A moulded part's surface is a copy of the cavity's surface, so specifying a finish is really specifying how much hand work goes into the steel. Polishing and texturing are separate operations applied after the cavity is machined, they are priced accordingly, and texture in particular reaches back into the part design because it demands more draft.
How finishes are specified
The industry works from published finish standards that group cavity surfaces into families: diamond polished for optical clarity, fine and medium polished for gloss, stone finished for a satin appearance, and dry blasted for matte. Each grade names the abrasive sequence used to produce it, so a specification is a description of the process rather than a roughness number.
Texture and where it comes from
Texture is chemically etched or laser engraved into the finished cavity as a separate operation, from a standard pattern catalogue or a bespoke artwork. It hides sink and flow marks, improves grip and disguises minor tool wear, which is why textured surfaces are so common on housings. It is charged per cavity and per pattern.
Texture demands more draft
A moulded texture is a field of small undercuts, so the part must have more draft to leave the tool without dragging. The deeper the texture, the more draft is needed, and the requirement rises quickly. A part designed with smooth surface draft and textured late in the programme is a common and expensive surprise, because the fix is either changing the part or accepting a shallower texture.
Choosing what to specify
High polish is expensive, shows every flow mark and scratches easily in service. Matte and textured finishes are cheaper to maintain, hide moulding marks and are more forgiving of tool wear. Specify the polish only where optical performance or the product's identity requires it, and let everything else be as machined or lightly textured.
Questions people ask about injection molding surface finish
How is a moulded surface finish specified?
Against a published set of standard cavity finishes, each of which names the polishing or blasting sequence used on the steel. Texture is specified separately by pattern reference.
Does texture need more draft?
Yes, and considerably more as the texture gets deeper. Texture is a field of tiny undercuts, so a textured face needs more draft than a smooth one.
Is a high polish expensive?
Yes, in hand work on the steel and in maintenance, and it shows every flow mark and scratch. Specify it only where the product genuinely needs it.
Can texture be added to an existing tool?
Usually yes, as a separate etching or engraving operation, provided the part has enough draft. That draft requirement is what most often blocks it.