Prepressing Plywood

The advantages of prepressing plywood are reduced rejects, faster panel handling, better adhesion, and improved press design. Disadvantages include inability to correct misaligned panels, increased possibility of panel dry-out, and possibly higher adhesive costs. The potential advantages of a prepress operation are dependent upon meeting certain requirements such as: well-planned equipment layout, selection of the proper adhesive system, adherence to good operating practices, and an awareness that the wood species involved will influence the prepress bond. The adhesive mix selected will depend upon the type of plywood to be produced. In all probability, the mix will be based on a urea-formaldehyde resin, a phenol-formaldehyde resin, or a soy-protein glue with known prepressing ability. The mix will undoubtedly be extended, contain fillers, and possibly catalysts. Each item should be selected to complement, or at least not detract from the resin’s prepress characteristics. An efficient prepress system requires close control of stock moisture content, adhesive spread, stand time before prepressing, prepress pressure and time, and stand time after prepressing. The interrelationship between these facets of the prepress operation and the adhesive mix and wood species must be considered when establishing the control system.

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