Thin-wall technology demands better design

There are several motivating factors driving thin-wall design. Material (and thus cost) savings is certainly one; lightweighting to benefit the consumer is another, and being able to fit more functional components into the same space can be a competitive edge. At the same time, thin walls also lead to faster moulding cycles, thus economizing in production costs.

Thin-wall technology means designing components so that they fulfill their function reliably, but use up no more material than is absolutely necessary. This is where the technological limits of conventional materials and moulding processes are reached. To produce thin-walled parts reliably at an acceptable cost, special attention must be paid to the design of the moulding and mould and to the injection moulding process.

At what point will the increased design costs and the higher mould costs often associated with this technology pay off? Even a few years ago, component design could be optimized empirically only through cost-intensive, time-consuming prototype construction or expensive arithmetical operations based on a high degree of experience. Now, sophisticated and reliable CAE programs, which provide highly accurate predictions, are available for these tasks.

Bayer, a supplier of engineering plastics, offers a technical service for optimizing component design that looks specifically at the criteria for thin-wall technology. The property profile of the intended materials is considered, and not merely for cost reduction. If thin-wall technology increases the utility of a component, the per-kilogram costs recede into the background.

The mobile telephone industry owes its rapid success to miniaturization of the telephone itself. As the components undergo miniaturization and weight reduction, it is also necessary to design housings that are smaller and more lightweight. Here then, thin-wall technology not only reduces the weight of an article, but also provides more space for electronic components within the same article size. As the technology allows moulders to produce housing parts for mobile telephones with wall thicknesses of .8 mm, some 17 cm3 of extra assembly space are gained within the same outer dimensions as earlier designs with conventional wall thicknesses. Additional benefits include a 30 percent weight savings over housing parts of conventional design and 10-second faster cycles.

These mini-telephones are exposed to shock loads at different temperatures, dust, and moisture. The most important task of the plastics housing is to protect the electronic components from these effects of the service environment. This, in turn, means that exceptional requirements are specified for the properties of the housing materials.

Assistance from CAE Programs

With the aid of flow simulation programs, the ideal gate position can be reliably determined. By predicting flow behaviour, it is possible at an early stage to check and optimize the design of an article with a view to its successful technical implementation. Figure 1 shows a Moldflow simulation program for a mobile telephone fed through a single gate. It can be clearly seen that the weld lines in the lower region freeze before the cavity has been completely filled. A much more favourable picture emerges if the cavity is filled via two gates (Figure 2). The mould can then be filled completely and with sufficient uniformity.

The ideal filling situation is achieved by a combination of direct gating in the keyboard area with a cold runner that distributes the melt to two points (Figure 3). This gating arrangement results in a good pressure curve and balanced loading of the mould (Figure 4). The dark-blue areas indicate low pressure.

High Demands on Mould Design

Mould design and the location of the gating system require special attention. Because of the low wall thickness, enormously high injection pressures are necessary. For thin-walled housings, pressures of about 200 MPa are usual. The pressure level is twice as high as in conventional injection moulding and consequently doubles the load on the mould. Different criteria, therefore, have to be applied in mould design, and generally, the moulds must be more rigid.

A mould for thin-walled parts that is designed on the basis of traditional criteria in terms of article size will exhibit excessive deformation and cause ejection problems. In an extreme case, it may even break. However, it is not usually necessary to double the mould dimensions. With thoughtful arrangement of the cavities and special measures to increase mould rigidity, sufficient production reliability can be ensured.

Mould rigidity, ejector systems, and the gating and cooling system are crucial to achievement of the fast cycles required.

Gate position is very important: this determines the filling pressure, the filling pattern with its flow fronts, and the pressure distribution in the mould. Because of the high pressures, unfavourably located gates have an increased adverse effect and impair article quality. For example, quality is impaired if one area of the cavity is overpacked while at another point, the melt still flows. In the area already filled, the extremely high injection pressure is fully exerted, while at the other point, there are pressure losses between the flow front and gate. This leads to asymmetrical loading of the mould.

Material Selection

The thinner the required wall, the better the flowability of the plastic melt has to be without compromising physical properties of the material. This calls for engineering plastics characterized by high impact strength, rigidity, heat deflection temperature, and dimensional accuracy. The materials must offer a wide processing range and allow parts made from them to be metallized on the internal surfaces to screen against electromagnetic waves.

Amorphous thermoplastics show a balanced combination of these properties. Within this class of materials, special material grades have been developed to meet the exceptional requirements of thin-wall technology. The most important material classes are ABS, PC+ABS blend, and PC.

The designer has the task of optimizing both the moulding and the injection mould to take account of the special flow conditions and pressure requirements of thin-wall technology.

Product Design

The rigidity of the components is an important aspect meriting special attention with the continual reduction in the wall thickness of housing parts. In the case of mobile telephones, however, the rigidity of the complete, assembled component is the crucial factor, while the individual housing halves can be quite flexible. The component must offer adequate rigidity to protect the telephone from critical deformation on exposure to mechanical loads.

The rigidity of a component does not solely depend on its wall thickness. The effect of internally fitted reinforcing elements, for example the baseplate, and the way in which the individual parts, including the housing, are connected together are also crucial. With mobile telephones, there are no generally specified guide values for rigidity. Ultimately, the initial prototype trials show whether the properties required by the manufacturer have been delivered.

Another aspect relating to mechanical properties is the behaviour of the component on exposure to shock loads. The component must not break if dropped. Fulfillment of this requirement depends both on the impact strength of the material and the component design. For example, abrupt changes in rigidity cause stress concentration points that can lead to housing fracture, even at low loads.

On the other hand, sink marks and shrinkage are less of a problem. At the point where a reinforcing rib connects to the base, a material accumulation generally occurs. Depending on the thickness of this material accumulation and the pressure conditions in the mould, sink marks can form at these points.

With thin-wall mouldings, the filling pressure is very high and the melt partially freezes under this high filling pressure. In addition, the material accumulation is small only because of the low base wall thickness. With amorphous thermoplastics, therefore, a rib/wall thickness ratio of 1:1 is possible.

Shrinkage depends not only on the material used but also on the design of the moulding and mould and on processing parameters. The high filling pressure and characteristic freezing behaviour have a beneficial effect so far as shrinkage of thin-walled structures is concerned. They shrink far less than conventional part designs.

Reliable Prediction

With the experience gained in numerous applications over the last few years and the support of simulation software, it is now possible to meet the high demands placed on moulding and mould design by thin-wall technology. On the material development side too, a specially tailored range of materials with suitable property profile is now available. A wide spectrum of applications is therefore opened up for this design principle that offers so much promise in so many directions.