Plastic hot air staking for accelerator pedals

Automotive OEMs and tier one suppliers around the world are harnessing the benefits of Phasa plastic hot air staking technology in the production of latest generation accelerator pedal assemblies.

Automotive OEMs and tier one suppliers around the world are harnessing the benefits of Phasa plastic hot air staking technology in the production of latest generation accelerator pedal assemblies.

Not only is the fastening method being used in light duty applications to retain stainless steel tread plates on a variety of cable operated throttle systems, but it is also being applied at the heart of new designs incorporating safety-critical drive-by-wire control units.

Here electrical contacts mounted on pivoting wiper arms are used to bridge across conductive tracks on ceramic wafers in order to provide a rheostat action as the pedal is depressed.

Retention of the ceramic components is just one application handled by the Phasa technology.

Typically, three point fixing is preferred to ensure positive, accurate fixing, while also minimising the risk of introducing twisting loads or residual stresses to the brittle wafers.

The thermoplastic accelerator housings are first positioned in purpose-designed fixtures in the Phasa machine, then the ceramic components are located by hand on three plain pegs moulded in the control unit's body.

On initiation of the automatic assembly cycle, hot air at between 150 and 400C is directed accurately on to the pre-moulded pegs to bring them to their plastic state.

Cold forming tools then re-shape the pegs into the required profile - while also clamping the components together - to enable a permanent, vibration and tamper resistant fixing to be achieved as the material solidifies and cools.

The same principle is employed to retain the stainless steel contacts to the pivot arms used to sweep across the ceramic wafers' conductive tracks.

Currently, all applications feature three contacts, which are each retained by pairs of pegs measuring 0.8 mm diameter before processing.

Drive-by-wire control units are subsequently completed by fixing the top cover to the body.

Here, between 8 and 10 further 2.5 mm diameter pins moulded into the body - depending on vehicle type - are staked simultaneously to provide a secure, sealed unit.

Not that hot air assembly is limited to the processing of a dozen or so stakes at once.

A further automotive application about to go into full production involves the retention of small circuit boards in automatic lock key fobs for a leading Scandinavian vehicle manufacturer.

In operation, motherboards containing 30 units are produced using the same three-point fastening method mentioned above - resulting in 90 staked fixings in a single 15 second cycle.

The Phasa system is most frequently employed for the production of simple rivet heads.

However, it is equally suitable for the retention of bearings, securing threaded inserts or pre-loading seals.

Its use in this way can be demonstrated by yet another accelerator pedal application - this time for one of the UK's best known light commercial vehicles.

Custom-designed thermoplastic pins have been specified as pivot shafts for the accelerator mechanism, each secured by the hot air staking method.

The process's controllability and repeatability enables the degree of roll-over to be maintained with consistent accuracy, ensuring reliable fastening without the risk of excessive clamping, which could result in a jerky throttle response.

According to Phasa Developments' John Neugebauer: "Plastic hot air staking is growing in importance as a mainstream fastening method within the automotive sector.

To date, it has been most widely used to assemble and secure components such as vehicle door trim and fascia components.

However, because of the method's ability to de-skill potentially complex assembly operations, users can be assured of achieving extremely high levels of repeatability and consistency in their finished products.

"This is borne out by a series of process studies conducted in conjunction with the Faculty of Engineering at the University of Hertfordshire, which demonstrated that hot air staking will regularly produce Cpk values in excess of 2.0 - the equivalent of fewer than one reject per million operations." As a result, Phasa assembly technology is increasingly employed on more intricate, complex or safety-critical applications.

In the automotive sector, this not only includes its adoption for the production of accelerator pedals, but is typified by its use in the production of airbag component assemblies, as well as its recent specification for the production of fast-response, ultra-compact actuators in a new generation of passenger restraint systems.

"The technology's inherent cost-effectiveness and ease-of-use are backed by an ability to achieve significant year-on-year cost reductions; typically through the use of multi-impression tooling," adds Mr Neugebauer.

"These fundamental benefits - combined with the on-going explosion in the use of thermoplastic materials, not to mention developments to further reduce cycle times - look certain to reinforce plastic hot air staking's position as a primary assembly method throughout an ever wider cross-section of manufacturing industry.".

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