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The process of Metal Injection Molding

Writer: admin Source: MDM Metal Date:2022-06-30
The process of Metal Injection Molding

Metal injection molding(MIM) is a new type of powder metallurgy near netshaping technology that extended from the plastic injection molding industry. As we all know, plastic injection molding can produce various kinds of parts with very complex shape with pretty cheap prices, but plastic products’ strength is too low, to improve the strength we can add metal/ceramic powder in the plastic to get products with better strength and better wear resistance. 

The processes for MIM are: first is to choose the metal powder and binder as required; Then use the appropriate method under a certain temperature to mix powder and binder into uniform feeding; Granulating and then injection molding; Decreasing the blank parts and sintering densification into finished products. 

1. MIM powder and powdering technology

MIM has high requirements for raw material, so we should choose the raw material that good for mixing, injection,degreasing and sintering. While this is often contradictory.  The research for MIM powder  including: powder shape, particle size and particle size composition, surface, etc.
Because of MIM powder raw material requirement is very fine, the MIM powder raw material prices generally higher, some prices even up to 10 times of the traditional PM powder, this is one of the key factors limiting the widely use of MIM technology at present. The methods used to produce  MIM powder including: carbonyl method, ultra-high pressure water atomization method, high pressure gas atomization method, etc.

2, Binder 

Binder is the core of MIM technology, it has two most basic functions: enhancing liquidity to suitable for injection molding and maintaining compact shape. In addition it should be easy removal, no pollution, non_toxic, and reasonable cost etc. Therefore all kinds of binder appeared, in recent years it has been developing towards good direction. 
Binder components is generally composed of low molecular macromolecule component, plus some necessary additives. Low molecular with low viscosity and good liquidity, is easy to remove; macromolecule component with high strength and will keep the intensity of black parts. The two mix in appropriate proportion collocation in order to obtain high powder loading capacity, finally get high uniformity and high accuracy products.

3. Mixing

Mixing is the process to get uniform feedstock by mixing metal powder and and binder. Due to the nature of the feedstock performance determines the final injection molding products, so the mixing the process step is very important. This involves the binder and the powder joining way and the sequence and the  the mixing temperature, characteristics ofmixing device a variety of factors. The process currently stuck in fumble level based on the experience, an important index to evaluate the mixing process of is feedstock's uniformity and consistency. As to shear stress, it varies according to the difference of the mixing way.

The mixing of feedstock is under the combined effect of thermal effect and shear force. Mixing temperature can not too high, otherwise the binder may decompose or two phase separation phenomenon will happen between powder and binder due to the low viscosity. Common used mixing device: twin_screw extruder, Z impeller mixer, single screw extruder, plunger extruder, dual planetary mixer, double CAM mixer and so on, all these mixing device is suitable for the preparation of viscosity in the 1-1000 pa s. 

Mixing method is usually first join high_melting component, and then cooling,  join the low melting component, and then in batches to join metal powder.This can prevent the gasification and decomposition of low melting point component, partial add metal powder can prevent the cooling fast shock caused by the torque, reduce equipment loss.

4. Injection molding

The purpose of the injection molding is to obtain the required shape with zero defect, particle uniformly arranged of MIM black part. First heat the granular feedstock to a certain temperature to make it liquidity, then inject it into mold cavity and cooling down to get the required shape with certain rigid body, finally eject the rigid body from the mold and that is the black part. This process is consistent with traditional plastic injection forming process, but as a result of the MIM feedstock high powder content, make the injection process have big difference in parameters and other aspects, improper control will lead to all kinds of defects easily.

5. Degreasing
Since the MIM technology was generated, with the different binder system, a variety of MIM technology pathes were formed, also for degreasing methods. Degreasing time reduced by the first few days to a few hours now. All the degreasing methods can be roughly divided into two categories: one is the two-step degreasing method.Two_step degreasing method including solvent degreasing + thermal degreasing, siphon degreasing+thermal degreasing, etc.

6. Sintering

Sintering is the last step in the MIM process, sintering removes pore between powder particles, make MIM products to achieve full density or close to full densification. Metal injection molding uses a large amount of binder, so the shrinkage is very large when sintering, commonly the shrinkage rate is between 13%~25%, so there is a deformation control and the dimensional accuracy control problem. For MIM products, most parts are with irregular and complex shape, so the deformation problem is more outstanding. Uniform feedstock is a key factor for the final sintered product size precision and deformation control. For iron and stainless steel products, there is a carbon potential control problem. Because the price of fine powder is higher, researching the strengthen sintering technology of coarse powder compact is an important way to reduce the production cost of powder injection molding, The technology is currently an important aspect in the research of metal powder injection molding.

MIM products due to the complex shape, big sintering shrinkage, most products need to do post_processing after sintering. Including reshaping, heat treatment(carburizing, nitriding, carbon nitrogen altogether permeability, etc.), surface treatment(fine grinding, ion nitriding, electroplating, shot peening hardening, etc.), etc.

Other Links - Want to Learn More?

  • What is MIM(Metal Injection Molding)
  • What's PM(Powder Metallurgy)
  • MIM technology application material system
  • CNC machining part-Watchcase
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