• DuPont Plastics

• Design Tips
  • Comparison of materials
  • Material selection
  • Wall thickness
  • Ribbing
  • Gate positioning
  • Cost-saving designs
  • General assembly technology
  • Welding technology
  • Tolerances
  • Check list
• Moulding Problems
  • Moisture in the Granules
  • Feed System Too Small
  • Wrong Gate Position
  • Hold Time Too Short
  • Wrong Melt Temperature
  • Wrong Tool Temperature
  • Poor Surface Finish
  • Problems with Hot Runners
  • Warpage
  • Mould Deposit
• What Can Plastics Do?
  • Chapter  1
  • Chapter  2
  • Chapter  3
  • Chapter  4
  • Chapter  5
  • Chapter  6
  • Chapter  7
  • Chapter  8
  • Chapter  9
  • Chapter 10
  • Chapter 11
  • Chapter 12

As much as necessary – as little as possible

In designing components made from engineering plastics, experience has shown that certain design points arise time and again, and can be reduced to simple design guidelines. One such point is wall thickness design, which has an important influence on component quality.

Effect on specific component criteria

Changing the wall thickness of a component has a significant effect on the following key properties:

• component weight
• achievable flow lengths in the mould
• production cycle time of the component
• rigidity of the moulded part
• tolerances
• quality of the component in terms of surface finish, warpage and voids.

— Fig. 2 —

— Fig. 1 —

Ratio of flow path to wall thickness

At an early design stage, it is important to review the question whether the required wall thicknesses can be achieved with the desired material. The ratio of flow path to wall thickness has a critical influence on mould cavity filling in the injection-moulding process. If long flow paths combined with low wall thickness are to be achieved in an injection mould, only a polymer with relatively low melt viscosity (easy-flowing melt) is suitable. To gain an insight into the flow behaviour of polymer melts, flow lengths can be determined using a special mould (Fig. 1 & 2).

Flexural modulus as a function of wall thickness

The flexural rigidity of a flat sheet is determined by the material-specific elastic modulus and the moment of inertia of the sheet cross section. If wall thickness is automatically increased to improve the rigidity of plastic components without any thought given to the consequences, this can very often lead to serious problems with partially crystalline materials. In the case of glass-fibre-reinforced materials, changing the wall thickness also influences the orientation of the glass fibres. Close to the mould wall, the fibres are oriented in the direction of flow. On the other hand, in the centre of the wall cross section, random fibre orientation occurs as a result of turbulent flow. By increasing wall thickness, it is mainly the cross-sectional area of randomly oriented glass fibres that is enlarged. On the other hand, the thickness of the zone in which the fibres are oriented in the direction of flow remains largely the same (Fig. 3).

— Fig. 4 —

— Fig. 3 —

This boundary zone, which critically determines component rigidity in the case of glass-fibre-reinforced plastics, is thus reduced in proportion to the overall wall thickness. This explains the decline in the flexural modulus (Fig. 4) when wall thickness is increased. The strength values determined on standard test bars (3,2 mm) are not therefore directly applicable to wall thicknesses deviating from this. To estimate component behaviour, it is essential to make use of safety factors.

So by increasing wall thickness without consideration of the consequences, material and production costs are increased without a significant improvement in rigidity.

— Fig. 6 —

— Fig. 5 —

Increase wall thickness?

An increase in wall thickness not only crucially determines mechanical properties but also the quality of the finished product. In the design of plastics components, it is important to aim for uniform wall thickness. Different wall thicknesses in the same component cause differential shrinkage which, depending on component rigidity, can lead to serious warpage and dimensional accuracy problems (Fig. 6). To attain uniform wall thickness, thick-walled areas of the moulding should be cored (Fig. 5). In this way it is possible to prevent the risk of void formation and reduce internal stresses. Furthermore, the tendency to warp is minimized. Voids and microporosity in the component severely reduce its mechanical properties by cross-sectional narrowing, high internal stresses and in some cases notch effects.