Testing of Thin Sheeting and Plastic Films

Plastic films have become an integral part of daily life. We use them to package food, carry our groceries home from the store, and cover vegetables grown in greenhouses and fields. Film packaging is common in the medical engineering industry and often saves lives. A wide variety of films are also used in construction. Moisture barriers ensure that our roofs do not leak and that water does not damage the walls of our homes. Sophisticated plastic films are used to cover sports arenas, opening up new possibilities for architects.
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Films today are high-tech products that have a wide variety of characteristics. Mass production uses cost-effective plastics, such as high- and low-density PE, PP, PVC, and PS. In cases where special characteristics such as toughness, permeability, or light resistance are required, high-quality polymers such as various polyesters, PTFE, or ETFE are used. Films such as EPDM and EVA are used to waterproof roofs. Multilayer films have various characteristics such as strength, impermeability in regard to gas and bacteria, and suitability for contact with food.

Thin sheeting with a thickness of up to 1 mm is a constituent material for many hollow bodies in packaging technology, which are formed by means of processes such as thermoforming or compression molding.

Due to the broad scope of film manufacture and its downstream processing, requirements of the applicable test method vary greatly. The objectives of the tests can be very different in nature.

In an incoming goods inspection, the first step involves testing raw material. This step determines the melt index—meaning the melt mass-flow rate—of plastic granulate.

In the second step, the plastic films are tested after the extrusion or blow molding process. In addition to testing film thickness, a variety of other mechanical characteristics are important. These are strength, strain, yield point, and the tensile modulus in the longitudinal and transverse directions of the film. Toughness is also important, which is determined during impact tests. In the case of multilayer films, adhesion between the layers is crucial.

Yet the focus lies on other characteristics during and after processing, including adhesive strength, weldability, and strength of adhesives and joints. If sharp objects are packaged, the puncture resistance of the film is measured. Understanding the coefficient of friction is interesting for packing machines or in film printing. Changes in light and moisture is also important when evaluating thin sheeting and plastic films.

Zwick testing systems are specifically designed for these requirements and address a wide range of mechanical tests:

  • Extrusion plastometers characterize the materials of incoming goods.
  • Universal testing machines provide accurate, reliable measurement of values for stress-strain characteristics, adhesion forces between two layers, coefficients of friction, penetration resistance and flexure characteristics.
  • Creep test benches measure the creep behavior at room temperature and at operating temperature.
  • Pendulum impact testers are used to determine impact strength values, typically in an impact tensile test.