Design Fundamentals

Micro molding is not simply traditional injection molding scaled down — it’s a discipline where every detail matters at the micron level. For injection moulding of micro features and micro parts, extreme processes are involved such as high shear rate and high thermal gradient, which influence the morphology and properties of micro parts.

Key Differences from Traditional Molding

The Physics of Micro Scale

Rapid Heat Transfer

When the feature size reduces to the micro/nanometre scale, the surface-to-volume ratio increases dramatically. Once the molten polymer contacts the cold mould, it freezes almost instantaneously. This creates unique challenges:

• Hesitation effect: When polymer melt is injected into a cavity with micro features on the surface, flow hesitates at the entrance of micro features until a much thicker substrate is filled. The hesitation duration is often longer than the critical cooling time of micro features.
• Freeze-off: Premature solidification before complete filling
• Skin layer formation: Rapid cooling creates oriented skin layers affecting properties

Rheological Behavior at Micro Scale

Research has shown that polymer melt viscosity in micro-channels can be 29-35% lower than in conventional channels due to:

• Wall slip effects becoming more significant
• Shear thinning at high shear rates (up to 10⁶ s⁻¹)
• Viscous heating through small orifices

Understanding polymer melt rheology at the micro/nano scale is critical for quality control, process design, and simulation.

Critical Design Considerations

1. Scale Changes Everything

At the micro scale, small variations have outsized impacts:

• A 0.0005” (0.013 mm) wall thickness change on a 0.004” wall represents a 12% variation
• This can mean the difference between a cavity filling reliably or short-shotting
• Traditional rules of thumb must be recalibrated for micro dimensions

2. Aspect Ratios

Feature aspect ratios (height-to-width) are typically limited to:

Higher aspect ratios increase the risk of:

• Incomplete filling due to premature freeze-off
• Difficult ejection and feature damage
• Lateral collapse of high features

3. Tolerance Stack-Up

Micro molding tolerances compound from multiple sources:

Critical insight: In micro molding, it’s common to have more error in the measurement than in the actual parts.

4. Structure Formation

The three transport phenomena (flow, heat transfer, and crystallisation kinetics) are involved in structure formation during processing:

• Flow causes macroscopic heat and momentum transport
• Flow influences crystallisation kinetics by controlling stress, strain, and strain rates
• This microstructure ultimately determines product properties

For semi-crystalline polymers, cooling rate dramatically affects crystallinity:

• Higher cooling rates → lower crystallinity → different mechanical properties
• Mold temperature controls crystal growth rate and spherulite size

The DfMM Process

Phase 1: Concept Review

• Define critical dimensions and tolerances
• Identify micro features requiring special attention
• Establish material requirements (biocompatibility, temperature resistance, etc.)
• Consider the three pillars: micro size, micro features, or micro tolerances

Phase 2: Manufacturability Analysis

• Assess moldability using flow simulation (Moldflow, Moldex3D)
• Identify potential filling, cooling, or ejection challenges
• Predict weld line locations and their impact on strength
• Recommend design modifications if needed

Phase 3: Tooling Strategy

• Determine optimal gate location and size (60-200 µm typical)
• Plan parting line and ejection approach
• Specify mold materials and surface treatments (DLC coating for release)
• Consider conformal cooling for complex geometries

Phase 4: Process Development

• Establish process window through DOE (Design of Experiments)
• Key parameters: melt temperature, mold temperature, injection speed, holding pressure, cooling time
• Validate dimensional capability (Cpk ≥ 1.33)
• Document control parameters for production

Common DfMM Challenges

The Hesitation Effect

When polymer melt flows into a cavity, it preferentially fills areas with less resistance. In parts with micro features on the surface:

1. Flow hesitates at micro feature entrances
2. Main cavity fills while features remain empty
3. By the time pressure builds, features have frozen

Solutions:

• Position gates to fill micro features first
• Use variotherm (rapid mold heating) to delay freeze-off
• Increase melt and mold temperatures
• Use vacuum venting to reduce air resistance

Thick-to-Thin Transitions

Abrupt wall thickness changes cause:

• Uneven cooling and shrinkage (leading to warpage)
• Sink marks on thick sections
• Stress concentrations and potential failure points

Solution: Use gradual tapers (3:1 minimum transition ratio) between thick and thin sections. No wall should have thickness less than 40-60% of adjacent walls.

Micro Feature Filling

Very small features may not fill completely due to:

• Premature freeze-off (dominant factor)
• Insufficient packing pressure
• Trapped air (micro-venting critical)
• Surface tension effects in nano-scale features

Research finding: Increasing injection speed and temperatures helps the reduced viscosity overcome extreme conditions and improves filling against the high cooling rate typical of micro injection moulding.

Ejection Without Damage

Micro parts are fragile and easily damaged during ejection:

• Ejector pins can be as small as 0.25 mm (0.010”)
• Pin placement is highly constrained by feature locations
• Insufficient draft causes sticking and surface damage
• Vacuum under the part can prevent release

Solutions:

• Maximize draft angles (0.5° minimum for polished, more for texture)
• Use air-assist ejection
• Consider stripper plates for delicate parts
• Apply DLC or other release coatings to mold surfaces

Process Parameters and Their Effects

Research has identified the relative importance of key parameters:

Key finding: Increasing mold temperature and melt temperature reduces thermal residual stresses and improves uniformity. However, increasing packing pressure can intensify shear and increase molecular orientation stresses.

Design Review Checklist

Before submitting a design for micro molding, verify:

• Wall thickness meets minimum requirements (≥0.004” / 0.1mm)
• Wall thickness is uniform or transitions are gradual (3:1 ratio)
• Adjacent walls are within 40-60% of each other
• Aspect ratios are within acceptable limits (≤6:1 to 8:1)
• Draft angles are specified on all vertical surfaces
• Critical tolerances are identified and achievable for chosen material
• Material is selected considering flow, shrinkage, and function
• Gate location preferences consider filling sequence
• Weld line locations are acceptable for strength requirements
• Measurement method exists for critical dimensions

Next Steps

• Review Material Selection to choose appropriate thermoplastics
• See Part Design for specific dimensional guidelines
• Consult Tooling & Mold Design for gate and runner considerations