FINISH
Machining process
Complex and highly stressed plastic components, such as those used in the machines of the Food and packaging technology or conveyor technology are often manufactured using machining technology.
The tolerance requirements, the mechanical loads and the wall thicknesses required for these components, as well as the usually small quantities, make these processes sensible.
The semi-finished plastic products required for machining also usually have better mechanical properties than those used in the machining process. Injection moulding granulates used. Injection mouldable plastics require a lower viscosity due to the process and generally have a lower degree of crystallisation. Direct comparisons have shown that, for example, from Polyamide 6.6 machined plain bearings have a 30 to 40% longer service life than the comparable injection-moulded version.
Advantages of machining technology
Depending on the material, extremely tight tolerances can be achieved with machining technology for plastics. Component tolerances to ensure a high level of quality. For this reason, injection-moulded or pressed components are sometimes reworked by machining.
Another advantage of machining technology is the ability to produce components with large wall thicknesses. This means that pieces with cross-sections several centimetres thick can also be produced. Undercuts, metal inserts and complex boreholes that are subsequently closed again are also technically possible.
The extruded or pressed semi-finished products required for machining also usually have better mechanical properties than the granulates used in injection moulding technology. Injection mouldable plastics require a lower viscosity due to the process and generally have a lower degree of crystallisation. Extruded semi-finished products often show significantly better properties here. Direct comparisons have shown that, for example, bearings turned from semi-finished products have a 30 to 40 per cent longer service life than the comparable injection-moulded version.
Restrictions
Although machining technology enables a relatively free design, there are also some restrictions to be observed here.
The surfaces that can be achieved with machining are generally worse than with injection moulding or compression moulding. Roughness values of Rz 10 to 6.3 are possible. Particularly high-quality surfaces (less than Rz 6.3) can only be achieved using complex mechanical and sometimes manual processes (grinding, polishing, lapping, etc.), which significantly increase the processing costs.
Machined plastic components must always be manufactured from appropriately sized semi-finished parts. There is a risk of material stresses being released during machining. In the worst case scenario, the component can become extremely warped. Even after weeks, significant dimensional and shape changes can still occur, especially under the influence of temperature. Such stress-induced component changes can be minimised through complex processing with resting times and/or intermediate thermal treatment.
Taking all aspects into account when designing components is difficult and time-consuming and should therefore only be carried out by designers experienced with plastics. POLYTRON helps with the plastic-compatible design of components.
Combinations of spraying and machining
In the case of high-performance plastics, which often cost more than fifty euros per kilogramme, a combination of machining and injection moulding technology is sometimes used: in order to save high material and tool costs, a simple blank is produced using the injection moulding process. This is then subsequently machined to shape and tolerance. In this way, the advantages of both processing methods are optimally combined.