Plastics

Plastics are synthetic or semi synthetic viscoelastic materials composed of organic polymers and organic or synthetic additives. The behavior of plastics under stress depends on the length of time they are exposed to the stress and the temperature the exposure occurs. The applications for plastic materials are broad because the mechanical properties of plastics are defined by their molecular makeup which may be customized to meet the requirements of the desired application. The more common forms of plastics and their applications are styrene foam or Styrofoam which is used as insulation and packaging, PVC which is used in plumbing and nylon which is a flexible yet strong material used in the clothing industry.

Plastic Material Testing Goal:

Testing a plastic’s mechanical properties determines if it will meet the requirements of the desired application. Plastics are similar to metals in that they can be manufactured or produced using a wide range of procedures and material components in order to achieve the desired material characteristics. Typically the desired characteristics of plastic products are low cost, low weight, high toughness, high elasticity and ductility, and high strength in tension, compression, flexure, torsion and shear. These properties are defined by the chemical makeup of the material components used to produce the plastic and are more specifically dependent upon their molecular weight, hardness, density, and resistance to heat and solvents.

Common Plastic Test Methods:

Depending upon the expected stresses and loads of the application any number of tests will be performed upon a plastic material to predict its behavior under those forces. The most common tests of plastics are tensile tests, compressive tests, flexure tests, shear tests and torsion tests. Tension and compression tests involve the loading of the plastic along its longitudinal axis, but where tension tests pull the ends away from each other compression tests push them together. Shear testing can be similar to tension and compression tests but the key difference is in the offsetting of the ends of the test sample so that when the load is applied the outer surfaces of the sample move in opposing direction causing the sample to fail in a sliding motion. Flexure tests require the load to be applied to the midsection of the test sample so that it is bent into a “U” or “V” shape with the outside surface experiencing tensile forces and the inside face experiencing compressive forces. Torsion testing simply requires a rotational force or torque to be applied to one end of the sample while the other end is anchored. All of these tests are run until the sample breaks or ruptures with the stress and strain values recorded throughout the duration.

Plastic Testing Specimens:

There are four common methods for producing plastic test specimens: machining, die-cutting, injection molding and compression molding. Machining test specimens is achieved by taking a straight sample of the test material and shaping it into the desired shape by grinding or cutting away the undesired material and is usually done for more rigid plastics that may be subject to the formation of defects is created using one of the other processes. Die cut test samples are fairly easy to produce as many samples can be cut from a single sheet of material almost like a cookie cutter cutting cookies from a piece of dough. Injection molded specimens are produced by heating the plastic material until it is a liquid and injecting it into the mold of the desired shape, which allows for the production of highly specialized shapes. Specimens that are compression molded are formed by pushing the material into a mold so that is deformed into the shape of the mold. Choosing which process to use depends entirely upon the characteristics of the material that the test is performed upon.

Applicable Standards

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