How to make and measure surface textures for plastics injection molding

All plastic molders understand that the surface texture of the mold cavity and core will transfer a texture to the corresponding molded part. But even experienced molders can be unclear about the exact technical language used to describe these finishes, how they’re created by toolmakers and how to measure them correctly in a production environment. At Star Rapid, we use the finishing standards created by the Plastics Industry Association (PLASTICS; Washington, DC). Although many reading this will be familiar with PLASTICS, for those that aren’t, it’s an independent trade organization, educational resource and regulatory body that supports the entire plastics chain. The association has created standards for surface finishes on injection molding tools, still commonly called SPI comparators based on their former name. These comparators are used to measure the common kinds of finishes for injection-molded tools and parts. Using this standard helps avoid gray areas not only for the manufacturer but also in communication with product designers, who need a standard way to describe the types of surfaces they want on their products. Surface finish categories In the PLASTICS classification system, there are four major grades: A, B, C and D. Group A finishes are made with diamond buffing; B with grit sandpaper; C with grit sanding stones; and D with pressure blasting using glass beads or aluminum oxide grains. Does surface finish impact product cost? As you go higher in the classification system, more time and effort is involved. In fact, each step of refinement requires that you work your way up from the lesser grades that preceded it. This is an unavoidable accumulation of effort and, of course, will impact the final cost of the tool. That cost must then be prorated into the piece price, which is less of a concern for larger orders. Category A: Finishes created by polishing 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, non-linear motion—this tends to scatter or reflect light without indicating a clear texture. There’s no direction or pattern to the waviness of the surface. With a category A finish, it should appear the same no matter the viewing angle or the angle of light. There are three levels in the A category, representing different degrees of polish, and within these levels an Ra value is specified for each. As an example, A-3 corresponds to Ra 2~3. Category B: Finishes made by sandpaper Category B is achieved using sandpaper. This surface finish is applied in a back-and-forth motion, which creates characteristic linear scratches in a very noticeable pattern. It’s possible to have a surface finish for B-1 which also measures a flatness of Ra 2~3, the same as A-3. However, this finish doesn’t look the same as A-3, nor would the part made from such a tool. A 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. These finishes are applied with sanding grit ranging from 600 grit for B-1, 400 for B-2 and 320 for B-3. Category C: Finishes made with grit stone The C-grade finishes are made with grit sanding stones. As with B-grade finishes, the grits of the stones also range from 600 to 320, though they 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. Note again that there is overlap in the Ra flatness of surfaces with very different textures. For example, a D-1 dry blasted finish can be as flat as a C-1 stone finish, but they will look and feel very different. Finishes made with EDM In addition to the four categories, there are similar results that can be produced using electrical discharge machining (EDM) or spark erosion. We use both approaches depending on the features and the type of metal being processed. Both processes are measured according to the PLASTICS standards. Expressing the shine of your part with gloss We advise clients, at minimum, to specify not only a PLASTICS surface finish but also gloss. Gloss is defined as the reflectance of a surface compared to a polished black glass standard. We use a simple gloss meter. This device bounces a light off the surface of a sample at different angles depending on the Ra value being tested. The measurement values are called gloss units (GUs). The black glass standard is rated as 100, so measured gloss units from a target part are compared to this reading. From these gloss readings, it’s then possible to describe a surface as high-gloss, semi-gloss, matte or flat. Obviously surface textures and finishes are designed to achieve cosmetic and functional effects on the part. Rougher textures provide better grip, and can also be used to hide minor molding defects like weld lines. But note that rougher surface textures are harder to release from the mold, so draft angles usually must be increased accordingly. That’s why it’s a good idea for toolmakers and product designers to work closely together in the early stages of product development, and why they need to speak a common language of surface finish terminology.