Fibre foaming process wins accolade

The M???llerTech AquaCell foam moulding process can be used for thick parts like this screen grid

The process uses release of water vapour from the moist fibres into the melt during injection moulding, triggered by the pressure drop as the compound enters the mould cavity.

M"llerTech developed the process out of its experience with the Sabiprop hemp reinforced PP that it introduced in 2003 and which found its first commercial application in a car storage part for Audi. Commercial growing of hemp fibre has been allowed in Germany since 1996.

Dr Marcel Salamon of M"llerTech said that standard screw designs are suitable for AquaCell, especially those intended for long glass fibres. He says the volume of gas available is up to 2.5 times that for conventional physical foaming methods and up to 9.5 times more than for chemical foaming agents, with the additional benefit that water vapour is harmless.

The process produces a foam structure with an integral outer skin using materials such as Ticona’s Celstran 40% long glass fibre reinforced PP or Basell’s Hostacom 30% short glass fibre reinforced PP. It enables parts to be produced with varying thicknesses without sink marks, and the technology helps speed cooling of moulded parts to the benefit of cycle time and energy savings.

In the first series application – a cover in the foot area of the VW Golf Plus in unreinforced PP with AquaCell fibres as foaming agent – Salamon said moulding of 500 parts saves 500kg of plastics over an equivalent compact part, 470 kWh of energy and 65kg of CO2 emissions.

At fibre doses of 3-10% in unreinforced PP, M"llerTech has seen increases of up to 50% in tensile strength.

Key to the AquaCell process is the selection of fine-grained “shives”, ligneous components of hemp fibre that arise as side products during hemp fibre production. These are capable of absorbing several times their own weight in water.

The shives are fed by a horizontal screw into weighing equipment into which the polymer is simultaneously dosed, the mixed polymer and shives being passed to a mixing chamber where water is added. The resulting free-flowing mixture rests in intermediate storage for the water to become evenly distributed and is then conveyed as needed by pipeline to a transport screw above the injection moulding machine for dosing into the melt.

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