19 Jan 20 Facts You Need to Know About Plastisol Inks
There are many misunderstandings about why plastisol ink fails. The injudicious passing along of widely held but false beliefs and ideas, misrepresentation of the truth, and exaggerated or idealized conceptions of ill-conceived thoughts of plastisol inks on social media are among the most rampant of myths in the screenprinting technologies.
Bill Hood, ASDPT Fellow
Having been a screenprinter for over seventy years, I have witnessed a great number of changes in the screenprinting industry. Perhaps the greatest of these was the advent of plastisol ink for printing on textile substrates. However, with all the benefits of plastisol, the headache of achieving cure continued. In fact, screenprinters have probably been troubled with curing ink since the beginning of screenprinting about 1852 and that is a long time to have a problem. Most novice screenprinters are under the mistaken assumption that all that is needed is for the ink deposit to reach a certain temperature for two minutes in the dryer to ensure a proper cure. Others believe that they can cure ink with a flash gel unit. If it were only that simple!
Curing screenprinting ink on textiles is one of the greatest challenges faced by many screenprinters and the return of uncured or over-cured prints degraded in the laundry of the consumer is a huge nightmare that many live with. This is totally unnecessary once you have the knowledge necessary to achieve the proper cure of screenprinting inks.
Achieving a full cure using screenprinting ink should, on the surface, be quite simple.
Most believe that simply turning on the dryer, placing the shirt on the belt, and watching it disappear into the dryer will suffice. However, there is much more to curing screenprinting ink than one might assume. There are hosts of variables that one must contend with in order to achieve a proper cure.
Take plastisol ink as an example. It is made of polyvinyl chloride resins suspended in plasticizer. Under the heat, the polyvinyl chloride resins will absorb the plasticizer. To fully fuse the plastisol to the garment, the entire ink deposit must be heated to a temperature specified by the ink manufacturer. This temperature is based on the resins used in the manufacturing of the ink. At the point when the entire thickness of the ink deposit reaches the required temperature, the ink will achieve adhesion to the garment and will provide maximum wash fastness.
Sound simple? Don’t believe it.
20 Facts You Need to Know About Plastisol Inks
Fact 1: There is no one temperature at which all plastisol will cure. Cure depend on the materials and chemistry of a particular ink. The size of the resin one of the most important, as a larger resin will require more temperature. Not adhering to the ink manufacturers’ specifications may very well result in a failure of the ink.
Fact 2: The use of non-curable reducer can extend the cure temperature, and excessive amounts can render the ink incurable, resulting in cracking.
Fact 3: Cracking is usually more prevalent when printing white ink on dark substrates, due to the thicker ink film. Especially when using a print/flash/print technique rather than using the correct mesh count and emulsion over mesh ratio to apply the ink with a single squeegee stroke.
Fact 4: Typically, the print/flash/print technique will have a 40% thicker ink layer than a single opaque layer of ink. On the thicker ink deposit, the top surface may become cured long before the bottom surface. The hardened top layer is sitting on a softer bottom layer and may crack.
Fact 5: Use a non-air-circulating flash gel unit or a handheld heat gun will risk curing the ink unevenly. The proper cure may be attained in small areas, but other areas will not be properly cured and are subject to failure in the laundry much sooner.
Fact 6: There will always be some moisture in the material. As the moisture in the substrate is heated to approximately 155˚F (68˚C), depending on the ambient relative humidity, and begins to evaporate, it cools the ink layer. This can cause plastisol not to cure fully. Garment dyed substrates may have an excess of moisture in them, as well as thicker substrates such as athletic wear. You can expect this to be more problematic when substrates are delivered in the rainy season and printed before they are allowed to acclimate to a dryer climate.
Fact 7: If your storage area for substrates is damp or the boxes are allowed to sit on the damp floor, you can expect to have problems, especially with those near the bottom of the box. When the moisture is excessive, and the dryer is short or has poor airflow, the moisture will be difficult to drive out and fully cure the ink.
Fact 8: Plastisol ink must properly and fully cured throughout the thickness of the film. A plastisol ink layer that is not fully cured (often erroneously referred to as under-cured) through the entire thickness will crack, just as well as an improperly cured ink layer. Plastisol will crack if not properly and fully cured because the top layer cures first and the underlying ink will still be soft. The top layer will be very much like a dry lake bed, which still has soft, moist soil underneath, but has become dry and cracked by the sun on the surface.
Fact 9: Plastisol will also crack if cured past the full cure (often erroneously referred to as over-cured), as the plasticizer in the resin particles is hardened. The now hardened surface will more easily crack over time from stretching. Think about where it cracks – in the low wales of the material where the ink deposit is less than on the surface of the threads of the material.
Fact 10: Properly cured plastisol becomes a layer of plastic. It should be evident that if the substrate is stretched too far the plastic will separate along the wales of the material.
Fact 11: Use a non-air-circulating flash gel unit or a handheld heat gun will risk curing the ink unevenly. The proper cure may be attained in small areas, but other areas will not be properly cured and are subject to failure in the laundry much sooner.
Fact 12: When using an infrared dryer different colors of substrates and inks will absorb the UV energy differently and need to have the dryer appropriately adjusted. Light ink colors reflect the heat and require a longer dwell time or higher temperature than dark ink colors, which more readily absorb heat.
Fact 13: Most white ink is pigmented with titanium white, which needs more heat than other pigments. Reflective or metallic inks, which will require more heat.
Fact 14: Stretching the printed material is not indicative of a properly cured ink. An ink layer that has been adequately or inadequately cured will crack easily as the ink as the fabric can always be stretch more than the ink layer.
Fact 15: The print does need to go through the laundry process to crack. The weave of the fabric in a garment can easily separate over time if placed on a hanger. This occurs as the weight of the garment will cause a more pronounced separation of the threads. This separation will cause the ink in the wales to separate (crack). The cracking will be more noticeable as the cracks are in a pattern reflective of the material.
Fact 16: Plastisol is either cured or not. You cannot afford to be off one way or the other. Get an Atkins Donut Probe and use it faithfully. It is the only tool that can accurately determine the cure. Let this job be a lesson learned. Don’t let it happen to you.
Fact 17: A non-contact hand-held infrared imager cannot measure temperature. They measure emissivity. The amount of radiation emitted by any object is influenced by the target surface condition, target shape, ambient relative humidity, ambient temperature of the air between tip of the imager and the target object, and the distance to the target. Radiation is also influenced by the viewing angle, which must be at a right angle to the tip of the imager. An object will reflect the infrared energy from its surroundings (reflectivity). Only by knowing the amount of infrared energy emitted by the object and its emissivity, could the object’s temperature be estimated within a certain range of its actual temperature.
Fact 18: Non-contact hand-held infrared imagers are designed to be used by HVAC technicians to look for hot spots in a room and only infers temperature. Furthermore, the devices are affected by frost, moisture, dust, fog, smoke, and other particles in the air. The instructions state that the device must be held at a right angle to the object being measured, which means you cannot aim it into the dryer to take a reading. The instructions farther state that if a very hot object is the target, then the measurement will include cooler elements outside the target as, well as the ambient air between the tip of the device and the target.
Fact 19: There is only one device that can actually test the temperature of the ink inside the dryer and that is the Atkins Digital Temperature Monitor and Donut Probe. The Thermo-Probe and Donut package brings a heightened awareness of plastisol curing and dryer performance.
Fact 20: The relative humidity (Rh) and ambient temperature in your shop can alter the curing rates. As Rh and temperature range throughout the day, you need to adjust the belt speed and or temperature to keep up with the changes. For example, if you set your dryer to cure plastisol ink early in the morning when the temperature is 36˚F (2˚C) at an RH of 60% and then as the day progresses the temperature rises to 60˚F (15˚C), and the RH drops to 30%, the ink may be taken past the proper curing temperature, leaving the ink brittle as the plasticizer is evaporated away from the ink. The substrates that were moist in the morning have acclimated during the day, and thus there will be less moisture to be evaporated.
Excerpted from The Knowledge Base at http://billhoodconsulting.com online. Gilbertson, Brad, Level III Certified Infrared Thermographer, “Avoiding Common Mistakes that Can Compromise and Infrared Inspection Program“, Sandhills Thermal Imaging, Fayetteville, NC.