MIM & PM DESIGN RESEARCH
MIM DESIGN GUIDE
I. Scope
The intent of this design guide is to gather details about the
capabilities of Metal Injection Molding (MIM). It will explain the criteria
useful in designing a part in order to optimize the MIM process and
achieve a cost effective product.
The nature of MIM allows a broad spectrum of applications
to be considered. However, key markets served are medical,
telecommunications,Defense,automobile and aerospace.
This document is only a basic design guide. If designs fall outside the
criteria set forth in the following pages it does not preclude the part
for manufacture using MIM.
II. Metal Injection Molding
Metal Injection Molding, or MIM, is the process in which a fine metal powder, typically in the sub 20μm range, is mixed
with a proprietary binder system to create a feedstock. Common binders are a combination of waxes and organics, which
may include thermoplastic or thermoset materials as well as surfactants and other additives. The feedstock is then
injected into a mold cavity using molding machines very similar to those used in plastic injection molding. The majority of
the binder system is then removed using a thermo or thermo/chemical operation.
The parts are then placed in a
sintering furnace where any remaining binder is removed and the parts are sintered to their final dimensions. The end
result is a near net shape part with a typical density of 98+%. Depending on the part, this can be the final step, unless
there are any secondary operations to be performed.
MIM has filled an important void in traditional metal processing methods. As seen in
most common metal processing methods fall into 3 categories, each with their
own benefits and drawbacks. Standard machining practices, whether automatic or
discrete, are useful for parts that are either high volume/relative low part complexity or
low volume/relatively high part complexity. For high volume production runs, with
limited part complexity, Pressed and Sintered operations are often used. However,
with more complex designs, Investment Castings are often used. The drawback being,
the cost of consumable investment cast molds becomes prohibitive in higher part
quantities. MIM becomes more advantageous where the volume of parts is relatively
high (typically +10,000 parts) with complex, difficult to machine geometries.
III. Design Guidelines
Designing your parts using the following guidelines will ensure you get the most out of the metal injection molding
process with regards to cost and overall manufacturability.
A. Material Selection
The following materials are readily available to the MIM process. Other materials are created upon request.
• 17-4 PH (as sintered) Stainless Steel
• 17-4 PH (H900 condition) Stainless Steel
• AISI 316L Stainless Steel
• AISI 304L Stainless Steel
• High Purity Cu
• Tungsten Heavy Metal (W-NI-Fe)
• Alloy F15 (Kovar)
• CuMo (85Cu15Mo)
B. Design Considerations
The key elements in considering a part for the MIM process are Size, Part Complexity, and Quantity.
High volume: Quantity plays a part in the cost effectiveness of MIM, as there always will be a NRE charge for a mold
which accompanies the per piece price. A typical low-end quantity for the process is 10,000 pieces.
Complex, difficult to machine geometries:
What makes MIM so attractive is the ability to
mold cross-drilled holes, radii, blind holes, internal features, etc with tight tolerances,
with little or no secondary machining. It is this aspect of the process that makes MIM so
versatile. If a part can be made by a simple stamping, casting or on a screw machine, it
is probably too simple to be a good MIM candidate. However, where there are complex
geometries in high volumes, MIM is often the process of choice.
Also, molding 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.
Sinterable Materials: First of all, the material must be available in a small powder (typically sub 20μm). Many, but not
all, of the common engineering materials are available in this form. If a small powder is available then sintering becomes
the next concern. For metals, sinterability usually means the powders have low contents of ingredients that prove
reactive, especially the strong oxide formers, reactive metals, volatile elements, and toxic materials. This usually means
PIM compositions avoid beryllium (toxic and easily oxidized), mercury (toxic and volatile), lead (toxic and volatile),
manganese (strong oxide former and both the metal and oxide are volatile), zinc (volatile), sodium (reactive), magnesium
(reactive and strong oxide former), aluminum (strong oxide former), tantalum (reactive), diamond (unstable during
sintering), oxides of metals such as indium and tin (unstable during sintering), and titanium (reactive and strong oxide
former).
Relatively small in size: The MIM process is capable of producing fairly large components, however the bigger the part
the more material cost plays into the equation, making MIM a less then optimal financial option. Parts weighing 100g or
less, can make extremely effective MIM parts.
There are, however, some cases, which the above may not apply and MIM may be the process of choice. For example,
there are some fairly large parts and parts in low quantities that are MIMed, simply because they are just too hard to
machine using traditional methods. Again, speaking with Applications Engineers will help aide in deciding
whether MIM is the optimal process for intended particular part.
C. Designing a Part for Metal Injection Molding
Once MIM has been identified as the process of choice, there are certain features that can be incorporated into the
design to increase the efficiency of the process.
Gate Location: Material needs to flow into the mold cavity through a gate. This ideally will be located somewhere on the
thickest section of the part and will allow the material to flow unhindered into the rest of the cavity.
Gates that are directly opposite walls or cores can result in stress concentrations in the material, which
can lead to voids, sinks and flow/witness lines. There is typically a gate vestige that will remain on the
part which can be trimmed off in the green state or machined off after sintering if necessary.
Ejectibility: Since MIM is less forgiving then plastics in the green state, a draft must be incorporated into the mold.
Typically this no greater than 2° and must be located on the part normal to the parting line.
Material Flow: As is standard in any type of fluid flow, sharp corners and edges should be avoided where possible.
Notches can be incorporated where 90° angles need to be maintained. We typically like to see .005”
radii on all corners and edges. It is preferable to have uniform wall thickness (with a preferred minimum
thickness of ≈ .030”). However, if wall thickness must vary, a gradual transition is desirable.
Flat Sides: This is not an absolute necessity, however, if there is a flat surface upon which the part can be placed
during sintering, it makes setting of the part that much easier. This also greatly reduces the amount of
engineering, and therefore cost, that goes into manufacturing a quality MIM part.
Threads: Both ID and OD threads are attainable with the MIM process. In the case of ID threads an unscrewing
core can be used or a hole can be molded in the part, which can be tapped to spec. as a secondary operation. OD threads can be formed along a parting line and a slight flat can be incorporated into the
design to allow for minimal parting line witness.
IV. Basic MIM FAQ
General Questions:
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 (discussed below) MIM offers substantial cost savings.
Q: What makes a part a good candidate for MIM?
A: 1.) Relatively small in size. One 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 weighing 100g or
less, can make extremely effective MIM parts.
2.) Complex, difficult to machine 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.
3.) High volume. 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.
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 a slurry and then injected into a mold.
There are several differences between the two processes, the
major ones are as follows; firstly, with P&S parts, the geometries
you can achieve are limited. Undercuts, cored sections, and sidethru-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. One can achieve
uniform densities consistently around 98+% where a P&S part maintains only roughly 85%. P&S parts can be
HIPed (Hot Isostatic Pressing), which can raise the densities to ~ 90% - 92% which typically only effects the
outer surfaces of the part and is not uniform throughout. Thirdly, MIM particles are on the sub 20μm 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 .
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 10k - 15k pieces/year.
Molding Specific Questions:
Q: What are the molds made of and how do they differ from Plastic Injection Molding tools?
A: Typically, the molds are made of A2 steel, a hardened tool steel. The only real difference between the molds that are
used 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, One 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
Q: What are typical tolerances you can achieve?
A: Typically ± 0.003" / 1.00" (i.e. on a 1.00” Ø we can hold ± .003”, on a .500” Ø we could hold ± .0015” etc.).
Q: What type of finish can I expect from a MIMed part?
A: One can get ≈ Ra 32 microinches (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 material dependant. 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 our MIMed parts are typically in the 97 - 99+% range.
Q: Can you MIM threads?
A: One 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 using an unscrewing core, or we can mold a hole and have it tapped to size as a
secondary operation.
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.
