Frequently Asked Questions: Metal Injection Molding (MIM)

Q: What is MIM?

A: MIM stands for Metal Injection Molding. It is very similar to plastic injection molding but with metal powder blended with a binder system (feedstock).

Q: Why MIM?

A: MIM is a cost effective method of producing small, complex metal components in high volume. With the right part MIM can offer substantial cost savings. In the right application it is a low cost alternative to traditional machining and casting processes and can produce net shape parts with little or no secondary machining.

Q: What makes a part a good candidate for MIM?

A: Typically we consider a part to be MIM-able if it is a small part (roughly the volume of a golf ball or deck of cards maximum) with a complex, hard to machine geometry in high quantities.

A: 1.) A relatively small part size: We can MIM about any size part required, however the larger the part, the more material cost plays into the equation, making MIM a less then optimal financial option. Parts with a volume of, or less then, a golf ball or deck of playing cards is where MIM is extremely effective.

2.) Complex, difficult to machine part geometries: A large factor that makes MIM so attractive is the ability to mold cross-drilled holes, radii, blind holes, internal features, etc with tight tolerances all in one shot, with little or no, secondary machining. The MIM process can't compete dollar-wise with simple stampings, castings or parts easily produced on a screw machine. But if the component has complex geometries in high volumes, MIM is a perfect fit.

We are able to mold several SST's (17-4 PH, 316L, 304L) along with other metals such as Kovar, Cu, and Tungsten alloys, which may pose problems when using traditional forming methods. Other materials are also available, so please contact us if you run into a specific material requirement.

3.) High volume parts: Quantity of parts plays a roll in the cost effectiveness of MIM. There is a NRE tooling charge accompanying a per piece price. We have found, quantities of 10,000+ pcs/year usually make the most sense when considering the cost of the tool and the piece price of the MIMed parts versus other forming processes.

There are, however, some cases, where the above may not apply, but MIM is still the process of choice. For example, there are some fairly large parts or parts in low quantities that are MIMed, simply because they are just too costly to machine using traditional methods. If you run into a part that fits the above criteria or you think, for certain reasons, it could be better made with the MIM process, please forward us a drawing or sketch via e-mail (sales@morganadvancedceramics.com) or fax (508.998.1962) and we will evaluate it.

Q: What densities can be expected from a MIMed part?

A: Densities of MIMed parts are typically 95 - 99+%.

Q: What differentiates MIM from the Pressed and Sintered process?

A: With P&S forming, a metal powder with a small amount of binder is poured into a mold, pressed and then sintered. Where MIM is more akin to plastic injection molding, where a powder with a binder is heated into slurry and then injected into a mold.

There are several differences between the two processes, the major ones are as follows; first, with P&S parts, the geometries you can achieve are limited. Undercuts, cored sections, and side thru-holes are unattainable with this type of process, where, with MIM, it is all formed in the mold without secondary machining. Second, there is a major difference is the density of the finished product. We can achieve uniform densities consistently around 98+% where a P&S part can get only around 85%. P&S parts can be HIPed (Hot Isostatic Pressing), which can raise the densities to roughly 90% - 92% but that typically only effects the outer surfaces of the part and is not uniform throughout. Third, MIM particles are on the sub 20 micron size and the P&S particles are in the 100 - 150 micron size.

Q: What materials are available for MIM?

A: 17-4 PH SST, 316L SST, 304L SST and F-15 Alloy are all commonly MIMed. Other materials and custom alloys can also be produced upon request. We can MIM just about any material that can be found in powder form.

Q: What is the minimum quantity you would consider?

A: As stated above, depending on the size and complexity, we usually have a cut off of l0k - 15k pieces/year. However if the part is very complex and/or big and it is cost prohibitive to machine, we'll consider projects in the 5000 parts range, or maybe less.

Molding Specific Questions:

Q: What are the molds made of and how do they differ from Plastic Injection Molding tools?

A: Our molds are made of A2 steel, a hardened tool steel. The only real difference between the molds we use and a plastic injection mold is the finish. We usually require a finish of 8 on our mold faces for easy removal of the part.

Q: What type of binder system is used in the feedstock?

A: The binder is a mix of wax and plastic and makes up roughly 50%, by volume, of the feedstock.

Q: What is the difference between a prototype tool and a production tool?

A: As far as the process is concerned, nothing. A prototype tool is an insert that will fit into a mold base that we maintain in house. Typically, you can't get all the features you may want in a production tool in a prototype tool. For example, a prototype tool may not have all the automatic core pulls that a production tool would have and some actions may need to be done manually. A prototype tool would not have complex gating or a hot runner system either. However, these mold inserts are easier to modify and adjust. Because of the manual nature of this type of mold, cycle times are typically longer than production tooling. If a customer has a new product or the design is still in flux, a prototype mold is often a cost effective way of proving out the part.

Another major benefit to going with a prototype mold is, once the design is set, we can continue shipping usable parts while a production mold is being made. As noted above, the rate of parts produced will be significantly reduced however the parts will be production quality. And, as stated above, the piece price will drop accordingly once the production tool is in place.

A minimum amount of sample parts (between 10 - 20) will be supplied with the purchase of a prototype mold. After that amount runs out, the customer will be required to pay a piece part price. This is typically higher then a part price out of a production tool simply for the reasons stated above.

Part Specific Questions:

Q: What are typical tolerances you can achieve?

A: Typically plus/minus 0.003" / 1.00" (i.e. on a 1.00" 0 we can hold plus/minus 003", on a .500" 0 we could hold +- .0015" etc.).

Q: What type of finish can I expect from a MIMed part?

A: We can get"" Ra 32 micro inches (Ra 0.8 microns) finish straight out of the mold and better with an optional secondary polishing operation.

Q: What hardness can I expect from the MIM process?

A: This is again material dependant. After the sintering process, the parts come out in a pseudo-annealed state. We can sometimes, again depending on the alloy, get the parts hardened as a secondary operation.

As a guideline, for 17-4 PH Stainless, we get an as-sintered part in the 29 - 32 HRC range and can have the part heat treated to achieve values in the 38 - 42 HRC range.

Q: What densities can be expected from a MIMed part?

A: Densities of MIMed parts are typically in the 97 - 99+% range.

Q: Can you MIM threads?

A: We can MIM OD threads but with some considerations. We'll typically mold a flat along the parting line of the mold. We can mold ID threads, however it is typically more cost effective to have these done as a secondary operation. In that case, we would mold holes to the required tap drill size

Q: Can you plate a MIMed part?

A: Yes. Basically you can do anything to a MIMed part that you would do with a part of the same material made from a more traditional forming method.