by John Kaverman
What constitutes a “good pad?” At first glance the two pads in example 1 may appear to be of similar in quality. Look again. The vacuum-formed pad on the left was manufactured utilizing a precision master. The pad on the right was manufactured using a vacuum-formed mold of a pad.
The telltale signs that a vacuum-formed mold was made from a pad instead of a precision master are the lack of sharply defined edges and the large taper at the base. Additionally, molds that are vacuum-formed using a pad as a master rarely will produce pads having a consistent height.
In example 2, the pad on the right is taller. While this might not present a problem if running this pad alone, it would most likely cause problems if it was used in tandem with the pad on the left because the optimal print strokes would be different. This would be especially troublesome if the pads were individually mounted in an assembly where parallelism is important.
As stated previously, and as example 3 illustrates, it is difficult to correctly mount a pad made in a mold that was vacuum-formed using a pad. The pad on the right was mounted crooked. It is approximately four degrees off vertical on the long axis. Additionally, in example 4, it is mounted crooked in the short axis by approximately four degrees, and as shown in example 5, the wood base was mounted off-center relative to the base of the pad.
Essentially, the manufacturers of the pad on the right (in example 3) did the following things incorrectly:
- Used a mold vacuum-formed from a pad rather than a precision machined master.
- Did not pour the mold to the correct height.
- Did not mount the pad straight in any of three different axes relative to the base/centerline.
The old saying “you get what you pay for” many times is transposed in pad printing to “you pay for what you get.” If the pad in question had been purchased from a discount vendor, you would have paid for it in lost time trying to get it to work.
How Pads Are Made
A properly manufactured pad mold (Figure 1) is CNC-machined from aluminum, poured, or vacuum-formed using a CNC-machined “precision master” pad. The surface characteristics of the mold will dictate the surface characteristics of the finished pad. For example, a highly polished mold will produce a shiny, smooth pad. Typically a “lip” is machined into the top edge of the mold to accept the pad base after the mold has been poured.
Pads are made of a mixture of three component parts: silicone-base material, catalyst, and silicone oil. The silicone-base material comes in numerous different blends and levels of quality. The higher the material grade, the better longevity and service-life you can expect from the pad. Most silicone-base materials have an appearance similar to wood (Elmer’s) glue. The catalyst is typically what gives the pad its color. For example, a blue catalyst produces a blue pad. Catalysts differ with respect to the length of time it takes them to cure. Silicone oil is what gives the pad its degree of hardness (expressed as “shore”). The more oil, the more pliable the resulting pad and the more easily/faster the pad releases the ink onto the substrate.
Once these three components are mixed together per the required formula, they are placed in a vacuum so that all of the air can be evacuated from the mixture. Once the air is removed, the material is poured into the mold (Figure 2).
Pad bases are typically made of either wood or aluminum. In order to get the pad to adhere to the base, the base first must be chemically primed (Figure 3).
Normally, bases have a hole drilled into the middle of them to allow excess silicone to escape from the mold once the base is installed and clamped (Figure 4).
The completed pad is removed after the curing schedule is realized. Excess silicone is trimmed from the base, and in some cases a threaded insert is placed into the hole to make mounting of the pads easier (Figure 5).
John Kaverman is the technology coordinator for Innovative Marking systems of Lowell, Mass. He holds a degree in printing from Ferris State University and has over 20 years of combined screen and pad printing process experience. He may be reached at [email protected].