A Designer's Guide to Part Aesthetics: Surface Textures, Finishes, and Weld Lines

Specifying Surface Textures and Finishes

When a mold is first machined, the steel cavity has a raw, tool-marked finish. The texture of your final part is created by applying a secondary finish to that steel surface. These are typically specified using industry standards, like the SPI (Society of the Plastics Industry) finish chart.

In simple terms, they range from:

• High Polish (SPI-A): A flawless, mirror-like finish created by diamond buffing. This is used for optically clear parts like lenses but is very expensive and will show any microscopic defect.
• Semi-Gloss (SPI-B): A smooth finish created by fine-grit sandpaper. Good for parts that need to look clean and smooth without the cost of a perfect polish.
• Matte (SPI-C): A fine, bead-blasted texture. This is one of the most common finishes as it provides a clean, non-glossy look and is excellent at hiding fingerprints and minor imperfections like sink marks.
• Textured (SPI-D): A rougher, dry-blasted finish.

The most important takeaway for designers: The more texture you add to a surface, the more draft angle it requires. The texture creates millions of tiny undercuts, and without sufficient draft, the finish will be scraped and dragged during ejection.

Managing Weld Lines: The Unavoidable Artifact

A weld line is a faint line on a part that forms where two or more fronts of molten plastic meet and fuse together inside the mold. This happens when the plastic has to flow around an obstacle, like a hole or a core pin, and then rejoins on the other side.

It's critical to understand that weld lines are a natural part of the molding process and usually cannot be entirely eliminated. They are a cosmetic issue, not typically a structural one (though they can be a weak point in some high-stress applications).

Our goal isn't to eliminate them, but to control where they form. By strategically placing the gate, we can influence the plastic's flow path and push the resulting weld lines to less visible, non-cosmetic areas of the part, effectively hiding them from the end user.

What's Next?

With aesthetics covered, it's time to build the hidden structure that makes your product robust. In our next guide, we cover the workhorses of plastic part design: How to Design Functional Features: Ribs and Bosses.