MIM Powder Metallurgy Injection Screw Barrel
Metal Injection Molding (MIM) is a powder metallurgy process that combines plastic injection molding with powdered metal sintering.
It uses fine metal powders (typically stainless steel 316L, 17-4PH, iron-based soft magnetic powders, titanium alloys, etc.) mixed with a binder to form a feedstock, which is then injected into a mold, debound, and sintered to produce complex, high-precision metal parts .
The global MIM market was valued at approximately USD 1.68 billion in 2025 and is expected to reach USD 2.95 billion by 2034, growing at a CAGR of 6.7% .
Why MIM Requires Dedicated Screw Barrels
The screw barrel assembly-consisting of the barrel (cylinder) and screw-is the core plasticizing unit of an injection molding machine. In MIM, the feedstock contains 55–70 vol% metal powder, which is extremely abrasive and challenging to the screw and barrel . Compared to conventional plastic injection molding, MIM feedstocks are:
Highly abrasive: metal particles act like sandpaper, causing severe wear
Shear-sensitive: excessive shear leads to powder-binder separation
High-viscosity: requiring stronger conveying capability
Thermally sensitive: binders can decompose under overheating
Therefore, standard plastic screws cannot meet MIM requirements; dedicated designs are essential .
Key Design Parameters
A typical MIM screw is a three-zone design (feed zone, compression zone, metering zone). Critical parameters differ from standard screws:
|
Parameter |
MIM Screw |
Standard Plastic Screw |
|---|---|---|
|
Compression Ratio |
1.7 – 2.2 |
Up to 3.0 |
|
L/D Ratio |
20 – 22 |
Varies |
|
Screw Clearance |
2× polymer standard |
SPI standard |
Data sources: ScienceDirect MIM molding reference ; industry technical guidelines
The lower compression ratio minimizes shear heating and prevents powder-binder separation, while the deeper feed zone ensures adequate melting time .
Materials & Wear Protection
To withstand extreme abrasion, MIM screw barrels employ advanced materials:
Barrels: Cobalt-based bimetallic barrels with 35% tungsten carbide in the inner lining, based on 40Cr steel substrate
Screws: Imported powder metallurgy steel bars, through quenching (HRC 52–63), nitrided to surface hardness HRC 63
Bimetallic options: Fe-based, Ni-based, Co-based, or NiCo-based alloys with a 2–3 mm alloy layer at HRC 58–67
Bimetallic screws and barrels can achieve 5–8 times longer service life than nitrided ones when processing highly abrasive feedstocks such as those containing metal powders .
Industry Applications & Trends
MIM screw barrels are primarily used in injection molding machines serving industries such as:
Consumer electronics: folding phone hinges, camera brackets, connectors (the largest application segment)
Medical devices: surgical instruments, orthopedic implants
Automotive & Aerospace: lightweight, complex small parts
Humanoid robots: emerging applications in reducer gears and dexterous hand joints
As MIM technology expands into high-end fields like titanium alloy components and humanoid robotics , the demand for high-performance, wear-resistant screw barrels continues to grow. Chinese manufacturers dominate the global MIM market with over 40% share , and domestic equipment suppliers are progressively improving screw barrel technology to match high-end applications.
Selection Considerations
When selecting an MIM screw barrel, manufacturers must consider:
Metal powder type and particle size
Binder system characteristics
Desired shot size and plasticizing capacity
Required wear resistance level (determining whether nitriding or bimetallic solution is needed)
Proper selection ensures stable injection, reduces downtime, and extends equipment life-critical factors in the capital-intensive MIM production environment where tooling costs can reach hundreds of thousands of dollars .