Application temperatures of thermoplastic polymers
If a plastic part is exposed to temperatures, then it is important to choose the right material. Different plastics have different maximum application temperatures. When this temperature is reached, the plastic becomes soft, because higher temperatures affect the mobility of the polymer chains, which changes the mechanical properties. When and how much the mobility of the polymer chain changes depends, among other things, on the chemical structure of the molecular chains. Thus, some polymers can withstand higher temperatures better than others.
In practice, two different methods are applied to determine the upper temperature resistance. These are the Vicat softening temperature (VST), and the heat deflection temperature (HDT). For both measurements, a sample is mechanically stressed and then constantly heated. As soon as a defined deformation occurs, the characteristic temperature is reached.
A third method is the dynamic mechanical analysis, which provides results on the material´s stiffness as a function of temperature. However, since this analysis is more complex and cost-intensive, the other two methods are used more frequently in practice.
Vicat softening temperature (VST)
To define the Vicat softening temperature, a circular punch with a cross-sectional area of 1 mm² is loaded with a force of 10 N (method A) or 50 N (method B). The temperature is increased continuously. For A and B, a temperature increase of 50°C or 120°C per hour is possible. As soon as the plunger is pressed into the sample by 1 mm, the value sought has been reached. In the Vicat test, the sample is already so soft on the surface that the material virtually flows away under the plunger.
Heat deflection temperature (HDT)
In the HDT test, a multi-purpose test sample measuring 80 x 10 x 4 mm³ is loaded in bending centrally between two supports. The sample is heated slowly and uniformly. Once the defined deflection is attained, the heat deflection temperature is determined using the HDT method.
Use of the characteristic values
Using these two measurement methods, it is determined up to which temperature the plastic can be used without deformation. The HDT temperature is lower than the Vicat softening temperature. Depending on how a component is loaded and what the criteria for failure are, either the HDT heat deflection temperature or the Vicat softening temperature should be used as the selection criterion. In case of doubt, the lower value is selected.
Among the most heat-resistant materials at Extrudr are GreenTEC, GreenTEC Pro, and GreenTEC Pro Carbon, DuraPro ASA CF, FLEX Medium, FLEX Hard, and FLEX Hard CF. The exact values for each material can be found in the data sheets.