Drying and Dispersion Control Specification for MIM Feedstock

MIM or ceramic powder injection molding feedstock consists of metallic/ceramic powder mixed with organic binder by compounding and pelletizing. Feedstock is highly sensitive to humidity, drying temperature, holding time and particle dispersion status. Improper material control brings bubbles, pinholes, silver streaks and uneven density on green parts. Defects will be further amplified after debinding and sintering. Standardized drying and dispersion control effectively reduces mass‑production rejects at source.

1. Raw‑material receiving and storage management

Check package integrity upon raw‑material arrival. Damaged packaging leads to moisture absorption and agglomeration. Different batches must be stored separately and cannot be blended arbitrarily. Wipe outer packages before unpacking to avoid dust contamination.

Workshop warehouse humidity shall be controlled below 60% RH. Seal leftover feedstock tightly. MIM pellets absorb moisture much faster than conventional thermoplastics. Even short‑time open‑air exposure brings hidden moisture risks.

Severely caked feedstock should be scrapped directly. Slightly agglomerated material needs independent evaluation and treatment and cannot be crushed and blended directly.

2. Standardized drying‑parameter control

Recommended drying temperature ranges for most MIM feedstock are 60‑80℃, while ceramic feedstock adopts 55‑70℃. Over‑high temperature softens and decomposes binders and causes pellet sticking. Too‑low temperature cannot remove moisture effectively and generates silver marks and bubbles.

Normal drying holding time is 2‑4 hours. Extended drying is allowed for heavily‑moisturized feedstock, yet endless heating is forbidden. Material layer thickness shall be kept between 3‑5 cm. Excessively thick layers result in uneven heating.

Hot‑air circulating ovens are preferred. Regular oven temperature calibration is required. Drying trays must stay clean to prevent cross‑contamination of different material grades.

Do not open oven covers immediately after drying. Cool material below 60℃ inside the oven before taking out. Hot pellets exposed to ambient air re‑absorb moisture quickly. Dried feedstock should be consumed within 8 hours. Material stored beyond time limit requires re‑drying.

3. Feeding and dispersion requirements

Visually inspect pellets before feeding for agglomeration and fine powder. Sieve loosely‑agglomerated feedstock with proper mesh screens. Check screen conditions periodically to prevent broken mesh fragments mixing into materials.

Thoroughly clean hoppers before production. Hopper dryers keep dried feedstock warm and moisture‑proof with moderate setting temperature to avoid bridging.

Add pellets evenly instead of bulk feeding. Never knock hoppers with hard metal tools to resolve bridging; vibration feeding mechanism is preferred.

Sprue and runner regrind cannot be mixed with virgin material without restriction. Regrind ratio is generally limited between 10%‑30%. Regrind must be re‑dried and sieved before blending. Direct crushing without pre‑treatment is prohibited.

4. Mixing and fine‑powder management

Use blenders to mix virgin material and regrind homogeneously. Optimize mixing duration. Excessive blending generates frictional heat and pellet agglomeration. Completely clean blenders while switching material grades.

Fine powder generated from repeated recycling absorbs moisture easily and disrupts stable feeding. Remove surplus fine powder via sieving before feeding.

Keep feeding area dust‑proof. Close hopper lids after feeding to minimize material exposure to ambient air.

5. Abnormality handling and routine inspection

Once silver streaks or bubbles occur, check drying parameters and material storage status at first instead of blindly modifying injection settings.

Record actual oven and hopper temperature during shift inspection. Document raw‑material batches, drying duration and regrind proportion.

Take residual material out from hoppers if machine stops longer than 2 hours. Seal them properly and re‑dry before reusing. Long‑time thermal holding inside hoppers accelerates binder degradation.

Summary

Most bubble and cavity defects of MIM green parts stem from moisture absorption and poor particle dispersion. Drying work covers temperature, holding time, layer thickness and anti‑re‑moisture protection. Dispersion management focuses on agglomerates, fine powder and regrind addition. Strict implementation of above‑mentioned specifications stabilizes feedstock status and lowers reject rate of subsequent debinding‑sintering processes.