By Larry Loffer
Feb. 3, 2015 - HMI Hardwood LLC, headquartered in Clinton, Michigan, has successfully developed and maintained a lumber quality control program that focuses on moisture content. As a result, it has improved its process efficiency, maximized output and, most important of all, optimized profits.
Over the years, this 100-person operation has expanded its facilities to include a sawmill, a drying complex and a planing/ripping line. Today, its kiln capacity is 700,000 board feet, pre-dryer capacity is 1.5 million board feet, and the monthly drying output is 1.5-1.8 million board feet. The mill processes hardwoods that include red and white oak, ash, hickory, maple and cherry.
The drying complex at HMI Hardwoods includes seven kilns that, on average, can process up to 420,000 board feet per week. Typically, it takes four hours to unload and reload a kiln. Aside from downtime to reload them, these kilns operate around the clock.
Unlike some mills, the team does not air-dry its lumber because of higher inventory requirements and the lack of climate control outside. Variations and excesses in temperature, humidity and wind velocity seasonally are factors that can cause lumber to degrade by 10 per cent or more.
Being able to screen out lumber with high moisture content has several benefits. It allows HMI to:
- Assure customers of high-quality lumber and protects them from the many problems that can result from moisture content that is too high
- Identify and more easily remove the occasional board that contains high levels of moisture, reducing the risk of extended kiln drying time that could over-dry the other wood in that charge
- Maximize its wood production and, at the same time, save 10 to 20 per cent of costs compared to not scanning the wood for moisture content.
Stickers are affixed to newly arrived green lumber, then that lumber is sorted by species and thickness and arranged into 80,000 board foot bays. Next, the bays are loaded into one of two pre-dryers that conduct temperature, humidity and moisture checks throughout each 24/7 period.
- Stain-prone species such as maple are loaded into a specially designed pre-dryer that has air velocities of 300 to 500 feet per minute and operating temperatures of 37 to 60° Celsius, according to Randy Rickmon, HMI’s Pre-Drier Manager. Typically, the process requires 10 to 12 days to complete.
- Check-prone species such as oak are loaded into a second pre-dryer with air velocities of 100 to 125 feet per minute and operating temperatures of 26 to 32° Celsius. Oak takes a longer time to process, typically 45 to 48 days.
Whitewoods are loaded into the pre-dryer for about 9 days, followed by four days in the kiln. Red oak takes much longer, going into the pre-dryer for about 36 days followed by 12 days in the kiln.
Quality control in the kiln
After the pre-drying process, HMI personnel move the charges of lumber into the kiln. They do quality and moisture checks and prepare two types of samples that they assign randomly.
The first sample, usually about 16 boards, is chosen based on how they schedule and operate the kiln. These samples come from about 12,000 boards that make up the kiln charge.
They use the second set of samples after they finish the kiln process to evaluate drying degradation and moisture distribution.
Moisture meters control quality after the kiln
One hundred boards are randomly tagged from the 80,000 in the bay after they emerge from the kilns. These boards are then sent to HMI’s quality lab for further evaluation.
After they are surfaced, employees scan each board with a Wagner 601-3 handheld moisture meter. They record the moisture content of each board and inspect them for drying defects like splitting, warping, checking, staining and stickermarking.
Each board is then rated on a scale of 0 to 4, with 4 being a perfect score for a board free of defects. They then prepare a report to summarize the moisture distribution and overall drying quality.
HMI surfaces most of its wood production. Unlike many mills, they plane or surface the lumber immediately following kiln-drying, and only then do they check the lumber’s moisture content. The order of when to test for moisture is reversed due to the mill’s layout.
After surfacing, each board passes over an in-line moisture detector. The ideal target for wood moisture is seven per cent, so any board that has a moisture content of 10 per cent or higher is spray marked. These boards are then pulled out of the sorting line for reprocessing.
Untagged wood goes through an in-line meter and scanner, which records each board.
Kiln size impacts wood moisture content. The bigger the kiln, the greater the variability of wood moisture content. Its in-line meter helps HMI catch boards that are too high or too low in moisture content.
Moisture and profitability
Quality control can impact production yield and profitability. There are many components to a quality control program, but moisture content is one issue that, if addressed throughout the entire production process, can greatly impact output as well as profits.
Developing and executing a quality control program that focuses on moisture measurement and analysis prior to, during, and after kiln drying can significantly improve results.
Using in-kiln, in-line and hand-held meters, HMI Hardwoods has seen a boost in its profits.
Larry Loffer is Senior Technician at Wagner Meters, Inc., where he has 26 years of experience in the Sawmill and Dry Kiln environment. Larry is involved in both hardware and software development, installation and training for sawmill operations, management and personnel.