Methods and Materials for Difficult Pad Printing Applications
by John Kaverman
Pad Print Pros LLC
Applications requiring pad printing on easy-to-adhere-to substrates are becoming increasingly rare as ABS, PC, PVC and other similar, ink-friendly substrates are being replaced with materials such as LDPE, silicone and TPEs (thermoplastic elastomers, a class of copolymers that consists of materials with both thermoplastic and elastomeric properties).
In most cases, a process needs to be developed to combine specific pre- and post-treatments with specific inks and additives to be successful. This article will review some methods and materials that should be considered.
Methods for cleaning and pre-treatment
Many of the more challenging materials have low surface energies and/or additives that can have a negative impact on adhesion. To combat these characteristics, methods for both cleaning and pre-treating the substrate should be investigated.
Oils, waxes, plasticizers and other problem contaminants often can be removed by cleaning the substrate prior to printing with isopropyl, denatured ethyl alcohol or cleaners that are formulated for cleaning specific materials. Ask an ink supplier for recommendations. Also, it is important to replace rags or wipes frequently to avoid transferring contaminants from dirty rags. Sometimes the rags are only effective for a single cleaning.
Galvanized/plated parts can be cleaned with distilled water prior to printing. Alcohol or other cleaners may not effectively dissolve residual salts and other contaminants. Distilled water is recommended, since most municipal water sources can contain chlorine, fluoride and other additives that adversely affect ink adhesion and performance.
Pre-treatment generally is required when surface energies are below 38 Dyne/cm. Optimally, a surface should be closer to 42 Dyne/cm to ensure adhesion. Pre-treatment can be accomplished using methods including primers, flame, corona, plasma and Reactive Gas Technology™.
- Liquid primers generally are manually applied, making their use time consuming. Even so, on specific materials such as EVA (ethylene vinyl acetate) a liquid primer may be the best solution.
- Flame is fast and, when properly controlled, works well for treating larger areas.
- Corona and plasma are excellent for smaller parts and/or more complex geometries.
- Reactive Gas Technology (offered by Inhance Technologies LLC) allows the treatment of all surfaces of the part at once.
The surface-modifying effects of these pre-treatment methods deteriorate over time at varying rates (except for the effects of Reactive Gas Technology, which are permanent). With the exception of liquid primers, pre-treatment processes can be integrated with pad printing equipment to eliminate the material handling associated with treating offline.
Complications with pad printing inks and additives
Ink manufacturers constantly work to formulate new inks for difficult applications. In researching ink solutions for new applications, it is highly recommended that the decorator submit physical sample parts to the ink manufacturer for testing rather than simply relying on an “ink compatibility” chart that may or may not be accurate. Most ink suppliers are happy to test-print their ink and return the parts for evaluation (preferably accompanied by a written report of how the manufacturer mixed, applied and dried/cured the ink). When submitting samples, the decorator should be sure to include all pertinent information regarding the ink performance, compliance, color and other requirements.
When printing onto UV-curable hard coats, a “window of opportunity” often exists before or after which pad printed ink will not adhere. This varies depending upon the specific formulation of the UV-curable hard coat. In some cases, when the hard coat is a component of the IML (in-mold label), it is necessary to pad print before the hard coat is cured.
Many issues with ink performance arise when users fail to follow the ink manufacturer’s recommendations for storing, mixing, applying and drying/curing the inks. It is imperative that technical data sheets be obtained and that manufacturers’ recommendations are followed for hardener/catalyst mixing ratios, compatible thinners and other additives, as well as drying and curing. This is especially true when using two-component inks, since mixing hardener/catalysts at incorrect ratios and/or failing to observe the specified cure schedules can result in the ink failing to meet requirements.
What’s new in pad printing ink?
Pad printing inks continue to evolve as substrates and part shapes change. A few of the newer developments include solvent-based pad printing inks with improved qualities for adhering to glass, ceramic and metal substrates.
While pad printing inks for plastic substrates continues to be the primary focus for this magazine, developments for textile and silicone substrates have implications for plastics decoration, too. New solvent-based inks created to address growth in textile label pad printing have been shown to work on some TPEs, as well as natural and synthetic rubber, artificial leather and EVA (ethylene vinyl acetate). New two-component inks were introduced in late 2015 for silicone; these inks have excellent transfer efficiency and come in a variety of colors.
Adherence always is an issue and in 2015, some inks had success on “borderline low” surface energy materials without pretreatment, especially when heat was applied in curing immediately after application. Advances in UV-curable inks have led to excellent chemical and mechanical resistance, even on powder-coated surfaces.
Compliance continues to drive ink developments, with newer releases that meet needs in the medical market (specifically, for applications requiring Medical Class 6 certification) and comply with REACH human health and environment regulations in the European Union (EU).
Post-treatment of pad-printed parts
Remember: Drying and curing are not the same thing. Drying simply removes the vehicle (thinner and/or retarder) from the printed ink film. Curing is achieved only after the ink has completed the respective chemical/physical changes required for maximum adhesion, as well as chemical- and mechanical-resistance. All inks continue to cure after drying. Conventional solvent-based inks and UV-curable inks frequently require 24 hours before they are considered “cured,” while two-component inks can require three to five days for the chemical reaction of the catalyst to be complete. Testing for adhesion and/or chemical- and mechanical-resistance prior to the completion of the recommended cure schedule may result in false negatives.
For conventional, solvent-based, single- and two-component inks, drying can be completed using industrial hot air, conveyor dryers, batch ovens or “air drying,” as indicated on the manufacturer’s technical data sheet. For UV-curable ink, it sometimes is necessary to first evacuate the thinner from the printed ink film using infrared drying prior to UV curing.
Whenever drying/curing any ink, it is important to know how much energy is really being applied, whether it be heat in conventional dryers or peak irradiance and energy density in UV-curing equipment. Most conventional dryers have a digital temperature indicator, and a laser thermometer can prove useful for double-checking peak temperature on the surface of parts while in the dryer. For UV curing, a belt radiometer provides useful information about the UV exposure of production parts when used regularly, with real-time monitoring of UV made possible by newer, integrated sensors.
When drying/curing some materials, such as fire-retardant ABS, it is important to realize that temperatures well below the molded resin’s actual recommended maximum can cause certain additives to migrate, which can result in the ink failing to adhere. In some applications, the ink may fail a day, weeks or months after it initially adhered and passed performance requirements, due to the migration of material additives.
As a wise person once said that some of the best solutions in pad printing don’t come in a box. Determining the best methods and materials for difficult pad printing applications always is easier when strong relationships have been developed with suppliers and other pad printing professionals who act as an extension of the decorator’s engineering department. Working toward a clear goal in conjunction with experienced partners always is less painful than going it alone, especially when working with a realistic timeline.
Thank you to Clif Treco, Marabu North America; Trent Pepicelli, Innovative Marking Systems; Nate Slating, Inhance Technologies LLC; Gordon Christensen, Pad Printing Services; Jeff Morris, RUCO; and retired pad printing guru Ulrich Auerswald for their contributions to this article.