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P20 steel is a popular, high-grade forged tool steel relatively free of defects; it is available in a prehardened steel. It can be textured or polished to almost any desired finish and it is a tough material. H13 is usually the next popular steel used. Stainless steel, such as 420SS, is the best choice for optimum polishing and corrosion resistance. Other steels and materials, such as copper alloys with fast cooling, aluminum with low cost and fast cooling, are used to meet specific requirements to meet tool life and cost. The choice of steel is often limited, particularly by the available sizes of blocks or plates that are required for the large tools.

As a tooling guide to life expectancy, consider P20 steel for long runs (1 million products), QC-7 aluminum for medium runs (250,000 products), sintered metal for short runs (100,000 products), and filled epoxy plastic for relatively shorter runs (50 to 200 products).

The flow surfaces of the tool usually have protective coatings, such as chrome plating, to provide corrosion resistance.  With proper chrome-plated surfaces, microcracks that may exist on the steels are usually covered. The exterior of the die is usually flash chrome plated to prevent rusting. Where chemical attack can be a severe problem (processing PVC, etc.), various grades of stainless steels are used with special coatings. Coatings will eventually wear, so it is important that a reliable plater properly recoat the tool; this is usually done by the original tool manufacturer.

The needs of the vast majority of materials, particularly steel, can be satisfied with a relatively small number of these materials. The most widely used steels have been given identifying numbers of the AISI. The properties of the tool material usually are as follows: (1) chemical compositions; (2) wear resistance to provide a long life; (3) toughness to withstand processing and particularly factory handling; (4) high modulus of elasticity so that the die channels do not deform under melt operating pressure and the die’s weight; (5) high compression strength, which is very important; (6) high uniform thermal conductivity; (7) machinability so that good surface finishes can be applied, particularly near the die exit; and (8) ability to be repaired.

Important requirements for the tools are high compression strength at the processing temperatures of the platics, wear resistance especially in regard to the increasing use of reinforcing fibers, adequate toughness, possibly corrosion resistance, and good thermal conductivity. In addition, so that the tools may be manufactured economically, good machinability is expected and, in certain cases, also cold hobbing is of less importance since wire and die sink electrical discharge machining (EDM) has taken over most of these applications that require hobbing. Dimensional stability during heat treatment generally is necessary.

Given that there is substantial interplay between the product design, mold design, and the injection molding process, an iterative mold development process is frequently used as shown in Fig. 1. To the extent possible, the product design should follow standard design for injection molding guidelines as described in the article of mould design standard . To reduce the product development time, the product design and mold design are often performed concurrently. In fact, a product designer may receive a reasonable cost estimate for a preliminary part design given only the part’s overall dimensions, thickness, material, and production quantity. Given this information, the mold designer develops a preliminary mold design and provides a preliminary quote as discussed in another article. This preliminary quote requires the molder and mold maker to not only develop a rough mold design but also to estimate important processing variables such as the required clamp tonnage, machine hourly rate, and cycle times.

Once a quote is accepted, the detailed engineering design of the mold can begin in earnest as indicated by the listed steps on the right side of Fig. 1. First, the mold designer will lay out the mould design by specifying the type of mold, the number and position of the mould cavities, and the size and thickness of the mould. After-wards, each of the required subsystems of the mould is designed, which sometimes requires the redesign of previously designed subsystems. For example, the placement of ejector(s) may require a redesign of the cooling system while the design of the feed system may affect the layout of the cavities and other mold components.

Multiple design iterations are typically conducted until a reasonable compromise  is achieved between size, cost, complexity, and function. To reduce the development time, the mold base, insert materials, hot runner system, and other components may be ordered and customized as the mold design is being fully detailed. Such concurrent engineering should not be applied to un- clear aspects of the design. However, many mold makers do order the mold base and plates upon confirmation of the order. As a result of concurrent engineering practices, mold development times are now typically measured in weeks rather than months [1]. Customers have traditionally placed a premium on quick mold delivery, and mold makers have traditionally charged more for faster service. With competition, however, customers are increasingly requiring guarantees on mold delivery and quality, with penalties applied to missed delivery times or poor quality levels.
After the mold is designed, machined, polished, and assembled, molding trials are performed to verify the basic functionality of the mold. If no significant deficien- cies are present, the moldings are sampled and their quality assessed relative to specifications. Usually, the mold and molding process are sound but must be tweaked to improve the product quality and reduce the product cost. However, sometimes molds include “fatal flaws” that are not easily correctable and may necessitate the scrapping of the mold and a complete redesign.