LOAD STRENGTH

At the outset, in addition to defining the amount of load, it is necessary to determine whether the part can deflect under load and, if so by how much.  In addition it is important to understand whether the load is frequent, infrequent or constant as this will have a bearing on part design and material selection.

Load strength is very similar to stiffness and ribs may be used to add strength.  For load bearing applications a cross-rib pattern is often used.

MATERIAL CONSIDERATIONS

As far as polymers are concerned, the tensile strength, strain and modulus figures are important.  These will illustrate breaking point, point of deformation with and without recovery.  In all cases the higher the figure, the better the performance under load.  What also needs to be borne in mind is that plastics can suffer permanent deformation, known as creep, under constant loading or loading beyond a certain point.  Using a glass fibre filled material will add to the stiffness of the part but will increase brittleness.  The two most popular glass-filled polymer families are polypropylene and nylon, choice will depend on what other factors need to be considered as there is a considerable difference in price.

Environmental factors must also be taken into account.  Temperature has an effect on polymers – the lower the temperature the more brittle the part will become whereas plastics become softer and more ductile at higher temperatures.  Materials vary widely in this respect, HDPE has particularly good low temperature performance, for example, but is relatively flexible.

Amorphous materials, such as ABS and Polycarbonate, are much stiffer by their nature and may be better suited to an application where little or no deformation is required.

FREE DESIGN GUIDE

For advice on all aspects of plastic injection moulding design request a free copy of the Rutland Plastics Design Guide.