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What’s The Best SPI Surface Finish For Your Injection Molded Part?

What’s The Best SPI Surface Finish For Your Injection Moulded Part?

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For the convenience of industrial designers, design engineers and mold manufacturers, SPI (the Society of the Plastic Industry) has set a standardized surface finish for injection molds, known as SPI mold finish.

This article covers the standard SPI mold finishes we offer, recommended finish for each material, and results you can expect to get the most out of injection molding.

 

What’s the SPI surface finish standard?

The SPI surface finish standard covers 12 SPI grades of polish finishes split across four categories: glossy (A), semi-glossy (B), matte (C) and textured (D). The SPI mold finish chart below describes all twelve SPI surface finishes, including the finishing method and typical surface roughness.

 

FinishSPI* standardFinishing methodSurface roughness Ra (μm)
Super High GlossyA-1Grade #3, 6000 Grit Diamond Buff0.012 – 0.025
High GlossyA-2Grade #6, 3000 Grit Diamond Buff0.025 – 0.05
Normal GlossyA-3Grade #15, 1200 Grit Diamond Buff0.05 – 0.10
Fine Semi-glossyB-1600 Grit Paper0.05 – 0.10
Medium Semi-glossyB-2400 Grit Paper0.10 – 0.15
Normal Semi-glossyB-3320 Grit Paper0.28 – 0.32
Fine MatteC-1600 Grit Stone0.35 – 0.40
Medium MatteC-2400 Grit Stone0.45 – 0.55
Normal MatteC-3320 Grit Stone0.63 – 0.70
Satin TexturedD-1Dry Blast Glass Bead #110.80 – 1.00
Dull TexturedD-2Dry Blast #240 Oxide1.00 – 2.80
Rough TexturedD-3Dry Blast #24 Oxide3.20 – 18.0
As machinedFinished to the machinist’s discretion3.20 (with visible machining marks)

 

 

What’s the right SPI surface finish for each material?

For product designers and design engineers, it’s essential to understand that SPI finishes produce different results depending on the injection molded plastic that you choose.

This section gives you an idea of what to expect when selecting different grades of SPI finishes for your plastic injection molded parts. It also includes a table of common injection molding plastic material and the recommended SPI finish for each material.

 

Glossy Finish

SPI glossy finishes are made with diamond buffing, which results in the shiniest and glossiest finish possible. Such finishes yield excellent results when paired with transparent materials like PMMA (Acrylic). Due to the high polish effect, it is great in applications such as injection-molded lenses, mirrors, and clear cases.

It is worth noting that it is virtually impossible to apply Grade A finishes to a material like TPU because such plastics have high abrasion resistance and are thus resistant to the buffing process.

 

SPI finishExcellentGoodAverageNot recommended
A-1 (Glossy)AcrylicABS, PC, PS, NylonHDPE, PP, TPU
A-2 (Glossy)AcrylicPCABS, HDPE, PP, PS, NylonTPU
A-3 (Glossy)Acylic, PCABS, PS, NylonHDPE, PPTPU

 

PC material injection molded part with spi-a3 glossy finish

 

Semi-Glossy Finish

SPI semi-glossy finishes are made with sandpaper, which results in some glossiness. Such finishes are great for concealing machining, moulding and tooling marks and can achieve good visual appearance. This makes it great for many consumer products.

Due to the sanding technique employed, Grade B finishes leaves behind characteristic linear scratches producing a very distinct pattern.

 

SPI finishExcellentGoodAverageNot recommended
B-1 (Semi-Glossy)PSABS, Acrylic, HDPE, PC, PP, Nylon
B-2 (Semi-Glossy)PS, NylonABS, Acrylic, HDPE, PC, PP, NylonTPU
B-3 (Semi-Glossy)ABS, HDPE, PP, PS, NylonAcrylicPC, TPU

 

Nylon material injection molded part with SPI grade b surface finish

 

Matte Finish

SPI matte finishes are made with grit sanding stones. This technique makes it possible to remove machining marks without creating a glossy finish. It can support product designs that need a more subdued or muted appearance.

Because Grade C finishes will reflect less light, fingerprints on products that are constantly handled are less noticeable than with a highly polished finish. This makes them great in applications such as keyboards and laptop frames.

 

SPI finishExcellentGoodAverageNot recommended
C-1 (Matte)ABS, HDPE, PS, PP, NylonAcrylic, TPUPC
C-2 (Matte)ABS, HDPE, PS, PS, NylonAcrylic, TPUPC
C-3 (Matte)ABS, HDPE, PS, PS, NylonAcrylic, TPUPC

 

Textured Finish

SPI textured finishes are made with pressure blasting, which results in a flat, dull and rougher finish. Such finishes can improve the end user’s ability to better grip the part and increase paint or sticker adhesion. This makes it great in applications like injection-molded steering wheels, handles and shop vac.

Although Grade D finishes provide excellent results with most plastics, designers should avoid selecting SPI D2 or SPI D3 finishes with Polycarbonate (PC) material.

 

SPI finishExcellentGoodAverageNot recommended
D-1 (Textured)ABS, HDPE, PC, PP, PS, Nylon, TPUAcrylic
D-2 (Textured)ABS, HDPE, PS, PS, Nylon, TPUAcrylicPC
D-3 (Textured)HDPE, PPABS, PS, Nylon, TPUAcrylicPC

 

Plastic injection molded part with SPI grade d textured finish

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Picking an SPI finish for your injection moulded part

Do you have a design you want to get injection moulded but are unsure which material or SPI grade to use for the best results? Picking between the different grades of finishes can be tricky. Drop us a message here and specify your project requirements. Our mould tool and injection mould engineers are available to help with your project from design to production.

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