'Flip' the melt for balanced mold filling.

Why do geometrically balanced multi-cavity molds sometimes refuse to fill symmetrically? Conventional wisdom blames filling imbalances on mold-temperature variations and mold deflection. But a new answer to the question was presented at this spring's SPE Antec meeting. John Beaumont, assistant professor at Penn State Erie, used mold trials and finite-element analysis to trace flow imbalances back to temperature stratification within the melt stream caused by asymmetrical shear distribution. Beaumont also introduced a simple runner-balancing solution that he calls the "melt flipper."

The melt-flipper is intended for H-pattern and other geometrically-balanced molds of eight cavities or more. Beaumont believes this approach will help permit use of higher cavitation in applications where quality requirements have kept cavitation low in the past. "The melt flipper might very well make an eight-cavity mold act like a four-cavity, or a 16-cavity act like an eight-cavity," he says.

Hotter melt takes short cut

According to Beaumont, when the melt passes through junctions in a runner system, a shear differential across the melt stream results in temperature and viscosity gradients. "Each time the flow is split at a runner branch, the hotter material will follow the inside edge of the new runner branch," he says. Shear-induced imbalance is most pronounced in molds with eight cavities or more because the melt splits not just once but several times. The same applies to four-cavity molds with parting-line injection.

As the melt stream splits, the higher-temperature, faster-flowing resin tends to follow a path to the inner cavities - those closest to the sprue. Parts from these cavities thus fill faster and end up bigger and heavier than parts from the outer cavities. Beaumont reports cases where the variation in mass volume flow rate can be as high as 19:1. Faster filling of the inner cavities can cause problems such as flashing of those cavities while the outer ones are still filling, increased clamp-tonnage requirement to fill the tool, and greater glass-fiber breakage in the inner cavities.

Enter the 'flipper'

The melt flipper addresses the imbalance by changing the orientation of the melt stream. "It doesn't get rid of the temperature gradients, but it changes their distribution," explains Beaumont.

Located at runner intersections, the flipper forces the melt stream through changes in level and direction as it splits. During this brief detour, the melt stream rotates in the runner. Temperature gradients in the melt rotate too, becoming symmetrical from left to right and ensuring that melt conditions in each branch remain identical once it splits again.

Most eight-cavity molds would require two flipper sections, one each at the two junctions of the primary and secondary runners. For 16 or more cavities, more flippers may be needed to achieve the tightest part tolerances.

The size of the melt flipper depends on the application. For example, a 1/8-in. runner would need about a 1/2-in. round flipper section.

Machining costs to incorporate flippers in a new tool are negligible, says Beaumont. For retrofits, the melt flipper is built into a round insert that drops into a machined pocket.

Beaumont licenses his patented technology for a fiat fee of $1200/tool through Beaumont Runner Technologies, Erie, Pa. The company also offers engineering services to match the flipper design to the application.

Who's using it

Early users of the melt flipper include Osram Sylvania of Warren, Pa. It used the flipper to reduce cavity-to-cavity part variation from 0.2 mm to under 0.04 mm on an automotive electrical connector. Lord Corp. in Erie has used it to mold thermoset rubber. Eastman Kodak and Packard-Electric Div. of General Motors are also trying out the melt flipper,

Beaumont's work has focused on cold-runner systems, but he says the flipper will also work with H-shaped hot runners. Beaumont has also modified the melt flipper for PVC and glass-filled materials, which require a more complex design with larger radiuses.