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.