Davis describes common modifiers available and when to use them.
The range of available plastisol products is broader than ever. This variety creates a challenge for printers, who not only must determine what ink line will work best in their shops, but what modifiers are compatible with that ink and when they are necessary. It's easy to assume that every ink will be ready for use straight out of the can, but this is not the reality of our industry. The fact is, as ink lines have become more diverse to address a wider range of applications and substrates, the list of modifiers to enhance printability and performance has also grown. Manufacturers take several approaches in offering ink-mixing systems and corresponding modifiers. The most common configurations include the following: * Ink-mixing systems targeted at one specific substrate with modifiers to enhance the ink's performance only on that substrate. * Ink-mixing systems targeted at one specific substrate with modifiers offered to expand the ink's applicability to other substrates. * General-purpose mixing systems that offer a variety of bases and provide the versatility to decorate a wide variety of substrates. While each of these systems approaches flexibility and versatility from a different angle, the modifiers they incorporate are similar. But does the availability of modifiers mean that you have to use them? Only your application requirements and the characteristics of the unmodified ink will determine when modification is necessary. Common modifiers All ink manufactures offer additives for their standard ink lines to help the printer satisfy key application parameters. The most popular additives available today include modifiers for viscosity, flashability, gloss, and loft. Additionally, some manufacturers also offer modifiers for reducing ink odor, increasing ink volume, adjusting ink opacity and color, and improving adhesion. Because they are more common, we'll focus on the first group. Viscosity modifiers I advise against using viscosity modifiers, particularly viscosity reducers, because printers frequently misuse them. A disturbing number of printers use the "glug" method to add viscosity reducer to their inks, instead of following the manufacturer's guidelines for measuring and mixing. The result is usually an ink with excessive plasticizer content, which means it will never cure properly, and the print will wash out. Today, most available ink lines are precisely manufactured in ready-for-use formulations that minimize the need for viscosity modifiers. But, over time, the PVC resin in these inks will absorb plasticizer, increasing the ink's viscosity. So while it's likely that recently manufactured ink will satisfy your requirements without modification, you can justify using a viscosity reducer on inks that exhibit signs of excessive viscosity after prolonged storage. In general, avoiding viscosity reducers is the safest course. And the easiest way to minimize the need for reducers is to carefully control how much ink you order and store. Keep enough ink on hand to satisfy your present production requirements, and avoid stockpiling ink for long periods. If you find that your ink is still too thick, consult with the manufacturer about the proper viscosity reducer to use. Also, make sure to strictly follow the manufacturer's mixing recommendations, and always measure the product with a scale to ensure that you do not over plasticize the ink. A related, yet less common, viscosity modifier is a thickening agent. As the name implies, this modifier is used to increase the viscosity of a given ink. However, I have never seen a situation where thickening a plastisol was truly required. If a plastisol exhibits low viscosity, chances are that the ink has been overloaded with reducer to begin with. Adding a thickening agent to such an ink would be pointless since the product would already be ruined by excessive plasticizer content. Flashing additives Flash additives are designed to minimize the degree of tack an ink exhibits after flash curing. The less tack, the easier it is to print over the ink. Flash additives were widely used when white inks with good flashing characteristics were hard to find. But today, a large assortment of high-opacity, low-bleed white inks are available with excellent flashing characteristics built in. Flashing additives may still be useful when you need to flash a colored ink used as an underbase or when you have to flash a color later in the print sequence. For example, if a job calls for a multicolor design with special-effect inks (e.g., glitter, metallic), the colors require flashing before you apply the special effects. But even in situations like this, you can often the avoid the need for a flash additive by using an ink that flashes well across the board. Dulling powders In the early history of plastisols, most ink lines were used primarily for athletic printing and offered a high gloss level. But as the market for screen-printed garments expanded beyond athletic applications, some printers wanted to reduce the gloss of their prints. So manufacturers introduced dulling powders that could be mixed with the inks. Today, however, most ink lines automatically provide a low gloss level after curing. And printers who want high-gloss athletic prints now turn to high-gloss athletic inks. Such inks are generally low-temperature, fast-fusing varieties that provide high gloss because of the "hot" plasticizers they contain. As you may have guessed, dulling powders are rarely needed. If you want high gloss, simply select a suitable athletic ink. Otherwise, stay with a conventional low-gloss plastisol. Puff additives Puff additives (blowing agents) can serve several functions when mixed with a plastisol. Their original purpose was to allow printers to create puff graphics using any color of ink and when used according to manufacturer guidelines, they work reasonably well. However, expansion of the blowing agent during the curing process tends to lighten the printed color, which could cause problems in color-critical applications. With white or light colors, adding a small amount of blowing agent can boost the ink's opacity. For this reason, some ink manufactures add small amounts of blowing agents to their white products during manufacturing. These inks are usually easy to identify by their flat, low-gloss appearance and slightly uneven surface texture after curing. Puff additives are also useful to decrease the amount of after-flash tack in a standard white plastisol. Conclusion To find out more about these and other modifiers, contact your ink supplier. The supplier can provide you with the details about the benefits and drawback of any modifier, and guide you to products that may benefit your operation. You may discover that some additives will enhance the performance of a given ink line for your particular applications, while others are either unnecessary or harmful to ink performance. And remember, if you decide that a particular modifier can help you improve the quality of your prints or efficiency of your production process, make sure to follow the manufacturer's recommendations for using the additive. Editor's note: This column was condensed from an original that appeared in the Nov. 1997 issue of Screen Printing.