How to Select the Right Tool Steel for Mold Cavities
With cavity steel or alloy selection, many variables 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.
YK30 (SK3) carbon tool steel, grade T10A.
Uses： Stripping, up and down pads, splints with higher requirements, female and male molds with lower requirements.
Mechanical properties: heat treatment quenching temperature 760°, return water temperature 140°~250°, hardness HRC50°~58°.
D3, alloy tool steel, grade C12.
Uses： Stripping, underlaying, female and male molds with lower requirements.
Mechanical properties: heat treatment quenching temperature 950°~980°, return water temperature 180°~250°, hardness HRC50°~56°.
SKD11, grade CR12M0V
Uses： Male molds, female molds, and molds with higher requirements, such as stripping of continuous molds of stainless steel shrapnel, etc.
Mechanical properties: heat treatment quenching temperature 980°~1050°, return water temperature 180°~250°, hardness HRC 55°~62°.
DC53 (SKD upgrade product)
Uses: Commonly used in male and female molds of higher density molds.
Mechanical properties: Hardness and wear resistance are better than SKD, deformation coefficient is less than SKD. Heat treatment quenching temperature 980°~1050°, return water temperature 180°~250°, hardness HRC55°~62°.
High-speed SKH-9, grade W6CrM05V2
Uses： It is often used in the master block (insert) of engineering molds or continuous molds and punches for punching small holes. Many tools such as ordinary milling cutters, drills, wiretaps, etc. are also suitable.
Mechanical properties: high hardness, good wear resistance, complicated heat-treatment process. Hardness HRC62°~64°.
NAK80, pre-hardened Die steel.
The electrical processing performance is good, the mechanical processing performance is poor, and no heat treatment is required after processing. Due to the compact internal molecular structure of the material, the polishing performance is good.
SS41 (also known as A3 steel) is a carbon structural steel.
Uses： upper and lower mold seat feet, heightening boards, supporting boards, lower backing boards, splints.
Gray cast iron (pig iron), commonly used grades of gray cast iron HT30~54.
Uses： Mould base, general machine bed.
45# steel is a high-quality carbon structural steel.
Uses： Commonly used as a material for shaft parts.
Mechanical properties: good mechanical processing performance and strong hardness, quenching and tempering treatment, quenching temperature 820°~850°, return water temperature 600°~650°, HRC40°.
There are pros and cons to each option, so it’s important to know all the angles to understand the long-term cost versus just the up-front tooling costs. If you need wear-resistant steel, you are looking at hardened tool steel. This will increase your tooling costs upfront 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 standard tool steel that will not be hardened, your cost will be lower upfront 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 hardened tool steel. Then there are options in aluminum and alloys with a much greater thermal conductivity that can have a big payback in cycle time. Again—pros and cons for each option.
WHAT ARE YOU MOLDING?
The first thing I take into consideration is the material being molded. With abrasive, 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 expectations, 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.
ALUMINUM & CONDUCTIVE ALLOYS
A few years back, there were many talks and studies about aluminum and its positive impacts on mold-build cost and cycle times, both of which can be significant. But there is always a negative that can offset the positive if all aspects are not considered. What’s more, there are many versions of aluminum, and each has very different properties of toughness and thermal conductivity, so never assume that “aluminum is aluminum.” Do your research.
Aluminum is often used for prototype tooling to keep costs down. And the production tooling is then built with more robust steel. Just keep in mind that the molding results will not be the same. And in some cases can be significantly different, depending on the part geometry and size. The main reason is the cooling factor, as aluminum has much greater thermal conductivity. I would not recommend using aluminum on high-volume parts with visual specs or tools with many lifters and slides. Aluminum is very soft and requires extra attention in the design to make sure it is robust enough, and extra care in the press to reduce cavity damage.
The most common, middle-of-the-road tool steel is P-20 or similar, which has 28-30 RC hardness. This steel also comes in a high-hard version (38-40 RC), and in-lens grade when high surface-finish requirements are needed. P-20 is the first choice in most cases when using plastics without abrasive additives. But as I mentioned earlier, unless it’s for very low-volume use, you will need to protect the steel against erosion with a coating or surface hardening. This has drawbacks, with which I am very familiar.
S-7 is a common choice
For very tight-tolerance and high-volume parts, S-7 is a common choice. It’s a very durable, impact-resistant tool steel that can be hardened up to 56 RC. This steel also is much more stable through the heat-treating process, shrinking or expanding less than H-13 or stainless steel.
In most cases, cavities are hardened to 50-52 RC. S-7 can also be used for slides or lifters and hardened to 54-56 RC. They have excellent wear properties working against the S-7 cavities at 50-52 RC. I’ve seen tools set up like this running every day all week with intricate lifters and never have an issue with wear or galling.
Stainless steel is commonly used not only for PVC but in medical tooling to provide highly polished cavities. The high end for hardness on stainless steel is 50-52 RC.
H-13 is typically the go-to choice for tool steel to address wear when running abrasive materials. In most cases, cavities are not hardened above 50 RC to reduce the chances of stress cracks. The typical range is 44-48 RC. I have had a tool with H-13 inserts running 33% glass-filled nylon for 1 million cycles without any wear issues. But I would not recommend this unless you take very careful consideration in the tool design.