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
Knowing whether a squeegee is NDI or MDI gives you a quick indicator of the expected performance level for chemical and abrasion resistance—and relative price. NDI is at the top of these three factors. The performance of an MDI material in all three areas could fall anywhere across a wide range, independent of each other. Therefore, you must conduct your own comparative evaluation to determine whether you’re getting the best balance of performance value for your dollar.
Some people erroneously assume that squeegee polyurethane is extruded. Extrusion of polyurethane is too slow for financial practicality and produces too inconsistent a finish for critical printing performance. The polyurethanes discussed here are all cast formulations. The terms cast and molded are used within the trade. The difference is that most squeegees are rotocast and very few are mold-cast.
In rotocasting, prepared raw material is poured into a heated, large-diameter centrifuge. Centrifugal force spreads the material evenly across the inside of the drum and forces entrapped air out. The resulting product looks like a giant rubber band. A cut made across the band allows the urethane to lay out as a flat sheet. Width and length are determined by the depth and circumference of the drum. The sheet is then placed into a heat-treatment oven. The heat-treatment parameters vary with formulation. MDI requires less time, while NDI requires cycles of treatment that can last up to several weeks. This also contributes to the higher cost of NDI products. After treatment, rotocast goods are cut into rolls. The rotocast material has two cut edges.
Molded blades, as the term is popularly used, are created one strip at a time by pouring the prepared material into a heated mold. It remains in the mold until set and is then removed and treated according to the manufacturer’s specification. The molded strip is a finished piece, unless it’s sharpened on the top side. The molded blade has one edge formed by the mold. As soon as that squeegee is sharpened once, the distinguishing feature of that molded edge no longer exists. The same is true when the squeegee is placed in a holder upside down.
Physical specification— the great inadequacy
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