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 veneerpress 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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