TOOLING: How to Select the Right Tool Steel for Mold Cavities

With cavity steel or alloy selection there are many variables that can dictate the best option. Things that need to be considered are the material you’ll be molding, cycle-time expectations, part criteria, expected volume, tooling costs, and maintenance. The goal here is not to suggest, recommend, or give preference to any specific steel or alloy, but to offer some guidance you should take into account when making the selection. I’m not an expert on all the steel types used around the world, so there will be some not mentioned—I’m only writing from my personal experience.

WHAT ARE YOU MOLDING?

The first thing I take into consideration is the material being molded. With abrasives and abrasives products, glass-filled materials, my focus would be on addressing concerns over wear and erosion unless the expected volume is extremely low. But with the most common glass-filled materials, cooling is more critical than with other materials, and the best steels to address wear have lower thermal conductivity. Carbide inserts are the exception; they have excellent wear properties along with great thermal conductivity, but the costs and lead times to replace these need to be considered.

With corrosive materials such as PVC, stainless steel is a common choice. Using cheaper options will require critical procedures to prevent corrosion. On parts that have very high surface-finish expectation, tool steels that have lens-grade specs should be considered. For molding materials that do not contain abrasives like glass fibers or corrosive ingredients, P-20 steel is the most common choice. But with smaller tools for high-volume production, hardened tool steels are always a good option to prolong the tool life with reduced maintenance. On the other hand, aluminum can be an excellent choice for lower-volume tools to reduce cycle times. But from a maintenance viewpoint, aluminum is not my friend.

Maintenance is the variable I focus on most in the steel-selection process. Most tool makers that build, maintain, and repair production tooling have definite opinions on what they like and don’t like in tool steels. I’ve heard so many varying viewpoints over the years, but I have tried to keep an open mind by taking the big picture into account. So at times I’d be willing to accept added maintenance on the tooling if the payback was faster cycles or lower tool costs. But price should never be the main factor. As I often say, “It’s cheaper but costs more.”

There are pros and cons to each and every option, so it’s important to know all the angles to understand the long-term cost vs. just the up-front tooling costs. If you need a steel that is wear-resistant, you are looking at a hardened tool steel. This will increase your tooling costs up front, but will reduce your maintenance costs in the long term. But here’s the twist: Hardened steels are less thermally conductive, which can impact cooling time if you don’t put extra focus on the tool design for cooling. If you go with a standard tool steel that will not be hardened, your cost will be lower up front but your long-term maintenance cost will be greater.

You can also apply a coating or surface treatment to reduce wear, which will still be cheaper than hardened steel but will put you at risk if the tool is damaged. Repairing coatings and surface hardening takes lots of time and money, especially for a part with visual requirements. But the thermal conductivity will be 10-15% greater than with a hardened tool steel. Then there are options in aluminum and alloys with much greater thermal conductivity that can have a big payback in cycle time. Again—pros and cons for each option.