Mold Steels offer Machinability and Weldability
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.
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
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
Table 1. Properties of prehardened
Tensile strength (kgf/mm2)
Yield strength (kgf/mm2)
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
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
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
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)