Buyers should specify surface finish by zone, not with a vague word such as smooth. Identify each surface purpose; define the mold finish or texture system, resin and colour; attach a reference plaque or approved sample; and state acceptance inspection. For precision injection molding, the supplier’s injection mold design review should confirm that draft, gating, venting, ejection, shrinkage and measurement datums support the requirement without compromising micro-features or dimensional stability.
A finish designation is not a complete acceptance standard. It becomes reproducible when the drawing locates the surface, the requirement states the comparison method, and trial evidence connects the tool surface to the molded-part result.
01
Begin with a zone-by-zone surface requirement
Surface appearance transfers from the cavity or core, but the purchased item is the molded part. Mark each controlled zone on CAD, a drawing view or a colour-coded map. Label it visible, touch/contact, sealing or adhesion, optical, hidden, or datum and functional. Show the viewing side, boundaries, permitted gate or ejector marks, and explicitly non-cosmetic surfaces. This prevents broad notes where release or inspection is impractical.
State the intended outcome before naming a grade: fingerprint masking, grip, low reflectance, paint adhesion, clear optics, or controlled mating friction. A polish or texture may support it, but does not define colour, gloss, blemish limits or tactile acceptance. For cross-border sourcing, a controlled English drawing, approved master and revision-controlled acceptance record should take precedence over local shorthand.
02
Select an SPI, texture or roughness reference that matches the need
SPI finish injection molding terminology is a common language for mold finishing. The conventional A, B, C and D groups describe different polishing or blast approaches, and published SPI guidance gives typical roughness ranges. It is useful shorthand, but not a universal promise that every resin, pigment, filler or production setting will create identical molded appearance.
Do not replace a texture pattern with one Ra value, or treat a texture code as a functional roughness limit. ISO 21920 provides methodology and terminology for communicating and measuring surface texture; it does not set the pass/fail requirement for a particular part. When roughness matters, specify the parameter, limit, sampling zone, trace direction, filtering or cut-off and comparison procedure. For visual appearance, a retained plaque or approved production-intent sample is often more useful than a number alone.
| Requirement type | Buyer callout | Approval evidence |
|---|---|---|
| High-polish visible zone | SPI grade or agreed polish reference, exact zone, resin and colour, permitted marks and viewing conditions | Tool-finish record and molded comparison sample under agreed light |
| Patterned grip or low-gloss zone | Texture supplier/code or agreed plaque, texture direction and boundary, draft review and allowed variation | Texture reference plus trial parts showing coverage and release |
| Functional contact or seal zone | Functional surface requirement, measurable profile parameter if needed, datum and inspection method | Defined measurement setup, results and functional test where relevant |
| Hidden zone | As-molded condition and exclusions from cosmetic inspection | First-article visual review appropriate to the stated purpose |
03
Review texture injection molding with geometry and micro-features
Texture changes release behaviour. A textured wall can resist ejection more than a polished one, so draw direction, draft, rib depth, steel access and ejector layout require review before steel is cut. Published guidance shows that a medium bead-blast finish can require more draft than a light finish, but values are supplier-specific. Document draft by surface and identify texture that must stop short of a shutoff, slide, parting line or deep rib.
For micro injection molding and other small features, do not add texture after geometry is frozen. Etching or blasting can soften edges and make narrow ribs, apertures and thin-wall transitions harder to reproduce or inspect. Define whether each micro-feature is excluded, polished, or textured. For a critical dimension, establish datum and measurement strategy before surface treatment is released.
04
Match finish approval to resin, colour and process stability
The same tool treatment can look different in different materials. Resin hardness, pigment, gloss, filler, flow direction and shrinkage can alter visible texture. Industry guidance cautions that filled resins can make fine finish differences difficult to distinguish and recommends a plaque in the actual material. Name resin grade, colour or masterbatch condition, filler status and any permitted recycled-content or regrind policy rather than approving finish on unspecified material.
During trials, compare parts from a documented process window, not one attractive shot. Gate location, melt condition, packing, cooling, venting and mold cleanliness can influence gloss, flow marks and texture definition. Stable appearance must coexist with approved dimensions, especially where a cosmetic face is also a locating face, seal land or assembly interface. Re-review dimensions and function if a cosmetic change alters a critical interface.
05
Specify inspection evidence a remote buyer can assess
A visual requirement should state where the part is viewed, at what distance and angle, under which lighting, after what conditioning, and against which approved standard. State whether the check is 100 percent, sampled, or first-article only; do not leave sampling implied by an SPI note. Record cavity identity and material lot where appearance traceability is needed on multi-cavity tools.
For a numeric roughness claim, require a calibrated instrument and a report naming the surface, trace direction, measurement length and filtering conditions. Measurement settings can change comparability, which is why ISO 21920 guidance emphasizes defined length, direction and evaluation conditions. For texture, retain the approved plaque, labelled sample and photographs as visual comparators. CMM evidence should verify dimensional CTQs and datum relationships; it is not proof that a texture is acceptable.
06
Turn the requirement into an RFQ and DFM decision
Provide native CAD, drawing revision, finish map, resin and colour, end-use intent, demand range, cosmetic acceptance standard and requested evidence in the RFQ. Ask the supplier for an injection mold design and DFM response separating confirmed items, assumptions, risks and alternatives. It should show finish zones, draw directions, draft changes, gate and ejector exclusions, polishing or texturing access, CTQ interfaces and trial evidence.
Do not request a blanket tool-life, cycle-time, capacity or tolerance promise because a finish is named. Those commitments depend on resin abrasiveness, steel and treatment, cavity count, geometry, process window, maintenance and production conditions. Set them after supplier review. Send the controlled design package and finish map to InjeForm through the request-a-quote route, and request written DFM feedback before authorising surface treatment or production tooling.
Practical takeaway
Before you move
to the next review.
- Mark every cosmetic, tactile, functional and non-cosmetic surface on a controlled drawing or 3D finish map.
- State the intended outcome and name the applicable SPI grade, texture code or approved reference plaque; do not use smooth alone.
- Name the resin grade, colour condition, filler status and any permitted material-content policy used for approval.
- Request DFM confirmation of draw direction, draft, parting lines, gates, ejectors, venting and access to micro-features before texture or polish is released.
- Protect dimensional CTQs, datums, sealing lands and mating surfaces from unreviewed finish changes.
- Define visual conditions and, where needed, a numeric surface-measurement setup rather than requesting a measurement report without a method.
- Approve retained plaques or samples and link them to tool revision, cavity identification and trial documentation.
FAQ
Common questions
Is an SPI designation enough for a purchase order?
No. An SPI designation describes a mold-finish category, but it does not locate the surface or define resin, colour, visual conditions, defect limits, part geometry or inspection method. Combine it with a finish map and approved comparator.
Should buyers specify SPI or a texture supplier code?
Use the system that matches the treatment. SPI is appropriate for an agreed polish or blast category. A patterned texture should identify the agreed texture supplier/code or physical reference. Either way, state the zone, boundaries and acceptance evidence.
Does a polished mold guarantee a polished-looking molded part?
No. Material formulation, filler, colour, flow, packing, cooling and mold condition all influence transferred appearance. Approve samples in the production-intent resin and colour, then record the relevant process and tool revision.
How should a textured micro-feature be specified?
First decide whether texture is necessary on that feature. If it is, identify the feature on CAD or drawing, define its datum and functional dimensions, request a draft and ejection review, and agree a practical visual comparator. Do not assume a general texture note fits every small rib or aperture.
What surface-finish evidence should be requested at first article?
Request the approved finish map, tool or texture record, material and colour identification, labelled molded samples, agreed visual photographs or plaque comparison, any defined roughness report, and dimensional results for affected CTQs. Review appearance and function together.
References
Source material
- [1] Fictiv, SPI Guidelines for Injection Molding Surface Finish
- [2] ZEISS, DIN EN ISO 21920: Surface Roughness Measurement
- [3] Protolabs, Injection Molding Surface Finishes
This guide is general information for early procurement and engineering discussion. It does not replace product-specific DFM, qualification, validation, regulatory assessment or legal advice.