The Identity of a Squeegee
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.
Many in the screen-printing industry assume that a squeegee is a squeegee is a squeegee. Printers’ lack of knowledge or concern about squeegees is tragically ironic, given how critical squeegees are to the successful application of the process. Even those who pay attention seldom invest the same effort in educating themselves about squeegees as they do for mesh, ink, or emulsion.
Yes, there are marked similarities among squeegees. But even if we limit the discussion to polyurethane squeegees, the scope of available formulations and the resulting balances of performance and cost can be extreme. The printer’s goal is to find which of those balances fits his definition of value. Getting a better handle (yes, the pun is intended) on reaching that goal requires knowledge about the different formulation groups, key terminology, and important criteria for practical evaluation. A good place to start is with the development of squeegee materials.
Probably 95%, or more, of printing squeegees used today are of some form of polyurethane, or urethane for short. Many materials can be squeegees, but the first popular choice was rubber. The term squeegee rubber is still used even though squeegee urethane or even squeegee blade would be more accurate. Natural rubber was used first, but rationing and material shortages during World War II spurred both the need for and development of synthetic alternatives to many materials—rubber among them. One of the new materials was polyurethane, which was developed by Bayer in Germany. However, some time would pass before polyurethane gained acceptance for use as squeegees.
The shift in the US gravitated toward materials that bore more resemblance to natural rubbers, which also remained in use once supplies were restored. These products seemed more familiar and were of lower cost, which seemed appropriate because screen printing was still a fairly crude process with very few technical performance demands. Next, ink systems began to evolve, using increasingly aggressive solvents and resins. Since that time, the performance needs for squeegees have continued to be influenced by developments in inks.
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.
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.
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
Specifications are the values or nomenclature about physical properties of an item used to accurately communicate material selection or requirements. The information generally used to specify a squeegee for ordering purposes covers only a small portion of the material properties. Information typically used includes:
• Profile, the cross-sectional shape (rectangular, single-bevel, etc.)
• Hardness, expressed in Shore A durometer (i.e., 75 durometer or 75A)
• Color, usually not an option, but included for accuracy of description
• Size, the overall dimensions of the cross-section (i.e., 0.375 x 2.0 in. or 9.5 x 50 mm) and the length of roll or precut piece.
You will note that hardness is the only true specification of a physical performance parameter. The physical performance criteria that have bearing on performance—and when comparing materials—are:
• Chemical resistance, no standardized test methodology for our industry
• Abrasion resistance, no standardized test methodology for our industry
• Hardness, the one value for which there is an applicable measurement
• Modulus, the great unknown—again, no standardized test methodology for our industry
Test procedures may prove a specific property related to some of the above, but the tests are structured for laboratory evaluation as opposed to being designed to provide usable comparative values for screen-printing production. The questions then become, how do we work around the lack of information, and what is this thing called modulus?
Everything in modulation
Let’s tackle the topic of modulus first. The term is short for flexible modulus, sometimes called Young’s modulus, which is an expression of the relationship between applied shear stress and resultant shear strain for a given material. Simply put, it is a means of gauging flexibility. Stiff is high modulus; a squeegee that folds over is low modulus. Again, our industry lacks a standardized test method for this.
A materials-science lab would design fixtures and methods for different material requirements and perform the measurements using recognized industrial testing tools. The lab then plugs the values obtained into a series of formulas to arrive at a specific value fo that specific situation. If we were to add in the factor of the dynamic force applied to the material during printing—compression, squeegee movement, friction—the mathematics would become truly daunting. I can honestly say that my memory of calculus is not sufficient.
The fact that screen printers are without a set of standardized tests for modulus does not diminish its importance in our industry. For years we have specified durometer and talked about the importance of soft and hard squeegees. Instead, hardness should be considered an index of compressibility.
Printers sometimes interchangeably think of durometer as an indicator of both hardness and modulus. But developments over time give evidence that even though we did not know the appropriate vocabulary, we did, at some level, understand some distinction between the two properties. Almost every twist on standard squeegee configuration has been an attempt to modify the squeegee to achieve a different balance of compression and flex.
Consider the following: Angled squeegees are created to change angle to the screen but, without printers realizing it, are sometimes used to achieve variance in flex and compression. Backing plates are inserted in the holder behind squeegees to stiffen a softer material. Urethane-tipped fiberglass—one of the most extreme examples—involves compressible urethane edge bonded to a dissimilar high-modulus material. Finally, dual/ triple-durometer blades use available urethane technology to couple a compressible outer skin to a stiff, high-durometer, high-modulus support layer.
You could even suppose that an angle adjustment on press can, in part, serve as compensation for a less than satisfactory modulus. The blade bends too much, so the angle is changed to compensate. Either that or put in a harder (stiffer) squeegee. Keep in mind that modulus is a measurement of force related to the cross-section of the material. By changing angle, you change the vector of force, thereby (slightly) changing the shear stress applied to the squeegee. Considering force to cross-section means that in a multi-durometer construction, the modulus will not default to the highest modulus material included. Higher and lower modulus materials can be played off against each other such that two different constructions could have different durometers but the same modulus; likewise, it is possible to use the same durometers, but change the amounts of each material to vary the modulus.
Obviously, there is a link between durometer and modulus (Figure 2). The relativity of the two is constant, but the specific modulus for a given durometer can change from one formulation to another. Such differences are typically minor, but they can be just enough for a printer to observe that one brand feels snappier than another. However, as with all properties, there is a trade-off: You cannot change the relative modulus without affecting another property. For instance, you may be able to increase the modulus-to-durometer, but that can have a negative impact on abrasion resistance.
If durometer is the only thing we can practically measure, and it is linked to modulus, is it linked to other criteria in a way that can be useful? Yes, in that there are some tendencies you can count on (Figure 3). For a given squeegee formula, from a single manufacturer, there is an increase in both chemical and abrasion resistance as the durometer increases. Additionally, as durometer increases (same brand and formula), the relative degree of improvement increases in a non-linear fashion. The relative increase in performance from 80A to 85A is greater than the increase in performance from 70A to 75A. That’s why creating an apples-to-apples comparison of durometer is critical when examining two different types of squeegees.
All manufacturers maintain a certain durometer tolerance. The typical range is ±5. A durometer gauge is essential for proper measurement. If you do not have one, talk to your supplier. Do not make assumptions about the fact that you specified 80A for your current squeegee and merely request a sample of the same durometer. For example, if the current product is an 80A that actually reads 84A (within tolerance), and the test sample is an 80A that reads 78A (even closer to spec), you will not obtain a fair comparison.The difference in durometer alone could cause you to perceive that the performance of the sample is not as good. For the test to be fair, the submitted material should be an 85A.
Your goal is to find a product that meets your needs and delivers what you feel is the best available value given your particular emphasis on performance or price. Some printers use extremely harsh ink chemistries, others print long runs, and some produce commodity goods and focus on margin. You’ll eventually reach the point when you won’t realize any improvements without doing some in-house comparison of materials. Here are some suggestions to help you gain the most from such comparisons.
Apples-to-apples A simple principle, but probably the most important one of all for testing. As mentioned earlier, make sure the durometers are equitable, but do not stop there. Make sure to start with fresh material of each brand and run in the same type of ink for the same type of print parameters for the same number of impressions. Make sure the number of impressions and stroke lengths are the same and that the times in and out of the ink are the same. The list can be as long as you care to make it, but the guiding principle is everything needs to be exactly the same for each sample being tested.
Chemical resistance and durometer If one squeegee just turns to jelly when exposed to an ink or chemical, the point is obvious. However, this is seldom the case. A good means of examining chemical resistance is to measure the durometer of fresh material for each brand right before printing. Put each on press under similar circumstances and set a fixed interval of time or impression count, pull each squeegee off of the press, and measure the durometer. For example, if your run length permits, run both squeegees for one shift and measure the durometer of each in two-hour intervals. Write down the number of impressions each has printed in case there’s a big difference. At the end of the day, measure, clean, and let the squeegees sit until the next day. Measure again to look at recovery. Different brands frequently show different rates of durometer loss for different inks, as well as different rates of recovery. This is extremely important for work such as four-color-process printing. The rate of durometer loss (Figure 4A) plays a role in the degree of dot gain during a run. You can cut samples, submerge them in ink and measure them at intervals, but you’ll often see that the dynamic results under printing are different from those obtained from static testing. Durometer measurements should be taken from at least three areas across the blade that are constantly exposed to ink, then the measurements should be averaged.
Swell Noticeable swelling has little impact on the performance of some squeegee materials, while for other formulations it means a trash can is in the near future (Figure 4B). Again, measure durometer when you see swelling, then visually examine the material and measure durometer after relaxing the squeegee at least overnight.
Abrasion resistance Evaluating abrasion resistance is the most subjective of all comparisons. It’s critical that you run each sample for an equal number of impressions on the same mesh counts and similar setups before making a visual or tactile comparison of the edge. At the same time, examine for chipping and make notes of any such problems during printing. Also note when you sharpen the blade. And when you sharpen, make sure that the two materials are sharpened to the same depth of cut. If you use an abrasive (wheel or belt) sharpener, be sure to use the same grit for the same number of passes and advance the same amount with each pass. If you see a difference in action on an abrasive sharpener, you should repeat that action and vary the grit and depth of cut. Some squeegees seem more difficult to sharpen than others, particularly when you take off a lot of material in one pass or use a finer grit. You’ll usually find that those squeegees are actually formulated to be more abrasion resistant than the other sample you’re testing—and your sharpener is doing exactly that, abrading the material. If you consult the manufacturer, you’ll probably find tha the company can indeed make a squeegee that sharpens easier, but it will wear faster on press. Sometimes on-press durability makes it worth adjusting your sharpening technique.
Cosmetics, packaging, and flatness Cosmetics in this case are dimples, surface irregularities, edge quality as supplied, bubbles, etc. These are all qualities that can waste material. If a roll has a flaw, point it out to the supplier and ask for another roll. Every company makes a bad roll now and then and will happily exchange it. You don’t want to see serious flaws on a regular basis. Packaging should protect the material from damage, but not coil the material so tightly that remains dramatically curled when you unroll it. Note that the thicker the material and the higher the durometer, the greater the natural tendency to curl.
Some printers may feel that these points for comparison are cumbersome and that the tests are a lot of work; for others, comparing and testing squeegee properties is cursory. But those who take the time to educate themselves about their tools and consumables, scrutinize their squeegees, and consider material qualities and performance are typically among the best in the business. Even if you feel that the exercises described here are more than your shop can handle, just keep in mind that carefully observing even the most basic aspects of squeegee evaluation will help you find better value and pave a path to an improvement in your printing.
James Elliott is the manager of screen-product sales for Pleiger Plastics Co. He spent the bulk of his career in the screen-printing industry, starting as an artist and printer, and has since worked with several international manufacturers.