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Plastics with good mechanical properties

In applications where plastic components are designed to withstand stress, the mechanical properties of polymers play a particularly important role. The fundamental mechanical material characteristics include:
  • Strength: measure of the resistance of a material to external stress
  • Stiffness: measure of the resistance of a material to deformation
  • Hardness: measure of the resistance of a material to deformation under concentrated compressive load
  • Toughness: measure of the energy absorption capacity of a material during impacts

Such properties can be investigated and compared between different products using standardised test methods. For example, tensile properties of plastics such as plastic tensile strength and rigidity can be determined according to DIN EN ISO 527 by briefly applying load in one direction with a tensile test. The possible results and typical values observed during such test, depending on material behaviour, are summarised in the following graph:

Ensinger tests the mechanical properties of all our semifinished products. This information is provided in our standard technical product datasheets. This information allows users to directly and reliably compare the physical properties of different engineering materials.

Users should note that when comparing Ensinger's values with those from other sources, apparently dissimilar results may be shown. This is probably due to different test methods, different testing speeds and different test specimens. The difference could stem from the fact that most of the published literature available on thermoplastic materials is based on results from injection moulded specimens, while the data provided on Ensinger datasheets is obtained from specimens machined from extruded samples. Crystallinity level and fibre orientation differs between extruded and injection moulded materials, which leads to important differences in values.

Strength - Tensile and flexural (mechanical resistance)

Flexural strength and tensile strength of plastic are two of the most commonly used values to compare materials. High tensile strength plastic products from Ensinger include:

The addition of carbon fibre and glass fibre reinforcement in extruded materials generally improves tensile and flexural strength, but the effect is more limited than in injection molded samples.

Stiffness - Modulus of elasticity in tension and flexion

The rigidity of unfilled materials is expressed by the tensile modulus. Ensinger products with the best stiffness values include TECASINT 4111 (PI), TECAPEEK (PEEK), TECAST (PA 6 C), TECAFORM AD (POM-H), and TECAPET (PET).

For maximum E-modulus values, carbon fibre and glass fibre filled materials are available, e.g.:

Compressive strength

Compressive strength [MPa]

Compressive strength gives a good indication of the short term loading capabilities of different plastic materials. It is measured by applying an increasing force on cylindrical or cubical specimens held between two plates, while measuring both pressure and elongation. 
 
With thermoplastics, the property of compressive strength at break is not always the relevant measurement, because for many ductile materials the specimen deforms without clear breakage. Excessive deformation under load would thus not be a good indication for success in any real industrial application. For these reasons, the compressive load is not normally given at break, but is instead given at a defined deformation point (commonly 1%, 2% or 10%). It is very important to check the test conditions before comparing compressive values from different sources!
 
It should also be noted that the addition of carbon fibre or glass fibre reinforcement generally improves the compressive strength of the polymer, but the macroscopic effect is more visible in the long term rather than in short term load performance, due to the improvement of creep properties.


Composite fibre-reinforced products:

Toughness - Impact strength

Strength / Stress [MPa]
 
The impact strength of thermoplastics is measured either by Charpy or Izod impact tests. A small rectangular rod is struck by a pendulum at high speed and the energy absorbed while breaking the specimen is measured; the higher the value, the better the impact resistance. In the case of high impact resistant plastics, where such samples do not break, resulting in no usable value, the test is repeated with a notch made on the specimen to obtain information under this more severe condition.


Typical high impact strength plastics are:
 

Hardness

Ball impression hardness [MPa]

While surface hardness can be measured in many different ways, one of the most common methods used to test thermoplastics is called “ball indentation hardness”.A metallic sphere of standard dimensions is pressed into the material with a defined force and for a defined time, the residual mark in the material defines the hardness value. Another common method of hardness testing is referred to as Rockwell Hardness testing.
 
Glass fibre and carbon fibre filled materials exhibit the highest surface hardness, while the best unfilled materials with regard to this property are: