Mold Surface Finish: Polishing, Texture and Specs

Two molds cut from the same steel, running the same resin on the same machine, will produce parts with visibly different surfaces if their cavities are finished differently. Surface finish is a mold specification that gets decided late, written vaguely as "smooth" or "textured," and then argued about during the first article inspection.

Figure 1. SPI mold finish grade ladder from mirror A-1 to matte B-3 with Ra values.

It should be the opposite. Surface finish determines whether the part releases cleanly, how the light reflects off it, how the texture hides or reveals weld lines and sink, and how much the cavity will resist wear and corrosion. This guide turns "make it look nice" into a specification the mold shop can execute.

Why surface finish is a real decision

Surface finish touches almost everything downstream:

• Gloss and appearance. The customer buys with their eyes; the finish sets the gloss band the part must fall into.
• Release and drag. A too-glossy cavity can stick; a too-rough cavity drags and creates ejection marks.
• Defect visibility. Glossy surfaces reveal every flow mark and sink. Textured surfaces hide small ones but show drag and polish lines.
• Flow resistance. Texture adds microscopic resistance to the melt front, which can change fill behavior in thin walls.
• Wear and maintenance. Deeper texture wears unevenly; mirror finishes scratch visibly and need careful handling.

The SPI finish scale

The most widely used reference is the SPI (Society of the Plastics Industry) mold finish scale, which maps a polishing sequence to a typical surface roughness and appearance.

The lesson from the table is that finish is a process, not a value. A-1 is reached by a determined sequence of stones, papers and diamond pastes; skipping a step leaves scratches that the next step cannot remove.

The polishing sequence, and why steps matter

Polishing removes material from the previous step's scratches by going one grade finer each time. The cardinal rule is to never jump more than one step.

• Cut with a stone or paper coarse enough to remove the machining marks from the previous operation (EDM recast, milling lines).
• Never skip a grade. A jump from 320 to 15 µm polish leaves deep scratches that appear only after molding, under the gloss.
• Change direction between steps (cross-hatch) so the previous scratches are easy to see.
• Keep the tool clean. A single embedded chip in a polishing pad gouges the whole cavity.
• Polish the ribs, slots and corners by hand — power tools round off edges and change the corner radius.

EDM is a special case. Electrical discharge machining leaves a brittle, recast "white layer" that is much harder than the base steel and that traps micro-cracks. Any cavity that will be polished or textured must have its EDM recast layer removed first, or the finish will chip and the texture will not be uniform.

Texture: what acid etching does

Texture is created by acid etching, and it is a controlled corrosion process, not a polishing process. The steps are:

1. The cavity is polished to a starting gloss level — this is the base for the texture.
2. A photoresist film carrying the texture pattern is applied to the polished surface.
3. The surface is exposed and developed so that the pattern defines where the acid will bite.
4. Acid etches the exposed steel, creating the texture depth.
5. The resist is stripped and the cavity is cleaned.

The reference standard for texture is commonly given by the VDI 3400 scale (or a supplier's own texture book), which correlates a texture code to depth and gloss.

Key facts about texture that catch designers out:

• Texture depth multiplies over the part surface. The applied depth is measured on the flat steel, but on the part the "as-molded" depth is roughly the same while the gloss and direction change the visual appearance considerably.
• Texture adds drag. A deep texture increases the force to eject a part with a rough, close-fitting surface.
• Texture is directional. Acid etching on a sloped wall can undercut, making release worse.
• The base gloss determines the final look. A texture applied over a rough polish looks duller and muddier than the same texture applied over a mirror base.
• Partial texturing needs masking. Clear windows and textured borders on the same part require masking during etch, and the transition line has to be planned.

Gloss control on the part

Gloss is not a property of the steel alone. It is the interaction of the steel finish, the resin, the mold temperature and the local flow.

• Higher mold temperature generally raises gloss.
• Semi-crystalline resins (PP, PA, POM) show a skin that scatters light; you rarely get a true mirror from unfilled PP regardless of the steel.
• Gate location and shear change gloss locally: a weld line will read as a dull band on an otherwise glossy part.
• Additives — matting agents, fillers, pigments — all change the achievable gloss.

Because of this, gloss must be specified as a target with a tolerance and a measurement geometry (typically 60° or 20° gloss meter), agreed on the real part, not only on a steel sample.

Specifying finish on the drawing

A usable finish specification includes:

• The SPI grade or VDI texture number, per surface if it varies.
• A roughness target (Ra) and the direction of polish, where relevant.
• The gloss target and measurement angle.
• Which surfaces are cosmetic (Class A) and which are functional.
• The transition or "break" line between gloss and texture.
• The material and hardness of the cavity and core that will carry the finish.

Vague words like "smooth" or "good finish" are not specifications and should be struck from drawings.

Defects traced back to surface finish

• Polishing lines / drag marks. Failure to remove the previous step's scratches; visible on glossy parts as directional hazing.
• Texture flaking or blotching. EDM recast layer not removed before etching; non-uniform steel hardness.
• Gloss variation across the part. Uneven steel polish, uneven mold temperature, or gate and flow effects.
• Sticking and ejection marks. Cavity too glossy, or not enough draft on a textured wall.
• Pitting and corrosion. Textured or blasted surfaces trap moisture and release agents; stainless or properly maintained steel is required.
• Photographing a texture that will not reproduce. Selecting a texture that is finer than the resin can fill, giving a washed-out molded look.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Finish specified by SPI or VDI grade, not by adjective
• Cosmetic Class A surfaces identified separately from functional ones
• Number of polish steps planned and the sequence documented
• EDM recast layer removed before any polish or texture
• Texture depth and base gloss agreed together, with a sample molded
• Gloss measured on the real part at a defined angle with a tolerance
• Draft allowance added for textured walls (roughly 1° per 0.025 mm of texture depth)
• Release agents prohibited or controlled on textured tools
• Steel grade and hardness matched to the finish goal
• Maintenance plan that preserves the finish over the tool's life

Treat the surface finish as a designed feature with a number and a process. When the drawing states the grade, the gloss target and the texture code, the mold shop can hit it, the quality team can measure it, and the customer gets the appearance they signed off on.