Today’s laminates require smooth surfaces for their application. Most particleboard and MDF products are produced with interior-use glues and are not designed to get wet. Therefore, to ensure the best surface quality of panels, units should be protected against exposure to both water and high humidity. 

The effect of water or high humidity on any product is directly related to the severity and length of the exposure. Tight straps, which may be indenting the upper corners of a bundle, are an indicator of built-up stresses induced by exposure to high humidity. Corner protectors on the tops of bundles will minimize such damage. 

While only the top and bottom panels of units have their surfaces exposed to the atmosphere, each panel has all four edges exposed. Most panels tend to absorb moisture into their edges more rapidly than through the panel surfaces. This unequal rate of moisture absorption can cause unequal stresses to build in the panels that will be relieved when the panels are cut. These differential stresses can become obvious when the panels are cut into long narrow strips. As the stresses are relieved through cutting, the strips from the outer edges of the panels may bend, creating what is commonly referred to as a “banana” cut. Allowing the cut part to come to equilibrium will normally straighten these panel strips. (Cutting panels before they are completely cooled may also result in banana cuts.)

Storage & stacking
Composite panels should never be stored outdoors. Indoor storage areas should be clean, dry, well-ventilated, and isolated from machining operations that create dust, dirt, or airborne particulates that could soil or contaminate the panels. Panels to be laminated are affected by sunlight, which can have a negative effect on some glue bonds and change the color of wood.

Panels are usually stacked in the storage area by unit sizes. Stacks should be limited to five units in height, but may also be dependent on the lifting capacity of the forklift, ceiling height, available floor space, and the load-bearing capacity of the floor.

It is important to stack panel units on a hard, level surface. For example, five 4-foot x 8-foot x 32-inch (1220mm x 2440mm x 813mm) units correctly stacked and weighing a total of 10 short tons (9 metric tons), at a panel density of 45 pounds per cubic foot, will exert an average of 3.2 short tons (3 metric tons) on each of three bottom support bolsters. Bolsters for a given unit must all be of equal thickness. The weight of several thousand pounds of material on panels with misaligned, uneven, or missing support bolsters can cause high bending stresses that could in turn result in a permanent set in the panel, warping and/or panel damage.

Cut-to-size panels should be kept off the floor, and the sequence of panel sizes should be arranged for uniform alignment and proper support from bolsters. It is a good idea to use 3/4 inch (19mm) or thicker bottom boards for component storage. Large sheets are often used for top and bottom boards to protect and consolidate smaller pieces.

Materials slated for gluing, laminating or other finishing processes may need a week or more before coming to temperature and moisture content equilibrium. They should be “conditioned” by separating the panels with clean, dry spacer sticks or by placing panels in a spacing rack to provide good air circulation across all surfaces. This will facilitate rapid and uniform conditioning.

Temperature, RH and MC
The temperature and moisture content of composite panels can affect panel gluing, finishing, and dimensional stability. Poor adhesive bonds, poor finish quality, steam blows, surface roughening, moderate swelling or shrinkage, unbalanced panel construction, and face checking of laminated veneers can occur if the panel moisture content is too high or too low. Variations in wood moisture content can be minimized if panels are held within desired limits by controlling the temperature, relative humidity, and rate of air movement in the storage area.

Dimensional changes also may occur in small parts that are cut or routed exactly to size if left in areas of high relative humidity.
For optimum panel finishing and fabrication, composite panels and laminate materials should be conditioned at 35% to 45% relative humidity and 70F (21C) to achieve an equilibrium moisture content (EMC) between 5% and 9%. If it is impractical to establish an acceptable EMC range in the main storage area, smaller, separate storage or conditioning area may be required. In-service dimensional stability is best achieved when: 

• The moisture content of panels and laminate materials are in the same range at the time of fabrication. 
• The moisture content of the finished product is approximately the same as the moisture content to be encountered in service.

Through experience, some fabricators have found that the conditioning of panels may not be required for some processing phases. For best results, however, it is always good practice to have materials enter the manufacturing area at a temperature in the range of 60 to 80F (16 to 27C) and a moisture content in the range of 5% to 9%.

When temperature extremes exist during shipment it can require several days for the center panels of large units to equalize to storage area temperatures. This can be especially important for just-in-time shipments, so adequate conditioning time should be scheduled into the fabrication process.

Source: Composite Panel Association. For information on panel products and decorative surfaces call CPA at 703-724-1128 or visit CompositePanel.org. Information excerpted from the Technical Bulletin: Storage and Handling of Particleboard and MDF.

Founded in 1960, the CPA represents the North American wood-based composite panel and decorative surfacing industries on on technical, regulatory, quality assurance and product acceptance issues. CPA General Members include the leading manufacturers of particleboard, medium density fiberboard (MDF), hardboard and engineered wood siding and trim, representing more than 92% of industry manufacturing capacity in the United States, Canada and Mexico.  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.