Pad printing’s a great way to transfer images to 3-dimensional parts. Its versatility handling shapes and substrates composition surpass almost every other automated marking method. However, getting that ink to ADHERE to all these parts can present a challenge, especially parts molded from Polyethylene (PE) or Polypropylene (PP).
These two substrates exhibit a low surface energy (usually measured and described as the “Dyne level”). There are other substrates like those possibly containing Teflon that are also problematic. But PE and PP are so widely used in different industries, we’ll stick (no pun intended) with them for the purposes of this discussion.
Most ink manufacturers provide inks that are “suitable” for PE and/or PP. In many cases you’d just treat it like any other pad printing job with no issues. However, depending on the customer’s expectations of image durability, this may not prove a viable option. It is, however, the first place to look.
Pre-treating for adhesion choices
There are four choices when faced with adhesion issues:
- Special ink
- Chemical wipe
- Flame treatment
- Corona discharge
Pre-wiping parts by hand with a chemical that allows the ink to bond is a “second-tier” option, best suited to prototypes and short runs. Many firms hesitate to introduce another chemical into their work flow, and you’ll be exposing operators while increasing run time as well.
For longer runs and constant production speeds, you really have to look at the final two options – flame or corona – usually in a print cell, or actually integrated either into the machine configuration or in-line.
Flame treatment is more cost-effective, easily integrated and in some cases, actually provides a better surface pre-treatment. However, some firms choose not to have an “open flame” in their workplace, regardless of the fact that most flamers are heavily guarded and the actual flame is no more than 1” to 2” in length.
Electrical discharge units (some call it corona while others refer to a plasma-jet) are just that: an electrical charge across or through the substrate that raises the energy level to a point where ink will cross-link and adhere. These are more costly to purchase, but better suited to some substrates.
Please note that there’s no “magic bullet” when it comes to pre-treatment technologies. Some work better with substrate “A” versus substrate “B.” To complicate matters even further, there are a large number of different blends of plastics, especially PE and PP, while other plastics may include recycled (“re-grind”) material; flame-retardant additives; colors, etc., all of which can affect adhesion characteristics.
To conclude: your choice is based on throughput speeds and budget, plus environmental issues.
Your ink or machine supplier will be able to assist you with testing and choices, as you need to find out what works best, will meet your production rates, and is most cost-effective.
One final note: If you are importing molded parts, be aware that the “blend” of materials may change without notice; what works this month may not work in the future. New batches may require new research. If you can, check with your parts manufacturer and bring that information back to your ink supplier.
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