“Melt Flipper” solves in-mold flow imbalances

Encountering molded parts heavier in the inside cavities than parts made in the outside cavities is not uncommon. The problem remains even when all runner branches are meticulously made the same length with symmetrical diameters, i.e., geometrically balanced runners. Whom to blame for this – the mold or the molder?

Perhaps neither. Dr. John P. Beaumont made a thorough study on this issue at the Penn State University Plastics Technical Center at the Behrend Campus in Erie Pennsylvania. His findings are shuffling a few preconceived ideas. The flow characteristics of plastic in a runner system are determined by a number of complex, interrelated factors. While plastic melt is in motion, the viscosity changes (flow imbalances occur) both along and across the runner channel. Flow imbalance stems from non-symmetrical shear distributions. The extent of imbalances can cause high-flow-rate melt feeding in the inner cavities of a multi-cavity mold resulting in mass volume flow rate variations between cavities as high as 19:1. In a four-cavity mold with a symmetrical “H” runner system, it’s no problem since there is only one intersection which contains molten plastic with flow properties in the same proportion. How about an eight-cavity (or more) mold that has more than one intersection?

Dr. Beaumont found out that the “Melt Flipper” could provide a solution to flow imbalances. His invention not only produces a balanced pressure between cavities but also balances melt temperature and mechanical properties of the molded part.

How does the flipper work? The “Melt Flipper” is inserted at the intersection of the primary and secondary runners in a standard “H” pattern runner. This rotates the melt approximately 90 degrees in the secondary runner, strategically rearranging the different melt regions which nullify the shear induced variation across the melt. As opposed to mold filling analysis (3D) or artificially balanced molds, “Melt Flipper” is said to be virtually immune to variations in process and temperature. This technology can be applied to both cold and hot runner molds and whether one is processing thermoplastic, thermosetting, powdered metal or ceramic materials. Presentation of this work at ANTEC ’98 has won SPE’s Injection Molding Division’s best paper award.

Further investigations revealed that part variations are due to factors either external to the mold and/or internal to the mold. Dr. Beaumont’s talk in upcoming ANTEC ’99 meeting in New York will address means of isolating and quantifying the primary causes of these variations. Let’s keep our eyes, ears and options open.

–By Prithu Mukhopadhyay, director, Society of Plastics Engineers, Quebec Section (For more information contact Prithu at prithu@plasti-world.com or 514/344-7860)