use inks or hardeners that are past their shelf life.
  9. Seal partly used tins of ink their shelf life is reduced once opened.
  10. Keep unused mixed ink in a closed container.
  11. Do not use mixed two component ink after pot life is expired.
  12. Do not use two component inks in closed cup printing systems.
  13. If single component ink is used continuously change every two days.
  14. Never mix ink types to achieve a colour match.
  15. If ink splashes onto your skin wipe off excess with tissue or cloth and the wash with soap and water or proprietary hand cleaner.
  16. Never use thinners to remove ink from skin.
  17. If ink or solvents enter your eye irrigate with water for 15 minutes and seek medical advice immediately.
  18. In cases of swallowing do not induce vomiting seek medical advice immediately.
  19. In both the above cases take the Health and Safety Data Sheet and the Technical Data Sheet to the medical authority.

DILUTING OR THINNING PAD PRINTING INKS

Pad printing inks are usually diluted somewhere between 10% and 20% with thinner. The best printing viscosity is subject to various factors such as hardness of the pad, size of the printed image and the type and nature (porous or non-porous) of the printing substrate

SHADE SELECTION AND SAFE INKS

Compared with screen printing, the theoretical ink lay down in pad printing is much less and inks that are formulated for screen printing applications are not suited for pad printing due to their lack of opacity in the ink. A true pad printing inks will have nearly 4 times the pigment content that a screen printing ink has.

The color selection is absolutely comparable to screen printing inks and virtually any color shade can be produced (PMS or to a color chip). Many pad printed items, such as advertising specialty items and children’s toys are subject to certain laws in the USA and Europe, that describe exactly the demands of the printed ink found on the product. Most specifically the EEC regulation EN 71, part 3, for the printing of toys and similar items must be observed. This law strictly forbids the use of coatings/ inks that contain more than trace amounts of heavy metals. Some ink shades have to be modified from their standard pigmentation to meet these strict requirements.


INK HANDLING ON THE PRODUCTION FLOOR

The operator is given the production control card. That details all the setting instructions and ink solvent mixes. Having gathered together all the elements necessary to set up the job the ink is mixed.
The operator puts on additional protective clothing. Goggles, gloves and an apron if necessary.
Read the labels on the ink, hardener, and solvents to ensure they are as specified on the production control card. Also checks the date to ensure that they are not outside their use by date.
The tin of ink is opened and the contents stirred vigorously to ensure that it is well mixed. The hardener is also opened and stirred. A suitable container is placed on the scales and the stated amount of ink is weighed in. The hardener is then added to the correct ratio, by weight, and stirred in. Then the mix of solvents is added and this is carefully stirred to ensure complete dispersion in the mix. At this stage some people will use a visco-spatula that gives a guide to viscosity. If your mixtures are correct by weight the viscosity will be acceptable in any case. The specified amount of ink is then poured into the ink reservoir. The remainder used for replenishing ink later on in the day is kept in a closed container.
The other machine settings are made as per the production control card and, surprise the machine will print first time. Assuming the solvent balance is maintained during the day with addition of measured amounts of solvent at specified intervals, it will run right on through. Depending on the ink system selected and the ambient temperatures, the ink will have to be completely replaced with a new batch in 8 to 10 hours time. This is because the ink will be curing as it is in the ink reservoir and would have changed its print characteristics by that time. In very high ambient temperatures it may be necessary to change the ink more often. Accurate estimation of ink used during the day is important, as mixed ink not used should be disposed of correctly and is expensive to waste. If you had been using an ink that did not use a catalyst the ink could be used for a much longer time as it does not cure in the ink tray. I would still recommend that it is changed after 48 hours, as there can be a build up of contaminants in the ink that could alter its properties. There also may be chemical changes if the inks are Oxidation curing. Often I hear printers say, ” If I use fresh ink it improves the print”.
Reactive (Two component) inks are not recommended for use in sealed ink cups as they gel if they are not agitated and will cure off completely if left in the cup for extended periods.
Care must also be taken when using reactive inks on screened plates as any trace left in the etched portion of the plate overnight will cure and be impossible to remove the next day. This applies to a greater or lesser extent with any part of the machine or jigging.

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Pad Printing Plate, Silicon Pad, Jig Fixture, Doctor Blade, Consumables

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Different sizes of Pad Printing Plates

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Different types of Pad Printing Silicon Pads


PAD PRINTING PLATE

Pad Printing Steel Plate

Closed Cup Machine or Open Ink Well Machine – Plates No Problem!!! We always have a ready stock of large variety of different sizes of Cliche Plates or Tampon Plates. We have around 40 odd sizes of plates ready for the customer. Our Steel cliches are especially suited to high quantity printing. Tell me more…

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PAD PRINTING DOCTOR BLADE

Doctor Blade, Ink Cups.

We manufacture Doctor Blades which are used for cleaning(Doctoring Operations) on the steel plate. We have various sizes depending on your requirements. We also provide Ink cups upon request. Various other small consumables are also readily available Tell me more…

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PAD PRINTING SILICONE PAD

Pad Printing Silicone Pad

We have around 400 odd different varieties of Printing Silicon Rubber Pads or Tampon pads. Tampons are available in three different Shore A Hardness namely soft, medium and Hard. Round, Square and Rooftop(long) shapes are cataloged. they can be downloaded in PDF format.Tell me more…

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CARROM STENCIL

Carrom Screen Printing

We make Textile Carrom Screen for Screen Printing of Professional Carrom Boards. We have more the 30 designs in carrom.We make carrom screen and supply to almost all carrom manufacturers. Carrom Screen are made for 12″, 15″, 18″, 21″, 24″, 27″, 30″, 36″ & 42″ (sizes in inches).  Tell me more…

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PAD PRINTING JIG FIXYURE

Epoxy, Acrylic Jig Fixture

The fixture is a female cavity of the substrate you want to print on. the accuracy of the fixture is very important. If it not accurate the substrate is move while printing and cause mis-printing.We make fixtures from 4 different types of materials Epoxy, Acrylic, Nylon and Teflon Tell me more…

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SCREEN PRINTING SUPPLIES

Screen Printing Supplies

e manufacture and sell screen stencils of various sizes in wooden or metal for screen printing machines.We manufacture our own Screen Emulsion. Screen Emulsion is available as a very reasonable rate. Indian as well as Imported Squeegee is also available with us.. Tell me more…

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Pad Printing Machines and Screen Printing Machines – by Print Link

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