Mold class refers to an industry standard provided by the Society of Plastics Industry (SPI) for classifying the quality and lifetime of molds.

• Class 105 (<=500 cycles): The least expensive type of mold, which can be constructed from cast metal or epoxy, and is to be used for prototypes only.
• Class 104 (<=100,000 cycles): A low priced mold with mold cavities typically constructed from aluminum or mild steel.
• Class 103 (<=500,000 cycles): A moderately priced mold, also the most common, requiring cavity and cores to be of a hardness of 28 R/C or higher.
• Class 102 (<=1,000,000 cycles): A high priced and high quality mold, requiring cavity and cores be hardened to 48 R/C and all other mold components be heat treated.
• Class 101 (>1,000,000 cycles): The most expensive and highest quality mold, requiring cavities and cores be hardened to at least 48 R/C and all other mold components be made of hardened tool steel.