Curing Comes Clean Through Wash Testing
Discover how to use wash testing to prevent failed prints and unhappy customers.
A few of my recent columns have addressed issues surrounding the use of offshore printing companies. In my work with such companies, I frequently see evidence that I'm dealing with young businesses still learning about the printing process and creating problems that could be avoided with some basic testing. One way such companies fall short is by failing to conduct wash tests to ensure a proper cure on the garments they print. It's a procedure you should be concerned about as well, whether you contract work to other foreign or domestic printers or handle production in your own facility. No matter where printed garments originate, the printing facility should have regular wash-testing procedures included as part of the production process. And you should know how to troubleshoot the results if garments fail the wash test. Wash testing must be a standard operating procedure, because it's the only reliable way to ensure that prints are properly cured. If you contract your printing work out, control of production parameters is out of your hands. So it's essential to have some way of checking your print provider's results. Requiring wash-tested samples is an excellent way to get assurance that the facility's curing methods meet your needs. Depending on the facility you are working with, you may discover that no wash-test procedures exist. Particularly in high-volume foreign plants, you'll find that wash-testing is often overlooked, and priority is given to simply printing and shipping the goods as rapidly as possible. Some printers might argue that they carefully measure and control their dryers to make sure prints are cured properly. While heat-sensitive tapes, probes, and non-contact pyrometers are excellent tools for ensuring consistent drying conditions, they don't tell you whether a print is actually cured. The only insurance you can have that prints are curing properly is for them to survive a wash-testing procedure. Understanding the ink film To understand why wash testing is such an important function, you must consider the characteristics of printed and cured plastisol ink and how it reacts during the washing process. When an ink film is cured after printing, the PVC resins in the ink absorbs most of the plasticizers contained in the ink film. But a small residual amount of the plasticizer remains unabsorbed after curing. This gives the printed ink film its elasticity and determines the print's washability. Each time a garment printed with plastisol is washed, a minute amount of the residual plasticizer is extracted from the ink film. As this process continues through repeated washings, the print will inevitability begin to lose its elasticity and will eventually become less flexible. With thicker ink films, you will begin to see cracking. While it's true that all plastisol prints will fail over time as plasticizers are drawn out during repeated washings, a good print should display bright color and resist flaking for dozens of washings. At the other extreme is having too much plasticizer left in the print, which would indicate that it was cured insufficiently. With such a print, excess plasticizer would likely lead to ink flaking off after the first washing. What you want is enough residual plasticizer available to help the print withstand multiple washings without becoming brittle, but not so much that the print won't remain adhered to the garment through a single wash cycle. The ability of a plastisol print to withstand numerous wash cycles is determined by the composition of the ink, thickness of the ink film, the amount and type of modifiers added to the ink, and the time and temperature of the curing process to which the ink film was exposed. Each of these factors must be considered as you adjust the curing process to provide the ideal level of residual plasticizer in the print. Results of wash testing will vary with ink type and application. With specialty inks like glitters, for example, you can expect a noticeable drop in glitter effect after the first washing as loose glitter flakes at the surface of the ink film are pulled away. But the effect should remain relatively consistent after subsequent washings if the ink film was properly cured. Also consider ink thickness, which can cause some misleading results, particularly when printing on ring-spun cotton garments. With such garments, loose fibers at the surface will often become exposed after the first washing, raising from thin ink films and dulling the color of the print. This phenomena, called fibrillation, is often misinterpreted as evidence of insufficient cure. But, the ink film isn't being washed away due to insufficient cure, it's simply being interrupted by protruding garment fibers at the print surface. While you can print thicker ink films that resist fibrillation better, it's important to understand the substrate's impact on the wash tests so you can draw the correct conclusions. Another thing to note about thinner inks films is that they tend to separate more easily when the garment fabric is stretched, particularly when they comprise halftones or other non-continuous printed areas. This shouldn't be mistaken for improper cure. The bottom line is that all garments should be wash tested for several cycles to get a thorough picture of print performance--I recommend a minimum of five. As with the glitter ink example, what you are looking for is color and other print characteristics to remain consistent after the initial washing, without ink cracking or clearly washing out. Using wash testing to determine whether thicker inks films are properly cured is somewhat easier than with thin films, because brittleness and wash-out of color is more readily identified. However, one common mistake that printers make in assessing the cure of thick prints is to stretch the prints. If cracking or peeling result after excessive stretching, the conclusion is often improper cure. But typically, these problems have nothing to do with the degree of cure within the ink film; rather, they involve its natural elasticity--all plastisol inks do not stretch to the same degree. Underbased ink films also require scrutiny since you actually have two ink films that must be properly fused together in order to withstand wear and tear. The most common curing issue with underbased prints has to do with the way they were flashed. When underbases are overexposed to heat in the flash-curing unit, they receive a more thorough surface cure. As a result, colors printed on top of the underbase are unable to bond strongly to this layer when the print undergoes thorough curing in the dryer. The problem will be easy to see because these overprinted colors will come off during the first wash-testing cycle. Putting the right spin on wash testing While wash testing may add a little time to your production process, it will leave you much more confident as you sign off on jobs for delivery to the customer. A wash-testing routine should be a regular part of every job you or your contract printers produce, because it's the only way to make sure you're providing consistent quality and protecting yourself from diminished returns.