Tek Tip - What Offset Printers Need to Know About Plastic Films
Plastic substrates have very different handling and printing characteristics when compared to paper products. If you are not familiar with printing on non-paper substrates please take note of the following:
- Plastics will not absorb fluids like paper.
- They restrict the flow of air and other gases between sheets of a stack.
- All plastics are not the same and will vary in ink receptivity and adhesion.
- Most plastics will attract and hold a static charge much easier than paper.
- Plastics generally are more expensive than paper products.
Because of this the processing window for plastic is much narrower than paper and mistakes can be costly. TEKRA has made every effort to provide you a quality substrate but realizes that the variation between products, and in some cases between lots of the same film, requires special care in processing. Unless otherwise noted, all of these products have been tested for printability by the offset process.
The following are recommendations that will help in processing plastic films:
- Have your ink supplier test each lot for printability and adhesion and reformulate his ink if necessary. Inks should:
- be 100% solids
- have high color strength
- dry by oxidation only
- have variable tack--descending from first color down
- be formulated for low water pickup
- dry quickly
- be compatible with the application--consider also adhesives used and end use environment.
Ask TEKRA to supply a pre-shipment sample to your ink supplier.
- Avoid solid coverage--use a screen whenever possible to simulate the appearance of a solid.
- Condition stock 48-72 hr. (unwrapped) to press room environment.
- Wipe down sides of stack with a damp cloth to remove static attracted dust.
- Use paper of comparable thickness for mechanical make ready. Coming up to color must be done on production stock.
- Use static eliminators such as ionized air jets and blowers on the feed and delivery end.
- On matte two-side material print the smoother side. If a job is two-sided, print smoother side first and print other side as soon as practical to minimize ghosting.
- Add ink after press is set up and feeding properly. Ink should be fresh.
- Run inks thin to facilitate drying and minimize offsetting.
- Dampening system--ask fountain solution supplier about:
- special formulation for non absorptive materials, if not run alcohol if possible substitutes may extend drying time.
- consider fountain drying stimulators.
- don’t use old solution that’s been running paper for a while. Dump tanks and start over. Clean rollers to remove build up of old solution.
- run as dry as possible--water has no where to go on plastics.
- control conductivity and pH consult ink manufacturer.
- Plastics do not compress like paper. Adjust the squeeze or blanket material when running plastic films.
- Use spray powders to facilitate drying-consult supplier.
- Use short lifts-about 2”-this will reduce the chance of offsetting.
- When possible wind sheets to aerate and accelerate the oxidative drying process.
- Aqueous over coats may protect the ink and prevent offsetting but may interfere with ink drying-consult ink manufacturer
- If adhesive is to be applied over ink have it tested for compatibility--some adhesives will remove ink from plastic film.
Keep a log of what you did on your job so the next set up will be faster and easier!