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Rotation at maximum speed

Innovative metal replacement with high-precision plastic parts made of PBT/ASA

When desig­ning a new rotating viewing window for deman­ding appli­ca­tions in mecha­nical enginee­ring, an intel­li­gent choice of materials and tool design was required. A tool concept with a tunnel connec­tion and symme­trical sprue distri­butor as well as sophisti­cated tempe­ra­ture control was designed and imple­mented.

Tips for practitioners

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The material matrix is intended to help developers who are not native to the plastics industry narrow down the right material. The matrix is deliberately kept simple, but is intended to help avoid major wrong decisions.

Practice has shown that new develo­p­ments often take wrong paths when it comes to material selec­tion, which are diffi­cult to correct later. If a new product is created, the materials must be largely narrowed down at an early stage of construc­tion, on which, for example, strength calcu­la­tions or shrin­kage are based. Some proper­ties are provided with averaged real values, others only with school grades in order to be able to compare (1: high value / 6: lowest value)

If the raw material price is considered too late because the material was selected based on purely technical aspects, this can endanger an entire project or at least delay it enorm­ously. However, prices are subject to a wide range of fluctua­tions, especi­ally for standard plastics. Shown here is a snapshot of medium quality, with purchase quanti­ties of around 1000 kg (as of September 2001).

In addition to calcu­la­ting its own weight, the density of the material is also included in the calcu­la­tion, since the volume of a plastic part is prede­ter­mined and the weight is only calcu­lated using the specific weight. Here, looking at the volume price is an interes­ting state­ment.

Strength here means the tensile strength, knowing the fact that plastic compon­ents are prima­rily calcu­lated accor­ding to the maximum elonga­tion. This means that you define the environ­mental influences and then look in the tables for the strengths that you can expect the compo­nent to have in order not to exceed the maximum limit elonga­tion.

The level of stress to which a compo­nent is exposed is crucial for its tempe­ra­ture resis­tance. It is also important how long a compo­nent is exposed to high tempe­ra­tures. The values given here are average values of the usage tempe­ra­ture over a medium exposure time and serve as a guide only.

It is important for the designer to know how far the material flows in the mold. Average flow lengths with 2 mm compo­nent thick­nesses are mentioned here. It is important to know that the flow paths increase dispro­por­tio­na­tely with thicker walls. Further­more, in every material group there are also types with better and worse flow proper­ties. And lastly, it must be mentioned that hot runner techno­logy with multiple connec­tions makes it possible to produce parts with a high wall thick­ness-flow path ratio.

For proces­sing shrin­kage during injec­tion molding, only average values are given for a wall thick­ness of 2 — 3 mm and optimal part proces­sing, which are based on years of experi­ence. Due to tool tempe­ra­tures that deviate from the manufacturer’s speci­fi­ca­tions, as well as inade­quate tempe­ra­ture control channels in the tool and extreme cycle times, the real shrin­kage values can deviate signi­fi­cantly from the values stated here. For this reason, the infor­ma­tion provided by raw material manufac­tu­rers often covers a wide range.

The draft angles are important for removing the compon­ents from the mold without drawing marks. Due to the diffe­rent shrin­kage of the materials, the required demoul­ding angles also vary. Further­more, thin walls require larger demol­ding trays than thicker walls. VDI level 30 corre­sponds to a medium struc­ture with a roughing depth of 3.5 μm, although the tool should be designed with VDI level 33 due to the somewhat reduced image.

The average cavity pressure is, on the one hand, a calcu­la­tion variable for the design of the injec­tion molding machine, where strong fluctua­tions can occur depen­ding on the geometry of the compo­nent, and on the other hand, the demol­ding forces also increase as the cavity pressure increases. The number and design of the ejectors must be adapted to these forces.

Impact strength is an important aspect for the suita­bi­lity of plastic compon­ents for everyday use. In addition, the struc­tural design contri­butes signi­fi­cantly to the durabi­lity of a compo­nent after impact stress. The influence of water absorp­tion in polyamide and the general possi­bi­lity of impact modifi­ca­tion with, for example, elastomer modifiers should be mentioned here.

The chemical resis­tance of a plastic is a very complex topic because, in addition to the large number of chemical substances, their concen­tra­tion and the ambient tempe­ra­ture also play a major role. In addition, most common chemi­cals are mixtures of a variety of indivi­dual substances. This means that when asses­sing chemical resis­tance you have to go through long lists of substances and the substances with which the compo­nent later comes into contact are often not tested or these substances are diffi­cult to deter­mine. In general and very simply, one can say: Amorphous plastics (mostly trans­pa­rent plastics, if not colored) have poor chemical resis­tance. Parti­ally crystal­line plastics (mostly opaque plastics, if not colored) have good chemical resis­tance.

Apart from special proces­sing processes (with very fast cooling speeds), all amorphous plastics are more or less trans­pa­rent and all semi-crystal­line plastics are opaque and trans­lu­cent if the wall thick­ness is small. An excep­tion here, however, is ABS, which is opaque although amorphous because it has been copoly­me­rized from diffe­rent raw materials, which influences the refrac­tion of light. Basically, it should be noted that the dimen­sional requi­re­ments are often too high for plastic parts. However, it is often possible to cope with larger tolerances through a plastic-friendly design.

Property matrix of the most common thermoplastics

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.