PROCESSING PLASTICS
Manufacturing tolerances
Due to their properties, plastics generally require greater manufacturing tolerances than metallic materials. In particular, the significantly higher thermal expansion in length and volume compared to metallic materials as well as moisture absorption have an influence on the dimensional accuracy of the plastics. In addition, all plastic products have more or less severe, production-related material stresses that inevitably arise from the temperature difference between core and outer skin of the solidifying plastic melt.
For the determination of manufacturing tolerances, it must also be taken into account that dimensional changes due to the before mentioned physical effects rarely occur isotropically, i.e. uniformly in all directions. Often these effects differ significantly along and across the processing direction. In the processing direction, the effects due to the strong main valence forces are generally smaller than transverse to it.
In addition, it should be noted that, as already mentioned, the (post-)machining process often breaks the stress equilibrium. This can lead to severe material distortion which often becomes apparent days after machining or when exposed to temperature.
For plastics therefore, the basic tolerance grades IT9 to IT12 specified in the ISO system for limit dimensions and fits are generally used. The tolerance grade depends on the selected material and the fillers used in it.
In the case of extremely dimensionally stable plastics, such as PI, basic tolerance levels of IT 9 or IT 10 should be adhered to. Extremely soft materials, such as PTFE, can only be used in accordance with IT 11 or IT 12. With certain high-performance plastics and assuming a suitable geometry, tolerance fields of less than 0.05 mm can be maintained for components.
However, tolerances suitable for plastics take into account possible dimensional changes due to temperature and humidity as well as material stresses and are usually in a range above 0.1 mm.
In injection molding and compression molding, the manufacturing tolerances do not only depent on the before mentioned parameters, but also on material shrinkage and the path-cross-section ratios, and are therefore even more difficult to define.
With materials such as Torlon® or Victrex® PEEK, tolerances of up to 0.03% of the nominal size can be achieved, but, however, respective efforts have to be made in design and manufacture. The press molding technology allows - depending on the material - only tolerances of 0.1 to 0.2 mm and more.
In order to take into account all essential aspects and to ensure the component function in the application, you should discuss the definition of the manufacturing tolerances with our application engineers. We are happy to take over the plastic-compatible component design for you and look forward to your inquiry!