Borealis delivers key technical support in new polypropylene-centred dishwasher development

Borealis, a leading supplier of polyolefins, working closely with major white goods manufacturer Bosch Siemens, has developed a polypropylene compound suitable for the replacement of stainless steel in the interior of a dishwasher.

Stainless steel is traditionally used for dishwasher tubs because it provides excellent corrosion resistance. However, the stainless steel tub is a contributory element to the high assembly costs in dishwasher manufacturing. In order to reduce this cost a review of alternative tub solutions produced two viable possibilities:

1. The combination of normal steel with a plastic coating

2. Moulding the tub in polypropylene

The second solution was considered by Bosch Siemens to be the most advantageous because of the significant cost savings a moulded polypropylene tub could deliver compared with stainless steel. Borealis therefore conducted research to develop a new polypropylene grade suitable for a dishwasher tub. The result was MB350WG, a talc-filled polypropylene with outstanding impact/stiffness behaviour, combined with an excellent stabilisation package to protect it against rinse aid, hot water and cleaner.

In addition to meeting the in-use, hostile environment performance criteria, the tub had also to satisfy a high level of robustness set by the customer. Key was to ensure that the dishwasher was capable of withstanding a wide range of handling and transportation events without impairment of operation on delivery.

A particular focus was resistance to dropping. The completely assembled dishwasher had to withstand a half-meter drop at an angle of inclination of 10 degrees onto a concrete surface, without the tub sustaining any damage. This involved a series of different impact situations:

• Impact on the front side edge

• Impact on the back edge

• Impact on the right side edge

As a cost and time saving aid for the project, Borealis decided to conduct the drop tests by means of dynamic computer simulation programs, thereby avoiding the longer and more expensive physical testing procedures.

However, the Computer Aided Engineering (CAE) tools for simulated testing, which are well-established and have been used for many years by the aviation and automotive industries, have been principally developed around the material characteristics of steel and other metals. And, because plastics have greater viscoelastic and viscoplastic behaviour than metals, this is not generally reflected in the material models within common CAE solver codes. So, simulations involving polymers, which are frequently conducted in respect of polymers used in structural applications, are dependent on detailed strain and temperature related deformation data for each specific material.

Therefore, in order to optimise the simulation program for drop testing involving MB350WG, as well as the performance of other dishwasher parts under dynamic loading, it was first necessary to obtain a range of data concerning the mechanical behaviour of the polymer at, for example, varying strain rates and temperatures.

This was made possible through co-operation between Borealis and the University of Leoben, Austria. The university developed a method that enabled Borealis to determine material data at high strain rates and these data formed the basis for the simulation calculations.

The computer model for the calculations was combined with data from the 29 most important parts of the dishwasher. The complete model consisted of approximately 26,000 elements which were subjected to finite element analysis to calculate the loads.

Having accurately determined the loads on the individual components, areas with too high plastic deformation were localised and improved by appropriate structural design changes. The dishwasher part then underwent final testing and passed without the necessity for practical physical trials.

Having played a leading role in this new challenging development project, Borealis created value both for its customer, Bosch Siemens, and for the wider white goods industry throu

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