Images_Digital_Edition_July_2019

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 Screen printing expert Tony Palmer advises on how to print polyester fabrics Managing migration 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. Cool down 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. Tiny straws 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 When heated, the dye that’s used to colour polyester fabric can run up the polyester fibres, like water drawn up a drinking straw. www.images-magazine.com 46 images JULY 2019 TIPS & TECHNIQUES