We know how to realise your requirements

Due to their properties, plastics generally require greater manufacturing tolerances than metallic materials. In particular, the significantly higher thermal linear and volumetric expansion compared to metallic materials and, where applicable, moisture absorption have an influence on the dimensional accuracy of plastics. In addition, all plastic products exhibit more or less strong, production-related material stresses, which are inevitably caused by the temperature difference between the core and the outer skin of the solidifying plastic melt.

When defining manufacturing tolerances, it must also be taken into account that dimensional changes rarely occur isotropically, i.e. evenly in all directions, due to the physical effects mentioned. These effects often differ significantly along and across the processing direction. In the processing direction, the effects are generally smaller than in the transverse direction due to the strong main valence forces.

It should also be noted that, as already mentioned, the stress equilibrium is often broken as a result of (re)machining. This can lead to severe material distortion, which often only becomes apparent days after machining or under the influence of temperature.

The basic tolerance grades IT 9 to IT 12 specified in the ISO system for tolerances and fits are therefore generally used for plastics. The degree of tolerance depends on the selected material and the fillers used.

For extremely dimensionally stable plastics, such as PI basic tolerance levels of IT 9 or IT 10 can be maintained. Extremely soft materials, such as PTFE can only be machined in accordance with IT 11 or IT 12. With certain high-performance plastics and assuming a suitable geometry, tolerance zones 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 moisture as well as material stresses and are generally in a range above 0.1 mm.

In the Injection moulding and the Press moulding technology the manufacturing tolerances depend not only on the parameters mentioned but also on the 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 dimension can be achieved. However, this requires a corresponding amount of effort in design and production. Depending on the material, press moulding technology only allows tolerances of 0.1 to 0.2 mm and more.

In order to take all essential aspects into account and to ensure the component function in the application, you should discuss the definition of the manufacturing tolerances with our application engineers.

We will be happy to take over the plastic-compatible component design for you and look forward to your enquiry. Enquiry.

Processing has a significant influence on the properties of plastics. Both plastics processing and mechanical processing influence the properties of plastics.

By designing the flow path accordingly, it is possible, for example, to control the molecular chains and thus align the component properties. The machining process can create additional Stresses in a material are introduced or released, which lead to distortion of the components.

In addition to the material properties, technical and economic feasibility also play a not insignificant role in the selection of a suitable processing method. Not every plastic can be processed with every process and certain technical requirements may require special processing methods.

Technical plastic components can essentially be manufactured using two processes: injection moulding, in which the components are moulded directly from the plasticised plastic under pressure, and machining, in which the components are shaped by machining (turning, milling, drilling, etc.). In some cases, both processes are used in combination to ensure efficient material utilisation on the one hand and tight manufacturing tolerances on the other.

There are also high-performance plastics such as PTFE or PI, which are not thermoplastic and therefore cannot be processed in the melt. Such materials are processed into semi-finished products for machining in a press/sintering process or moulded directly into components. This process allows the production of semi-finished products with more or less isotropic properties or the direct moulding of components with a simple geometry.

Due to the production-related material stresses that have already been mentioned several times, it is often necessary to thermally treat or post-treat the plastics.

Material stresses are caused by the rapid cooling of the outer skin of a plastic that is still hot in the core and partially plasticised during production. Although the stresses cannot be completely avoided by correct heat treatment, they can be significantly reduced. For this reason, semi-finished plastic products are always thermally post-treated.

However, heat treatment can also have disadvantages. Heat treatment that is too hot can cause lasting damage to the plastic. In addition, dimensional changes must be expected as a result of the process. When thermally treating hygroscopic plastics, it must be taken into account that the material is dried by the process. After heat treatment, the material will absorb moisture again until it reaches equilibrium. This results in an additional dimensional effect.

For complex components to be produced by machining, it sometimes makes sense to prefabricate the desired shape, treat the material thermally and only then finish it. This can accelerate the process of stress release so that the component remains more dimensionally stable.

The heat treatment of the polyamideimide Torlon® is of particular importance. Torlon® must undergo special heat treatment to ensure its outstanding mechanical and chemical properties. In particular, the exceptionally high wear resistance of Torlon® can only be achieved through thermal post-treatment.

In this thermal process, known as postcuring, a chemical process is triggered by precise temperature control. In addition to wear resistance, other mechanical properties are also positively influenced by the heat treatment. Strength and elasticity increase.

Postcuring also significantly increases the heat deflection temperature and the glass transition temperature (TG), but the process requires precise temperature control that is accurate to within ± 2K and dependent on the material cross-section, which should only be carried out by specialised companies.

Summary

When manufacturing technical components from high-performance plastics, the selection of a suitable processing method is of particular importance. In addition to the purely production-related aspects, economic considerations must also be taken into account.

Various processes are available, each with different advantages and disadvantages. Before designing technical components made of high-performance plastics, the necessary processing method should be selected and the design adapted accordingly. Our experienced application technology consultants offer support in this regard.