Key takeaways
- A molded part copies the surface of the steel, so the finish is a tooling decision that is expensive to change later.
- SPI grades A, B, C and D cover glossy, semi-glossy, matte and blasted finishes; VDI 3400 and Mold-Tech cover deeper textures.
- Texture needs extra draft, commonly about 1 to 1.5 degrees for every 0.001 inch (0.025 mm) of texture depth.
- Gloss shows sink, flow lines and fingerprints; texture hides them, so most consumer housings use texture on the visible faces.
Injection molding surface finish is specified on the mold, not the part. Every molded part copies the surface of the steel it was formed against, so a mirror-polished cavity gives a glossy part and a blasted or etched cavity gives a matte or textured one. The finish is called out per face on the drawing using a standard such as the SPI grades (A-1 to D-3) or a texture reference such as VDI 3400 or Mold-Tech.
Finish is one of the decisions covered briefly in the guide to plastic part manufacturing methods. This article goes deeper on the central contrast: a polished surface versus a textured one, and what each choice costs in tooling, draft and appearance.
Why finish is a tooling decision, not a part decision
The cavity surface is made once, when the tool is built. Changing it later means pulling the mold, reworking steel and sampling again.
- Polished finishes are produced by hand with diamond compounds, papers and stones, so a higher polish adds toolmaker hours and favors harder, polishable steel.
- Blasted finishes are produced by dry blasting with glass bead or aluminum oxide, which is quicker and hides minor tool marks.
- Patterned textures are etched chemically or produced by EDM, and the pattern is fixed into the steel once applied.
Finish is one of several inputs that move the tooling quote, alongside cavity count, undercuts and steel grade, all covered in injection mold tooling cost and lead time.
Free checklist: the 30 checks to run before you pay the factory deposit →The SPI finish grades in one table
The SPI system, now maintained by the Plastics Industry Association, groups mold finishes into four letters by method: A for diamond buff, B for paper, C for stone and D for dry blast. Within each letter, 1 is the finest and 3 the coarsest. The roughness ranges below are those commonly published by mold-finishing suppliers, and individual shops vary slightly.
| Grade | Finishing method | Typical Ra (µm) | Typical use |
|---|---|---|---|
| A-1 | Grade #3 diamond buff | 0.012 to 0.025 | Mirror and optical parts, clear lenses |
| A-2 | Grade #6 diamond buff | 0.025 to 0.05 | High-gloss cosmetic surfaces |
| A-3 | Grade #15 diamond buff | 0.05 to 0.10 | Glossy non-optical parts |
| B-1 | 600 grit paper | 0.05 to 0.10 | Medium polish |
| B-2 | 400 grit paper | 0.10 to 0.15 | Medium polish |
| B-3 | 320 grit paper | 0.28 to 0.32 | Medium to low polish |
| C-1 | 600 grit stone | 0.35 to 0.40 | Low polish, fine matte |
| C-2 | 400 grit stone | 0.45 to 0.55 | Matte |
| C-3 | 320 grit stone | 0.63 to 0.70 | Matte |
| D-1 | Dry blast, glass bead | 0.80 to 1.00 | Satin |
| D-2 | Dry blast, #240 oxide | 1.00 to 2.80 | Dull |
| D-3 | Dry blast, #24 oxide | 3.20 to 18.0 | Coarse dull, hides marks |
Roughness alone does not describe the look. A D-1 blasted surface and a C-1 stoned surface can measure similar Ra values and still look and feel different, so ask for physical sample plaques before you commit.
Beyond SPI: VDI 3400 and Mold-Tech textures
Many consumer products use textures deeper or more patterned than the SPI D grades.
- VDI 3400 is a German texture scale, usually produced by EDM, running from VDI 12 (fine, roughly comparable to SPI C-1) to VDI 45 (coarse). It is common on tools built in Europe and Asia.
- Mold-Tech is a commercial library of etched textures, from fine matte to leather grain and geometric patterns, each identified by a code such as MT-11010.
- Rough equivalence. Suppliers note that some textures across the systems look similar (for example, a coarse SPI D-3, VDI 33 and MT-11020), but the match is visual, not exact.
Texture depth decides the draft angle
A texture is a field of tiny undercuts. Without enough draft, the part drags against the steel on ejection and the texture scuffs or tears. The basic draft and wall rules every molded part follows are covered in how injection molding works; texture adds to them.
A common guideline is about 1 to 1.5 degrees of extra draft for every 0.001 inch (0.025 mm) of texture depth. In practice that looks like this:
| Surface | Approximate texture depth | Draft to plan for |
|---|---|---|
| Polished (SPI A or B) | Negligible | Base draft only, often around 1 degree |
| Light texture (for example MT-11010) | 0.001 in (0.025 mm) | About 1.5 degrees |
| Medium texture (for example MT-11020) | 0.0015 in (0.038 mm) | About 2.25 degrees |
| Leather grain | 0.005 in (0.13 mm) | About 7.5 degrees |
Choose the texture before the walls are drawn. A housing designed with near-vertical sides may not have room for the draft a leather grain demands.
What gloss shows and texture hides
The finish changes how forgiving the part is of normal molding variation and of daily handling.
- Gloss shows defects. Sink marks over ribs, flow lines, weld lines and gate blush all stand out on a polished face.
- Gloss shows use. Fingerprints and fine scratches appear quickly on high-gloss housings.
- Texture hides both, which is why most handheld products use a light texture on gripped faces.
- Texture changes color perception. A coarser texture reads as lower gloss and often slightly darker, so judge color on a textured sample.
- Material limits polish. Clear parts need A-1 or A-2 in an optical-capable resin such as polycarbonate or acrylic, while polypropylene is hard to bring to a mirror finish. The resin trade-offs are covered in ABS vs polycarbonate.
How to call out finish on your drawing
A clear callout prevents the most common dispute: parts that are dimensionally correct but cosmetically rejected.
- Call out each face separately. A visible top shell might be MT-11010, an inside face SPI C-3, and a lens window SPI A-2.
- Name the standard and grade, not a description such as “smooth” or “matte”.
- Request sample plaques in your resin and color before the texture is applied.
- Plan the timing. Many molders apply texture after the first tool trials, once dimensions are approved, because texture is hard to undo.
- Lock an approved sample as the cosmetic reference for incoming inspection, described in quality control in manufacturing.
Timing matters in another way, too. In our view at Inventornest, the most overrated step in development is polishing the product’s appearance before resolving its essential technical uncertainties. Finish callouts belong once the function and geometry are proven.
Frequently asked questions
What is the most common surface finish for consumer product housings?
A light texture, such as a fine Mold-Tech pattern or an SPI D-1 or D-2 blast, on visible faces is common because it hides fingerprints and minor molding marks. Glossy accents and clear windows are usually called out separately.
Does a higher polish make the mold more expensive?
Generally yes. Diamond polishing is skilled hand work and favors harder steel, so A-grade finishes add toolmaker time compared with stone or blasted finishes.
Can I change the texture after the mold is built?
A texture can often be made coarser or re-applied, but going from textured back to polished is difficult and may not be possible without significant steel work. Settle the finish before texturing starts.
What is the difference between SPI and VDI finishes?
SPI grades are defined by the polishing or blasting method, from mirror to coarse blast. VDI 3400 is a scale of EDM-produced textures defined by roughness, and it reaches coarser textures than SPI typically covers.
How much draft do I need for a textured part?
Plan for roughly 1 to 1.5 degrees of extra draft per 0.001 inch of texture depth, on top of your base draft. Confirm the figure with your molder and texture supplier for the exact pattern.
Where Inventornest fits
At Inventornest, our decision to move from 3D printing to injection molding depends on enclosure complexity, material, finish, tolerances, tooling cost and the difference in unit cost, so finish is part of the tooling conversation from the start. Our design-for-manufacturing checks consider manufacturing methods, assembly access and differences between prototype and intended production parts.
We retain responsibility for engineering coordination and design clarification when production is handled by our manufacturing partners, which includes clarifying questions with the factory and reviewing initial samples before scaling production. To see how this fits a full program, our product manufacturing support covers the path to production, or you can book a consultation to review the finish callouts on your part.
