PkD 128 tiL

where P is the applied pressure on the feedstock, L is the runner length, and '/is the feedstock viscosity. The rate of mold filling is very sensitive to the injection pressure and runner diameter. Usually a high feedstock flow rate is needed in order to fill the die before the feedstock cools.

The barrel holds the rotating screw and is surrounded by heaters that control the mixture temperature. There are often multiple heater zones to ensure temperature control during mold filling. Because cold feedstock is abrasive, the first heater zone is geared to rapidly heat the feedstock, and subsequent zones might be at lower temperatures. The materials used in constructing the screw and barrel are critical to long service without contamination. Hard materials and close tolerances are required to reduce abrasive wear. Tool steels containing vanadium carbide and boride-clad tool steels prove most durable. Similarly, other machine components in the flow path can exhibit considerable wear, resulting in a loss of machine control.

All of the molding steps are controlled by a computer that might even correct errors during molding. Besides the molding machine, coordination is required with the peripheral operations needed in automation schemes. For example, robots are used to stage the compacts for debinding. Other automation features include conveyor systems, parts and tooling storage with automated retrieval, and continuous feedstock preparation and component debinding steps.

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