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Metal Production Methods: A Guide to Finished Product Surface Finish

Introduction

A part’s surface finish (also known as surface roughness) not only affects its appearance and texture but also has a direct impact on its wear resistance, corrosion resistance, and fit accuracy. Different production processing methods can achieve vastly different initial surface finishes. Understanding these differences will help you set appropriate quality standards and plan necessary post-processing steps.

Detailed Explanation of Surface Finish by Method

Casting
The surface finish of a casting depends primarily on the mold material and production method.

Sand Casting: This method produces the roughest surface, with a roughness of approximately 12.5–50 μm. The finished surface exhibits a noticeable sand grain texture and typically requires post-processing such as sandblasting, grinding, or CNC machining to achieve a smooth appearance.

Shell Mold Casting: The finish is superior to traditional sand casting, with an Ra of approximately 6.3–25 μm. While still rough, it allows for better detail rendering.

Gravity/Low-Pressure Casting (GLC): Using metal molds, this method significantly improves surface finish, achieving Ra values of 3.2–6.3 μm, which is sufficient for many industrial applications.

Die Casting (Die Casting): Produces a very smooth surface, with Ra values of approximately 1.6–3.2 μm. This surface quality approaches the aesthetic requirements of consumer electronics.

Forging
Hot-forged parts have a layer of oxide scale on their raw surface, resulting in a poor surface condition; cold-forged parts, on the other hand, have a smoother surface. Forged parts focus on internal strength, and their surfaces typically require subsequent CNC machining to achieve final dimensional and finish requirements.

Metal Injection Molding (MIM)
The surface finish of MIM products is comparable to that of die-cast parts, with Ra values of approximately 1.6–3.2 μm. For smaller parts requiring even higher gloss, further polishing is possible.

Sheet Metal Forming & Extrusion
The surface finish of this process depends primarily on the quality of the raw material (sheet metal or extrusion die). The initial surface is typically quite smooth (Ra ≤ 1.6–3.2 μm) and can be directly processed by anodizing, painting, or brushing to achieve the desired final appearance.

CNC Machining
CNC machining offers excellent surface finish, with Ra values typically ranging from 0.8–3.2 μm. By controlling the tool path, speed, and feed, matte or glossy finishes can be achieved, offering the most flexible and precise surface quality control of all methods.

Key Concepts

Surface Finish/Roughness (Ra): Ra, short for “arithmetic mean roughness,” is the most commonly used indicator internationally to assess surface smoothness. It measures the average deviation in height at each point along a surface profile. A lower Ra value indicates a smoother and more even surface, while a lower Ra value indicates a rougher surface.

Conclusion

From the coarsest sand casting to the finest CNC machining, the surface finish achieved by different methods varies significantly. During the initial design phase, a mass-production method that meets basic appearance requirements should be selected based on the product’s positioning and budget. If a more advanced finish is desired, budget and allowance should be made for subsequent surface treatments such as polishing, sandblasting, painting, or electroplating.