Polymer bushings can be produced by a variety of methods, such as injection molding, casting, machining, or compression molding. In this post, we are going to compare the pros and cons of injection molded versus machined polymer bushings.
Injection molding is a manufacturing process whereby a polymer part is produced by forcing molten plastic into a precision mold at high pressures and temperatures. This manufacturing method is excellent for high volume production runs (i.e., 5,000+) of standard polymer bushing sizes and shapes. Injection molded plastic bushings are known for dimensional accuracy and a smooth surface finish, as well as consistency in the dimensions of parts even over extended production runs.
However, if a non-standard bushing is needed, then custom dies and molds must be made which in turn drives up manufacturing costs and can extend the lead time. There may also be issues involving residual stresses which can in turn affect the shape, dimensions, and stability of the part; there are post-processing techniques such as annealing that can minimize or even eliminate these issues, though.
It is possible to make extremely complex parts with injection molding, however the overall part cost is directly tied to the complexity of the part. Polymer bushings, however, are typically simple enough that this is not a problem and makes injection molding an economical choice.
When a bushing is machined, it is manufactured by cutting away unnecessary material from solid stock. This does result in some material waste and is not a net-shape process like injection molding. In general, machining can provide outstanding dimensional accuracy and a good surface finish. It generally requires little to no custom tooling.
It is more appropriate for low volume production runs (i.e., less than 1,000), and may be more expensive than injection molding. However, it is much easier to obtain non-standard bushing sizes and shapes. In addition, machining can make it possible to achieve much higher tolerances because there is no inherent shrinkage involved as in injection molding. Machining is not subject to the same limitations in wall thickness as injection molding, and has a considerably shorter lead time.
In short, injection molding is best for high production bushing runs while machining works well for lower-volume production runs. If there is the potential for major issues with residual stresses affecting the shape and dimensions of a bushing, then machining might be the better option even for large production runs.
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