As engineering plastics replace metals in many applications, engineers are having to come to grips with a whole new lexicon of performance parameters to ensure they specify the right material for the job. But before the enterprising engineer sets foot in this fascinating new world of high performance plastics, it is critical to become familiar with some of the key measures and limits of thermoplastic performance.
 Tensile Strength which is a major measure of strength (although strength can also refer to compression strength). The ability of a material to resist breaking under tensile stress is one of the most important and widely measured properties of materials used in structural applications. The force per unit area (MPa or psi) required to break a material in such a manner is the ultimate tensile strength.
A common engineering thermoplastic such as Ertalon 6PLA has a tensile strength of 85MPa, which is lower than commonly used metals, but with a density of only 1.15g/cm3, its strength-to-weight ratio is far better than the metal equivalent. Materials such as Celazole has a tensile strength of 140MPa, for those applications where extreme strengths are required.
Compression Strength, is the stress that a material can withstand under compression (squash). With plastics, there generally is no ultimate or yield compressive strength, because the material slowly deforms rather than suddenly failing. This strength is normally measured by 1, 2, 5 or 10% strains (squash or deformation). |
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This is an area where engineering thermoplastics are normally very suitable, i.e. wear pads, bushes and sheaves. One particular application that an engineering thermoplastic is often used is rollers or wheels. Because of the elastic nature of plastics compared to metals, when a plastic roller is loaded, the contact area between the roller and the counter face is larger than that compared to metals. As a result, the stress experienced by the plastic is lower than that of a metal roller under the same conditions.
 Temperature: referring to the max continuous (5000hrs) allowable operating temperature. This temperature is derived by noting when the tensile strength of the material is 50% of that when at normal room temp (23°C). (Materials can operate above this temperature, but the mechanical properties are significantly reduced).
Engineering thermoplastic materials normally can operate at continuous temperatures of 100-120°C , with short term (few hours) allowable operating temperatures up to 240°C. But, if required, there are engineering thermoplastic materials that can operate up to 310°C continuously, and 500°C for short periods. |
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 Thermal expansion refers to the amount of expansion (and contraction) of materials with change in temperature. These numbers are higher than those of metals (i.e. not as good), but the important fact is to design in accordance to these properties, e.g.: for long wear strips, incorporate scarf cuts (45 deg cut) to allow for thermal expansion; or for bearings design a running clearance that allows for thermal expansion, i.e. add a small additional gap between the ID of a bush and the shaft. This can be calculated by engineers such as those in our national network, or using CAMSAD (Computer Aided Material Selection And Design), which is a program designed to help design sleeve bearings, thrust washers and wear pads.
For comparative data on Engineering Thermoplastic properties discussed in this newsletter, please click on the below link
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