17 Dec Why Plastisol Ink Cracks
The curing process of plastisol ink is simple. During the curing process, the plastic resin is heated and opens up to absorb the liquid plasticizer. As the resin cools it once again retains its plastic form, with the plasticizer inside. However, curing has to be measured and controlled.
The three most common reasons for plastisol ink to crack in the laundry or over time are.
1. Plastisol will crack if not fully cured because the top layer cures first and the underlying ink will still be soft. The top layer will take on the appearance of a dry lake bed, which still has moisture underneath, but has become dry and cracked by the sun on the surface.
2. Plastisol will also crack if cured past the full cure, 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.
3. Fully 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.
Note: The print does need to go through the laundry process to crack. The weave of the fabric can easily separate over time and 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.
And don’t stretch the shirt to test for the cure as it is unreliable. An ink layer that has been adequately cured will crack just as easily as under- or over-cured ink as the fabric can always be stretch more than the ink layer.
Plastisol is either cured or not. You cannot 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.
Now, let’s talk about some less common reasons for plastisol ink to fail in the laundry.
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. 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, which leads to the same result as an under cured ink. The hardened top layer is sitting on a softer bottom layer and may crack.
You should be aware when using an infrared dryer that different colors of substrates and inks will absorb the UV energy differently and need to have the dryer appropriately adjusted. Dark colors reflect the heat and require a longer dwell time or higher temperature than light colors. Most white ink is pigmented with titanium white, which needs more heat than other pigments. This is also true of reflective or metallic inks.
There will always be some moisture in the material. As the moisture in the substrate is heated 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. 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.
Watch the relative humidity (RH) and ambient temperature in your shop as it can harm curing rates. If your climate has drastic RH and temperature ranges 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 45˚F (7˚C) at an RH of 60% and then as the day progresses the temperature rises to 75˚F (24˚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.
And, finally, the use of non-curable reducer can extend the cure temperature, and excessive amounts can render the ink incurable, resulting in cracking.