In MIM design there are many variations of the initial forming process where MIM manufacturers add their particular process differentiators. Whether your processes involves metals or ceramics, the key to reproducible production of precise metal parts requires a carefully planned and controlled sintering/debinding operation.
Molding plastic parts is similar to molding metal parts in many ways such as designing draft on features or radii on edges to improve the moldability of a part. But in the case of a metal injection molded part, greater attention needs to be paid towards the need for smooth material flow through the entire part creation process. This process also requires that the parts be carefully supported throughout the debinding and sintering process.
MIM Part Creation Overview
After the part design is agreed upon, the MIM tool is designed. During this process, gates and vents are added to the part, and ejector pins (to push the finished part out of the tool) are selected and placed. The mold designer also adds side-actions for any undercuts. MIM tools are fabricated using a combination of CNC milling and electrical-discharge machining (EDM). After milling, the tool is polished by hand to customer specifications. The finished tool is loaded into a metal injection molding press for green part production. A MIM press is nearly identical to a standard plastic injection-molding press, with a special screw and barrel designed to reduce separation of the binder and the metal powder during injection. Pellets of MIM feedstock are loaded into the hopper of the machine; they are then volumetrically metered into an injection barrel with a screw similar to an injection-molding press. Once the pellets are heated (through use of electric heaters and screw motion), the barrel is placed against the tool and the feedstock is injected. After solidification, the parts are ejected from the press and the cycle repeats.
The MIM Furnace’s Role in Sintering & Debinding
After ejection, green parts are de-gated and placed on ceramic substrates which help retain the shape of the part throughout the debinding and sintering process. Pallets of green parts are loaded into a debind oven (like our CM 400 continuous MIM furnace) to remove most of the binder that carries the metal powder through the injection-molding process; the binder is about 20 percent of the feedstock volume. The length of time required for debinding is a function of the thickest section of the part, as the binder must migrate all the way out of the part. At the end of the debinding process, the resulting brown part is approximately the same size as the green part, but only 80 percent dense. Just enough binder remains to keep the powder particles together, so the brown part is quite fragile. Typically, pallets of parts are moved directly from the debind oven into a sintering furnace. This can be accomplished automatically at high volume with a furnace like our CM 400 Continuuous Hydrogen Pusher Furnace Series . The furnace precisely controls the temperature, cover gas profile required to remove the remaining binder, and sinter the parts into the final product. During the final cycle, parts shrink about 20 percent into their final size. After sintering, secondary operations may be performed and the parts are ready for QA and shipping.
Advantages of MIM Part prototyping
- MIM offers rapid prototyping now, with costs much less than typical proto-metal houses.
- Ability to produce high volumes of complex parts
- Relatively low production cost compared to several other metalforming technologies
- If your part is destined for mass production in MIM, the ability to resolve manufacturability issues during the design/prototype stage using the same process as you will use in production, is a big advantage.
- Produces a clean surface finish
- MIM is a Consistent and reliable process.
For more information see our MIM Furnace Process Overview or contact a CM Furnace representative today.