In the world of injection molding, the surface of a part isn’t just about looks—it impacts functionality, customer appeal, and even mold design. SPI (Plastics Industry Institute) and VDI (German Engineering Association) standards are the two most trusted guides for getting injection molded part surfaces right. Whether you’re a designer aiming for a sleek mirror finish or a manufacturer needing a grippy matte texture, understanding these standards helps you avoid costly mistakes (like choosing the wrong polish for a food – safe part) and deliver products that meet client expectations.
Why Surface Treatment Matters for Injection Molded Parts
You might wonder: Why not just use the mold as – is? The truth is, unprocessed mold surfaces lead to flawed parts—and missed opportunities. Here’s why surface treatment is non – negotiable:
1. Fixes Mold Imperfections
Most injection molds are made from aluminum or steel. Even tiny machine tool marks (from mold manufacturing) or minor scratches on the mold will show up on every injected part.
- For example, a mold with 0.1mm deep tool marks will produce plastic parts with the exact same marks. If those parts are for a high – end electronics case, customers will reject them.
- Solution: Sandblasting or polishing the mold removes these marks. A small toy manufacturer once saved 20% of production costs by polishing their mold—they no longer had to discard parts with visible tool lines.
2. Balances Aesthetics and Cost
Polishing a mold to a mirror finish looks great, but it comes with a price: finer polishing means higher mold costs (up to 30% more) and longer lead times (adding 1–2 weeks to mold production).
- Practical Choice: For hidden parts (like the inside of a washing machine), low – level polishing (e.g., SPI C – 3) is enough—tool marks won’t be seen and won’t affect function.
- For visible parts (like a smartphone back cover), a high – level polish (e.g., SPI A – 1) is worth the investment—it makes the product feel premium.
3. Boosts Part Functionality
Surface texture isn’t just about appearance—it solves real problems:
- Better Adhesion: A matte texture (e.g., VDI 21) helps paint or labels stick. A furniture brand switched from a smooth SPI B – 1 finish to VDI 21 for their plastic chair armrests, and label peeling dropped by 80%.
- Easier Gripping: Textured surfaces (e.g., SPI D – 2) prevent slipping. A tool manufacturer used SPI D – 2 for their plastic tool handles, and customer complaints about slippery grips fell by 90%.
- Air Release: Well – designed textures help air escape from the mold during injection. This avoids “air traps” (bubbles or gaps in the part). A medical device maker added a VDI 18 texture to their syringe mold, eliminating air traps that had caused 5% of syringes to be defective.
Key Factors That Shape Injection Mold Surface Finish
Unlike machining or 3D printing (where surface treatment happens after the part is made), injection molding’s surface finish is determined by the mold itself. Two factors control the final result:
1. Mold Surface Treatment
The mold’s surface—whether polished, sandblasted, or textured—directly transfers to the part. If you want a matte part, you need a matte – textured mold. For example:
- To make a matte plastic cup, the mold’s inner surface is sandblasted (per SPI D – 1). Every cup made from that mold will have the same matte finish.
- A mistake here is costly: A company once polished a mold to SPI A – 1 (mirror finish) but needed a matte part. They had to re – sandblast the mold, adding $1,500 and a week of delay.
2. Material Compatibility
Not all plastics work with every surface finish. For example:
- Acrylic takes mirror finishes (SPI A – 1) beautifully—its clear, hard surface shows off the polish.
- TPU (a flexible plastic) is better for matte textures (SPI D – 3)—polishing it to a high shine is hard, and the finish wears off quickly.
The tables in the next sections (SPI and VDI) list which materials work with each finish to help you avoid this mistake.
Deep Dive into SPI Surface Finish Standards
The SPI standard (from the Plastics Industry Institute) is widely used in North America and Asia. It ranks finishes from ultra – smooth (mirror) to matte, with clear guidelines on tools, roughness, and material use.
SPI Surface Finish Table
| SPI Polishing Level | Tools Used | Purpose | Surface Roughness (Ra μm) | Compatible Materials |
|---|---|---|---|---|
| A – 1 | 3#, 6000 grit diamond paste | Mirror finish (highest shine) | 0.012 – 0.025 | Acrylic |
| A – 2 | 6#, 3000 grit diamond paste | Mirror finish | 0.025 – 0.05 | Acrylic, PC (Polycarbonate) |
| A – 3 | 12#, 1200 grit diamond paste | High polish (near – mirror) | 0.05 – 0.10 | ABS, Acrylic, PS (Polystyrene), Nylon, PC |
| B – 1 | 600# sandpaper | Medium polish | 0.05 – 0.10 | ABS, Acrylic, PP (Polypropylene), PS, HDPE, Nylon, PC |
| B – 2 | 400# sandpaper | Medium polish | 0.10 – 0.15 | ABS, Acrylic, PP, PS, HDPE, Nylon, PC |
| B – 3 | 320# sandpaper | Medium – low polish | 0.28 – 0.32 | ABS, Acrylic, PP, PS, HDPE, Nylon |
| C – 1 | 600 Whetstone | Low polish (basic smoothness) | 0.35 – 0.40 | ABS, Acrylic, PP, PS, HDPE, Nylon, TPU |
| C – 2 | 400 Whetstone | Low polish | 0.45 – 0.55 | ABS, Acrylic, PP, PS, HDPE, Nylon, TPU |
| C – 3 | 320 Whetstone | Low polish (minimal smoothing) | 0.63 – 0.70 | ABS, Acrylic, PP, PS, HDPE, Nylon, TPU |
| D – 1 | Sandblasting (coarse glass beads) | Matte finish (soft texture) | 0.80 – 1.00 | ABS, PP, PS, HDPE, Nylon, PC, TPU |
| D – 2 | #240 oxide sandblasting | Matte finish (slightly rougher) | 1.00 – 2.80 | ABS, PP, PS, HDPE, Nylon, TPU |
| D – 3 | #24 oxide sandblasting | Matte finish (roughest SPI) | 3.20 – 18 | ABS, PP, PS, HDPE, Nylon, TPU |
Real – World SPI Example
A consumer electronics company needed a clear PC phone case with a mirror finish. They chose SPI A – 2 (3000 grit diamond paste) for the mold. The result: The case had a sleek, reflective surface that matched the phone’s metal frame. Customers loved it, and sales of the case were 30% higher than their previous matte version.
Deep Dive into VDI Surface Finish Standards
The VDI 3400 standard (from the German Engineering Association) is popular in Europe and global industrial markets. It’s especially useful for parts made with EDM (Electrical Discharge Machining), but it also works with traditional polishing and sandblasting.
VDI Surface Finish Table
| VDI Parameter | Tools Used | Purpose | Surface Roughness (Ra μm) | Key Use Cases |
|---|---|---|---|---|
| 12 | 600 Whetstone | Low polish (basic smoothness) | 0.40 | Hidden parts (e.g., inside of a toaster) |
| 15 | 400 Whetstone | Low polish | 0.56 | Non – visible structural parts (e.g., plastic brackets) |
| 18 | Sandblasting (coarse glass beads) | Matte finish (soft texture) | 0.80 | Parts needing light grip (e.g., remote control bodies) |
| 21 | #24 Oxide Sandblasting | Matte finish (moderate grip) | 1.12 | Tool handles, small appliance exteriors |
| 24 | #24 Oxide Sandblasting | Matte finish | 1.60 | Furniture parts (e.g., chair armrests) |
| 27 | #24 Oxide Sandblasting | Dull finish (strong grip) | 2.24 | Heavy – duty tool grips (e.g., power drill handles) |
| 30 | #24 Oxide Sandblasting | Dull finish | 3.15 | Industrial container lids (anti – slip) |
| 33 | #24 Oxide Sandblasting | Dull finish | 4.50 | Large machinery parts (e.g., tractor plastic panels) |
| 36 | #24 Oxide Sandblasting | Dull finish | 6.30 | Outdoor equipment (resistant to scratches) |
| 39 | #24 Oxide Sandblasting | Dull finish | 9.00 | Heavy industrial parts (e.g., construction machine components) |
| 42 | #24 Oxide Sandblasting | Dull finish | 12.50 | High – wear parts (e.g., conveyor belt guides) |
| 45 | #24 Oxide Sandblasting | Dull finish (roughest VDI) | 18.00 | Parts needing maximum grip (e.g., safety glove clips) |
Real – World VDI Example
A European automotive supplier needed a plastic grip for a car’s emergency brake. They chose VDI 27 (#24 oxide sandblasting) for the mold. The texture was rough enough to provide a secure grip (even with wet hands) but not so rough that it damaged gloves. The supplier reported zero customer complaints about the grip over 2 years.
SPI vs. VDI: How to Choose the Right Standard
Choosing between SPI and VDI often comes down to your location, industry, and part needs. Here’s a simple comparison to help you decide:
| Factor | SPI Standard | VDI Standard |
|---|---|---|
| Regional Use | North America, Asia | Europe, global industrial markets |
| Best For | Consumer products (e.g., electronics, toys) | Industrial parts, EDM – made components |
| Surface Range | Mirror (A – 1) to rough matte (D – 3) | Low polish (12) to very rough (45) |
| Roughness Match | SPI C – 1 (0.35–0.40 μm) ≈ VDI 12 (0.40 μm) | SPI D – 1 (0.80–1.00 μm) ≈ VDI 18 (0.80 μm) |
Pro Tip
If you work with global clients, ask their preference. For example:
- A U.S. client may specify SPI A – 2 for a cosmetic container.
- A German client may ask for VDI 24 for the same type of container.
Yigu Technology’s Perspective on Injection Mold Surface Treatment
At Yigu Technology, we see SPI and VDI standards as essential tools for delivering consistent, high – quality injection molded parts. They eliminate guesswork—whether our clients need a mirror – smooth acrylic lens (SPI A – 1) or a grippy TPU handle (VDI 27). We’ve helped clients cut costs by matching finishes to part needs: for example, using SPI C – 3 instead of B – 1 for hidden parts saved one client 15% on mold costs. As injection molding evolves, we’ll keep leveraging these standards to balance aesthetics, functionality, and affordability for our global customers.
FAQ
- Can I use SPI A – 1 (mirror finish) for TPU parts?
No, TPU (a flexible plastic) isn’t ideal for SPI A – 1. It’s hard to polish TPU to a mirror shine, and the finish will wear off quickly with use. For TPU, choose a matte finish like SPI D – 2 or D – 3—they’re easier to achieve and more durable. - How do I know if I should use SPI or VDI for my project?
Start with your client’s location and industry. If your client is in the U.S. or makes consumer products (e.g., phones, toys), use SPI. If they’re in Europe or make industrial parts (e.g., machinery components), use VDI. If you’re unsure, ask—most clients will have a preference based on their market. - Does a higher – level polish (e.g., SPI A – 1) always mean a better part?
No, it depends on the part’s purpose. A mirror finish (SPI A – 1) is great for visible parts like a laptop cover, but it’s a waste of money for hidden parts like a printer’s internal bracket. For hidden parts, a low – level polish (e.g., SPI C – 2) is enough—it’s cheaper and faster to produce, and it won’t affect the part’s function.