Mould Tool Finishes: SPI and VDI Explained

Surface finish plays a crucial role in the final appearance, performance, and functionality of an injection-moulded part. At Apex Manufacturing, we offer a full range of industry-standard finishes based on the SPI (Society of the Plastics Industry) and VDI (Verein Deutscher Ingenieure) standards, helping you achieve the perfect balance between cosmetic quality and manufacturability.

SPI Finishes – Cosmetic Control for Gloss and Texture

SPI finishes are categorised from A1 to D3, based on the polishing process applied to the tool steel. These finishes are commonly used throughout the UK and North America, especially for customer-facing products where gloss level and surface feel are important.

SPI finish	Description	Applications	Suitable materials	Surface roughness (Ra um)
A-1	Grade #3, 6000 Grit Diamond Buff	High polish parts	Acrylic	0.012- 0.025
A-2	Grade #6, 3000 Grit Diamond Buff	High polish parts	Acrylic, PC	0.025-0.05
A-3	Grade #15, 1200 Grit Diamond Buff	High low polish parts	ABS, Acrylic. PS, Nylon, PC	0.05-0.10
B-1	600 Grit Paper	Medium polish parts	ABS, Acrylic, PP. PS, HDPE, Nylon, PC	0.05-0.10
B-2	400 Grit Paper	Medium polish parts	ABS, Acrylic, PP. PS, HDPE, Nylon, PC	0.10- 0.15
B-3	320 Grit Paper	Medium low polish parts	ABS, Acrylic, PP. PS, HDPE, Nylon	0.28-0.32
C-1	600 Stone	Low polish parts	ABS, Acrylic, PP. PS, HDPE, Nylon, TPU	0.35-0.40
C-2	400 Stone	Low polish parts	ABS, Acrylic, PP. PS, HDPE, Nylon, TPU	0.45-0.55
C-3	320 Stone	Low polish parts	ABS, Acrylic, PP. PS, HDPE, Nylon, TPU	0.63-0.70
D-1	Dry Blast Glass Bead	Satin finish	ABS, PP. PS, HDPE, Nylon, PC, TPU	0.80-1.00
D-2	Dry Blast #240 Oxide	Dull finish	ABS, PP. PS, HDPE, Nylon, TPU	1.00-2.80
D-3	Dry Blast #24 Oxide	Dull finish	ABS, PP. PS, HDPE, Nylon, TPU	3.20-18

VDI Finishes – Textural Engineering with Repeatable Roughness

VDI finishes, most commonly defined under VDI 3400, specify measured surface roughness levels (Ra). These are achieved using EDM (electrical discharge machining) or chemical etching. VDI finishes are preferred in automotive, industrial, and European tooling environments, where tactile control and mould release performance are often more important than visual polish.

VDI numbers range from around VDI 12 to VDI 45, with higher numbers producing rougher, more grippy textures:

VDI Value	Description	Applications	Surface roughness (Ra um)
12	600 Stone	Low polish parts	0.4
15	400 Stone	Low polish parts	0.56
18	Dry Blast Glass Bead	Satin finish	0.8
21	Dry Blast # 240 Oxide	Dull finish	1.12
24	Dry Blast # 240 Oxide	Dull finish	1.6
27	Dry Blast # 240 Oxide	Dull finish	2.24
30	Dry Blast # 24 Oxide	Dull finish	3.15
33	Dry Blast # 24 Oxide	Dull finish	4.5
36	Dry Blast # 24 Oxide	Dull finish	6.3
39	Dry Blast # 24 Oxide	Dull finish	9
42	Dry Blast # 24 Oxide	Dull finish	12.5
45	Dry Blast # 24 Oxide	Dull finish	18

Surface isn’t an afterthought. It’s identity.

SPI vs VDI: What's Right for Your Project?

SPI finishes are ideal if you need high-gloss, cosmetic surfaces or are working to a design spec familiar with UK/US standards. VDI finishes offer greater control over functional texture, making them suitable for parts where release performance, tactile feel, or flow uniformity are priorities.

At Apex, we work closely with clients to determine the best finish based on material, part geometry, and use case. Whether you're prototyping a medical device or producing industrial-grade enclosures, we ensure the finish supports both your technical requirements and brand aesthetic.

Need Help Choosing?

Send us your part drawing or get in touch with our design-for-manufacture team. We can recommend suitable finishes early in the tooling process to save time and cost downstream.