Hot air staking ideal for fastening armature

Manufacturer of innovative electro-mechanical and electro-magnetic devices BLP's investigations into the use of Phasa's hot air staking system proved it had the makings of an ideal solution

The production of miniature armature assemblies for use in safety-critical vehicle passenger restraint systems demands total confidence in the integrity of the fastening method selected.

But that's only part of the story.

Due to their compact size and close operational tolerances, positional accuracy is also a key process requirement - along with all the normal automotive industry demands of: cost-effectiveness, ease of use and the potential for year-on-year cost reductions.

As Newmarket-based BLP has found out, the selection of plastic hot air staking from Phasa Developments meets all these criteria and more.

Part of the multi-million pound Roxboro Group, BLP is a world leader in the design and manufacturer of innovative electro-mechanical and electro-magnetic devices.

Its Automotive Division works closely with vehicle manufacturers and Tier 1 suppliers to develop stand-alone devices such as solenoids, actuators and relays, as well as devices that are integrated into complete systems for steering or transmission interlocks, door locking actuators and seat belt restraint mechanisms.

As part of the development of a new safety restraint system in conjunction with a leading seat belt manufacturer, BLP has devised an ultra-compact actuator system which mounts within the seat belt retractor.

This fast-acting electronic unit provides a 7 millisecond response to on-board motion sensors.

But unlike most other passenger restraint systems, it is unaffected by the vehicle's attitude.

At the heart of the new system is a fixed core and moving armature assembly.

This measures 20 mm x 18 mm and, in order to meet the stringent response time characteristics, has a stroke of just 2mm.

It consists of a moulded body to which a metallic armature is permanently attached.

It also incorporates a cam profile that locks against a moulded ratchet gear at the limit of its travel.

According to BLP's Head of Design, Barry Beard: "The securing of the armature to the moulding body is crucial to the operation and reliability of the finished units." In use, actuators are subjected to rapid variations in acceleration and deceleration.

Therefore, a precise air gap must also be maintained to help eliminate noise.

"Because the system is in operation all the time the car is moving, any audible clicking or chattering would be totally unacceptable," he explains.

BLP initially investigated the use of ultrasonic welding to fix the armature, but for a number of reasons this proved unsuccessful.

Firstly, the resilience of the material needed to meet the noise requirements caused inconsistent results.

"Another problem emerged through the fact that this was a new product development exercise," says Beard.

"Each time components were modified, new horns for the ultrasonic welder would be required, which not only proved to be time consuming, but costly as well." Subsequently, the decision was taken to secure the metal armature by incorporating three pegs on the moulding, which could be heated and reformed to produce rivet head profiles.

Contact heating and swaging of the pegs was evaluated, but this too raised drawbacks of its own.

"The heat sink effect of the metallic component had a tendency to cause stringing of the thermo-plastic material," reports Beard.

"Crucially, this could form flash that either encroached into the mechanism's air gap or could even break free to create loose debris within the unit.

"Clearly, on such a safety critical piece of equipment, that would be a major problem." By contrast, BLP's investigations into the use of Phasa's hot air staking system proved it had the makings of an ideal solution.

Plastic hot air assembly uses the principle of directing air at between 150 and 400C on to pre-moulded pegs, ribs or collars - to bring them to their plastic state for re-shaping, clamping and subsequently cooling into new forms.

It is widely employed throughout the automotive, IT, telecomms equipment and domestic appliance sectors for the production of simple rivet heads, but is equally suitable for the retention of bearings, securing threaded inserts or pre-loading seals.

"Right from day one we were impressed by Phasa's track record and the co-operation we received from their engineers.

As a result, the finished assembly's design has been developed jointly with them to incorporate some novel features," Beard explains.

Among these is the inclusion of a countersunk form on the metal armatures and modification of the tooling to provide a flush rivet profile eliminating the risk of material protruding into the critical air gap.

A further feature of the hot air staking process is that as the pegs cool, they contract slightly.

This not only ensures that the two components are clenched together firmly, but also provides high dimensional consistency of the finished assemblies.

"The fact we are working to tolerances of +0.05mm on the moulded pegs will give some idea of the overall accuracy and repeatability we aim to achieve," adds Beard.

Following a series of trials, the solution based on a Phasa 10/30 machine equipped with twin impression tooling was selected - and is now in production at BLP's Newmarket site.

"Since the equipment's installation last year, the application has been trouble-free," reports Beard, "enabling us to build a high level of confidence in its performance.

"Clearly, this is extremely important with products like these, which have a design life of greater than 1 million cycles and are 100% tested to ensure zero defects prior to despatch to our customers." BLP's process studies confirm the excellent capabilities of the Phasa assembly method.

This bears out studies conducted by the company in conjunction with the Faculty of Engineering at the University of Hertfordshire, which demonstrate that hot air staking will regularly produce Cpk values in excess of 2.0 - the equivalent of fewer than one reject per million operations.

A further benefit identified by BLP has been the controllability of the system, with no evidence of heat marking on any of its process components during any pilot production runs.

Full production of the new actuator is due to commence shortly, with most units destined for a luxury model due to be introduced by a major European vehicle manufacturer later this year.

But, according to Beard, the potential for the new actuator could be as high as 2.8 million per year.

"Production costs and process times are always major factors in any automotive sector application," he continues.

"So, it's no surprise that we are currently targeting 3 per cent year-on-year cost reductions across all our operations.

"With the demand for the new seat belt system set to grow, investment in multi-impression assembly tooling - perhaps with automated parts handling equipment - will provide us with the means to continue driving down piece part costs on this product." "Indeed, the Phasa system's scalability points to another of its key benefits; as it means expenditure can be matched closely to market demand - avoiding the need for major capital investment right at the start of any product development programme," Beard concludes.

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