Computer simulation discovers best plastic alternative to brass valve body
02 August 2010
An efficient design process is not the exclusive preserve of the automotive industry. Manufacturers of more commonplace domestic items are beginning to recognise the benefits of computer simulation when developing new products – particularly when it comes to substituting plastics for metal. Martin Bussmann reports
Dispense with those expensive physical prototypes and save time and money designing new plastic parts with the aid of computer simulation. That’s what BASF and Austrian company Multiplast did when they worked on a plastic alternative to a brass bodied valve for a new generation of fire extinguishers.
The main valve body of the new fire extinguisher is now moulded from BASF’s Ultramid polyamide and is the first volume application of this high glass content reinforced plastic. Tyco Fire Suppression & Building Products, one of the world's largest manufacturers of fire extinguishers, now uses the new plastic valve on about 90 percent of its product range, including a family of cartridge-operated fire extinguishers.
The valve is a multi-function component that integrates connections for the internal riser pipe, the hose with the spray gun, the trigger mechanism and the handle. To operate reliably, it has to withstand a pressure of 80bar between -30 and +60 degrees C over a service life of 15 to 20 years. Inconsistent quality of the imported metal was one of the major drawbacks of the brass valve for manufacturer Multiplast.
The injection moulded alternative, however, required a plastic with high heat resistance, high strength and stiffness as well as high dimensional stability. A standard polyamide is unsuitable for this application, as Multiplast discovered when it conducted tests on early prototypes to ascertain service life, burst pressure, resistance to the fire extinguishing agent and stability following long-term exposure to UV-light.
In order to develop this complex component as efficiently and quickly as possible, BASF employed its UltraSIM simulation tool, which is able to determine the orientation of the fibres as a function of filling pressure and gate location, and then optimise the mechanical performance of the component on the basis of this information.
This is no easy task with a glass fibre content of 50 percent and significant anisotropy in the component. Simulation of three load situations was of major importance. This included the valve’s behaviour at a gauge pressure of 120bar, its behaviour under a bending load applied by the pressurised cartridge when in the horizontal position and the effect of a dynamic load applied by the handle when the fire extinguisher is lifted abruptly.
In terms of the pressure peak, a ‘weak spot’ had to be designed into the valve that would rupture at a pressure above 105 to 110bar to avoid the entire extinguisher bursting. UltraSIM enabled the location of this weak spot to be determined and the result was confirmed experimentally.
When the fire extinguisher is stored in a horizontal position, the fire extinguishing agent, be it powder or liquid, is in continuous contact with the valve. The Ultramid valve tolerated this condition in both the simulation and in reality. And as to failure of the device when lifted abruptly by the handle, this posed a challenge for the lever and for simulation, since strain rate-dependent material properties are involved. Here, too, there was significant agreement between both simulation and experiment.
With this knowledge, it was possible to establish the design of the complicated part exactly, with all of its integrated additional functions and without metal inserts, for production in a single injection moulding step. Even the four integrated metric threads are of plastic.
The final version of the valve, with an operating window of minus 40 to 80 degrees C, achieved a burst pressure increase to 100bar and resistance to pressure spikes of up to 250bar, considerably better characteristics than those of the brass part. Resistance to the fire extinguishing agent, environmental factors such as UV radiation and ozone must not be overlooked either. Last, but not least, the carrying characteristics and handle performance of the portable fire extinguisher were distinctly improved.
Dr Martin Bussmann is with BASF’s Engineering Plastics Europe Division
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