Steel Selection Can Overcome Mold Problems
Frederick T. Gerson, F.T. Gerson Ltd. (Toronto, Ontario)
Originally published by The Plastics Molders & Manufacturers Group of the Society of
Manufacturing Engineers in Plastic Parts Production & Design, Vol. 1, No. 4., Fourth Quarter, 1996.
Reproduced here with permission.
Mold designers and mold makers are problem solvers. In each stage of mold creation there can be problems, and the experienced mold professional knows all manner of remedies to solve them. Design changes become necessary when detailed checking shows a problem, and even the best design is apt to need "tweaking" on the CRT or during mold tryout.
Mold makers are tweakers par excellence: relieve a radius, adjust a tolerance, open up a channel, beef up a section, or polish out a rough report the list is as long as the skill, experience, and courage of the mold maker.
SCOPE
The following examples relate to solving difficulties encountered in injection molds -- difficulties
that were readily and economically solved by specifying appropriate mold alloys to provide the required
performance level. To put it another way, conventional alloys those normally preferred by the mold maker in
question had been found to be inadequate at the design stage or during commissioning of the mold, and they
were replaced by more suitable alloys.
Rather than describing optimized solutions, the case histories describe steps that were taken to solve one or more mold problems, bearing in mind that alloy selection may depend on product availability and other factors in addition to performance and cost. Alloy cost will be discussed in detail later.
CAM ROLLER
The roller had been specified in AISI type 420 (UNS S42000) stainless steel to provide resistance to moderate
airborne corrosion. The alloy was chosen because the mold maker had used it previously at HRC 45. At the
available bearing area, the roller deformed by flattening. The problem was solved by switching the alloy to
AISI type 440C (UNS S 44003), which could be safely hardened to HRC 55. Figure 1 explains the rationale of
this simple substitution by comparing alloy chemistry. The carbon content of AISI 440C is about three times
greater than that of AISI 420, providing increased hardenability and resistance to rolling wear. To maintain
sufficient corrosion resistance in the presence of higher carbon content, the chromium content of AISI 440C
is increased and 0.75% of molybdenum is added. Even so, the corrosion resistance of AISI 440C was probably
inferior to that of AISI 420 as a result of the heat treatment required to achieve the stated HRC
values.
Figure 1. Composition of alloys
| Alloy Designation | C | Mn | Si | Cr | Mo | V | Co | Ni | Cu | Nb/Al | Ti | Al | |
| UNSS42000 | AISI 420 | 0.35 | 1.0 | 1.0 | 13.0 | ||||||||
| UNS S44004 | AISI440C | 1.1 | 1.0 | 1.0 | 17.0 | 0.75 | |||||||
| UNS T30402 | AISI D2 | 1.5 | 0.6 | 0.6 | 12.0 | 1.9 | 1.1 | 1.0 | |||||
| UNS T51620 | AISI P20 | 0.35 | 0.75 | 0.5 | 1.7 | 0.4 | |||||||
| UNS T15500 | AISI PH 15-5 | 0.04 | 1.0 | 1.0 | 15.0 | 4.0 | 3.5 | QS | |||||
| UNS T20813 | AISI H13 | 0.35 | 0.35 | 1.0 | 5.0 | 1.5 | 1.0 | 0.3 | |||||
| MAR18 (250) | 0.03 | 0.1 | 0.1 | 5.0 | 8.5 | 18.0 | 0.4 | 0.1 | |||||
| Cobalt-free | MAR 18(250) | 0.03 | 0.1 | 0.1 | 3.0 | 18.5 | 1.4 | 0.1 |
Why not try AISI D2 (UNS T30402) in this case, given that it provides high carbon with chromium, vanadium, and cobalt? Answer: Although the wear resistance of AISI D2 is superb (0.5 mg/1000 cycles), its corrosion resistance may have proved inadequate for this application.
MOLD CAVITY FOR FAN SHROUD
The original design called for chromium-plated AISI type P20 (UNS T51620). After approximately 2000 cycles, a
crack appeared next to a boss at the center of the cavity. Stress calculations at this section, as well as in
the surrounding area, indicated generous safety margins.
Initially, differences in section thickness were thought to have contributed to the failure. Radii near the boss were therefore enlarged and section thickness increased to the limits permitted by the geometry of the part. An almost identical crack appeared in the same location, but it was discovered earlier because the molder made more frequent and closer inspections, identifying the crack when it was still minute.
The fan shroud was redesigned to eliminate the "offending" boss, but nevertheless the crack reappeared. The cavity steel was then changed to a commercially available alloy similar to AISI type PH15-5 (UNS S15500), after which the mold ran satisfactorily and no further cracking was experienced.
As noted from Figure 1, the change in the composition of the original and the substituted alloy was more radical in this case than in the preceding example of the cam roller. Clearly, carbon does not account for the hardenability of the PH15-5 alloy wherein carbon is kept as low as possible so as not to reduce the amount of chromium available to ensure corrosion resistance. This alloy hardens by quenching and tempering. In this application, it was hardened to HRC 42 and provided more than double the fracture toughness of AISI P20.
Fracture toughness
Because it is a critical property of mold steel, it is necessary to digress for a brief discussion of
fracture toughness. Fracture toughness is the property that indicates the inherent resistance of an alloy to
the onset and growth of cracks. The higher the fracture toughness of an alloy, the greater the strength to
which it may be safely hat-treated without undue concern about brittleness and failure.
It is helpful to keep in mind the difference between fracture toughness and impact toughness. Impact toughness is usually reported as Izod or Charpy V-Notch (CVN) values, which are determined by breaking a specimen with a swinging pendulum. But in the extruder or molding press, the type of loading is gradual and sustained; hence, behavior under impact is not the best method of gauging the performance of a given cavity alloy.
In contrast, fracture toughness is a shape- and material-specific property, which defines the ability of an alloy to sustain a load in the presence of a notch, micro crack, or other stress raiser. It is much more appropriate than impact toughness as a measure of alloy performance in plastic molds.
Fracture toughness of a given mold block is not only a function of its chemical composition and metallurgical structure but also of its heat history and the state of strain within the block. In other words, the mold maker would be helped by knowing the fracture toughness of a particular block of steel before beginning to work on it -- before investing what might aggregate to be many thousands of dollars in labor and overhead to produce a cavity or core.
Testing for fracture toughness would have been impracticable until recently because it was very expensive and time consuming (about US$100 to test a single specimen) compared to an Izod or CVN impact test (about US$5). But, a quick and easy fracture toughness test has been developed, which will be discussed later.
First, a quick look at some fracture toughness values of different mold steels. As shown in Figure 2,
precipitation hardening grades offer about 50% more resistance to cracking than conventional
quench-and-temper alloys such as AISI P20, H13, or 420 stainless. Maraging steels are more than twice as
resistant.
Figure 2. Fracture toughness as a function of tensile strength
In a very practical sense, some awkward problems can be overcome by switching to an alloy that provides
higher fracture toughness. For instance, suppose there is a need to beef up the section thickness in a
critical part of a mold but space limitation won't permit it. Tests have shown that, at a loading of 1725 MPa
(250 ksi), a given thickness can be made twice or even three times more resistant to cracking by switching
from P20 to a better mold steel.
Consider the next example:
SHOULDER PIN
A 3.5 mm (0.15") diameter pin in AISI H13 at HRC 52 suffered a fatigue crack after two weeks of operation --
corresponding to approximately 125,000 cycles. Proprietary surface treatment provided only marginal
improvement but consumed valuable time. The problem needed to be solved quickly.
The cycle load in the pin varied from 0 to 450 kg (992 lb), corresponding to a unit stress of
approximately 460 MPa (67,000 psi), which is about half the nominal tensile strength and falls well within
the design stress for the alloy. However, as shown in Figure 3, this is not far below the fatigue limit
between 104 and 105 cycles.
Figure 3. Fatigue endurance limit
Replacement pins in MAR (250) at HRC 50 ran without interruption for more than 4 million cycles. At a given
load, a steel pin is unlikely to fail in fatigue after it has survived 2 million cycles. MAR (250) has an
ultimate fatigue endurance limit of 760 MPa (110,000 psi) vs. only 410 MPA (60,000 psi) for H13. See Figure 1
for the composition of these alloys.
In regard to fatigue and wear resistance (mentioned earlier), it is practical to note that alloy supplier literature lists nominal or typical values for the properties and characteristics of their products. These are average values obtained during several test series recording better and worse results, and such tests tend to be conducted under controlled conditions. In an actual manufacturing context, more stringent conditions may apply. Therefore, while the values given in published literature provide valuable guidelines, the mold designer and mold maker always applies safety factors based on judgment and experience.
TYPICAL APPLICATIONS
Rather than continue with individual examples, here are two other problem situations in which
appropriate alloy selection has helped to save the day.
Controlled expansion alloys
In molds for long or large-area components such as auto parts, critical differences can exist
between the coefficient of thermal expansion (CTE) of the resin to be formed and the material from which the
mold is constructed. The CTE of mold steel varies in a range between 10 and 12 X 10-6/°K. The corresponding
value of a typical glass or carbon fiber filled composite would be 1.5 X 10-6/°K. Given the range of
temperatures at which some of these resins are cured, the linear interference could be large enough to snap
off surface ribs or distort ridges. Borrowing techniques developed in the aircraft industry, the mold
designer can specify one of several low-expansion alloys that permit very close matching to the CTE of any
resin or composite, starting well below 1 X 10-6/°K.
Other specialty alloys
Commercially available alloys offer a wide variety of characteristics in addition to high strength,
toughness, polishability, machinability, ease of heat treatment, and resistance to wear and corrosion not
only for cores and cavities but also for holder plates, mold frames, pins, and other standard components.
Several series of proprietary alloys are promoted with the claim that they are easy to weld and repolish without leaving visible traces of the repair work. Nevertheless, welding is resorted to only emergencies because the heat-affected weld zones cannot fully match the properties of the base alloy. Welding problems are minimized in molds made from precipitation hardening alloys because in these the structure of the entire part -- not only the welded section -- can be renewed by means of solution treatment and age hardening at moderate temperatures.
As mentioned, designers and mold makers have not always availed themselves of the broad spectrum of capabilities available in mold alloys. By contrast, manufacturers of hot runner systems have attained a high level of sophistication in the use of modern materials. One reason frequently given for the mold maker's reluctance to use more advanced alloys is that they cost more than those that have always been used.
In many mold shops, alloy selection is based on lowest unit cost and that can be a costly mistake. The advanced mold alloys cost more than their conventional forerunners. One reason for this is the competition among steel suppliers to capture a share of the large market for AISI P20, which is still the largest single segment of the market.
ELEMENTS OF MOLD COST
Yet, as is obvious from Figure 4, the first cost of mold steel has little influence on the cost of the
finished mold and even less on the value or total cost of that mold. The left stack in Figure 4 shows the
elements of mold cost in the hands of the mold maker. Note that mold material accounts for only about 5% of
the total cost of the mold as its leaves the mold maker's facility. The right stack in the illustration
indicates that, in the hands of the mold user, the first cost represents less than a third of the total cost
of the mold. If doubling material cost only increases utilization by a small margin, then the upgrade is well
worth it. Or, putting it another way, a single breakdown or unforeseen maintenance requirement wastes much
more money than a good mold alloy would have cost in the first place. That statement assumes even greater
importance on failures occurring in an integrated manufacturing system where outages and interruptions are
particularly costly.
Figure 4. Elements of mold cost
FRACTURE TOUGHNESS TESTING MADE EASY
Having earlier discussed the importance of fracture toughness, a simple rig for determining it is presented.
Developed at the University of Rochester, the rig uses a notched four-point test specimen, an ordinary shop
vise, and a torque wrench tool, which can be put together for a few hundred dollars or purchased from the
developers in Rochester. Figure 5 is a line drawing of the torque wrench for toughness testing.
Figure 5. Schematic of toughness tool
To perform the test, the operator exerts a steadily increasing force to the upper head by pushing down on the
operating handle of the tool. A plastic marker indicates the amount of force that was needed to break the
specimen. For virtually all alloy grades used in a steel mold, that value is a good indicator of fracture
toughness. By exerting a force of about 270 N (60 lb) on the operating handle, the notched specimen held in
the fixture will be broken. Both the theory and practice underlying the simplified test is detailed in a
paper by Quesnel and Stromswold (Engineering Fracture Mechanics, v41, n3, 1992), which also contains 23
useful references showing the evolution of fracture toughness determination.
The torque wrench method is so simple, fast, and inexpensive that it can be used for materials characterization for acceptance testing and as a routine quality assurance procedure. It enables the mold maker to verify the fracture toughness of a mold block at the workbench. It could become part of the routine quality control reports made by steel producers.
CONCLUSION
New applications under development will allow making injection molds not merely for glass and carbide-filled
compounds but also for concrete, ceramics, and even for nonferrous metals.
Tough mold alloys will become an everyday necessity because these unusual molding compounds are both abrasive and corrosive, while requiring injection pressures and temperature ranges beyond any experienced to date.
No one disputes that the selection of appropriate mold alloys is but one of many resources and skills with which mold professionals solve problems. Against that background, this article has described a few simple cases that demonstrate the role that alloy selection can play in solving mold problems. It mentioned some developments that drive the trend toward newer and better quality mold alloys. It also seeks to encourage the mold designer, mold builder, and mold user to gain practical experience with the more advanced mold alloys that will be critical for success in tomorrow's market.
