When heat pressing, place the teflon sheet on the substrate, which can prevent the heating mat from the pollution of the image on the substrate. Teflon is an integral part in heat pressing. Teflon offers the maximum stain resistance and is perfect for keeping your heat press and garment clean. The Teflon is 5-mil and will last 2-3 times longer than 3mil. Teflon sheets are re-usable and can be used on either sided.