Sintering is a wonderful way to improve part properties via powder metallurgy (PM). But like any other manufacturing process, it’s not perfect. A component that goes through a sintering furnace will change size or otherwise become distorted.
Sizing in powder metallurgy is a specific secondary operation that brings a part back to the correct dimensional specifications. While achieving a net shape part is possible with powder metallurgy, sometimes even PM needs a little help. Here’s how sizing lends a hand:
What Is Sizing in Powder Metallurgy?
Sizing is all about shape and size control. It’s a repressing operation that “requalifies” the part so it can hold tighter tolerances.
Sizing is a post-sintering operation done “cold.” The amount of change your component undergoes in the furnace depends on its material composition.
Sizing Vs. Coining
Many makers of metal parts have heard of coining. Sizing? Not so much. While they are similar operations, there are important nuances of each:
- Sizing is the primary choice for net-shaping needs.
- Sizing can use much higher pressures in initial powder compaction -- often up to 60 tsi. This is because the tooling is much more sturdy.
- Neither increases the density all that much; you’ll instead rely on the double-press/double-sinter technique for density improvements.
- Both coining and sizing can increase mechanical properties. Your manufacturer is basically cold working the part, which provides a slight strength increase.
Benefits of Sizing in Powder Metallurgy
Sizing is a way to reduce the dimensional variation from part to part. Even better, sizing lets you use lower-cost processes to achieve those tight component tolerances.
If your component requires smaller or tighter tolerances than can be achieved with conventional pressing and sintering, your best option is to size the part. Simply putting the component in a uniquely shaped die and repressing it improves dimensional precision by up to 50%.
Sizing allows you to add certain features to a part:
- Rounded-off edges
- Raised or grooved features (coining)
- Other surface features (dependent on material choice)
Sizing primarily exists to improve dimensional precision in parts. However, your components will see several additional improvements:
- Better surface finish
- Improved inner and outer diameter tolerances
- Flatter and improved thickness tolerances
Related Article: Design Considerations for Powder Metallurgy: Overcoming Tolerances
Caveats to Sizing
This is manufacturing -- nothing is ever perfect! There are a few limitations and disadvantages of sizing a powder metal part:
- You can’t size a sinter hardened material. The Rb80-90 range can be sized, but that’s the limit.
- If you size a magnetic part, it’ll lose magnetic performance and require a second thermal process.
Alternatives to Sizing in Powder Metallurgy
Some engineers think of double-press, double-sinter (DPDS) as a sizing operation. In reality, it’s not.
So what is it really?
Double-press, double-sinter is a powder metallurgy technique in which the material is compacted again to substantially increase the density of the PM part. Remember that DPDS uses a presintering operation to remove the lube, anneal the initially compacted iron, but not combine the carbon content (added by the graphite content). Thus, the second pressing operation can significantly increase the density, but requires a second set of tools and a second sintering operation.
There’s typically a 50% cost penalty for such projects. But today, there’s an alternative.
Advanced compaction technology can negate some of the rationale for relying on expensive double-press/double-sinter processing. With advanced compaction, you can achieve DPDS-level densities without the added cost of a second set of tools and a second sintering operation. The dimensional stability of both processes are going to be roughly the same.
Other Design Considerations in Powder Metallurgy
Use sizing to hold tighter tolerances, improve surface finish, and add advantageous features. Of course, sizing is just one of many design considerations in powder metallurgy and its tooling.
If your needs fall outside the capabilities of sizing, there may be another option. To learn more about improving your part’s performance with certain powder metallurgy processes, ask an engineer a question directly: