Die Casting Surface Finishes

Tweeler AB is a precision aluminium die casting company based in Stockholm, Sweden, specialising in custom mould design, HPDC, re-engineering, and global logistics.

The surface finish on an aluminium die casting affects appearance, corrosion resistance, hardness, and in some cases electrical properties. Choosing the right finish — or combination of finishes — is as important as alloy selection. This guide covers the most common post-casting surface treatments and helps you match them to your application.

As-Cast Surface

Fresh from the die casting mould, aluminium parts have a smooth metallic surface with parting line flash, ejector pin marks, and gate witness marks removed by deburring and trimming. As-cast surfaces typically have a roughness of Ra 1.6–3.2 µm. For many internal or non-visible components, the as-cast surface is entirely acceptable and requires no further treatment.

Shot Blasting

Shot blasting propels small steel or glass beads at the part surface at high velocity, creating a uniform matte texture and removing surface oxides. It is usually the first step before any coating process, as it improves coating adhesion.

Anodizing

Anodizing is an electrochemical process that converts the aluminium surface into a hard, dense aluminium oxide layer. It significantly improves corrosion resistance and surface hardness, and can accept dyes for colour. There are three main types:

• Clear / Type II anodize — 8–12 µm layer. Good corrosion resistance and a natural aluminium appearance.
• Colour anodize — Type II with an added dye step. Black, gold, bronze, red, blue and other colours. UV-stable for outdoor use.
• Hard anodize / Type III — 25–50 µm layer. Extremely hard surface for wear-resistant applications.

Note: not all HPDC alloys anodize equally well. High-silicon alloys like ADC12 produce a darker, less consistent anodize than low-silicon alloys like A360.

Powder Coating

Powder coating applies a dry polymer powder electrostatically and then cures it in an oven, forming a continuous, thick coating (typically 60–100 µm). It provides excellent corrosion resistance, a wide range of colours and textures, and is highly UV-resistant for outdoor applications. It covers parting lines and surface defects well and is cost-effective for high volumes.

E-coating (Electrocoating)

E-coating deposits a thin, even paint layer (15–25 µm) using an electric current in a liquid paint bath. Because the coating is applied electrically, it reaches into recesses, holes, and internal surfaces that spray painting cannot access. Commonly used as a primer under powder coating or paint, particularly in the automotive industry.

Chemical Conversion Coating (Chromating / Alodine)

Chemical conversion coating applies a thin chemical layer that improves corrosion resistance and paint adhesion without adding significant thickness. Used as a pre-treatment for painting or as a standalone protective layer for electronics applications. RoHS-compliant trivalent chromate versions are available.

Choosing the Right Finish