{"id":2293,"date":"2026-05-20T09:06:22","date_gmt":"2026-05-20T01:06:22","guid":{"rendered":"https:\/\/www.jcinjectionmolding.com\/?p=2293"},"modified":"2026-07-01T09:32:26","modified_gmt":"2026-07-01T01:32:26","slug":"spi-polishing","status":"publish","type":"process","link":"https:\/\/www.jcinjectionmolding.com\/ja\/service\/spi-polishing\/","title":{"rendered":"SPI Polishing"},"content":{"rendered":"
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SPI Polishing Classifications & Surface Roughness Matrix<\/h2>\n

The Society of the Plastics Industry (SPI) defines 12 standard mold finish grades grouped into four distinct categories, ranging from high-gloss mirror finishes to matte stone-ground textures:<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n
SPI Grade<\/th>\nPolishing Method \/ Medium<\/th>\nRoughness (Ra)<\/th>\nRequired Mold Steel<\/th>\n<\/tr>\n<\/thead>\n
A-1 (Super Mirror)<\/td>\nGrade #3 Diamond Buffing Compound<\/td>\n0.012 – 0.025 \u00b5m<\/td>\nHardened S136 \/ NAK80 (52 HRC)<\/td>\n<\/tr>\n
A-2 (High Mirror)<\/td>\nGrade #6 Diamond Buffing Compound<\/td>\n0.025 – 0.050 \u00b5m<\/td>\nHardened S136 \/ H13<\/td>\n<\/tr>\n
A-3 (Standard Mirror)<\/td>\nGrade #15 Diamond Buffing Compound<\/td>\n0.050 – 0.100 \u00b5m<\/td>\nS136 \/ NAK80<\/td>\n<\/tr>\n
B-1 (Fine Semi-Gloss)<\/td>\n600 Grit Paper Polishing<\/td>\n0.050 – 0.100 \u00b5m<\/td>\nS136 \/ H13 \/ NAK80<\/td>\n<\/tr>\n
B-2 (Medium Semi-Gloss)<\/td>\n400 Grit Paper Polishing<\/td>\n0.100 – 0.150 \u00b5m<\/td>\nStandard P20 \/ 718H<\/td>\n<\/tr>\n
B-3 (Regular Semi-Gloss)<\/td>\n320 Grit Paper Polishing<\/td>\n0.150 – 0.200 \u00b5m<\/td>\nStandard P20 \/ 718H<\/td>\n<\/tr>\n
C-1 (Fine Matte)<\/td>\n600 Grit Stone Polishing<\/td>\n0.350 – 0.400 \u00b5m<\/td>\nStandard P20 \/ 718H<\/td>\n<\/tr>\n
C-2 (Medium Matte)<\/td>\n400 Grit Stone Polishing<\/td>\n0.450 – 0.550 \u00b5m<\/td>\nStandard P20 \/ 718H<\/td>\n<\/tr>\n
C-3 (Regular Matte)<\/td>\n320 Grit Stone Polishing<\/td>\n0.650 – 0.750 \u00b5m<\/td>\nStandard P20 \/ 718H<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/section>\n

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SPI Polish Grade Applications & Material Compatibility<\/h2>\n

Selecting the correct SPI polishing grade is critical to prevent dynamic friction, parting line flash, or ejection demolding defects:<\/p>\n\n\n\n\n\n\n\n
Application Class<\/th>\nTarget SPI Grade<\/th>\nCompatible Polymers & Assemblies<\/th>\n<\/tr>\n<\/thead>\n
Clear Optics & Lenses<\/td>\nSPI A-1 \/ A-2<\/td>\nHighly compatible with transparent resins like PMMA, COC\/COP, and optical PC (Makrolon 2407). Used for surveillance domes, camera prisms, and lenses.<\/td>\n<\/tr>\n
High-Gloss Cosmetics<\/td>\nSPI A-3 \/ B-1<\/td>\nSpecified for high-gloss black ABS housings (POLYLAC PA-757), consumer electronics, luxury cosmetic jars, and automotive trim bezels.<\/td>\n<\/tr>\n
Functional Matte & Sliding Parts<\/td>\nSPI B-2 \/ C-1<\/td>\nIdeal for low-friction POM sliders (Delrin 500P), structural Nylon connectors (Zytel 101L), and internal brackets where high gloss is not required.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/section>\n

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Dynamic Molding: Tooling and Flow Control for Mirror Finishes<\/h2>\n

Achieving a flawless SPI A-1 mirror finish requires deep-rooted expertise in polymer crystallization and precision tool steel treatment:<\/p>\n

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Hardened S136 Tool Steel Core<\/p>\n

To prevent microscopic pitting or micro-voids during diamond buffing, we use high-purity S136 stainless steel heat-treated to 52 HRC. This hardened substrate allows our master polishers to reach a flawless Ra 0.012 \u00b5m surface roughness, crucial for medical and optical components.<\/p>\n<\/div>\n

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Conformal Cooling & Stress Relief<\/p>\n

Highly polished parts are prone to stress cracking and internal birefringence. Using our 5-axis CNC machines, we mill uniform conformal cooling lines directly behind cavity walls. This prevents uneven cooling, controls high volumetric shrinkage, and eliminates thermal warpage.<\/p>\n<\/div>\n

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Ejection and Draft Angle Optimization<\/p>\n

High-gloss A-1 parts create strong vacuum suction against core blocks. We design generous draft angles (minimum 1.5\u00b0 to 2\u00b0) and integrate wide air-assisted ejection poppets or stripper plates. This prevents dynamic release drag marks and scuffs from ruining highly polished surfaces.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<\/div>","protected":false},"excerpt":{"rendered":"

SPI Polishing Classifications & Surface Roughness Matrix The Society of the Plastics Industry (SPI) defines 12 standard mold finish grades grouped into four distinct categories, ranging from high-gloss mirror finishes to matte stone-ground textures: SPI Grade Polishing Method \/ Medium Roughness (Ra) Required Mold Steel A-1 (Super Mirror) Grade #3 Diamond Buffing Compound 0.012 – 0.025 \u00b5m Hardened S136 \/ NAK80 (52 HRC) A-2 (High Mirror) Grade #6 Diamond Buffing Compound 0.025 – 0.050 \u00b5m Hardened S136 \/ H13 A-3 (Standard Mirror) Grade #15 Diamond Buffing Compound 0.050 – 0.100 \u00b5m S136 \/ NAK80 B-1 (Fine Semi-Gloss) 600 Grit Paper Polishing 0.050 – 0.100 \u00b5m S136 \/ H13 \/ NAK80 […]<\/p>\n","protected":false},"featured_media":2588,"template":"","process_category":[25],"class_list":["post-2293","process","type-process","status-publish","has-post-thumbnail","hentry","process_category-surface-finishes"],"_links":{"self":[{"href":"https:\/\/www.jcinjectionmolding.com\/ja\/wp-json\/wp\/v2\/process\/2293","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.jcinjectionmolding.com\/ja\/wp-json\/wp\/v2\/process"}],"about":[{"href":"https:\/\/www.jcinjectionmolding.com\/ja\/wp-json\/wp\/v2\/types\/process"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.jcinjectionmolding.com\/ja\/wp-json\/wp\/v2\/media\/2588"}],"wp:attachment":[{"href":"https:\/\/www.jcinjectionmolding.com\/ja\/wp-json\/wp\/v2\/media?parent=2293"}],"wp:term":[{"taxonomy":"process_category","embeddable":true,"href":"https:\/\/www.jcinjectionmolding.com\/ja\/wp-json\/wp\/v2\/process_category?post=2293"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}