Some screen printers, who print directly onto t-shirts or who print plastisol heat transfers, will stretch the garment before printing or heat pressing. By doing this, the weave of the shirt opens up allowing the ink to better penetrate the fibers of the fabric. This practice is believed to prevent cracking of the ink.
While stretching garments may work when printing t-shirts or heat pressing plastisol heat transfers, you should not stretch shirts prior to heat pressing a polyurethane heat transfer film. Here’s why. After the shirt is washed, it will shrink back to its original shape. When this happens, the polyurethane applique will contract, resulting in the puckering of the heat transfer film. See the photo below.
Instead of stretching a garment prior to pressing, you should pre-press it. This causes the shirt to shrink. In addition, if there is any moisture in the fabric, pre-pressing will dry the shirt. Moisture can potentially cause adhesion problems. Pressing the garment also provides a smooth surface for the application of the heat transfer applique.