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Plastisol Curing Facts You Should Know

Achieving a full cure using plastisol ink should, on the surface, be a simple proposition. Turn on your dryer to the proper temperature, lay the shirt on the belt, and send it on its way. But there are rules, and there are complications we must deal with to be certain of a proper cure.

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At the point when the entire thickness of the plastisol ink film reaches curing temperature, you will achieve the ideal adhesion to the garment and maximum wash fastness. In other words, the plastisol ink will be fully cured.

If the ink film feels dry to the touch but has not achieved a minimum cure temperature, it may give the appearance of being properly cured, but the ink may simply have gelled. Ink will gel at 180-240 degrees, or partially fuse at 280 degrees. But in either case, without reaching the minimum cure temperature, the garment is not ready for delivery to the customer because it cannot withstand wear or laundering.

Flash drying

The gel stage is an important function of plastisol ink for the purposes of producing an underbase print. Textile inks are made to print on textiles, and not on a plastic sheet of cured plastisol. When an underbase is completely cured (or even partially fused) rather than gelled, you are in fact printing your overprint colors onto plastic. This full cure of the underbase can result in the overprinted colors washing out during laundering. To ensure you are not fully curing your underbase, flash dry only until the ink is dry to the touch, and test your results. With a flash cure unit set on high, and at a distance from the garment of about ¾”, flashing should take about 10-15 seconds.

Apparent curing

A phenomenon of "apparent" curing occurs when the top surface of the ink film has reached the optimum cure temperature, but the ink closest to the garment has not. This results from the garment itself not reaching the required temperature. A printed garment needs the right combination of time, temperature and air movement to ensure the ink and fabric fuse together. If the physical garment does not have time to reach the correct curing temperature, the ink at the garment's surface (where it touches the ink film) will not reach that point either. The result will be an ink surface cure, but undercuring where the ink physically touches the garment.

Over-curing

Over-curing textile inks can also cause problems with adhesion and wearability. Temperatures in excess of 350 degrees can result in a loss of elasticity in the cured ink. When the plasticizer-resin chemical interface is destroyed by excessive heat, the ink can crack or fade when washed. When an underbase is over-cured and then cured again in the dryer after the overprint colors are added, "fisheyes" in the print may occur when the underbase is literally boiled. Frequently, the overprinted ink is blamed for the pock-marked surface, but the problem is actually rooted in the over-curing of the underbase ink.

Moisture in the garment

Occasionally problems with curing may result from residual moisture in your garment, rather than from dryer temperature or air movement within the drying chamber. Moisture can hinder the garment from reaching its proper cure temperature. As a result, this moisture must be driven off before the ink film can reach proper curing temperature. The temperature rise in a moisture laden garment will level off (below curing temperature) until the moisture evaporates, which is the reason that forced air can be an important part of a good dryer setup. The hot, forced air continuously evaporates and then exhausts the moisture laden air away from the garment, enabling it to reach the desired temperature.

Fabric type will determine how susceptible the garment will be to moisture retention. Natural fabrics tend to hold moisture while synthetics resist it. As a result, curing time for heavyweight 100% cotton shirts can be greater than the time for 50/50 shirts. It is important to monitor your curing time and temperature, particularly with fabrics such as cotton if you print in an area of high humidity and moisture.

For persistent garment moisture problems, and short of replacing drying equipment, you have two options for attacking the problem before the actual printing process. You can run your garments through the conveyor dryer prior to printing to drive off some of the residual moisture, or you can use your flash unit to preheat the garment and accelerate the evaporation process before actually printing the product.

Garment color may hold some significance when it comes to curing time as well. Dark colors absorb heat faster than light colors. Black and navy blue shirts should in turn reach full curing temperature more quickly than whites and pastels. Here again, you will need to test for differences and record your results for use in actual production.

Curing time

It's safe to say that varied curing times are a result of the combination of all the other variables in the job. You are striving for total ink film temperature. Once plastisols reach 310-320 degrees through the entire ink film thickness, the ink will fuse evenly. The variables we've discussed impact this. Therefore, there are no hard and fast curing times that can be printed on your dryer at the factory. You must take into account the variables for your shop, and calculate your own best curing times.
Terry Combs has managed production shops large and small across the country. He has written hundreds of management and technical articles for screen printing publications, speaks at industry trade events nationwide, and publishes the free newsletter, Screen Print Weekly. Training and consulting information is available at TerryCombs.com.
 

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In my experience flashing should not take 10-15 seconds with a flash unit at 3/4" from a shirt. I can flash at 4" for 5 seconds and get successful to the touch results. Seems like a typo.
I agree, but I am sure it depends on the flash unit. Mine is about 3-4 inches off the shirt and normally is 6-7 seconds... No way I could leave it for 10-15 sec.
 

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Thanks! I've heard there are options for using heat presses as well, and have seen a few threads on this. What are the rough guidelines for using a heat press to hover and cure multicolored plastisol prints (meaning, you've used flash heating on multiple colors)? How do you avoid over-heating in this case?

Thanks so much! Have a blessed and beautiful day.
 
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