Deposit formation and wear during injection moulding with recycled plastics

The composition of recycled plastics fluctuates from batch to batch. In addition to the changing properties of the recyclates, impurities also have to be dealt with. These can accumulate in the recyclate despite the melt filtration during the regranulation step. According to reports from recyclate processors, this can lead to the formation of massive deposits on the mould surfaces and an increase in demoulding forces, which ultimately results in damage to the injection-moulded parts. Increasingly thicker deposits reduce the component quality regardless of the demoulding forces due to the deviating geometry. In addition, the deposits can lead to corrosion of the mould steel. Additional cleaning intervals are necessary to counteract the effects of the deposits described above. These and any increased wear that may still occur will result in additional costs for processors. Exact dependencies are not currently known.

Rapid test for melt-steel interactions

The aim of the proposed joint research project is to gain a deeper insight into the melt-steel interactions that take place during the processing of recyclates and to develop a rapid test based on this. This should make it possible, for example, to assess a given batch of recyclate regarding the tendency to deposit formation and corrosiveness during the incoming goods inspection by easy-to-use procedures. For this purpose, the process and material parameters with a particularly high damage potential, i.e. those that lead to a pronounced reduction in service life of the mould equipment, are to be identified using a larger number of recyclate batches.

Project focus & approach

Deposit formation, demoulding forces and wear caused at the tool steels by corrosive and abrasive plastics melts has been a field of work at the LBF for quite some time. We will focus on injection moulding tests with varying process parameters and carry out accompanying characterisations.

A special mould developed at the LBF equipped with force transducers will be used. It enables recording of the demoulding force during the demoulding of a test specimen in high temporal resolution.

During the deposit formation, the demoulding force profile changes over time, making it possible to detect the deposits and their effects (e.g. increase in demoulding force) at an early stage. This in turn enables a higher number of trials (with different moulding compounds and process parameters) within a given time frame. The injection moulding tool also has interchangeable inserts so that different steels can also be tested

The corrosiveness of the batches is to be specifically determined using the platelet test.

The batches will also be characterised, e.g. using thermogravimetry, DSC, MVR, etc. The deposits that form during the injection moulding tests will be examined using Raman microscopy (provides spatially resolved chemical information, e.g. about organic components) or SEM/EDX (identification of chemical elements, e.g. phosphorus in flame retardants).

Finally, an assessment tool for estimating the damage potential of a given batch is to be developed by combining all the test results.