Rongjiang Bai
3 min read
12 Jun

What is Injection Mold Finish?

When we talk about Injection mold finish here, we are referring to the texture of the product's surface. It is well-known that injection-molded products are created by injecting molten plastic material into a mold to form a specific shape, which then cools and solidifies into the final product. The surface texture is directly produced by the mold itself, meaning no additional post-production work such as polishing, painting, or oxidation is required. Therefore, the surface texture of an injection-molded product corresponds to the processing done on the mold's surface, specifically the polishing and finishing of the mold during its production. Understanding this, we can now look at some common surface treatment methods we use.

VDI 3400 Mold Finish

SPI Mold Finishes

The most common finishes used are standardized by the Society of the Plastics Industry (SPI). These finishes range from the highest grade mirror-like surface (SPI A) to semi-gloss surfaces (SPI B) and fine and coarse matte textures (SPI C and SPI D), which can enhance the cosmetic reflection or transparency of parts in transparent polymers. Mirror finishes are ideal for parts requiring a shiny or transparent appearance, but they are also more prone to showing usage marks and generally require longer processing times and higher costs. On the other hand, matte finishes provide a softer appearance and help hide fingerprints or scratches on frequently handled items. They have shorter processing times and are more cost-effective.

Type of Finish
SPI Number
(Ra μm)

SPI-A1#3 Diamond Buff
High polish or transparent parts, optically clear
0.012 - 0.025
#6 Diamond Buff
High polish or transparent parts
0.025 - 0.050

SPI-A3#15 Diamond Buff
High-to-medium polish parts, non-optical lenses
0.050 - 0.100

SPI-B1600 Grit
Medium polish parts
0.05 - 0.10
400 GritMedium polish parts
0.10 - 0.15

SPI-B3320 GritMedium - low polish parts
0.28 - 0.32

SPI-C1600 StoneLow polish parts
0.35 - 0.40
StoneSPI-C2400 StoneLow polish parts
0.35 - 0.40

SPI-C3320 StoneLow polish parts
0.63 - 0.70

SPI-D1# 11 Glass BeadSatin finish
0.80 - 1.00
Dry BlastedSPI-D2#240 Aluminum Oxide BlastDull finish
1.00 - 2.80

SPI-D3#24 Aluminum oxide blastDull finish
3.20 - 18.00

VDI 3400 Mold Texture

VDI 3400 Surface Finish refers to the texture standard Verein Deutscher Ingenieure, the Society of German Engineers. VDI has set a standard of mold cavity surface for Europe market or even the whole world. The VDI 3400 is mainly processed by EDM while machining the mold. VDI consist of different electric spark particle in size, depth and density.

VDI ValueDescriptionApplicationRoughness (Ra µm) 
VDI 12
600 Stone
Low polish parts
VDI 15400 Stone
Low polish parts
VDI 18Dry Blast Glass Bead
Satin finish
VDI 21Dry Blast # 240 Oxide
Dull finish
VDI 24
Dry Blast # 240 Oxide
Dull finish
VDI 27Dry Blast # 240 Oxide
Dull finish
VDI 30Dry Blast # 24 Oxide
Dull finish
VDI 33Dry Blast # 24 Oxide
Dull finish
VDI 36Dry Blast # 24 Oxide
Dull finish
VDI 39Dry Blast # 24 Oxide
Dull finish
VDI 42Dry Blast # 24 Oxide
Dull finish
VDI 45Dry Blast # 24 Oxide
Dull finish

The VDI 3400 surface finish standard, primarily achieved through Electrical Discharge Machining (EDM), is an international guideline for creating matte textures on molds. EDM can produce a range of textures from fine to coarse, depending on the selection. Some textures have close equivalents, such as SPI D-3 and VDI 33, which appear quite similar. Similarly, VDI 12 corresponds to SPI C-1 in texture quality.

SPI Mold finish

Injection Molding Finish Optionals

The smaller the finish value, the finer the mold and product surface, which translates to longer mold production times and higher costs. Therefore, when selecting a mold finish, it's not always necessary to choose the highest grade. The decision should be based on the product's usage environment and specific requirements. For instance, for plastic parts that are not externally visible, an SPI-B2 finish (the industry default if not otherwise specified) or lower can be adequate. For products requiring a transparent or mirror-like appearance, standards above SPI-A3 are necessary.

Finally, the surface treatment of the mold is significantly influenced by the mold production method, especially concerning the product's draft angle. In injection molding, almost every product requires a draft angle (which helps in the removal of the product from the mold), typically ranging from 1 to 10 degrees. The rougher the product's surface, the greater the required draft angle. Consequently, if you need a product with surfaces that are nearly perpendicular to the mold's opening direction, you cannot opt for a very rough surface finish. If you're unsure about the appropriate mold finish for your product, feel free to consult with Ideal Pro's professional engineers and consultants. We are delighted to offer you free consultation and assistance, let's get started!