Material selection for stars for screening:
NBR or TPU, which is more suitable?

Which material is more suitable for the production of stars for screening depends on the project budget, the production volume, the actual requirements and the geometry of the stars.

If we look from the calculatory side with regard to the production volume, the following point becomes clear: Vulcanizing molds for NBR components are less expensive than those used for TPU molded parts in the injection molding process. However, the process costs in TPU injection molding are typically lower for plastics above a certain series size (depending on the geometry). For small series, NBR is therefore more suitable because the mold recoups very quickly. For large series, on the other hand, injection-molded TPU is preferable. For the sake of completeness, PU-cast stars should also be mentioned. Here, the mold costs are also low, but the unit costs are higher than for the comparable product made of NBR. The biggest disadvantage here, however, is the long cycle time per product.

The mechanical and chemical properties of the materials also play a major role in material selection, especially hydrolysis resistance. If the feed material is potentially moist (e.g. organic material such as prunings or compost), stars for screening made of NBR or ether TPU are useful. These are less prone to molecular damage when in contact with water and are less susceptible to microbes. For oily feed materials (e.g., seeds), stars for screening made of NBR are well suited, but certain TPU compounds can also be oil-resistant.

However, the specific chemical and mechanical requirements for the star material are often difficult to define. In forestry, for example, the extent to which the feed material is moist or dry depends on the weather, and oils can also leak out if seeds happen to be present.

In addition, desired service life influences material selection. High abrasion means that the stars for screening wear out more quickly and thus lose their efficiency. They must be replaced at shorter intervals, which increases maintenance costs. However, abrasion, and thus service life, depends on the feed material. A screen deck that handles abrasive materials (e.g., boulders) will wear out faster than one that handles soft wood. In some cases, it may also make sense to reinforce stars for screening with metal to make them more resistant (so-called “armored stars for screening”).

Furthermore, the geometry of the stars is relevant for the material selection, especially wall thickness and weight. The wall thickness plays a role because it influences the cycle times during production. Solid NBR stars for screening have to be heated for a longer time period until the vulcanization of the material is completed than thin-walled versions. Vice versa, a TPU star for screening must be cooled in the injection molding process so that the end product retains its shape. A decision must be made on a case-by-case basis.