Brian Lessard

Dial-In Your Dryer Using A Thermocouple Instrument & Screen Printer’s Temperature Probe

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Brian Lessard | April 15, 2022

ROQ Screenprinting Dryer

Properly curing printed garments is one of the most critical steps in the production process to ensure a durable ink film that will look great after multiple washes. The best way to ensure you’re reaching optimal curing temperature is to test your dryer & ink film using a thermocouple kit specifically designed for screen printing. The kit includes a thermocouple instrument and a screen printer’s temperature probe, aka a donut probe, capable of reading a max temperature of 400ºF (205ºC).

Thermocouple Kit

The advantage of using a donut probe over other temperature readers, e.g., infrared thermometers or heat strips, is the ability to read the actual ink film temperature throughout the heating chamber instead of just one surface reading.

Testing Ink Temperature

Begin by setting the dryer temperature 10–20º higher than the required temperature to cure the ink being printed; this is just a starting point and may be adjusted based on results. For this test, we’re using white plastisol ink with a cure temperature of 320ºF; however, this test will work with other types of ink.

For belt speed, you want the garment to be in the chamber as long as possible to ensure a good cure without excessive heat, so for plastisol ink, start with at least 1 minute dwell time. Dryer belt speed is often determined by the size of the drying chamber combined with the volume of garments being fed through, so try to have as long a dwell time as possible to ensure a good cure.

Plug the donut probe into the thermocouple instrument, turn it on, and set the temperature reading to Fahrenheit or Celcius. Next, place the donut (white disc) with crosshairs facing down directly on the wet ink, and press the crosshairs into the ink film. Pressing the crosshairs into the ink enables you to read the temperature of the ink film, not just the surface. Let the printed garment and donut pass through the dryer while holding the thermocouple instrument and monitor the temperature. If you receive an error on the thermocouple screen at any point during testing, make sure the metal crosshairs are touching in the middle. If the wires are loose, tighten them using the two screws on the side until the wires are making contact in the middle. For plastisol ink, you want to reach the target temperature and remain at or above that temperature for 5–7 seconds. Think of the curing process as baking the ink; a large circle of thick ink will begin to cure on the outside edges first and work towards the middle and top to bottom.

Temperature Probe on wet ink

Shirt with Donut Probe entering Dryer

Thermocoupler reading

If your thermocouple reading is lower than the target ink temperature, you can increase the dryer temperature or slow down the belt to give the ink more time to reach the target temperature. Continue running the test until the ink film reaches the target temperature for 5–7 seconds. Be sure to use a new print for each test so the crosshairs can be placed onto wet ink each time. Once you have the settings dialed in, you can run this same test on 100% polyester, fleece, blends, nylon, or any other fabrics or inks you print often. Polyester and poly/cotton blends contain less moisture than 100% cotton and will heat up much faster in the dryer, allowing you to lower the temperature. Document all the results and post the setting on or near the dryer for daily reference.

Control Display of ROQ printing press

Testing Internal Dryer Temperature

Now that we have the ideal settings for curing the ink, we can check the consistency of the drying chamber by testing the ambient temperature. This procedure is not recommended for an infra-red dryer as they do not heat the air.

Plug the donut probe into the thermocouple instrument, turn it on, and set the temperature reading to Fahrenheit or Celcius. Next, place the donut (white disc) on the dryer belt (infeed) with the metal crosshair wires up; face down will read the belt temperature. Allow the probe to pass through the dryer while holding the thermocouple instrument and record the temperatures in 5-second intervals. This should be done on the left, right, and center of the conveyor belt to determine if the dryer is heating consistently. Document the results and save them for future reference. This test can be performed as a daily maintenance procedure to ensure the dryer is functionally properly and heating consistently.

donut probe on dryer belt

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