To the point

Optimal sprue position and gate shape for plastic parts

Long-term, often very expen­sive practical experi­ence shows that most errors in tool design are made in the area of injec­tion. Cost-inten­sive correc­tions can be avoided if a design check­list is syste­ma­ti­cally worked through before the tool is designed. The practi­tioner with the relevant profes­sional experi­ence certainly knows and takes into account all the construc­tive aspects mentioned here. Nevert­heless, banal errors still occur in day-to-day business, the number of which can be reduced by consis­t­ently working with the questions compiled here (Table 1). If just one of these questions is answered with “no”, this can lead to subse­quent, expen­sive changes to the tool. The check­list can also be incor­po­rated into a failure mode and effects analysis (FMEA) for tool design. The indivi­dual problems are explained below.

economics

If rework is required to remove the sprue, this must be taken into account when calcu­la­ting the part price. The most common self-separa­ting sprue system with only one parting level that can be operated fully automa­ti­cally is the tunnel sprue. The optimal gate shape with regard to gentle material proces­sing is the bar gate, which, however, is material and post-proces­sing inten­sive and increases manufac­tu­ring costs due to the exten­sion of the cooling time.

Optical aspects

Often the optimal gate location from a technical point of view cannot be realized because the part has to meet deman­ding optical requi­re­ments at this point. In some circum­s­tances, the use of a needle gate nozzle may be considered.

Mold filling

The relati­onship between flow path and wall thick­ness must enable complete mold filling. In addition to complex simula­tion software, the flow curves offered by raw material manufac­tu­rers have proven useful here. These — possibly supple­mented with the graphical filling image method — allow an initial reliable check of the mold filling as long as the geome­tries are relatively simple with uniform wall thick­nesses.

Weld lines

Weld lines should be located in areas of the injec­tion 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 signi­fi­cantly increases tooling costs.

Air pockets

It must be ensured that the air in the cavity can escape. We recom­mend air grooves on all ejectors as well as a circum­fe­ren­tial venti­la­tion groove around the entire tool cavity, which is connected to the outside. Ejectors are the best form of venti­la­tion because they clean themselves through their movement. In contrast, sintered inserts or capil­lary inserts have the disad­van­tage that they quickly become clogged due to outgas­sing from the material, thereby affec­ting produc­tion relia­bi­lity.

Free jet bonding

To avoid a free jet, the injec­tion must be carried out against a tool wall and not into the free cavity. This is actually a banal error, but one that is nevert­heless frequently observed (Figure 1). Although the free jet forma­tion can be mitigated by injec­ting with a profile, this modifi­ca­tion of the injec­tion should be reserved for optimi­zing the process and should not be used to conceal tool errors. If neces­sary, the tool must be supple­mented with an auxiliary core in the mold. This may possibly be withdrawn during the reprint phase.

Shear heating

The shearing of the thermo­pla­stic material during injec­tion must not lead to extreme heating of indivi­dual tool parts. To avoid high tempe­ra­tures, tempe­ra­ture control channels should be provided near the gate. If it is justi­fiable with the tool contour and function, a tempe­ra­ture control hole should be provided with a separate connec­tion opposite the injec­tion point, if possible, so that the shear heat can be speci­fi­cally dissi­pated here. In principle, there must be a tempe­ra­ture control hole in the injec­tion area of hot runner nozzles in order to be able to dissi­pate the inevi­table excess heat from the electri­cally heated nozzle.

Shear stress

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 there­fore be made as large as possible. It may also be possible to split a gate into two or more gates to reduce shear. The forma­tion of a weld seam between the cuts must be taken into account. Develo­p­ments in the hot runner sector, especi­ally in extern­ally heated systems, now also allow the proces­sing of relatively tempe­ra­ture-sensi­tive materials. Another option that is still rarely used are liquid-tempered hot runner systems.

Balanced sprue system

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 deter­mine graduated distri­butor cross sections (Figure 2). In general, a naturally balanced sprue system should be used — regard­less of whether it is hot or cold — and only in excep­tional cases should balan­cing be carried out using diffe­ren­tiated distri­butor cross sections, since the visco­sity of the plastic melt depends on the shear that occurs and the tempe­ra­ture. This means that the balan­cing is only exact in one point, namely the calcu­lated point. If the proces­sing parame­ters are subse­quently corrected, a correc­tion of the cross sections is usually also unavo­idable.

Balanced sprue system

Hesitation effect

To avoid freezing of the flow front due to the delay effect (hesita­tion effect), the mold must be filled from large wall thick­nesses towards small wall thick­nesses. Thin-walled areas near the gate (e.g. film hinge) should be avoided. The old rule of plastic compo­nent construc­tion of prefer­ring uniform wall thick­nesses is justi­fied here. With diffe­rent wall thick­nesses, the melt only flows along the path of least resis­tance, i.e. in the direc­tion of the greater wall thick­ness. However, the melt then remains on the parts of the smaller wall thick­nesses until suffi­cient pressure has built up. The shear rate approa­ches zero due to the standstill of the mass and the melt front freezes due to its intrinsic visco­sity (Figure 3).

Reprint

To avoid voids, sink marks, etc., suffi­cient pressure must be ensured. The sprue position should there­fore be chosen so that you are spraying onto a thick wall that is prone to sink marks. Another possi­bi­lity is to intro­duce flow aids between the sprue and mass accumu­la­tions to ensure the supply of additional pressure.

Default

By optimally choosing the sprue position (e.g. injec­tion of a ruler on the narrow side), distor­tion can be largely avoided. Especi­ally with fiber-reinforced materials, but also with semi-crystal­line thermo­pla­s­tics without reinforce­ment, it is important to take the question of poten­tial distor­tion into account when selec­ting the correct injec­tion point. Often several cuts or a film sprue have the effect of reducing distor­tion.

Pictures: Europlast Ep-Kunst­stoff­technik GmbH

author

Dipl.-Ing. Elmar Nachts­heim, born in 1959, trained as a toolmaker before working as a designer at Zeller Plastik KG, Zell/Mosel. After studying plastics techno­logy at Darmstadt Univer­sity of Applied Sciences, he was respon­sible for the injec­tion molding techno­logy division at Formplast-Reichel GmbH, Besig­heim. Since 1998 he has been managing director of Europlast Ep-Kunst­stoff­technik GmbH, Ilsfeld.