Selecting plastic

It is unfortunate that plastics do not have all the advantages and none of the disadvantages of other materials but often overlooked is the fact that there are no materials that do not suffer from some disadvantages or limitations. The faults of materials known and utilized for hundreds of years are often overlooked; the faults of the new materials (plastics) are often over-emphasized.

As examples, steel is attacked by the elements of fire [1500 to 2500°F (815 to 1370°C)]. They lose all their strength, modulus of elasticity, etc. Common protective practice includes the use of protective coatings (plastic, cement, etc.) and then forgetting their susceptibility to attack is all too prevalent. Wood and concrete are useful materials yet who has not seen a rotted board (wood on fire, etc.) and cracked concrete. Regardless this lack of perfection does not mean that no steel, wood, or concrete should be used. The same reasoning should apply to plastics. In many respects, the gains made with plastics in a short span of time far outdistance the advances made in these other technologies.

To significantly extend the life of structural beams, hardwood (thicker than steel, etc.) can be used; thus people can escape even though the wood slowly burns. The more useful and reliable structural beams would be using reinforced plastics (RPs) that meet structural performance requirements with even a more extended supporting life than wood. To date these RPs are not used in this type of fire environment primarily because of their high cost.

Even though the range of plastics continues to be large and the levels of their properties so varied that in any proposed application only a few of the many plastics will be suitable. A compromise among properties, cost, and manufacturing process generally determines the material of construction. Selecting a plastic is very similar to selecting a metal. Even within one class, plastics differ because of varying formulations (Chapter 1), just as steel compositions vary (tool steel, stainless steel, etc.).

For many applications plastics have superseded metal, wood, glass, natural fibers, etc. Many developments in the electronics and transportation industries and in packaging and domestic goods have been made possible by the availability of suitable plastics. Thus comes the question of whether to use a plastic and if so, which one.

As an initial step, the product designer must know and/or anticipate the conditions of use and the performance requirements of the product, considering such factors as life expectancy, size, condition of use, shape, color, strength, and stiffness. These end use requirements can be ascertained through market analysis, surveys, examinations of similar products, testing, and/or experience. A clear definition of product requirements will often lead directiy to choice of the material of construction. At times incomplete or improper product requirement analysis is the cause for a product to fail.

As a general rule, until experience is developed, it is considered desirable to examine the properties of three or more materials before making a final choice. Material suppliers should be asked to participate in type and grade selection so that their experience is part of the input. The technology of manufacturing plastic materials, as with other materials (steel, wood, etc.) results in that the same plastic compounds supplied from various sources will generally not deliver the same results in a product. As a matter of record, even each individual supplier furnishes their product under a batch number, so that any variation can be tied down to the exact condition of the raw-material production. Taking into account manufacturing tolerances of the plastics, plus variables of equipment and procedure, it becomes apparent that checking several types of materials from the same and/or from different sources is an important part of material selection.

Experience has proven that the so-called interchangeable grades of materials have to be evaluated carefully as to their affect on the quality of a product. Another important consideration as far as equivalent grade of material is concerned is its processing characteristic. There can be large differences in properties of a product and test data if the processability features vary from grade to grade. It must always be remembered that test data has been obtained from simple and easy to process shapes and does not necessarily reflect results in complex product configurations. This situation is similar to those encountered with other materials (steel, wood, glass, etc.).

Most plastics are used to produce products because they have desirable mechanical properties at an economical cost. For this reason their mechanical properties may be considered the most important of all the physical, chemical, electrical, and other consideradons for most applicadons. Thus, everyone designing with such materials needs at least some elementary knowledge of their mechanical behavior and how they can be modified by the numerous structural geometric shape factors that can be in plastics.

0 0

Post a comment