Simulation | product development
Belleville chair uses full gas moulding simulation
Resin producer BASF broke new ground with its propri- etary Ultrasim software in the development of Vitra’s new Belleville gas injection moulded chair design, taking the precise shape of the gas bubbles into account in the structural analysis of a glass reinforced polyamide structure for the first time.
Virtual simulation is an increasingly important element in designing components and tools, says BASF, allowing component geometry to be optimised and the precise dimensions of mould tools determined prior to production. In many cases it can eliminate the need for prototype tooling, which for furniture manufac- turers such as Switzerland- based Vitra can result in significant cost and time savings in the design process. BASF has already applied
its Ultrasim CAE (computer- aided engineering) expertise in a variety of injection moulded seating projects, including the MYTO cantilever chair by Konstantin Grcic and the A-Chair from Brunner. However, both of these designs were produced by conventional compact injection moulding. The Belleville design was to be manufactured using the more complex GIT (gas-injection technology) process. The Belleville chair is the
work of the French designer duo Ronan and Erwan Bouroullec. It consists of two
www.injectionworld.com Vitra’s Belleville chair: BASF used actual GIT bubble predictions in its Ultrasim structural analysis
separate components: a frame structure and seat shell. Both are produced in BASF’s Ultramid B3EG6 SI, a 30% glass reinforced grade offering a very high quality surface finish. A key feature of the design is the elegant frame, which meets the structural stability requirements with minimal material content through the use of the GIT moulding technique. The GIT process adopted for
the Belleville frame involves moulding a solid part then injecting nitrogen gas to displace the melt and create a hollow core. In comparison to conventional injection moulding, the technique allows a considerable weight saving. However, it presents a significant challenge in terms of calculating component geometry as the size and position of the gas bubbles have a major impact on the strength of the part. If the prediction is not correct,
subsequent mould modifica- tion costs can be considerable. BASF says the usual simulation approach for GIT moulding projects is to estimate the size and shape of the gas bubbles and insert these idealised features into the CAD drawing. However, the company says the simplifica- tions involved can introduce significant uncertainty and frequently result in substantial deviation from reality. As a con- sequence, components such as chairs that are subjected to significant loads have to be over-dimensioned, sometimes compromising on both aesthetics and weight savings. In the Belleville design,
BASF’s engineers used the Ultrasim simulation for the first time to account for the actual shape of the gas bubbles in the structural simu- lation. In a realistic virtual moulding process, the frame was first volumetrically filled with melt then the gas
injected. The internal bubble geometry generated through the GIT process simulation was then used for the strength calculation in the structure simulation. “In contrast to the idealised
process, this precisely takes account of areas of differing wall thicknesses, rounding and transitions,” says Bła˙ Paluszy´
zej nski, an Ultrasim
expert at BASF. “The precision of our calculations was demonstrated in a cross-sec- tion of the manufactured chair frame. It reflected the simulation results with great precision.” Vitra launched the Belleville chair, which is available with and without arms and is named after the Paris artists’ quarter, at the Milan Furniture Week in Italy last year.
Click on the links for more information: ❙
www.ultramid-si.com ❙
www.ultrasim.basf.com
January/February 2016 | INJECTION WORLD 53
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