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Phasa keeps Philips out of hot water
Phasa keeps Philips out of hot water
Keywords: Assembly, Sealing
The speed, versatility and reliability advantages of plastic hot air forming are all highlighted through its adoption by domestic appliance manufacturer, Philips, for the critical assembly of the company's "Filterline" plastic-bodied, electric kettle range.
The method is not just employed to secure new technology, flat disc heating elements in the base of the 1.5 litre capacity kettles, but also to provide reliable, water-proof sealing under extreme operating conditions (see Plate 10).
"The flat element design was a totally new concept for domestic kettles," says Philips' senior production engineer, Colin Tomlins. "In operation, they reach 250°C and raise the temperature of the polypropylene body, and the water it contains, from mains water temperature to 100°C in a matter of seconds.
The PHASA method provides a cost-effective and reliable semi-automatic assembly process capable of maintaining sealing integrity over the kettle's operating design life of 22,000 heating and cooling cycles. Here a finished assembly is shown alongside the heating element and associated seal, retaining ring and plastic kettle body
"In determining the most suitable assembly method, our task was to find a cost-effective and reliable process that would maintain sealing integrity over the kettle's operating design life of 22,000 heating and cooling cycles."
On the way to the preferred solution, Tomlins and his colleagues investigated a series of alternative approaches.
Threaded mechanical fasteners were quickly ruled out both on the grounds of cost and that they were prone to loosening as the plastic bodies heated and cooled.
"We also looked at induction welding, but were concerned about the process's repeatability," Tomlins adds. "Similarly, we felt that induction welding could not match the cycle times we believed were achievable using hot air forming. We had already used this method on other assemblies, with good results, and subsequently entered detailed discussions with PHASA Developments of Bishop's Stortford, the UK's recognised leader in this field.
"Even so, our new application posed severe design problems, which have effectively helped to push back the boundaries of this technology."
Typically, hot air staking is used in fastening applications where pre-moulded pegs in thermoplastic base materials are heated; then clamped, re-shaped and cooled to produce a variety of rivet head forms.
"While the system had proved successful for creating a variety of features such as peripheral securing tabs and collars, as well as for the retention of bearings and seals, this was the first time that such a large and critical diameter component had been tackled by PHASA," Tomlins reports.
A joint development project team was formed to address the problems and through an intensive nine-month development programme, the process was optimised to fully meet Philips' requirements.
One of the installation's key features is the design of the shaping tools, which are water-cooled both to improve performance and to minimise cycle times.
In operation, the flat disc heating elements are first fitted with a circumferential silicon rubber sealing ring and located by hand into the base of the kettle bodies. An additional nylon retaining ring is then placed in position and the PHASA assembly machine's automatic cycle is initiated.
The kettle is then transferred to the equipment's heating station. Here, a manifold system directs hot air precisely over the lipped kettle base, before the custom-engineered forming tools roll over the base moulding section, which subsequemly cools to provide a secure and water-tight seal.
"The heater temperature is particularly critical, as are the pre-set heating and forming times," says Tomlins. "The controllability and repeatability of the PHASA equipment have been important factors in enabling us to maintain consistency throughout production runs.
"We use SPC methods and have imposed a tight tolerance of only 0.3mm on the critical finished assembly height. Anything below bottom limit will lead to over-stressing of the heating element, while assemblies above top limit are prone to water seepage."