Classes of Plastic Injection Molds

Classes of Plastic Injection Molds

At HTI, we work with our customers to help them determine the type of mold that will suit their product best. Whether it's a new design or an existing design that we're helping make "moldable," we'll ask a series of questions to determine the best type of mold for the product.

What Are We Making?

What is it used for? What types of environments will the product endure? Is it cosmetic? We can offer suggestions on material types, wall thickness, placement of ribs, or where the part will be gated. Are there tight tolerances that need to be held? What is the planned life cycle of the product? Answering these questions and more helps us determine proper mold selection for each product we make.

The Society of Plastics Industry has classifications of injection molds to help serve as a guideline when quoting or placing orders for uniform types of molds. In defining these classifications, we'll refer to the SPI-designated mold class, as well as the materials that go into making that mold. Hardness is measured using the Rockwell scale, a scale used across a wide variety of industries and materials.

A Note About the Rockwell Scale

To determine the Rockwell hardness measurement of a material, a test instrument impresses a piece of diamond into it using a given weight, and depth of the impression is measured. It's expressed as R(scale letter), with A, B, and C the most common scales for steel. We use Rockwell hardness measurements to determine the types of steel used in our molds.

Classes of MoldsHTI Plastics Injection mold

Class 101 Mold
This mold will run one million or more cycles. Built for high production and fast cycle times, it will be made of the highest quality materials, with hardened steel cavities, cores, and wear items of minimum hardness 48 Rc with provision for temperature control of individual cavities.

Class 102 Mold
This mold is capable of up to one million cycles. Class 102 molds are good for abrasive materials or parts requiring close tolerances. Cores and cavities on this high-quality, fairly high-priced mold will be made of hardened steel minimum 48 Rc.

Class 103 Mold
This is a medium-production mold with cycles not to exceed 500K. It's quite popular for low- to medium-production products and fits nicely into the most common price range. Class 103 molds may have hardened steel cavities and cores if required, minimum 28 Rc.

Class 104 Mold
This is a low-production mold for products running less than 100K cycles, generally used in projects using non-abrasive materials. The mold base may be of steel or aluminum, and cavities and cores may be cut directly in the mold base plates or inserts made from steel, aluminum, or another material.

Class 105 Mold
This is a low-cost prototype mold for running less than 500 cycles. It's built in the most inexpensive manner possible and designed to produce a very limited number of parts. The mold may be cast metal, epoxy, or any other material offering sufficient strength to produce minimum prototype pieces.

Once we understand the product’s design, purpose and objectives, we’ll work with you on the determining the right class of mold needed. While there are different molds for different applications, each of these mold classifications is capable of producing a quality product given the limitations of the mold life cycle identified for each classification.