Design engineers can often draw on Mother Nature for help, as she often has the solution to design problems ready to hand. Even in our modern world, dominated as it is by computer technology, it is often worth looking at Nature's bag of tricks for new ideas. Like any other material, thermoplastics have their own profile of characteristics and it has to be taken into account when components are being designed. Once the area of use has been defined, it will be possible to specify the requirements for the component.

Fig. 1 Molded part

[ Strengths ]

• It enables plastic components to be designed in the right way for the plastic concerned, in order to take mechanical and rheological requirements into account
• It minimizes development costs
• Faults in existing components can be rectified quickly and at minimal cost.

[ Profile of requirements ]

Whether the final product is a suction tube or a monitor housing, the console of a washing machine or the fascia of a radio, every component has a profile of requirements specific to its application, and the design engineer has to take it into account in designing the component. With existing applications, he can often make use of previous experience or an existing re-quirement specification, but with a new application the conditions of operation have to be summarized in a new specification. The meticulous compilation of a production specification is the basis for the successful development of a component, and has a direct influence on the design of the mold.

Fig. 2 Requirement profile

[ Choice of materials ]

As part of the requirement profile, the design engineer is also given clear instructions on the choice of material. This defines not only the mechanical loads but also presents the environ-mental factors. In order to prevent any failure in use, the following points have to be clarified in advance.

• Mechanical load
  Compared with the other materials typically used by design engineers, thermoplastic materials display a dependence on time and temperature. It is therefore important to know whether the loads will be imposed at room temperature or at higher temperatures. Also, the duration or dynamic loads have to be assessed differently from forces which only have a single impact. Partly crystalline thermoplastic materials often represent the ideal solution to the problem of permanent dynamic loads; all according to the application involved, a choice can be made between non-strengthened, mineral-reinforced, or glass-fiber reinforced products. If the focus is on cosmetic requirements combined with moderate temperatures and loads, amorphous materials will often be used.
• Resistance to chemicals
  Contact with other chemicals can have disastrous consequences for the plastic that is being used. Whereas, for instance, polycarbonate is an outstandingly good impact-resistant product, it must never be allowed to come into contact with fuel because tension cracks can be caused in the plastic and its mechanical load-bearing capacity will be reduced.

When the choice of material has been made, work can start on the design of the component. Each material has its own profile of characteristics and it has to be taken into account in the design.

Fig. 3 Choice of material

[ Wall thickness ]

Generally speaking, the aim should be to achieve a low wall thickness but without ignoring the mechanical and rheological requirements. Although materials costs are often the prime consideration, component quality must not be neglected. Wall thicknesses of 2 to 3 mm have proved their value for heavy-duty components, both from the mechanical and from the rheo-logical point of view. For special applications in which stringent requirements are made on component weight, wall thickness of less than 1 mm can also be used although they require additional costs in processing. Differences in wall thickness in the main structure can have critical consequences. Differences in pressure build-up during filling lead to differences in the molding quality of the surface structure.

Fig. 4 Filling pressure curve

[ Surfaces ]

Because of their low structural rigidity, flat surfaces are more vulnerable to distortion than slightly curved ones. However, if a flat surface is required or necessary, the rigidity of the surface can be increased by means of ribbing on the its inside. The ratio of ribs to wall thick-ness must be retained as much as possible, in order to prevent surface disturbances.

Fig. 5 Surface design

[ Ribs and integrated supplementary functions ]

From the mechanical point of view, it would make sense to keep to a continuous wall thick-ness even in the ribs. However, the surface quality must also be included in the design, par-ticularly if the component has to meet visual requirements. At the connecting point of the ribs, if the wall thickness of the ribs and the main structure is constant, an accumulation of material occurs at in the area of the join which can cause an eruption on the visible surface. It is there-fore expedient to reduce the wall thickness of the ribs in order to avoid this accumulation. In practice, a ratio of rib to main wall thickness of 2:3 has proved its worth. As the eruption is due to differences in shrinkage, the position of the incisions can play an important role. Dur-ing the secondary pressure phase, the secondary pressure is used to compensate for part of the volumetric shrinkage by feeding in additional material. The eruption will be influenced by the effectiveness of the secondary pressure in the molding. If the rib lies at the end of the flow path, more visible eruption can be expected than with a rib near the beginning of the incision.

Fig. 6 Wall-thickness/rib ratio

[ Corners and edges ]

From the mechanical point of view, corners and sharp edges represent critical areas for all materials. Very special attention must be paid to such areas when a material is being used with mechanical properties which are dependent on the design of notches, as can be observed with thermoplastics. This applies in particular to the class of amorphous thermoplastics, where the dependence is very great. It is therefore advisable to provide all corners and edges with radii. Even breaking the edge with a radius of 0.1 mm will have consequences for the mechanical behavior of the component.

Fig. 7 Influence of notch shape on notch impact resistance

[ Accumulations ]

When the molding has finished being filled, thermoplastics undergo a dimensional change: shrinkage. This is dependent on wall thickness, which means that differences in shrinkage behavior can be seen with different wall thicknesses and accumulations of material. This leads to eruption points and, in extreme cases, to lumps in the component, which limit its perform-ance capability.

Fig. 8 Avoidance of melt accumulations

[ Demolding behavior ]

When the cooling phase has finished, the tool opens and the finished molding is thrown out by the ejector. During the filling phase, the cavity is completely filled with plastic under high pressure. Although the plastic shrinks back, all according to the design of the tool wall, the same process can cause it to shrink back onto the injection core. The resultant clamping forces can be so great that the ejector pin penetrates the component when the mold opens and the component is left hanging from the core. In order to avoid this phenomenon, walls and ribs lying at right-angles to the direction of ejection are provided with an ejection bevel which will enable the tool surface to be extracted more quickly. The deeper the surface notch pattern, the greater the ejection bevel must be in order to prevent the tips from shearing off.

Fig. 9 Demolding angle

[ Joining elements ]

Plastic components mostly consist of a number of individual parts which are either all made of plastic or of other materials. A number of different processes are suitable for connecting the parts together, and these can be separated into two main groups: permanent connections, and those that can be released.

• Screw fastenings
  The distinction is made here between metric and self-tapping screws. With the metric ones, a nut thread has to be pressed into the relevant bush in the plastic part. It must be pressed in at as high a temperature as possible, in order to avoid tension forces in the plastic which could cause it to fail later. The ATI "Embedding of connection elements by means of ultrasonic welding" provides additional information on this subject. In the case of self-tapping screws, the screw and the bush must be designed for one another in order once again to avoid excessive loads on the plastic. Relations and demandings placed on both, the screws and plastic bushes are described in ATI 1114 "Self-tapping screws for thermoplastic polymers"
• Snap fastenings
  There is a large number of possible ways of creating a snap fastening, but one factor is com-mon to all of them: during the joining process, a rear cut has to be overcome, during which process one join partner has to be pressed out of its resting position. This distortion creates tension forces in the connecting point of the join partner to the main part, which must not be allowed to exceed the permissible values of the material in use. Suggested desingns and mathematical methods for various snap systems are detailed in ATI 1119 " Snap joints and spring elements in plastics".

 ATI 1114: Self-tapping screws for thermoplastic polymers

 ATI 1119: Snap joints and spring elements in plastics

Fig. 10 Various snap joints

Fig. 11 Design of a screw dome