This is the most common issue when baking sublimation cups for too long. Sublimation coating is not meant to withstand too much heat for long periods of time. Another cause for paint chipping is that the cups are not being rotated every 2 minutes. The reason rotating cups in the oven is important is because the longer they sit in the oven, the more the heat is attacking one area which will cause the paint to get weak and end up cracking, chipping, or even forming bubbles.