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Monitoring moisture content can reduce costs & improve quality.

High-quality furniture must be constructed with wooden components that are at the appropriate moisture content.

In most homes, wood will reach an equilibrium moisture content of 7 percent. Therefore, parts should be machined and assembled at a moisture content near 7 percent. Parts assembled at different moisture contents shrink or swell and change shape as they equalize to 7 percent in service. As a result, joints can fail, drawers stick, doors drag, tops split and surfaces become uneven. Failures due to moisture content changes are expensive to correct and can damage the manufacturer's reputation for quality.

Lumber with improper moisture content also causes problems during processing that result in rejected components and subassemblies, increased processing times and production schedule glitches. Parts that are too dry are more likely to experience machining damage such as torn grain. Parts that are too wet are difficult to glue. Often glue-line problems develop in panels and parts during processing, assembly or finishing. Such failures may be preferable to failures that occur to furniture in storage or service, but they increase production costs and frustrations for both workers and managers.

Moisture variations

Moisture content control usually depends upon the use of kiln control samples or upon electronic moisture meter readings of selected pieces of lumber entering the plant. Such techniques, conscientiously applied, prevent deviations from average moisture content standards for different kiln charges or from different suppliers. However, they do not prevent individual pieces with improper moisture content from entering the process.

A study of moisture content of oak lumber used by six Mississippi rough mills showed considerable variation. Figure 1) shows the range of moisture contents for lumber processed at these mills. The average was near the 6 percent to 8 percent moisture content suggested by the USDA Forest Products Laboratory. However, many pieces were above 1 0 percent and 75 pieces were above 12 percent.

Board-to-board moisture content differences in dried lumber can occur from numerous causes. Kilns are designed to subject each piece of lumber to the same conditions during drying, but individual boards dry at different rates. Thick boards dry more slowly than quarter sawn boards; wide boards dry more slowly than narrow boards; heartwood may dry differently than sapwood; and high-density pieces may dry differently than other boards.

The process of equalizing in dry kilns can prevent over drying of dried pieces while continuing to dry wet pieces. However, the process is slow and equalizing until total uniformity of moisture content is reached is not economically practical.

Kiln drying barriers

One barrier to uniform drying is that kilns may not subject every piece of lumber to the same drying conditions. Temperature, humidity and air flow may be different in different parts of kilns. Such differences may result from problems with kiln fans, heating systems, vents or controls. They may also result from improper stacking of lumber or improper loading of the kiln. The result is that some boards are either over dried or wet when the majority of the charge is at the appropriate moisture content.

A 2 percent change in moisture content of a 3-inch-wide read oak part causes a dimension change of 0.022 inch. This is enough change in size to be readily visible and to cause problems with the fitting and gluing of parts. Furniture glues will fill a gap of about 0.009 inch. therefore, in addition to processing charges of lumber with the appropriate average moisture content, it is important that manufacturers do not allow individual pieces with high or low moisture content to be introduced into the processing stream.

Workers or managers are not able to recognize individual pieces of wood that are too wet or too dry. Pieces in the 12 to 15 percent moisture content range usually do not feel, machine or look different than pieces in the 6 to 8 percent range. However, continuous moisture meters can detect wet pieces and mark them before they are cut into parts. The use of such equipment in furniture plants would improve product quality, increase productivity and reduce scheduling problems.

Continuous metering

A test of effectiveness of one commercial moisture monitor, a Wagner Electronics Continual Guard 682, was conducted by the Mississippi Forest Products Laboratory. The sensing head of the instrument (6 inches by 24 inches) was placed under the transfer chain conveying lumber from the unstacker to the cut-off saw stations. During the test, all pieces registering 10 percent or higher on the monitor and a sampling of pieces in the acceptable range were removed from the chain for further testing. A moisture content sample was cut from the portion of the test boards that passed over the sensor for determining exact moisture content by oven drying.

The tests showed that the continuous meter was effective in sensing boards with high moisture content in both 4/4 and 5/4 red oak and white oak lumber. A graph of data for 5/4 red oak and white oak lumber is presented to show the relationships between the meter and the moisture content determined by oven drying

Meter values were, on average, slightly lower than actual moisture contents determined by oven drying the samples, but all boards with oven dried moisture content above 9 percent were detected by the continuous meter.

Hand-held resistance meters are widely used to estimate moisture content of hardwood lumber in the furniture industry. They are quite useful for checking samples of lumber or for testing pieces that are suspected to have high moisture. However, hand-held meters that require pins to be driven into the wood are not readily adaptable for continuous monitoring of high volumes of lumber.

Continuous meters are widely used by the softwood industry. They may be programmed to tally the number of boards passing the sensor. They can record the number of boards in discrete moisture content categories. They may also be set to spray dye on boards of any moisture content class or to activate equipment which diverts boards in specific moisture content classes to different machines. Such marking or diverting of wet boards from the processing line of furniture plants appears to be very useful for preventing many processing and quality problems.
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Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:lumber
Author:Steele, Phil
Publication:Wood & Wood Products
Date:Jul 1, 1992
Words:1029
Previous Article:Dust collection considerations.
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