Critical Requirements for Molds in Micro Injection Molding

Critical Requirements for Molds in Micro Injection Molding

Micro injection molding (injection mass <1 gram, even milligram-level) is essential for manufacturing medical devices (e.g., catheter connectors, micro gears), electronic components (connectors, sensor housings), and optical parts (lenses, fiber optic connectors). Its demands on molds far exceed those of conventional injection molding, requiring advancements in materials, design, processing, and control.

1. Material Performance Requirements

1. High Hardness and Wear Resistance

• Mold Steel Selection: Powder metallurgy steels (e.g., ASP-23, Vanadis 10) or tungsten carbide (Co-WC), hardness ≥60 HRC, to resist wear from glass/mineral-filled materials.

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• Surface Coatings: Diamond-like carbon (DLC) or titanium nitride (TiN) coatings (2-5 μm thick) reduce friction coefficient (≤0.1), extending mold life to over 1 million cycles.

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2. Corrosion Resistance

• Medical-grade materials (e.g., PEEK, biocompatible LCP) may release acidic gases, requiring stainless steel molds (e.g., Stavax ESR) or anti-corrosion coatings (CrN).

2. Mold Design Essentials

1. Runner and Gate Design

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• Hot Runner Systems: Valve-gated hot runners (diameter ≤1 mm) precisely control micro-shot volumes, preventing cold slugs and drooling.

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• Gate Types: Laser-drilled pin-point gates (diameter 0.1-0.3 mm) or fan gates minimize shear heat and residual stress.
  Example: Insulin pump gears molded with 0.15 mm pin-point gates achieve weight±0.5%.

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3. Venting System

• Nano-Scale Vents: Depth 0.003-0.01 mm, located at melt flow ends and insert interfaces, preventing air traps and burns.

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• Vacuum Venting: In-mold vacuum ≤0.1 MPa ensures complete gas evacuation.

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• Temperature Control

• Conformal Cooling Channels: 3D-printed stainless steel channels follow cavity contours, temperature fluctuation ≤0.5°C, reducing warpage.

• Zone Control: Independent temperature control for cores and inserts (e.g., PID精度 ±0.1°C), adapting to crystalline materials (e.g., PBT) shrinkage.

3. Machining and Assembly Precision

1. Micro-Machining Technologies

• Cavity Machining: Five-axis micro-milling (tool diameter ≤0.1 mm) or EDM, surface roughness Ra≤0.02 μm.

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• Polishing: Diamond paste mirror polishing (Ra≤0.01 μm) reduces demolding resistance.

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3. Dimensional Tolerances

• Core-cavity clearance ≤2 μm prevents flash (e.g., micro-lens mold tolerance ±1 μm).

• Core alignment ≤0.005 mm ensures balanced filling in multi-cavity molds.

4. Process Control and Monitoring

1. Injection Machine Parameters

• Screw Design: Small diameter (14-18 mm), high compression ratio (≥2.5) screws improve plastification uniformity.

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• Injection Speed: High-speed injection (500-1000 mm/s) fills micro-cavities before premature solidification.

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2. Real-Time Monitoring

• In-Mold Sensors: Piezoelectric pressure sensors (accuracy ±0.1 bar) monitor cavity pressure profiles.

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• Vision Inspection: 100% online dimensional checks (optical accuracy ±1 μm) and defect detection (short shots, flow marks).

5. Typical Applications and Cases

1. Medical Catheter Connectors

• Materials: PEEK or PEI (low flowability), requiring mold heating to 160°C and DLC coatings to prevent sticking.

• Challenges: Wall thickness 0.2 mm, weight 0.02 grams, necessitating vacuum venting to avoid short shots.

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2. Micro Gear Transmission Systems

• Materials: LCP (30% glass-filled), mold hardness ≥62 HRC, pin-point gates to reduce weld lines.

• Quality Metrics: Tooth profile error ≤5 μm, cycle repeatability 99.9%.

6. Future Technology Trends

1. Smart Molds

• IoT sensors monitor wear and temperature in real-time, enabling predictive maintenance.

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2. Hybrid Manufacturing

• 3D-printed tungsten carbide inserts + CNC finishing, combining conformal cooling and ultra-hardness.

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3. Super-Hydrophobic Coatings

• Nanostructured coatings (e.g., fluorosilane) enable mold release agent-free production, avoiding part contamination.

Conclusion

Molds for micro injection molding are the "chip-level" core of precision manufacturing. Requirements can be summarized as:

• Material Extremes (high hardness, wear/corrosion resistance);

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• Nano-Scale Design (micro-gating, precision venting, uniform temperature control);

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• Digital Control (real-time sensing, AI optimization).
  Only through interdisciplinary collaboration (materials science, mechanical engineering, data science) can stable mass production of milligram-level parts be achieved, advancing miniaturization in medical and electronics fields.