Advanced Tool Motions

A common limitation of some rigid tooling systems is that part features not perpendicular to the direction of pressing cannot be compacted and stripped. Frequently, it is cost effective to form features such as cross holes and threads by machining. Other nonperpendicular features, notably helix shapes and hidden flanges, can be formed using complex tool motions. Another type of advanced tooling system permits production of complex shapes with magnetic orientation of the microstructure.

Helical shapes, typically helical spur gears, are produced in rigid compaction tool sets with punch rotation capability. In a simple system, a helical form lower punch is engaged in a die with a matching gear form. In such a system, the lower punch remains engaged in the die at all times, as is common practice for all rigid tool systems, so that indexing rotation of the punch to the die is avoided. The die acts as a guide. Rotation is carried out on a thrust bearing, which rests on the punch platen that supports the lower punch. An upper punch is not required, because the top of the die cavity is closed by an upper anvil, which does not enter the die cavity. Central core rods, with or without additional features such as splines and key forms, are commonly operated in this helical tool system.

Helical gears made in this manner are limited to helix angles of —25° and a thickness of 32 mm (14 in.) due to fill limitations along the helix tooth form. More complex helical gear tooling systems have been developed for routine production using helical upper punches, driven by follower cams for indexed die entry, with inner and outer lower helical punches for stepped helical gears.

Split Die Systems. Another rigid tooling system that avoids some through-cavity limitations is known as the split die, or "double die," system. It enables the compaction of parts with completely asymmetric upper and lower sections in the pressing direction. Figure 12 shows typical tool motions in split die compaction. This system requires two die-holding platens to carry the upper and lower die. Each platen is controlled and moved independently.

Fig. 12 Split die compaction sequence

Wet magnetic compaction (Fig. 13) has enjoyed wide usage in the production of magnetically oriented ferrite shapes. In this production process, a feed shoe is not required. Instead, the die cavity is injected with an aqueous slip (slurry) that has a high concentration of ferrite powder, with the addition of green binders as required. Typically, the die filling pressure is 35 MPa (5000 psi). By using an aqueous slip, many of the gravity die fill problems, such as attainment of uniform powder density and filling the areas that are difficult for the powder to reach, are avoided.

Fig. 13 Wet magnetic compaction. (a) Force-time diagram for magnet presses. (b) Schematic of press tool for chamber-filling method designed for withdrawal operation

Following die fill injection, an orienting magnetic field is applied to the slip, resulting in magnetic polarization of the individual ferrite particles, which remain mobile at this point. The optimal orientation of the ferrite particles directly determines the quality of the finished permanent magnet. After magnetic orientation, the main pressing load is applied, densifying the ferrite mass and causing the suspending aqueous carrier to be expelled through drainage ports. The compact is imparted with the precision shape and dimensions of both the upper and lower dies, plus any core rods that may be inserted. The cycle is completed by separation of the press platens and ejection of the compacted ferrite shape.

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