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
Methylenebisdiphenyl diisocyanate (MDI) The majority of the squeegees on the market are formulated from this group. It has the widest range of variants available, leading to a huge range of performance from very poor to very good. It is the foundation for the materials that the majority of end users will consider to have an acceptable balance of value between performance and price.
Naphthalene diisocyanate (NDI) This group has been used for high-performance squeegees for decades and provides the highest level of chemical and abrasion resistance of all three. NDI is notably higher than MDI formulas in cost.
MDI and NDI are the two urethanes screen printers are likely to encounter (Figure 1), and the MDI group probably defines at least 90% of the market in North America and Europe. Several manufacturers sell more than one formulation of squeegee.
You may recognize the name Vulkollan. It represents the original, trademarked NDI technology developed and owned by Bayer. Only licensed manufacturers have access to the technology. Some choose to use the registered mark openly, while others do not. Other than Bayer, very few manufacturers of NDI variants exist. So far, other manufacturers’ prepolymers have not proven to have the same performance level and do not provide cost savings equal to the drop in performance. This means that nearly all NDI squeegees are Vulkollan technology and are very similar in performance. The licensed manufacturers have limited rights of adjustment to their specific formula and parameters, but the performance characteristics are at a level where most end users will discern little functional difference.
The polyol component of the prepolymer is either an ether or an ester. Esters offer better chemical resistance and better resistance to sliding abrasion, such as when the squeegee moves across the screen. Ethers provide better resistance to moist environments and impingement abrasion. In other words, an ether formula performs reasonably in a water-based system but has poor solvent resistance and wears down faster in printing. Ester is the predominant squeegee polyol because most shops use solvents of some kinds (most water-based systems also contain solvent) and printers want their squeegees to last on press.
The selection of curative canfurther influence specific physical or processing properties, such as hardness, flexibility, set time, and more. Finally, additives are used to influence color, UV (light) resistance, mold release, and other characteristics.
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