Take the sweat out of PET preform molding.

The secret of success may be 10% inspiration and 90% perspiration, but sweat is not something you want in your process if it affects product quality. In the past three years, a growing number of injection molders of PET preforms for stretch-blow molded bottles have been turning to dehumidified molding areas to prevent condensation on their molds.

Mold sweating, caused when the dewpoint of ambient air is higher than the temperature of the mold cooling water, can create a host of preform quality and handling problems. One solution is to raise the mold temperature - if you are willing to sacrifice cycle time. Some molders have erected an enclosure around the press and installed an air conditioner inside the space. But more processors are choosing dehumidification systems to keep both mold sweat and cycle times to a minimum.

Mold sweat ruins production

Water marks on the preform caused by mold sweat can lead to preforms that wind up sticking to the mold core or to the robot, and can create surface blemishes that lead to an inferior bottle, says Fred Coppes, plant manager of the West Chicago, Ill., plant of Constar, the plastics subsidiary of Philadelphia-based Crown Cork & Seal, said to be the world's largest supplier of PET bottles.

Around the clock at West Chicago, Constar operates three 600-ton and four 300-ton preform injection machines, as well as three Sidel blow molders. The plant typically experiences mold-sweating conditions from May to September of each year, when warm temperatures combine with relative humidity levels of 80-90%. The mold-cooling temperature is usually 55 F - lower than the prevailing ambient dewpoint. "Without a mold dehumidification system, we'd probably not be running," Coppes says.

To run year-round with low humidity indoors, Constar installed a liquid-desiccant plant dehumidifier system made by Kathabar Inc., New Brunswick, N.J. The system uses a water solution of lithium chloride salt, called Kathene, that removes water vapor from the air by chemical absorption. The concentration of Kathene and the solution temperature control humidity and temperature of delivered air. Kathabar's Kathapac 2500FV conditioner dehumidifies plant air and a 6FP regenerator unit regenerates the desiccant in a separate stream to prevent cross contamination.

The Kathabar system reduces the delivered air to 20% RH and a 15 F dewpoint in order to maintain the molding area at 35 F dewpoint. "Preform quality problems are virtually non-existent" says Coppes. What's more, the low dewpoint allows the cooling-water temperature to be dropped from 55 F down to 44 F, resulting in faster cycle times than before.