Case Study: Increasing Quality Through Controlled Drying

A temperature- and humidity-controlled dryer has enabled Isolantite Manufacturing Co. to increase product quality while significantly decreasing drying time.

The drying process is one of the most important steps to control in order to ensure a high quality product. Many of the defects that appear after firing (such as cracks, warping and distortion) are a direct result of a poor drying atmosphere. For this reason, it is essential to develop a proper drying procedure that will eliminate waste, increase productivity and, most importantly, improve the quality of the ware.

It was this objective that compelled Isolantite Manufacturing Co., Inc., a Stirling, N.J.-based producer of precision ceramic electrical insulation components, to investigate a different drying process for its operations. The company was drying its extruded products on large racks in the middle of the shop floor. Depending on the weather conditions, the temperature in the plant ranged from 60 to 90?F, with humidity anywhere from 30 to 60%. When trying to dry larger-diameter stock, which required a slower drying rate, the company placed plastic or wet burlap around the racks to keep the humidity at higher levels. These drying methods yielded non-satisfactory stock that tended to be warped and cracked due to non-uniform drying, and the process required a tremendous amount of time. Depending on the product’s size, drying on the shop floor took anywhere from two to six weeks. In the meantime, other extruded products built up, and space became limited.

The company was also having trouble drying its slip cast products, which were placed in a separate drying room for up to two weeks. Many problems arose, including warping, slumping and cracking, and the company had to reject nearly 100% of its products. Knowing that a controlled environment would dramatically improve the yield, the company developed a makeshift dryer with a control to adjust the temperature and humidity. But while the dryer enabled the company to increase production by 50%, these results were not completely satisfactory. The company decided that it needed a more sophisticated dryer that would provide better control of the drying atmosphere.

After evaluating its options, Isolantite purchased and installed a temperature- and humidity- controlled dryer* that was designed specifically for its needs.

Developing Dryer Profiles

Slip Cast Pieces
Once the new dryer was in place, the next step was to create dryer profiles for each product. Although the exact time in which a particular ceramic is going to dry cannot be calculated, assumptions can be made based on what is known about the theory of drying.

The goal in drying technical ceramic components is to dry the green ceramic until it is machinable, normally with 1.0 to 1.5% moisture left. By the time the part is finished being machined, it is safe to fire. Based upon the theory of drying, the drying cycle should begin with a relatively warm, wet flow of air, after which the atmosphere in the dryer is held at a constant temperature and humidity. This stage of the drying cycle is called the conditioning stage. There is no dramatic loss in moisture—instead, this stage simply allows the internal temperature of the ceramic to equalize to the temperature of the exterior. As the temperature of the water inside the body increases, the viscosity decreases, allowing the water to migrate to the surface more readily. The profile should then continue with a changing atmosphere to enable the moisture to evaporate from the surface of the ceramic.

Isolantite first decided to develop a drying profile for its slip-cast pieces—large, hollow bowls that are 10 in. in diameter with a two-in. wall thickness. The first attempt at a profile included a 36-hour conditioning stage at 75?F and 90% relative humidity (RH). The profile ramped to 100?F and 25% RH at different rates and finally soaked at those static conditions. The entire drying cycle lasted six days.

Although the resulting parts were dry enough to fire, the drying time was extremely long and way too conservative. The starting relative humidity was too high, and the conditioning stage was also too long. The company decided to try to cut the drying time in half.

The second attempt at a profile lasted three days. The conditioning stage was decreased to 20 hours but at 75?F and 80% RH. The final temperature and humidity were 100?F and 10% RH.

The resulting castings were dry, and no dramatic cracking occurred. However, the drying profiles needed to be more aggressive. The company knew that the conditioning stage and the starting humidity could both be decreased, and the ending temperature could be increased by 10∞. However, increasing the ending temperature any further would bring the dryer close to its maximum operating temperature of 120∞F, above which clay loses its plasticity. Since the company was considering implementing a waste-reclamation process in the future, it wanted to be careful not to change the properties of its material. Additionally, it knew that ceramics are quite capable of drying successfully in temperatures lower than the boiling point of water.

After two more attempts, a final temperature-humidity profile was developed that dried the slip castings in approximately 12 hours. The conditioning stage lasts for four hours at 75?F and 70% RH. It ramps to 110?F and 10% RH at different rates. The castings are then held at those static conditions for up to 12 hours until green machining, depending on the production schedule. A graph of the two initial attempts at creating a drying profile, as well as the current drying cycle, is presented in Figure 1.

Extruded Pieces
The company also experimented with drying profiles for its extruded products. Two different extrusions were used—a 1-1⁄2-in. circular extrusion and a 3 x 3-1⁄4-in. rectangular extrusion. The weight losses plotted with the actual temperatures and humidity profiles for each size are presented in Figures 2 and 3. Where it used to take two weeks for the smaller circular extrusion to dry on the shop floor, the first attempt for a drying cycle in the temperature- and humidity-controlled dryer lasted just two and a half days. The samples were completely dry after 60 hours.

Additional changes could be made to decrease the time even more. The conditioning stage could be cut back to four hours and then ramped slowly for another eight hours. Finally, with a dramatic drop in the humidity to 10% and a hold until the stock can be machined, this size extrusion could be easily dried in less than 24 hours.

The same changes could be made to the drying profile for the rectangular extrusion, except that it would be a little less aggressive since it is more than 5% larger. This extrusion usually takes a month to dry in the shop atmosphere. The initial drying cycle lasted almost eight days. The next profile planned consists of a 12-hour conditioning stage and a total drying cycle of 36 hours.

Increasing Quality and Efficiency

The drying cycles that Isolantite developed are on a continual reconstruction. Each drying cycle begins with a conditioning stage at 75?F and 65 to 75% RH, and lasts anywhere between one to six hours, depending on the size of the ceramic. The temperature then ramps up slowly, approximately 2? per hour, until the body has lost 50% of its moisture content. Finally, the atmosphere in the drying chamber is dramatically changed to a high temperature and very low humidity to remove the residual water. At that time, the green ceramic is left in the dryer at the static conditions for up to 12 hours, until it is ready to be machined.

The temperature- and humidity-controlled dryer has been a beneficial addition to Isolantite’s manufacturing process. With the new dryer, the company has increased production of certain parts by 50%, mainly due to the faster drying cycles. The rejection of slip castings due to drying problems has decreased from 100% to 0%, and there have also been fewer rejected lots due to improper drying techniques.

For More Information

For more information about temperature- and humidity-controlled dryers, contact Ceramic Services, Inc., 1060 Park Ave., Bensalem, PA 19020; (215) 245-4040; fax (215) 638-1812; e-mail kilns@kilnman.com; or visit www.kilnman.com.

For more information about Isolantite Manufacturing Co., Inc., contact the company at 337 Warren Ave., Stirling, NJ 07980; (908) 647-3333; fax (908) 580-0936; or e-mail isol@rcn.com.

*Pottery Pro 2000, supplied by Ceramic Services, Inc., in Bensalem, Pa.