Techniques in Hardwood Plywood Quality Control

Quality control in plywood manufacture cannot be achieved simply with inspection of the final product but must be applied to the entire production operation. Quality control normally includes testing and inspection. Testing in the laboratory. determines the strength of glue bonds, moisture contents of veneer, etc. Inspection includes checking the accuracy and efficiency of the drying, clipping, taping and splicing, and gluing operations. To be efficient and economical, inspection must be a sampling process, and it is important to decide how intensive the sampling should be. Statistical quality control methods can be useful in improving average quality and uniformity of quality of a product. An application of the Shewhart control chart to control veneer moisture content is examined as an indication of how this method can be used. Data are charted for an operation in which the manufacturer was attempting to achieve a 10 percent moisture content. The chart indicated which aspects of the operation were out of control and action was taken to solve these problems. Frequently, a process may be brought into statistical control, but the control limits may exceed the specification limits. In this case, either the process must be refined, or the specification limits must be re-examined and possibly changed. With the statistical method, a supervisor can determine whether his operation is in control and whether the control is being accomplished within acceptable limits; it can then guide him to corrective action. It can indicate that excessive variation is due to assignable causes which can be corrected by closer supervision or by adjusting or repairing a machine; on the other hand, it can also indicate that the material being processed is itself so variable that the operation cannot be controlled within proper limits, or that the equipment being used is not accurate enough to satisfy the specifications. Other aspects of plywood processing analyzable in this way include thickness of veneer, dimensions in clipping, glue spread, glue-joint quality, and panel dimensions. A control chart for attributes can be used where inspection is not for measuring but for classifying; the “control chart for fraction defective” can indicate whether products are acceptable or rejectable. The method of preparing quality control charts and the statistical theory behind them are amply covered in other published material.

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