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SPI Finish Standards

The Plastics Industry Institute (SPI) has developed a set of precision machining process standards to represent the surface finish of finished parts. The mold can be polished or textured by mechanical or manual means. The specific standards are shown in the table below:

SPI A10.012 to 0.0256000 Grit DiamondSuper High Glossy finishS136 (54HRC) or 8407(52HRC)
SPI A20.012 to 0.0253000 Grit DiamondHigh Glossy finishS136 (54HRC) or 8407(52HRC)
SPI A30.05 to 0.101200 Grit DiamondNormal Glossy finishS136 (300HB) or 718H
SPI B10.05 to 0.10600 Grit PaperFine Semi-glossy finish718H
SPI B20.10 to 0.15400 Grit PaperMedium Semi-glossy finish718H
SPI B30.28 to 0.32320 Grit PaperNormal Semi-glossy finish718H
SPI C10.35 to 0.40600 Grit StoneFine Matte finish718H
SPI C20.45 to 0.55400 Grit StoneMedium Matte finish718H
SPI C30.63 to 0.70320 Grit StoneNormal Matte finish718H
SPI D10.80 to 1.00Dry Blast Glass BeadSatin Textured finish718H
SPI D21.00 to 2.80​Dry BlastDull Textured finish718H
SPI D13.20 to 18.0Dry BlastRough Textured finish718H

VDI Standards

The VDI standard uses different parameters to represent the surface finish of components. Comparing the two tables, you can see the surface treatment effects corresponding to the parameters.

VDI parameterInstructionsApplicationSurface roughness (Ra um)
12600 oil stone Low Polishing Requirements0.40
15 400 oil stone Low Polishing Requirements0.56
18#24 Sandblasting (coarse glass beads)Sandblasting (coarse glass beads)0.80
21#24 Sandblasting (coarse glass beads)24 Oxide Sandblasting1.12
24#24 Sandblasting (coarse glass beads)24 Oxide Sandblasting1.60
27#24 Sandblasting (coarse glass beads)Dull Finish2.24
30#24 Oxide Sandblasting (coarse glass beads)Dull Finish3.15
33#24 OxideSandblasting (coarse glass beads)Dull Finish4.50
36#24 Oxide Sandblasting (coarse glass beads)Dull Finish6.30
39#24 OxideSandblasting (coarse glass beads)Dull Finish9.00
42#24 Oxide Sandblasting (coarse glass beads)Dull Finish12.50
45#24 Oxide Sandblasting (coarse glass beads)Dull Finish18.00

The molds used in injection molding processes are mostly made of aluminum and/or steel. As long as there are imperfections on the mold surface, even small imperfections can be reflected in the final molded parts. If these marks are not removed by sandblasting or polishing, the mechanical tool marks will remain on the finished product. In most cases, if the interior of the part is not visible to the naked eye, there is no problem with tool marks, which will not affect the appearance or function of the part

However, those exposed surfaces often require additional processing. Almost all molds are polished to varying degrees, but of course, the more refined the polishing, the higher the cost of the mold and the longer the manufacturing time of the mold.

The key factors that determine surface treatment

Injection molding processes have different surface treatment processes than parts produced by machining or 3D printing. The surface finishing of the latter two is performed after the parts are manufactured, which is a post-processing step. For injection molding processes, the mold itself must undergo such treatment (or add texture), and the molded parts are the finished products. For example, if the finished part requires a matte surface, the interior of the mold must be textured first to achieve the matte effect on the molded part. The key factors determining surface treatment are the following two factors:

Improve the appearance of parts

The designer of the part may choose the texture of the finished product based on different aesthetic considerations. A smooth or matte texture on the surface of the part not only improves the appearance and gives a sense of quality, but also covers minor defects such as flow lines, weaves, dents, or shadow marks caused during the injection molding process. From a commercial perspective, parts with better surface finish are also more attractive to customers.

Improve part function

The texture treatment of the part surface can improve the adhesion of paint or stickers. For example, parts with a matte texture are easier to paint, and the durability of the paint is also better. The matte texture is also more suitable for sticking labels. Texture treatment can also make parts easier to grip. By surface treating the mold, we can also avoid over-injection or under-injection defects of plastic parts. A well-designed part with texture treatment can also help discharge air inside the mold to avoid trapped air.

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