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An Overview of Injection Molded Surface Finishes

Xometry offers SPI injection molding finishes and Mold-Tech and VDI injection molding finish options for polished, matte, to textured molded parts.

Greg Paulsen - Xometry Contributor
By Greg Paulsen
October 7, 2020
 3 min read
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What injection mold finish should you choose for your part?

A common question we hear at Xometry is, "What do your molded parts look like?" The answer is: however you want it! Xometry has access to a diverse marketplace of molding suppliers and all standard mold finishing options. Our mold finishes include SPI, MoldTech, VDI (EDM), and others. Mold finishes can range from glossy to matte to even patterned. Finishes applied to the surface of the mold cavity transfer to the surface of the molded part. For example, a mold surface that is polished to be reflective will make a reflective (or transparent) part. Mold finishes are different from finishes for CNC machined, or 3D printed parts, where each component is treated individually. Adding a finish or texture can change the tool's price depending on the process required, but it does not alter the price of the individual parts.

The most common finishes used include those from the Society of the Plastics Industry (SPI), which are a set of standard mold finishes starting from heavily polished (SPI A) to semi-gloss (SPI B) to fine and coarse matte (SPI C and SPI D). Where polished textures can increase a parts' cosmetic reflection or transparency on transparent polymers, matte finishes can help provide more subdued tones and even help prevent fingerprints on handled products (e.g., your laptop's frame and keyboard).

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SPI, Mold-Tech, and VDI 3400 injection molding finishes
SPI, Mold-Tech, and VDI 3400 injection molding finishes
SPI, Mold-Tech, and VDI 3400 injection molding finishes

Injection molded finishes guide: SPI, Mold-Tech, and VDI 3400

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Society of Plastics Industry (SPI) Mold Finishes

Standard SPI Injection Molding Finishes
SPI FinishDescriptionTypical ApplicationsSurface Roughness (Ra µm)
SPI Finish

SPI A-1

Description

Grade #3, 6000 Grit Diamond Buff

Typical Applications

High polish or transparent parts, optically clear

Surface Roughness (Ra µm)

0.012 - 0.025

SPI Finish

SPI A-2

Description

Grade #6, 3000 Grit Diamond Buff

Typical Applications

High polish or transparent parts

Surface Roughness (Ra µm)

0.025 - 0.050

SPI Finish

SPI A-3

Description

Grade #15, 1200 Grit Diamond Buff

Typical Applications

High-to-medium polish parts, non-optical lenses

Surface Roughness (Ra µm)

0.050 - 0.100

SPI Finish

SPI B-1

Description

600 Grit Paper

Typical Applications

Medium polish parts

Surface Roughness (Ra µm)

0.05 - 0.10

SPI Finish

SPI B-2

Description

400 Grit Paper

Typical Applications

Medium polish parts

Surface Roughness (Ra µm)

0.10 - 0.15

SPI Finish

SPI B-3

Description

320 Grit Paper

Typical Applications

Medium - low polish parts

Surface Roughness (Ra µm)

0.28 - 0.32

SPI Finish

SPI C-1

Description

600 Stone

Typical Applications

Low polish parts

Surface Roughness (Ra µm)

0.35 - 0.40

SPI Finish

SPI C-2

Description

400 Stone

Typical Applications

Low polish parts

Surface Roughness (Ra µm)

0.45 - 0.55

SPI Finish

SPI C-3

Description

320 Stone

Typical Applications

Low polish parts

Surface Roughness (Ra µm)

0.63 - 0.70

SPI Finish

SPI D-1

Description

Dry Blast Glass Bead

Typical Applications

Satin finish

Surface Roughness (Ra µm)

0.80 - 1.00

SPI Finish

SPI D-2

Description

Dry Blast #240 Oxide

Typical Applications

Dull finish

Surface Roughness (Ra µm)

1.00 - 2.80

SPI Finish

SPI D-3

Description

Dry Blast #24 Oxide

Typical Applications

Dull finish

Surface Roughness (Ra µm)

3.20 - 18.00

Mold-Tech Textured Finishes

Standex Engraving Mold-Tech is a standardized mold texturing option typically used for commercial or saleable goods. Mold-Tech is serialized in four series, A through D. Most commonly the Mold-Tech Series A finishes are used on products as it has a range of fine to coarse matte finishes that do not require laser, masked chemical etching, or other engineered texturing processes. Mold-Tech finishes are categorized by their serial number and texture depth. A more aggressive draft angle is needed with Mold-Tech because mold texturing has more coarse features than SPI finishes, it is recommended to add 1.5° of draft for every 0.001" of texture depth.

Mold-Tech A Finishes

Common Mold-Tech Injection Molding Finishes
Mold-Tech Serial NumberTexture Depth (inch)ApplicationMinimum Draft
Mold-Tech Serial Number

MT-11010

Texture Depth (inch)

0.001"

Application

Dull matte finish

Minimum Draft

1.5°

Mold-Tech Serial Number

MT-11020

Texture Depth (inch)

0.0015"

Application

Coarse matte finish

Minimum Draft

2.25°

Mold-Tech Serial Number

MT-11030

Texture Depth (inch)

0.002"

Application

Coarse matte finish

Minimum Draft

VDI 3400 Surface Finish (known as VDI surface finish) refers to the mold texture standard Verein Deutscher Ingenieure (VDI), the Society of German Engineers. The VDI 3400 surface finish is mainly processed by Electrical Discharge Machining (EDM) when mold machining. VDI is an international standard for mold texturing of matte surfaces. The EDM surface gives a fine-to-coarse grain depending on the selection. VDI is not the same as Mold-Tech, which is applied through various techniques from manual finishing to laser etching to chemical etching with patterns. However, some textures may have near equivalencies, such as SPI D-3, VDI 33, and MT-11020, all looking very similar. In similar aspects, VDI 12 and SPI C-1 are equivalent.

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SPI Mold Finishes

VDI (EDM) Mold Texture Values

Common VDI Injection Molding Finishes
VDI ValueDescriptionApplicationsSurface Roughness (Ra µm)
VDI Value

VDI 12

Description

600 Stone

Applications

Low polish parts

Surface Roughness (Ra µm)

0.40

VDI Value

VDI 15

Description

400 Stone

Applications

Low polish parts

Surface Roughness (Ra µm)

0.56

VDI Value

VDI 18

Description

Dry Blast Glass Bead

Applications

Satin finish

Surface Roughness (Ra µm)

0.80

VDI Value

VDI 21

Description

Dry Blast # 240 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

1.12

VDI Value

VDI 24

Description

Dry Blast # 240 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

1.60

VDI Value

VDI 27

Description

Dry Blast # 240 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

2.24

VDI Value

VDI 30

Description

Dry Blast # 24 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

3.15

VDI Value

VDI 33

Description

Dry Blast # 24 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

4.50

VDI Value

VDI 36

Description

Dry Blast # 24 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

6.30

VDI Value

VDI 39

Description

Dry Blast # 24 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

9.00

VDI Value

VDI 42

Description

Dry Blast # 24 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

12.50

VDI Value

VDI 45

Description

Dry Blast # 24 Oxide

Applications

Dull finish

Surface Roughness (Ra µm)

18.00

Unless otherwise specified, an injection molded part will typically have an SPI B-2 default finish to enable better part release from the mold and remove tool marks. Surface finishes can be applied to some or all faces; for example, an opaque molded part can have a custom outer surface. The interior, non-cosmetic surface, can remain default. Multiple finishes and texturing can be applied to a single tool cavity if required. Transparent parts are typically polished on both sides to achieve the highest clarity.

Xometry offers a wide variety of surface finishes and additional mold textures to ensure that your parts meet all of their structural and cosmetic requirements. Learn more about our injection molding service.

Greg Paulsen - Xometry Contributor
Greg Paulsen
They call me the Director of Application Engineering at Xometry. This means I not only get to produce great design-for-manufacturing content, but also consult on a variety of custom manufacturing projects using CNC machining, additive manufacturing, sheet metal, urethane casting, and injection molding. If you have a question, I'm your guy.