The centre armrests for automotive interiors become ultralight with Flexflow

The production of a lightweight centre armrest for the automotive interior is one of the latest examples of the use of the innovative Flexflow technology from Italian hot runner specialist, HRSflow. The multi-step process combines the thermoforming of a composite sheet with the injection moulding of two different thermoplastics. Development partners in the successful implementation of the project included injection moulding machine manufacturer, KraussMaffei, as well as US mould manufacturer, Proper Tooling.

The load-bearing structure of the centre armrest was created with the aid of the FiberForm process developed by KraussMaffei in which a thin-wall, fibre-reinforced composite sheet is heated, inserted into the mould and thermoformed. The two subsequent injection moulding processes are carried out according to the principle of the swivel-platen technique simultaneously in two opposite parts of the same mould. In one step, the carrier is first encapsulated with polypropylene (PP). This results in the near-final geometry of the part, including the elements needed for subsequent assembly. This PP structure is then overmoulded with a thermoplastic elastomer (TPV) in the opposite section of the mould. This produces the flexible visible surface with the fine grain that is later responsible for the high optical and tactile quality of the centre armrest.

With both injection moulding steps, a Flexflow hot runner system is used, with two or three hot runner nozzles. Their servo-electric, individually controlled valve pins ensure optimum filling of the cavities in each case, and they also contribute to the reliability and economic viability of the process.

The integrated servo-electric valve gate system - the characteristic of the innovative Flexflow technology - opens up a variety of possibilities for adjusting the process parameters. It is possible, for example, to control the individual pins of a hot runner system independently of one another with regard to their stroke, velocity and force. This means that users can control the pressures and flow rates during the entire mould-filling process very accurately, easily and flexibly and thus optimize the quality of their injection-moulded parts. The attainable advantages over conventional pneumatically or hydraulically driven valve gate systems include the possibility for producing streak-free Class A surfaces and reducing warpage. Another benefit is its maintenance and user friendliness. The clamping force can be lowered by around 20 %, and the part weight by up to 5 % while ensuring the outstandingly high reproducibility that is absolutely essential for large-series production.