Why do cavities crack? Consider the steel

Mold cracking was discussed in the April 1996 issue of IMM when Bill Tobin and Don Groves listed many reasons for mold cracking and pointed out that high-quality steel was an important safeguard against cracks. Consultant Frederick T. Gerson (Toronto) offers some additional thoughts for the reader who wants to know a bit more about mold steel selection as a means to preventing cracks in cavities and cores. Here's what he has to say.

Fracture Toughness

Fracture toughness is a critical property of mold steel that indicates the basic resistance of a steel (or other alloy) to the start and further growth of cracks. Cracks often occur because a mold has been made from a grade of steel that provides insufficient fracture toughness.

The higher the fracture toughness of an alloy, the greater the hardness and strength to which it may be safely heat treated without undue concern about brittleness and failure.

Fracture toughness is different from impact toughness (Charpy V-Notch or CVN), which is the value usually reported in supplier specifications for mold steel. CVN is determined by a hammer blow from a swinging pendulum. But in an injection press, mold steel is exposed to gradual and sustained loading, and therefore resistance to impact is not the best measure of mold steel performance - not nearly as reliable as fracture toughness, which depends on the composition, shape, and heat history of a given alloy.

At one and the same hardness, steels in the precipitation hardening group (including P-21, PH 15-5 and NAK 55) provide roughly twice the fracture toughness of conventionally heat treated mold steels (P-20 or H-13); maraging steels, such as MAR (18) 250, provide three times greater fracture toughness and therefore offer practical advantages to the mold designer and builder. Suppose, for instance, you need to beef up the section thickness in a critical part of a mold but space limitations won't permit this. Tests have shown that a given section thickness can be made two times, even three times, more resistant to cracking if P-20 is replaced by a mold steel that provides higher fracture toughness.

Until recently, impact strength was much cheaper to determine than fracture toughness, but a quick and easy fracture toughness test has been developed at the University of Rochester in Rochester, NY. It uses a simple test piece - an ordinary shop vise and a torque wrench - that can be put together for a few hundred dollars or purchased from the developers in Rochester. (See previous discussion of the instrument, September 1993 IMM.)

Metal Fatigue

Metal fatigue is the cause of many different mold failures. The fatigue strength of a steel (or other alloy) is defined as the maximum stress that can be sustained without failure for a specified number of cycles in which the stress is reversed. Here's why fatigue strength matters in a mold steel. Consider that an injection mold running continuously at short cycles can pile up almost a million stress reversals in as short a time as a month. Mold pins, especially slender ones, must have high fatigue strength because they are often subject to high unit stress.

Figure 3 compares the endurance limit of H-13 at a stress of 60,000 psi to that of MAR (18) 250 at 110,000 psi. Its endurance limit is the maximum stress that a steel will withstand without failure no matter how many cycles it is exposed to. In other words, once a given alloy has withstood 10 million cycles, it won't ever fail due to metal fatigue. Figure 3 also shows that the endurance limit of maraging steel is almost twice as great as that of H-13.

All the alloys mentioned here are commercially available from mold steel producers and distributors in a variety of grades and trade names. When considering mold steel properties, such as fracture toughness or fatigue strength, it helps to remember that values given in reference books or supplier literature represent averages obtained during carefully controlled test runs in which both higher and lower values were recorded. On the manufacturing floor, more stringent conditions may exist. Therefore, although published values provide valuable guidance, the moldmaker and designer always apply safety factors based on judgment and experience.