Fundamentals of Surface Finish in Injected Molded Parts

 

Once a part is molded, most plastic parts require additional processing for completion. Many plastics finishing processes are performed for appearances purposes or aesthetics, such as surface finishing. Surface finish can be used to give an injected molded part a certain look, feel, or texture. The surface finish can be a critical factor in either the appearance or performance of your product as it serves both a cosmetic and functional purpose.

SPI Mold Finish Standards

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—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Displaced Standard—SPI mold finish standards have superseded other finishing standards, but customers sometimes use old finish grades when writing mold specifications.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.

Since each SPI classification has unique parameters, it’s essential that you partner with a molder that is knowledgeable across the SPI classification spectrum and can determine the best type of mold that meets your production life, cost, and quality objectives.

Visual Appearance or Functionality?

Part designers may choose the texture for several aesthetic reasons. Texture finished are valuable because they can be used to hide imperfections like flow lines, blush marks, sinks, and shadow marks. Other advantages that texture can provide a surface finish to withstand contact damage in shipping and smudges from fingerprints.

Other additional advantages of texture include:

• Plastic Flow Creases—These creases can be eliminated through the additional of textured thickness while also adding strength and non-slip qualities.
• Improved Grip—Adding texture makes the part easier to hold, improving usability and increase of safety in certain applications.
• Paint Adhesion—Paint holds firmly to a textured part during additional molding operations.
• Making Undercuts—If you have a part that will not consistently come across to the moving half of a mold, texture on some surface could give you the pull you need.

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 the experts at Midstate Mold!