Posted May 18, 2021

Surface Finishes for Plastic

CNC machine surface finishing plastic.

The demand for precision plastic parts has increased in the 21st century, with most manufacturers using injection molds, 3D printing, and CNC machining. Industrial-grade polymers became important in the modern industry due to their durability, as well as having a lighter weight and lower manufacturing costs than metal. Plastics are also indispensable in prototyping, where their ease of workmanship allows for faster product development of the eventual metal part.

Like any other component, plastic parts can have a surface finish applied to them, but those differ from the finishes used on metal parts. The finishes also vary depending on the manufacturing method used to fabricate the part. It is essential for design engineers to understand these differences when creating their parts to make the right plastic material choices and production methods. Here is some more information on surface finishes for plastic parts that should help. 

The Multiple Uses of Plastic in Manufacturing Precision Parts

The physical properties inherent in many plastic polymers make them suitable for a wide range of applications. They can withstand harsh chemicals, lack electrical conductivity, and resistance to friction. Here are a few examples of the industries that benefit from plastic components:

  • Food services: The self-lubricating nature of plastic materials makes them a natural fit for roller, sprockets, augers, and other food processor components.
  • Oil and gas: Plastic’s natural resistance to high pressures, temperatures, and corrosive chemicals makes it an excellent material for seals and other fluid transportation components.
  • Medial: Plastic’s ability to withstand harsh chemical cleaners has helped prevent the spread of infection between patients using medical devices with plastic components.
  • Electronics: Many components used on a circuit board, such as plastic leaded chip carriers (PLCCs), rely on the non-conductive properties of plastic for their packaging.
  • Aerospace: In addition to its chemical resistance and reduced friction, plastic’s light weight makes it essential for aviation and aerospace components.

Not every plastic part requires a surface finish, such as internal components hidden during regular use. But many part designers desire a smoother finish for visible parts, or parts intended for user interaction. However, the type of surface finish depends a lot on the manufacturing process used to fabricate the part. Next, we will look at the three main plastic component manufacturing methods in use and what type of surface finishes they can support.

Different Surface Finishes for Plastic Parts

The type of surface finishing for plastic parts varies with the method used to fabricate the part. The following includes some of the different surface finishing options for CNC machining, injection molding, and 3D printing.

CNC machining

CNC machined plastic parts will have tool marks on them unless they are reduced or removed with finishing steps. In some cases, machine shops won’t apply surface finishes on components used internally due to the part being out of sight. For components that do need finishing, however, the following options are available:

  • As machined: A shop will run the part through the CNC machining process again with a smaller tool to remove the roughness of the initial machining. The part will still have machine marks on it, but they won’t be as pronounced.
  • Polishing & buffing: The component can be polished to create a smooth finish on a machined plastic part. This process includes either a flame treatment, which provides the clearest finish or a more complicated chemical vapor method that provides a high-quality finish. The part can also be polished or buffed mechanically but will leave fine marks on the surface of the plastic.

In addition to surface finishing, plastic machined parts have other post-processing steps for cleanup and strengthening. The part will undergo deburring either mechanically or chemically to remove the burrs left over from the machining operation. Shops may also put plastic parts through an annealing process to strengthen and reduce their internal stress by heating them evenly in an oven.

Injection molding

The surface finish used on injection-molded plastic parts is entirely different from a machined part. A manufacturer will fabricate the polishing and texturing into the mold, which transfers to the part during the molding process. Because of this, post-processing finishes are not available, and one should consider the material’s ability to take on the molded finish and how it will appear when choosing it. According to the Plastics Industry Association, there are four main classifications of surface finishes for injection molded parts:

  • Group A, diamond buff: A rotary tool with diamond buffing paste polishes the mold’s interior surface in random patterns to look the same no matter the viewing angle or light. Diamond buffing gives the molded part the shiniest or glossy appearance.
  • Group B, grit sandpaper: Sandpaper creates a semi-gloss finish for parts by hiding manufacturing marks in the mold. A back and forth motion with the sandpaper will create linear scratches in an obvious pattern in the mold.
  • Group C, stone polish: Polishing the mold with sanding stones will quickly erase any manufacturing marks left in the mold. This finish is much rougher and less flat than the first two groups, leaving a matte finish on the final plastic part.
  • Group D, bead blast: A rougher look can be created by bead blasting the mold. Texturing the mold’s interior surface will result in a dull or flat finish on the final part.

All of these groups break down further into various degrees of the applicable finish. Each successive group of surface finishes diminishes in cost from the preceding group. There is also the choice of not applying a surface finish when the part’s appearance is not a factor in its use.

3D printing

Plastic parts produced with 3D printing over several different surface finish options:

  • Plating: A plastic part can be covered with a coat of metal using either an electrical or chemical process, but one must first treat the part with conductive paint for the plating to work.
  • Sanding: Sandpaper can remove manufacturing imperfections, and the part smoothed out with an even surface.
  • Polishing: After sanding, applying a plastic polish will give the part a glossy surface finish.
  • Bead blasting: Like sanding, bead blasting will smooth out the surface of a part and give it a matte finish.
  • Vibration & tumbling: Vibrating or tumbling the part in a container with ground-up material such as ceramic or a synthetic medium will polish it with repeated impacts.
  • Vapor smoothing & solvent dripping: Solvent vapors or a bath can melt the part’s surface, producing a shiny finish. It will also fill pores and seal the surface of the part. However, this process doesn’t apply to all plastics, and its use can harm the part if not carefully monitored.
  • Epoxy coating: Epoxy will also seal up a part and costs less as a tech usually applies it by hand. However, it may slightly change the part’s dimensions, and users should carefully monitor its application.
  • Painting & dyeing: One may also paint plastic parts for an aesthetic appearance or immerse it in a dye bath.

With all of these different options available, it is crucial to decide on suitable plastic materials, manufacturing methods, and surface finishes before committing to having a part built.

How Your Local Machine Shop Can Help You Make the Best Surface Finish Choices

When you need a plastic component precisely built, you should have as much information upfront as possible before committing resources to the project. The expertise of a local machine shop can offer the necessary help to make your project a successful one. With their experience in building parts, they will advise you on the different manufacturing methods available and advise you on the right material choices. They will have the information to make the correct surface finish choices ensuring that your component turns out as expected.

At Plethora, we specialize in CNC machining to manufacture precision plastic components and fabricate metal injection molds to your exacting standards. Our production teams have years of experience working with different plastics and will help you make the best material and finishing choices for your needs. We are ISO 9001 certified, supporting our primary goal of manufacturing your parts and injection molds to the highest level of quality. To get started, upload your design files to Quote My Part or call us at 415-726-2256.

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The Plethora Team

The Plethora team is your go-to CNC manufacturer for hardware done right the first time. We have the tools and experience needed to create high quality custom parts quickly and with precision, whether you need a prototype or production run.

Topics: Design, Manufacturing, finishing, CNC machining, Quality, Prototyping, manufacturing trends