Polymer bushings are found in everything from the food processing industry, where they meet the challenging demands of sanitation and sterilization, to the rugged environments and high shock loads encountered by rear hub bushings on bicycles.
Quick Review: Why Polymer Bushings?
Bushings, which are also referred to as plain bearings, are used to reduce the level of friction between two surfaces that are in rotating or sliding contact with one another. They can also serve secondary functions, such as providing additional support and alignment. Polymer bushings are replacing more and more metal bushings because polymers are typically lighter-weight, are more corrosion resistant, lower friction, often dry-running, and can be enhanced with fillers to improve properties such as wear-resistance and strength.
Manufacturing Polymer Bushings
When it comes to manufacturing polymer bushings, there are two primary methods to choose from: machining and injection molding. Both of these methods can generate reliable bearings to extremely tight tolerances, but they have significant differences and situations where one is preferred over the other.
Machining Polymer Bushings
When someone refers to machining, most people think of working with metals such as steel and aluminum, but plastics can also be machined. Machining is a material removal process where the material that is not needed in the final part geometry is removed with a cutting tool and may involve numerous steps.
Depending on the part geometry, this may involve a lathe (for parts with rotational symmetry). Most modern machining facilities use CNC (Computer Numerical Control) and CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing) to make parts that meet strict tolerances and are consistent in their dimensions. For polymer bushings, CNC lathes are typically used and multiple process steps are required to achieve the final part geometry and surface finish, including boring, reaming, and facing.
Benefits to machining include a short lead time and cost-effectiveness for low volume production runs of less than 1,000. Machining also avoids residual stresses (which we will discuss in a moment) that can warp the bushing and works extremely well when there are very tight tolerances or thin walls. Machining is also well adapted for situations where there is a need for non-standard shapes or bushing dimensions.
Injection Molding Polymer Bushings
Injection molding is a commonly used polymer manufacturing process that forces molten plastic into steel molds at extremely high pressures. Injection molding machines use a screw system to transport the plastic at high pressure into detailed molds that are typically designed to make multiple parts at a single time. The major expense in injection molding lies in the engineering and fabrication of the molds.
Injection molding of bushings is fast once the tooling is engineered and machined, ideal for production runs over 5,000 parts, and works exceptionally well for bushings that are standard-sized. However, injection molding does have its drawbacks. The most problematic drawback in the context of polymer bushings is residual stresses that develop as the part cools, but it is possible to eliminate these stresses through plastic annealing.
There can also be issues with shrinkage and dimensional change, which can make it a poor option if the plastic bushings need to meet high tolerances. In addition, injection molding is not a reliable approach if the bushings have thin walls.
The demand for polymer bushings and plain bearings continues to rise. When it comes time to specify the bushings for an application, it is important to choose the best manufacturing method. For situations with small production runs, non-standard dimensions, tight tolerances, or thin walls, polymer bushings should be machined. When a large production run is involved and the bushings have standard dimensions and/or geometries, injection molding is the best option.