While all types of glue set faster in warm atmospheres, thermoplastic glues, i.e. heat-melting glues such as hide glue, will soften or liquefy under pronounced heat; but thermosetting glues will cure much more rapidly either by quicker evaporation of the moisture content, as in modified animal glues, or by a definite heat reaction in the case of synthetic resin glues. As this rapid acceleration of glue sets is of great importance to furniture-manufacturers seeking quick release of expensive tools, jigs and equipment, considerable research has been directed into the study of the most effective methods of applying heat without scorching the wood or disrupting its fibres, and various processes have been perfected which are now used extensively in industry for edge banding, laminating, scarfing, veneering and general assembly-work. It should be pointed out, however, that while the various processes are simple enough to operate, the more sophisticated methods require the initial services of skilled electricians and expert jig-makers, and as such are only applicable to production in quantity. It is, therefore, proposed to refer to them in broad outline only.
Various methods of applying heat to glue-lines are practised: (a) space heating of the surrounding air by any convenient heat source (hot-air blowers, radiant heat sources, etc.) which relies upon the conductivity of the warmed-up wood to transmit sufficient heat to cure the glue; (b) contact heating, also actively heating the wood substance either by hot cauls, steam or electrically heated platens or strip heaters (low-voltage heating). (Low-voltage heating of glue-lines was first perfected by Aero Research Ltd [now CIBA/ARL Ltd] in 1939, and was used extensively in the production of wartime Mosquito aircraft, gliders and assault craft). (c) radio-frequency heating, whereby the wood molecules are actively distorted and rotated, thus causing friction and inducing heat according to the electrical resistance of the materials (wood and glue).
Of these methods only low-voltage and radio-frequency heating require further explanation.
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