Glossary

What Is an SPI B1 Finish? Injection Molding Explained

SPI B1 is semi gloss polish for large visible cavity-side land, not mirror gloss for whole enclosures.

Published October 15, 2026

Engineering note

SPI B1 is semi gloss mold polish, not a resin property

Use B1 when you need a visible housing finish with less glare than A grades and less texture than C grades.

On the SPI chart, B1 is produced with fine abrasive polish on cavity steel, commonly described as 600 grit paper.

The molded part reflects that steel preparation. Gloss still varies with resin, color, fill, gate location, and pack profile.

Treat B1 as a mold scope and cosmetic acceptance target. Quote it with per face callouts, not a blanket note on the title block alone.

Reference

SPI B1 vs nearby grades (typical mold steel targets)

Ra ranges are industry shorthand for steel prep, not automatic part Ra. The same B1 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.

Finish	Method	Gloss	Ra (approx.)	Typical use
SPI A2	Fine diamond polish (pre mirror sequence)	High gloss	0.025 to 0.05 um	Premium cosmetic housings, high reflection
SPI B1	600 grit paper polish on cavity steel	Semi gloss	0.05 to 0.10 um	Large visible covers, reduced glare vs A2
SPI C1	600 grit stone	Matte	0.35 to 0.40 um	Low glare exteriors, hides minor flow witness

Selection

When SPI B1 is a strong choice

B1 sits between polish cost and defect visibility for many consumer and industrial shells.

Large flat faces viewed under diffuse office or factory lighting, not mirror spotlights
Handheld or desktop enclosures where A2 glare reads as cheap or distracting
Programs that need a visible exterior without full A grade polish labor on every cavity
Paint or coating planned after molding where minor steel texture is acceptable
Mixed finish tools when B1 Class A faces meet C or D utility zones with defined shutoffs

Selection

When to avoid SPI B1 or pick another grade

Escalate gloss down or up before steel polish is locked.

Specify B1 by habit

Optical windows or lens zones that need A1 or dedicated optical polish
Heavy flow lines or sink on long flows where matte C or texture hides witness better
High touch wear areas that need MoldTech or VDI etch instead of polished gloss
One global SPI note with no face map while geometry mixes ribs, textures, and logos

Match grade to face role

A2 or A1 on small hero faces only, B1 on large flats if budget allows split polish
C1 or etched texture where glare and flow visibility matter more than sheen
Drawing shows SPI per face, gate, and parting line rules before mold build

DFM

How B1 changes mold and part risk

Gloss bands amplify fill pattern and steel defects that matte grades hide.

Gate blush and weld lines show more on B1 than on C1. Plan gate style, location, or cosmetic hide zones early.
Mixed B1 with texture or stone finishes needs steel shutoffs and clear parting line witness rules.
Ejector pin marks and parting line flash targets must be defined because polish makes them visible.
Draft angles stay resin driven, but neighbors in heavy texture need extra draft to avoid drag on B1 lands.

Injection mold design guide SPI, texture, and material on one RFQ

Power and hand tool molded plastic housings and grips

Materials

Resin and color effects on B1 appearance

Not every family reads semi gloss on B1 steel. Match material to the cosmetic target.

PC, PC/ABS, and high gloss ABS often suit B1 visible land when filler and gate plan are controlled.
Unfilled PP, soft TPU, and glass filled PA on large B1 faces often look hazy or show fiber or flow.
Dark colors often hide minor witness better than light or transparent grades on B1.
Regrind and color letdown shifts can change gloss batch to batch. Lock cosmetic resin spec when B1 is acceptance critical.

Drawings

How to call out SPI B1 on CAD and PDF

Give mold makers one primary finish system per face to avoid double polish scope.

Label each cosmetic face: SPI B1 (or your company SPI B-1 syntax) on the drawing or PMI, not only in a general note
Attach a finish plan view or color coded CAD faces when more than one SPI band appears on one part
State gate location constraints relative to B1 zones and allowed vestige treatment
Reference the SPI chart spoke or customer finish standard. Cross reference MoldTech or VDI only when those faces are etched
Define cosmetic acceptance for flow marks, sink, and scratch limits on B1 surfaces for T1 signoff
State that cosmetic signoff uses physical samples or a finish plaque, not Ra numbers alone on the drawing

Go deeper