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