Welcome to this week’s installment “From the Dugout” covering kiln drying and why it matters to your bat. Kiln drying is a necessary evil in every area of wood manufacturing, often overlooked and outsourced by many baseball bat manufacturers. Most will say, a baseball bat requires a kiln dried billet, free of cracks and with as little discoloration as possible. The quality and performance is determined by the straightness and density of the grain and nothing more. All of this is true, but the way the billet was kiln dried may have the largest impact on bat performance and fracture mechanics, of any stage of manufacturing. Kiln drying can significantly aid or hinder performance, durability and safety on the field.
Before diving deeper into the effects of kiln drying, lets develop a baseline understanding of what kiln drying is and why it is necessary. Timber is referred to as “green” prior to kiln drying and has a moisture content in the 30-40 range. To be manufactured into finished products, more specifically baseball bats, diffuse porous woods (maple & yellow birch) must be kiln dried to 6-8, and ring porous woods (ash) must be kiln dried to 8-10. This number is a ratio of water content to wood. For reference, a piece of wood with a moisture reading of 100 has a 1:1 ratio of water content to wood (50% water, 50% wood).
This stabilizes the wood such that it can be milled into a baseball bat. It also significantly reduces the weight of the material. But, kiln drying puts the wood under extreme stress and weakens it significantly. Minimizing these negative effects has been Leadbury’s mission from its conception. If the wood is not dried enough, it will continue to lose moisture overtime and could warp, crack or check as this moisture releases. If over dried, the wood becomes brittle and will take on moisture from its environment which can also lead to warping and other potential issues.
There are two primary methods of kiln drying available in the industry. These are known as conventional kilns and vacuum kilns. Conventional drying uses ambient heat to raise the temperature of the wood such that the moisture evaporates out of the material. Dehumidifiers are placed inside the kiln to remove the moisture from the air. Conventional kilns are very simple and cheap to operate; however, they have several disadvantages. The high temperatures required to remove the moisture place extreme stress on the grain structure of the wood. This causes significant warping, cracking, checking, uneven moisture content and material yield issues. The high temperatures also cause significant wood discoloration which is not desirable for a premium baseball bat. Batch times range from six to eighteen weeks.
Currently, the industry standard for kiln drying baseball bat billets is vacuum drying. Vacuum kilns work in essentially the same manner as conventional kilns, but the chamber is under a nearly complete vacuum. This lowers the boiling point of water significantly, and thus far lower temperatures can be used to remove the moisture from the wood. Higher yields, less wood discolouration, and shorter drying times in the two to four week range have made vacuum drying the choice of all premium bat manufacturers.
To most, vacuum kilns check all the boxes for drying billets. Reasonable batch times, very little wood discolouration, and far less cracks and defects than what is common in a conventional kiln. What is happening on the microscopic level is overlooked, along with potential performance gains that can be made. Billets are still dried using ambient heat, and thus uneven moisture content in the billet is still present. This means that there is undesirable internal stress across the cross section of your bat, which can cause unwanted failure.
Leadbury bats are manufactured using a proprietary kiln drying process developed in house by Leadbury Bat Co that uses alternative methods to heat the wood and a cycling vacuum as opposed to ambient heat. We combine wood technology with vacuum technology, mechanical technology and thermodynamic technology to create a superior process for drying wood. It is a technique based on the study of physical change of wood and thermal exchange between the inner and outer surface of wood during drying together with the process of water extraction under vacuum. This allows for lower drying temperatures, precise temperature and vacuum ramp rate control which places wood under far less internal stress than the competitive processes and is far more repeatable in a manufacturing setting.
The reduction in internal stress when drying allows the wood to retain more strength and elasticity than competitive kiln drying processes allow. This not only improves performance but reduces the tendency for a weak shear plane to develop, leading to a catastrophic bat failure. Leadbury bats are proving to hold together far better when they do break. When a bat fractures, it usually starts with a small tensile failure on the outer surface of the handle. If this tensile failure encounters a weak plane in the grain structure of the wood, the crack will propagate along that plane, sometimes all the way to the barrel.
This is where slope of grain becomes important. If the slope of grain as indicated by the ink dot is good, this crack will propagate along the centre axis of the bat. If not, expect a catastrophic two-piece failure. Sometimes, even with ideal slope of grain a two piece failure can occur, as shown below. This competitor bat failed into two pieces on the three point bend test, even with perfect slope of grain. This is called a horizontal grain tension fracture, and as long as the crack does not propagate so far is a more desirable failure mode.
Worse yet, bats can undergo what is called a brash tension failure. shown below, where the bat fails in tension across the entire cross section, again leading to a dangerous two piece failure. This was another competitor bat that failed into two pieces on the three point bend test.
Leadbury bats are consistently showing increased bending strength and flex in the labs, as shown by this load vs. displacement curve from one of several three-point bend tests conducted at Western University. A Leadbury 271 Maple and competitors 271 Maple were tested; a 20% strength and 30% flex increase was observed on the three point bend test.
Failure modes have also been of extreme importance in our research and development, both in the labs an on the field. Almost all broken Leadbury bats have been collected from players and each failure has been examined. In 2018, not a single two-piece failure was reported. We have also collected several broken competitor bats, many of which were two-piece failures. These failures occurred due to a combination of poor wood grain structure, but more importantly, internal weakness caused by poor kiln drying. Shown below is a Leadbury bat which failed on the bend test in the most desirable fashion, a simple tension failure.
In conclusion, the importance of kiln drying cannot be overlooked by the manufacturer, and thus it matters to the customer. At Leadbury, our innovative methods and precise supply chain management give us the ability to reduce the negative effects kiln drying has on the strength and durability of your bat, increasing on field performance and safety. Leadbury bats truly are Engineered to Outpeform. There is lots more to come in the near future on the subject of kiln drying and fracture mechanics as our thesis research as Western University concludes in the spring.