Plywood Core veneer Lay-up Joint Types Scarf joint
The scarf joint is commonly used to construct a large-size Plywood core veneer structure piece by piece or to repair a damaged veneer structure. Because the interface of the scarf joint does not have any continuous reinforcement from one part to the next, it is generally the weakest link of a veneer composite structure. QC should check details of the the strength of the scarf jointof a veneer composite structure. For study, the general interface strength is first discussed and then the scarf joint is presented. The numerical inspections focuses on proper modeling of the interface and the scarf joint in order to predict the interface strength. Furthermore, a technique is presented to enhance the interface strength of the scarf joint using nano-size particles.
Scarf joints are overlap core veneer joints where the angle (θ) between the axis of the adhesive layer and the axis of the adherends (of equal width and thickness) is greater than 0º (butt joint) and less than 90º, through varying the scarf angle the ratio of normal stresses to shear stresses within the bondline can be controlled. This control over stress ratios provides an extremely useful tool for exploring yield and failure criteria in adhesives and can be used to validate constitutive vener core layup.
Scarf jointsare used to provide structural integrity and lengths can exceed the length of the logs from which the components were derived. While individual pieces often come from different trees and positions, leading to variable performance in strength, stability and appearance, manufactures have an opportunity to match colour and grain characteristics to suit markets by segregating.
An extremely important aim in plywood and LVL production is to maximise the yield of useful veneer that becomes part of the product. The need to clip out defected parts of veneer and the possible requirement to peel shorter logs can result in the necessity to join several smaller pieces together to produce veneer sheets that are of a suitable size for plywood or LVL production. Veneers can be joined in the direction of the grain by creating scarf joints that allow a sufficient overlap to ensure that good bonding takes place. In this way, veneers can be joined together (which is limited by the length of the log) to an almost unlimited length. Scarfing is impractical for joining veneers across the grain and so to join veneers in this direction, smaller veneers are composed using hot melt glue to butt joint the veneers to form a continuous mat. Hot melt strings are then used to reinforce the joint to give it added integrity. The larger veneer sheets created by jointing and/or composing can then be cut to the desired dimensions for plywood manufacture. Generally jointed or composed veneers are used in the core of plywood. Any remaining defects, such as loose knots, open holes or pitch pockets, are detected using machine vision and the veneers are repaired by replacing the defected portion of veneer with a patch of defect-free veneer.