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Glossary

What Is an SPI A2 Finish? Injection Molding Explained

SPI A2 is high gloss polish for visible housing land on the cavity side, not a default for whole enclosures or optical windows.

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Engineering note

SPI A2 is high gloss mold polish, not a resin property

A2 is the common Class A compromise: strong reflection without full A1 mirror labor on every face.

A2 uses fine diamond polish on cavity steel, stopping before the last mirror passes required for A1.

SPI A2 describes high gloss cavity polish. It is not the same as an optical surface specification for imaging lenses.

Gloss on the part still depends on resin, color, fill, gate location, venting, and pack profile.

Call out A2 per face on drawings and RFQs. A title block note alone is not enough for mixed finish molds.

Reference

SPI A2 vs A1 and A3 (typical mold steel targets)

Ra ranges are steel prep shorthand, not automatic part Ra. The same A2 steel can look different by resin, color, and gate location. Mass production acceptance should use signed T0 parts or a finish plaque, not Ra alone.

FinishMethodGlossRa (approx.)Typical use
SPI A1Grade A diamond polishMirror gloss0.012 to 0.025 umLenses and mirror cosmetic land
SPI A2Fine diamond polish (pre mirror sequence)High gloss0.025 to 0.05 umVisible housings, premium consumer shells
SPI A3Fine abrasive polishMedium gloss (below A2)0.05 to 0.10 umNon hero glossy faces, moderated glare vs A2

Selection

When SPI A2 is a strong choice

A2 balances visibility and polish cost on many programs.

  • Consumer electronics and appliance housings seen daily under normal indoor lighting

  • Branded exterior faces that need high gloss without lens grade mirror scope

  • Programs stepping down from A1 on large flats while keeping hero areas glossy

  • Paint or coating over molded parts where minor steel texture is acceptable after prep

Selection

When to avoid SPI A2 or pick another grade

Step up to A1 or down to B1 or C1 based on reflection and defect visibility.

Specify A2 by default on all exteriors

  • True optical windows that need A1 or dedicated optical polish beyond SPI
  • Long flow paths with visible weld lines where C1 or texture hides witness better
  • Large enclosures under harsh spot lighting where B1 reduces glare and fingerprint show
  • One global gloss note with no face map while the part mixes ribs and logos

Match grade to each face

  • A1 on small hero land, A2 on adjacent visible faces when budget allows split polish
  • B1 on large flat covers when high gloss highlights sink and flow
  • Drawing maps SPI per face, gate rules, and parting line cosmetics before steel polish

DFM

How A2 changes mold and part risk

High gloss still amplifies fill pattern and steel defects.

  • Gate blush, weld lines, and sink remain visible on A2. Plan gate style, location, or hide zones early.
  • A2 needs hardened cavity steel that survives fine diamond polish and maintenance. Pre hardened mold steel can be polished toward A2 on limited land but often does not hold stable high gloss on large hero faces through high volume runs.
  • Corrosion sensitive or long life polish programs often review stainless cavity steel with the mold maker. Link steel choice in the mold spec before A2 polish hours are quoted.
  • A2 cosmetic land is usually on the cavity (A side). Map which faces are A side polish before shutoff design.
  • Mixed A2 with stone or etched finishes needs shutoffs and parting line witness rules.
  • Ejector, slide marks, and parting line flash on A2 land need hide features or written acceptance limits.
  • Valve gate timing and pack profile matter for glossy faces even when steel is correct.
A side and B side glossaryStainless mold tooling glossary
3D CAD, material, quantity, tolerance band, and insert notes for an instant injection molding quote

Materials

Resin and color effects on A2 appearance

Not every family reads high gloss on A2 steel. Match material to the cosmetic target.

  • Clear PC, PMMA, and high gloss ABS often suit visible A2 land when grade, drying, and gate plan are controlled.

  • Unfilled PP, soft TPU, and glass filled PA on large A2 faces often look hazy or show fiber or flow.

  • PC and PC/ABS often deliver strong high gloss on A2 steel when color and drying are controlled.

  • Light colors and metallics highlight sink and flow more than dark colors on A2.

  • Lock cosmetic resin and color for production. Regrind and letdown shifts can change gloss.

Drawings

How to call out SPI A2 on CAD and PDF

Give mold makers one finish system per face before polish scope is bid.

  • Label each cosmetic face SPI A2 on the drawing or PMI, not only in a general note

  • Provide a finish plan view or color coded CAD when multiple SPI bands appear on one part

  • State gate location constraints and allowed vestige treatment on A2 zones

  • Define T1 cosmetic acceptance for flow marks, sink, and scratch on A2 surfaces

  • Attach plaque or prior part reference when matching an existing glossy standard

  • State that cosmetic signoff uses physical samples or a finish plaque, not Ra numbers alone on the drawing

Go deeper

Where this term connects

Each link is a different angle on the same program, not a duplicate definition.

Visual reference

Photos from molding and tooling programs

Images are selected from the NetProto Forge media library for this topic.

SPI A2 high gloss on molded housing exterior

SPI A2 high gloss on molded housing exterior

SPI A2 high gloss on molded housing exterior

Cavity-side glossy plastic after diamond polish

Cavity-side glossy plastic after diamond polish

Cavity-side glossy plastic after diamond polish

Inspection lighting on SPI A2 cosmetic surface

Inspection lighting on SPI A2 cosmetic surface

Inspection lighting on SPI A2 cosmetic surface

FAQ

SPI A2 questions

Is SPI A2 an optical specification?

No. SPI A2 is a mold polish band. Optical lens specs are separate from SPI high gloss labels.

How does SPI A2 compare to A3?

A2 is finer and higher gloss than A3. A3 is usually cheaper polish and is often enough for non hero glossy faces that do not need full high gloss.

Is A2 acceptable for lens windows?

Optical programs often need A1 or dedicated optical polish beyond SPI labels.

Can A2 and texture coexist on one mold?

Yes with shutoffs when each face is dimensioned separately.

Does resin choice change A2 appearance?

Yes. Gloss and fill pattern vary by material, filler, and color.

Does A2 cost less than A1 or A3?

Usually less than A1 mirror polish and more than A3 abrasive polish on the same geometry. A2 saves mirror pass labor, but if the part still needs T0 plaques, gate study, and strict cosmetic limits, total program cost may not drop much.

How should we accept A2 cosmetics in production?

Use signed T0 samples or an agreed finish plaque. Ra on the drawing supports steel prep but does not replace visual acceptance.

Should I pick A2 to hide weld lines?

Often no. Matte C grades or light texture hide flow witness better than high gloss.

Next step

Lock SPI A2 before cavity polish

Upload CAD with per face finish, resin, and gate notes for DFM and quote alignment.

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