By Sue Stuckey
Cambridgeshire composites specialist Cellbond celebrates 10 years in business by making materials history in collaboration with car maker BMW.
In the first application of honeycomb aluminium used in volume car production, Cellbond will supply headliners made from an aluminium honeycomb filled with a low density polyurethane foam to BMW’s assembly line for the new Series 3 models destined for the US.
The material developed by Cellbond with BMW meets stringent US RMVSS 201 regulations relating to head injury protection.
European laws are less onerous, but customer pressure is expected to drive demand for safer cars, posing a problem for designers.
Currently available rigid foams on their own are relatively poor shock absorbers. These are around 50% efficient compared with 85% for a low density soft foam, or 87-88% for the new composite made from foam-filled honeycomb.
Foams alone pose problems. Unlike honeycomb, they are temperature sensitive and deteriorate in the heat of a car standing in the midday sun. Foams lack ‘crushability’ and when compressed by 50% they become ineffective shock absorbers. Honeycomb continues to absorb impact shock up to 88% compression. Another problem with foam is that it ‘kicks back’ on recovery which can cause injury.
Because of its higher performance characteristics, honeycomb meets safety regulations without adding bulk, taking half the volume of traditional car padding. The material also suppresses noise.
Novel production methods include the use of robots to automate volume manufacture of a safety-critical component.
In a fully tooled situation, one robot services two turntables each with 16 moulds containing an unfilled honeycomb headliner. At each indexable station the robot fills a mould with polyurethane resin. When set, it removes the part from the mould using a vacuum lifting head, then places it in a trimming press. The headliner is taken to an inkjet label printer before being placed in a packing box.
The cycle time is 45 seconds per headliner excluding the resin cure time of five minutes.
Cellbond is using waterjet cutting to profile headliners from the unfilled honeycomb from 700mm thick blocks of the extruded material before they are put in the moulds. A thin polyester wadding placed around the honeycomb provides extra protection for the finished component.
The new honeycomb is more than twice as expensive as traditional padding material. But, given safety and space-saving features, cost is not an issue says Dr Mike Asmead, Cellbond managing director. On an £800 pilot’s helmet, the honeycomb absorber accounts for £20.