Quality & Metrology
Micro molding presents unique measurement and validation challenges. If you cannot measure it, you cannot make it reliably. In micro molding, it’s common to have more error in the measurement than in the actual parts.
The Metrology Challenge
Micro molded parts can have:
- Dimensions in microns (±2-5 µm tolerances achievable)
- Features requiring 10x+ magnification to see
- Tolerances tighter than many measurement systems can reliably verify
- Internal features impossible to inspect without destructive testing
Key Principle: Your measurement capability must exceed your tolerance requirements by a significant margin (10:1 ratio preferred, 4:1 minimum acceptable).
Measurement Technologies
Contact Measurement
| Technology | Resolution | Best For | Limitations |
|---|---|---|---|
| CMM (Coordinate Measuring Machine) | 1-2 µm | 3D geometry, GD&T | Probe size limits access to micro features |
| Micro-CMM | 0.1-0.5 µm | Small parts, micro features | Specialized, expensive equipment |
| Profilometer (stylus) | 0.01-0.1 µm | Surface roughness, profiles | 2D measurement, may damage soft materials |
| Touch probe systems | 1-5 µm | On-machine verification | Limited to accessible surfaces |
Non-Contact Measurement
| Technology | Resolution | Throughput | Best For |
|---|---|---|---|
| Vision systems | 1-5 µm | High | 2D dimensions, edge detection, 100% inspection |
| Laser scanning | 5-25 µm | Medium | Complex 3D surfaces, point clouds |
| White light interferometry | Sub-nm | Low | Surface topography, optical components |
| Confocal microscopy | 0.1-1 µm | Low | Surface features, step heights, transparent materials |
| CT scanning | 5-50 µm | Low | Internal features, assemblies, wall thickness |
CT Scanning for Micro Molding
CT (Computed Tomography) scanning has become essential for micro molding quality:
Advantages:
- Non-destructive measurement of internal features
- Complete 3D data capture in single scan
- Measure wall thickness, voids, and internal geometry
- Compare to CAD for deviation analysis
Capabilities:
- Resolution down to 5 µm with micro-CT systems
- Measure features impossible to probe
- Detect internal voids and porosity
- Verify internal channel dimensions in microfluidics
Limitations:
- Slower than vision systems (not suited for 100% inspection)
- Cost per scan higher than other methods
- Material density affects image quality
- Requires operator expertise for interpretation
Specialized Micro Metrology
For the most demanding applications:
| Technology | Resolution | Application |
|---|---|---|
| Zygo NewView | Sub-nm | Optical surface quality |
| High-resolution CT | <5 µm | Internal features, micro-channels |
| SEM | 1-10 nm | Nano-scale features, surface analysis |
| AFM (Atomic Force Microscopy) | Sub-nm | Nano-topography |
For micro-optics: Peak-to-valley measurements of ~1 micron standard, down to 200 nm for critical imaging optics. Surface finishes of 80-100 angstroms standard, 20 angstroms for precision.
Measurement System Validation
Gage R&R Studies
Before trusting measurement data, validate the measurement system:
Gage Repeatability and Reproducibility (Gage R&R) quantifies:
- Repeatability — Same operator, same part, multiple measurements
- Reproducibility — Different operators measuring same parts
- Total measurement variation as percentage of tolerance
| Gage R&R Result | Interpretation | Action |
|---|---|---|
| <10% | Excellent | Acceptable for precision work |
| 10-20% | Acceptable | Use with caution |
| 20-30% | Marginal | Improve or use for screening only |
| >30% | Unacceptable | Do not use; measurement system needs work |
For micro molding: Target <10% Gage R&R. A measurement system with 30% R&R consuming 30% of your tolerance leaves only 70% for actual part variation.
Measurement Uncertainty
Every measurement has uncertainty. For micro molding:
| Factor | Typical Contribution |
|---|---|
| Equipment repeatability | ±0.5-2 µm |
| Temperature effects | ±0.1 µm per °C on steel |
| Operator technique | ±1-5 µm |
| Part fixturing | ±1-3 µm |
| Calibration uncertainty | Per certificate |
Best practices:
- Document measurement uncertainty for each inspection method
- Ensure uncertainty is small relative to tolerance (<10%)
- Control environment (temperature 20±1°C, humidity, vibration)
- Maintain calibration traceability to NIST
Process Validation
Medical device micro molding requires rigorous process validation per FDA 21 CFR Part 820 and ISO 13485.
IQ/OQ/PQ Protocol
| Phase | Purpose | Key Activities |
|---|---|---|
| IQ (Installation Qualification) | Verify equipment installation | Document machine specs, utilities, calibration records, software validation |
| OQ (Operational Qualification) | Define operating limits | DOE to establish process window, challenge extreme parameters, verify safety interlocks |
| PQ (Performance Qualification) | Confirm production capability | Extended runs (typically 3 lots), demonstrate Cpk ≥ 1.33, verify all acceptance criteria |
Process Capability Studies
Statistical process capability validates that the process can consistently meet specifications:
| Metric | Formula | Interpretation |
|---|---|---|
| Cp | (USL - LSL) / 6σ | Process spread relative to tolerance (assumes centered) |
| Cpk | Min[(USL - µ) / 3σ, (µ - LSL) / 3σ] | Accounts for process centering |
| Ppk | Same as Cpk using actual (not predicted) data | Actual performance over time |
Capability Requirements:
| Cpk Value | Yield | Application |
|---|---|---|
| 1.00 | 99.73% | Minimum acceptable |
| 1.33 | 99.994% | Standard requirement |
| 1.67 | 99.99994% | Critical dimensions |
| 2.00 | 99.9999998% | Six Sigma |
Design of Experiments (DOE)
DOE systematically varies process parameters to understand their effects:
Typical DOE factors for micro molding:
- Melt temperature (most significant effect on filling)
- Mold temperature (second most significant; affects crystallinity)
- Injection speed/pressure (critical for micro features)
- Packing pressure (affects shrinkage, sink marks)
- Packing time (significant for dimensional stability)
- Cooling time (affects crystallinity, cycle time)
Research finding: Studies show melt temperature and mold temperature consistently rank as the two most influential parameters for dimensional accuracy and part quality.
Statistical Process Control (SPC)
Control Charts
Monitor process stability during production:
| Chart Type | Best For | Sample Size |
|---|---|---|
| X-bar/R | Most applications | 3-5 parts per sample |
| X-bar/S | More sensitivity needed | ≥10 parts per sample |
| Individual/Moving Range (I-MR) | Slow processes, expensive measurement | 1 part |
| p-chart | Attribute data (pass/fail) | 50+ parts |
Control Limits vs Specification Limits
| Limit Type | Source | Purpose |
|---|---|---|
| UCL/LCL | Calculated from process data (±3σ) | Detect process shifts |
| USL/LSL | Customer specification | Define acceptable parts |
Process must be in control (within UCL/LCL) before assessing capability against specifications.
Out-of-Control Response
When control limits are exceeded:
- Stop and assess — Determine if special cause or false alarm
- Quarantine — Isolate suspect production
- Investigate — Find root cause (5-Why, fishbone diagram)
- Correct — Implement fix and verify effectiveness
- Document — Record cause, action, and outcome
- Resume — Continue production with increased monitoring
In-Process Monitoring
Cavity Pressure Monitoring
Real-time process insight through pressure sensors in the mold cavity:
| Metric | Indicates |
|---|---|
| Peak pressure | Fill completeness, packing |
| Pressure at gate seal | End of packing phase |
| Pressure curve integral | Total energy, part weight correlation |
| Pressure uniformity (multi-cavity) | Balanced filling |
Applications:
- Automatic part sorting (good/suspect based on pressure signature)
- Process trending and early warning
- Scientific molding optimization
- Mold protection (detect short shots, flash)
Process Parameter Monitoring
Log and trend critical parameters:
| Parameter | Target Variation |
|---|---|
| Melt temperature | ±2°C |
| Mold temperature | ±1°C |
| Injection pressure | ±2% |
| Cycle time | ±1% |
| Shot size | ±0.5% |
Inspection Strategies
Sampling Plans (AQL-Based)
| Inspection Level | Application | Typical Plan |
|---|---|---|
| 100% inspection | Launch phase, critical dimensions | Every part |
| Tightened | New process, quality issues | 2x normal sample |
| Normal (AQL 1.0) | Mature process | Per ANSI/ASQ Z1.4 |
| Reduced (AQL 2.5) | Proven history | 0.4x normal sample |
First Article Inspection (FAI)
Complete dimensional verification required before production release:
- Measure all drawing dimensions (not just critical)
- Document actual values vs. specification
- Calculate Cpk for critical dimensions (minimum 30 samples)
- Visual inspection to cosmetic standards
- Material certification verification
- Formal approval signature before production
Production Inspection
| Frequency | Dimensions | Method |
|---|---|---|
| Every shot | Visual, gross defects | Operator, camera |
| Per hour | Critical dimensions | Gage, vision system |
| Per lot | All dimensions | CMM, CT scan |
| Per shift | Process capability | SPC analysis |
Medical Device Requirements
FDA 21 CFR Part 820 Requirements
Key quality system requirements for medical device molding:
| Section | Requirement |
|---|---|
| 820.70 | Production and process controls |
| 820.72 | Inspection, measuring, and test equipment |
| 820.75 | Process validation |
| 820.90 | Nonconforming product |
| 820.184 | Device history record |
ISO 13485 Alignment
| ISO 13485 Section | Requirement |
|---|---|
| 7.5.6 | Validation of processes |
| 7.5.9 | Traceability |
| 7.6 | Control of monitoring and measuring equipment |
| 8.2.4 | Monitoring and measurement of product |
Cleanroom Requirements
For medical micro molding:
| ISO Class | Particles/m³ (≥0.5µm) | Application |
|---|---|---|
| ISO 7 (Class 10,000) | 352,000 | Standard medical |
| ISO 6 (Class 1,000) | 35,200 | Implants, drug delivery |
| ISO 5 (Class 100) | 3,520 | Critical implants |
Documentation Requirements
Device History Record (DHR)
For each production lot, maintain:
- Material lot numbers and certificates
- Process parameters (actual vs. setpoint)
- Inspection results (dimensional, visual)
- Operator and equipment identification
- Date/time stamps
- Non-conformance records
Traceability Requirements
| Level | Traceability |
|---|---|
| Material | Lot → supplier certificate → raw material source |
| Process | Part → machine, mold, cavity, process conditions |
| Inspection | Result → equipment, calibration status, operator |
| Release | Part → approval signature, date, criteria |
Quality Checklist
Ensure your quality system addresses:
Measurement:
- Measurement equipment selected for tolerance requirements
- Gage R&R completed (<10% preferred)
- Measurement uncertainty documented
- Environment controlled (temperature, humidity, vibration)
Process:
- Process validated (IQ/OQ/PQ complete)
- Process capability demonstrated (Cpk ≥ 1.33)
- DOE completed to identify critical parameters
- Process window documented
Control:
- Control plans established
- SPC implemented for critical dimensions
- In-process monitoring defined
- Reaction plans documented
Documentation:
- Inspection procedures documented
- Traceability system in place
- Non-conformance procedures defined
- Training records current
- DHR format established
Next Steps
- Review Defect Troubleshooting for quality problem resolution
- Return to Part Design if design changes are needed
- See Tooling & Mold Design for mold qualification requirements