As full sized race car constructors strive for speed and reliability so too do engineers of scale model versions. Hornby Hobbies, makers of Scalextric slot racing cars, has a strong commitment to continuous improvement, and recently has evaluated a series of alternative fastening methods to retain the electrical pick-up contacts at the front of the vehicles, as well as to secure small magnets used to improve cornering grip on the 1/32nd scale models.
Cold forming was initially used for contact assembly, but over a period of time problems were experienced as the plastic material relaxed. The solution was plastic hot air staking, which the company says provides permanent, high strength assemblies with excellent vibration resistance.
The process uses the principle of directing air at between 150 degrees C and 400 degrees C onto pre-formed pegs, ribs or collars in thermoplastic components to bring them to their plastic state for reshaping, clamping and cooling into new forms. It is a process that has been pioneered by PHASA Developments.
Hot air staking is most commonly used to produce simple rivet heads in thermoplastic materials. This is the technique used to secure the cars’ electrical pick-up contacts – even though there are three sizes.
The retention of the magnets was a different matter. The initial approach was a clip arrangement which was unable to accommodate the variety of configurations brought about by the introduction of new models. Drawbacks were also apparent from using adhesives.
Again the answer was hot air staking, only this time the lips of the undertray recesses have been extended to form tabs, which are heated and folded over the magnets.
Additional benefits are that both operations can be undertaken simultaneously, and they have been combined into a single, semi-automatic production cell, using a custom-engineered machine equipped with a four station rotary table.
At the first assembly station, cars are loaded in pairs onto special purpose jigs. The magnets are placed into position and the spring contacts are located over 2mm diameter plain pegs moulded in the ABS undertray.
On initiation of the automatic cycle, the workpieces are indexed through 90 degrees to the heating position. Hot air is directed accurately over the pegs and the magnet retaining lugs. At the next station, forming tools simultaneously turn over the lugs and reshape the softened pegs to produce 4mm diameter rivet heads. The completed assemblies then cool, before being ejected automatically at the final indexing position.
PHASA Developments Tel: 01279 657711