Not all fabrics are able to withstand the heat needed to transfer iron-on decals or patches to their surfaces. It is important that you read each garment's label before attempting to affix an iron-on decal. The labels will indicate the garment's composition as well as the maximum iron heat you can use on the material.
Fabric Types
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Cotton is the best fabric on which to use an iron-on patch or decal. Cotton is a natural fiber and is able to withstand heat up to 400 degrees Fahrenheit. To preserve the integrity of the fabric, don't iron garments with a wrinkle-resistant finish. The label will state whether this is the case. Some cotton blends, such as poly cotton, are able to withstand heat almost as high as cotton itself. Use a medium setting if you are going to use an iron-on on poly cotton.
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Material Textures
Though different fibers dictate whether a material can withstand the correct amount of heat, some fabric textures prevent this from being a possibility. Not only will some lose their natural texture in the iron-on process, such as seersucker, some will not take the transfer at all. Loosely woven fabrics and pique knits fall into this category. For best results, use smooth, tightly-woven fabrics and standard knits, such as T-shirts.
Iron-On Temperature
Read the iron-on manufacturer's instructions for application. If the temperature ranges are compatible with your garment, then it is safe to use it. Wash the garment before beginning to ensure that any shrinkage occurs before you apply the transfer. Place the iron-on patch or decal on the garment with the adhesive side down and iron the back for the recommended amount of time. Peel the backing off the iron-on to complete the process.
Testing Your Garment
If you have made your own garment, test the ironing temperature on a scrap of fabric before adding the iron-on. If you are using store bought clothing, you may still wish to test the integrity of the fabric. Using an iron set to the transfer's recommended temperature, test the heat on the inside hemline of the garment where the hemline and side seam meet. There is more fabric here than in the rest of your garment, making it difficult to see any singes from the outside.