A Wave of Improvements in Mixing Tips.

* A new family of V-mixing sections for improved distributive mixing achieves intense chaotic mixing through "wavy" roots in flight lands and channels. Spirex Corp., Youngstown, Ohio, sold the first basic V-Mixer screw tip three years ago. A few dozen of these have been sold mostly as injection molding retrofits. Since then, its V-Mixers (U.S. Patent #5798077, Aug. 25, 1998) have blossomed into an extended family of new mixing tips, and more are in development. Spirex president Paul Colby says the tips are useful for extrusion, injection, and blow molding.

Ride the wave

The basic idea of the V-Mixer is to create side-to-side ("wavy wall") movement of polymer flow, as well as an up-and-down ("wavy root") movement. The side-to-side flow accomplishes four to five times more distributive mixing than a standard single-stage screw, Spirex says.

"Shallows in the channels give dispersive mixing, where shear develops and breaks up agglomerates," says Timothy Womer, a consultant in Edinburg, Pa., who is one of the inventors of the V-Mixer tip. Tips can be designed for different shear rates according to the application. Depth and pattern of the wavy designs also varies. In the first V-Mixer, helical depressions in the land area between flights were slight dimples like thumb prints on the shaft. Later iterations use different "V" shapes or triangles and greatly increase the slopes to the deepest points in the waves.

The latest cut is the Interrupted V-Mixer, or V-2. Its Vs are connected by random channels cut through the spiral flights, which allow 10-15% cross flow from channel to channel to give better distributive mixing, Womer says. The first Interrupted V-Mixer will be shipped this month for a 2.5-in. lab extruder at Dow Plastics, Midland, Mich.

Spirex has also experimented with flight lands at 30[degrees], 45[degrees], and 60[degrees] angles and with single, double, and triple flight leads. Testing different combinations of these features yielded some unexpected results. "We thought the 60[degrees] flight angle would inherently give the best throughput rate and the worst mixing, and that the 30[degrees] angle would be the reverse--the poorest rate and the best mixing," says Spirex R&D engineer Gary Harrab. "But it ended up that the 45[degrees] flights gave the best mixing and the best rates." The Interrupted V shown in the photo for the first time also takes advantage of this discovery. [Circle 17]

Prime cuts

The key to the burst of new screw-tip developments is Spirex's six-axis CNC grinding machine, said to be the only one in North America. Spirex has two of these machines, which cut 3-D contours with accuracy of [plus or minus]0.001 in. They allow wavy designs with contoured depth to be cut continuously. A conventional CNC milling machine can only cut in planes parallel to the screw axis, Spirex notes.