by John Kaverman
Pad printing is a means of placing a high-quality decoration or mark on a variety of products. But what happens when the ink doesn’t adhere properly? The following tips can help pad printers overcome ink adhesion problems.
1. Printing on a substrate that is difficult or impossible to print on.
Some examples of difficult or impossible substrates include TPU (thermoplastic elastomers), soft-touch paints, P.T.F.E. (Teflon) and silicone. A simple test, for any substrate, involves using a fingernail to scratch the surface. Is the surface easily abraded? If it is, printing on this substrate will not make it any more durable.
Most of these flexible materials have plasticizers in them to make them pliable. These additives can make the surface very slippery to the touch. While you can sometimes pre-wipe these materials to remove excess additives from the surface, they usually leech back out of the material over time. If printing on a questionable substrate, test it thoroughly.
Print several with a pre-wipe step (I recommend using denatured alcohol to clean the surface) and several without. Air dry some and send others through your dryer to see if exposure to elevated temperature causes additives to leech to the surface. Set them aside and test them for adhesion at regular intervals such as at 12, 24, 36, 48 and 72 hours. Always try to retain at least one sample from each production run in case there is an issue later, so you can come back to it for reference.
2. Using the wrong ink.
The safest thing to do is send samples to your pad printing equipment/ink supplier for test printing. Let them determine the best ink, as well as whether or not the parts require pre- or post-treatment.
No time for that or dont have samples? When you know what your specific substrate is, such as ABS, polycarbonate, PVC, polyethylene or polypropylene you can usually refer to the “ink compatibility” information available on most ink manufacturers websites to see what they recommend.
3. Failure to pretreat substrates that require pretreatment.
Some plastics require pretreatment for the ink to adhere. Two of the most popular substrates are polypropylene (PP) and polyethylene (PE). Molders are using more and more of these materials because they are less expensive than easily printed materials such as styrene, ABS, polycarbonate and blends.
Do your homework on substrates. Test Dyne levels using a Dyne test kit or pen. Consult your equipment manufacturer (or a company specializing in pretreatment solutions) about potential methods for pretreating your parts.
Typically, pretreatment will involve one of the following: liquid primer (not recommended for environmental/worker safety considerations), flame or corona/plasma. There are others, but those are the most popular.
Contamination is a problem that is often overlooked. No matter the source of the contamination, the parts need to be cleaned.
Use a cleaner such as denatured alcohol solvent and a hefty supply of clean, dry shop cloths or low-lint paper towels. Wipe the print area thoroughly and allow it to dry. Change cloths or towels frequently to avoid simply transferring contaminants from one part to another.
5. Failure to cure.
People often confuse the words “dry” and “cured”, but they are two completely different things. Most pad printed images are dry within 30 seconds to a minute of printing because the inks have extremely fast evaporating solvents. Cured means that the ink has completed the chemical-physical reactions necessary to make it perform optimally. Ink should never be expected to exhibit optimal adhesion, abrasion and chemical resistance prior to the completion of the cure schedule.
All conventional, solvent-based pad printing inks have a cure schedule. Typically, a single component ink (not having a hardener/catalyst added) will require up to about 24 hours to completely cure. Two-component ink (ink with a hardener/catalyst added) will require anywhere from three to five days to completely cure.
In addition, if parts printed with two-component inks are submitted to temperatures below 60 degrees Fahrenheit, this may cause the chemical cross-linking of the ink and hardener to STOP. If it stops, it cannot be restarted, meaning the ink will never completely cure.
John Kaverman is the owner of Pad Print Pros LLC, an independent sales and consulting firm, specializing in pad printing and digital thermal transfer marking, and a regular contributor to Plastics Decorating.