6.3 Materials selection
Good design fulfils the product specification under the required service conditions as well as contributing to the cost effectiveness of its manufacture and maintenance. The product specification itself must be an interpretation of the market needs. Hence good design means giving product appeal at the point of sale. Selecting the polymer is just one stage in this design exercise, both in terms of information on various properties of materials, as well as the detailed evaluation and selection of the best material for the product in question (Figure 60). It is obviously an important stage, however, and the variety of polymer types plus the many different grades within each type make that selection relatively difficult. Furthermore, each materials manufacturer supplies only proprietary products. Although a polymer type will be available from several sources the detailed characteristics of each grade will be unique to the supplier concerned. Hence a systematic approach to material selection is sensible.
The first selection stage is to eliminate those materials which do not have reasonable mechanical properties at the extremes of the anticipated service temperatures and when in contact with any fluids, greases, etc., which may be encountered. Note the concentration on what the materials cannot do. Understandably, but unfortunately, data from materials suppliers tends to concentrate on the positive qualities of their materials. Frequently there are clear-cut requirements for the material, e.g. it must be transparent, or must meet a fire performance specification of the Underwriters’ Laboratory. These go / no-go factors should also be considered at an early stage in order to reduce further the number of candidate materials. Some of the most important properties used for such initial screening include density, short-term tensile modulus and tensile strength at ambient temperature. Detailed property data for design must be obtained from the suppliers of the materials and may need to be supplemented by in-house testing. However, materials selection cannot be finalised without making and testing prototypes.