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Curing Methods
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The reason you have to cure your prints is to ensure that your inks bond with the fabric that they are printed onto and don't crack or fade. Your method of curing depends on what ink you've used as well as what your set up, print quantity and budget is.
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Water based inks
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Water based inks have to be completely dry before you cure them. To cure water based inks you need to heat them up to a certain temperature for a certain amount of time. There are binding agents in water based inks which are activated by heat, once they reach this temperature a thing called cross linking occurs. If an ink still has moisture in it due to not being left to dry for long enough or due to it being left in a cold room the print could end up being only 80% cured.
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You can use a drying tunnel but they are better suited to plastisol inks. If you use one for water based inks you would have to put the shirts through twice, once to dry and the second time to cure. You would have to adjust the belt speed and do wash tests.
Irons
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I started screen printing at home and started with clear/translucent water based inks which I printed onto light and white T-shirts. These I would leave to air dry, usually overnight and would then iron them with a hot iron in the morning (don't use steam). Sometimes I would iron too long and the fabric would start to go brown so I learnt to cover the print with a tea towel or piece of old T-shirt and then iron the print. It's difficult to judge how long to iron for so it's good to iron one print and then do a wash test to see if you did it for long enough. Wait at least an hour before washing the cured sample as cross linking/curing is still happening after the heat is applied. You'll get a good idea of how long to iron for, but a guide would be to iron each print for 2-3 minutes.
Heat presses
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This is a great way to cure a water based ink. You can set the temperature and time and have very consistent results.
Curing times and temperatures for water based inks are:
- clear/transparent water based ink - 30 seconds at 160 degrees
- opaque onto dark fabric - 40 seconds at 160 degrees
Do a wash test and make sure these times work for you and your ink.
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The only drawback of using a heat press is that you have to wait for inks to be completely dry before you cure them. If you take the printed fabric out of the press and see a wet area in the shape of the print on the rubber mat, your print wasn't completely dry before curing, this will only be an 80% cure. You would need to dry the shirts completely and cure again.
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Things to keep in mind when curing :
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1. Be careful of white fabrics that have been bleached white as heat will make the fabric turn brown, for fabrics like these you either need to cure at a lower temperature for a little longer or use a Room Temperature Cure.
2. When using a heat press, orange fabric and red fabric will change colour and look darker but will return to the normal colour once they cool.
3. Always check for garments that are not prepared for printing like polycotton fabrics with a lot of excess dye in them, cheap polyester fabrics with Red, Navy or Dark green. Any fabric with excess dye should be prewashed to remove the excess dyes. If you print a white ink onto a fabric that has excess dye, during curing, the heat gives the excess dye a vehicle to migrate into your ink - these have been referred to as fugitive dyes. Your bright white print on the back of a red football shirt has now become pink. Your bright yellow print on the back of black overalls has now faded into a greyish black. If you have a fabric that has excess dye, you can print with a Room Temperature cure mixed into your ink which will allow the ink to be cured without heat to prevent the fugutive dye migrating into your ink.
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Room Temperature Cure / Cold Cure
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A RTC is a liquid that you add to your ink which causes cross linking to occur at a much lower temperature. You add 5% (check your suppliers instructions) to your ink. I would use teaspoons (5ml) and dessert spoons (10ml) to measure out the RTC. Once you mix the RTC into your ink you have to use it up as the ink will become thick and be ruined after a few hours. Printed garments can be left to air dry and should be left for 3-5 days before being washed, again check your supplier's instructions.
A RTC can be quite expensive but it lasts a long time and works amazingly well.
If you have fabric that has dye migrating into your prints during curing a RTC is a good way to avoid that problem. If you have to print a lot of single colour prints and they are going to take ages to heat press and cure, pop in some RTC into your ink and leave the prints to air dry, no curing necessary. People printing at festivals or at shows/markets can print with a RTC and let the prints air dry, tell people to not wash the garment for a few days and they curing will occur by itself. One drawback of using an RTC is that the ghost image on your screen will probably never go away and if you let your screen block and dry or don't clean it down properly after printing you might need to have the mesh replaced and restretched. I have printed some long jobs and not cleaned my screen properly when I was finished and cleaning the screen was hard work.
Flash Dryers
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You get two kinds of flash dryers, ones with a heating elements that heat up slowly and another type with bulbs that heat up instantly - these can sometimes have a sensor that switches the unit on when a platten on a carousel moves underneath it. BBC Technologies have a great flash dryer called a Black Flash. These work well for drying inks between print layers and also between colours.
If you want to cure using a flash dryer it would be good to get a laser temperature gun to measure the temperature of the ink while you cure it. I've only used them for drying layers of inks and speeding things up and never solely for curing. If you start out with clear water based inks on light fabrics you wouldn't need to dry inks between colours as you can print wet on wet. It's when you start to print with opaque water based inks that you start to need to dry between printing layers of inks and between colours. If you are starting out a Heat gun is great for drying inks but once you start to get busier a flash dryer is a great investment. For curing a water based ink leaving it to dry in the air is much easier than drying it with a flash dryer. Once it is dry you could put it under the flash dryer to cure.
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Curing Plastisol Inks
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Plastisol inks need to reach 160 Degrees Celsius to dry. You can use a heat gun to cure plastisol inks but it would be difficult to get consistent results, you could use a heat gun to dry and then a heat press to cure properly. A flash dryer for drying would be better as it covers a bigger area and is consistent. It's difficult to judge exactly how long to do this for but you can get a laser heat temperature gun so that you can check to see that your ink reaches the right temperature and you can also check if your plastisol ink is cured by stretching the print, if it cracks then it isn't cured properly.
The best way would be to use a tunnel dryer and get a more consistent result by changing the speed of the belt depending on the amount of ink being printed.
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When drying plastisol ink with a heat gun and then curing with a heat press, you need use a teflon or silicone sheet between the print and the heat press platen as the plastisol will melt and smear if it isn't covered. Remember to put a "Do not iron on print" sticker on your shirt if you print with plastisols.
Drying tunnel
Outsourcing your curing :
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If you get to in an 'in between' stage of hobby and printing jobs for customers and get a larger job that can't be cured with an iron, you can always approach a local screen printing company and ask them what they would charge to run your prints through their tunnel dryer. I used to do this in the beginning when I had larger print runs which had bigger prints and were difficult and time consuming to iron- they would charge me a small price per shirt and this saved time and also made sure my inks were well cured.
You have to make sure that you cure your prints properly - this is a really important part of printing onto fabric. If a customer returns their order due to their shirts not washing well, you will not only loose repeat business, but it can be very costly and time consuming as you will not only have to reprint the job, but also have to replace the garments.