Tooling for MIM
The tool cavity or mold for MIM is constructed as an enlargement of the final part. The space taken up by binder in the feedstock is annihilated by sintering. This is evident in that the final component is usually about 20% smaller than the tool cavity.
1+1 “Family” mold, ejector side of tool.
Left: Sintered part to the right of the cavity.
Right: Green part to the left of the cavity.
Although tool dimensions are expanded to allow for sintering shrinkage, angles are generally preserved. However, mold design is much more complicated than simply increasing the dimensions. Many critical decisions are required on features that include the following:
- Parting line location; where will the mold open to extract the component
- Gate size and location; how will the feedstock enter the mold
- Taper or draft; can a small taper be included to ease ejection from the tooling
- Ejector pins; where and how many ejector pins are required for part ejection
- Slides; are tool motions required to add features perpendicular to the parting line
- Motions; for example are threads to be added using unscrewing motions.
Molds for MIM are constructed in a manner similar to how plastic injection tooling is formed, so MIM vendors rely on the same tool and die industry. Mold design and construction can take some time. It is common to see tool design and tool construction take several weeks. For features at or close to the industry standard tolerances of +/-0.5% of the dimension or +/-.00!” outer dimensions may be set to the lower end of the component tolerance band and the inner dimensions may be set to the high end. This “steel safe” approach allows for final mold size adjustments after first test pieces are sintered if necessary. Once precise shrinkage factors for each dimension are known, the final mold cavity dimensions can be modified by final machining.
Within a mold, the number of cavities ranges from 1 on up. A single cavity tool is satisfactory for low production quantities. The lowest project cost depends on the number of parts per year to be produced. In rough terms here are some typical break points:
- Below 150,000 parts per year – 1 cavity.
- 75,000 – 250,000 parts per year – 2 cavities.
- 200,000 – 750,000 parts per year – 4 cavities.
- 500,000 – 2,000,000 parts per year – 8 cavities.
- 1,000,000 – 5,000,000 parts per year – 12 or 16 cavities
- 5,000,000 or more parts per year – 16, 32 cavities or multiple lower cavity count tools.
These are not fixed rules, since many factors are involved; for example the maximum clamping force on the molding machine is a factor to consider. In addition if there are very close tolerance and dimensional requirements on the components multiple lower cavity count tools may be a better path to keep process capability and controls as tight as possible.
MIM tooling is often hardened steel, such as S7 or H13. For lower volume or “bridge” tooling P20 can be used, when heat treated, this steel has some wear resistance. Harder tool steels are used for tooling for high production quantity situations. It is possible to have from 100,000 to 2,000,000 shots before a mold requires refurbishing, depending on the powder and component geometry. Ceramic particles are very hard and angular, a combination that may require more frequent mold maintainence and refurbishing.