{"id":17364,"date":"2021-07-21T13:18:47","date_gmt":"2021-07-21T11:18:47","guid":{"rendered":"http:\/\/s522541291.online.de\/?p=17364"},"modified":"2024-10-25T15:02:03","modified_gmt":"2024-10-25T13:02:03","slug":"europlast-kunststoffe-ausgabe-10-98","status":"publish","type":"post","link":"https:\/\/ep-kunststofftechnik.de\/en\/europlast-kunststoffe-ausgabe-10-98\/","title":{"rendered":"EUROplast \u201cPlastics\u201d Issue 10\/98"},"content":{"rendered":"

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To the point
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Optimal sprue position and gate shape for plastic parts<\/h5>\n\n\n\n

Long-term, often very expen\u00adsive practical experi\u00adence shows that most errors in tool design are made in the area of injec\u00adtion. Cost-inten\u00adsive correc\u00adtions can be avoided if a design check\u00adlist is syste\u00adma\u00adti\u00adcally worked through before the tool is designed. The practi\u00adtioner with the relevant profes\u00adsional experi\u00adence certainly knows and takes into account all the construc\u00adtive aspects mentioned here. Nevert\u00adheless, banal errors still occur in day-to-day business, the number of which can be reduced by consis\u00adt\u00adently working with the questions compiled here (Table 1). If just one of these questions is answered with \u201cno\u201d, this can lead to subse\u00adquent, expen\u00adsive changes to the tool. The check\u00adlist can also be incor\u00adpo\u00adrated into a failure mode and effects analysis (FMEA) for tool design. The indivi\u00addual problems are explained below.<\/p>\n\n\n\n

<\/figure>\n\n\n\n
economics<\/h5>\n\n\n\n

If rework is required to remove the sprue, this must be taken into account when calcu\u00adla\u00adting the part price. The most common self-separa\u00adting sprue system with only one parting level that can be operated fully automa\u00adti\u00adcally is the tunnel sprue. The optimal gate shape with regard to gentle material proces\u00adsing is the bar gate, which, however, is material and post-proces\u00adsing inten\u00adsive and increases manufac\u00adtu\u00adring costs due to the exten\u00adsion of the cooling time. <\/p>\n\n\n\n

Optical aspects<\/h5>\n\n\n\n

Often the optimal gate location from a technical point of view cannot be realized because the part has to meet deman\u00adding optical requi\u00adre\u00adments at this point. In some circum\u00ads\u00adtances, the use of a needle gate nozzle may be considered. <\/p>\n\n\n\n

Mold filling<\/h5>\n\n\n\n

The relati\u00adonship between flow path and wall thick\u00adness must enable complete mold filling. In addition to complex simula\u00adtion software, the flow curves offered by raw material manufac\u00adtu\u00adrers have proven useful here. These \u2014 possibly supple\u00admented with the graphical filling image method \u2014 allow an initial reliable check of the mold filling as long as the geome\u00adtries are relatively simple with uniform wall thick\u00adnesses.<\/p>\n\n\n\n

Weld lines<\/h5>\n\n\n\n

Weld lines should be located in areas of the injec\u00adtion molded part where they cannot lead to strength problems. For complex molded parts, a mold filling study to find the weld seam areas can be worthwhile. A cascade gate system can avoid weld lines, but this signi\u00adfi\u00adcantly increases tooling costs.<\/p>\n\n\n\n

Air pockets<\/h5>\n\n\n\n

It must be ensured that the air in the cavity can escape. We recom\u00admend air grooves on all ejectors as well as a circum\u00adfe\u00adren\u00adtial venti\u00adla\u00adtion groove around the entire tool cavity, which is connected to the outside. Ejectors are the best form of venti\u00adla\u00adtion because they clean themselves through their movement. In contrast, sintered inserts or capil\u00adlary inserts have the disad\u00advan\u00adtage that they quickly become clogged due to outgas\u00adsing from the material, thereby affec\u00adting produc\u00adtion relia\u00adbi\u00adlity.<\/p>\n\n\n\n

Free jet bonding<\/h5>\n\n\n\n

To avoid a free jet, the injec\u00adtion must be carried out against a tool wall and not into the free cavity. This is actually a banal error, but one that is nevert\u00adheless frequently observed (Figure 1). Although the free jet forma\u00adtion can be mitigated by injec\u00adting with a profile, this modifi\u00adca\u00adtion of the injec\u00adtion should be reserved for optimi\u00adzing the process and should not be used to conceal tool errors. If neces\u00adsary, the tool must be supple\u00admented with an auxiliary core in the mold. This may possibly be withdrawn during the reprint phase.<\/p>\n\n\n\n

Figure 1: Injec\u00adtion into the free cavity leads to the forma\u00adtion of a free jet instead of a melt front<\/span><\/figcaption><\/figure>\n\n\n\n
Shear heating<\/h5>\n\n\n\n

The shearing of the thermo\u00adpla\u00adstic material during injec\u00adtion must not lead to extreme heating of indivi\u00addual tool parts. To avoid high tempe\u00adra\u00adtures, tempe\u00adra\u00adture control channels should be provided near the gate. If it is justi\u00adfiable with the tool contour and function, a tempe\u00adra\u00adture control hole should be provided with a separate connec\u00adtion opposite the injec\u00adtion point, if possible, so that the shear heat can be speci\u00adfi\u00adcally dissi\u00adpated here. In principle, there must be a tempe\u00adra\u00adture control hole in the injec\u00adtion area of hot runner nozzles in order to be able to dissi\u00adpate the inevi\u00adtable excess heat from the electri\u00adcally heated nozzle.<\/p>\n\n\n\n

Shear stress<\/h5>\n\n\n\n

The gate geometry (e.g. tunnel gate) must not lead to such a high shear stress that the processed material is damaged. Basically, the bleed should there\u00adfore be made as large as possible. It may also be possible to split a gate into two or more gates to reduce shear. The forma\u00adtion of a weld seam between the cuts must be taken into account. Develo\u00adp\u00adments in the hot runner sector, especi\u00adally in extern\u00adally heated systems, now also allow the proces\u00adsing of relatively tempe\u00adra\u00adture-sensi\u00adtive materials. Another option that is still rarely used are liquid-tempered hot runner systems.<\/p>\n\n\n\n

Balanced sprue system<\/h5>\n\n\n\n

For multiple tools or parts with multiple gates, the gating system must of course be balanced. If this is not possible, a mold filling study must be carried out to deter\u00admine graduated distri\u00adbutor cross sections (Figure 2). In general, a naturally balanced sprue system should be used \u2014 regard\u00adless of whether it is hot or cold \u2014 and only in excep\u00adtional cases should balan\u00adcing be carried out using diffe\u00adren\u00adtiated distri\u00adbutor cross sections, since the visco\u00adsity of the plastic melt depends on the shear that occurs and the tempe\u00adra\u00adture. This means that the balan\u00adcing is only exact in one point, namely the calcu\u00adlated point. If the proces\u00adsing parame\u00adters are subse\u00adquently corrected, a correc\u00adtion of the cross sections is usually also unavo\u00adidable.<\/p>\n\n\n\n

Balanced sprue system<\/h5>\n\n\n\n
Figure 2: If the sprue system is not naturally balanced, atten\u00adtion must be paid to graduated distri\u00adbutor cross sections<\/span><\/figcaption><\/figure>\n\n\n\n
Hesitation effect<\/h5>\n\n\n\n

To avoid freezing of the flow front due to the delay effect (hesita\u00adtion effect), the mold must be filled from large wall thick\u00adnesses towards small wall thick\u00adnesses. Thin-walled areas near the gate (e.g. film hinge) should be avoided. The old rule of plastic compo\u00adnent construc\u00adtion of prefer\u00adring uniform wall thick\u00adnesses is justi\u00adfied here. With diffe\u00adrent wall thick\u00adnesses, the melt only flows along the path of least resis\u00adtance, i.e. in the direc\u00adtion of the greater wall thick\u00adness. However, the melt then remains on the parts of the smaller wall thick\u00adnesses until suffi\u00adcient pressure has built up. The shear rate approa\u00adches zero due to the standstill of the mass and the melt front freezes due to its intrinsic visco\u00adsity (Figure 3).<\/p>\n\n\n\n

Figure 3: The freezing of the melt due to the hesti\u00adta\u00adtion effect leads to incom\u00adplete mold filling in the thin-walled area<\/span><\/figcaption><\/figure>\n\n\n\n
Reprint<\/h5>\n\n\n\n

To avoid voids, sink marks, etc., suffi\u00adcient pressure must be ensured. The sprue position should there\u00adfore be chosen so that you are spraying onto a thick wall that is prone to sink marks. Another possi\u00adbi\u00adlity is to intro\u00adduce flow aids between the sprue and mass accumu\u00adla\u00adtions to ensure the supply of additional pressure.<\/p>\n\n\n\n

Default<\/h5>\n\n\n\n

By optimally choosing the sprue position (e.g. injec\u00adtion of a ruler on the narrow side), distor\u00adtion can be largely avoided. Especi\u00adally with fiber-reinforced materials, but also with semi-crystal\u00adline thermo\u00adpla\u00ads\u00adtics without reinforce\u00adment, it is important to take the question of poten\u00adtial distor\u00adtion into account when selec\u00adting the correct injec\u00adtion point. Often several cuts or a film sprue have the effect of reducing distor\u00adtion.<\/p>\n\n\n\n

<\/p>\n\n\n\n

Pictures: Europlast Ep-Kunst\u00adstoff\u00adtechnik GmbH <\/p>\n\n\n\n

author<\/h5>\n\n\n\n

Dipl.-Ing. Elmar Nachts\u00adheim, born in 1959, trained as a toolmaker before working as a designer at Zeller Plastik KG, Zell\/Mosel. After studying plastics techno\u00adlogy at Darmstadt Univer\u00adsity of Applied Sciences, he was respon\u00adsible for the injec\u00adtion molding techno\u00adlogy division at Formplast-Reichel GmbH, Besig\u00adheim. Since 1998 he has been managing director of Europlast Ep-Kunst\u00adstoff\u00adtechnik GmbH, Ilsfeld.<\/p>","protected":false},"excerpt":{"rendered":"

Our specia\u00adlist article on the optimal sprue position and gate shape for plastic parts was published in the specia\u00adlist magazine Kunst\u00adstoffe (issue 10\/98).<\/p>","protected":false},"author":1,"featured_media":17391,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"brizy-blank-template.php","format":"standard","meta":{"wp_typography_post_enhancements_disabled":false,"footnotes":""},"categories":[10,8],"tags":[15,17,16],"class_list":["post-17364","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-aktuelles","category-presse","tag-events-2","tag-innovation-2","tag-technologie-2"],"brizy_media":[],"_links":{"self":[{"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/posts\/17364","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/comments?post=17364"}],"version-history":[{"count":32,"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/posts\/17364\/revisions"}],"predecessor-version":[{"id":24452,"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/posts\/17364\/revisions\/24452"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/media\/17391"}],"wp:attachment":[{"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/media?parent=17364"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/categories?post=17364"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ep-kunststofftechnik.de\/en\/wp-json\/wp\/v2\/tags?post=17364"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}