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The vast majority of molds for the plastic injection molding industry have been traditionally made of conventional steel grades—such as AISI P20—but an alternative steel1 that uses an innovative metallurgy very different from that of the family of AISI P20 was developed to break the mold of material for mold manufacture with the intention of satisfying the specific needs of the plastics industry.

Conventional steels draw their characteristics from the hardening effect of carbon combined with elements—such as chromium or molybdenum. Although giving satisfactory results for several decades, with new polymers being introduced to the market and increased requirements on tools, these materials started to show their limits: closer inspections pointed out a consistent drop in hardness in the middle of thick blocks, which depending on the application was not acceptable.

On top of that, moldmakers have been facing a variety of problems during machining—such as hard spots during gun drilling, strong vibrations during milling, etc. Therefore, carbon-based metallurgy does not prove to be the best choice any more. Shops need to reduce internal costs while keeping their good reputation and position in the market place. In order to succeed, they have to minimize the risk of failure during manufacturing. For moldmakers and molders one of the contributing factors is choosing an improved steel grade.

Figure 2. Hardness.

The Carbon Challenge
Since carbon is responsible for the weaknesses of P20-type steels, one solution is to limit the carbon content to the minimum possible—without having to make up for this reduction by significant addition of expensive alloying elements such as nickel, chromium or molybdenum.

The alternative is utilizing boron metallurgy. Added in small quantities, boron guarantees levels of hardness of 300 HB with excellent homogeneity—even for 800 mm (31.5") thick blocks. Years of research on how to optimize its benefits lead to this improved grade of steel (see Figures 1 and 2).

Effects of Metallurgy
Since metallurgy is only the means, it is of utmost importance to be interested in its effects.

Machinability
The first argument for the use of a steel grade light on carbon and prehardened to 300 HB is improved machinability and the related cost reduction.

A trial with a car fender mold resulted in a reduction in roughing time of 33 percent. The operation was performed in 60 hours with conventional steel and was reduced to 40 hours with the use of a steel grade light on carbon and prehardened to 300 HB. The final result was a substantial reduction in machining cost (see Figures 3 and 4).

Figure 4. Chip removal rate.

Homogeneity
Improved homogeneity is another advantage, which manifests itself, on one hand with the absence of low hardness in mid-sections, but also importantly with the absence of hard spots that cause drills to break during gun drilling, and surface defects during chemical etching operations and polishing.

Figure 5. Improved homogeneity manifests itself as the absence of hard spots that cause surface defects during chemical etching operations and polishing.

Hard spots (also called segregations) are areas with increased carbon content, which form during solidification of liquid steel. The advantage of having a grade light on carbon seems to be obvious. These segregations—which are much harder than the rest of material—can create deviations of the polished surfaces as well as waves, especially visible on pieces with large surfaces (see Figure 5).

This type of defect is most visible after painting, so it directly affects the body pieces. Using a steel grade light on carbon and prehardened to 300 HB for molds on automobile body pieces allows for perfect geometry of the surface of the pieces.

Weldability and Thermal Conductivity
Other advantages include improved weldability as well as a 15 to 20 percent improved thermal conductivity in comparison with conventional P20 grades. Improved thermal conductivity can bring significant contributions in reduction of injection cycle times; however, such potential might be slightly diminished in the case of poor thermal conductivity of the plastic itself (thick wall section), lack of contact between the steel and plastic (flat plastic pieces) or any problem related to cooling channels or their plugging.

Summary
As moldmakers and molders seek alternative mold materials to improve performance and reduce costs, a look at the machinability, homogeneity, weldability and thermal conductivity characteristics of any mold material is key to finding the right solution. Improved steel grades light on carbon (with a boron metallurgy instead) and prehardened to 300 HB offer the benefits for which today’s shops are searching.

References
1 Superplast® 300(P), Patented Superplast® 300, prehardened to 300 HB. Superplast® products are available in blocks from 20 mm (0.75”) up to 810 mm (32”) thick, and are distributed in North America by the Burger Iron Company (BICO) and Sturdell Industries, Inc. ® Industeel Trademark. (P) Patented grade.