Compounding is not simple stirring. Under elevated temperature and high shear force, ultrafine metal powder is blended with a multi-component binder system to form an injectable homogeneous mixture. Every powder particle is fully wetted and encapsulated by the binder, with no agglomeration, stable rheology and uniform powder loading. The mixture is then cooled and pelletized into feedstock granules.
Defects formed at the feedstock stage CANNOT be corrected in subsequent injection molding, debinding or sintering processes.
1. Objectives of Compounding
1. Break up powder agglomerates, uniformly coat each powder particle with binder, and eliminate local powder-rich or binder-rich zones.
2. Stabilize powder loading (volume fraction) and batch consistency to guarantee predictable sintering shrinkage.
3. Achieve targeted rheological behavior: pseudoplastic fluid, good mold filling and shape retention at elevated temperature, resistant to phase separation.
4. Efficient degassing to reduce voids, delamination and blisters in green parts.
5. Produce free-flowing uniform pellets suitable for feeding into injection molding machines.
2. Raw Material System
(1) Metal Powder
- Particle size: typical 5–20 μm; μMIM down to 1–5 μm. Mainstream: gas-atomized spherical powder, water-atomized irregular powder.
- Key impacts: Spherical powder delivers higher packing density and higher critical loading. Irregular powder tends to agglomerate easily and causes sharp viscosity rise.
- Pre-treatment: oven drying to remove moisture (moisture leads to poor wetting, voids, rusting and debinding blistering). Pre-dispersion of powder is required for certain grades.
(3) Powder Loading (Core parameter, volume fraction, NOT mass fraction)
- Normal production window: 60–63 vol%. Gas-atomized spherical powder upper limit: 64–65 vol%. Water-atomized irregular powder: mostly 58–61 vol%.
- CPVC (Critical Powder Volume Concentration): the loading where powder is densely packed with interstitial space filled by binder. In mass production, loading is normally set 2~3 vol% below CPVC.
- Over-loading: sharp viscosity surge, difficult mold filling, high injection pressure, severe mold abrasion.
- Under-loading: excessive binder, larger and scattered sintering shrinkage, high risk of distortion & slumping, longer production cycle.
3. Two Main Compounding Equipment Routes (Batch / Continuous)
- Advantages: continuous discharge, superior consistency, high output for mature feedstock mass manufacturing.
- Challenges: extremely precise feeding metering (loss-in-weight feeders control feed rate of powder and each binder component; formula drift results in scrapped batches).
- Auxiliary units: pre-mixer (V-type high-speed blender for dry mixing powder + stearic acid), cooling water bath, air conveying line, pelletizer, vibrating sieve.
4. Full Process Flow (Standard Paraffin Wax System, Batch Internal Mixing Version)
1. Weighing & Pre-drying
Powder dried at 80~100℃ for 2–4 h to remove adsorbed moisture. All components are weighed precisely per volumetric formula. Dry pre-mixing: powder + surfactant high-speed dry blending for 5–15 min, to attach coupling agent onto powder surface and mitigate agglomeration.
2. Equipment Preheating
Chamber heated to binder melting range (140～170℃ for PW-based formulas, adjustable per recipe).
3. Feeding Sequence (Critical – wrong sequence causes severe agglomeration)
Recommended sequence: add skeleton polymer first → melt → add wax low-molecular carrier → mix until molten → feed metal powder slowly in batches (never dump large volume powder at once, which causes instant agglomeration and trapped air) → finally add minor additives & antioxidants.
4. Shear Compounding & Dispersion
Control temperature and rotor speed. Torque rises rapidly at early stage (agglomerate breaking & wetting). Maintain shear while activating vacuum degassing (vacuum: 0.085~0.098 MPa) to remove trapped air.
Mixing cycle per batch: 30–60 min. End point is reached when torque stabilizes with minimal fluctuation.
Discharged as continuous mass / strand. Rapid water or air cooling to prevent binder segregation and wax phase floating.
6. Pelletizing
Strands fed into pelletizer to produce uniform cylindrical pellets (φ2~4 mm, length 2~5 mm). Sieved to remove fines and long connected strands.
Excessive fines lead to unstable feeding, bridging and inconsistent shot weight during injection molding.
7. Post-treatment & Testing
Sealed and moisture-proof packaging. Some formulas require aging stabilization before rheology testing.
Continuous TSE workflow: Loss-in-weight feeders meter pre-blended powder, binder pellets and liquid additives synchronously → main & side feeding into barrel → staged heating, melting and shearing → mid-barrel vacuum degassing → strand extrusion through die → cooling, pelletizing, sieving.
5. Key Process Control Parameters
1. Temperature: Above binder melting point but not excessively high. Over-temperature triggers polymer degradation, wax volatilization, binder composition shift and rheology drift. Too low temperature raises viscosity and insufficient shear, leaving agglomerates unresolved. Typical PW range: 140–170℃; PEG system runs cooler; POM catalytic system runs hotter.
2. Shear Speed: Higher speed delivers stronger shear but generates more frictional heat and risks thermal degradation. Speed cannot be increased blindly for high-density, high-loading feedstock.
3. Vacuum Degassing: Mandatory. Entrapped air creates pinholes, voids and delamination in green parts.
4. Feeding Rate: Powder feed speed must match binder wetting rate. Fast feeding leads to dry agglomerates and “dry powder core” defects.
6. Mandatory Factory QC Tests for Finished Feedstock
1. Capillary rheology test (160/170℃, viscosity curve at varied shear rates, to assess flowability, pseudoplasticity and phase separation risk)
2. TGA (Thermogravimetric Analysis): solid content measurement, verify actual powder loading against formula target
3. Density test + sintering shrinkage calibration (standard tensile bar injection & sintering, obtain shrinkage factor for mold scaling)
4. Metallographic cross-section: microscopic inspection for powder distribution, agglomerates and binder-rich zones
5. Moisture content test
Powder Metallurgy | Metal Injection Molding | Powder Metallurgy Gears
Guangming Group is a professional manufacturer, factory and reliable supplier of metal forming and wear-resistant materials from China. We specialize in R&D and manufacturing of powder metallurgy components via conventional compaction and MIM.
We maintain a high-caliber technical team. Our founding team originates from Guangdong Institute of Non-Ferrous Metals, a former national ministerial research institute, with rich industrial experience and strong R&D capacity to sustain continuous innovation.
The group holds 5 invention patents and over 30 utility model patents. We have obtained IATF16949, ISO9001-2015, ISO14001 and Intellectual Property Management System certifications, together with international trademark registration.
We are an Innovative Enterprise of Guangdong Province, a Little Giant Sci-Tech Enterprise of Guangzhou, director member of Guangdong Powder Metallurgy Industry Technology Innovation Alliance, and initiator of Guangzhou Advanced Powder Metallurgy Cluster Promotion Center.
We use cookies to offer you a better browsing experience, analyze site traffic and personalize content. By using this site, you agree to our use of cookies. Privacy Policy
