There is a recipe to follow whenever you are using a heat press. The ingredients consist of temperature, time, and pressure. The first thing to check is the temperature. You can purchase heat strips from your supplier. Put the strip on the platen and lock down the press. If you set the press for 375 degrees F and the heat strip measures 350 degrees F, you know you’ve got a temperature problem.
If the temperature is off, contact the manufacturer who will either teach you how to calibrate your press or have you return the press for factory calibration. In a pinch, you can raise or lower the temperature by the amount the heat strip says the press is off as a temporary measure to fix the problem until you can get it properly adjusted.
Time is the second thing to check. Most transfer manufacturers will give you a window of time to apply a transfer. Experiment to see if more or less time makes a difference. Transfer manufacturers provide a range because the amount of time depends on what make heat press you are using and what type of substrate you are embellishing (cotton versus polyester, for example). If you are putting a bright white or gold design on a black shirt, you want to stay on the lower end of the time range. If not, you may develop strikethrough. Strikethrough is what occurs when a garment is pressed for too long. This drives the transfer too far into the garment. The result is seeing the garment color through the transfer. This may turn your white transfer pink on a red shirt or gray on a black shirt.
Pressure is the most confusing element of heat-seal printing. A lot of manufacturers use a pounds per square inch (PSI) measurement which is easy to adjust and monitor on a pneumatic air press because there is a pressure gauge. However, on most manual presses this is very hard to measure. If the press has a digital pressure display, you at least have a reference point as the gauge will reflect a number between one and nine. What the operator needs to do is observe what amount of pressure gives the best results, and record that number. Without a gauge, this is done strictly by feel and usually denoted as light, medium, or heavy.
There is a pressure knob on a heat transfer press that can be turned clockwise or counterclockwise to apply more or less pressure. This knob will need to be adjusted as you switch from doing a thin T-shirt to a thick sweatshirt. Ideally, a recipe book is kept by the press and the proper time, temperature, and pressure settings should be recorded each time a new transfer and garment combination is successfully handled. This cuts down on experimentation and misprints as the same successful settings are used every time.