Clear Plastic Injection Molding – PMMA & PC Optical Parts
Injection molding of transparent PMMA (acrylic) and optical-grade polycarbonate parts with controlled weld lines, polished tooling and dryness discipline. Lenses, light guides, windows and display components at 90%+ light transmittance – from our China workshop.
Free Consultation90%+
Light Transmittance
±0.02mm
Dimensional Tolerance
SPI A2
Tooling Polish
3-4 Weeks
Prototype Lead Time
Materials
| Grade | Properties | Typical Applications |
|---|---|---|
| PMMA (Acrylic) | Light transmittance 92-93%, the highest of common plastics. Hard, scratch-resistant surface, excellent weathering. Not impact-resistant – brittle compared to PC. Requires thorough drying (2-4h at 80-100°C) and high mold temperatures. | Lenses, light guides, display windows, cosmetic packaging, signage, optical diffusers |
| PC (Polycarbonate, Optical Grade) | Light transmittance 87-89%, outstanding impact strength (virtually unbreakable), service temperature to 120°C. Hygroscopic – must be dried to <0.02% moisture or hydrolysis degrades the chain. Tighter process window than PMMA. | Impact windows, safety shields, automotive lenses, LED covers, medical device windows |
| GPPS / HIPS (Clear Polystyrene) | Transmittance 88-90%, low cost, easy to process, crystal clarity. Brittle and poor in weathering – best for indoor, short-lifecycle or cost-driven clear parts. | Display boxes, cosmetic packaging, clear trays, stationery, sample containers |
| PETG / PET (Clear) | Excellent clarity with better chemical resistance than PMMA, good toughness, FDA food-contact grades available. Shrinks more than PMMA; PET preform grades are the standard for bottle applications. | Bottles, containers, medical packaging, food-contact clear parts, display parts |
| TPU (Clear Grade) | Transparent thermoplastic polyurethane combining clarity with elasticity and abrasion resistance. Yellows under UV unless stabilized. Bonds well to ABS and PC substrates. | Clear overmolds, protective covers, flexible windows, wearable device straps |
| COC / COP (Cyclic Olefin) | Specialty optical resins with very low birefringence and high heat resistance, used where optical distortion matters. Higher cost; quoted case by case. | Precision optical components, medical diagnostic parts, lens arrays |
Process Parameters
| Parameter | Standard Range |
|---|---|
| Light Transmittance (3mm plaque) | PMMA 92-93%; PC 87-89%; GPPS 88-90% |
| Haze (Optical Grades) | ≤1% PMMA; ≤2% PC optical grade |
| Dimensional Tolerance | ±0.02mm standard; ±0.01mm on critical features |
| Wall Thickness Range | 0.8mm – 4mm; uniformity is more critical than the value itself |
| Mold Temperature (PMMA) | 80-100°C – high mold temperature is mandatory for flow and gloss |
| Mold Temperature (PC) | 80-120°C depending on grade and geometry |
| Material Drying Requirement | PMMA 2-4h @ 80-100°C; PC 3-4h @ 120°C (moisture <0.02%) |
| Tooling Surface Finish | SPI A2 (grade #6000 diamond polish) standard; SPI A1 for optical surfaces |
| Maximum Part Size | 400mm x 300mm depending on tonnage |
| Prototype Tooling Lead Time | 3-4 weeks single-cavity polished tool; 5-6 weeks production |
Surface Finishes
Polished (SPI A2 / A1)
The tool cavity is diamond-polished to SPI A2 (Ra ≤0.012μm) or A1 optical grade, so the molded surface needs no post-processing. Polish level directly determines clarity and gloss – any tool mark replicates into every part.
Vapor Polishing (Post-Mold)
Chemical vapor treatment that smooths machined or textured surfaces of PMMA/PC and restores optical clarity at cut edges and machined features. Useful for prototypes and for parts machined after molding.
Anti-Reflective & Hard Coating
AR coatings reduce surface reflection from ~4% to <1% per surface; hard coats add scratch resistance to PMMA windows. Applied by coating partners after molding, on clean, polished surfaces.
Anti-Fog & UV Treatments
Anti-fog coatings for windows and shields in humid environments; UV-stabilized grades and coatings for outdoor exposure where uncoated PMMA would craze and yellow.
Textured / Frosted
Light-diffusing matte or frosted texture molded directly into the surface for LED covers, indicator windows and privacy panels – texture replaces polish and hides fingerprints and scratches.
Printing & Coating
Pad printing, screen printing or vacuum metallizing on clear parts for logos, graduation marks and reflective bezels. Flexible inks formulated for PMMA/PC adhesion are used and cure-tested.
Design for Manufacturing (DFM)
- Specify uniform wall thickness – optical parts punish thickness variation: differential shrinkage creates lens distortion, visible as waviness in windows and focus shift in lenses
- Design gates and flow so weld lines never cross an optical zone: a weld line scatters light and is always visible through clear material. We position gates at the part edge, never mid-lens
- Add generous draft (1.5-3°) on polished optical surfaces: polished tools resist ejection, and drag marks on a lens surface are scrap, not rework
- Avoid deep draws with high polish: polishing access into the tool limits how deep a cavity can be polished to SPI A2. Split complex optics into simpler mating components
- Keep ribs and bosses thin (≤60% of wall) and few: thick ribs sink-mark through clear material and distort optical surfaces
- Eliminate all release agent contact on optical cavities – silicones migrate and cause crazing and bond failure. Tooling is maintained with dry-cleaning protocols only
- Plan material drying into every production plan: undried PMMA splay-silvers and undried PC hydrolyzes – both defects are invisible at the gate and obvious in the finished part
- Vent thoroughly along the parting line and at flow ends: trapped air in a clear part shows as burn marks or silver streaks that ruin optical clarity
- For light guides, design the light-extraction features (prisms, dots, V-cuts) in the tool, and let us review total internal reflection geometry before steel is cut
- Mark the optical zone explicitly on the drawing – which surfaces are functional optics and which are hidden structure – so inspection and defect criteria match your intent
Tooling & Capacity
| Type | Lead Time | Suitable For |
|---|---|---|
| Single-Cavity Polished Prototype Tool | 3-4 weeks | Optical prototypes and pilot runs; SPI A2 polish, allows geometry and optical validation before production tooling |
| Multi-Cavity Production Tool (Hot Runner) | 5-6 weeks | Volume production of lenses, windows and covers; sequence-valve gates control fill and weld-line position cavity by cavity |
| Optical Component Tooling | 5-7 weeks | Light guides, lens arrays and precision optical parts with polished or diamond-turned inserts |
| Clear Substrate Tooling for Overmolding | 4-6 weeks | Clear substrates (PC/PMMA) that receive elastomer overmolding for combined transparency and soft-touch function |
Quality Assurance
- Optical inspection under backlit and dark-field conditions on every batch: weld lines, flow marks, silver streaks, bubbles and inclusions are graded against an approved boundary sample
- Light transmittance measurement with a spectrophotometer on reference parts from each material batch
- Haze measurement where diffusion or clarity is specified
- Surface quality grading against a defect catalog (scratch standard, sink standard, flow-line standard) agreed at reference-sample approval
- Dimensional inspection (CMM) on mounting features – clear parts are usually positioned optically, so mounting geometry carries the tolerance
- Polish level verification of tooling before production release, re-checked at defined shot counts as polish wears
- Material dryness discipline: dew-point and drying-time records maintained per batch; dried-material hopper coverage enforced
- Birefringence check where stress-induced distortion matters (lenses, polarizing applications)
- Adhesion and cure test for coated or printed surfaces (cross-hatch tape test, abrasion rub test)
- Batch traceability: material lot, drying record, tool number and shot count retained for every production run
Applications
- Lighting: LED lenses, light guides, diffusers, covers, streetlight and automotive lamp components
- Displays and instrumentation: windows, touch-panel covers, gauge faces, screen protectors
- Consumer electronics: transparent housings, camera windows, speaker grilles, wearable lenses
- Medical devices: fluid windows, specimen viewers, diagnostic device covers, sterile barriers
- Automotive: signal lenses, sensor windows, interior trim lenses, headlamp components
- Safety equipment: face shields, goggles, machine guards, protective windows
- Packaging: cosmetic jars, clear boxes, premium containers, sample displays
- Industrial: sight glasses, level indicators, flow windows, machine covers
Why Choose FHY Precision
Optical process discipline: dried-material protocols, high mold temperatures, silicone-free tooling maintenance and vented polished tooling – the four things that separate clear molding from ordinary molding
Weld-line control by design: gate and flow simulation for every clear part so the weld line lands at the edge, never across the optical zone
Polished tooling in-house: SPI A2 diamond polishing is standard, with polish verification and re-check schedules during production runs
Both PMMA and PC expertise: we mold the high-clarity, scratch-resistant choice and the high-impact choice, and we will tell you honestly which your application needs
Optical verification equipment: spectrophotometer transmittance and haze measurement, backlit inspection benches, agreed defect catalogs – clarity is measured, not eyeballed
Reference-sample approval before production: boundary samples for scratches, sinks and flow lines are approved by you, so "clear enough" is defined once, in writing
Post-molding options: vapor polishing, AR/hard/anti-fog coatings and printing through qualified partners, on surfaces we prepared
Fast optical prototyping: single-cavity polished tools get functional clear parts in your hands in 3-4 weeks for optical and fit validation
FAQ
1. PMMA or polycarbonate – which material should I choose?
Choose PMMA (acrylic) when clarity and surface hardness dominate: 92-93% transmittance, scratch-resistant, excellent weathering, lower cost. Choose PC when impact resistance dominates: virtually unbreakable, 87-89% transmittance, service to 120°C, but softer and prone to scratching without hard coat. Many products use both – PMMA where it is seen, PC where it is stressed. We will recommend per function.
2. Why do clear parts show weld lines and how do you prevent them?
Weld lines form where two flow fronts meet after passing around a hole or dividing at the gate. In clear material the meeting line scatters light and is always visible. Prevention is flow design: gate position and count are chosen by simulation so fronts meet at the part edge, outside the optical zone. Where geometry forces a weld line, we will show you where it lands before tooling – sometimes a design change moves it.
3. What causes silver streaks and cloudiness in clear molded parts?
Silver streaks are usually moisture: undried PMMA or PC vaporizes at the melt front and leaves streaks. Cloudiness (haze) typically comes from too-low mold temperature, contamination, or degraded material from excessive shear heat. Our controls: enforced drying cycles with dew-point records, mold temperature verified per material, and clean-room-style material handling with no regrind unless you approve it.
4. What light transmittance can you achieve?
On 3mm plaques: PMMA 92-93%, PC optical grade 87-89%, GPPS 88-90%. For finished parts, transmittance depends on wall thickness and surface condition – every surface reflection costs about 4%, so thick multi-surface optics transmit less than the material number suggests. We measure and report actual values on reference parts.
5. Can you mold clear ABS or clear nylon?
Clear ABS does not exist as a true optical material – ABS is inherently opaque; "transparent ABS" grades are translucent at best and yellow over time. Clear polyamide (PA12 amorphous) exists but has lower clarity and moisture sensitivity. For genuine transparency we steer customers to PMMA, PC, GPPS or PETG, which are proven optical materials.
6. What tolerances are achievable on clear optical parts?
±0.02mm standard, ±0.01mm on critical mounting and positioning features. Note that optical performance depends on wall uniformity as much as dimensions: a lens within dimensional tolerance can still distort if wall thickness varies. We control thickness with simulation and tool design, and verify with sectioned samples on critical optics.
7. Do you offer anti-reflective or hard coatings?
Yes, through qualified coating partners. AR coating cuts surface reflection from about 4% to under 1% per surface – valuable for displays, shields and multi-surface optics. Hard coating restores scratch resistance to PC windows. We prepare the surfaces (polish, cleanliness) before coating and adhesion/abrasion test the results.
8. How do you inspect clear parts for defects?
Every batch is inspected under backlit and dark-field lighting against a boundary sample that you approved before production. The catalog covers scratches, sink marks, flow lines, bubbles, inclusions, weld lines and discoloration – each with accept/reject limits agreed in advance. Transmittance and haze are measured with instruments on reference parts, not judged by eye.
9. Can you machine clear parts after molding?
Yes – PMMA and PC machine well, and vapor polishing afterward restores optical clarity at cut edges. This is common for prototypes and for parts needing post-molded holes or threads. Machined features are specified to ±0.05mm, and we will note which surfaces must remain untouched to preserve optical quality.
10. What is the minimum order quantity for clear molded parts?
Production MOQ is typically 1,000 pieces with steel tooling. Single-cavity polished prototype tools are available for 50-100 piece pilot runs in 3-4 weeks – useful for optical validation, trade shows and early customer samples. We quote pilot and production routes side by side.
11. How do you handle light guides specifically?
Light guides are total-internal-reflection devices: the extraction geometry (prisms, dots, V-cuts) and the input face finish determine uniformity. We review your optical geometry before tooling, polish the input face to optical grade, control material dryness strictly (yellowing and haze kill guide efficiency), and can supply test reports on luminance uniformity for pilot runs.
12. What lead times should I plan for clear molded parts?
Single-cavity polished prototype tools: 3-4 weeks plus trial and reference-sample approval. Production multi-cavity tools: 5-6 weeks. After tool release, production runs complete in 2-4 weeks depending on quantity. Coating and printing add 3-7 days through our partners.
13. How dry do PC and PMMA need to be before molding?
Very dry – as little as 0.02% moisture causes silver streaks, bubbles and haze in clear parts. PC needs 120°C for 4-6 hours; PMMA needs 80-90°C for 3-4 hours. Use a dehumidifying dryer with filtered air, not a hot-air dryer that reintroduces ambient humidity. Keep dried material sealed in a closed hopper and clean conveying lines regularly. If a production stop occurs, drop barrel temperature below about 160°C to limit degradation while material waits.
14. What mold finish do clear parts require?
The cavity copies itself onto the part – every machining mark, EDM texture and polishing defect transfers. Optical and high-clarity surfaces call for mirror polishing to SPI A1-A2 grade, cavity roughness below Ra 0.8μm and in many cases much finer. Polishing cannot fix a bad gate location or trapped gas, though: gate position, venting and flow balance are designed in before steel. For non-optical surfaces, polishing to an unnecessarily high grade adds cost without function – we agree which surfaces are optical and which are merely cosmetic before tooling.
15. How do you minimize weld lines in clear parts?
Weld lines form where melt fronts meet and are especially objectionable in clear parts. The strongest lever is gate design: fan gates spread flow across flat parts, diaphragm or ring gates suit round components, and gate position is chosen so flow fronts meet away from critical viewing or optical areas. Where a weld is unavoidable, raising mold temperature and injection speed promotes molecular diffusion across the interface, improving strength and visibility. Mold flow simulation before steel predicts weld location and lets us move it deliberately.
16. Should clear parts be annealed after molding?
Often yes for PC. Residual stress may not be visible initially but causes cracking later, especially when the part contacts cleaning agents or chemicals. Annealing means heating to just below the deflection temperature – around 80°C for PC – and cooling slowly, roughly one hour per millimeter of thickness. The trade-off is cycle time and handling, so we apply it where function demands: parts facing chemical exposure, assembly loads or field cracking risk. We will recommend annealing case by case, not blanket-apply it.