Metal injection molding
The metal powder injection molding , also MIM process (English for Metal Injection Mo (u) lding ) is a primary molding process for the production of metallic components of complex geometry and has its origin in the injection molding technology of plastics.
Since metals and ceramics are used in conventional manufacturing processes such as B. Machining or casting much more difficult to process than plastics, especially with complex geometries, the desire to combine the simplicity of the shaping in injection molding with the better material properties of metals or ceramics: The idea of powder injection molding, or PIM for short for powder injection molding ), was born.
In powder injection molding , fine metal or ceramic powder is mixed with an organic binder and then shaped on an injection molding machine. The binder is then removed again and the component is sintered in an oven at a high temperature . The result is a purely metallic or ceramic end product that combines the mechanical advantages of sintered components with the great variety of shapes of injection molding (see figure on the right, spectacle hinge). Powder injection molding has been known since around 1920, when ceramic housings for spark plugs were first manufactured using this process. Powder injection molding has only been able to establish itself industrially since around 1980 with the production of small, high-precision metallic components for industrial goods and handguns.
Today, powder metallurgical injection molding is an economical manufacturing process for large-scale products, which is mainly used in the manufacture of small to medium-sized components with a rather complex geometry and a weight of 0.1 to around 150 grams.
A major advantage of this process is that components with complex geometries that can only be manufactured in several parts using conventional processes can be manufactured in a single piece (see middle figure on the right). The main areas of application are automotive and mechanical engineering, measurement and control technology, precision mechanics and weapons manufacturing, the lock and fittings industry, tool technology and household appliances.
The powder injection molding process
The powder injection molding is one according to DIN 8580 . Manufacturing technology to the molding method and represents a subset of injection molding the basis of the processed material groups divided it into the metal powder injection molding ( Metal Injection Molding MIM ) and the ceramic powder injection molding ( Ceramic Injection Molding; CIM ).
The process steps of powder injection molding
Powder injection molding consists of four consecutive process steps
- Feedstock production,
- Injection molding,
- Debinding
- Sintering,
which are shown schematically in the adjacent figure. The finished component can then be subjected to an after-treatment.
Feedstock production
In the first process step, a fine metal or ceramic powder is mixed with an organic binder to form a homogeneous mass, which can be processed in an injection molding process similar to plastics processing. This metal / plastic mixture is called a feedstock.
Injection molding
This feedstock is injected into a closed mold in liquefied form (usually at an elevated temperature), where it first completely fills the mold (cavity) through targeted temperature control and then plasticizes it. The resulting molded body (green part or green part) already has all the typical geometric features of the finished component.
Debinding
After removing the green part from the injection molding machine, the binder is removed again in a two-stage process, the debinding. A purely metallic or ceramic component is created.
Sintering
The porous shaped body remaining after debinding, now called brown part or brown part, is compacted by sintering at high temperature to form a component with its final geometric and mechanical properties.
post processing
Basically, metal powder injection molding is a near-net-shape or true-to-net shape manufacturing process, the components have a high density (typically> 96%). Due to the complex manufacturing process, attempts are made to reduce the post-processing steps and subsequent processes to a minimum as early as the design phase.
In principle, MIM parts can be subjected to all conceivable heat and surface treatments that can also be used with conventionally manufactured metallic or ceramic components. These are for example
- Surface finish: sandblasting , vibratory grinding , polishing , lapping
- Coating technology: thin film technology, electroplating , painting
- Connection technology: laser welding , soldering , assembly
- Heat treatment: hardening , HIP (hot isostatic pressing)
- Machining: turning , milling , drilling , grinding , reaming , honing , thread cutting
- Forming technology: bending , calibration
Differentiation from other manufacturing processes
Metal powder injection molding (MIM process) competes with machining, press sintering and investment casting.
In comparison to the machining or press sintering of simple, rotationally symmetrical components, it is clearly at a disadvantage in terms of cost. However, the MIM process always has advantages wherever complex geometries, special material requirements, high precision and low error rates are required or expensive reworking is to be avoided.
Undercuts, holes, threads, logos or other labels can be integrated directly into the component or entire assemblies can be manufactured in one piece. The MIM process is therefore a near-net-shape manufacturing process.
The MIM process is often confused with conventional press sintering, in which - based on the cross section - comparatively flat components are pressed into die tools by powder compacting and then sintered. However, this means that far less complex component geometries can be produced. Due to the uniaxial compression, there is also a density gradient over the component cross-section during press sintering, while MIM components have a very even density distribution.
Since the MIM process is a tool-related process, the tool costs must be amortized over the number of pieces. Depending on the application, these can range from a few thousand to several million components per year.
The adjacent shows schematically the delimitation of the manufacturing processes competing with the MIM process, whereby the boundaries are fluid and always depend on the component geometry and the material used.
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- Carlo Burkhardt: Metallpulverspritzguss In: Die Bibliothek der Technik 237. Süddeutscher Verlag onpact, Munich 2013, ISBN 978-3-86236-056-7 .
- The history of the company OBE Ohnmacht & Baumgärtner GmbH & Co. KG, Ispringen 2004