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Surface Finishing for Injection‑Molded Parts: Texture, Polishing & Post‑Processing Options

Created on 05.21

Meta Description: A guide to injection-molded part surface finishing, comparing mold texture (VDI standards), polishing grades, and post-processing options, helping you choose the right finish for your product positioning.

Surface finishing is a critical step in injection molding—it enhances product appearance, improves functionality (e.g., scratch resistance, grip), and aligns with your product positioning (matte, texture, high-gloss). However, many customers struggle to choose the right finish: some select a finish that doesn’t match their application, while others face dimensional deviations after post-processing. This guide breaks down common surface finishing options, their benefits, and ideal applications to help you make the right choice.

1. Mold Texture (In-Mold Finishing)

Mold texture is the most common surface finishing method—textures are applied to the mold cavity, and the plastic part inherits the texture during molding. It’s cost-effective, consistent, and avoids post-processing dimensional deviations.

VDI Texture Standards (Common Grades)

VDI (Verein Deutscher Ingenieure) is the global standard for mold textures, with grades ranging from VDI 10 (smooth) to VDI 80 (rough). Common grades for injection molding:

• VDI 10-20: Smooth texture, close to polished finish (surface roughness Ra 0.2-0.8μm)—ideal for high-gloss parts (e.g., consumer electronics enclosures), processing cost: $500-$1,500 per cavity.

• VDI 30-40: Fine matte texture (Ra 1.0-2.0μm)—hides scratches and fingerprints, suitable for automotive interior parts, medical devices, processing cost: $300-$800 per cavity.

• VDI 50-60: Medium texture (Ra 2.5-4.0μm)—provides grip, ideal for handles, buttons, and industrial parts, processing cost: $200-$500 per cavity.

• VDI 70-80: Rough texture (Ra 5.0-8.0μm)—used for non-decorative parts (e.g., internal components) or parts requiring adhesion for post-processing, processing cost: $100-$300 per cavity.

2. Mold Polishing (High-Gloss Finishing)

Mold polishing involves polishing the mold cavity to create a high-gloss, mirror-like finish on the part. It’s ideal for parts requiring a premium appearance (e.g., consumer electronics, cosmetic packaging).

Polishing Grades (From Low to High):SPI B1-B3: Basic polishing (Ra 0.8-1.6μm), slight gloss—suitable for non-decorative parts, processing cost: $200-$500 per cavity.SPI A1-A3: High-gloss polishing (Ra 0.02-0.2μm), mirror finish—ideal for premium decorative parts, processing cost: $800-$2,000 per cavity.

Note: High-gloss polishing requires high-precision mold manufacturing—any mold surface defects (e.g., scratches, pits) will be visible on the part.

3. Post-Processing Finishing (Secondary Treatments)

For parts requiring additional functionality or appearance, post-processing is an option. Common post-processing methods:

Painting & Spraying

Provides a uniform color and improves scratch resistance. Ideal for parts requiring custom colors (e.g., automotive parts, consumer products). Choose between matte, semi-gloss, or high-gloss paint.

Screen Printing & Pad Printing

Used to add logos, text, or patterns to the part. Suitable for small-batch or custom parts (e.g., medical devices, electronic components).

Laser Marking

Creates permanent marks (logos, serial numbers) without damaging the part. Ideal for high-precision parts (e.g., medical implants, electronic components) and parts requiring traceability.

Coats the part with a metal layer (e.g., chrome: $1.50-$3.00 per piece, nickel: $1.00-$2.00 per piece, gold: $5.00-$10.00 per piece) to improve appearance and corrosion resistance. Suitable for premium parts (e.g., decorative components, automotive trim). Note: Plating requires pre-treatment (degreasing, etching) to ensure adhesion.

Coats the part with a metal layer (e.g., chrome, nickel) to improve appearance and corrosion resistance. Suitable for premium parts (e.g., decorative components, automotive trim).

How to Choose the Right Surface Finish

• For decorative parts: Choose VDI 10-20 texture or SPI A1-A3 polishing, with optional painting/plating.

• For functional parts: Choose VDI 30-60 texture for grip or scratch resistance; avoid high-gloss finishes that show wear.

• For medical/industrial parts: Choose VDI 30-40 fine texture for easy cleaning; avoid porous textures that trap dirt.

Our team can help you select the right surface finish, leveraging our mold texture and post-processing capabilities to ensure your part meets your product positioning and functional needs.