Evaluating squeegee materials for quality and performance on press is one of the keys to successful screen printing. This article describes material formulations, essential vocabulary, and criteria for making effective comparisons.
By James Elliot
One of the most popular synthetics was Neoprene, but Nitrile (or Buna-N) and EPDM, among others, have all been employed at some point. Different combinations of squeegee materials and inks led to squeegees that showed vastly different degrees of hardening, softening, crazing, swelling, and utter breakdown. At the same time, durability became of increasing concern because economic growth, coupled with improvements in press and stencil technology, was creating longer run lengths at higher speeds.
The industry needed a material technology that could provide a better cross-the-board answer for squeegees. Each available material yielded good chemical resistance to certain inks, but not others, and all had severe limitations in abrasion resistance. Printers who used three, four, or more ink systems did not want to maintain a squeegee selection in each of two or three different materials. Urethanes became increasingly popular in the 1970s. They provided superior abrasion resistance and at least acceptable levels of performance with a wide range of ink systems. Improved production and manufacturing processes and a broader range of urethane formulations reduced costs and improved selection.
Polyurethane could be more easily colored, leading to the color-to-durometer coding that is common today. It provides more range of formula manipulation for specific characteristics. Also, it can be more easily adjusted to specific criteria in batch sizes suitable to the industry’s consumption. The net result has been better, more affordable materials for screen printing.
Types of polyurethane
Not all polyurethanes are the same. Performance can vary significantly. Polyurethane is composed of two main ingredients, the prepolymer and the curative. Prepolymers comprise two principal items: isocyanate and polyol. For relevant discussion, the isocyanate defines the type of urethane employed. There are three groups used for squeegees:
Toluene diisocyanate (TDI) This is the easiest of all the groups to process, yielding few cosmetic flaws for rejection. It has good resistance to hydrolysis hydrolysis (water reaction) but the poorest chemical resistance. TDI is used in some parts of the world to produce extremely low-cost squeegee material that has correspondingly poor performance. It is seldom, if ever, seen in squeegees produced in North America or Europe, so we’ll exclude it from further discussion.
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