• There are 5 classes of standard molds defined by the Plastics Industry Association, formerly known as the Society of Plastics Industry, or SPI.  See the table below for further details.

• For higher volumes, it is recommended to use harder more expensive steels due to long production runs and high service life.

• P-20 is a very common tool steel for non-abrasive and non-corrosive resins.

• P-20 High Hard (HH) (approximately 20% harder than P-20) is also a common choice as it provides more compressive strength than standard P-20.  We typically spec this material for most new tools if using resins without abrasive additives.

• Harder tool steels like H13 are a common choice for abrasive resins such as glass filled, or to prolong tool life and reduce maintenance.

• Stainless steel is a common choice for corrosive resins such as PVC.

• S-7 is typically used for small-tolerance and very high-volume applications.

• Aluminum tools can be a good choice for low-volume tools, although they can produce many thousands of parts with proper design and meticulous care.

• Hardening, plating, and surface treatments may also be required, depending on the resin and application.

• Higher wear resistance and higher hardness tool generally equals a more brittle tool.  And likewise, greater toughness generally equals less wear and abrasion resistance.

​