In today's article, we will be discussing an important part of materials development and proper materials selection, i.e., testing and standardization of polymers with the Polymer Testing Equipment. The latter part of this article is therefore devoted to this aspect. It presents schematically (in simplified form) a number of standard test methods for plastics, highlighting the principles of the tests and the properties measured by them.
There are two stages in the process of becoming familiar with plastics. The first is rather general and involves an introduction to the unique molecular structures of polymers, their physical states, and transitions which have marked influence on their behavior. These have been dealt with in article "What is a Polymer". The second stage, which will be treated in this article, is more specific in that it involves a study of the specific properties of plastics which dictate their applications.
Besides the relative ease of molding and fabrication, many plastics offer a range of important advantages in terms of high strength/weight ratio, toughness, corrosion and abrasion resistance, low friction, and excellent electrical resistance. These qualities have made plastics acceptable as materials for a wide variety of engineering applications. It is important therefore that an engineer be aware of the performance characteristics and significant properties of plastics.
In this article plastics have been generally dealt with in respect to broad categories of properties, namely, mechanical, electrical, thermal, and optical. In this treatment the most characteristic features of plastic materials have been highlighted.
Testing for Mechanical Properties Of Plastic
Several unfamiliar aspects of material behavior of plastic need to be appreciated, the most important probably being that, in contrast to most metals at room temperature, the properties of plastics are time dependent [1-4]. Then superimposed on this aspect are the effects of the level of stress, the temperature of the material, and its structure (such as molecular weight, molecular orientation, and density). For example, with polypropylene an increase in temperature from 20 to 608C may typically cause a 50% decrease in the allowable design stress. In addition, for each 0.001 g/cm3 change in density of this material there is a corresponding 4% change in design stress. The material, moreover, will have enhanced strength in the direction of molecular alignment (that is, in the direction of flow in the mold) and less in the transverse direction.
Because of the influence of so many additional factors on the behavior of plastics, properties (such as modulus) quoted as a single value will be applicable only for the conditions at which they are measured.
Properties measured as single values following standard test procedures are therefore useful only as a means of quality control. They would be useless as far as design in concerned, because to design a plastic component it is necessary to have complete information, at the relevant service temperature, on the timedependent behavior (viscoelastic behavior) of the material over the whole range of stresses to be experienced by the component.
Testing for Stress and Strain limits of Polymers
Any force or load acting on a body results in stress and strain in the body. Stress represents the intensity of the force at any point in the body and is measured as the force acting per unit area of a plane. The deformation or alteration in shape or dimensions of the body resulting from the stress is called strain. Strain is expressed in dimensionless units, such as cm/cm, in./in., or in percentage.
Corresponding to the three main types of stress?tensile, compressive, and shear?three types of strain can be distinguished.
1. Tensile strain is expressed as elongation per unit length
2. Compressive strain as contraction per unit length
3. Shear strain
That is the end of our discussion about "Polymer Testing Equipment" and you should have a fairly good idea about the various aspects involved in Polymer Testing and the sort of Equipment required to carry it out. If you have any tips or suggestions that you think might be useful for other Plastic Injection Molding Design or Processing related engineers, then do mention them so all can benefit from them. Leave a comment below for such design or processing tips.
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