The metal powder is mixed with a thermoplastic polymer (usually called a binder) in a heated state, so that the surface of each metal particle is evenly covered with a layer of binder, and the resulting mixture is called feed. After the feed is cooled, it is prepared into granules with a particle size of several millimeters by a granulator, which is used as the injection molding material of the injection molding machine.
The feed material is heated in the loading barrel to melt it, and the reciprocating screw is forced to feed the material into the furnace through the gate and into the mold. After cooling, the formed part is released from the mold to obtain a green stage material.
Special degreasing equipment is used to process the green stage material after injection molding to remove 90% of the binder in the components and the remaining 10% of the residual binder plays a supporting role, so that the degreasing part can be transferred to the next sintering process. The remaining binder will be completely removed during the thermal release stage of the sintering process.
Sintering is to put the degreased product on a ceramic plate and put it into a sintering furnace. Through precise temperature control, the furnace temperature is gradually increased to about 85% of the melting point of the material. The relative density of the sintered product is about 98%, and its performance is similar to that of the parts obtained after bar cutting. At the same time, sometimes in order to obtain more precise tolerances or increase material properties, the parts can be shaped, machined, heat treated and plating after sintering.
Since MIM products are formed by a mold similar to an injection mold, a high degree of freedom in geometric design can be obtained. As long as the injection mold can form a product, it can basically be manufactured by using the MIM process.
Since the MIM raw material powders are all the powders with D90 below 18um, the surface roughness of the sintered body can reach about Ra0.8, which brings convenience for subsequent polishing, plating and other processing.
The high density of MIM products makes them have excellent material properties, which are basically close to the properties of sheet materials, so they are especially suitable for parts with special requirements for product function.
It has been proved that almost all materials can be produced by MIM process, but it is best to use stainless steel for production.
The general production lead time of MIM process is 5-7 days, and products can be supplied continuously after one production cycle. The daily output can meet the demand from several hundred to several hundreds thousand per day.
The dimensional accuracy of powder injection molding is generally around +/-0.5%, and it can be +/-0.3% in special cases.
The MIM process can achieve an annual output from as a few thousand to several million. However, because the MIM process must use molds, the MIM process is usually not used for parts with a small output due to mold costs. Generally speaking, for products with an annual output of 20,000 or more, the MIM process has more advantages, and for products with an annual output of less than 20,000, it is necessary to evaluate the investment in mold costs.
Powder injection molding is formed by filling the raw materials into a mold similar to plastic products through an injection molding machine, so it can carry out complex shape design like traditional plastic products. When the part has holes, grooves, ribs, bosses and other obvious features, the MIM process will become more useful.
a. Uniform wall thickness change
b. Maximum size is less than 125 mm
c. All features greater than 0.1 mm
d. Weight mass is less than 1 kg
e. Thickness is less than 10 mm
f. There is at least one plane as the sintering support surface
g. Allow the existence of parting line, gate mark and thimble mark
It has been proved that almost all materials can be produced by MIM process, but it is best to use stainless steel for production. This is because more than half of MIM parts are made of stainless steel, which means that it hasfundation of the largest sup base, and due to the large output, the price of the material is correspondingly lower.
The dimensional accuracy of powder injection molding is generally around +/-0.5%, and it can be +/-0.3% under special circumstances. Higher accuracy requirements mean increased costs. Therefore, it is recommended to relax some tolerances under the premise of meeting the requirements. The following table is the tolerance comparison table.
| Size | General MIM enterprise manufacturing tolerance | Yibi manufacturing tolerance | |
| General case | Special case | ||
| 1-4mm | +/-0.05 | +/-0.05 | +/-0.03 |
| 4-10mm | +/-0.10 | +/-0.07 | +/-0.05 |
| 10-20mm | +/-0.15 | +/-0.10 | +/-0.08 |
| 20-30mm | +/-0.25 | +/-0.15 | +/-0.10 |
| feature | General range | Special range | |
| Part size | 0.2-50g | 0.09-200g | |
| Wall thickness | 0.5mm or more | 0.25 or more | |
Note: Special tolerance requirements can be achieved through secondary processing, such as: shaping, CNC.
Compared with traditional powder metallurgy parts, MIM parts have a higher sintered density (96-100% of the theoretical density). Therefore, the typical chemical, physical and basic thermodynamic properties of PIM materials are the same as those in the sheet material manual. Is consistent. So if performance is important, the performance of MIM products will be more competitive.
Metal injection molding (MIM) is a net forming process, which was first used in the 1920s. The origin of some current MIMs can also be traced back to the 1960s. However, it lacked the necessary industrial foundation at that time and has not been widely used. In recent years, due to the leading role of 3C products, the MIM industry has developed rapidly and is widely used in 3C electronics, hardware accessories, power tools, lock accessories, smart wear, jewelry accessories, medical equipment and other industries. The main processes of the MIM process include: mixing, injection, debinding, sintering, and appropriate post-treatments if necessary. The MIM process has the characteristics of high cost performance, high product performance and the ability to form complex parts.
aMIM generally does not require subsequent processing
bMIM parts have excellent corrosion resistance, strength and toughness
cMIM parts have excellent magnetic properties
aMIM design can save material and weight
bMIM can shape difficult-to-cut materials into the final shape
cMIM design can save multiple assembly operations
aMIM can produce smaller thickness
bMIM roughness is better
cThe small diameter half holes and through-holes of MIM parts have better Process-ability
dMIM parts can greatly reduce the required finishing machining
eMIM produces small parts at lower costs and faster delivery.
