Mold Materials Selection

At this time (or maybe even earlier, while designing the stack), the designer will think of the materials (steels, etc.) to be used for the mold. (See also Chapter 9)

5.4.1 Effect of Expected Production

Before making any decision, the designer must again consider the lifetime production expected from the mold. There is no point in specifying the best possible (and expensive) materials if the mold will be required for a small production. Also, there is a difference whether, for example, 24 million pieces are to be produced in a 24- or an 8-cavity mold. With 8 cavities, the mold will operate 3 million cycles; with 24 cavities, it will operate only 1 million cycles. This requires the designer to consider fatigue in metals, as discussed in Section 5.4.3.3.

5.4.2 Forces in Molds

The designer must know what forces are present within the mold when deciding on the strength of the mold component to resist these forces. The most important forces acting within the mold affect these strengths:

(1) Tension: the forces created by the injection pressure of the plastic inside the runner system and in the cavity space, usually requiring high tensile strength

(2) Compression: the compressive strength required to counteract the clamp force of the machine, typically, the forces on the P/L, and the forces seen where inserts are supported by plates, and so on

(3) Bending (or de¯ection): the forces seen by cores, and by all plates, especially the ejector and stripper plates

(4) Wear: the forces created by wedge action, as in stripper rings and so on, or tapers and wedges for alignment, which create wear on the matching surfaces

(5) Torsion: the forces seen by coil springs and in mold features, such as unscrewing, or in some robots

(6) Shear: forces seen by dowels, or by the backup of wedges

Note that in many cases, we have combinations of any of the above forces. 5.4.3 Characteristics of Steels and Other Mold Materials

For every mold part the following must be considered: which of these characteristics are most important? Unfortunately, some of them are directly opposite to each other (e.g., toughness and hardness) and compromises are necessary.

5.4.3.1 Availability

This applies not only to selected raw materials, but also to hardware items: the designer must make sure that any material, hardware, or standard mold component intended to be speci®ed is also available when required. Many items are often shown in catalogues or other listings as ``standard'' but this does not always mean that they are readily available, on the shelf, in the desired size, and in the quantities needed.

5.4.3.2 Strength of Material

This applies to steel, BeCu, aluminum, bronze, and so on. Strength is speci®ed by its tensile strength; compressive strength is often but not always about the same. Shear and torsional strength is about one-half the tensile strength. The designer should always get the exact values from a machinery handbook or from the supplier.

Always watch whether the values given are in ISO or in inch systems. The strength values are given either in kPa (kilopascal) or in psi (pound/in2_). 5.4.3.3 Fatigue (See ME, Chapter 18)

The strength ®gures for steel and other metals are arrived at from stressing a test sample, for one cycle only. The results of such tests are satisfactory for steady loads, such as seen, for example, by preloaded screws, but molds often operate many, sometimes millions of cycles. If there are more cycles, the rated strength gradually declines.

This decline is usually shown, as in Fig. 5.15, in logarithmic graphs, as a straight line declining from the rated strength (e.g., tensile or yield strength) for one cycle to a point where the value remains the same regardless of the additional number of cycles; this is for all steels at about 2 million cycles. The

strength of the material, after 2 million cycles (the fatigue strength) depends very much on the material and hardness selected, but also on features such as notches, holes drilled into it, and surface ®nish. The fatigue strength can be as low as 15±20% of the yield strength (yield, in hardened mold steels, is only a little less than the tensile strength; many data are given in yield rather than tensile strength). Note that so-called machinery steels, but also the related P20 or P20PQ, do not lose as much strength as hard mold steels.

The fatigue strength is equivalent to the safety factor often used by designers (frequently, 5) when calculating the strength of a part. The problem is that all force calculations depend on an assumption of the injection pressure, as discussed in Section 4.6.1. But we know that the forces will be greater for thin- wall molding, and since most of them are designed for a very large number of cycles, the selection of only the very best materials with appropriate strength and hardness is suggested.

Note that springs inside molds (sometimes speci®ed for ejector plate return) are especially sensitive to cycling. When designing for springs, use the manufacturer's suggested values for maximum compression and load of the selected spring.

5.4.3.4 Wear

Some materials are better for wear than others. Lubrication (or the lack of it) can be a decisive factor. Wear points could be steel on steel, steel on bronze, steel on hard plastic, and so on. Hard steels are always better, but the designer must never use the same alloy for both members rubbing against each other, as in wedges or

taper locks, except if the wear points can be lubricated. Each alloy has a distinct, different grain structure, and the problem is that when using identical grain structures, the surfaces will lock (seize) when sliding under pressure, and damage (tear) the surfaces. Hardness differences alone are no substitute for different grain structure, except where one of the rubbing surfaces is treated with methods such as nitriding. In nitriding, very hard nitrogen compounds enter between the grains and alter the surface of the steel. Lubrication in molds is never permitted where it could contaminate the molded products, especially for pharmaceutical and food use.