Defects In Plywoods

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Certain defects regularly occur during or after manufacture, and should be diligently searched for otherwise they may give endless trouble. Figure 26 gives the most common defects. Ripples or bumpiness (26:1) caused by swelling of the core strips in laminated construction can be detected with a knife-edge straight edge, or passing the palm of the hand over the surface (closing the eyes increases the sensibility). Planing off the bumps is not always satisfactory, as the cores may shrink back in drier conditions, forming undulating hollows; therefore it is better to reject bumpy boards for face-work and use them only for unseen interiors. Figure 26:2 shows hidden blisters which can only be detected by tapping the surface with the fingernail for any hollow sound; they must be glued back or subsequent veneering is bound to lift them. Overlapping veneers are shown in 26:3, and will eventually show as a long spine under a high polish. Long splits in the face (26:4) are easily rectified, but hidden gaps in the core (26:5) are often difficult to detect and may only declare themselves at later stages. Here again laboriously tapping with the finger-nail is the only answer, while all freshly cut ends should be critically examined. Figure 26:6 shows a method adopted in some constructions, in which the core strips are grooved across to take a string

26 Defects in plywoods, etc.

binder during assembly, and such boards should not be used in face-work. All these defects may not necessarily detract from the structural strength, but will certainly mar the surface quality if not rectified. Badly warped boards, particularly in the thicker grades, should be rejected out of hand, for it is never wise to assume that holding frameworks will pull them straight or subsequent veneering correct them. More often than not the whole carcass will be pulled in wind.

All prefabricated woods are dried to a moisture content of around 9 per cent during manufacture. They pick up moisture slowly but also give it up as slowly, therefore they should be stored flat on level foundations and in dry conditions.

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