Surface finish
Injection mold surface finish, SPI grades explained
A finish callout is a tool decision as much as a cosmetic one. It changes which steel the cavity can be cut from, how much draft the part needs to release cleanly, and how many hours go into polishing before the tool ever sees plastic. This guide covers how the SPI A to D grades work, why the tool steel and the resin decide which grade a tool can actually hold, and how to call finish out on a drawing so it prices against what you meant rather than our default assumption.
The SPI standard, in plain terms
SPI grades a mold finish by how the cavity surface was worked, not by a single number describing the plastic part. There are four families, each with three grades running finest to coarsest:
| Grade | How it is made | Look |
|---|---|---|
| A-1, A-2, A-3 | Diamond buffed by hand under magnification, grit stepping down from #3 to #15 | Mirror to high gloss, the finest finish specified. Checked against a bright reference light rather than a single roughness reading. |
| B-1, B-2, B-3 | Sanded with abrasive paper, 600 down to 320 grit | Semi-gloss to satin. Roughly Ra 0.2, 0.4 and 0.8 µm by grade. |
| C-1, C-2, C-3 | Stoned with an abrasive block, 600 down to 320 grit | Matte, dull. Roughly Ra 1.6, 3.2 and 6.3 µm by grade. |
| D-1, D-2, D-3 | Dry or wet blasted with glass bead or oxide media, fine to coarse | Textured, from a light stipple to a heavy, leather-like grain. Roughly Ra 12.5, 25 and 50 µm by grade. |
Treat the Ra figures as reference points for comparing suppliers, not a guaranteed number on your own part. The finish that actually comes off a cavity also depends on the steel it is cut into and the resin run against it, both covered below.
Which finish fits which part
- Cosmetic surfaces a customer sees or touches usually take an A-grade gloss or a B-grade satin, chosen for how the product is meant to present, not by default.
- Functional or hidden surfaces, the inside of a housing, a bracket, anything that mates but is never seen, are usually left at C-grade. It is the standard finish, it costs less to produce, and it hides minor tool marks that would show under a gloss.
- Parts that need grip, or need to hide fingerprints and light scratches in the field, take a D-grade texture. The texture itself does the work a coating or a soft-touch material might otherwise be asked to do.
- Small features such as snap fits, threads, and the zone around an ejector pin are usually left at a standard machine finish regardless of the callout on the rest of the part. Polishing or texturing a feature that small rarely shows once it is molded, and asking for it adds cost without a visible result.
Finish is a tool steel decision before it is a polish decision
Not every steel we cut a cavity from can hold every grade. Our own tool steel table is built around this: aluminum (7075, QC-10) is fast and cheap to machine but has a limited texture range and does not hold a fine polish over a long production run, so it is not where an A-grade gloss belongs on a tool meant to run for volume. A durable mirror or high-gloss finish is cut into P20 pre-hardened or H13 hardened steel instead, or into 420 stainless where the part also has to resist a corrosive resin, because those steels hold the polish shot after shot rather than dulling early. We choose the steel to match the finish and the shot count the part needs, not the other way round, so a gloss callout on a quote can change which steel is proposed for the tool.
Finish and resin choice interact too
The same wear mechanism that shortens tool life on an abrasive glass or mineral filled resin also dulls a fine polish faster than the same grade running against an unfilled resin in the same cavity. A filled grade can also flatten or emphasise a texture differently than an unfilled one does, so a D-grade texture proven on an unfilled nylon is not guaranteed to look identical once the resin changes to a glass-filled version of it. Weld lines and flow marks are also simply more visible on a glossy A-grade surface than on a matte or textured one, which is worth knowing before a part with a complex fill pattern is specified glossy purely for appearance.
Texture and cycle time
A coarser texture needs more draft to release cleanly. A vertical wall textured to a heavy D-grade grain can drag or scuff on ejection at a draft angle that would release a smooth or lightly textured wall without issue, so texture has to be decided with the part's own draft, not added to a design that was drawn assuming a machine finish. On the other side of the mold, a glossy surface shows ejector-pin witness marks and sink more readily than a matte or textured one, so ejector layout is planned around a gloss callout rather than fitted to it afterward.
How to call out finish on your drawing
- State the grade explicitly, for example "SPI B-2" or "matte, no texture required," rather than "polished" alone. "Polished" means different things to different molders and prices against an assumption rather than a spec.
- Flag finish per surface where it matters, rather than one grade for the whole part. A part is rarely seen and handled identically on every face, and a uniform high-gloss callout on faces nobody will ever see adds polishing cost for no visible benefit.
- If you are working from a VDI 3400 number instead, state it as VDI, not as an assumed SPI equivalent. The two scales are not a direct conversion, and treating them as interchangeable is a common, avoidable mismatch between what was drawn and what the tool actually produces.
- Ask for a reference chip or an early sample in your own resin when finish is critical to how the product presents. A grade name is a starting point for discussion, not a guaranteed visual match without a check sample, especially once a filled or tinted resin is involved.
- Tell us the resin grade alongside the finish call, since how long a finish holds up, and in some cases how it looks at all, depends on what is run against it, not on the grade number alone.
A drawing that states grade, surface, and resin together prices faster because nothing about the finish has to be assumed, and it samples faster because the tool was cut against a real target instead of a guess. For the rest of what a quote needs, see how to get an accurate injection molding quote.
Questions
Frequently asked
What buyers ask before they put a finish grade on a drawing.
Ask an engineerWhat does an SPI finish grade actually specify?
It specifies how the cavity surface itself was worked, diamond buffed, sanded with abrasive paper, stoned, or blasted, not a property of the plastic. Whatever the steel carries gets replicated onto every part molded in that cavity, which is why the grade is chosen and cut into the tool before the first shot, not adjusted afterward on the part.
What is the difference between SPI and VDI finish standards?
SPI is the US letter-and-number scale, A1 through D3, based on the polishing or blasting method used. VDI 3400 is the German scale, a single number tied to the depth of a spark-erosion texture, common on tools built or specified outside North America. The two are not a direct one-to-one conversion, so asking for a stated VDI number as the equivalent of an SPI grade without a reference sample risks a visible mismatch.
Can any tool steel take any surface finish?
No. Aluminum tooling is fast and cheap to cut but has a limited texture range and does not hold a fine polish over a long run. A durable A-grade mirror or gloss finish is cut into a pre-hardened or hardened steel such as P20 or H13, or into 420 stainless where the part also needs corrosion resistance, because those steels hold the polish over the life of the tool rather than dulling early.
Does a coarser texture cost less than a mirror polish?
Usually, on the hand-polishing labor alone, since a mirror finish is polished under magnification to a bright reference light while a textured grade is blasted. But a heavier texture is not free: it needs more draft to release cleanly, which can mean a design change if the part was not drawn for it, and the etching or blasting itself takes its own tooling time.
Will my part actually look like the SPI grade I ask for?
Only if the steel and the resin match what the grade assumes. The same nominal grade looks different in an unfilled resin than in a glass-filled one, because a filled grade is more abrasive and wears a fine polish faster, and because filler can flatten or emphasise a texture differently than an unfilled resin does. A physical reference chip or an early sample in your own material is the only way to confirm a finish before a production run, not the grade name by itself.
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