Once a part is molded, most plastic parts require additional processing for completion. The surface finish of your injection molded part is an important design consideration. Identifying the right surface finish is dependent on a few important elements, including the manufacturing materials and their end-use applications. The surface finish can be a critical factor in the appearance and performance of your product.

Surface Finish Options

Injection molding offers numerous thermoplastic resins that are flexible, strong and can have fine cosmetic finishes. Many surface features must abide by the industry standards of the Society of the Plastics Industry, or SPI. These standards in the US plastics industry identifies the cosmetic quality of plastics. The SPI standards specify 12 grades of mold finish in four different categories that range from mirror-perfect to dull.

  • Shiny Finish – SPI grades A-1, A-2, and A-3 specify high gloss finished on hardened tool steel molds buffed to a mirror luster with fine diamond powder suspended in oil. These finishes are super smooth, shiny, and expensive that make plastic mirrors, visors, and other optical goods.
  • Semi-Gloss Finish – SPI grades B-1, B-2, and B-3 specify semi-gloss finishes with some sheen on hardened tool steel molds polished with ultra-fine grit sandpaper or emery cloth. These plastic parts can’t show mold, tool, or machining marks.
  • Matte to Dull Finish – SPI grades C-1, C-2, and C-3 specify matte finished from steel molds polished with fine stone powders. These finished can’t show mold, tool, or machining marks. These finishes are used on diecast or thermoset plastic industrial parts.
  • Displaced Standard – SPI mold finish standards have superseded other finishing standards, but customers sometimes use old finish grades when writing mold specifications. Society of Plastics Engineers from the 1960s-1980s offered a 1-6 scale to specify mold finishes ranging from high-sheen diamond polish to rough blasted surfaces.

Using Texture for Visual Appeal and Functionality

Many part designers consider adding texture to their surface finish. Texture finishes are valuable because they can be used to hide imperfections from the manufacturing process such as flow lines, sinks and shadow marks. It can also give a part the appearance of depth.

Beyond the simple aesthetic considerations of texture, it also has a number of functional benefits.

  • Use texture to make undercuts.
  • Use texture to improve paint adhesion.
  • Use texture to improve product grip, usability and safety.
  • Improve sticker adhesion.

As with many choices during the plastic injection molding process, the different elements used to create your product are best determined by the intended use of that product. that’s why it’s so important to have a trusted plastic molding partner to work with. Working with a skilled plastic injection molder will help you to make the right decision on the surface finish for your plastic part. Considering the surface finish will impact the type of material used, tooling, and other process decision: It’s important to determine this feature early in the design phase to achieve your desired results. Want to learn more about surface finishing options for your plastic part? Contact a representative at Bangor Plastics today!