Understanding the Pad Printing Pad

Shape and Size of The Pad Printing Pad.

Shape is the most important variable in selecting a pad.

It's important to choose a pad shape that will achieve a "rolling" action when the ink is both picked up from the cliche and deposited on the substrate. Without this roiling actjun, air can be trapped between the pad and either the cliche or the substrate, causing print distortion and pin-holes due to irregular ink pickup or depasition.

In order to achieve a satisfactory print, the pad surface must roll onto the cliche (plate) and the image area of the substrate, as shown in Figure 1.

The shape of the pad largely determines how well the pad will achieve this rolling action. This makes shape the most important variable in selecting a pad. Most pad suppliers have hundreds of pad shapes in their standard inventory. But most are based on these three basic shapes
(see Figure 2):

Figure 2:
Although hundreds of pad shapes are available, most are based on these three shapes: round, rectangular, or bar. Regardless of what pad shape you choose for a job, avoid pads with perfectly flat profiIes, since these can trap air during ink pickup or deposition. For best printing results, use the largest pad size that is practical for - the item to be printed.

Each of these shapes could have either curved or flat printing surfaces, depending on the nature of the part to be printed. But regardless of what shape the pad has, it must roll onto both the cliche and substrate for good printing results. Try to avoid flat-bottomed pads, as they have a tendency to trap air when they come in contact with the cyiche, hampering ink pickup. Again, the more roiling action that is achieved, the more ink that will be transferred.

Another important variable to consider is pad size relative to image size. I n screen printing, the larger the screen is in relation to the image size, the less distortion that will occur. The same holds true in pad printing.

The larger the pad, the less the image is likely to distort. Often, the distance between the cliche and the body of the machine (sometimes called the "throat" of the machine) will determine the maximum pad size you can use.

Generally, most pad-printing shops will have a favourite shape that covers 90% of their applications. But standard shapes are available for all sorts of unusual applications. For example, many suppliers have standard shapes that will print onto oven-control knobs. These special pads have holes in them to accommodate the embossed portion of the knobs and allow the print to be applied to the bevelled edge. So before you go to the expense of having a custom pad rnouId made, check with your suppliers to be certain that no such shape already exists, Nowadays, someone, somewhere should have a pad shape to suit your job. Remember, if you do pay to have a special mould made, your supplier will probably include the item in the next edition of its catalogue.

For very unusual parts, custom pads will sometimes be made that combine two different profiles, as in the top illustration in Figure 3.

For unusual imaging needs, a custom pad may be used that combines two different profiles, as the one in the top illustration.

Such pads are expensive and must be carefully designed to avoid print distortion. Often, a better alternative is to use two different pads and mount them together on the machine, as suggested in the lower illustration.

These "combination" pads are worth considering, but they can be expensive and they are prone to print distortions unless they are very carefully designed. A preferable solution is to use two separate pads and mount them close together on a single machine. The lower half on Figure 3 shows how the combination pad above could have been designed as two separate pads. Another advantage of using two pads is that if one is damaged, the cost of replacing it is much less. Always use as litt/e pressure as possible to pick up and print the image.

Use these guidelines when choosing a pad shape for a particular job:

  • First, try your standard pads that you think would do the job for this particular part. Do a test print to verify that the proposed print area is imaged accurately.
  • If the pad shape you have chosen provides a satisfactory print over just a part of the area, look for similar pad shapes that extend the profile in a way that will cover the entire image. Distortion at the image edges is almost always caused by undersized pads.
  • If the obvious pads fail, try ones that appear to be unsuitable. Maybe the pad has a sharper angle than would seem to be appropriate, or is clearly too large for the image. It still may solve the problem.
  • Irregular ink pickup during the test print usually means that air is being trapped between the pad surface and the cliche. Watch carefully as the pad is being imaged to be sure that a rolling action is occurring.
  • Whenever possible, ensure that the point or apex of the pad does not come into contact with the image area of the cliche. This tends to thin the ink at that point, causing an inconsistent ink deposit.
  • If the pad is "overstressed" (that is, too small for the image) or the image is too close to the edge of the pad, distortion is likely to occur. Always use as little pressure as possible to pick up and print the image. If the machine is running too fast, excessive pad pressure can cause distortion as well as poor ink transfer.
  • If your experimentation doesn't reduce the print distortion to an acceptable level, and a custom pad is out of the question, your last resort is to distort the image on the cliche to compensate. This is often done by printing a grid onto the substrate and measuring the distortion of the grid to guide you in the alterations that must be made to the original artwork. This will shorten the time it takes you in test printing, but it won't eliminate the trial-and-error altogether. This method also leads to ongoing problems since positioning of the part and the pad (relative to the image on the cliche) must be absolutely dead on each time the job is set up to avoid distortion. We have heard that computer software packages are available that will do this work, but none are known to us.

Shape and Size of The Pad Printing Pad.

Shape is the most important variable in selecting a pad.

Hardness of the Pad Printing Pad

The hardness of the pad is normally determined by the amount of silicon oil used when the pad is moulded.

Surface Finish of the Pad Printing Pad

Throughout the pad-printing industry, the custom practice among pad manufacturers is to furnish pads with a high gloss finish.

Material of the Pad Printing Pad

This topic refers not only to the material of the pad itself, but also the base onto which the pad is mounted.

Special Printing Pads for Large Images

In some situations, a large image area must be printed and the machine does not have the power to compress such a heavy pad in a smooth motion.

Quality Control of Pad Printing Pads

Poor-quality consurnables like pads can destroy the performance of the printing machine.

Pad Printing Pad Life

Next to "What should I use?", the most difficult question to answer is "How long should a pad last?"