Choosing a texture is an engineering decision, not a styling afterthought
The first thing a person notices when they pick up your product is not the function — it is the surface. Mold texture selection controls grip, glare, perceived quality, how well molding defects hide, how much draft the part needs, and how much the tool costs — so it belongs in the design phase, not the week before steel is cut. Texture is machined or etched into the cavity wall and copied onto every shot, which means changing it later means reworking the tool.
Three specification systems come up constantly, and they answer different questions: SPI grades describe how polished or blasted the steel is, VDI 3400 numbers describe measured roughness, and Mold-Tech style codes describe decorative patterns.
SPI finishes: the polish scale
The SPI system, published by the plastics industry association, is the shorthand US toolmakers use for the base finish of a cavity:
- A-1 to A-3 — diamond-buffed, mirror finishes. Required for optical parts, light pipes, and lenses. Expensive, slow, and unforgiving: every sink mark, flow line, and weld line shows.
- B-1 to B-3 — paper-stoned, semi-gloss. The default for a clean cosmetic surface that will be painted, plated, or left as a satin sheen.
- C-1 to C-3 — stone-finished, matte. A practical, economical general-purpose finish for non-critical surfaces.
- D-1 to D-3 — dry blasted, from satin to a dull, sandy texture. Hides a great deal, resists fingerprints, and adds grip.
SPI grades are about gloss and smoothness, not pattern. A leather grain or a technical stipple is a separate specification on top of the base finish.
VDI 3400: numbers instead of adjectives
VDI 3400 assigns a number to a measured surface roughness, produced by EDM or blasting. Low numbers are near-smooth; high numbers are coarse. Because the scale is metrological rather than descriptive, it travels between a US designer and an overseas toolmaker with far less ambiguity than the word "matte." If you work with Asian tooling suppliers, expect VDI numbers to appear on their quotes even when your drawing says SPI. Ask for a conversion note in writing rather than assuming a mapping.
Mold-Tech and pattern families
Decorative patterns — leather grains, sand, linen, stipples, geometric micro-patterns — are usually called out by a texture house's catalog code. Each code has a defined depth. Three production methods dominate:
- Chemical etching. The traditional approach: mask the steel, expose a pattern, etch with acid. Organic and rich; the natural home of leather and wood grains.
- Laser texturing. A digital file is engraved into the steel. Repeatable, capable of gradients and logos, able to blend a pattern smoothly across a parting line, and easy to reproduce on a second tool years later.
- Blasting and EDM. Uniform, non-directional matte surfaces, cheap and quick, with no decorative pattern.
The draft rule nobody remembers until it hurts
A textured cavity grips the part. The working rule of thumb toolmakers use is roughly one additional degree of draft for every thousandth of an inch of texture depth — so a deep grain can demand several degrees more than a polished wall. Ignore this and the part drags, scuffs, or gouges on ejection, and the fix is either more draft (a geometry change) or a shallower texture (a cosmetic change). This is why texture and draft angles and parting lines have to be resolved together.
What texture hides, and what it cannot
Texture is the cheapest cosmetic insurance in injection molding. Sink marks over ribs, gate blush, flow lines, weld lines, and slight parting-line mismatch all become dramatically less visible on a matte, patterned surface. What it will not fix is warp, short shots, dimensional error, or a bad gate location — see common injection molding defects. Texture also interacts with resin choice: the same cavity finish reads differently in a filled nylon than in a high-gloss ABS, and colorant changes the apparent gloss. Material selection comes first — start with how to choose a plastic for your product.
Surfaces that must stay untextured
- Sealing faces for gaskets and O-rings.
- Ultrasonic weld joints and adhesive bond lines.
- Bearing and sliding surfaces, and press-fit features.
- Windows, lenses, and any surface light passes through.
- Areas receiving pad printing or in-mold labels, unless the vendor confirms otherwise.
Cost, lead time, and reversibility
Texturing is a modest line item next to the tool itself — typically a small fraction of tooling cost, plus days of lead time while the cavity goes out for etching or laser work. What is not modest is changing your mind. Going coarser is usually possible. Going smoother means polishing steel away, which changes dimensions and can require welding and re-cutting. Sequence the decision before the cavity is finished, alongside the rest of your injection molding cost planning.
How to actually approve a finish
- Decide what the surface must do: hide defects, provide grip, kill glare, or read as premium.
- Shortlist two or three candidate codes — not twenty.
- Order texture plaques in your actual resin and your actual color. A catalog photo or a plaque in generic black tells you almost nothing about how it will look in your product.
- Handle the plaques with wet hands and gloves if grip matters, and photograph them under the lighting your marketing images will use.
- Write the approved code, the base SPI or VDI grade, and the affected surfaces onto the drawing, and keep a signed plaque as the reference sample.
Texture is one leg of a wider finish strategy that also covers color and coating — the framework is in CMF design, and for painted or coated parts compare with powder coating vs wet paint. More background is in our industrial design section.
Get the finish decided before the steel is cut
Projects House designs plastic parts for US clients and specifies finishes, draft, and tooling requirements as one connected decision, with plaque approval built into the schedule. If you have a part heading into tooling and the surface is still undefined, send us the details through our contact form.