Moulding Compound Test
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Moulding compound test
Methods of moulding compound testing
The properties of the moulding compound are essentially determined by its chemical composition and the associated manufacturing processes, and are therefore largely independent of the material’s geometry and history. However, this is only true if the manufacturing process does not involve extrusion followed by granulation using processing aids or the incorporation of reinforcing agents or fillers.
Moulding compound testing therefore constitutes an analytical task that provides information not only on the chemical and physical structure of the plastic, but also on its rheological and processing characteristic values. The physical testing methods used are not only employed for analytical characterisation in the context of plastics analysis, but also form the crucial basis for establishing correlations between the structure of macromolecular materials, their manufacturing and processing conditions, and their technological properties.
Typical identification methods used in incoming inspections in industrial practice, which provide characteristic information on the composition or differentiation of plastics, include density measurement, determination of melting and softening temperatures, ash analysis, combustion tests and/or pyrolysis tests, which can be supplemented where necessary by spectroscopic testing methods such as infrared spectroscopy (FTIR) or thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and thermomechanical analysis (TMA), as only small sample quantities are required here.
The processability of plastics can be assessed using either simple technological methods or more complex rheological test procedures, depending on the specific requirements and the type of material. From an industrial perspective, particle size analysis and the measurement of viscosity or melt flow rate (MVR, MFR) are particularly important. Due to the known correlations between molecular weight, molecular weight distribution and the structure of macromolecules with application-related properties such as strength, ductility, toughness and density, it is possible to draw certain conclusions from the determined levels of these characteristic values regarding the influence of the processing and manufacturing process on chemical degradation effects [1].
Methodology of specimen extraction
Sampling plays an important role in characterising the properties of moulding compounds, as the statistical sample taken – usually consisting of small quantities of material (Fig. 1) – is intended to represent the entire population of properties. The accuracy of the characterisation of these properties depends not only on the measurement techniques used, but also significantly on the method of sampling.
| Fig. 1: | Single-type coloured plastic granules (a) and pulverised or shredded plastic regranulate (b) |
If no suitable sample divider is available, the total quantity to be characterised must be thoroughly mixed, and samples must then be taken at three points sufficiently far from the surface. Sampling at different positions is intended to compensate for changes in particle size distribution and moisture content caused by transport and storage, as well as segregation effects. If mixing is not possible, as is the case with silo storage, for example, the statistical samples should be taken evenly from several depths. Moisture measurement can also be carried out online, e.g. using a moisture sensor. A common sampling technique for granules and powders is the so-called quartering method [2, 3]. For the purposes of traceability and accountability, the test report to be drawn up must document all details identifying the plastic (type of material, delivery date, bag number, filling date, type and condition of packaging, etc.). As with the processing of the material, a material-specific pre-treatment must be carried out prior to testing in order, for example, to remove condensation or foreign particles resulting from storage and to establish a defined reference condition.
Manufacturing process for test specimens
In order to characterise the properties of polymer moulding compounds using mechanical, thermal or electrical parameters, precisely specified test specimens are required in accordance with the relevant standards; these must meet defined requirements regarding their dimensions and condition. These test specimens can be produced separately or together with a structural or moulded part, or can be taken from such a part, e.g. for investigating the property profile within the moulded part or for failure analysis. The direct and indirect methods of moulding and forming technology commonly used for test specimen production are listed below, whereby test specimens made from pressed or injection-moulded sheets are to be preferred due to their low residual stresses and the absence or dominance of orientation:
- Direct forming processes
- Injection moulding
- Injection stamping
- Compression moulding
- Casting
- Indirect forming processes
- Extrusion
- Calendering
- Stamping
- Cutting
Influence of the inner state of order
Regardless of the type of shaping process, material-dependent elastic, entropic and viscous deformations occur during manufacture. These deformations are caused by shearing, e.g. during the injection and flow process, stretching and elongation of macromolecules, as well as cooling and solidification processes within the mould, and have a dominant influence on the subsequent internal state of the component or test specimen. Energy-elastic deformation is based on reversible changes in the vibrational and rotational states of atoms and parts of the macromolecule and is therefore time-independent. Entropic-elastic deformations correspond to changes in entropy, i.e. in the internal state of order, with translational movements of chain segments occurring at elevated temperatures. These processes are reversible, but depend on time and temperature. Irreversible viscous deformations result from the slippage of macromolecules due to shear and/or stretching during the manufacturing process.
As an additional deformation, a material-dependent volume contraction occurs during the transition from the molten to the solid state; this is also referred to as processing shrinkage and must be taken into account by allowing for an appropriate amount of material allowance in the mould design. Shrinkage affects dimensional accuracy and tolerances, and is generally less pronounced in filled or reinforced materials than in the matrix material.
Depending on the complexity of the moulded part, these various processes generally result in an uneven distribution of internal stresses (residual stresses), matrix and filler orientations, and morphological parameters of the plastic. This makes it clear that the characteristic values determined on test specimens do not, as a rule, represent the properties of the moulded material, but rather characterise the properties of the test specimen, which is in a specific state determined by the manufacturing process. Determining material-specific characteristic values therefore requires fundamental information on the state of the test specimen and the selected test conditions.
See also
References
| [1] | Bierögel, C.: Preparation of Specimen. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022), 3rd Edition, pp. 18–19 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 23) |
| [2] | Brown, R. (Ed.): Handbook of Polymer Testing: Physical Methods. Marcel Dekker, New York Basel (1999) (ISBN 0-8247-0171-2; see AMK-Library under C 5) |
| [3] | Brown, R.: Handbook of Polymer Testing – Short-Term Mechanical Tests. Rapra Technology Limited, Shawbury (2002) (ISBN 1-85957-324-X) |