What type of steel is used for molds?

(Updated April 2019)

When it comes to injection mold making, choosing the right tool steel can make a huge difference. Making an incorrect choice can cause disasters that fly-in-the-face of many hard hours of work. For example, selecting the wrong tool steel for your injection mold can mean a cracked core or cavity, causing it to wear out long before it is expected it to. To help avoid this problem, ask yourself these questions before making your tool steel choice:

• How many parts is the mold expected to produce?
• What is the desired surface finish of the molded part?
• Are there any shut offs that could wear?
• What cycle time is expected?
• Are there any long cores that there is no way of getting cooling into?
• Will there be thin steel areas vulnerable to cracking?

Common Injection Molding Steels

When in the process of considering which steel to choose for your injection molding process, there are generally two types to choose from: hardened steel and pre-hardened steel.  The commonly used hardened tool steels will contain S-7, H-13, 420 Stainless Steel, while the pre-hardened tool steels are comprised of P-20 and pre-hardened stainless steel. Additionally, specialty steels, such as Maraging 300, can be used due to their desired physical properties in thin areas.  Below is some additional information about the most common steels used in injection molding.

• Pre-hardened steels are typically used for lower production tools.  Many times, the mold plate is P-20 steel and the molding can be cut solid into the plates.  Areas of the plate could be inserted with hard steel if needed for shut offs or wear surfaces.
• Hard stainless steel tooling, such as 420 stainless steel, is used to minimize corrosion, either from cooling channels or corrosive materials such as PVC.  Stainless steel will crack quicker than other hardened steels and the thermal conductivity is not as good.  Stainless steel will not hold a sharp edge.  Stainless steel will be used for high quality surface finish needed to produce lenses and clear parts.
• H-13 and S-7 steels are tough materials.  These materials hold up well to wear and constant pressures of injection and the mold closing.  Special care must be taken for corrosion as water channels will rust in time.
• PAS940 is used for transferring heat.  The material is not very hard so plating is sometimes used to add surface hardness.
• Maraging 300 is used for thin steel areas and for strength and toughness.

Choosing The Right Steel is Cost Effective

While having proper materials inevitably improves the design, build and repair processes for specialty injection mold making and timely product delivery, it often also saves both time and money. This is what keeps injection mold making able to remain competitive.

Mold makers and tooling specialists alike agree on the huge impact choosing the right materials can make. Each engineer has their own experience of the risks, factors to consider and scope of results obtained when doing their own evaluations of the success of their injection mold making processes. When it comes to tracking, maintenance costs, tooling - and when considering wear resistances, part geometry, cooling and part stability - even cycle times - all of these considerations become essential.

Evaluations and materials results can differ from machine to machine and from process to process. Some major manufacturers will swear by the evaluation of their part geometry.  They also consider the cycle time impact and the nuances of part stability from cooling processes and materials selections. Mold material impact is always reinforced with these outcomes and they also have a big impact when it comes to cooling and water channels.

In addition to tool steels - choosing the right materials for other aspects of your injection mold making processes also need to be considered.

If you have any uncertainty at all about which choices to make when it comes to tooling steel, materials or more efficient processes for your next injection molding project - be sure to contact Crescent Industries. Crescent Industries has been offering in-house tooling solutions for over 75 years with specialization in building new tooling along with comprehensive maintenance, repairs, and revisions of existing tools to ensure the life of our customers tools. Our journeymen mold builders produce injection molds that provide cavity-to-cavity consistency and repeatability by utilizing high-speed CNC mills, die-sink EDM, and wire-sink EDM.

For additional information, please fill out the form below to get our white paper “10 Factors that Impact Injection Mold Costs”.

This article was composed from notes taken from posts in globalplasticinjectionmolding.com. Notes were also taken from the article: "Which Tool Steel is Right for Your Plastic Injection Mold?"  Other notes were also taken from the article published in an issue of MoldMaking Technology, on October 12, 2012 called "Becoming a Master of Mold Material."

In this article, we will provide a general introduction and guide to understanding common tool steels for injection molds. In this article, we are going to discuss the most common steels used when building injection molds. Each of the materials have pros and cons. One that works well for one application doesn’t necessarily mean it will work well for another. There is no simple formula to make this determination. Instead, several factors contribute to the ultimate choice of steel.

The first two questions a toolmaker will ask:

1) What is the annual volume and life expectancy of the part?  A part that only requires 1000 parts per year, will have different steel requirements than a part that runs millions of parts per year.

2) What polymer have you chosen for your part? If the material is a filled resin (glass/carbon etc.), those materials are abrasive and will require steel that has good wear properties. High-temperature materials will also require steels that tolerate the required higher processing temperatures.

Other key discussion points will include:

• Tolerances: If your part has tolerances +/-.002 or smaller, the part will be considered a tight tolerance part. Wear of the steel will be a main concern and the right steel choice will reduce future repair/maintenance costs on the tool.
• Surface finish will influence steel choice as well. For example, if you have a high gloss finish, you will need steel that is capable of being highly polished and hard enough to maintain that finish.
• The overall design of the product itself. A highly detailed part that requires slides/side actions needs to take into consideration thermal conductivity and moving parts to avoid issues during production.

Common Tool Steels

The most common choices are Aluminum, P-20, H-13, S-7, and 420 SS. When matched properly with the application, each of these materials will perform well. However, if you opt for aluminum based on its affordability and you are running a glass-filled material, that tool is not going to yield thousands of parts.  While the price point may be attractive up front, you may need further investment to produce more parts due to wear and dimensional issues. Here is a brief description of each:

Aluminum

Aluminum is a common choice for low-volume applications and prototyping. It is best suited for unfilled commodity or engineering-type resins. Generous tolerances fit best with this steel choice. Because it is a less dense steel, it machines much faster than hardened steel thereby reducing machining time, lead times as well as reduced labor costs. While it is low cost, it will not offer you longevity. This is a true prototype/low-volume choice.

P-20

P-20 is a semi-hardened tool steel. This is a step up from aluminum and is usually worth the minor increase in cost. P-20 is going to give you a longer life and will perform well with abrasive materials (glass and/or carbon filled). For some applications, P-20 is enough for the entire life of the program. As a general rule, P-20 is good for volumes up to 50,000. When given the option to build a first-stage tool before production, P-20 is a good choice. It can then serve as a backup tool if an unforeseen issue comes up with the production tool.

H-13

H-13 is a hardened steel. Prior to heat treat it machines well. It works well in high-volume applications and can last dimensionally for over 1 million shots.  H-13 is also a choice when it comes to highly filled (abrasive) materials. It also works well for materials that require high tool temperatures like polyamide-imide (PAI), polyaryletherketones (PAEK/PEEK), and Liquid Crystal Polymer (LCP).  It is also a good option for optical quality type parts that require high polish and/or a mirror-like finish.

S-7

S-7 is a pre-hardened tool steel. This steel is suited for both high volumes and tight tolerances. It can stand up to abrasive filled materials and can be welded successfully. In addition, S-7 polishes well to a high luster making it a good choice for aesthetics including glossy finishes.

420 SS

420 SS is a grade of stainless steel. Its hardness is excellent in maintaining tight tolerances and has high wear resistance with abrasive materials.  420SS is a good choice for PVC and it performs well in mildly corrosive environments.

Interested in learning more? Here is a link to a great resource on mold making. https://moldmakingresource.com/injection-mold-tool-steel/

Here is a matrix that you can reference as you head down the design and product development path:

 

If you found value in this article, consider signing up for our blog!  At Plastics Manufacturing Resources, our Blog focuses on issues to help you succeed.  Stay tuned…..next month we will be writing about “Questions You Should Ask About Injection Molders Quality Systems”

If you have a current project, reach out to the Professionals here at PMR. They can guide you through the complexities of supplier selection and will be your single largest ally in making your next project a success!

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