MIM
Metal Injection Moulding
MIM - Metal Injection Moulding
"Think plastic but manufacture steel"
Metal Injection Molding, or MIM, was developed in 1985 in the USA. This method of metal powders mixed with organic binders - is used to obtain plastic-like characteristics and then injected into molds. The parts then undergo a debinding process to remove the organic binders. The final stage is sintering, where the parts shrink to their final dimensions and density. The parts can then undergo various surface and other required treatments, resulting in a completed ready-to-assemble component, or a final product. This method for producing powdered metal parts is applicable where complex shapes are required to be manufactured out of metal alloys. The characteristics of the feedstock make it possible to inject the mixture into molds, similar to those used by the plastic industry.
MIM technology offers significant advantages:
- High precision.
- Metal parts can be machined, heat-treated, coated or plated.
- Ability to produce complex and intricate parts in large quantities.
- High level of surface finish.
- Tight tolerances of up to +/- 0.5% of the desired measurements.
- High density of 97% and more of the theoretical density of the material.
- Lower cost per unit.
It is important to note that the economical advantage of MIM over other technologies is especially significant for orders of large quantities (5000+) of complex small metal parts which weigh less than 5 gram to 50 grams. (The savings can be up to 50%). With the MIM technology, very near net shape parts are obtained and therefore is most advantageous for parts normally produced entirely by machining or where machining is necessary as a secondary operation.
Size
Even though it is possible to produce parts larger than 100 mm with a weight of 100 grams upwards, our experience shows that a cost advantage is reached mainly for smaller parts. Tolerances As in other production technologies, it is always advisable to mention tolerances which are not tighter than needed in order to lower the production cost. In the MIM technology it is usually possible to get to precision in the tolerance of up to 0.3% of the dimension, but not less than 0.02 mm. Higher precision requires grinding as a secondary operation.
MIM process is based on 5 main stages:
1) Preparation of the Feedstock: Metal powders (with particle size of a few microns) are mixed hot with organic binders (typically wax, thermoplastic resins and other materials) until a homogenous mixture is obtained. After cooling, the mixture is granulated, to allow it to be fed into an injection molding machine. If necessary, it can then be stored for a practically unlimited amount of time.
2) Injection Molding - Injection of the feedstock is done using an injection molding machine, similar to those used in the plastic industry. The feedstock is melted in the cylinder at a temperature of approximately 165°C and then injected into the mold. The injected metal parts (called "green parts") are formed into the desired geometry, with allowance for shrinkage. They then undergo visual inspection by means of a stereoscope and are weighed and checked for density. Rejected parts are granulated and recycled.
3) Debinding - After producing the green parts it is necessary to remove the organic binders before sintering. Here technologies vary. Metaor uses a process which consists of two stages: 1 - removal of the binder with an organic solvent, and 2 - thermal burnoff of the plasticizers.
4) Sintering - The final stage of the metal sintering process is sintering which is performed in a high temperature - controlled atmosphere furnace or in a vacuum furnace. In this stage the residual binder is removed and shrinkage occurs. At the end of the sintering process, the parts have shrunk by 15-18% and reach the precise desired dimensions. Shrinkage is unique for each metal alloy and is similar along the three axes.
5) Secondary operations are required for some parts. These may include:
- deburring - removal of flashes that occur during injection - done after debindingand before sintering.
- calibration - especially for parts with thin and long walls that could be slightly warped during sintering.
- thermal treatments
- surface treatments
Materials
Here are some typical materials that are used by the MIM method. For further information concerning materia properties please contact us.
Stainless Steel:
AISI 316L (DIN 1.4436.1.4404), AISI 304L (DIN 1.4306)
AISI 420 (DIN 1.4021), AISI 440C (DIN 1.4125)
AISI 430L (DIN 1.4016), AISI 17-4PH (DIN 1.4542,1.4548)
Ultra High strenght Low Alloy Steel:
AISI 4340 (DIN 1.6565)
AISI 4140 (DIN 1.7225)
AISI H13 (DIN 1.2344)
Low Alloy Steel:
7% Ni-Fe
2% Ni-Fe
AISI 8620 (DIN 1.6543, 1.6523, 1.6526)
Soft Magnetic Materials:
2% Ni-Fe, 40% Ni-Fe, 50% Ni-Fe (DIN 1.3927),
80% Ni-Fe, 3% Si-Fe, 7% Si-Fe, Iron, 2V Permendur,
AISI 430L (DIN 1.4016)
Controlled Expansion & Sealing Materials:
36% Ni-Fe (Invar)
42% Ni-Fe
ASTM F 15 (Kovar)
Other Materials:
Copper
Brass
Titanium
Illustration of some parts manufactured by MIM - Metal Injection Moulding
