Plastic Surface Finishes for Automotive Lighting: The Engineer’s Guide

Why Surface Finish Matters for Automotive Lighting

• Optics first. Lenses and light guides need ultra‑smooth surfaces (Ra ≤ 10–20 nm) so light stays clean and bright. Bezels and housings can use micro‑textures to hide flow lines and weld marks.

• Premium feel. A consistent satin or fine VDI texture makes parts look expensive and hides small scuffs from handling.

• Real‑world durability. The right micro‑topography camouflages scratches and fingerprints.

• Smoother production. Match draft to texture and parts eject cleanly—no blush, no drag.

Designing headlamps or tail lamps? Treat material + finish + gating/cooling as one system from day one. See Auto lamp mold and Injection molding.

Common Finish Standards (SPI, VDI, Mold‑Tech)

Finish Selection by Resin & Part Function

Quick notes—always confirm with supplier plaques on your exact resin and color.

Texture vs. Gloss: Design Trade‑offs

• High gloss looks premium but exposes sink, waviness, and dust. Requires a stable window and careful handling.

• Matte/satin hides defects and fingerprints but may increase ejection friction.

Draft allowance vs. texture depth (rule‑of‑thumb)

How Finish Is Made on the Mold Surface

1. CNC machining builds the shape. Finer scallops mean higher potential gloss.

2. Stoning/sanding erases tool marks and sets the pre‑polish baseline.

3. Diamond polishing (SPI A) climbs through compounds to reach mirror.

4. EDM delivers controlled matte that maps to VDI values.

5. Chemical etch/laser texture creates repeatable grains and patterns (Mold‑Tech, VDI).

6. Verification uses interferometry, gloss and haze meters, and replica tape for texture depth.

Care tip: etched surfaces dislike abrasive cleaning. Use mild detergent and lint‑free cloths. Store optical cavities with anti‑rust paper in a clean area.

Tolerances, DFMA, Risk Control

• Draft early. Add texture‑driven draft in CAD, not at T1.

• Keep A‑surfaces clean. Move gates/ejectors off glossy areas; for optics use sleeve or air ejection.

• Parting‑line discipline. Align textures across splits; laser blend where needed.

• Hide the weld. Combine micro‑texture, balanced flow, and packing control.

• Plan for repairs. Use inserts so etched areas can be re‑worked locally.

Two‑Shot Parts: Seam Control, Light Leakage

• Align texture across both shots to keep a consistent look.

• Shut‑off precision prevents whitening and flashing.

• Resin pairing + flow tuning reduces interface stress and knit issues.

• Local masking/micro‑texture blends the boundary where needed.

Learn more: Two‑shot mold.

Optical Parts: A‑grade Surfaces for Lenses and Light Guides

• Ra/Haze targets: Ra ≤ 10–20 nm for light guides and lenses; control haze and stray reflections.

• Steel: S136/420SS or Ni‑P plated inserts for polish retention and corrosion resistance.

• Polishing ladder: 600/800# → diamond 3 µm → 1 µm → 0.25 µm; verify with white‑light interferometry and haze testing.

• Processing: Proper drying, low‑shear gating, and uniform mold temperature to minimize birefringence.

• Cleanliness: Handle in Class‑10k‑level facilities with clean packaging to avoid dust‑encoded defects.

Related: Auto lamp mold.

Metrology, Validation, Maintenance

Key metrics: Gloss (GU), Haze (%), Ra/Rz (nm/µm), texture depth (µm), and ΔE (color). Tools: White‑light interferometer, haze meter, gloss meter, replica tape for texture depth.

1. Compare first‑off parts to the master plaque/golden sample.

2. Record Ra, GU, haze, and ΔE under specified conditions.

3. For optics, re‑inspect under the intended lighting environment.

4. For textured parts, sample texture depth and uniformity.

5. Maintenance: gentle scheduled cleaning; re‑etch/laser re‑texture after end‑of‑life thresholds.

Cost Impact, Lead Time, ROI

• Where cost comes from: polishing hours, laser/etch lead time, and insertability for repairs. Optics may need cleaner rooms and packaging.

• Where you save: use texture to skip painting, add generous radii to shorten polishing, and build modular inserts for fast refurbishment.

• Why it pays: the right finish reduces scrap and rework, prevents warranty pain, and lifts perceived quality—especially bezels and touch areas.

Case Study: Tail Lamp Bezel Texture Upgrade

Problem. Under showroom lights, a satin bezel showed faint weld‑line ghosts.
Fix. Switched to VDI 21 micro‑grain, added +1° draft, re‑balanced the runner, and blended the texture across the split.
Result. Weld marks disappeared to the eye; scrap fell from 3.2% → 0.6%; paint step removed.

Practical Checklist for Designers

• On the drawing, specify standard + code + Ra/texture depth.

• Add extra draft for textures and simulate ejection early.

• Keep A‑surfaces free of gates/ejectors; agree a clear repair plan.

• Build master plaques with the actual resin and lighting conditions.

• For optics, define cleanliness, packaging, and maintenance rules.