Often it is not so clear which material would work best for your injection molding, besides the fact that there are so many choices available. Choosing the wrong tool steel can be devastating to the outcome of a plastic molded part, so it is important to make an informed decision.

Injection moulding

To be clear, this article focuses on the actual core and cavity of a plastic injection mold that is producing thermoplastic parts. There are so many other ways to mold plastic and so many types of molding that one answer does not always cover every technology.

Factors in choosing material for injection molds

• What type of plastic are you injection molding? This makes a huge difference. Some plastics have very abrasive qualities, others require high temperatures to process in the mold, others need very high pressures to perform correctly. The plastics supplier has recommendations that are very good starting points.
• How detailed is the plastic part? Tiny, fine detailed molds obviously need different treatment than a mold for a mop bucket, for example.
• How many parts do you intend to make? A mold that produces a million shots just cannot be made out of aluminum or even pre-hardened steel. There are always exceptions to this however, and most molders can tell you stories of the prototype mold that has been running for 5 years now.
• How long do you have to build the mold? It takes many man and machine hours to produce a quality injection mold, and this process can be shortened by skipping the heat treating step. For some reason, companies that do not make prototype tooling feel the need to use hardened tool steel for everything, maybe because that is what they are used to. They could make more money by using P20, NAK55 or PX5 and still have a very durable tool.

Characteristics of mold steels

• S7 is probably the most common steel used in the USA for plastic injection molds, or moulds for the rest of the world. This steel is relatively easy to machine, can be hardened to Rockwell 54-59, which is certainly hard enough for a mold. It is easy to polish to a high lustre, is easy to weld and is relatively tough.
• H13 steel is often chosen because it is excellent for diamond polishing. Mold for lenses and other plastic parts that require mirror finishes are often made of H13. It can be hardened to Rockwell 52 and is very versatile, with high hardness, toughness, and high temperature strength.
• 420SS is popular as a corrosion and wear resistant tool steel that is often used in medical and highly polished applications. It is easy to weld, which makes it a good choice for certain projects. 420 stainless is typically hardened to 50 Rockwell.
• NAK 55 is a pre-hard tool steel which is easy to machine, hardened to 38-42 Rockwell, easy to weld and polishes nicely.
• NAK 80 is a pre-hard tool steel, somewhat harder and tougher than NAK 55. It can be polished to a high lustre finish, making it popular for medical and optical plastic parts.
• P20 is the old standard pre-hard steel, and it is still quite popular. It does have a tendency to work harden and contain hard spots, but, with carbide tooling so commonly used, this is not much of an issue. It is approximately 32 Rockwell and is easy to weld and polish.
• Aluminum is used more often now that mold grade alloys have been developed. These metals can be much more durable than in the past and are commonly used in prototype tooling and short-run projects. QC7 is probably the most popular mold grade aluminum. It is very easy to machine, easy to weld and can be polished to a high lustre.