What Every Garment Screen Printer Should Know about Fabric Testing
This article identifies the key fabric characteristics you should assess and outlines the procedures to use when testing various materials.
One of the most common themes I come across in my discussions with garment printers is how an ink did not work right on a particular fabric. Invariably, as these discussions progress, it becomes clear that the printers did absolutely nothing in advance to make sure that the substrates, inks, and processing conditions they used were a good match for each other.
I am going to go out on a limb here and make a broad statement: Not all fabrics will accept plastisol. Or to put it another way, not all plastisols will work on all fabrics. When we consider the costs associated with the goods we are printing, it becomes clear that testing is imperative in order to make sure that everything we expect will work actually does.
Of course, we want to avoid going over the top with our testing regimen. For example, if we are printing standard white or black cotton T-shirts from a name-brand mill, then testing each batch is a bit excessive. However, I would say that whenever the garment fabric contains other natural and man-made fibers, such as polyester, rayon, polynoisic, silk, nylon, etc., tests for ink compatibility and print performance should always be performed. I also recommend testing whenever a job comes along that involves a fabric weight different than what we're accustomed to printing. So if our regular work usually involves 5.5-oz T-shirts and we receive an order that involves 12-oz, 80/20 fleece, it's time to test.
Kinds of tests
What types of testing are we talking about? The following list covers variables that are worthy of consideration for all garment screen printers:
* fabric composition and color fastness
* fabric coating/treatment
* fabric weight
* moisture retention
* heat sensitivity (dryer and flash)
* dye sublimation
* intended ink suitability
* wash fastness
Note that not all of these variables will come into play in every garment job we face. But some will always apply.
Fabric composition and color fastness
What is the fabric made of? Is it polyester? Is it a blend? More importantly, is it labeled correctly? Determining what the fabric is composed of must always be the first step in deciding whether we can print on it or not. Once we have ascertained the composition, we can decide which other tests are necessary and what ink or ink/catalyst combination is best suited for the job.
For instance, if the fabric is labeled as 100% polyester, then we know that that ink adhesion will not likely be an issue. We also know that a catalyst probably won't be necessary--unless the fabric has a waterproof coating, in which case a catalyst may be required to boost adhesion. If we don't test for such a coating, however, we are just shooting in the dark and may end up eating the goods if we make a mistake.
If the fabric is polyester, we also need to see if it is overly susceptible to bleeding and dye sublimation. If the fabric was properly heat set when it was produced, then good quality, low-bleed plastisols designed for 100% polyester should work fine. However, there is currently a wave of improperly set polyester flooding the market, and it will sublimate and bleed even when you do everything correctly. That's why testing is imperative on all goods that contain polyester.
Color fastness can be evaluated by performing a crock test. A simplified crock test consists of rubbing a piece of white-cotton fabric against the fabric. Give the fabric three hard rubs with the dry cotton material, then wet the cotton and give the polyester fabric three more hard rubs. If a large amount of dye transfers to the white cotton, further testing is essential to ensure that the dye will not migrate through the plastisol when you print the fabric. We'll look at a more conclusive test for dye migration and sublimation later in this article.
Note that crock testing should be done on all types of colored fabric. Even 100% cotton can show signs of dye migration if there is unbound dye on the material when it is printed. A good example of this phenomenon is indigo-dyed denim. Excess indigo dye can indeed migrate through plastisol ink and ruin any print you produce on this material.
Most fabric coatings are designed for moisture blocking. However, some of the new treatments are also designed as stain blockers. In either case, the coatings can greatly affect the adhesion of ink.
The first test to perform in order to determine whether a coating is present is to see if water soaks into the fabric. We just sprinkle a few drops of water on the fabric. If the water penetrates after a few seconds, it is unlikely that the fabric is coated. We can further verify the presence or absence of a coating by creating a test print on the fabric. First, we print the material with an uncatalyzed plastisol and cure the print as specified by the ink manufacturer. If the ink peels off, we repeat the test with an appropriate catalyst, such as Nylobond. The catalyzed print must be allowed to stand for at least 24 hours, and preferably 72 hours, to fully cure. Then, if the ink does not peel off, the goods can be released for production. If the ink does peel off, even with a catalyst, we should not print these goods.
Fabric weight is important because of the difference in heat energy that may be required to elevate the fabric to the proper temperature for curing. If a shop is used to running T-shirts and, all of a sudden, heavy coveralls or sweatshirts are introduced, the dryer settings have to be changed. We cannot expect a dryer that is set for 200-gram T-shirts to handle moisture-laden 2000-gram coveralls at the same belt speed and temperature.
Moisture is the most overlooked factor in plastisol curing. Most printers, especially the ones who have not been burned by this factor yet, do not realize that moisture can greatly effect the cure rate of plastisol. The reason is that when you introduce a damp garment to a dryer, the fabric and ink temperature do not rise to the proper level because the dryer energy is being expended just to get rid of the moisture.
I have seen heavyweight sweatshirts, with a high cotton content (cotton holds more moisture than polyester) drag the heat right out of a moderate-sized electric dryer. As few as six sweats, one after the other, dropped the dryer temperature by more than 50°F. Fortunately, we caught it and realized what was happening. It also became clear why some of the better dryers out there feature alarms that warn of unexpected temperature drops.
We can perform a real simple test to check the moisture content of fabric. We simply weigh one garment, run it through the dryer, and then reweigh the piece. The difference in the weight is the moisture removed. For example, if you have a sweatshirt that weighs 560 grams before the dryer and 525 grams after the dryer, the moisture content equals 6.25% by weight. Another way to look at this situation is that if each garment has that much moisture, it means the dryer has to get rid of a liter of water for every 29 shirts placed through the unit. That is a lot of moisture, and it really puts a strain on the dryer.
Many specialty fabrics are much more sensitive to the normal dryer and flash temperatures than what most shops are used to. Cotton T-shirts can handle a fairly wide range of temperature without showing ill effects. But if we try to dry or flash nylon panels at across the same temperature range, we would end up frying them. The nylon will take the same ink and cure just fine, but it must be processed at the low end of the ink-curing range, not the high end.
Most printers are familiar with the care that must be taken with temperature in regards to polyester: too hot, and the dye may sublimate. Most of the other fabrics that are coming into our world these days are also much more sensitive and require a narrower curing and flashing range than is acceptable for cotton.
Earlier, we looked at the issue of color fastness and touched on dye sublimation. We can perform a specific test to ascertain the dye-sublimation potential of polyester and similar man-made fabrics. The test requires a piece of white cotton or cotton/poly fabric, a sample of the polyester substrate, a heat-transfer press, and a small amount of plasticizer. Because few shops keep pure plasticizer reducer in stock (and for good reason too!), I suggest using a small amount of a gel clear ink instead. Plastisol gel inks contain an aggressive plasticizer and will help give a quick response to this test.
We start by placing a small drop of gel ink on the test fabric. Next, we place the white test swatch on top of the fabric and place the sandwich of fabrics in a heat press at 350°F for approximately 15 seconds. The time can vary but should be consistent from test to test to ensure continuity. For the same reason, we should always use the same ink for the test.
After removing the fabric and test swatch from the heat press, peel the materials apart. A small amount of dye transfer into the gel ink on the white test swatch indicates a normal, well-prepared polyester fabric. A large amount of dye transfer indicates a problem. Examples of the results that can be achieved with this test are depicted in Figure 1.
If a large amount of dye transfers, we should print a white, low-bleed ink designed for 100% polyester on another piece of the fabric and cure the sample as recommended by the manufacturer. Then we place the sample in a warm place for 24 hours (a car dash board, office heat register, or a warm dryer top make ideal test-holding areas). If the dye sublimates through the ink, the fabric is unsuitable for screen printing with plastisol.
Intended ink suitability
If a particular ink is specified for any job, it is important to make sure that ink will adhere to the fabric, will not allow dye migration, and will stand up to the environment for which the product is intended. This last item may seem like a no-brainer, but I cannot count the number of times printers have told me that the general purpose plastisol that they printed on mesh football jerseys did not hold up very well. Duh! It was never formulated for that application. In this situation, athletic plastisols designed to resist abrasion would have been a much better choice.
The most common catalyst for plastisol is a nylon bonding agent, such as Nylobond. Fortunately, this catalyst will work in a wide range of applications other than nylon bonding. For instance, a waterproof polyester-blend jacket can most likely be printed with the addition of Nylobond to the ink. Several very heat-sensitive fabrics may also be printable with plastisol as long as Nylobond is added. The addition will allow the print to be cured at the very low end of the plastisol cure range and therefore not damage the fabric. Still, we must create test prints to make sure the catalyst works before committing to production.
If there is any possible question about ink adhesion to a particular fabric, a wash test is still the absolute best test you can run. If you cannot afford to have a washer in your shop, make sure that there is a regular procedure for taking the samples home or to a coin laundry for testing when necessary.
Fibrillation is the result of printing with high mesh counts on ring-spun fabric. The fabric fibers lift through the ink film after the first wash. Some customers will describe the print as "faded." In reality, the ink is all there, but the ink film was not strong enough to prevent the fibers from breaking through. If you have any concern about a particular fabric, or a particular customer, run a wash test on a sample that is printed in the same manner as the final product will be (use the same inks and mesh counts). Make sure that the customer understands the consequences and the limitations before proceeding with production.
Only a matter of time
One issue that all these tests have in common is that they take time to perform. If you were to run all the tests listed here, it might take the better part of a day or even two days to get a satisfactory answer about a fabric's printability and its compatibility with particular inks and additives. However, if you think that you cannot afford to take the time to do these tests, ask yourself the following two questions.
1. Can you afford to buy the goods when the customer rejects the order?
2. Do you have the time and money to run the job again...properly?
The old adage, "never enough time to do it right, but always enough time to do it over" really applies to screen printing. You need to develop the discipline to conduct proper tests on questionable fabrics. I would recommend that you publish a statement on whatever documentation your customer may see that indicates that testing may be required. If you publish a price list, put it there. Make a sign and put it in the office where everyone can see it. If you have a company Website, put it there as well. Even a sales brochure should have a statement that explains your testing policy. Here is an example of what such a policy statement might say:
Due to the nature of modern fabrics, it may be necessary to conduct testing to ascertain the printability of an order. These tests take from one to three days, which must be added to the time required to process your order. In addition, if a fabric is found suitable for printing but requires extra steps, additional charges may be required. We reserve the right to reject orders on fabrics that we deem unsuitable for screen printing.
The statement could also incorporate the following concept: Please note that if we determine that a fabric requires testing before printing, it will be necessary to use one or more of the pieces to conduct the test(s). Please make sure to include extras with your order.
If we are asked to print on a customer's $200 jackets, we may find the customer reluctant to waste one for testing. This dilemma can be addressed in two ways: We can ask the customer to get us a sample of the fabric from the manufacturer, along with instructions for printing it. Or we can offer to give the job our best shot with no testing or guarantees.
If your customer accepts the offer to print without testing, your responsibility is to keep the waste to a minimum. You would need to probably set up the job twice. First, print one and wait and see what happens. Then set up again and print the rest if the first one was okay. In this scenario, the customer will have to realize that they need to pay extra for the second setup. For expensive garments, this may still be an acceptable way to go. It's not perfect, but it protects you. It's better not to complete the job and have a moderately angry customer than to ruin the garments and have an irate customer demanding compensation.
The test department
Setting up a testing department is not expensive, because the needs in this department are very basic. A small shop does not need a separate setup at all. Since everything should be fairly close together, walking around to the different tools needed for testing should be simple. Larger shops, however, can benefit from a purpose-built test area, which would include the following components:
* heat-transfer press for dye-sublimation tests
* various screens for generating table-top test prints
* a washer and dryer
* counter space or table for holding equipment and filling out paperwork
I did not include a conveyor dryer because I think it is essential that tests for curing, heat sensitivity, and moisture be conducted using the production dryers. A small lab dryer will never give the correct data to compare to the actual shop conditions.
To ensure that we leave no stones unturned in our fabric testing, it's also useful to create a standard form on which we can record the results of our tests, such as the example shown in Figure 2. This form has been filled out with the results of tests conducted on a batch of heavy sweats. The most important aspect is the comments made by the tester, which should be transferred to the job ticket. In fact, a good policy would be for the test form to be a regular part of the job ticket so there is never any question about the suitability of a fabric for printing.
Testing will require more work on the front end but save time and money over the long haul. It takes only a few returned orders and unhappy customers to justify testing on a regular basis.
About the Author
Mike Ukena is a 15-year screen-printing veteran who has owned a textile-printing company and worked in technical services for the Specialty Graphic Imaging Association Int'l as the director of education. A member of the Academy of Screen Printing Technology, Ukena is a frequent speaker on technical and management topics at industry events. He is currently a technical sales representative for Union Ink Co., Inc.