Many printers have at least dipped a toe into the world of single color plastisol heat transfers. They're quick, easy, and great for taking on the go. What many printers haven't tried is a multi-color heat transfer. Most printers just aren't sure how to do it right, so they don't do it at all. As with most things screen printing related, once you know the correct method, all it takes is a little time and practice and you'll be printing like a pro. Knowing how to produce multi-color transfers increases your shop's overall versatility and opens you up for more potential live event opportunities.
So, how do you print a multi-color heat transfer?
Multi-color transfers start the same as a single color, with the artwork. For heat transfers, your artwork needs to be reversed so that when you press the transfer onto your shirt, it's correctly oriented. You can do this by either printing your film positive reversed or by flipping your film around when you expose your screen. Choose a screen that will allow a good deposit of ink through. Doing a 2/1 coat of emulsion on your screen is a good idea to make sure you get a good ink layer down on the transfer.
A little bit of spray tack will work great for putting your transfers in place on your platen. If you are considering longer and more frequent runs of transfers, however, a vacuum platen is a very useful tool that negates the need for adhesive spray when doing transfer printing.
One major thing to keep in mind with multi-color transfers is the order of your colors. The simplest and most common multi-color transfer is a single color and a white underbase. This combination allows you to print on darker garments without worrying about the opacity of your color. Assuming you are printing a white underbase and a color, you need to consider how to make sure your underbase ends up under the color when you heat press. This means printing your color first, and your underbase on top of it. If you're working with a design with more colors, the same principle applies. Print what will be the bottom color on the garment last, and the top color first.
Get your transfer paper loaded onto your platen, ink up your screen, and lay down your first color. One pass should be fine, but as with a standard print, check the ink layer and make sure you don't need a second pass. After you've printed your color, then you'll want to flash it before printing the underbase, or next color. This will need to be a slightly quicker and cooler flash to make sure you don't shrink and peel the paper. Raise your flash a little to keep it cooler and use a laser temperature gun to make sure your ink doesn't get hotter than 280F. After you've flashed the color, then go ahead and print your underbase or next color.
With your first color flashed and your underbased/final color printed, take your transfer to where you have your adhesion powder set up and coat it evenly. Don't forget to tap off any extra powder, back into the container you're using!
At this point is when you'll do your gel cure. The goal here is not to fully cure the ink, as then you wouldn't be able to transfer the ink onto the garment, but to get it into an almost cured, "gel" state. Plastisol ink gels at around 270-280F, so you will want to put it back under your flash or through your dryer until it hits that point. Temp the surface of the ink, and adjust your dryer settings as necessary to get an even gel cure. If you are doing a transfer with more than just two colors, you may want to increase your dwell time. If the bottom colors do not gel, then you can end up with a partial transfer, with some of the ink on the shirt, and some on the paper.
The very first transfer you print is a good opportunity to do a little testing to make sure your cure is good. After you've gotten through your gel cure phase, grab the transfer and try to peel up the corner of it. If it peels up cleanly, then that means you're good. If it crumbles away, or leaves slightly wet ink on your fingers, then that is a sign of undercuring. If it won't peel away from the paper at all, then its been cured too much. Get your dwell time and temperature dialed in on the first print, and all the rest will be much easier.
After gel curing and testing, the only thing left is to actually heat press your transfer. Load up the garment, lay down the transfer, and don't forget to use your teflon sheet to avoid scorched fabric! Set your heat press to 320-330F and press the transfer for 10-12 seconds. As these are hot peel transfer papers, simply peel the paper back as soon as you lift the lid.
If all was done correctly, the print should release onto the garment smoothly, and leave only a ghost of ink behind on the paper itself. If it overcured, the ink may not release from the transfer paper. Undercuring would come in the form of a split print, with only some of the ink making it onto the garment and the rest being left on the transfer paper.
Screen printing plastisol heat transfers can be a great way to bring your shop with you, without having to take all of the equipment. Knowing how to effectively produce multi-color transfers opens up the world of on-the-go printing immensely.