Surface finish is the only specification your customer can judge with their fingernail. Yet most drawings carry a two-letter guess — “SPI-B2” — scribbled next to a view, and both sides pretend they mean the same thing. They usually don’t. The SPI surface finish system (twelve grades, four families, from mirror to satin blast) is a language, and like any language it only works when both speakers are fluent.SPI (Society of the Plastics Industry, now the Plastics Industry Association)

This article is a curator’s walk through that gallery: four rooms, one placard each — how the finish is made, what it costs, what it hides, what it exposes — followed by the eight deliverables you are entitled to receive from any serious custom plastic injection molding company, and six case files where the finish, not the dimension, was the whole battle.

🖼️ Room 1 · Grade A — The Mirror Room

Placard: Hand-polished with progressive diamond paste down to #1; Ra 0.012–0.025 µm at A-1. The surface becomes a mirror — literally: it reflects your process sins back at you.

Best for: optical lenses, reflectors, clear parts, pre-plating surfaces, luxury caps.

What it exposes: every flow line, weld line, sink and gate blush — Grade A is a truth serum (see why blush survives here in our gate blush clinic↗).

The price: 8–40 hours of hand polish per mold; the highest mold-temp discipline; glass-filled resins are not admitted past ~A-3.

🖼️ Room 2 · Grade B — The Workhorse Hall

Placard: Semi-gloss from graded abrasive paper; Ra 0.05–0.25 µm across B-1 to B-3. The finish of most consumer electronics you own.

Best for: enclosures, bezels, appliance panels — where gloss must look intentional but defects must stay quiet.

What it hides: minor flow and ejection witnesses; what it still shows: deep sinks (pair with our sink & warpage audit↗ before blaming the finish).

The price: 4–12 hours; the best cost-to-appearance ratio in the gallery.

🖼️ Room 3 · Grade C — The Matte Gallery

Placard: Matte from grit stone work; Ra 0.35–0.90 µm. Light scatters instead of reflecting — which is exactly why sins disappear.

Best for: tool housings, grips, medical shells, any part where function outranks shine.

What it hides: sinks, flow, polish direction — the gallery’s best camouflage.

The price: 4–10 hours; slightly more ejection friction, so mind draft and release.

🖼️ Room 4 · Grade D — The Satin Annex

Placard: Dry media blast (glass bead or oxide) at controlled pressure; Ra 0.80–3.50 µm. A uniform satin that forgives almost everything — including the mold’s age.

Best for: industrial parts, interiors, surfaces that must look identical cavity to cavity and year to year.

The annex next door: chemical textures (VDI 3400, Mold-Tech) — a different language for leather, grain and geometric patterns; remember each 25 µm of texture depth asks for ~1.5° more draft.

📜 The Twelve Grades on One Wall

Values are typical SPI chart ranges; the only number that legally binds anyone is the signed finish plaque exchanged at kickoff.

💰 What the Finish Costs — Before Anyone Quotes It

Run the appearance decision through the same ledger as everything else with our Injection Molding Cost Calculator — finish is a line item, not a vibe.

🎁 The Eight Deliverables — What a Real Custom Plastic Injection Molding Company Hands You

🗂 Case Files — Six Battles Fought on the Surface

❓ FAQ (Snippet-Optimized)

Q1: What is an SPI A-2 surface finish?
A near-mirror polish made with #3 diamond paste, Ra 0.025–0.05 µm — one step below the A-1 mirror; typical for lenses, reflectors and premium cosmetic surfaces.

Q2: Which SPI finish hides sink marks and flow lines best?
Matte Grade C and chemical textures scatter light and hide sinks and flow; glossy Grade A reveals every defect, which is why it demands the best process.

Q3: Can glass-filled resin reach a mirror finish?
No — exposed fibers cap practical polish around A-3/B grades. Specify mirrors on unfilled resins or plan a coating/plating route instead.

Q4: How should SPI Surface finishes be called out on a drawing?
Per surface, not per part: “SPI-B-2, polish direction X” on each visible face, with an A/B-surface map and a signed plaque reference attached.

Q5: What is the difference between SPI, VDI 3400 and Mold-Tech?
SPI grades describe polished and blasted surfaces; VDI 3400 and Mold-Tech describe chemically etched textures. Ask your molder for plaques in both languages.

🎯 CTA — Free SPI Surface Finish Map

Send the part and one sentence of appearance intent (“looks like a phone, hides like a tool”). Within 48 hours Ulite returns a free SPI surface finish Map: SPI callouts per surface, polish directions, texture draft analysis, replication limits for your resin, and the plaque set we’ll sign together. Ulite — a custom plastic injection molding company that speaks finish fluently.

💬 Real-time: The online chat window in the lower-right corner

📧 Email: inquiry@ulitemech.com

🌐 Visit: https://ulitemech.com

📍 Headquarters: Shenzhen, China ( China plastic injection molding company)

     Factory 1: Dongguan, China ; Factory 2: Hung Yen, Vietnam

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