DESIGN & FINISHING GUIDE

A Guide to Injection Mold Finishes (SPI Standards)

The finish of your plastic part is a direct replica of the finish on the mold itself. This guide explains the industry-standard SPI finishes to help you specify the exact appearance you need.

What are SPI Finishes?

The Society of the Plastics Industry (SPI) created a set of standard classifications for mold finishes that are used throughout the industry. These standards define the process used to finish the mold cavity and the resulting appearance on the plastic part. Specifying an SPI finish ensures a consistent and predictable cosmetic outcome.

The standards are grouped into four main categories: Shiny Finishes (A), Semi-Gloss Finishes (B), Matte Finishes (C), and Textured Finishes (D).

A part with a high-gloss, premium finish, representing an SPI-A level

The Four Categories of SPI Finishes

Grade A: High Gloss

This is the highest quality and most expensive finish. The steel mold is polished with fine diamond powder to a perfect, mirror-like finish. The resulting plastic part will be extremely glossy. (e.g., SPI-A1, SPI-A2).

Grade B: Semi-Gloss

The mold is polished with fine-grit sandpaper, leaving no visible tool marks. The resulting plastic part will have a semi-gloss or satin finish. This is a good choice for high-quality cosmetic parts where a mirror polish is not needed (e.g., SPI-B1, SPI-B2).

Grade C: Matte

The mold is polished with fine stone powder, leaving a fine, uniform matte texture. This is a good choice for parts that need to be non-reflective and hide fingerprints. (e.g., SPI-C1, SPI-C2).

Grade D: Textured

The mold is bead blasted with abrasives to create a rough, textured surface. This is used for parts that need a heavy matte, satin, or textured grip finish. (e.g., SPI-D1, SPI-D2).

Mold Finish FAQ

How does the plastic material affect the final gloss?

The material plays a huge role. Even with a highly polished SPI-A1 mold, a material like glass-filled Nylon will not produce a high-gloss part because the glass fibers disrupt the surface. In contrast, materials like ABS and Polycarbonate can replicate the mold finish almost perfectly, resulting in a beautiful high-gloss part.

How do I specify an SPI finish?

You should add a note to your 2D drawing that calls out the specific SPI finish you require for the cosmetic surfaces of your part. For example, "ALL EXTERIOR SURFACES TO HAVE SPI-B1 FINISH."

Does a high-polish finish increase the cost?

Yes, significantly. Achieving a high-polish SPI-A finish requires many hours of skilled manual labor by a mold polisher. A standard machine-finished mold (similar to an SPI-C or D finish) is the most cost-effective option.