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Minimizing Warp in Laminated Particleboard and Medium Density Fiberboard

technical BULLETIN

Introduction used to measure warp, and “ASTM Test Method D-1037,” which describes the test Industrial grade particleboard (PB) and method used to measure moisture content. medium density fiberboard (MDF) are widely recognized as ideal substrates for laminated Balanced Construction panel products. Their properties - smooth surface, uniform density and thickness, One of the most common causes of warp in dimensional stability, strength, stiffness, laminated panel products is unbalanced panel flatness, screw holding strength, and construction. As different materials are machinability - contribute to the ease of rigidly bonded together, moisture content fabrication and to the ultimate performance changes may occur. In response to the of laminated panel products such as table changes in moisture content, the materials tops, countertops, kitchen cabinets, and attempt to change dimensions. When that stereo and television cabinets. These happens, stresses can accumulate. Warp applications require laminated panels to results when these stresses become remain flat. excessive and are no longer balanced on the two surfaces. Warp is defined as the “deviation of the geometry of a panel from an initial state of This imbalance may be caused by a number flatness.” This document outlines some of factors, including selection of laminate, factors (balanced construction; material laminating environment, component handling and storage; and proper laminating, conditioning, product design, installation, fabricating, and installation techniques) that and application. Selection of laminates and affect warp, and steps a laminator or balanced construction go hand-in-hand. The fabricator can take to minimize it. laminates and/or coatings applied to each side of a PB or MDF substrate should be SM Two other useful documents are “CPA similar in properties. Generally, this is best Standard Method for Measuring Warp in achieved by using the same material to cover Particleboard and Medium Density both sides of a substrate. Fiberboard,” which describes the test method Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin

Unusually moist or dry conditions should be avoided in the laminating and storage environments. The moisture content of wood-based materials and laminates is dependent on the amount of moisture in the air. When these materials are moved from Panel conditioning prior to final installation The information one environment to another, the moisture improves reliability. in this document content changes. Resulting dimensional is believed to be reliable. changes can be substantial. The design of laminated panel applications However, the must also consider the service environment. determination of Once assembled, differences in the Applications that expose one surface of a suitability of expansion or shrinkage characteristics of the panel to a warm humid atmosphere and the this information laminate and substrate can produce stresses other to a dry atmosphere will result in for any use or which cause warped panels. Ideally, moisture unbalance that can cause warp. application re- laminates and substrates should be stored Long expanses of panels, such as display mains the sole and assembled in conditions similar to the cases or walls, may warp if they do not responsibility of finished product application environment. include expansion joints or other design the user. considerations allowing for dimensional It is unlikely that the moisture content of changes resulting from seasonal variations No guarantees, the laminate and the substrate will be in in the environment. Panels that are butted representations equilibrium with the laminating environment edge-to-edge and rigidly fastened may or express or when they are delivered. Allow sufficient buckle (a form of restrained warping) due to implied warran- time for the laminate and the substrate to expansion stresses as moisture content ties are made adapt to the laminating environment. It can increases. regarding the take two or more weeks to reach a accuracy and satisfactory equilibrium. Seasonal changes A balanced laminated panel is one that will completeness of and air circulation around the materials will not warp when subjected to forces induced this information, and the Com- influence the time it takes. by uniformly distributed moisture changes. posite Panel As- Balanced panel construction and subsequent sociation (CPA) Laminates at one equilibrium moisture dimensional stability in service are best assumes no re- content (EMC) condition should not be achieved when: sponsibility or applied to PB/MDF of a different EMC liability for any condition. If they are, as the moisture • The moisture content of the PB/MDF loss or damage content equalizes, the PB/MDF substrate substrate and laminate materials are through reliance may expand or contract while the laminate similar at the time of lamination. upon it. seeks the opposite. When bonded with rigid adhesives, the components cannot move in • Materials with similar expansion/ relation to each other. This creates stresses shrinkage coefficients are utilized. at the substrate/laminate interface which can result in a warped panel. • The moisture content of the finished product is similar to the moisture Service Environment content to be encountered in service.

Even with a perfectly balanced panel, A well-balanced laminated panel can exhibit installation and application conditions can temporary warp due to unequal rates of cause moisture unbalance that results in moisture gain or loss by the face and back warp. The laminated panel should not be laminates. However, as soon as the exposed to extreme variations in humidity laminates equalize, these stresses diminish, during final installation. Changes in and the panel returns to its flat condition. humidity at the installation site can result in This ability to equalize and return to the flat stresses that cause warp as the entire panel condition at any humidity is an important equalizes to the new service environment. attribute of the balanced panel.

Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Composite Panel Association Substrates

Selection of PB/MDF for laminating applications should follow basic guidelines. Always select flat panels for substrates. Flatness indicates that the substrate is balanced and free of stress. Consider the substrate properties, including stiffness (MOE), thickness, linear expansion, and uniformity. These can be evaluated from the manufacturer’s specifications or standards. The greater the MOE or thickness of the material, the better it will resist moisture related expansion stresses. Measure the panel moisture content and set guidelines of acceptability. Evaluate all of these properties with respect to laminates that will be Approximate equilibrium moisture content zones in the United States for wood-based products. Values may applied. vary with local and seasonal conditions.

Storage and Handling applied to PB/MDF substrates by the laminator.

Rules for substrate and laminate handling and storage High-Pressure Laminates (HPL) are generally the same. Materials should be stored flat and kept dry. For best performance: High-pressure laminates consist of multiple layers of

kraft paper saturated with phenolic resin, a decorative 1. Do not store materials outside or in locations layer of paper saturated with melamine resin, and a where they may be exposed to water or high very thin top sheet of paper heavily saturated with humidity. melamine resin. As does any wood-based product,

HPLs expand and contract with changes in moisture 2. Keep material off the floor, use bolsters of the content. same thickness, and allow adequate space between units. The HPL and the substrate materials should be 3. Avoid storage conditions where extremes of brought to equilibrium at the same humidity and temperature and humidity can occur. temperature before laminating. This conditioning period may be two weeks or even longer. If laminated 4. Before final assembly, allow materials a panels are fabricated from materials conditioned at satisfactory conditioning period to equalize. radically different humidities, warp will most likely occur when the finished panel comes to equilibrium with the surrounding environment. Laminates The key factors for obtaining balanced HPL panels are: High-pressure laminates, resin-saturated papers, vinyl films, heat transfer foils, decorative papers, and wood 1. Proper conditioning of component materials before veneers comprise types of overlay materials commonly laminating. Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin

2. Selection of HPL faces and backer sheets when using different papers. with similar linear expansion, stiffness and thickness properties. 5. Avoid degradation of the substrate when using phenolic papers with elevated press 3. Lamination of HPL face and backer sheets temperatures and extended press times. with machine directions parallel. Proper cooling of the panels is critical.

4. Following the adhesive manufacturer’s 6. The linear expansion properties of resin- recommendations for storage, method of saturated papers are often greater than that application, spread rates and end use. of particleboard and MDF under the same conditions. In some applications, such as countertops, it may be appropriate to use a face laminate 7. Use as few press openings as possible: too without a backer sheet. In these cases, using an many openings tend to increase precure by elastic adhesive (for example, rubber-based slowing closing time. contact cement) reduces the amount of stress transferred to the substrate by the face laminate Wood Veneers and thereby reduces the tendency to warp. Attached countertop installations further reduce A major application of wood Composite Panel Association the tendency to warp by restraining residual veneer is as a decorative stresses that may be transferred to the laminate material over PB/MDF substrate. substrates. The general handling and storage requirements Saturated Papers discussed previously also apply to veneer and PB / MDF Saturated papers, or low-pressure laminates combinations. In addition, the (LPL), are preprinted or solid color decorative materials should be laminated papers that have been saturated with either a with their moisture content in the melamine, a phenolic, or a polyester resin. 6-9 percent range. And, since LPLs, like HPLs, are wood-based products and balanced panel construction is will shrink and expand with changes in moisture essential to prevent warp, the content. same thickness and grade veneer should be used on both sides. In addition to the key factors listed for the successful use of HPLs, the following should be Different veneer species can be kept in mind: used, but they must have similar strength properties and dimensional behavior patterns. 1. During assembly, load and close the press as Problem areas, such as tension wood, burls, and quickly as possible. Use a cool caul on the knots, and their effect on stress must be bottom, when cauls are used for panel specifically considered. Finally, the glue spread insertion. rate should be uniform.

2. Hot boards should be stacked flat and be well Other Overlays supported while cooling. Units with fewer pieces cool more evenly. Avoid cooling Vinyl films, low-basis-weight papers, and foils panels too rapidly. Cool panels uniformly on should all be applied using good balanced- both sides. lamination practices. Generally, the application

of a film or paper to one side only, or different 3. Follow manufacturer’s instructions for storing overlays on each side of a PB/MDF substrate will papers. Do not use papers that are overaged not result in warp. However, one-sided or dried out. application of any laminate may act as a

moisture barrier creating a transitory imbalance 4. Be sure that press platen temperature and which can result in warp over time. conditions are set to properly cure both sides Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin Technical Bulletin

Composite Panel Association

Summary natural variability of the laminate and substrate properties is a common cause of An imbalance in moisture-related expansion warping, particularly in the case of a thin or contraction frequently causes warping of substrate with relatively thick laminate laminated panels. Such an imbalance is faces. Controlling the variability between activated by changes in moisture content. A the laminates can effectively reduce warp in change might be temporary, as in the case laminated panels.

of wetting one side of a flat panel. The The concept of balance does not end with resulting “transient warping” is beyond the the manufacture of a balanced panel. The control of the laminated-panel installation and the end-use environments manufacturer. In theory, “structural warp” can also be sources of moisture imbalance resulting from a built-in imbalance can that create internal stresses resulting in sometimes be prevented. warp. To ensure acceptable laminated product performance, design and Balanced lamination is the key to engineering must consider the product consistently manufacturing flat panels. The application and environment. REFERENCES Composite Panel Association

Medium Density Fiberboard From Start to The Composite Panel Association (CPA), Finish. Composite Panel Association, founded in 1960, represents the North Gaithersburg, Maryland. 1998 American composite panel industry on technical, regulatory, quality assurance and Particleboard From Start to Finish. product acceptance issues. CPA General Composite Panel Association, Members include 38 of the leading Gaithersburg, Maryland. 1996 manufacturers of particleboard, medium density fiberboard and hardboard and other Standard Method for Measurement of compatible products. Together they Warp in Particleboard and MDF. represent nearly 95% of the total Composite Panel Association, manufacturing capacity throughout North Gaithersburg, Maryland. America.

Suchsland, Otto and Hong Xu, The Linear CPA also brings together the complete Expansion of Commercial Particleboard and value chain affiliated with the composite its Measurement. Composite Panel Association, Gaithersburg, Maryland. panel industry. CPA’s 200 members worldwide, primarily in the US and Canada, Suchsland, Otto, and J.D. McNatt, Computer are committed to product advancement Simulation of Laminated Wood Panel and industry competitiveness. Associate Warping. Forest Products Journal, 36 Members include manufacturers of (11/12):16-23, 1986 furniture, cabinets, decorative surfaces and equipment as well as laminators and Suchsland, Otto, and J.D. McNatt, On the distributors. Warping of Laminated Wood Panels. Michigan State University, East Lansing, CPA is a vital resource for both producers Michigan. 1985 and users of industry products. As an

accredited standards developer, CPA writes Forest Products Laboratory, Wood and publishes industry product standards. Handbook: Wood as an Engineering Material. A.H. 72, Washington, DC. 1987 It also participates in the standards development work of ANSI, ASTM and Heebink, B.G. Importance of Balanced others, sponsors product acceptance Construction in Plastic-Faced Wood Panels. activities and works with federal and state U.S. Forest Service Research Note FPL-021. agencies and model building code bodies. 1963 In addition, CPA conducts product testing and third-party certification programs, while helping manufacturers create in-plant quality control programs. Composite Panel Association Outreach and education are also focal Main Office points of the CPA. The Association 18922 Premiere Court publishes industry performance data and Gaithersburg, Maryland 20879-1574 conducts seminars to assist specifiers, 301-670-0604 ▪ Fax 301-840-1252 manufacturers and other users of composite panels. CPA produces a series of 99 Bank Street, Suite 701 technical bulletins and develops Ottawa, Ontario K1P 6B9 publications, videos, and other materials to 613-232-6782 ▪ Fax 613-232-8386 inform key audiences about the attributes

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