furniture industry uses overlaid panels as flat, straight elements in furniture and cabinet construction. The panels are often in 3-ply or 5-ply construction with a thick core and thin overlays. Occasionally, 2-ply overlaid panels, which are particleboard or medium density fiberboard overlaid on the visible face only, are used for economical reasons.
These panels sometimes warp unexpectedly and severely after being assembled, even though they left the manufacturing plant in a perfectly flat condition. Such warping cannot be easily corrected because the forces that cause the warping are of considerable magnitude. Often the entire panel or the entire product must be replaced with no guarantee that the replacement will perform better than the original. Severe warping of finished products may well damage a company’s reputation and even lead to lawsuits.
The potential to warp is often built into the panel during manufacture. This potential may be triggered by changes in the moisture content of the panel components in response to longterm variations of the relative humidity of the air.
The process of bringing moisture content of a material or a panel to a desired level is called conditioning. If raw materials, such as veneers or particleboard, used in the manufacture of panels are conditioned before they are assembled, the process is called preconditioning. During manufacturing, all raw materials are normally preconditioned to ensure that they all have a similar moisture content. After a panel is made, manufacturers normally condition the panel again, called reconditioning, to ensure that the panel’s moisture content is similar to those at the places where the panel will be used.
This article highlights the relative importance of the various factors involved and the development of a computer-aid tool that the manufacturer can use for analysis in design and manufacture. It is hoped that the effort will foster a better understanding of the technology of furniture panel design and reduce panel warping.Warping causes
Warping in overlaid panels is almost always caused by differences in shrinkage or swelling from one side of the panel to the other. This imbalance in shrinkage or swelling can be caused by panel structure imbalance, which means a panel was constructed with different face and back materials. It also can be caused by moisture penetrating into the panel unevenly from the panel face and back. Sometimes a combination of both is found. These two examples illustrate how these imbalances occur.
Figure 1a shows a typical veneered 5-ply furniture panel. It consists of a 1/64-inch thick mahogany veneer face, a 1/32-inch thick yellow poplar cross-band, a 1/2- inch particleboard core, a 1/32-inch yellow poplar cross-band and a 1/32-inch yellow poplar veneer back.
Figure 1b shows a typical 3-ply overlaid panel. This panel consists of a high pressure laminate ( HPL) face overlay (1/20-inch thick), a particleboard core (3/4-inch thick) and a thin HPL back (1/50-inch thick).
Both panels have two imbalances: 1) differences in wood species (Figure 1a) or materials (Figure 1b) between face and back and 2) differences in veneer or overlay thickness between face and back.
In addition, different face and back materials cause different moisture sorption rates from the face and the back. This causes the third imbalance: moisture gradient across panel thickness. As a result, both panels will warp after being exposed to high or low humidity conditions.
For veneered panels, a difference in grain angle, or grain deviation, from one side of the panel to the other also leads to warping, even if both face and back veneers are from the same type of wood. Warping Analysis
The magnitude of warping of a laminated panel is a relatively complex function of layer thickness, stiffness, moisture expansion and relative position of the layer in the panel. Under swelling or shrinking conditions, these variables interact and result in a complex pattern of warping for a given panel. To facilitate the calculation of the warping of a multi-layer panel, the author developed a computer program named WARP. The program is based on one-dimensional beam bending equations and predicts center deflection or radius of curvature (Figure 2) of a multi-layer laminated beam over a given span. The center deflection is used to indicate the degree of warping in the panel. The program takes input information on material properties including material type (solid wood, particleboard, medium density fiberboard or overlay), moisture expansion, stiffness, grain angle for solid wood, layer thickness and moisture content change of individual layers forming the panel. Most of the variables can be either measured directly or estimated from published data. The panel can be re-analyzed by changing the construction parameters without exiting the program. The program can be run in any IBM-compatible system. Case study
The following analysis uses the panel shown in Figure 1a to illustrate the importance of various factors in controlling the warping of the panel. Two cases of moisture content change are considered:
- Case 1: Moisture content change is +3.0 percent in all veneers and +2.0 percent in panel core
- Case 2: Moisture content change is +3.0 percent in all veneers and +4.0 percent in panel core
Case 1 corresponds to a relative humidity change from 30 percent to 65 percent where the moisture content of the veneers increases about 3 percent and the particleboard core by 2 percent. Case 2 corresponds to a condition where all materials are preconditioned at 30 percent relative humidity, and the finished panel is then reconditioned at 65 percent relative humidity. This gives a 3 percent moisture content increase in veneers and a 4 percent moisture content increase of the particleboard core.
The WARP program was used to predict the warping behavior of the above panel. With straight-grained mahogany face veneer (grain angle = 0 degrees), significant warp develops in the panel ( Figure 3) because of its imbalanced structure. This indicates the importance of panel construction on its warp behavior. For economical reasons, expensive hardwood veneers, such as mahogany veneers, will usually not be used in the back side of a panel. Under such circumstances, the effect of material differences can be compensated by varying veneer thickness at the front and back faces. A computer simulation program such as WARP is a useful tool in determining the appropriate thickness for a given panel structure.
To show the effect of grain deviation in the mahogany face veneer on panel warp, the WARP program was run at various grain angles. Figure 3 shows the warping of the panel under the two prescribed exposure conditions. As the grain angle of the face veneer deviates from 0 degrees (grain angle = 10, 20, 30 and 50 degrees), severe warp develops in the panel. For example, at a grain angle of 50 degrees in the mahogany veneer, about 0.5- and 1-inch deflections (over a 48-inch span) develop under Case 1 and Case 2, respectively. The large amount of panel deflection will significantly affect the performance of the product. Case 2, with a larger core moisture content change, has a more severe problem than Case 1. Thus, both panel construction and extent of moisture content change affect the warping of the panel significantly.
Manufacturers need to pay close attention to the moisture content of panel components. They need to look for consistency of moisture content in the materials they use. The WARP program can help them determine if different materials will behave in the same manner. This will help in controlling warp problems. The WARP program is available to manufacturers through the Louisiana Forest Products Laboratory, a division of the LSU Agricultural Center. Acknowledgment
The author wishes to thank Otto Suchsland, Department of Forestry, Michigan State University, for his cooperative work in this field. Qinglin Wu
, Assistant Professor, Louisiana Forest Products Laboratory, LSU Agricultural Center, Baton Rouge, La. (This article was published in the summer 1999 issue of Louisiana Agriculture)