Warp is defined as the “deviation of the geometry of a panel from an initial state of flatness.” There are several factors (balanced construction; material handling and storage; and proper laminating, fabricating, and installation techniques) that affect warp. Following are steps a laminator or fabricator can take to minimize it.

Balanced construction

One of the most common causes of warp in laminated panel products is unbalanced panel construction. As different materials are rigidly bonded together, moisture content changes may occur. In response to the changes in moisture content, the materials attempt to change dimensions. When that happens, stresses can accumulate. Warp results when these stresses become excessive and are no longer balanced on the two surfaces.

This imbalance may be caused by a number of factors, including selection of laminate, laminating environment, component conditioning, product design, installation, and application. Selection of laminates and balanced construction go hand-in-hand. The laminates and/or coatings applied to each side of a PB or MDF substrate should be similar in properties. Generally, this is best achieved by using the same material to cover both sides of a substrate.

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 one environment to another, the moisture content changes. Resulting dimensional changes can be substantial.

Once assembled, differences in the expansion or shrinkage characteristics of the laminate and substrate can produce stresses which cause warped panels. Ideally, laminates and substrates should be stored and assembled in conditions similar to the finished product application environment.

Allow sufficient time for the laminate and the substrate to adapt to the laminating environment. It can take two or more weeks to reach a satisfactory equilibrium. Seasonal changes and air circulation around the materials will influence the time it takes.

Service Environment

Even with a perfectly balanced panel, installation and application conditions can cause moisture unbalance that results in warp. The laminated panel should not be exposed to extreme variations in humidity during final installation. Changes in humidity at the installation site can result in stresses that cause warp as the entire panel equalizes to the new service environment.

Panel conditioning prior to final installation improves reliability.

The design of laminated panel applications must also consider the service environment. Applications that expose one surface of a panel to a warm humid atmosphere and the other to a dry atmosphere will result in moisture unbalance that can cause warp. Long expanses of panels, such as display cases or walls, may warp if they do not include expansion joints or other design considerations allowing for dimensional changes resulting from seasonal variations in the environment. Panels that are butted edge-to-edge and rigidly fastened may buckle (a form of restrained warping) due to expansion stresses as moisture content increases.

A balanced laminated panel is one that will not warp when subjected to forces induced by uniformly distributed moisture changes. Balanced panel construction and subsequent dimensional stability in service are best achieved when:
• The moisture content of the PB/MDF substrate and laminate materials are similar at the time of lamination.
• Materials with similar expansion/ shrinkage coefficients are utilized.
• The moisture content of the finished product is like the moisture content to be encountered in service.

A well-balanced laminated panel can exhibit temporary warp due to unequal rates of moisture gain or loss by the face and back laminates. However, as soon as the laminates equalize, these stresses diminish, and the panel returns to its flat condition. This ability to equalize and return to the flat condition at any humidity is an important attribute of the balanced panel.

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 applied.

Storage & handling

Rules for substrate and laminate handling and storage are generally the same. Materials should be stored flat and kept dry. For best performance:
1. Do not store materials outside or in locations where they may be exposed to water or high humidity.
2. Keep material off the floor, use bolsters of the same thickness, and allow adequate space between units.
3. Avoid storage conditions where extremes of temperature and humidity can occur.
4. Before final assembly, allow materials a satisfactory conditioning period to equalize.

Laminates

High-pressure laminates, resin-saturated papers, vinyl films, heat transfer foils, decorative papers, and wood veneers comprise types of overlay materials commonly
 Approximate equilibrium moisture content zones in the United States for wood-based products. Values may vary with local and seasonal conditions.
applied to PB/MDF substrates by the laminator.

High-pressure laminates (HPL)

High-pressure laminates consist of multiple layers of kraft paper saturated with phenolic resin, a decorative layer of paper saturated with melamine resin, and a very thin top sheet of paper heavily saturated with melamine resin. As does any wood-based product, HPLs expand and contract with changes in moisture content.

The HPL and the substrate materials should be brought to equilibrium at the same humidity and temperature before laminating. This conditioning period may be two weeks or even longer. If laminated panels are fabricated from materials conditioned at radically different humidities, warp will most likely occur when the finished panel comes to equilibrium with the surrounding environment.

The key factors for obtaining balanced HPL panels are:
1. Proper conditioning of component materials before laminating.
2. Selection of HPL faces and backer sheets with similar linear expansion, stiffness and thickness properties.
3. Lamination of HPL face and backer sheets with machine directions parallel.
4. Following the adhesive manufacturer’s recommendations for storage, method of application, spread rates and end use.

In some applications, such as countertops, it may be appropriate to use a face laminate without a backer sheet. In these cases, using an elastic adhesive (for example, rubber-based contact cement) reduces the amount of stress transferred to the substrate by the face laminate and thereby reduces the tendency to warp. Attached countertop installations further reduce the tendency to warp by restraining residual stresses that may be transferred to the substrate.

Wood veneers

A major application of wood veneer is as a decorative laminate material over PB/MDF substrates. The general handling and storage requirements discussed previously also apply to veneer and PB / MDF combinations. In addition, the materials should be laminated with their moisture content in the 6-9 percent range. And, since balanced panel construction is essential to prevent warp, the same thickness and grade veneer should be used on both sides.

Different veneer species can be used, but they must have similar strength properties and dimensional behavior patterns. Problem areas, such as tension wood, burls, and knots, and their effect on stress must be specifically considered. Finally, the glue spread rate should be uniform.

Balanced lamination is the key to consistently manufacturing flat panels. The natural variability of the laminate and substrate properties is a common cause of warping, particularly in the case of a thin substrate with relatively thick laminate faces. Controlling the variability between the laminates can effectively reduce warp in laminated panels.

Source: Composite Panel Association. For information on panel products and decorative surfaces call 703-724-1128 or visit CompositePanel.org.

Founded in 1960, the CPA represents the North American wood-based composite panel and decorative surfacing industries on technical standards, industry regulation, and product acceptance. CPA General Members include the leading manufacturers of particleboard, medium density fiberboard (MDF), hardboard and engineered wood siding/trim in North America, representing more than 92% of industry manufacturing capacity.  CPA also operates the largest laboratory testing and certification program for composite panel products in North America, and the first one approved by the California Air Resources Board.