The mold steel selectioncan be as critical to the success of a plastics application as the selection of resin is to the performance requirements of the plastic part it produces. Just as resins are formulated to meet performance requirements in plastics applications, steels are also alloyed to meet specific performance requirements. Some applications may require a mold steel with high hardnessand wear resistance for parting line durability, while others will require a mold steel 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, and in almost all cases, a steel with greater toughness will deliver some reduction in resistance to steel-to-steel wear (adhesive wear) and abrasive resistance to resins containing glass fibers or mineral fillers. A mold maker, for example, may select a stainless steel for a resin that could be corrosive to most other steels. Parting line integrity will typically be greater with higher hardness steels (Rockwell 55 or higher), and 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. For abrasion protection from glass or mineral filled resins, it is suggested that gate inserts of A-2, D-2 or M-2 steel be considered with an abrasive-resistant steel be inserted in the mold core opposite the gate.
P-20 Steel While there is no “general purpose” steel for plastic plastic injection molds, P-20 steel has been regarded as the workhorse of the industry. Supplied in the pre-hardened state at Rc 30-32, it is very tough, yet fairly easily machined. It is a good steel to consider in applications where cavity sizes exceed 12x12x12 inches (303.6x303.6×303.6 mm), since the cost and associated risks of heat treating blocks of this size may be prohibitive. P-20 steel is also chosen in 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. When constructing a mold of P-20 steel where slides, lifters or other cams or moving components are necessary, it is suggested that these moving steel components be made of steels with different alloying and hardness to reduce galling or high adhesive wear. A common practice in large plastic injection molds of P-20 steel is to employ slides or lifters of H-13 steel that is heat treated to Rc 50-52 or to employ localized wearing surfaces of steels in the Rc 55 through Rc 58 ranges, or both.
H-13 and S-7 Steels These steels offer an extremely high degree of toughness and mechanical fatigue resistance with a perceived higher toughness in H-13 (Rc 50-52) but better durability in S-7 because of higher hardness (Rc 55-57). Neither exhibits exceptional abrasion resistance from glass or mineral resin fillers.
It is common for H-13 to be chosen in cavities larger than 8 ´ 8´ 8 inches (202.4 ´ 202.4 ´ 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. S-7 can be heat treated in an air quench in small cross sections of 2 1/2 inches (63.25 mm) or less, and offers very good dimensional stability through this process. Large cross-sections of H-13 and S-7 must typically be quenched in oil.
Corrosion Protection 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 have an aggressive (PVC) component. It is generally suggested that, if there may be occasions of longterm mold storage, where corrosion protection beyond preventative spraying may be necessary, nickel plating may be employed. Electroless nickel plating offers excellent chemical protection and is relatively inexpensive when compared to chrome or other techniques.
Finally, nickel plating can allow for steel selection offering highermechanical properties such as toughness, hardness, abrasion or adhesion wear resistance, and higher thermal conductivity than stainless steels.