Injection Molding

Water-line venting saves molding job

Bleeds trapped air in cavity that caused part burns

Using a water-coolant line to vent trapped gas from an injection-mold cavity would normally be a ticket to an underwater adventure, but Apollo Molded Products will tell you "not so." The Rockville, CT. based custom molder took this unconventional route to mold venting and saved a job that might otherwise have been undeliverable.

The part was a rectangular polypropylene overcap for a cologne bottle, relatively deep in relation to its width. For cosmetic reasons, the customer refused to permit the part to be center-gated on the top and insisted on filling the mold from a submarine edge gate at the bottom edge of the cap.

The result was predictable: An air pocket formed at the top edge of the cap, at the point of the longest flow from the gate. What wasn't expected, though, was that the trapped air would get hot enough to burn a hole through the material and ruin almost every cap in the 12-cavity mold. The usual remedies, including varying the wall thickness to alter the melt path, merely shifted the burn point.

With the first shipment due in eight weeks, it looked like a lost cause until Plant Manager Bob Gelina and Manufacturing Engineer Ralph Duell figured out how to vent the cavity without disfiguring the part: Do it through the core. The idea was to suck the trapped air from each cavity into a water line in its core by applying negative pressure to the cooling system. The ingenuity paid off: Apollo is now producing the "unmoldable" caps at the rate of two million a year.

As shown in the drawing, the escaping air reaches the water line through a set of tiny passageways drilled into the core. A fine-mesh stainless-steel screen prevents the polymer from plugging the vent holes. Because the water isn't supplied by a traditional pump, but is circulated under negative pressure, there is no leakage into the cavity. To eliminate water- vapor accumulation, a timer shuts off the water just before the mold opens to permit the negative pressure to remove any water that might have reached the passageways and screens.

The water-line venting technique is an application of Logic Seals, a negative-pressure water circulation system designed to prevent cooling water from flooding the mold in case of seal failure. In addition to gas burns, the venting also can prevent short shots, flashing, and plateout. The system is a product of Logic Corporation. The firm also makes a negative/positive-pressure mold heater that it says can substantially reduce cycle times.

These processes may be used only with Logic Corporation equipment with the serial number specified on the license granted free of charge for use on Logic Corporation equipment