Material Selection
Material selection in micro molding directly impacts tolerance capability, feature replication, strength, and cost. The correct material choice is crucial to the manufacturability of micro molded products. Unlike conventional injection molding, micro molding places extreme demands on polymer flow behavior due to rapid cooling and high shear rates.
Why Material Selection is Critical in Micro Molding
In micro molding, the relationship between material properties and process outcomes is amplified:
- Viscosity at micro scale: Research shows polymer melt viscosity in micro-channels can be 29-35% lower than in conventional channels due to wall slip and shear thinning effects
- Shrinkage precision: A material with 0.5% shrinkage variation on a 1mm feature creates ±5 µm variation
- Flow behavior: High shear rates (up to 10⁶ s⁻¹) during filling cause significant shear thinning
- Crystallization kinetics: Rapid cooling rates in thin sections affect morphology and properties
Material Categories
High-Performance Engineering Thermoplastics
These materials offer superior mechanical properties, chemical resistance, and temperature performance:
| Material | Trade Names | Key Properties | Shrinkage | Applications |
|---|---|---|---|---|
| PEEK | Victrex, Ketaspire | High strength, chemical resistant, biocompatible, 260°C reflow capable | 1.0-1.5% | Medical implants, aerospace, semiconductor |
| PEI | Ultem® (1000, XH6050) | High heat resistance, dimensional stability, transparent grades available | 0.5-0.7% | Electronics, medical devices, optical |
| LCP | Vectra®, Zenite | Excellent flow, lowest shrinkage, minimal warpage, high stiffness | 0.1-0.5% | Connectors, micro-optics, high-precision |
| PPS | Ryton®, Fortron | Chemical resistance, dimensional stability, high temperature | 0.2-0.5% | Automotive sensors, electronics |
| PAI | Torlon® | Highest performance thermoplastic, extreme conditions | 0.5-0.8% | Aerospace, semiconductor |
| PEKK/PAEK | Kepstan | Balance of PEEK properties with better processing | 0.8-1.2% | Medical, industrial |
Why LCP excels in micro molding: LCP has the lowest viscosity of engineering thermoplastics, enabling filling of features as small as 100 µm. Its near-zero shrinkage (0.1-0.3% isotropic in some grades) makes it ideal for tight-tolerance applications.
Standard Engineering Thermoplastics
Suitable for many micro molding applications with good performance-to-cost ratio:
| Material | Key Properties | Shrinkage | Weld Line Strength | Applications |
|---|---|---|---|---|
| Polycarbonate (PC) | Optical clarity, impact resistance | 0.5-0.7% | 90-95% of base | Optical components, housings |
| Acetal (POM) | Low friction, dimensional stability | 1.8-2.2% | 70-80% of base | Gears, bearings, mechanisms |
| Nylon (PA) | Strength, wear resistance | 1.0-2.5% | 60-80% of base | Structural components |
| ABS | Good balance of properties | 0.4-0.7% | 85-90% of base | General purpose, housings |
| COC/COP | Optical clarity, low moisture absorption, biocompatible | 0.4-0.6% | 85-90% of base | Microfluidics, optical, diagnostics |
| PMMA | Optical clarity, UV stable | 0.4-0.6% | 80-85% of base | Lenses, light pipes |
Specialty Materials
| Material | Key Properties | Applications |
|---|---|---|
| Liquid Silicone Rubber (LSR) | Flexible, biocompatible, addition-cure (no byproducts) | Medical seals, overmolded grips, implants |
| Fluoropolymers (PTFE, FEP, PFA) | Chemical resistance, low friction, high purity | Chemical handling, semiconductor |
| Bioresorbable polymers (PLA, PGA, PLGA) | Dissolve over time in body | Implantable devices, drug delivery |
| Thermoplastic Elastomers (TPE/TPU) | Flexible, soft touch | Overmolding, grips, seals |
Selection Criteria
1. Flow & Processing Characteristics
This is the most critical factor for micro molding success:
| Property | Impact on Micro Molding | Preferred Values |
|---|---|---|
| Melt flow index (MFI) | Higher = better filling | >10 g/10min preferred |
| Viscosity at processing temp | Lower = fills finer features | Material dependent |
| Shear thinning behavior | More thinning aids high-speed filling | Significant beneficial |
| No-flow temperature | Higher = longer filling time before freeze | As low as practical |
Research finding: For micro-injection-molded devices, accuracy of channel width and depth increased with increasing mold temperature, melt temperature, injection velocity, and packing pressure within the regular processing window.
2. Shrinkage Characteristics
Material shrinkage directly affects achievable tolerances:
| Material Type | Typical Shrinkage | Shrinkage Variation | Tolerance Impact |
|---|---|---|---|
| LCP | 0.1-0.5% | ±0.05% | Best for precision (±0.003 mm achievable) |
| Amorphous (PC, ABS, COC) | 0.4-0.8% | ±0.1% | Good tolerance control |
| Semi-crystalline (Nylon, POM) | 1.0-2.5% | ±0.3% | More challenging |
| PEEK | 1.0-1.5% | ±0.2% | Good with optimized process |
Key insight: Amorphous polymers (PC, COC, PMMA) offer more dimensional stability because they lack the crystallization that causes shrinkage variation in semi-crystalline materials.
3. Weld Line Strength
Weld lines form where flow fronts meet and can have significantly reduced strength:
| Material | Weld Line Strength (% of base) | Notes |
|---|---|---|
| Amorphous unfilled | 85-95% | Best performance |
| Semi-crystalline unfilled | 70-85% | Perpendicular chain orientation |
| Glass-filled (any) | 40-60% | Fibers don’t cross weld line |
| Carbon-filled | 30-50% | Most significant reduction |
Research finding: The presence of large concentrations of solid fillers greatly influences the formation of entanglements across the weld line interface. Strength at the weld line can be as little as 20% of nominal in worst cases—or 100% in optimized conditions.
4. Crystallization Behavior
For semi-crystalline polymers, cooling rate affects properties:
- Higher cooling rates (typical in micro molding) → lower crystallinity
- Lower crystallinity → reduced stiffness, different shrinkage
- Mold temperature controls crystal growth rate and spherulite size
| Cooling Rate Effect | Crystallinity | Tensile Strength | Modulus |
|---|---|---|---|
| Fast (typical micro) | Lower | Lower | Lower |
| Slow (heated mold) | Higher | Higher | Higher |
5. Thermal Requirements
- Operating temperature range — Continuous use temperature
- Heat deflection temperature (HDT) — Structural integrity under load
- Coefficient of thermal expansion (CTE) — Dimensional stability with temperature
- Reflow compatibility — For electronics assembly (JEDEC 260°C survival)
6. Chemical Resistance
- Exposure to solvents, fuels, or cleaning agents
- Sterilization compatibility
- Biocompatibility requirements
Material Selection for Medical Devices
Medical micro molding requires additional considerations:
Biocompatibility Testing Hierarchy
| Standard | Testing Scope | Duration | Application |
|---|---|---|---|
| USP Class VI | Systemic toxicity, intracutaneous, implantation | 5 days | Basic biocompatibility, ≤30 day implants |
| ISO 10993 | Comprehensive series of 20+ tests | Risk-based | Required for device approval |
ISO 10993 is more comprehensive than USP Class VI and is the current global standard recognized by FDA and other regulatory bodies.
Biocompatibility Categories (ISO 10993)
| Contact Type | Duration | Required Testing |
|---|---|---|
| Surface (skin) | Limited (<24h) | Cytotoxicity, sensitization |
| Surface (skin) | Prolonged (1-30 days) | Add irritation, systemic toxicity |
| External communicating | Permanent (>30 days) | Add genotoxicity, implantation |
| Implant (tissue/bone) | Permanent | Full testing battery |
Sterilization Compatibility
| Sterilization Method | Temperature | Compatible Materials | Cautions |
|---|---|---|---|
| Autoclave (steam) | 121-134°C | PEEK, PEI, PSU, LCP, LSR | Not for PC, POM |
| EtO (ethylene oxide) | 37-63°C | Most thermoplastics | Residue concerns |
| Gamma radiation | Ambient | PP, PEEK, PC (some grades) | POM degrades, PE crosslinks |
| E-beam | Ambient | Similar to gamma | Faster, less penetration |
| VHP (H₂O₂) | Low temp | Most materials | Surface only |
Common Medical-Grade Materials
| Application | Recommended Materials | Notes |
|---|---|---|
| Surgical instruments | PEEK, PEI, PSU | Autoclavable, chemical resistant |
| Diagnostic disposables | COC, COP, PS | Optical clarity, low extractables |
| Drug delivery | COC, PEEK, LSR | Drug compatibility critical |
| Implants (<30 days) | Medical-grade silicone, PEEK | USP Class VI minimum |
| Long-term implants | PEEK, UHMWPE | Full ISO 10993 |
Glass and Carbon Fiber Reinforcement
Adding reinforcement affects micro moldability significantly:
Benefits
- Increased stiffness and strength (2-3x improvement possible)
- Reduced shrinkage (30-50% reduction)
- Improved dimensional stability
- Higher heat deflection temperature
Challenges
- Increased viscosity — Harder to fill micro features
- Fiber orientation — Anisotropic shrinkage and properties
- Weld line weakness — 40-60% strength reduction
- Increased tool wear — Especially with glass
- Surface finish impact — Fiber read-through
Micro Molding Recommendations
| Fill Level | Application | Considerations |
|---|---|---|
| 10-15% | Micro features present | Minimal flow impact |
| 15-20% | Standard micro parts | Good balance |
| 20-30% | Structural, no micro features | Flow may be limited |
| >30% | Generally not recommended | Flow restrictions |
Optical Materials for Micro-Optics
Micro-optics requires specialized material consideration:
| Material | Refractive Index | Abbe Number | Key Properties |
|---|---|---|---|
| PMMA | 1.49 | 57 | Low birefringence, UV stable |
| PC | 1.58 | 30 | Impact resistant, stress birefringence |
| COC/COP | 1.53 | 56 | Low birefringence, moisture stable |
| PS | 1.59 | 31 | Low cost, good clarity |
Critical for optics: Birefringence (stress-induced optical distortion) is a major concern. COC and PMMA have the lowest tendency to develop birefringence under molding stresses.
Tolerances achievable:
- Peak-to-valley: ~1 micron standard, 200 nm for critical optics
- Surface finish: 80-100 angstroms standard, 20 angstroms for precision
- Positional tolerance: ±1 micron, repeatable to 0.5 micron
Material Selection Workflow
- Define functional requirements — Mechanical, thermal, chemical, optical
- Identify regulatory constraints — Biocompatibility, food contact, UL ratings
- Assess flow requirements — Can the material fill your geometry?
- Evaluate shrinkage tolerance — Does predicted shrinkage meet tolerance needs?
- Consider weld line locations — Will material provide adequate weld strength?
- Check sterilization compatibility — For medical applications
- Evaluate cost and availability — Material cost, minimum order quantities, lead times
- Prototype and validate — Test material performance in actual application
Material Selection Quick Reference
| Priority | Best Choice | Second Choice | Avoid |
|---|---|---|---|
| Tightest tolerances | LCP | COC, PC | Nylon, POM |
| Highest temperature | PEEK, PAI | PPS, PEI | ABS, PC |
| Best flow (finest features) | LCP | COC, PC | PEEK, filled materials |
| Optical clarity | COC, PMMA | PC (low stress) | Filled, crystalline |
| Weld line strength | Unfilled amorphous | Unfilled semi-crystalline | Filled materials |
| Medical implant | Medical PEEK, LSR | PEI | Non-medical grades |
| Cost-effective | ABS, PC | COC, Nylon | PEEK, LCP |
Next Steps
- See Part Design for geometry guidelines specific to different materials
- Review Quality & Metrology for tolerance validation approaches
- Consult Tooling & Mold Design for gate sizing based on material viscosity