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