By Pete Baxer
First of all, let’s define the subject. By “soft touch” I am not referring to a particular chemical formulation but simply to the coatings that feel rubbery and soft on touch. Such coatings can be found on travel mugs, pens, memory sticks, jump drives, knives as well as other promotional, household and electronic products. Soft touch substrates can have a range of chemical formulations, including regrind – a mixture of different plastic leftovers. Soft touch can be particularly difficult to print on because of their elasticity, varied chemical composition and high sensitivity to thinners. However, with the right ink, thinner and a few tricks one can get excellent images!
Having worked for years with many ink products, I can say with confidence that Sapphire series is a top quality product line. For flexible and rubber-like products I recommend Sapphire SB series. SB has been very popular with our customers thanks to the ease of use and crisp durable prints. If you are in doubt, send me your product for free ink adhesion testing.
The best formulation that we have found for printing on soft touch is:
Sapphire SB Series Ink + 15% PP1 Adhesion Promoter + 15% Hardener + no more than 20% Thinner
In this formula, the adhesion promoter helps ink binding up with the difficult plastic; the hardener gives mechanical abrasion resistance. The resulting imprints are very durable and resistant to dishwashing.
Most if not all soft-touch surfaces are thinner-sensitive: if you do not evaporate the thinner quickly, it starts to erode the finish leaving ‘orange peel’ or cracking effect. So it is necessary to use a dryer or oven or put imprinted product in front of a fan to speed up evaporation.
A lot of people try to put 20-25% thinner in the mixture to be able to use the ink longer. This method does NOT work with soft-touch products! Do not use more than 20% thinner – otherwise over thinning will cause the finish to crack.
Thinners differ by evaporation speed. The choice of thinner depends on many factors. A general rule is to pick the fastest thinner suited for the application.
For pad printing, usually medium speed thinner is best. In 4-color or multiple color process, it is recommended to put SBM (medium) thinner in the ink cups with 1st and 2nd color and SB-MS (medium-slow) and EB (slow) thinner in the following cups. Because the thinner in the last cups has more time to evaporate, this method levels the amount of thinner left by each cup on the imprinted surface, which yields better prints.
For screen printing, the recommended thinners are SP-5 and EB or S-3. These are slower thinners; faster ones may evaporate on the screen and clog it.
Aside for application, the choice of thinner speed will depend on humidity (the more humidity, the slower the thinner), printing machine speed (the faster the machine, the faster the thinner) and other environmental factors.
Always allow for full cure before testing an imprinted sample. Curing takes approximately 6 days on shelf or 30 minutes at 300º in oven. When testing the ink by scratching, do not try too hard or you will simply dig into the finish!
If you need help, call me.
Pete Baxer is ICN’s Chief Ink Specialist and Tech Support Manager
with years of experience and an outgoing personality