Drying and Curing Inks Equipment and Procedure

silk screen ink

Plastisol, water based, solvent based and UV inks each are dried or cured by different equipment and methods.

Plastisol inks are cured and not dried. There is no water in plastisol inks. Plastisol inks can be the PVC inks (plastic) sold for years, or the new non-phthalate inks. The curing procedure is the same. Non-phthalate inks simply include a different dispersing agent for the pigment, one considered safer for infants who put their mouths on images.

Plastisol inks are printed as a film of ink where the bottom of the film must reach a temperature in order to melt into the garment. Garments that are cold, wet, thicker or composed of better insulating materials like cotton will require more time for the ink to cure. The bottom of the ink film cannot melt until the material under the ink film reaches the temperature required for the ink to melt. That explains why fleece takes longer to cure than T-shirts, and garments with moisture might have to be run through a conveyor more than once. PVC inks can be fully cured under a flash dryer, but process is slower and the cure might not be as consistent.

When PVC inks do not reach the cure temperature through the entire ink film thickness, then the image will crack and fade over time. After a garment has been cured and returns to room temperature two tests should be applied. If the ink picks off the garment with a fingernail, the ink did not reach the cure temperature at the bottom of the ink film. PVC inks are designed to expand 100% in size without cracking. The second test is to stretch the image up to 100% of the printed size. If any ink picks off or cracks, the solution is to put the garment back in the conveyor to finish the curing.

PVC inks are cured with infra-red equipment often referred to as dryers. These units do not dry. They cure. Curing is like cooking a thick piece of meat. If you want the meat well done, then you would turn the temperature down and leave the meat in the oven for a long time. That is the way to cure inks. Dwell time is required rather than high heat. If the meat is cooked fast under high heat, the outside will be cooked, but the center uncooked. That is the ink that picks off the garment and cracks when stretched.

Water based inks require two stages. The first is to dry the water out of the ink so the garment and pigment are dry. The second stage cures the pigment. The pigment is fused to the garment, but does not have the advantage of PVC to bond the pigment to the garment. Without PVC, the pigment will not be as durable or opaque.

To dry water, the best method is lots of air with some heat. A lot of heat is not required. Volumes of air will remove the water. As water evaporates it cools the surface, and ink will not fuse to the garment or cure. Conveyors that both dry the water and then cure the pigment under one hood are less efficient than equipment where the two stages are separate. A single hood unit would require a lot of convection to remove the moisture. Of course, heat is being removed at the same time, and that is a waste of energy. If water based inks are not fused with heat after drying, then the ink will come out in the wash.

Solvent based inks have a strong odor similar to mineral spirits. This odor can be very objectionable to people in the immediate area. The solvent or co-solvent in the ink etches the surface so the pigment can bond to the surface. Such inks typically air dry quickly and do not require a conveyor. No heat is required.

UV inks have a slight odor, but that is not objectionable. UV inks do not cure with heat or dry. UV inks are cured by a tuned ray of light. Such equipment is typically expensive, and the light must be shielded from human eyes. UV equipment is used on objects like poster boards where the distance of the light ray to the object is precise and consistent.