Strategic Mold-Material Selection

Molding a successful part goes beyond the design of the part. Molding a successful part requires a quality mold. Working with a team that has extensive knowledge of mold materials can help ensure that an end product comes in on budget and on time.

Molds require a material that can stand up to thermal shock, abrasive resins, part details, moving mold components and heat cycles. Improperly selecting a mold material can lead to galling, warp, sink, slow cycle times and steel fatigue, which results in premature wear, breakage and steel erosion around the gate (or gate washout).

It’s important to consider part finish, production volume, part material, cycle time and mold details such as the molding resin, gate style, print tolerances, finish requirements and part configuration when determining the right mold material. Understanding the composition of a part and general industry uses of certain materials can aid in the selection process, ensuring that the team arrives at the best decision for its shop to produce a superior part.

Stainless steel is used in plastic injection molding because of its corrosion resistance, hardness and polishability. It comes in grades like 420 and 440, which lends itself to projects that must withstand corrosive or abrasive materials, high- or low-volume molding and tight-tolerance parts. Molds for transparent parts are commonly built out of stainless steel because of its polishability.

When determining the right mold material, it’s important to consider mold details such as the molding resin, gate style, print tolerances, finish requirements and part configuration.

Hardness, compressive strength and machinability are key attributes of H13 and S7. The characteristics of these steels have been formulated over time to have the right metallurgical compositions to withstand abrasive glass or mineral-filled materials, endure long runs of a million (or more) parts and maintain the proper cavity finish. These two steel types are commonly used in cavities, cores, inserts for smaller areas, high-cavitation molds where extended mold life is required, projects involving abrasive materials and parts demanding a high polish.

Another commonly used steel for cavities and cores is P20. It is generally used for large molds because of its availability in larger sizes and for projects that permit shorter mold life. P20 has a Rockwell hardness of 28–36 RC, which makes it easy to machine and does not require post-heat treatment. However, someone looking to prevent premature mold damage from resin erosion should use plastics without abrasive additives.

Metals such as beryllium copper, Ampcoloy and aluminum are also used, especially when there are concerns about mold cooling, as these materials have better thermal conductivity in comparison to common mold steels. The result of improperly cooled molds is warp, sink and long cycle times, all of which can ultimately increase the cost of the project and delay its completion. Better thermal conductivity means a more efficient cooling process, faster cycle times and enhanced part quality.