Custom plastic cups are made from a type of plastic known as PET, or polyethylene terephthalate. This material is strong and lightweight, making it ideal for use in a variety of products, including clear plastic cups. PET is also clear, so you can see the contents of your cup. However, this transparency means that heat can easily pass through the walls of the cup. As a result, clear plastic cups are not well insulated and should not be used for hot beverages. The coffee will cool quickly, and the cup may become deformed if the liquid is too hot. Stick to using custom plastic cups for cold drinks only.