Chocolate moulds are subjected to severe stress when running through dosing systems. While chemical influences from cleaning agents and mechanical influences resulting from impacts, twists, friction and temperature fluctuations have been adequately documented, the influence of vibrations is nevertheless frequently underestimated.

The company Hans Brunner GmbH has heightened its focus on this issue in order to improve the moulds and provide customers with instructions for handling them. Together with the University of Stuttgart Institute for Plastics Engineering (IKT) and as part of the ZIM project (Central Innovation Programme for small and medium-sized enterprises) sponsored by Germany’s Federal Ministry for Economic Affairs and Energy (BMWi), the company has analysed the correlations and emergence of vibration-related damage symptoms in moulds.

In rare cases, it was observed that moulds show severe crack formation even after only a short period of use. In these cases, manufacturing-related tensions or weld seams were able to be ruled out as the cause of the defects, and the cleaning parameters were equally unremarkable.

In all of these cases, intensive vibrations in the dosing system vibration stretches were particularly conspicuous. The vibration occurred immediately after the dosing in which the chocolate mass is distributed in the mould cavities. The observations were also able to establish that this occurred mostly with tablets or bars in which the dosed chocolate mass featured high viscosity, and the articles tended to be flat and cover a large surface area. In these cases, the distribution of the chocolate is a genuine challenge.

For the analysis, the moulds were equipped with a high-resolution data logger to register acceleration values throughout the entire production cycle. The inspection revealed that large vibration amplitudes were present, but no excitation frequencies. The data loggers registered extremely high acceleration values with varying indicators, which explains the large amplitudes. The accelerations that were measured stretched up to 200-times the level of gravity-related acceleration.

It is in this manner that cracks occur after only a few cycles, which can ultimately lead to mould breakage. In contrast to classic fatigue breakage, the culprit here is vibration breakage as a result of overloading. The logger data aided in identifying the vibration stretch as a critical part in the chocolate production process for the vibration load. The vibration amplitude exerted the greatest influence by far on the formation of cracks, and the solution approaches are subsequently focused in this direction.

Simulations demonstrated that extremely stiffened moulds vibrate with small amplitudes. Even slightly smaller amplitudes significantly increase the life expectancy of the moulds. This state can be achieved with a specially adapted geometry of the rib structure, but there are heavy restrictions due to the injection moulding procedure and the requirements of the moulding machinery.

Measures can also be implemented in the moulding process that ease the distribution of the chocolate mass in the cavity. To reduce the viscosity of the chocolate mass, the moulds can be pre-heated, or additives can be fed into to the chocolate to make it more fluid. Dosing with several nozzles and the lateral or length-wise procedure of the nozzles during the dosing process are appropriate measures for distributing the chocolate in the cavities more favourably.

Subsequently, when possible, the entire length of the vibration stretch should be used to its full extent in order to completely fill the cavity only shortly before the end of the stretch. This would mean that the vibration intensity is reduced to decrease the damaging influence of extreme vibration amplitudes.

One customer has already used these findings and reduced the heavy load on its moulds by making simple organizational changes. Originally, the vibration stretch was fundamentally set to “worst case”. All of the other articles on this system were then equally produced with these parameters.

The “worst case” product was a tablet of white chocolate with a high proportion of various solid ingredients. This highly viscous mass is very difficult to distribute evenly in the cavities. After the vibration intensity was initially reduced as much as possible for this article, some of the vibration parameters for the less critical articles were significantly reduced. This resulted in a notably longer service life of the moulds.

In the final part of the research project, testing will be conducted with a test bench simulating the vibration conditions in the dosing systems. The expectation of the mould service life is to be determined in a mechanical test regardless of the vibration amplitude and compared with the simulation results. The aim is to provide customers with an idea of how the vibration amplitudes influence the service life of the moulds. The subsequent findings can serve as the basis for investment decisions aimed at process improvement and can also reduce the risk occurring from defective moulds.