All plastic injection molders know that the surface texture of the mold cavity and core will transfer a texture to the molded part. But even experienced molders can be unclear about the exact technical language used to classify these finishes, how they’re engineered by toolmakers and how to measure them accurately in a production environment.
Smart molders will use the finishing standards created by the Plastics Industry Association, an independent trade organization, educational resource and regulatory body responsible for the entire plastics chain. The association has created criteria for surface finishes on injection molding tools. These criteria are used to measure the most popular kinds finishes for injection-molded tools and parts. Using this criteria helps avoid gray areas not only for the manufacturer but also product designers, who need a standard way to describe the types of surfaces they want on their products.
Surface finish categories
In this classification system, there are four major categories: A, B, C and D. “A” finishes are created with diamond buffing; “B” with sandpaper; “C” with grit sanding stones; and “D” with pressure blasting using glass beads or aluminum oxide grains.
Surface finish impacts product cost
As you move from “A” to “D” in the classification system, more time, effort and money is involved. What’s more you have to move in stair-step fashion, from “D” to “C” to “B” to “A”. The final cost must then be reflected in the piece price, which is less of a concern for longer runs, obviously.
Category A: The Highest Grade
Category A is made with diamond buffing, and is considered to be the highest standard grade. A fine diamond buffing paste is applied with a rotary tool in a random manner — this will scatter or reflect light without indicating a clear texture. There’s no pattern to the waviness of the surface. With a Category A finish, the piece should appear the same no matter the viewing angle or the light.
Category B: Finishes created by sandpaper
Category B is achieved using sandpaper. This surface finish is applied in a back-and-forth motion, which creates linear scratches in a very obvious pattern.
B-grade finish is considered semi-gloss. It’s fine enough to hide mold defects, tooling and machining marks, while being more affordable to produce than A-grade.
Category C: Finishes created with grit stone
The C-grade finishes are made with grit sanding stones. The grits of the stones also range from and leave behind a surface that is rougher to the touch and less flat because the stones are more aggressive in their cutting action. Such a process is used to quickly erase tooling and mold marks, and will leave behind a matte finish on plastic parts.
Category D: Finishes made by blasting
Rougher finishes are made by abrasive blasting with sand or glass beads. These correspond to class D. Due to the random nature of the spraying, the resulting finish is uniform and non-directional, so there is no discernable pattern. This classification is used to produce dull or flat finishes.