pad-printing-silicon-rubber-pad

Selecting Pad Printing Tampon

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

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

PAD LIFE

Next to “What pad should I use?” the next question is usually “How long should a pad last?” Pads are like most things in life: Just like the human body, the better you treat it; the longer it will last. Mechanical damage, aggressive solvents, and poor storage all take their toll on pad life. But the real killer in pad life is a careless operator. 

Use a softer durometer pad if the power of your machine cannot suffiiently compress the pad to achieve a satisfactory rolling action.

In many applications, it is not unusual for a silicone pad to last up to 50,000 imprints, but going much beyond 50,000 prints is not commonplace. Conversely, some pads are completely ruined before they even print a single part. This disaster usually happens during set-up, when the downward motion of the pad (either during ink pickup or transfer to the part) has been set at too long of a stroke distance and the pad crashes hard into the cliché or part. The end result can be the total destruction of the pad.

Always remember to back-off the stroke distances when setting up any new project.

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