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Glossary

What Is an SPI C1 Finish? Injection Molding Explained

SPI C1 is fine stone matte on cavity-side land for low glare, not high gloss retail branding on whole shells.

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Engineering note

SPI C1 is fine stone matte on mold steel, not a plastic additive

Use C1 when low glare and hiding minor flow witness matter more than polished sheen.

C1 is produced with fine stone finishing steps that leave uniform matte microtexture on cavity steel.

The part reads matte and diffuses reflection compared with A or B polish bands.

Stone matte increases pull versus polished steel. Draft, ejection, and texture transitions need DFM review.

Reference

SPI C1 vs nearby grades (typical mold steel targets)

Ra ranges are steel prep shorthand, not guaranteed part Ra. The same C1 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 B1600 grit paper polish on cavity steelSemi gloss0.05 to 0.10 umVisible housings with moderate sheen
SPI C1Fine stone on cavity steelMatte0.35 to 0.40 umLow glare exteriors, hides minor flow witness
SPI C2400 grit stoneMedium matte texture0.40 to 0.50 umGeneral purpose matte, more hide than C1

Selection

When SPI C1 is a strong choice

C1 trades gloss for defect tolerance on many functional exteriors.

  • Large flat faces where polish would show sink, flow, and gate blush under use lighting

  • Interior covers and low glare exteriors that still need uniform appearance

  • Handheld devices where fingerprints and smudges on B or A polish are unacceptable

  • Mixed finish tools with glossy hero zones shut off from C1 utility land

Selection

When to avoid SPI C1 or pick another grade

Move up to B or A for sheen, or down to D blast for heavy utility hide.

Avoid C1 when the product needs high gloss branding

  • Class A logos or bezels that marketing expects to read glossy under retail lighting
  • Optical or mirror zones that need A grades or dedicated optical polish
  • Touch surfaces that need MoldTech or VDI etch for grip or wear rather than stone matte alone
  • Hidden functional faces where D2 blast is cheaper and acceptance is looser

Pick the right matte level

  • B1 when some sheen is still desired on visible exteriors
  • C2 or C3 when heavier stone hide is needed on the same cost tier
  • MoldTech or VDI when a defined grain pattern is part of the design intent

DFM

How C1 changes mold and part risk

Matte stone adds texture pull and shows depth differences at transitions.

  • Increase draft on C1 sidewalls versus polished neighbors unless resin and depth are verified.
  • Shutoffs between C1 and A or B polish need clear parting lines to avoid gloss bleed or texture drag.
  • Ejection may need more area or venting on deep C1 textures to avoid scuff on first pull.
  • Gate vestige is still visible as texture disturbance. Plan gate style and trim locations.
Matte finish spokeMoldTech texture glossary
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Materials

Resin and color effects on C1 appearance

Matte steel reduces gloss spread but fill and color still show through texture.

  • Glass filled resins can read grainy on stone finishes. Note filler when cosmetic acceptance is tight.

  • Dark colors often hide flow better than light colors on C1 faces.

  • Soft TPE or PP may look chalky on fine stone compared with PC/ABS on the same SPI callout.

  • Paint and coating can mask C1 steel texture when adhesion prep is defined in the RFQ.

Drawings

How to call out SPI C1 on CAD and PDF

Separate stone matte from polish and etched grain on the same part.

  • Label C1 faces on CAD or PMI and note transitions to B, A, or MoldTech zones

  • State minimum draft after C1 texture where sidewalls are long or deep

  • Define cosmetic acceptance for flow, sink, and scratch on matte faces at T1

  • If MoldTech or VDI applies on other faces, map which standard controls each zone

  • Reference SPI chart for C band context and link bead blast for D band utility faces if mixed

  • 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 C1 finish on molded housing exterior with stone matte texture

SPI C1 finish on molded housing exterior with stone matte te

SPI C1 finish on molded housing exterior with stone matte texture

Cavity-side stone matte plastic surface

Cavity-side stone matte plastic surface

Cavity-side stone matte plastic surface

Low glare SPI C1 finish on molded cover

Low glare SPI C1 finish on molded cover

Low glare SPI C1 finish on molded cover

FAQ

SPI C1 questions

Is SPI C1 the same as MoldTech texture?

No. C1 is fine stone finishing on cavity steel. MoldTech is etched grain from numbered plaques.

When should the drawing call MoldTech instead of SPI C1?

Use MoldTech when a specific numbered grain must match design or touch feel. Use C1 when uniform stone matte without plaque matching is enough. MoldTech adds etch scope and plaque cost versus stone-only C bands.

Should I pick C1 or C2 for my part?

C1 gives finer matte with slightly higher gloss than C2. Pick C1 when the face is visible and needs uniform low glare. Pick C2 when hiding flow marks or sink matters more than fine appearance. See the SPI finish chart for C2 and C3 Ra and method columns.

Can C1 meet A2 on one part?

Yes with steel shutoffs when the drawing defines each face and transition.

Does C1 need less draft than VDI texture?

C1 is lighter than heavy VDI but still needs more draft than polished B or A faces.

Is C1 the same as part Ra?

No. SPI labels steel prep. Part texture depends on resin and process.

How should we accept C1 cosmetics in production?

Use signed T0 samples or an agreed finish plaque. Visual acceptance matters more than Ra alone.

Does C1 hide all weld lines?

It hides minor witness better than gloss but deep hesitation may still show.

Next step

Lock SPI C1 before stone polish

Upload CAD with matte surface callouts, draft notes, and resin spec for DFM review.

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