Injection Molding Mold Material
The process of injection molding starts with a metal mold. Injection molds can be made of steel, aluminum, or beryllium copper. ICOMold by Fathom uses only high-strength steel when tooling molds.
The type of steel selected for the mold can be one of the most critical choices when entering into an injection molding project. The correct injection mold material is critical to the success of a plastics application just as the correct plastic resin is critical to the performance of the final part.
Steel: The Best Choice for Mold Material
ICOMold by Fathom uses only high-strength steel as an injection mold material. Steel has considerable advantages over other mold materials like aluminum and beryllium copper. Steel molds will last longer, ensure greater durability, and can even be repaired in certain circumstances.
Steel can be also used in alloys, allowing for a custom mold material that will meet the exact specifications of the project. Steels are alloyed to meet specific performance requirements of the project in much the same way resins are formulated to meet performance requirements of the final plastic part. Some applications may require a steel mold with high hardness and wear resistance for parting line durability, while others will require a steel mold with higher toughness for resistance to mechanical fatigue.
In general, injection mold materials delivering higher hardness and wear resistance properties are those that tend to be more brittle. In almost all cases, a steel material with greater toughness will suffer from some reduction in resistance to steel-on-steel wear (adhesive wear) and resistance to resins containing glass fibers or mineral fillers. ICOMold by Fathom has the knowledge and experience to select the proper injection molding mold material for your next project.
A mold maker, for example, may select a stainless steel material for a resin that would be corrosive to most other steels. Parting line integrity will typically be greater with higher hardness steels (Rockwell 55 or higher) and at points where steel-to-steel shut-offs produce coring. One or both steel faces should be in the hardness ranges of Rockwell 55 to Rockwell 58. Gate inserts of A-2 steel, D-2 steel, or M-2 steel should be considered along with an abrasive-resistant steel mold material inserted in the mold core opposite the gate for abrasion protection from glass or mineral filled resins.
P-20 steel is regarded as the industry workhorse of the injection molding mold material. P-20 steel supplied in the pre-hardened state at Rockwell 30-32 is very tough, yet fairly easy to machine. It is a good steel to consider in applications where the cavity size exceeds 12 in. x 12 in. x 12 in. (303.6 mm × 303.6 mm × 303.6 mm), since the cost and associated risks of heat-treating blocks of this size may be prohibitive. P-20 steel is also chosen for molds with smaller cavity sizes to eliminate the time and expense of heat treatment when it is anticipated that the mold will not exceed 500,000 cycles.
Slides, lifters, cams, or other moving components in a P-20 steel mold should be made out of a steel with a different alloying and hardness. This is to reduce galling or high adhesive wear. Slides or lifters of H-13 steel that are heat treated to Rockwell 50-52 are commonly used in P-20 steel molds. Localized wear surfaces of steel in the Rockwell 55 through Rockwell 58 ranges are another option.
H-13 and S-7 Steels
H-13 steel and S-7 steel offer excellent durability and mechanical fatigue resistance. H-13 steel has a greater toughness at Rockwell 50-52, while S-7 has better durability because of higher hardness rating at Rockwell 55-57. H-13 steel and S-7 steel do not exhibit exceptional abrasion resistance from glass or mineral resin fillers.
H-13 is a common mold material for cavities larger than 8 in. x 8 in. x 8 in. (202.4 mm x 202.4 mm x 202.4 mm) where a higher degree of hardness and toughness over P-20 is required.
Smaller cavities and cores are commonly constructed of S-7 steel. S-7 steel can be heat treated with an air quench in small cross sections of 2.5 in. (63.25 mm) or less, and offers very good dimensional stability through this process. Large cross-sections of H-13 and S-7 are typically quenched in oil.
Nickel plating or stainless steels may be needed to help prevent mold corrosion when molding in a high humidity environment. Corrosion is most likely to occur with a cold plastic injection mold where condensation, then oxidation may occur, or when using a material that may emit a gas that is aggressive to most steels.
Nickel plated or stainless steel plastic injection molds are not normally required to mold GE resins because mold temperatures should be no cooler than 140°F (60°C) and only a few grades of GELOY resin, like PVC, have aggressive components.
Nickel-plating the mold material is a typical practice for long-term mold storage or where corrosion protection beyond preventative spraying may be necessary. Electroless nickel plating offers excellent chemical protection and is relatively inexpensive when compared to chrome or other protection techniques for the mold material. The benefits of nickel-plating include higher mechanical properties such as toughness, hardness, abrasion or adhesion wear resistance, and better thermal conductivity than stainless steels.
The experts at ICOMold by Fathom are ready to help you select the correct injection molding mold material for your next project.