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PM&E Plastics Machinery & Equipment

Prehardened Mold Steels offer Machinability and Weldability
By Stuart Caren, contributing editor

For decades P-20 has been the most widely used steel for making large plastic injection molds. It offers a good combination of hardness, machinability, and toughness, but its properties are not always consistent. Frequently, the hardness isn't uniform thoughout a block, which leads to machining problems, and it is difficult to weld.

In the past few years, tool steels that are easier to machine and weld have been developed. One of these new materials is a precipitation hardening steel of uniform microstructure and hardness that was developed by Prof. Asada and Dr. Watanabe of Daido Industrial College in Nagoya, Japan. This material was introduced to the plastics industry in 1965 as NAK 80. Although NAK 80 is considerably harder than P-20 (40 Rockwell C compared with 32 Rockwell C for P-20), it reportedly machines 15-20% faster than P-20 and can be polished to a Class I optic-quality finish.  

To further improve machinability, a small amount of sulfur was added to NAK 80 to produce a new grade of steel called NAK 55. Some NAK 55 users report as much as a 50% reduction in machining time. According to Dave Hunt, vice president of engineering at Hunt Machine (Talmadge, OH), NAK 55 is "the best machining prehardened steel I've ever cut."

The success of NAK 80 and NAK 55 is only partially attributable to their excellent machinability. Usually, a mistake in the mold or part design must be corrected by welding. It is very difficult to weld, polish, and retexture the surface of P-20 without the welded area being visible in the steel and in the molded part. If the mistake occurs in a highly polished or textured area, the entire cavity may need to be scrapped. The mold made from NAK, when proper procedures and NAK-W welding rods are used, will have uniform hardness across the weld. This allows the toolmaker to purposefully match the polish or etch.

Roland Krevitt, tooling engineer from Apple Computer (Santa Clara, CA), explains it this way: "In today's business environment, it is crucial that we shorten the time to market for new products as much as is humanly possible. Because of this constant pressure, we are usually forced to build molds while the parts are being designed. This means we make a lot of changes to our molds, both during the construction and after the try-out. If NAK was not available, we would be in trouble. Our ability to make these changes, sometimes in an already textured area, has saved us countless times. We have also found that NAK etches much better than other mold steels."

Then combination of machinability and weldability is the main reason moldmakers select NAK over P-20 in spite of it's higher cost. According to Tom Schade, vice president  and general manager for International Mold Steels (Erlanger, KY), the U.S. distributor for Daido Steel Co. Ltd., NAK 55 and NAK 80 are used for over 90% of all prehardened molds build in Japan and are rapidly becoming the materials of choice for moldmakers around the world. This is confirmed by people like Dave Hunt of Hunt Machine and Ed Noggle of Southern Plastic Mold (Anaheim, CA).

Since they were introduced to the United States in 1982, NAK steels have been used for thousands of molds. Companies such as Apple Computer and General Motor's Inland Fisher Guide Division (Dayton, OH) often specify NAK because of anticipated design changes. Design changes to new parts typically require modification of the tool.

Design changes are also often made during the life of the part. An example is a horn pad that Hunt Machine built for General Motors. After the mold was completed, designers decided to relocate the bugles. "If we cut it in P-20, we would have had to throw away the mold and start again" said Paul Bryan, a tooling engineer at General Motors.

At Mold Expo '93 in Detroit, MI. last month, International Mold Steels introduced another prehardened steel for injection molds. Called PK5, it combines the weldability and machinability of NAK with added toughness (Table 1).

Table 1. Properties of prehardened mold steels

Steel Tensile strength (kgf/mm2) Yield strength (kgf/mm2) Hardness* Toughness
PX5 100 88 32 8
NAK 80 128 103 40 2
P-20 100 88 32 5
AISI 4140 95 75 25 5

* Rockwell C
Charpy impact test

PX5 has the same hardness as P-20 (32 Rockwell C), but the hardness is more consistent through the entire thickness. The steel's tensile and yield strengths are similar to NAK and P-20. Although a quench and temper heat-treating process is used in manufacturing PX5, the steel is quite stable. It can be machined to size without the need to relieve stress on the block before the finish cut. Figure 1 compares the machinability of PX5 with that of several common mold steels.

The sacrifice in hardness from NAK (40 Rockwell C) is made up for by higher toughness. PX5 is 60% tougher than P-20. This improved toughness allows increased design creativity and flexibility.

What really makes PX5 unique is its weldability. Welding this steel requires neither preheating nor postheating (Figure 2), which greatly reduces welding time and, of course, cost.

According to Ernie Beutel, vice president of technical services for International Mold Steels, the increase in hardness in the area heated during welding is very low and distortion is minimal, resulting in low overall repair time and cost. As with NAK steels, the weld can be textured or polished with no observable difference in the surface.

Since PX5 was introduced in Japan about two years ago, hundreds of molds have been built with this new steel. Hunt Machine learned about it several months ago and has already built a mold for General Motors. According to Dave Hunt, PX5 has lived up to its claims, and he confirms that it is easy to machine and weld.

As with any new material, at least one drawback must offset the benefits. The only one we heard about is the cost. PX5 costs about twice as much as P-20. But because the steel cost is only 5-10% of the total mold cost, the savings in machining time and superior weldability more than make up the difference.

Copyright 1993 Advanstar Inc.  All rights reserved.

FIGURE 1. PX5 machines about 30% faster than P-20 in both end mill (top) and drill (bottom) comparisons.
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