Metal powder injection molding is a hybrid between plastic molding and sintered powder metallurgy. The tolerance capabilities are also a hybrid. Plastic molding generally has a tolerance range from ±0.05 to ±0.5 mm (±0.002″ to ±0.020″ inch). Sintered powder metallurgy technologies generally have a tolerance range from ±0.5% of the nominal dimension down to ±0.038mm or ±0.0015″ on features of 7.5mm or 0.300″ or less, and sintered ceramics are larger.
There are two issues. The first is the location of the mean size for the specified feature – accuracy. The second issue is the variation about the mean – precision. The further the mean is from the target size, the smaller the allowed variation or tolerance zone. Adjustments to the mean size are easier to accommodate in MIM than adjustments to the variability. For example, changes in molding conditions, solids loading, sintering temperature, or sintering time can be used to relocate the mean size of the product.
MIM component production ranges over a large size range, making it difficult to address tolerances directly over such a wide range. In some instances, the choice has been to create a table of tolerance versus size, showing capabilities as follows: These tolerances can be obtained within a single batch but wider variation is usually observed over time.
- ± 0.38mm (0.0015″ inch) for features below 7.5mm (0.300″ inch)
- ± 0.05 mm (0.002″ inch) for features of 10 mm (0.400″ inch)
- ± 0.125mm (0.005″ inch) for features of 25 mm (1.000″ inch).
The coefficient of variation is used to normalize dimensional variability. Statistically the coefficient of variation CV is defined as the standard deviation divided by the mean dimension, often given as a percentage.
For the more typical size range encountered in MIM production, near 25 mm (1 inch), the general industry capability is ±0.125 mm (±0.005″ inch), or a coefficient of variation ±0.5%. This is generally reflective of the technology once the production process is tuned to center the dimensional variation on the desired mean size. It may be possible to maintain some features at lower tolerances but this cannot be determined until tooling has been built and Process Capability Studies have been completed. Closer tolerances can also be achieved by finish machining some features (MIM/Machining hybrid solution).
In large volume production operations tool wear becomes an issue. To avoid the expense of mold replacement or refurbishment, the desire is to have the ±0.5% tolerance band to allow for tool wear. This ensures a high process yield of acceptable parts over a longer time. Many industrial firms quote process capabilities based on at least six standard deviations (the mean plus and minus three standard deviations, which includes 99.74% of the product).
In production, the sources of variation are:
- Feedstock – mixture, lot-to-lot feedstock variations.
- Molding – inherent process variation shot-to-shot associated with the molding machine
and its sophistication as evident by the integrated controls.
- Cavity – largely from variations between tool cavities, systematic filling differences, and
cavity wear over time
- Furnace – associated with furnace-to-furnace differences in de-binding and sintering
- Placement – variation due to location differences within the thermal processing equipment
- Day – normal daily fluctuations, including operator, humidity, handling, tool wear
- Vendor – vendor-to-vendor differences.
Audits on these factors show vendor differences are the largest. One consequence is that molds often cannot be transferred between vendors with success. The next largest factor is feedstock variation. Although the sintering furnace is often blamed for dimensional variations, in reality sintering simply amplifies earlier defects. Sintering transforms subtle molding variations into dimensional scatter. In repeated statistical surveys, molding accounts for 60 to 80% of the MIM dimensional variation, most of which becomes evident after sintering.
A factor in determining dimensional tolerances is surface roughness. Nominally the tolerance specification cannot be any tighter than ten-fold the surface roughness. Most MIM operations deliver an average surface roughness near 1 µm or 32 RMS. On this basis surface roughness limits the tolerances to probably ±10 µm at best for MIM.
An option for tighter tolerances is to machine critical surfaces after sintering. Post-sintering machining is often used in MIM, ensuring precise final dimensions, but it adds to the fabrication expense. SMP has CNC machining capabilities in house and established external sources for more elaborate machining requirements.
The fabrication of sharp corners and small features is limited by the particle size. Indeed, if the particles are large compared to the feature, then the particles simply will not fill out the feature. As a rule of thumb, no feature should be specified to a size that is not at least ten-fold larger than the particle size. For a sharp corner fabricated from 10 µm powder this would say that the corner radius should be 100 µm or larger (0.1 mm or 0.004 inch). Attempts to form dead sharp edges on cutting tools, knives, or scissors using MIM have been unsuccessful since the particle size limits the sintered edge retention.