Understanding the Pad in Pad Printing

by Peter Kiddell, PDS Consulting

One of the most difficult questions to answer about the pad printing process is, “How do I determine what pad to use?” One answer won’t apply for every shop or application. Rather than lay down inflexible rules, this article will provide a methodology for determining the optimum pad characteristics and a framework you can work within to get the best performance from your pads time after time.

The key to good pad printing is to reduce and control the variables. Though the function of the pad seems simple – to transfer the image from the printing plate onto the substrate – it is subject to print quality fluctuations if the correct pad type is not chosen or used properly.

Five key pad characteristics can affect the quality of the printed image. These include shape, size, hardness, surface finish, and material. Vary any one of these and the print quality will shift. These factors must be taken into account when planning the job and ordering pads from suppliers.

Shape and Size

Shape is the most important variable in selecting a pad. In order to achieve a satisfactory print, the pad surface must roll onto the plate and the image area of the substrate. 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 the majority are based on three basic shapes: round, rectangular, and bar (see Figure 1).

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

Another important variable to consider is pad size relative to image size. In screenprinting, 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 plate and the body of the machine (sometimes called the ‘throat’ of the machine) will determine the maximum pad size to use.

For very unusual parts, custom pads sometimes will be made that combine two different profiles, as in the top illustration in Figure 2. These “combination” pads are worth considering, but they can be expensive and 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 2 shows how the combination pad above could have been designed as two separate pads. Another advantage to using two pads is that if one is damaged, the cost of replacing it is much less than replacing a combination pad.

Use the following guidelines when choosing a pad shape for a particular job:

• First, try whichever standard pads you think would do the job for the particular part. Do a test print to verify that the proposed print area is imaged accurately.

• If the pad shape 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. Undersized pads almost always are the cause of distortion at the edges of the image.

• 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 pick-up during the test print usually means that air is being trapped between the pad surface and the plate. 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 plate. 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 if 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 plate to compensate. Often, this is achieved by printing a grid onto the substrate and measuring the distortion of the grid to guide in the alterations that must be made to the original artwork. This will shorten the time taken 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 plate) must be absolutely dead-on each time the job is set up to avoid distortion.

Hardness

The hardness of the pad normally is determined by the amount of silicon oil used when the pad is molded, i.e., the harder the pad, the less silicone oil that was added. Four basic pad hardnesses are standard in the industry and cover most applications. Custom pad hardnesses are available through most pad suppliers. Many pad manufacturers color code the four standard hardnesses by adding pigment to the silicone itself or by coloring the pad base. Not all manufacturers use the same code, but the typical color designations by pad hardness are listed in Figure 3.

    Figure 3

As a general rule, the harder the pad, the better it will perform. However, a hard pad may be impractical in some applications, such as when using a low-power machine or printing onto a delicate item. Choosing the proper pad hardness for a job is a matter of experimentation and experience.
Special 55 Shore pads are available for printing onto abrasive substrates and textured finishes. Two such applications include the turn-signal control arms and windshield-wiper control arms of automobiles, which are molded in glass-filled nylon. A heavy white ink is required, and the pad must resist the abrasive nature of the substrate./p>

A useful tool for all pad printers is a durometer gauge for determining pad hardness. This simple tool (the same one you would use to measure the Shore hardness of a squeegee) is available through silicone-rubber suppliers and many general service dealers in the screen and pad printing industries. Maintaining accurate pad hardness can be problematic for some pad manufacturers, thereby making the durometer gauge an ideal quality control device for incoming pads.

Use the guidelines on the next page for pad hardness when selecting pads:

• Hard pads are most suitable for textured surfaces. They also can be used when printing an image in a recessed area next to a raised surface where the pad will have to roll over the ‘step’.

• Hard pads also can be utilized in a pad ‘nest’ or matrix, when a single machine must be fit with numerous pads that are spaced with small gaps between them (for example, when printing computer keyboards).

• Use softer pads when printing heavily-contoured surfaces or fragile items.

• A softer pad must be used if the power of the machine can’t compress the pad sufficiently to achieve a satisfactory rolling action.

• Avoid using pads of different hardnesses on the same application, or the thickness of the ink deposit will vary on the substrate. This is particularly true when dealing with a pad matrix.

Surface Finish /p>

Throughout the pad printing industry, the custom practice among pad manufacturers is to furnish pads with a high gloss finish. Users have had to ‘matte’ the pad surface – that is, remove the excess silicone oil that creates the glossy appearance – to enable the pad to pick up and transfer ink during the printing process. Typically, pad printers will use a strong solvent such as a fast thinner for the initial silicone oil removal. But excessive use of such a strong solvent damages the pad and shortens its life.

Pads with a ‘ready to use’ finish virtually eliminate the need to matte the pad. With these pads, the base rubber material is very close to the desired pad hardness, so the manufacturer adds much less silicone oil, if any.

After matting the pad (if necessary), the only other step that must be taken prior to production is to wipe the pad gently with an alcohol-based pad cleaning fluid. This removes any free silicone oil that can sometimes leach out of the pad. However, once the pad has been used, the best way to remove solid debris, dried ink, and dust is with a quality brown packaging adhesive tape. By following this simple procedure, you will improve your print quality, reduce downtime, and prolong the life of the pad.

Some pad suppliers provide ‘rejuvenating oil’. Basically, this is a silicone spray that can be applied to the pad surface when it becomes dry due to the removal of silicone oil by aggressive thinners. It can help prolong pad life, but spraying silicone spray anywhere near a surface that has to be decorated is a recipe for disaster. Because of the potential problems, using such oils is not recommended.

We have come across two extreme cases of pad abuse. The first company soaks its new pads in a solvent tank for four hours prior to use, while the second company has a press dedicated to ‘running in’ new pads before they are used in production. Pads have a limited print life; they are susceptible to mechanical damage; and they are not cheap. Such extreme measures make very poor use of your investment.

Material

This topic refers not only to the material of the pad itself but also, to the base onto which the pad is mounted. For example, you may order pads mounted onto aluminum bases rather than wooden ones, the advantage being that the pads can be mounted very accurately onto the machine. Pads with wooden bases rarely are supplied with drilled holes for attaching to the pad holder of the machine. This means that the printer usually screws the pad in himself, making it difficult to get repeatable pad positioning. Also, wood screws are used with wooden bases. If these screws are taken on and off several times, the base becomes loose. On several occasions, we have seen pads literally fall off the machine during the print cycle because of this problem.

Aluminum bases are better because they come with predrilled holes, so there is only one position to mount the pad on the holder or backing plate. Also, unless you have a very heavy-handed press operator, the threads of the screws are not easily damaged. This means that it is simple and quick for press operators to mount or replace pads. Similarly, if a set-up requires multiple pads (such as a matrix), aluminum bases are preferable because they will make pad positioning easier and more repeatable. One further advantage of aluminum bases is that they can be recycled with your pad supplier.

If you are using a long-bar pad that does not have an aluminum backing, then back the pad with either aluminum or a thicker piece of wood. This will prevent the base from bending when it comes into contact with either the plate or the substrate. Such bending can cause print distortion or prevent the pad from picking up the image entirely.

Recently, pads have appeared on the market that are molded onto a nylon-type base material, offering the advantages of aluminum at less expense. These pads also are delivered in a protective clear vacuum-formed plastic that can be used to store the pads when not in use.

Certain pads can be recycled (sometimes called ‘recovered’ or ‘resheathed’). Large pads are very expensive. To reduce the cost of replacing pads, some printers will send their worn or damaged pads back to their supplier. The supplier cuts the surface of the pad away and inserts the remaining rubber into the mold where the replacement pads are created, reducing the amount of new silicone rubber that is needed. The new material cures and bonds onto the old rubber and the finished pad performs like new, with a savings of up to 50 percent over the cost of a new pad. If the pad has been split or broken down internally, however, it cannot be recycled. Some pad manufacturers will use recycled material and mix it with new silicone rubber. As long as the pad surface is ‘virgin’ silicone rubber, this practice is perfectly acceptable.

Special Pads for Large Images

In some situations, a large image area must be printed but the machine does not have the power to compress such a heavy pad in a smooth motion. Two solutions to this problem are available.

The first is to use a pad with a hollow interior that provides the same surface hardness. This technique also reduces the cost of silicone rubber for such a large pad. The second option is a dual-hardness pad, where the core of the pad is made of a softer material and the outer layer is the harder rubber. Both methods can help, but the second produces a more stable pad. (A third option, of course, would be to use a different imaging process such as screenprinting for larger image areas.)

Quality Control of Pads

Poor-quality consumables like pads can destroy the performance of the printing machine. When you receive pads from your supplier, it’s vitally important before accepting them to check the pads for the following:

• Blemishes on the print surface.

• Foreign particles in the print surface, such as wood splinters.

• ‘Nipples’ on the print surface.

• 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 +2 Shore (using your durometer gauge).

• Positioning on the backing plate. (It should be concentric, with its vertical center line at a 90° angle to the backing.)

• Height (which is particularly important in multiple-pad applications).

Report any defects to your pad supplier immediately so that replacement pads can be furnished.

Pad Life

Next to “What should I use?”, the most difficult question to answer is “How long should a pad last?” Pads are like most things in life: the better you treat them, the longer they will last. Mechanical damage, aggressive solvents, and poor storage all take their toll. But the real killer is a careless operator. Some press operators have been known to pull a pad completely off its mounting plate during cleaning. If you make your staff aware of how much pads cost, they may treat them with more care.

It isn’t unusual for pads to last 50,000 prints; however, going substantially over that amount is not common. Conversely, some pads are irreparably damaged before they print a single item. Usually, this results from a poor set-up, when the downward motion of the pad during ink pick-up or deposition is far too long, resulting in the destruction of the pad.

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 initial removal of the silicone oil on the surface.

• Use a mild solvent such as alcohol, or preferably an adhesive tape, if the pad must be cleaned during production.

• Always use an adhesive tape to remove debris and dried ink before starting a production run.

• Don’t use too much pad pressure.

• Ensure that the substrate is free of debris, particularly sharp particles, before printing.

• With wooden-backed pads, don’t allow the mounting screws to penetrate the rubber.

• When possible, avoid printing near sharp substrate edges.

• Use as large a pad as is reasonable for the job.

• Never store a pad on top of another.

• If pads are supplied in a protective shell, use it when storing the pads in your shop.

• Handle and store the pads very carefully.

Certain inks have aggressive solvents that will be absorbed by the pad during printing, much as squeegees will absorb solvents during long print runs. This absorption will cause the image to “grow” on the pad, to the point that it eventually will affect the print quality. At this point, you must stop the machine and replace the pad. This isn’t a permanent condition, though. If you allow the original pad to stand, the solvents that have penetrated into it will evaporate, returning the pad to its normal surface finish. You can accelerate this process by warming the pad.

AltAlthough the importance of pads is sometimes overlooked in the field, remember that the process derives its name from these silicone-rubber image carriers. Correct selection and care of pads is essential. As with any process, pad printing has its limitations and it’s best to understand those before you choose a pad. Common sense and experimentation will guide you. Overall, keep the pad surface in good condition and it should serve you well.

Peter dell, PDS Consulting, is an international consultant and trainer in pad printing, screenprinting, and digital printing. For more information, e-mail [email protected] or visit www.pdsconsulting.co.uk.