SPI Surface Finish For Injection Molded Parts- SPI Mold Finish Chart

Injection-molded plastic parts replicate the Injection mold surface finish (Refer to SPI Surface Finish Chart). Therefore injection mold core and cavity surface finish are as per the specified part drawing.

SPI (Society of Plastic Industry) has defined a standardized surface finish for injection molds to facilitate industrial designers, design engineers, and mold manufacturers. These standardized finishes are known as SPI mold finish. This article covers various SPI surface finish, their roughness values, and recommendation for plastic parts.

SPI Surface Finish For Injection Molded Plastic Parts

Designers specify SPI surface finish in a part drawing. Injection molds are polished as per injection molded part surface finish requirements. As a result, the mold SPI finish replicates on injection molded plastic parts. 

Following SPI mold finish options are available to finish injection molded plastic parts according to SPI standards.

SPI Surface Finish Chart
The below chart shows the SPI finish along with the finish method and surface roughness values.

FinishSPI StandardFinish MethodSurface Roughness (Ra Value)
Very High Glossy FinishA16000 Grit diamond buff0.012 to 0.025
High Glossy FinishA23000 Grit diamond buff0.025 to 0.05
Normal Glossy FinishA31200 Grit diamond buff0.05 to 0.1
Fine Semi Glossy FinishB1600 Grit Paper0.05 to 0.1
Medium Semi Glossy FinishB2400 Grit Paper0.1 to 0.15
Normal Semi Glossy FinishB3320 Grit Paper0.28 to 0.32
Fine Matte FinishC1600 Grit Stone0.35 to 0.4
Medium Matte FinishC2400 Grit Stone0.45 to 0.55
Normal Matte FinishC3320 Grit Stone0.63 to 0.70
Satin Textured FinishD1600 stone prior to dry blast glass bead #110.8 to 1.0
Dry Textured FinishD2400 stone prior to dry blast #240 oxide1.0 to 2.8
Rough Textured FinishD3320 stone prior to dry blast #24 oxide3.2 to 18.0

SPI Surface Finish Recommendations For Plastic Part

SPI Mold Finish look, feel, and quality depends on the thermoplastic material’s mechanical properties and part color. Plastic material properties also impact the selection of the injection molding surface finish. For example, engineers do not recommend highly glossy finishes on the P22 tool material.

The below table can give you an idea about recommended SPI mold finish for given plastic material.

SPI Finish Chart
A1A2A3B1B2B3C1C2C3D1D2D3
MaterialSPI Finish Rating (X: Not Recommended; 1: Excellent; 2: Good; 3: Average;)
ABS3212
Poly Carbonate32123X1X
PVCX321
Acrylic123
Acetal321-
Polyester3 21212X1-
PolyethyleneX321-

To sum up, SPI finish is one of the best ways to convey the required plastic finish to the manufacturer. SPI mold finishes are accepted all around the world. Ra and VDI surface finish are good alternatives to the SPI Finish standard. 

We will keep adding more information on SPI surface finish for injection molding plastic parts. Add your suggestion, comments, or questions on the SPI surface finish in the comment box.

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