A Designer's Guide to Part Aesthetics: Surface Textures, Finishes, and Weld Lines
By Ichiplas Design Engineering Team · Oct 2025 · Updated Nov 2026 · Reviewed by the Ichiplas engineering team
Functionality is paramount, but aesthetics are what connect a customer to your product. The final look and feel of an injection-molded part isn't an accident; it's a series of deliberate design choices.
Understanding how to specify surface finishes and how to manage the unavoidable cosmetic artifact known as a “weld line” is key to achieving your aesthetic vision.
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 — see our mold design and fabrication services for the full range of options. 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 matte finish created by polishing the steel with abrasive stones. It gives a clean, non-glossy look and is good at hiding fingerprints and minor tool marks, which makes it a common choice for industrial and internal parts.
- Textured (SPI-D): A satin-to-rough texture created by dry blasting the steel with glass bead or aluminium oxide. This is one of the most common finishes for consumer housings because it hides sink marks, flow lines and handling scratches well.
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.
SPI Mold Finish Chart
The SPI (now PLASTICS Industry Association) finish standard is the most common way to specify a mold surface in North America. Each grade has a typical surface roughness range.
| SPI grade | Finishing method | Typical Ra (µm) | Typical use |
|---|---|---|---|
| A-1 | Grade #3 diamond buff | 0.012 – 0.025 | Lenses, mirrors, optical parts |
| A-2 | Grade #6 diamond buff | 0.025 – 0.05 | High-gloss cosmetic parts |
| A-3 | Grade #15 diamond buff | 0.05 – 0.10 | Glossy housings and covers |
| B-1 | 600-grit paper | 0.05 – 0.10 | Smooth, medium-gloss parts |
| B-2 | 400-grit paper | 0.10 – 0.15 | Semi-gloss consumer parts |
| B-3 | 320-grit paper | 0.28 – 0.32 | Low-gloss, hides minor tool marks |
| C-1 | 600-grit stone | 0.35 – 0.40 | Matte industrial parts |
| C-2 | 400-grit stone | 0.45 – 0.55 | Matte, hides fingerprints |
| C-3 | 320-grit stone | 0.63 – 0.70 | Non-cosmetic and internal parts |
| D-1 | Dry blast, glass bead | 0.80 – 1.00 | Satin texture |
| D-2 | Dry blast, #240 oxide | 1.00 – 2.80 | Dull texture, hides sink and scratches |
| D-3 | Dry blast, #24 oxide | 3.20 – 18.0 | Rough, grippy texture |
Finer finishes cost more to produce and maintain, and expose every flow mark and sink. Coarser finishes need more draft. European projects often specify VDI 3400 or a Mold-Tech pattern instead.
Frequently Asked Questions
What is the difference between SPI and VDI 3400 finishes?
SPI is a polish-based standard common in North America, graded A (diamond buffed) through D (dry blasted). VDI 3400 is a German standard for spark-eroded (EDM) textures, numbered roughly 12 to 45, where higher numbers are rougher. Both describe mold surface finish; which one you use usually depends on your market and your design team's convention.
Which mold finish hides sink marks and scratches best?
Textured finishes such as SPI C-2 to D-2, VDI 3400 grades in the high 20s to 30s, or a Mold-Tech grain hide sink marks, flow lines, fingerprints and handling scratches far better than gloss finishes. High-polish A-grade finishes show every imperfection.
What finish should I specify for a clear plastic part?
Optically clear parts in PC or PMMA normally need an SPI A-1 or A-2 diamond-buffed finish on the light-transmitting faces, molded in a polishable stainless mold steel. Non-optical faces can use a cheaper finish to save polishing cost.
Can weld lines be completely eliminated?
Usually not, wherever plastic flows around a hole or core pin. They can be made much less visible by raising melt and mold temperature, improving venting and moving the gate so the weld line forms in a non-cosmetic or low-stress area. Textured surfaces and darker colors also make them harder to see.
How do I specify surface finish on a drawing?
Call out the finish standard and grade for each surface — for example 'SPI B-2 on show surfaces, SPI C-3 elsewhere' or a specific Mold-Tech or VDI 3400 reference — and mark cosmetic faces clearly. Ask your molder for finish sample plaques in your chosen resin, because the same finish looks different in different materials and colors.
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.
Getting your design ready for production can feel complex. Our team in Johor Bahru lives and breathes this process every day. If you want to ensure your design is optimized for manufacturing from the start, reach out to us. We're here to help.
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