Graham Packaging is the world's largest processor of high density polyethylene (HDPE) products. When Graham acquired their Port Allen, La. facility from another company several years ago, production capacity for onequart bottles they manufacture to carry Castrol motor oil was limited by the existing out-dated equipment. Graham quickly replaced the original blow-molders with machines that they designed and built internally. These state-of-the- art molders theoretically tripled the plant's capacity. Achieving that higher production rate in practice, however, depends partly on rapidly cooling the plastic below its softening point by using low-temperature water-glycol mixture for cooling the bottle molds.

These cold mold surfaces can create problems during the humid weather Louisiana is famous for. When chilled molds open to release the bottles, water condenses out of the humid air onto the cold surface. This condensation, often called "mold sweat", causes several problems:

• Bottle surface looks like "orange peel" because water droplets form bumps on the mold surface;
• Condensation accelerates corrosion, which increases maintenance costs;
• Most importantly, surface deformities forming around the water droplets prevent the proper adhesion of labels.

Graham's problems are not unique. They are, in fact, common wherever plastic-product manufacturers use cooled molding equipment.

In many plants, production managers opt to reduce mold sweat by holding their chilled-water temperature above the ambient dew point. Doing so, however, reduces the speed at which the thermoplastic hardens, ultimately slowing down production.

Rather than accept such a low production rate, Graham decided to bring in desiccant dehumidifiers manufactured by Munters Corporation in Amesbury, Mass. to dry out the air surrounding the molds. These units reduce the dew point of the air surrounding the molds, eliminating condensation and allowing continuous, high-speed operation.

Molding Process
Rather than accept such a low production rate, Graham decided to bring in desiccant dehumidifiers manufactured by Munters Corporation in Amesbury, Mass. to dry out the air surrounding the molds. These units reduce the dew point of the air surrounding the molds, eliminating condensation and allowing continuous, high-speed operation.

The blow molding process begins with a "pre-mold," which is an open tube of thermoplastic extruded through a die. The material comes from a pressurized hopper heated to several hundred Fahrenheit-above the material's plastic temperature.

As the hot material leaves the die, two metal mold halves, shown in Fig. 1, clamp around it. Compressed air blown into the pre-mold tube's interior forces the material against the mold's interior surface. The plastic gives up heat to the cooler mold surface until it cools to below its plastic temperature and hardens.