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

20.10.2021   |   Hen­ning Schröer

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Wher­ev­er screen­ing and sort­ing is required, rub­ber stars are in use. The mate­ri­als con­veyed come from a wide vari­ety of sources — for exam­ple, recy­cled mate­r­i­al, organ­ic mat­ter from com­post­ing, or stony debris. In order to achieve the longest pos­si­ble ser­vice life, they should be made of a mate­r­i­al that is opti­mal­ly suit­ed for the respec­tive area of appli­ca­tion. Jäger can rely on a broad base of suc­cess­ful projects and make rec­om­men­da­tions based on experience.

Which mate­ri­als are avail­able for stars for screening?

Two main mate­r­i­al groups have become estab­lished for indus­tri­al rub­ber stars: Nitrile rub­ber (NBR) and ther­mo­plas­tic elas­tomers, most­ly ure­thane-based (TPU). Both are char­ac­ter­ized by dif­fer­ent prop­er­ties in pro­duc­tion and prac­ti­cal use.

Nitrile rub­ber is a syn­thet­ic rub­ber with a low pur­chase price and high resis­tance to water, oils and greas­es. How­ev­er, design­ers have to make slight con­ces­sions when it comes to abra­sion. Good com­pounds show excel­lent val­ues here, but these are high­er than those of PU plastics.

Ther­mo­plas­tic polyurethane is avail­able in two vari­ants: ether-based and ester-based. Both mate­ri­als have sim­i­lar char­ac­ter­is­tics, but dif­fer specif­i­cal­ly in terms of mechan­i­cal abra­sion and their resis­tance to hydrol­y­sis. Ether-based ther­mo­plas­tic polyurethane (or ether TPU) is resis­tant to water and microbes, but has less good mechan­i­cal prop­er­ties in com­par­i­son, espe­cial­ly some­what high­er abra­sion. For ester-based TPU (or also ester TPU), the behav­ior is reversed.

Which fac­tors influ­ence the choice of material?

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.

Func­tion­al­i­ty of a screen deck

Guide: How do I choose the right material?

Choos­ing the right mate­r­i­al for a screen deck essen­tial­ly depends on four key questions:

1. What is the bud­get for the project?

2. How large do you want the screen deck to be?
These two ques­tions are relat­ed, as they both involve the mon­e­tary side of the man­u­fac­tur­ing project and they are often cor­re­lat­ed. If it is a sin­gle screen deck with only a few stars for screen­ing, NBR is the bet­ter mate­r­i­al because of the cheap­er mold. Fur­ther­more the increased process costs are some­what rel­a­tivized with low pro­duc­tion vol­umes. In the case of large or mul­ti­ple screen decks, on the oth­er hand, TPU is to be pre­ferred, since the mold recoups over time and the low process costs come into sharp­er focus.

3. Which mate­r­i­al is con­veyed?
The feed mate­r­i­al influ­ences which media the stars for screen­ing come into con­tact with, which again has con­se­quences for the mate­r­i­al selec­tion. For wet mate­ri­als, NBR or ether TPU are prefer­able because of their hydrol­y­sis resis­tance. With dry mate­r­i­al, on the oth­er hand, Ester-TPU is the bet­ter choice, as the bet­ter mechan­i­cal prop­er­ties get more impor­tant. If, on the oth­er hand, the feed mate­r­i­al con­tains oil, the stars for screen­ing should either be made of NBR or of cer­tain TPU com­pounds spe­cial­ly designed for oily materials.

4. What does the geom­e­try look like?
The thick­er the wall, the longer the cycle per unit pro­duced in gen­er­al. In this case, it must be decid­ed on a case-by-case basis, depend­ing on the mate­r­i­al and pro­duc­tion vol­ume, which pro­duc­tion method is suitable.

These ques­tions give a direc­tion for the main influ­ences. How­ev­er, the choice of mate­r­i­al is not always clear. It hap­pens that dif­fer­ent mate­ri­als seem to do the job; for exam­ple, if a large screen deck with mas­sive stars for screen­ing and long ser­vice life is desired, which is to process moist cut­tings that some­times also con­tain oily seeds — and all this with a low project bud­get. In such a case, it is advis­able to call on the con­sult­ing ser­vices of a rub­ber and plas­tics pro­duc­er to select the right mate­r­i­al in each indi­vid­ual case.

Con­clu­sion

On the sur­face, mate­r­i­al selec­tion for stars for screen­ing is quite sim­ple, as there are often only three mate­ri­als to con­sid­er: nitrile rub­ber (NBR) and ether- and ester-based ther­mo­plas­tic polyurethane (TPU). All three dif­fer in terms of their cost struc­ture and their chem­i­cal and phys­i­cal prop­er­ties. Which mate­r­i­al is most suit­able always depends on the con­text. It is there­fore advis­able to ana­lyze the appli­ca­tion area of the screen deck in detail before select­ing the material.

Whitepa­per:
Secure the Start of Production 

Learn which fac­tors influ­ence your SOP! 

Author: Hen­ning Schröer

Hen­ning Schröer stud­ied M. Sc. Indus­tri­al Engi­neer­ing at the Leib­niz Uni­ver­si­ty of Han­nover with a focus on pro­duc­tion engi­neer­ing. He joined Jäger in 2019 as a sales engi­neer for the Hanover site. 

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