Process
The MIM process starts with proper part design. Anyone familiar with designing parts for injection molding should find designing a MIM part to be very similar. However, in the case of a metal injection molded part, greater attention needs to be paid towards the need for smooth material flow through the part (thus, greater importance to thick and thin transitions, and proper filleting of joints). This process also requires that the parts be supported throughout debinding and sintering.
Mold production
When an order is placed, Proto Labs will design a metal injection molding tool. 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 designer also adds side-actions for any undercuts. MIM tools are made of aluminum and use Protomold technology. Like a standard Protomold aluminum tool, a MIM tool is fabricated using a combination of CNC milling and CNC electrical-discharge machining (EDM). After milling, the tool is polished by hand to customer specifications.Part production
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. Those pellets 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. After ejection, green parts are de-gated and placed on ceramic substrates (or setters), which help retain the shape of the part throughout the debinding and sintering process.Debinding
Pallets of green parts are loaded into a debind oven 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. At Proto Labs, we use a feedstock that is debinded using a catalytic process. The length of time required for debinding is a function of the thickest section of your 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.Sintering
Typically, pallets of parts are moved directly from the debind oven into a sintering furnace. The furnace precisely controls the temperature, cover gas and vacuum profile required to remove the remaining binder, and sinter the parts into the final product. During the furnace cycle, parts shrink about 20 percent into their final size. After sintering, any secondary operations are performed and the parts are complete.Design Guidelines
Design guidelines for metal injection molding are very similar to guidelines for standard injection-molded parts, with one major difference — the parts must be designed to retain their geometry during the sintering process.
Size
Maximum part size limits at this time are approximately:- 4 in. by 4 in. by 4 in. (10.1 cm. by 10.1 cm. by 10.1 cm.)*
- No deeper than 2 in. (5.1 cm.) from any parting line
- Maximum projected mold area of 10 sq. in. (64.5 sq. cm.)
- Maximum part volume less than 1.25 cu. in. (20.48 cc.)
Under some circumstances it may be possible to extend these limits, but there are tradeoffs to consider. If you need larger parts, please discuss your design and application with a Proto Labs engineer.
*Smaller if side-actions are required.
Recommended wall and rib thicknesses
Walls as thin as 0.040 in. (0.10 cm.) are possible, depending on the size of the wall and the location of adjacent thicker sections. Wall thicknesses generally should not exceed 0.5 in. (1.27 cm.). Rib thickness should be from 0.5 to 1.0 times the adjoining wall thickness. The radius of inside fillets should typically be approximately equal to the wall thickness. Radii that are much larger or smaller than this may cause porosity.Uniform wall thickness
Typically, rapid changes in wall thickness can cause porosity and internal shrinkage. Porosity can develop in thick sections or where there is insufficient flow path from the gate to a thick section. If thick and thin sections are required, we recommend a gradual transition between the two. A good rule of thumb is to make the length of the transition three times the thickness of the thicker section.Undercuts
Some simple undercuts on your part can be created using one of two methods: side-actions or pick-outs. Proto Labs can add simple side-actions to your tool. They must move perpendicular to the tool pull direction and must be on the parting line. During analysis of your part, we can determine if the location and depth of the undercut can be accommodated with a side-action. In some cases, Proto Labs can add manual features such as pick-outs to a MIM tool to create undercuts that are not otherwise possible. Pick-outs increase both the tool cost and the per-unit cost.Part ejection
Ejector pins are required to eject your part after molding. Due to the constraints of the molding press, pins will always be placed on the B-side of the tool. The B-side will be the side of the tool where the part is retained when the mold is opened, and retention is generally governed by the geometry of the part (generally, the side with more cores). Proto Labs will propose ejector pin locations before manufacturing your tool.Ejector pins range in size from 0.047 in. (0.118 cm.) to 0.375 in. (0.953 cm.), with sizes larger than 0.063 in. (0.158 cm.) preferred.
Draft
Generally, parts produced from metal injection molding need less draft than what is used in plastic injection molding, however, some draft may still be required in order for Proto Labs to machine the tool. A good rule of thumb is 0.5 degrees of draft per inch (2.54 cm.) of depth (depth is considered at the molded size, which is roughly 15 to 20 percent bigger than the finished size). While 0.5 degree of draft is common, zero draft can occasionally be tolerated on shallow parts with good surfaces for ejection.Gating and venting
Metal injection molded parts require relatively large gates compared to plastic injection-molded parts due to the high metal content of the feedstock. Generally, a gate should feed into the thickest cross-section of the part, although feedstock flow considerations also need to be accounted for. Proto Labs can provide several types of gates, including hot-tip gates, edge gates (or tab gates), tunnel gates (or sub gates) and post gates. Gates will leave a vestige or blemish, so they should be placed on a surface that is not dimensionally or cosmetically critical, or a recess should be provided for gating.In some cases, Proto Labs will need to add vents to allow air to escape while the part is filling. Often, vents can be added at the parting line, typically leaving a vestige too small to notice. In some cases, a venting pin must be added to allow otherwise trapped air to escape, leaving a blemish that looks like an ejector pin mark. Proto Labs will propose gate and vent locations (that require your approval) before manufacturing your tool.
Tool and part finishes
Finishes on MIM parts generally replicate the finish of the tool used to mold the green part. Proto Labs offers the following finishes on MIM tools:- SPI-C1 600-grit stone
- SPI-B1 600-grit paper
Expected tolerances
Final dimensions are highly dependent on how well the part can be fixtured during sintering. The efficacy of sintering fixtures, in turn, is highly dependent on the geometry of the part. The skill of the original part designer in developing a part that can be sintered and fixtured properly is a major driver of meeting part tolerance requirements (see Fixturing guidelines). Typical achieved tolerances for a well-designed part are linear tolerances of ±0.005 in. (0.023 cm.), plus 0.001 in. (0.003 cm.) for each inch of dimension.Material selection
Proto Labs currently offers the following metal injection molding materials: 316L and 17-4PH stainless steels. Consult the available 316L stainless steel data sheet and 17-4PH stainless steel data sheet for more information. These materials are standard offerings of BASF and can be sourced worldwide.Fixturing
Two unavoidable facts greatly determine the success or failure of a metal injection molding project: first, a MIM part shrinks by about 20 percent during the debinding and sintering processes; and second, the MIM part becomes soft and responds to gravity during the sintering process. Proper MIM part design enables effective fixture design to overcome both of these phenomena. If necessary, Proto Labs will design a sintering fixture to support your part during sintering.
The basic challenge is that as a brown part is sintered, it shrinks to the final dimensions. At the same time, the part becomes soft as the metal powders partially melt and join together, and gravity pulls overhanging sections out of position. For the purposes of visualization, think of the part reaching the consistency and flexibility of children’s modeling clay.
As a part shrinks, some of it slides on the supporting surface to reach the final position. An ideal radially symmetric part would ideally shrink uniformly to the center, so the outside edges would move the most, the center would not move at all, and the center of mass would stay in the same spot during the process. The moving parts do experience some friction, which is often immaterial but can sometimes pose a problem.
The easiest parts to fixture have a common co-planar surface that can rest flat on a ceramic substrate or setter. Ideally, no portion of the part overhangs the planar surface, and there are no concave portions in the bounding polygon. Discs, or stacks of discs (e.g., visualize a wedding cake) are perfect. Unfortunately, the world only needs so many discs, and so we must develop ways to process real-world metal injection molded parts.
The basic challenge is that as a brown part is sintered, it shrinks to the final dimensions. At the same time, the part becomes soft as the metal powders partially melt and join together, and gravity pulls overhanging sections out of position. For the purposes of visualization, think of the part reaching the consistency and flexibility of children’s modeling clay.
As a part shrinks, some of it slides on the supporting surface to reach the final position. An ideal radially symmetric part would ideally shrink uniformly to the center, so the outside edges would move the most, the center would not move at all, and the center of mass would stay in the same spot during the process. The moving parts do experience some friction, which is often immaterial but can sometimes pose a problem.
The easiest parts to fixture have a common co-planar surface that can rest flat on a ceramic substrate or setter. Ideally, no portion of the part overhangs the planar surface, and there are no concave portions in the bounding polygon. Discs, or stacks of discs (e.g., visualize a wedding cake) are perfect. Unfortunately, the world only needs so many discs, and so we must develop ways to process real-world metal injection molded parts.
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