The Technique of FURNITURE MAKING

PRESSURE VENEERING WITH RESIN GLUES

Resin glues offer very definite advantages, for the glues set at ordinary room temperatures and the application of heat is optional according to the required rapidity of the set. As the veneers do not have to be dampened no water is used except as a solvent for the glue, therefore expansion and subsequent shrinkage is negligible; moreover, there is or should be little, if any, penetration to the wood surface, the pores of the wood are not choked with glue, and all the lustre is retained. Edges do not have to be protected as with hide glues, and in fact veneered surfaces can be shaped, moulded and run out to a feather edge without the veneers lifting, while the surfaces are harder, with greater wear resistance, and are impervious to moderate heat and moisture. Lastly, veneers can be cut square, assembled in complex patterns, taped together and laid as a single sheet with the certainty that if properly prepared there will be no gapping of joints, with a good joint in straight-grained boards almost invisible to the naked eye.

Pressure is always necessary with synthetic resin glues for they have no natural suck (impact glues suitable for laying formica, etc. are quite unsuitable for permanent veneer-work), but it need only be sufficient to bring both surfaces into intimate contact and to maintain that contact until the glue has set. Various forms of veneer-press are used and Figures 284-6 show representative examples. The principle is the same throughout, i. e. to maintain sufficient positive pressure to bend or flatten the veneer onto its former or groundwork (form or substrate): excessive pressure over and above the minimum requirements should be avoided or glue-starved joints will result. A thin skin only of glue should be used, and the object, therefore, is not to squeeze out the surplus glue as in hand-veneering but merely to establish the requisite intimate contact. Some form of veneer-press is virtually indispensable for every trade-shop no matter how small, for it will soon pay for its keep, and one of the most suitable for the general run of work is the hand-operated screw veneer-press (284) with open ends. With this type extra-long work beyond the capacity of the standard daylight press can be veneered in sections, or the overhang at the ends clamped between bearers and G-cramps/C-clamps. Alternatively, very long panels can be slit into convenient sections in the long-grain direction

of the face veneers, grooved for loose tongues and each section veneered and then assembled. The process is simple enough and highly effective, but the grooving must be very accurately cut if the veneered surfaces are to line up. Failing any kind of press large areas can be satisfactorily veneered with nothing more elaborate than heavy cross-bearers and sash - cramps as in caul veneering, but some local blistering must be expected which, incidentally, is not as difficult to lay as some writers would make out. A simple way of checking that the cross-bearers are exerting full pressure in the centre of the panel is to slip tags of newspaper under the bearers at intervals. A slight pull on the tags will disclose whether the pressure is uniform throughout.

Preparation and laying (resin glues)

The groundwork/substrate is prepared in the same way as for hammer veneering, laying the veneeers 'with the grain' on solid wood and 'across the grain' of plywoods, etc. The surfaces should not be roughened or keyed except for a light sanding to break through any surface glazing or grime, followed by a thorough dusting to clean the fibres. Any general - purpose single-application resin glue can be used for cold pressing at room temperatures, but special veneering glues such as Aerolite KL (CIbA (ARL) Ltd) with gap-filling powder hardener are preferable. The glue should be mixed exactly in accordance with the maker's instructions and applied to the groundwork in a thin coat, spreading it evenly with spatula or palette-knife, or more satisfactorily rolling it out with a photographic rubber roller or trough hand spreader, carefully checking for bare spots, as there will be no surplus glue to level them out in the pressing, and allowing a few minutes air drying according to the make of glue. Cramping/clamping pressure in handwork can be as for caul veneering with hide glue, but the bearers need not be rounded, always providing that they press in the centre, and a good test of this is to insert tags of newspaper under the bearers which should be firmly gripped when the cramps are tightened. As there is plenty of time for assembly both face and counter (backing) veneers can be laid at the same time with an interleaving of paper, cellophane, or waxed hardboard to prevent the veneers sticking to the cover boards or cauls. Setting time will be according to the type of glue and the working temperature, but work under strain should have more than the minimum before the cramps are released. For more detailed information as to working procedures, reference should be made to the printed instructions of the manufacturers concerned.