Why is it said that the surface quality of injection molded (MIM) parts is good
Metal Injection Molding (MIM) technology involves mixing metal powder with a binder, injection molding, and then using processes such as degreasing and sintering to prepare components. The reasons for its excellent surface quality can be analyzed in depth from the dimensions of process principles, material properties, and process control, as follows:
Metal Injection Molding (MIM) technology involves mixing metal powder with a binder, injection molding, and then using processes such as degreasing and sintering to prepare components. The reasons for its excellent surface quality can be analyzed in depth from the dimensions of process principles, material properties, and process control, as follows:
1、 High precision replication capability of forming process
1. Precision machining foundation of injection molds
MIM molds are usually manufactured using electrical discharge machining (EDM), slow wire cutting, or five axis machining centers, with an accuracy of ± 0.01mm and a surface roughness (Ra) that can be controlled below 0.2 μ m. The high precision of the mold cavity directly determines the surface detail replication ability of the formed blank, for example:
Complex patterns (such as gear tooth surfaces and threads), small holes (diameter ≥ 0.3mm), thin-walled structures (thickness ≥ 0.2mm), and other features can be fully replicated, reducing the need for subsequent machining.
2. Uniform filling characteristics during injection process
The "feed" formed by mixing metal powder and binder has a flowability similar to plastic, and can fill the fine structure of the mold cavity under injection pressure (50-150MPa), avoiding the "insufficient filling" or "cold shut" defects in traditional casting. For example:
The flow rate of the feed in the mold cavity is uniform, which can reduce surface weld marks (common defects in injection molding processes) and make the surface of the blank smoother.
2、 The surface optimization effect of degreasing and sintering processes
1. Surface defect control during degreasing process
The degreasing process (thermal degreasing, solvent degreasing, or catalytic degreasing) can gradually remove the binder in the blank, avoiding surface cracking or bubbling caused by rapid volatilization of the binder. For example:
Catalytic debinding decomposes the polyoxymethylene component in the binder with nitric acid, resulting in a mild reaction and low residual carbon content on the surface of the blank, laying the foundation for subsequent sintering.
2. Surface densification during sintering process
During high-temperature sintering (1000-1400 ℃), metal powder particles achieve densification through diffusion and recrystallization (with a density of up to 95% -99%), filling surface micro defects (such as small pits) and significantly reducing roughness. For example:
For stainless steel MIM components, the surface Ra value after sintering can be reduced from 1.6 μ m of the blank to below 0.8 μ m, approaching the effect of machining and polishing;
Some materials, such as tungsten alloys, may form an oxide layer on their surface during sintering, but this can be further improved through subsequent acid washing or electrolytic polishing.
3、 Detailed control of material and feed preparation
1. The advantage of fine particle size of metal powder
The commonly used metal powder particle size for MIM is 5-20 μ m (traditional powder metallurgy is 50-100 μ m). Fine powders have a large contact area and fast diffusion rate during sintering, making it easier to form smooth metallurgical bonds on the surface. For example:
After sintering, the surface grain boundaries of stainless steel powder become finer, reducing the uneven microstructure.
2. The impact of feeding uniformity
During feeding preparation, a twin-screw mixer is used to achieve uniform dispersion of metal powder and binder, avoiding local powder agglomeration or binder segregation. If the feeding is uneven, the formed blank may have surface "patterns" or density fluctuations, while MIM's mixing process can control the powder dispersion within ± 2% to ensure surface consistency.
4、 Auxiliary optimization of post-processing technology
1. Chemical polishing and electrolytic polishing
For components with high surface quality requirements, such as medical devices and watch cases, the roughness can be further reduced through chemical polishing (nitric acid+hydrofluoric acid solution) or electrolytic polishing (sulfuric acid+phosphoric acid system). For example:
Electrolytic polishing can reduce the surface Ra value of MIM stainless steel components to below 0.1 μ m, achieving a mirror effect.
2. Surface coating and passivation treatment
MIM components can be directly electroplated (such as nickel and chromium), PVD coated (such as TiN), or passivated after sintering. The coating has strong adhesion to the substrate (due to the dense and pore free sintered surface), further improving surface smoothness and corrosion resistance.
Home
Product
Wechat
Telephone