Micro Molding Guide — Injection Molding for Ultra-Small Parts
Micro molding (also called micro injection molding) produces parts weighing less than 0.1g or having features smaller than 0.2mm. It is a specialized discipline requiring dedicated micro injection molding machines, precision tooling at micron-level tolerances, and cleanroom or near-cleanroom conditions.
This guide covers what qualifies as micro molding, the equipment and mold design differences, and what buyers should verify when sourcing micro molded parts from China.
What Qualifies as Micro Molding?
| Metric | Micro Molding | Standard Molding |
|---|
| Shot weight | < 0.1g (often 0.01-0.05g) | 1g – 500g |
| Part volume | < 50 mm³ | 500 – 500,000 mm³ |
| Wall thickness | 0.05 – 0.3 mm | 0.8 – 5.0 mm |
| Feature size (ribs, holes) | 0.02 – 0.2 mm | 0.5 – 10 mm |
| Tolerance | ±0.005 – ±0.02 mm | ±0.05 – ±0.25 mm |
| Gate size | 0.1 – 0.3 mm diameter | 0.5 – 3.0 mm diameter |
Buyer's Tip: Here's the biggest trap in micro molding sourcing from China: many factories claim "micro molding capability" but use standard injection molding machines with standard screws to make small parts. Standard machines cannot accurately meter shot sizes below 0.5g — the screw's L/D ratio and check ring design produce inconsistent shot volumes. A legitimate micro molding machine uses a plunger-based injection system (not a screw) specifically designed for 0.01-1g shot sizes. When evaluating a Chinese micro molder, ask: "What is your smallest injection machine's screw/plunger diameter? What is the minimum shot size you can hold within ±1%?" If they can't answer these questions, they are using standard machines for micro parts — expect 5-15% scrap from shot weight variation alone.
Micro Molding Machine Requirements
Plunger vs. Screw Injection
Standard reciprocating screw injection units cannot accurately control shot sizes under 1g due to melt compressibility and check ring leakage. Dedicated micro molding machines use:
- Plunger injection: A separate plasticizing screw feeds a measured melt pool into a plunger chamber. The plunger injects with precision independent of the screw. Shot size accuracy: ±0.2%.
- Micro screw (12-16mm diameter): Some machines use a very small screw with a specialized check ring. Works for shots down to 0.1g. Less accurate than plunger but more versatile.
- Two-stage injection: A plasticizing screw feeds into a micro plunger. The plunger size can be as small as 4mm diameter. Used for ultra-precision micro molding (watch components, medical implants).
Mold Design for Micro Parts
Runner and Gate Scaling
Scaling a standard mold design down for micro parts does not work. The runner system must be completely redesigned:
- Runner diameter: 1.0-2.0mm for micro parts (vs. 4-8mm for standard). The runner must not overfill relative to the cavity.
- Gate dimensions: 0.1-0.3mm diameter or less. Gate land length: 0.1-0.3mm — any longer and the gate pressure drop prevents cavity fill.
- Cold slug well: Must be proportionally sized. A 0.5mm cold slug well is often sufficient — but many designers make it too small or skip it entirely.
Venting for Micro Molds
Micro cavities trap air that cannot escape through standard venting because the flow front is too thin to push air out. Solutions:
- Vacuum venting: The mold cavity is evacuated before injection. Essential for sub-0.1mm wall thickness parts. Adds $3,000-8,000 to mold cost.
- Sintered metal vents: Porous steel inserts allow air to escape through microscopic pores. Used for micro parts with thin, complex geometry.
- Vent lands at 0.005-0.01mm: Standard vent depths (0.02-0.05mm) are too deep for micro parts — they allow flash. Micro vents must be ground to 0.005mm depth, requiring specialized grinding equipment.
Materials for Micro Molding
Not all materials flow well enough for micro molding. The material must have a low melt viscosity at the available injection speed:
| Material | Micro Moldability | Notes |
|---|
| LCP | Excellent | Lowest viscosity; ideal for thin walls |
| PEEK | Good | High temp required; good flow with plunger injection |
| PA12 | Excellent | Low moisture sensitivity; good flow |
| POM | Good | Moderate viscosity; crystalline shrinkage challenges |
| PC | Moderate | High viscosity limits very thin walls |
| PP | Moderate | Good flow but high shrinkage for micro tolerances |
| ABS | Moderate | Used only for thicker micro parts (0.2mm+) |
| PE | Poor | High shrinkage; inconsistent in micro cavities |
Quality Control for Micro Parts
Standard measurement tools (calipers, micrometers) cannot measure micro parts. Specialized equipment is required:
- Optical comparators / vision systems: Measure part dimensions from 2D imagery. Accuracy: ±0.002mm.
- CT scanning (micro-CT): X-ray tomography to measure internal features and detect voids or incomplete fill. Essential for micro fluidic channels and medical components.
- Scanning electron microscopy (SEM): For surface quality inspection on sub-0.1mm features.
- Weight-based shot control: Micro parts are measured by weight (microgram precision) as a proxy for dimensional consistency — faster and more practical than optical measurement for every shot.
What This Means for Your Project: Micro molding is not just "small injection molding" — it is a fundamentally different process requiring specialized machines (plunger injection, not screw), dedicated mold design (vacuum venting, micro gates), and advanced metrology (vision systems, micro-CT). When sourcing micro molded parts from China, the supplier qualification checklist must include: machine plunger diameter (≤4mm for true micro), minimum shot size capability (certified, not claimed), and cleanliness level (micro parts require at least Class 100,000 cleanroom). Do not accept a quote from a factory that only has standard injection machines — even the best operator cannot produce consistent micro parts on equipment designed for gram-scale shots. The mold cost for micro parts is typically 2-4x higher than an equivalent-sized standard mold due to precision requirements and specialized components.
Related Guides