SPI Finish surface finish
SPI Finish applied by YiguProto

Surface finish

SPI Finish for CNC & Molded Parts

SPI finish is the standard mold-surface spec. It defines exactly how smooth or textured a mold cavity should be.

  • Industry-standard mold surface grades.
  • Predictable look and release for molded parts.
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What it is

What Is SPI Finish?

SPI finish is the Society of the Plastics Industry surface standard for molds. It uses grades from A (mirror) to D (textured). Each grade maps to a specific polish or bead-blast method. Specifying an SPI grade tells the mold maker exactly what surface to deliver.

We machine and polish mold cavities to the SPI grade your part drawing requires.

How it works

How SPI Finish Works

Review SPI grade

We read the required grade (A, B, C, or D) from your drawing.

Machine cavity

CNC brings the cavity close to final form.

Polish or texture

We polish (A/B) or bead-blast (C/D) to spec.

Verify

We check roughness and compare to the SPI standard.

A1–A3 are diamond-polished mirrors; B1–B3 are paper-polished; C1–C3 are stone/blast; D1–D3 are textured.

Why choose it

Benefits of SPI Finish

Standardized

Same result across shops and runs.

Predictable look

Known gloss and texture per grade.

Better release

Smooth cavities eject parts cleanly.

Repeatable

Easy to re-order to the same spec.

Typical specs

Typical SPI Finish Specifications

SpecificationTypical range / valueNotes
GradesA1–A3, B1–B3, C1–C3, D1–D3
A gradeDiamond polish, mirror
D gradeTextured / bead blast
Base materialsSteel, aluminium, pre-hardened mold steel

Best applications

When to Use SPI Finish

Injection molds

Cavities for plastic parts.

Optical lenses

High-gloss A-grade surfaces.

Consumer goods

Soft-touch textured housings.

Sealing surfaces

Controlled finish for function.

Materials

Compatible Materials for SPI Finish

We apply SPI Finish to the materials below. Not sure if your material qualifies? Our engineers will confirm it for free.

P20 steelH13 steelStainless mold steelAluminium (prototype molds)

Our solution

Our SPI Finish Solution

We build and finish injection molds to the exact SPI grade on your drawing. We verify roughness so the molded part looks and releases as expected.

  • All SPI grades A through D.
  • Mirror polish with documented Ra.
  • Texture options for grip and hide.
  • Mold making and finishing in one shop.
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Always note the SPI grade on the part drawing so the cavity matches your cosmetic need.

Design tips

Design & DFM Tips for SPI Finish

Choose by look

A for gloss, D for texture or to hide flaws.

Use texture to hide

D grades mask flow and sink marks.

Allow polish stock

Leave material for polishing to grade.

FAQ

SPI Finish — Frequently Asked Questions

Send your drawings or 3D files. Most quotes come back within a few hours on business days, never more than 24 hours.
Yes. All customer files and ideas are treated as confidential. We sign NDAs when needed.
Yes. Our engineers review your design and suggest changes that cut cost, improve quality, or speed up delivery.
For SPI Finish, we support one-off prototypes and full production runs. There is no strict MOQ.
We can make samples for your approval. Cost depends on the process, and it is credited back on a production order.
A standard mold-surface grade from mirror (A) to textured (D).
A1–A3 are the mirror, diamond-polished grades.
For texture, grip, or to hide surface defects on molded parts.
Yes, smoother grades (A/B) release molded parts more easily.

Need SPI Finish on your parts?

Send your files and get a finish recommendation + factory-direct quote.

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