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Paint and Coating Selection for Plastic Parts

Painting and coating plastic parts is one of the most quality-sensitive steps in manufacturing. Unlike metal, plastic surfaces are low-energy and chemically inert — meaning paint does not bond easily.

This guide covers key coating technologies available in China, what to specify in your drawings, and how to verify quality before shipment.

Why Plastic Coating Is Different

Plastic has low surface energy (30-40 dynes/cm for untreated PP). Below 38 dynes/cm, flame/corona treatment or primer is required. Most Chinese suppliers skip this unless specified.

Common Coating Systems

Coating Type	Best For	Durability	Cost/M² (China)
Solvent-based 2K PU	Automotive, consumer goods	★★★★★	¥8-15
Water-based acrylic	Medical, toys	★★★☆☆	¥5-10
UV-curable coating	High-gloss electronics	★★★★☆	¥12-25
Rubberized/soft-touch	Power tools, handles	★★★☆☆	¥10-20
EMI/RFI shielding	Electronic enclosures	★★★★☆	¥15-30
Anti-scratch hard coat	Optical, displays	★★★★★	¥18-35

Buyer's Tip: Request a cross-hatch adhesion test (ASTM D3359) on the first article. This tape test reveals if paint has bonded properly. Also ask for solvent rub tests to verify cure. Many suppliers accelerate curing by raising oven temperature, causing micro-cracking that appears weeks later.

Surface Preparation

Degrease → DI water wash → dry → surface treatment (flame/corona/plasma) → primer → paint → cure. Mold release agents are the #1 cause of paint failure.

Surface Energy Targets

Plastic	Min Dynes/cm	Treatment
PP	42+	Flame or corona
PE	42+	Corona or plasma
ABS	38+	Primer sufficient
PC	38+	Solvent wipe + primer
PA (Nylon)	40+	Plasma or chemical etch
POM (Acetal)	44+	Chemical etching required

QC Checklist

Adhesion: Cross-hatch test ASTM D3359, Class 4B+
Gloss: Glossmeter at 60°
Color: Delta E ≤ 1.0 under D65
Hardness: Pencil hardness ASTM D3363
Environmental: Thermal shock, humidity, UV

What This Means for Your Project: For a typical ABS/PC-ABS enclosure, budget ¥8-15 per part for 2K PU coating. Ensure factory documents surface energy readings. Request 48-hour humidity test on pre-production samples. Avoid solvent-based paints on PC — use water-based or UV-curable systems instead.

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