The staff of Union Ink Company fields hundreds of questions annually about the use of their printing inks. Questions like "Why is my design cracking?" and "Why is the ink coming off after washing?" make up the majority of problems screen printers encounter. In this paper we are going to describe why the design cracks or washes off the garment, the stages the ink and garment go through during the curing process as well as what you can do to ensure the garments you deliver to your customers are cured.
What is plastisol ink?
Plastisol inks are simple compounds mainly made up of PVC (polyvinyl chloride) resin, plasticizers and pigments. Other components such as fillers which may be used for opacity, modifiers which may be used to adjust viscosity or components to give an ink its low-bleed characteristics may be added to the ink by the manufacturer depending upon the particular application for the ink. Plastisol inks do not air dry and only through the introduction of heat do they become solid and washfast.
What are the stages a plastisol ink goes through during the curing process?
A plastisol ink goes through three stages. The first stage is the wet stage which is the state of a plastiol ink in the container or just after it has been printed on the garment prior to putting it through the dryer. The second stage is the gel stage which occurs somewhere between 180°250°F depending upon the type of plastisol. The gel stage is the state you are trying to reach with a flash-cured underbase or conveyor-cured transfer, dry to the touch on the surface but not fully cured. The third and final stage is the cure stage. The final cure occurs when the entire ink film from top to bottom reaches 280°320°F. and again depends upon the type of plastisol. Even though the vast majority of Union plastisols cure at 300° F., some special effects inks such as Hi-Square, Flash-Black, Shimmer, Suede or Glitter have higher recommended curing temperatures. Always consult the Union Ink Technical Data Sheet to detemine the cure temperature for the ink you are printing prior to any production run.
Are there differences in testing direct printed garments and transfers for gel and cure?
Yes, there is a major difference between the desired cure of a direct print on a garment and the desired cure on a plastisol transfer. While most people believe you cannot overcure a direct print (except an underbase), overcuring a transfer may result in difficult or faulty application of the later transfer of the print to a garment. Inks on a transfer should only be partially cured (gelled) to the extent that they can be stacked, cut, packaged and shipped without sticking together. The full cure of the plastisol occurs later at the time of transfer.
Temperature measuring devices do not apply here. Standard procedure is to decrease conveyor heat until you observe some ink set-off on the backs of the sheets at the delivery end and then raise the temperature or lower the conveyor speed until the set-off disappears.
How does a plastisol ink cure?
How a plastisol ink cures is another very simple process. Remember the PVC resins we spoke of earlier? Once these PVC resins used in the ink are exposed to heat, they begin to swell and react like small sponges to absorb the plasticizer. When all the plasticizer in the ink is absorbed by the resins you have a flexible, solid film called an "elastomer" which is tightly bound to the garment and washfast.
Are there other variables besides ink to consider?
Yes,there are many other considerations when curing a garment printed with plastiol ink.
Many printers think that heat is heat and the ink will cure at the exact same dryer temperature setting and belt speed for all garments. Many printers have also discovered this is a myth after undercuring garments and having to reprint the job. Fabric content will determine the length of time it takes to evaporate the moisture from the garments. Garments made from 100% cotton take longer because cotton fibers absorb and hold moisture unlike fibers of polyester or nylon.
Fabric color must also be considered. The same reason you dont want to wear a black t-shirt to the beach on the hottest summer day is the same reason dark colored garments cure quicker. Dark fabrics absorb heat instead of reflecting and will come up to temperature at a faster rate than light colored fabrics.
The weight of the garment is another factor to consider in the curing process. Are you printing t-shirts, four-ply nylon reversible basketball jerseys, football jerseys or pants, sweatshirts or cut pieces that are printed prior to being sewn into a finished garment. All of these garments are of different thicknesses and the curing time for the garments will be different. The thicker the fabric the longer it takes to bring the fabric under the print up to temperature. Again, remember the moisture content we spoke of earlier. The heavyweight 100% cotton t-shirts not only take longer to bring up to temperature due to the fabric thickness but also have more surface area in which to hold in moisture which has to be evaporated before you can bring the heavier fabric under the print up to temperature.
Ink Film Thickness And Type of Ink
Different ink film thicknesses cure differently for the same reason different fabric thicknesses cure differently. There is more volume to be brought up to temperature. Even though the recommended cure temperature is 300° F for both inks, the thick athletic uniform print requires more time and heat than a soft hand print on a white t-shirt. Glitter or shimmer inks require more time and temperature but for a different reason. The metallic characteristics of these inks tend to reflect infra-red heat and require a longer time in the dryer as well as more heat to reach their required curing temperatures.
One variable that often gets overlooked is what I refer to as the Spring/Fall Syndrome. Spring and Fall is the time of year I receive the most complaints about ink performance resulting from undercure. In the South, it is often too nice to have the the heat on in early Spring but not yet hot enough to use the air conditioning. Likewise, in the Fall , it is not warm enough to justify using the air conditioning and not cool enough to need the heat. During these two periods I have seen many print shops open the doors and windows to enjoy the fresh air and also let Mother Nature provide the climate control for the shop. Having the doors and windows open creates a draft that sucks heat from your dryer and out the open windows or doors. The printed garment now has to compete with the outside elements to receive enough heat to become fully cured. This is the same principle that happens when our childrens friends come over in January and have a full conversation at the front door with one child inside and the other standing outside while all the heat rushes out the door.
How do I measure ink film temperature?
Most shops measure their ink film temperature with either a donut probe, temperature tapes or a non-contact infra-red thermometer. However, these methods only provide a reference for setting dryer temperatures and should not be used as a final test for determining cure. There are marked differences of opinion throughout the industry as to which of these types is most effective in helping the screen printer set his equipment temperatures most accurately.
Temperature Tapes Pressure sensitive pieces of tape that are placed next to the print. One model has a coating with one temperature (270° F.) that is grey in appearance at room temperature but turns black and stays black when the temperature reaches that particular level. The second type has five fixed temperature levels on each tape. These five levels come in temperature ranges of 240280°, 290330° F., and 340380° F. and also blacken and stay black when the temperature reaches that level. Even though many say these tapes are inaccurate because they only measure the temperature of the tape and not the ink film they are still commonly used due to their low cost. However, it is difficult to dispute chemists and printers who say that the technique of placing the temperature tape inside the garment underneath the ink area is the only way you can prove the entire ink film above the tape has reached the required temperature. Many industry experts suspect the accuracy of these tapes to be somewhere around + or 10% so you must be careful that an indication of 330° F. is not actually 10 percent less or closer to 297° F. where the ink film may not fully cure.
Donut ProbeA round, teflon ring with two wires that cross in the center of the ring. The ring is connected to a hand-held thermometer by a teflon coated wire. The donut is placed on the print with the two wires actually sitting in the wet ink. As the garment travels though the dryer the actual ink film temperature appears on the screen of the hand held thermometer. This is considered to be a very accurate measurement of ink film temperature because you are seeing the temperature deeply into the as the wires sink through the ink and touch the garment. However, the accuracy of this method has also proven to be somewhat suspect depending upon the length of the dryer and by how well shops keep the contact point (where the wires cross) free of oil and ink residue to prevent false readings.
Non-Contact Infra-Red Thermometer (sometimes called a raygun because of the manufacturers name)These devices are hand-held and when aimed at the print from a distance of 6-12 inches provide a good reference point for the temperature of your ink film. Like the previous two methods the non-contact thermometer is not without a disclaimer also. It only measures the surface temperature of the ink film and does not tell you the temperature in the middle of the ink film or at the bottom where the ink film actually touches the shirt. You must make sure you are aiming accurately at the printed design. Even with these disclaimers, there are many who claim that its speed and accuracy make it the current way to go.
What is the best way to determine if my garments are fully cured?
While all the temperature measuring devices described above work well, they should only be used to provide a good reference point setting dryer settings. The definitive test to determine a fully cured print is still by washing the printed garment a minimum of three times. To perform this test use two garments that have been printed during a full production run. Do not use a shirt that has been printed and put through the oven by itself. Instead use a fully loaded belt to simulate a production situation. Unlike a conveyor belt loaded with multiple garments, a single garment requires less energy to cure because it does not have to share the heat with other garments. Turn one shirt inside out and leave the other garment right side out and wash with at least 5 pounds of other laundry to simulate the exact conditions in which your customer will launder the garment. One printer I know cuts the design in half and places it inside another t-shirt to simulate turning the shirt inside out. After each cycle he compares the washed printed with the unwashed print to detect any differences in the prints. While this method is time consuming, it is the only true test of whether the printed ink is fully cured and meets the requirements of the ultimate user. There are those who are proponents of a chemical test or stretching the garment to observe relative ink cracking, but the garment wearer is more interested in whether or not the print fails when washed than if it withstands ethyl acetate, etc.
Can overcuring and overflashing become a problem when printing plastisol inks?
Yes, if you are printing on polyester/cotton blended or 100% polyester garments too much heat can cause the dyes in the garment to migrate through the ink film. Just as undercuring the ink film can cause the dyes to sublimate and migrate through the ink film turning the white ink printed on the red garment pink, too much heat can produce the same result. One major uniform manufacturer in the Southeast recommends that the temperature of their 100% polyester uniforms not exceed 330° F. Even though the ink film is fully cured the components used to give the inks their low-bleed characteristics do not stand a chance if too much heat is applied to the garment. As one of our technical representatives says "Remember these inks are low-bleed, not no bleed". When printing any polyester blended garment you will not only have to ensure a full cure but also ensure that you do not overcure.
Overflashing can cause a problem with the adhesion of the top colors adhering to the underbase color. If for some reason a printer would actually cure the underbase print he would get what is called poor "intercoat adhesion" of the top colors to the underbase colors. Recommended gel or flash cure temperatures of inks commonly used to underbase are approximately 240250°
If the entire print looks faded, is this undercure?
More than likely if the entire print is faded evenly you are seeing a phenomena call fibrillation. Simply speaking fibrillation is caused when shirt fibers raise up through the print because there is not enough ink on the shirt to hold down the fibers. Fibrillation mainly occurs on white or light colored garments made from 100% cotton where the design has been printed with fine mesh counts to produce a soft print. It is also common on four color process prints for the same reasons. It is not seen on dark colored garments because the thicker ink deposits printed to cover the dark background hold down the fibers.
A simple test to determine whether you have fibrillation or undercure is to wet your thumb and rub it across the print and observe to see if the deeper color returns. You can also look at the shirt immediately after it is washed because the wetness of the wash cycle will bring back the color. Fibrillation can be corrected by utilizing coarser mesh counts to print a thicker ink deposit or by flash-curing and making an extra screen that allows you to overprint the entire design Mixopake (MIXO-9070) Soft Hand Transparent Base to hold down the fibers.
For more information
This paper was written and produced for your information by the staff of Union Ink Company. For more information about this product, process or any other Union Ink product please call 1-800-526-0455. You may also receive technical information via the World Wide Web at www.unionink.com or by e-mailing your request to firstname.lastname@example.org.
Customer testing is required and should be mandatory with this product or any new product or process before beginning production. Our technical advice and recommendations given verbally, in writing, or by trials are believed to be correct. There are certain personal opinions expressed above which should be evaluated by the printer in his/her own environment and circumstances. They are not binding also with regard to the possible rights of third parties and do not exempt you from your task of examining the suitability of our products for the intended use. We cannot accept any responsibility for application and processing methods which are beyond our control, nor can we accept responsibility for misuse by you of the products or use by you of the products outside the specified written instructions given with the products.
Union Ink Company will replace or refund any defective product returned to us within 1 year of the date of purchase. This warranty is in lieu of any implied warranty of merchantability or fitness, and no other warranty shall apply. The user is responsible to determine whether the product is suitable for each particular substrate and application. The user must test thoroughly (including wash and storage tests) before using in production. In no event will Union Ink Company be responsible for indirect or consequential damages such as damaged substrates or printing labor.