Screen printing expert Tony Palmer advises on how to print polyester fabrics
I don’t know the precise origins of polyester, but at some point people who are cleverer than me decided it required less work to take a by-product of the petrochemical industry and make a running vest out of it than grow cotton, pick it and spin it. As a printer, I would love to meet those people and have a few stern words with them in a dark, quiet room. Nevertheless, in spite of the challenges it poses for screen printers, polyester makes up more than half of the clothing manufactured now – the exact percentage varies depending on which report you read – so we must all get accustomed to printing on it.
Get used to it
Polyester consists of super long fibres. It’s made in lots of different ways, with different manufacturers using different methods and shapes of fibres: some are round, some are triangular, some are multifaceted. The fibres are, however, uniform and smooth which actually makes them really good for printing onto; we all hate those little cotton fibres that stick up through our pristine prints and make them look as fluffy as freshly brushed spaniel’s ears. Why, then, is polyester printing the thing that strikes more fear into printers than even a friend saying: “Oh, you print shirts? Can you print some for me?” (I now tell people I am a gardener, because I would rather dig someone’s garden than print shirts for a friend. It always ends in disaster.) The answer is simple: heat management.
Normal printing inks are either solvent- based or water-based. Solvent inks require a specific temperature to link onto the fibres of the shirt, usually 160°C. Polyester is hydrophobic (I actually sound like a scientist now) – basically it is plastic and doesn’t accept water without chemical additives. So we’ll concentrate on solvent-based inks. Polyester is a great fabric that can withstand lots of laundry cycles at a full range of temperatures, however the disperse dye that is used to colour most polyester fibres starts to get seriously annoyed with high temperatures. This is what makes printing on polyester such a challenge. We need to heat the ink film, which we have sold our souls to make pristine and perfectly aligned, to a high enough temperature to ensure the shirt doesn’t end up on a Facebook page titled “Don’t buy from this person, their prints are temporary”. But at the same time we have to try not to annoy the polyester. It’s like a teenager without wi-fi: it looks okay right now, but it could blow at any moment.
Grey blocker slows the passage of dyes along the polyester fibres but can also alter the colour of inks printed on top
Use a thermoprobe to measure ink film temperature rather than ambient temperature in the oven
The way I try to describe polyester printing is to imagine the ink layer as like a million tiny straws laid on top of the shirt facing upward. When you apply heat to this mixture, the polyester dye starts to run up the straws like marker-pen ink up chromatography paper (or like the guy at the end of the dryer running to get his coat on as soon as the press goes into outlet mode.) The secret is to stop the dye before it reaches the top of the straws. If it stops halfway up, the divine white you took hours to place perfectly on the ‘Codfather Fish & Chip Shop’ sponsored navy running vests won’t turn a delightful shade of ice blue right before your eyes.
There are many products on the market that claim to be suitable for screen printing on 100% polyester and I am sure they are. They have probably been tested in the best labs in the world by all the people that did so much better than me at school and became scientists rather than practising the dark art of squeegee pulling (don’t be a pusher, it’s just wrong), but my best advice would be: find the one that works for you. Grey blocker base is fantastic for slowing down the rampant march of the eager-to-be-seen dyes as it contains chemicals that basically stick a foot out near the door and trip up the running catcher before he can clock out. It can, however, change the colour of inks printed on top, which means using an extra screen for white base, which also means extra heat to gel it… The endless cycle continues.
A better option is to lower the required temperature needed to get the ink to stick to the substrate (the scientists’ word for vest). This can be done with either a ready-to-use ink that cures at or around the 140°C mark (there are plenty on the market now) or by adding a catalyst to the ink: this will allow it to cure at really low temperatures, but it will also shorten the shelf life of the ink to hours instead of years.
It’s imperative that your dryer is reliable. If it says 140°C on the display, it’s important that it doesn’t really reach 160°C inside the oven. Use a donut-type thermoprobe as this will accurately tell you what the ink film temperature is rather than the ambient temperature in the oven. The heat management doesn’t stop there: flash times need to be low to avoid shrinking and at the end of the dryer the ‘Codfather’ vests need to be stacked in very small piles. If you have the room for a table, make ten stacks and place a vest on each pile in succession – this allows the heat to dissipate quickly. We have all come into the print shop on a cold morning and felt the heat still locked into that huge stack of shirts that were printed the previous day. The heat locked into the shirts is basically the fuel for the dye to keep running up the straws. Kill the heat as quickly as possible: use a fan, hook up an airline, put the table on wheels and run it round the car park… Whatever it takes. Just get the vests up to temperature to cure the ink, and then cool them down as quickly as possible. In one shop, I even saw a chest freezer used to cool down wayward prints.
In summary: control the heat and you control the bleedin’ polyester – there are tools out there to help you, so use them. Keep the temperature under control and you will be happier than the teenager when the wi-fi returns to full strength. A mixture of dye blockers, low-cure additives and finely tuned drying equipment will ensure you don’t walk into your shop the following day after a successful print run, open the box and utter those immortal words: “Are these Codfather vests supposed to be light blue?”
Tony Palmer has been in the garment decoration industry for over 30 years and is now an independent print consultant working closely with print shops to get the most from existing processes and techniques. Tony is passionate about keeping and enhancing production skill levels within the industry. He is the owner and consultant at Palmprint Consultants, offering practical help and assistance to garment decorators all over the globe.