Engineered Systems NEWSCase in Point

Desiccants a perfect fit for injection-molding operation

Constar International is the world's leading producer of polyethylene terephthalate (PET) plastic containers for the beverage, food, and household products markets. A wholly-owned subsidiary of Crown Cork & Seal Company, Inc., the world's largest supplier of packaging products, Constar has manufacturing facilities throughout the United States and Europe, and in Mexico, Thailand, and China.

At Constar's injection molding plant in Jackson, MS, more than one billion plastic preforms, components used to produce plastic containers, are manufactured each year. The preforms are shipped to other plants worldwide to make plastic beverage bottles ranging in size from 12 oz to three liters, as well as others designed to contain water, liquor, salad dressing, and other food products.

During the summer of 1996, Constar recognized that an opportunity existed to increase production and reduce product defects by decreasing the amount of humidity in the plant. Executives and engineers decided to explore options that would achieve these goals.

The Need for Humidity Control

In Constar's injection molding operations, plastic is injected into a mold to make the preform, while chilled water flows inside the mold to rapidly cool the plastic preforms being created. The colder the water, the faster the preforms can set, creating stronger, better-quality products. A faster cooling period also allows product to be removed from the molds more quickly so new batches can be manufactured, increasing production volume. However, Constar experienced production problems, particularly in the summer months, due to high humidity levels.

According to Wayne Gresham, project engineer for Constar, conventional air conditioning systems were installed at the plant to control operating parameters and maintain desirable working conditions for personnel. These systems used a cool-reheat principle for dehumidification, which could maintain a temperature of 75°F but only at a 50°F dewpoint. Because the air dewpoint temperature was higher than the surface temperature of the mold, condensation formed on the mold, causing it to sometimes get stuck in the mold, causing cracks, blemishes, and other defects in the preforms, and altogether weakening the product.

"While raising the temperature of the water in the molds eliminated defects, it also slowed production because of the longer set-up (cooling process)," Gresham said. "In the competitive plastics market, a decrease in productivity can make the difference in whether or not this plant is profitable."

"We also realized that over time the condensation could cause the molds to become corroded and deteriorate, which could mean costly equipment replacements," Gresham added.

Additionally, in the summer months, when the demand for plastic bottles is at its highest, the plant's production would be at its lowest because the season's high dewpoints brought more humidity into the plant environment.

This added moisture from the season put an extra load on the air conditioning system, making it work much harder with less effectiveness. Under these circumstances, the air conditioner was delivering the desired temperature but was unable to maintain the necessary dewpoint.

"We needed to find a way to control the dewpoint level and maintain production rates year round," Gresham said.

Constar contacted Process Cooling Systems, Inc., an Atlanta-based engineering firm specializing in equipment sales. After careful analysis, Process Cooling and its president Phillip Fender, who assisted Constar in maintaining environmental conditions in other plants, suggested installing two Cargocaire HCD-9000 desiccant dehumidifiers from Munters Corp. to control humidity and help eliminate the damaging condensation.

How Desiccant Dehumidification Works

Desiccant dehumidifiers attract moisture from the air by creating an area of low vapor pressure at the surface of the desiccant. The pressure exerted by the water in the air is higher, so the water molecules move from the air to the desiccant to make the air drier.

In Cargocaire dehumidifiers, the desiccant support structure is a HoneyCombe¨ wheel with either silica gel, lithium chloride, or molecular sieve as the desiccant. The wheel rotates slowly between the process and reactivation air streams.

At Constar, the two desiccant dehumidifiers were positioned outside the plant. The dehumidifiers work in conjunction with the plant's existing air-handling system by removing moisture from the inside of the plant and depositing dry air into the return of the plant's distribution air. No moisture in the air means no condensation on the molds, regardless of the chilled water and air temperatures.

The units allow water in the mold to circulate below the necessary 50°F temperature by keeping the plant's dewpoint temperature at or below 40°F.

Installation of the units was a snap, according to Fender and Gresham.

"We simply introduced dry air into an existing air handling duct," Fender said. "In comparison to other plants, this solution was easy and convenient, and required very little modification to the existing system."

Desiccant Benefits at Constar

Since installing the two Cargoaire desiccant dehumidifiers during the summer of 1996, Constar has experienced benefits in both production and profits.

"Because the room dewpoint is controlled, we've been able to maintain constant production, even during the summer months," Gresham said. "We're producing more preforms per hour per day and the preforms are of better quality."

Constar also benefits through lower utility bills because the desiccant dehumidifiers utilize natural gas rather than electricity for reactivation.

"By using the Cargoaire units we found that our total hvac load is decreased," Gresham said. "The hvac unit takes care of the sensible load, while the desiccant units take care of the latent load."