Quality Control for Edge Glued Panel Manufacturing
Craig L. Forbes
Assistant Professor and Wood Products Extension Specialist
North Carolina State University
Edge glued lumber panels are an important component of many high quality furniture designs. Edged glued panels may be used for exposed furniture parts (such as doors), face glued into squares for turning or used as core material for hardwood plywood. In any case, it is imperative that the panels be manufactured with strong glue bonds, closed joints, smooth surfaces, and be free of cup and warp. To insure panels of consistent good quality, it is important that the manufacturer carefully monitor several crucial aspects of the edge gluing process. When edge gluing difficult species, it is especially crucial that manufacturers follow strict quality control practices. The purpose of this report is to discuss propermanufacturing techniques necessary for quality panels.
Quality edge glued panels cannot be manufactured from improperly manufactured lumber. The lumber must be dried to a moisture content appropriate for the environment of its intended end-use (6-8% for most of the United States). The lumber must be properly equalized to insure the moisture content is uniform, and properly conditioned to eliminate drying stresses in the lumber which may result in warped panel components when the lumber is cut into parts.
Proper moisture content must be maintained during wood storage and manufacturing. It is surprising how quickly dry lumber can pick up moisture from humid air. Often in furniture plants, dried lumber is stored on sticks after drying in open air storage sheds. Although this practice reduces handling of the lumber, it allows humid air to contact the dry lumber more effectively, increasing the rate of moisture gain. Lumber and wood parts should be stored in an environmentally controlled warehouse to eliminate the changes in moisture content. If this is not possible, lumber should be removed from sticks before storage, and storage time should be minimal (practice first-in/first-out inventory management). Keeping the temperature of the storage area approximately 20o F over the outside air temperature, will help slow down moisture content changes in wood not stored in sheds with humidity controls.
Keep in mind that one board of improper moisture content, when cut up, will result in many wet panel components. One wet component in a panel is all that is needed to cause a joint to open and the panel to become faulty. It is imperative that panel components are of proper moisture content. Wet parts may result in joint failure, cupping, warping, sunken faces, and poor machinability.
Machining of Panel Components
Panel components must be machined properly to achieve a quality panel. Machined parts must be straight, have parallel and square edges, and surfaces of good quality. Improperly machined components may result in cupped panels and joint failure.
Improperly maintained or adjusted machinery may result in components with non-parallel edges. Non-parallel edges result in uneven glue spread and pressure. If the piece is machined properly, the width of the component should measure the same in the middle and at each end. Any deviation is compounded as faulty components are assembled. In other words, if six components, each with 1/16" excess on one end, comprise the same panel, there could be as much as 3/8" total difference between the widths at each end of the panel. A severe form of non-parallel edges is snipe. Snipe occurs when more wood is removed from one edge of a piece due to a deviation as the saw blade enters the piece, or the piece does not meet a cutter head on a level plane. Figure 1 illustrates snipe and how it affects panel quality.
It is also important that the edges are machined square. When edges are not square, a gap occurs along one edge of the glue line. When this gap is on the face of the panel (in view of the operator), the operator will often apply extra pressure to force the components together. If the gap is on the bottom, the panel may buckle. In practice when a panel buckles, the clamp operator usually forces the panel flat by hammering. Whether the gap is on top or bottom the result is the same. As the gap is closed, the wood along the opposite side of the gap compresses (Figure 2). When the glue cures an apparently good panel results. However, if the panel is exposed to moisture changes, the compressed wood expands and a cupped panel results. Excessive pressure cannot be used to compensate for poor machining. The rip saw operator should check squareness of the cut often.
It also important the surfaces to be glued are of good quality. Surfaces should be smooth with no burnishing, fuzziness, or deep tooth marks. Burnished surfaces do not allow for good glue penetration and thus weak glue bonds are formed. Stray tooth marks or fuzzy surfaces do not allow for mating pieces to fit together tightly. The result is uneven pressure and poor surface contact, both of which contribute to poor glue bonds.
Finally, components should be glued together shortly after machining. Components in storage can get dirty and pick up moisture, both of which affect glue bond performance. As parts pick up moisture, they may warp or change dimensions due to unequal shrinkage or swelling, almost guaranteeing a defective panel. Components should be used as soon after machining as possible and the oldest parts should be used first.
Gluing and Clamping
Components must be properly glued to achieve a quality panel. It is important the proper glue formulation be used for the species, plant conditions, clamp cycle, and target moisture content of the components. If a new species of wood is to be glued, or if the manufacturing process is to be altered, it is best to contact the adhesive supplier and ask for advice. Very rarely are panel problems related to bad glue. However, it is necessary to have good quality, properly mixed glue. If glue is purchased pre-mixed, make sure the proper adhesive has been delivered to your plant. If the adhesive is mixed in-plant, follow the supplier's instructions carefully as to proper mixing procedures and quality control testing. Likewise, follow the adhesive supplier's instructions pertaining to spread rate and cycle times. Make sure clamp operators visually check each component to assure proper spread has been achieved.
Panels should be clamped at the appropriate pressure, usually 100-200 psi (higher density wood requires higher pressure). It is important the panel remain under pressure for the proper length of time. Proper time is dependent on the adhesive and process used as well as panel dimensions. Consult with the adhesive supplier to determine the correct press (clamp) time.
Glued panels must be properly conditioned before machining. Conditioning allows the glue to fully cure and the moisture content of the panel to equalize. Moisture is adsorbed into the wood from the glue as it cures. As moisture is adsorbed, the wood along the glue line expands. Conditioning allows the moisture to escape from the expanded wood along the glue line which allows the wood to shrink to its original state. If the wood is surfaced before the wood has returned to its original dimensions, sunken joints may occur (Figure 3). Conditioning generally requires at least two days but may vary depending on the adhesive used, plant conditions, and panel dimensions. Consult the adhesive supplier about proper conditioning times.
Summary of Quality Control Requirements
To reduce defective panels, the manufacturer must implement a rigid quality control program. The program should include a process for monitoring the moisture content of the lumber and panel components throughout the manufacturing process. If lumber or parts are purchased from an outside supplier, these too should be closely monitored.
Machining quality should be thoroughly monitored. Quality checks should include inspections of machined surface quality, straightness and flatness of components, squareness of component edges, accuracy of dimensions, and length of time between machining and gluing. Machine preventative maintenance should be enforced since machining quality is greatly influenced by machine performance.
Glue quality and spread levels should be regularly monitored. Spreader bearings and rolls should be maintained. Supervisors should observe clamp operators to assure operators are attentive and visually check glue spreads on each component. Pressure levels, cycle times (stand time and press time), and conditioning times should be monitored.
Proper manufacturing techniques are essential for quality edge glued panels. To insure proper techniques are followed, a structured quality control program must be in place. This report discusses the most important manufacturing concerns related to achieving quality panels and offers suggestions for designing a quality control program. The manufacturer must take this information and design a program which best fits their process needs and concerns. Equally important is that the manufacturer monitor and maintain their manufacturing equipment. Broken clamps, worn bearings, missing machine parts, and poorly adjusted machines are common causes of poorly manufactured panels. The cost of neglecting equipment is high. Likewise, manufacturers should seek advice from their adhesive suppliers and equipment manufacturers. Many companies offer good technical support and educational materials as a service to customers. Much can be learned from the experiences of those who deal with similar problems on a daily basis.