Hot air stacking for fire alarms

Citation

(1999), "Hot air stacking for fire alarms", Assembly Automation, Vol. 19 No. 3. https://doi.org/10.1108/aa.1999.03319caf.006

Hot air stacking for fire alarms

Hot air stacking for fire alarms

Keywords: Plastics, Welding

KAC Alarm Co. manufacture fire alarm callpoints - those familiar "break the glass" red boxes used in buildings to signal emergencies.

Based in Alcester and Redditch in the Midlands, KAC Alarm Co. Ltd has come a long way since its formation in 1972. Today, this privately-owned company has a 160-strong workforce and its call point products can be found in more than 170 countries around the world. In fact, over 60 per cent of production goes for export these days and, having maintained an annual growth rate of 7 per cent during the past ten years, KAC is confidently forecasting that sales will top £7 million during 1999.

Their new products have been designed to utilise snap-together assembly wherever possible, giving the combined benefit of minimising parts count and associated costs, while also speeding assembly. These same benefits also account for the adoption of plastic hot air staking to retain the all-important microswitches at the heart of each "World Series" unit.

Previously, the company employed a cyanoacrylate based adhesive to bond microswitches in place. In addition to the hazard presented by the use of such compounds, production was limited by the need to clamp each microswitch in position for up to 45 minutes while the adhesive fully cured. Ambient conditions also affected the process, not only influencing the adhesive's shelf life and setting time, but also leading to surface discoloration of assemblies on hot days.

By contrast, the use of hot air staking methods has transformed this former batch operation into the central element of a flow line process capable of producing up to 2,500 units/day.

The system employs the principle of directing air at between 150 and 400úC on to pre-formed pegs, ribs or collars in order to bring them to their plastic state for re-shaping, clamping and, subsequently, cooling into new forms (see Plate 3).

The technology has been pioneered by PHASA Developments of Bishop's Stortford, a member of the Flexible Lamps group, and over the past 15 years has been refined to the point that hot air staking provides an ideal method of achieving low-cost, vibration and tamper-proof fixings without the need for secondary fasteners.

Plate 3 Close-up of call point assembly using the PHASA method. Castings with pre-located microswitches are automatically fed to the assembly position, where they are initially clamped in position (diagonally applied tool in photo) while localised heat is applied to the pegs. Finally, cold forming tools produce the required rivet heads before solidification is completed and the finished assembly continues to the next station

A simple modification to the mould tools was all that was required for KAC to implement the change. It enabled two plain pegs to be incorporated into the body mouldings, which now provide a positive and accurate location for the microswitches. During manufacture, components are pre-assembled by hand before being track fed into a PHASA type 20/40 staking machine for automatic processing. Once the machine senses the parts are in the correct position, a clamp holds the microswitch in place, while localised heat is applied to the top of the pegs. Finally, cold forming tools produce the required rivet head configurations before solidification takes place and the finished assembly continues to the next station.

With a cycle time of just six seconds per unit, the new method provides a dramatic improvement in throughput. It also ensures more accurate location of the switches and effectively has de-skilled the operation, bringing a noticeable improvement in "right first time" product quality.

Furthermore, elimination of the adhesive has reinforced the new system's manufacturing cost advantages, while removing the need for controlled storage conditions and the use of protective clothing by operators.

For more information, contact PHASA Developments, 1 Raynham Road, Bishop's Stortford, Hertfordshire CM23 5PB, UK. Tel: +44 (0)1279 657711; Fax: +44 (0)1279 655911.