Testing thermoplastic & thermosetting molding materials

Selected test methods which can be carried out using Zwick testing equipment

What are molding materials?

The expression 'molding materials' is used for plastics before the actual forming process (e.g. compression molding, injection molding or extruding) takes place. These plastics are mostly in the form of granules or powder, as regrind or pellets, to enable them to be transported conveniently and conveyed to the next stage of production. Descriptions and specific testing conditions for many polymers are contained in the ISO standards relating to molding materials.

Examples of molding materials standards:

  • ISO 17855-1 and 2: Polyethylene (PE) moulding and extrusion materials
  • ISO 19069-1 and 2: Polypropylene (PP) moulding and extrusion materials
  • ISO 1622-1 and 2: Polystyrene (PS) moulding and extrusion materials
  • ISO 1874-1 and 2: Polyamide (PA) moulding and extrusion materials
  • ISO 7391-1 and 2: Polycarbonate (PC) moulding and extrusion materials
  • ISO 2898-1 and 2: Plasticised poly(vinyl chloride) (PVC-P) moulding and extrusion materials

Manufacturing specimens

Tests are carried out on standardized specimens produced under exactly defined conditions. This ensures that test results genuinely represent a property of the polymer and that comparability is not significantly impaired by processing parameters such as different orientation of chain molecules. This approach allows comparable test data to be obtained.

Standards for production of test specimens

  • ISO 294-1: Injection moulding - general principles
  • ISO 294-2: Injection moulding - small tensile bars
  • ISO 294-3: Injection moulding - small plates
  • ISO 294-4: Injection moulding - determination of moulding shrinkage
  • ISO 295: Compression moulding of test specimens of thermosetting materials
  • ISO 2818: Preparation of test specimens by machining
  • ISO 20753: Test specimens
  • ISO 3167: Multi-purpose test specimens

Test sequence and results evaluation

In some instances, test standards describe the test sequence and evaluation method in general terms. These standards can therefore be used both for molding materials and for specimens taken from semi-finished products or components. A high level of test result comparability is a primary factor in the characterization of molding materials; additional detailed specifications have therefore been drawn up:

  • ISO 10350-1: Comparable single-point data
  • ISO 11403: Comparable multi-point data
  • ISO 17282: Plastics - guide to the acquisition and presentation of design data.

Many ISO standards provide precision data for the test method which they describe. These precision data are usually determined in international cross-validations based on round-robin tests. Repeatability and reproducibility are shown.

Repeatability describes the spread of results determined in a testing laboratory by a single operator using the same testing equipment under the same environmental conditions. This value is relevant for comparability within an organization, for example in the context of quality assurance.

Reproducibility describes the spread of results determined in multiple testing laboratories by multiple operators using different testing equipment under the environmental conditions specified in the standards. This value thus describes a level of comparability such as is typically expected in the customer-supplier relationship. It is obvious that reproducibility usually greatly exceeds repeatability.

Zwick testing equipment

Zwick supplies reliable materials testing machines and extensometers for tensile, flexure and compression tests, together with pendulum impact testers, hardness testing machines, extrusion plastometers and HDT/Vicat testers in manual and fully automated versions. Attention to detailed design at many different locations ensures that operator influence on results is minimal and is crucial to the very high levels of repeatability and reproducibility achieved by Zwick testing equipment.