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Preventing warp in laminated particleboard and MDF.


Industrial grade particleboard (PBI) and medium density fiberboard (MDF) are widely recognized as ideal substrates for laminated panel products. Their properties - dimensional stability, strength, stiffness, flatness, screw holding power, and machinability - contribute to the ease of fabrication and to the ultimate performance of laminated panel products such as tabletops, countertops, kitchen cabinets, and stereo and television cabinets.

Because of such use, laminated panels must remain flat under normal service conditions. To ensure the panels remain flat, proper storage, lamination, fabrication and product application rules must be observed. Warped panels may result if the rules are ignored.

Warping is any deviation of a panel from an initial state of flatness. Its causes need to be recognized and understood to minimize their occurrence before, during and after fabrication. This article addresses those causes. It provides the laminator and the fabricator with information needed to manufacture a panel to minimize the potential for warp.

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 equal (balanced) on the two surfaces.

This unbalance 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 PBI 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 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 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.

It is unlikely the moisture content of the laminate and the substrate will be in equilibrium with the laminating environment when they are delivered. 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.

Specifically, laminates at one equilibrium moisture content (EMC) condition should not be applied to PBI/MDF or a different EMC condition. If they are, as the moisture content equalizes, the PBI/MDF may expand or contract while the laminate seeks the opposite. When bonded with rigid adhesives, however, the components cannot move in relation to each other. This creates stresses at the substrate/laminate interface which can result in a warped panel.

Even with a perfectly balanced panel, installation and application conditions can cause moisture unbalance that results in warp. The laminated panel should 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 product 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 in 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 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:

* 1. The moisture condition of the PBI/MDF substrate and laminate materials are similar at the time of lamination.

* 2. The moisture condition of the finished product is similar to the moisture condition to be encountered in service.

A well-balanced laminate 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.

Selecting the substrate

Selection of PBI/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 manufacturers 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 the laminates that will be applied.


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 bolster of the same thickness; space properly between units, and align carefully in the stacking process.

* 3. Avoid storage conditions where extremes of temperature and humidity can occur.

* 4. Before final assembly, allow materials a satisfactory conditioning period to equalize.


High-pressure laminates, resin-saturated papers, vinyl films, heat transfer foils, decorative papers, and wood veneers comprise the types of overlay materials applied to PBI/MDF substrate by the laminator. Because these materials have different properties, they introduce variables that affect the laminating process and have an influence on the expected performance of the laminated product.

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 woodbased 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.

Saturated papers

Saturated papers, or low-pressure laminates (LPL), are preprinted or solid color decorative papers that have been saturated with either a melamine, a phenolic, or a polyester resin. LPLs, like HPLs, are wood-based products and will shrink and expand with changes in moisture content.

In addition to the key factors listed for successful use of HPLs, the following should be kept in mind:

* 1. During assembly, load and close the press as quickly as possible. Use a cool caul on the bottom, when cauls are used for panel insertion.

* 2. Hot boards should be stacked flat and be well supported while cooling. Units with fewer pieces cool more uniformly. Avoid cooling panels to rapidly. Cool panels uniformly on both sides of the panel.

* 3. Retain adequate volatile material in the paper; be sure they are not overage and dried out.

* 4. Be sure that press platen temperature and conditions are set to properly cure both sides when using different papers.

* 5. When using phenolic papers with elevated temperatures and extended press times, avoid degradation of the substrate. Proper cooling of the panels is critical.

Wood veneer

A major application of wood veneer is as a decorative laminate material over PBI/MDF substrates. The general handling and storage requirements discussed previously also apply to veneer PBI/MDF combinations. In addition, the materials should be laminated with their moisture content at or near 6 percent. And, since balanced panel construction is essential to prevent warp, the same thickness veneer should be used on both sides. Different veneer species can be used, but if used 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, a proper glue spread rate should be uniform for proper adhesion.

Other overlays

Vinyl films, low-basis-weight papers and foils should all be applied using good balanced-lamination practices. In most cases, the application of a film or paper to one side only, or different overlays on each side of a substrate will not result in warp. However, one sided application of any laminate may act as a moisture barrier creating a transitory unbalance which can result in warp over an extended period of time.


Moisture can plague laminated panels. An unbalance in moisture-related expansion or contraction frequently causes warping of laminated panels. Such unbalance is activated by changes in moisture content and/or temperature. A change might be temporary, as in the case of wetting one side of a flat panel. The end result is called "transient warping" and is beyond the control of the laminated-panel manufacturer. In theory, "structural warp" resulting from a built-in unbalance can sometimes be prevented.

The perfectly balanced, warp-resistant panel is the exception rather than the rule. The natural variability of the laminate and the 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. Balanced lamination is the key to consistently manufacturing flat panels.

The concept of balance does not end with the manufacture of a balanced panel. The installation and the end-use environments can also be sources of moisture unbalance that can create internal stresses which could result in warp. To ensure acceptable laminated product performance, design and engineering must consider the product application and environment. (*) Although high pressure laminates are the only true laminates referenced in this bulletin, the other overlays, both wood veneer and man made decorative overlays, are grouped collectively as laminates.
COPYRIGHT 1991 Vance Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:medium density fiberboard
Publication:Wood & Wood Products
Date:Apr 1, 1991
Previous Article:Panel industry targets value-added services.
Next Article:How to find particleboard defects.

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