Fig 3 Feedstock pellets and worms for molding

Pelletized feedstock is injection molded into the desired shape by heating it in the molding machine and hot ramming it under pressure into the tool cavity. By virtue of the binder, the feedstock becomes low enough in viscosity that it can flow into the die cavity under pressure. Cooling channels in the die extract heat and solidify the polymer to preserve the molded shape. The shaping equipment is the same as that used for plastic injection molding. It consists of a die filled through a sprue, runner, and gate from a heated barrel. Most popular is molding in a reciprocating screw machine. Here the screw in the barrel stirs the feedstock while it is melting and acts as a plunger to generate the pressure needed to fill the die. In the actual molding stroke, the molten feedstock is rammed forward to fill the cold die in a split second. Molding pressures depend on several parameters, but might be 60 MPa or more. Pressure is maintained on the feedstock during cooling until the gate freezes to reduce the formation of sink marks and shrinkage voids. After cooling in the die, the component is ejected and the cycle is repeated.

Tooling. The tool materials used in PIM are similar to those encountered in many metal working, plastic injection molding, and powder metallurgy operations. Table 2 identifies some of the common tooling materials. The tool material choice depends on the anticipated number of molding cycles and the required wear resistance. On the one hand, machining difficulty and material costs need to be considered. For molding tools, P-20 is the most common material, because of the combination of strength and cost. Yet wear concerns with PIM make the selection of higher-hardness tool steels most common. Rapid prototype tool materials, including epoxy, have been used in pilot production. Soft alloys of aluminum, zinc, or bismuth are used during tool development because of easy machining. Cemented carbides are useful where wear is a primary concern, but tool fabrication is expensive and tool damage is a problem because of the low toughness. Material cost varies by a factor of ten between these materials. Tool steels are best because of the combined strength, toughness, hardness, and machinability.

Table 2 Construction materials for injection molding tools

Material

Composition

Hardness, HRC

Suggested applications

420 stainless

Fe-14Cr-1Si-1Mn-0.3C

50

Corrosion-resistant cavities, cores, inserts

440C stainless

Fe-18Cr-1Si-1Mn-1C

57

Wear-resistant, small inserts, cavities, cores

H13 tool

Fe-5Cr-1.5Mo-1Si-1V-0.4Mn-0.4C

50

Larger or intricate cavities, high toughness, low wear

M2 tool

Fe-6W-5Mo-4Cr-2V-0.3Mn-0.8C

61

Core and ejector pins

P20 steel

Fe-1.7Cr-0.8Mn-0.5Mo-0.4V-0.35C

30

General purpose, hot runner, large cavities

Cemented carbide

WC-10Co

80

High wear, compressively loaded small inserts

Tool fabrication occurs in a machining center via progressive removal of material from an initially oversized block of material. Most machining is computerized, but there is still the necessity to hand-finish critical components or dimensions in the tool set. A final surface roughness of 0.2 /'m (8 /'in.) is typical, but smoother finishes are used in selected applications. The desired tool hardness is typically more than 30 HRC, which is satisfied by many heat-treated stainless steels or tool steels. Under normal conditions, an injection molding tool set can mold up to one million parts. With soft tool materials like aluminum, the life is less, at 1,000 to 10,000 cycles.

The tool set has the cavity and further consists of the pathway for filling the cavity with ejectors for extracting the component from the cavity. In most instances, the tool set consists of a single cavity. This cavity captures the component shape, and it is oversized to allow for component shrinkage during sintering. Around the cavity are several tool parts needed for opening and closing the cavity, ejecting the component, aligning the die sections, moving inserts, cooling the component, and locating the sprue, runner, and gate. Figure 4 is a sketch of a molding tool set with ejector pins, ejector plate, and keyed slides to ensure proper closure of internal die components. Many operations use a three-plate mold for automated removal of the gate on mold opening.

Fig. 4 A sketch of the tool set for powder injection molding, showing major components

A primary concern in designing injection molding tooling is component shrinkage. On a volume basis, the typical feedstock contains 60% solid and 40% binder. To attain the desired final component properties, the linear shrinkage during sintering may be 15%. The shrinkage in dimensions is known as the shrinkage factor Y, calculated from the solids loading, and the sintered fractional density, P/PT:

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