The Technique of FURNITURE MAKING

HEAT ACCELERATION OF GLUE SETS

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 radio­frequency heating require further explanation.

Low-voltage (LF) heating

This method relies upon the conductivity of the wood to transmit heat from the contact elements to the glue-line. The actual elements can be 26 gauge stainless steel or mild sheet steel plated against corrosion, and are heated by low - voltage high-amperage currents from a suitable stepdown transformer giving a range of tappings of low voltages suitable for normal work. The heat is generated by the resistance of the metal sheet to the passage of the current, and while stainless steel has more resistance and therefore more heat generated for a given power-supply, mild steel is cheaper and more readily obtainable. In practice, therefore, narrow elements up to 6 in (152 mm) wide used for edge banding, etc. are generally in mild steel, while wider elements are made of stainless steel, otherwise the amperage required to attain the degree of heat necessary to cure the glue would probably be in excess of the capacity of the transformer.

The metal sheets or 'elements', which can be either flat or curved to conform to whatever shape of jig is used, are laid in contact with the work, and it has been established in practice that various power ratings per square foot are necessary for differently shaped elements.

It follows, therefore, that as the amperage increases rapidly with the width of the element, some means must be adopted of reducing the amount of current required, and two standard methods are used. As the voltage requirements are low the length of the element can be increased at the expense of the width, either by (a) connecting a series of narrow strips of identical width with brass plates, nuts and bolts (325:1) so that the square element now becomes a continuous ribbon, or (b) cutting slots in a square sheet alternately from each side to form the continuous ribbon (325:2). If the latter method is adopted the ends of the cuts must be pronged as shown in 325:4 with each prong one - third of the width (W) and at an angle (A) of 90° to each other, or local hot spots will occur and the end of the strip will be much cooler. For all practical purposes method (a) is better, but whichever is used the strips must be identical in width, and cable connections, brass plates, bolts, etc. clean and firmly seated. Measurement of the effective temperature in any part of the continuous strip can be made with a thermo-couple; while paints and crayons which change colour when the temperature reaches certain values are obtainable.

Jigs and platens for LF heating

Narrow strip elements can be laid directly against the veneer or laminate in edge banding, etc. but some softening is often necessary to absorb inequalities. If a strip of old bag press rubber is available this can be placed either between the element and the veneer or against the pressure member, with a thin piece of heat - resistant material to conserve the heat. Figure 325:3 shows a typical disposition. Continuous strip elements (325:1, 2) in panel form cannot be laid directly against the veneer as there are gaps between the strips, and in order to spread the pressure a sheet of 20 gauge aluminium is often used with an interleaving of 1/16 in (1.5 mm) laminated plastic to insulate the strips. The elements can be attached to the wooden pressure former with again a thin layer of some type of heat-resistant material to conserve the heat, and a softening pad if necessary (325:5). Platens or jigs can be made of plywood or blockboard on a rigid softwood foundation, and shaped to fit for curved work. Pressure must be positive and of the order of 50 lbpersq. in (3.515 Kgf/cm2), for inadequate pressures will allow thick glue-lines to form which will boil under heat, forming an air-filled froth which will eventually collapse. Raising the pressure naturally raises the boiling-point of the glue, and if 50 lb per sq. in (3.515 Kgf/cm2) cannot be attained then the amount of heat will have to be reduced well below the boiling-point of the glue at the particular pressure applied, with consequent longer pressing times. With the working temperature of the element within the range 212°F (100°C) to 230°F (110°C), which is well below the scorching temperature of the wood, a normal synthetic resin glue will set in about 30 seconds plus the time taken for the heat to travel from the element to the glue-line. A general rule for this heat transfer is 1 min. per mm of thickness separating the element from the furthest glue-line up to a total of 6 mm, and I1/2 min. per mm for thickness up to 12 mm; therefore a built-up lamination 12 mm thick with core veneers 2 mm thick and face veneer 1 mm thick will take 11 x I1/2 = 161/2 min., plus the actual setting time of the glue. For thicknesses over 12 mm it is better to place elements on either side of the lamination, in which case the time is calculated for only half the thickness. In constructional work where there is a considerable thickness of timber in the joint it may only be necessary to cure the glue part way into the joint before it is removed from the jig or press, and to allow residual heat to set the remainder, always providing no great stresses are involved, otherwise the joint will spring. Care must be taken in the choice of glue for all low-voltage heating, for a very fast - working glue may have a tendency to precure before the pressure has built up sufficiently, particularly with hand-operated screw presses or jigs, and in case of doubt the advice of the manufacturers should be sought.

Although low-voltage installations are inexpensive and very economical in power they are, as already mentioned, more applicable to quantity production, for individual elements will have to be made up for each particular platen or jig, calling for the services of both jig - maker and electrician; but a heated platen for a standard single daylight screw-operated veneer - press can easily be made which will give all the advantages of a hot press for flat veneering in a small workshop. Another technique is the incorporation of a length of resistance or Eureka wire in a tongued and grooved joint during assembly, which when coupled to a step - down transformer will heat the glue sufficiently for the cramps/clamps to be withdrawn within a few minutes; the ends of the wire are then snipped off with the length embedded in the

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GLUE LINE

Y"

325 Glue-line heating

joint unnoticeable. The voltage requirements will depend upon both the thickness and the length of the wire, and must be either calculated by a competent electrician or arrived at by trial and error. Mention should also be made of flexible heating mats after the style of electric blankets which run from standard voltages and do not require transformers, but as the wiring elements are very thin they will not stand rough usage, nor can the mats themselves be cut to shape.