Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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starting up industry & technology news /FX 8BZ UP $IBSBDUFSJ[F .PMEBCJMJUZ Of Resins & Part/Tool Designs Experienced injection molders have long known that a resin's melt flow index (MFI), measured under very low-shear condi- tions, provides very little information about how it will process under many orders of magnitude higher shear in an actual mold. They know, for example, that two materials of the same MFI can process quite differently, and a material of a lower MFI can sometimes process much easier than one with a higher MFI. A better alternative is to use a capillary rheometer to pro- duce curves of viscosity vs. shear rate, which clearly show changing viscosity caused by shear thinning. However, such curves provide mainly relative moldability comparisons; the units of measurement (Pascal-seconds for viscosity and recipro- cal seconds for shear rate) are of no practical value in judging a resin's process window. A novel solution to this problem was presented at NPE2012 in Orlando, Fla., last month by Beaumont Technologies Inc., Erie, Pa. As explained by CEO Dr. John Beaumont, his new Therma- Flo "moldometer" not only characterizes the injection moldabil- ity of polymer melts in a practically useful form, but it goes be- yond all existing rheometers by measuring flow under the thermal conditions the melt "sees" when flowing through a cooled mold. Beaumont's moldometer uses an actual injection molding ma- chine, which fills a "mold-like apparatus" that contains 15 differ- ent geometries—some that resemble a modified trapezoidal runner, ranging in diameter from 0.040 to 0.180 in.; and some that resemble a cavity geometry from 0.020 to 0.120 in. thick. The system is designed to index automatically through the vari- ous geometries, filling one at a time, and molding parts in each geometry over a range of 10 injection speeds. With five repeti- tions for each speed, that amounts to 750 molding trials for each material. The flow channels have direct melt sensors to measure pressure and velocity with a high-speed data-collection system. The accompanying graph shows a characteristic "moldom- eter flow sheet," which is unlike anything produced by a typical rheometer. It shows the melt pressure in psi/in. re- quired to flow a material through cavities of different thick- ness. The curves shown here for ABS display a typical drop in pressure required to go from very low speed to higher speed, as a result of shear thinning. More surprising is the fact that, after the initial drop, the curves tend to level off, requiring little or no increase or decrease in pressure per inch of flow at higher and higher speeds. This would not be predicted from capillary rheometer curves, which show steadily declining viscosity at higher shear rates (related to flow speed). Beau- 4 MAY 2012 PLASTICS TECHNOLOGY ABS Pressure/Inch vs. Velocity Sorted by Cavity Thickness (Inch) 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 .02 .03 .04 .06 .08 .1 0 10 20 30 40 Velocity (in/sec) mont believes that further research will establish a material's "critical velocity" at which the moldometer's psi/in. data can be extrapolated to predict filling pressures in molds of any size. The "critical velocity" is the condition where the melt will not freeze off during filling because heat loss to the mold is exactly balanced by shear heating. Beaumont sees many applications for his new material char- acterization method: comparing flowability of different materi- als or formulations; predicting ability to fill a mold within the pressure limitations of a given injection machine; and verifying the predictions of computer mold simulations. t CFBVNPOUJOD DPN Bayer to Buy Arkema's PC Sheet Business Bayer MaterialScience, Pittsburgh, will acquire the U.S. poly- carbonate sheet business of Arkema Inc., Philadelphia. The acquisition includes Arkema's Tuffak line of products for aero- space, transportation, and heavy equipment. Bayer, already a major producer of PC sheet, will also acquire sheet production equipment from Arkema. The deal is expected to be finalized this month. Tuffak is produced by Arkema's Altuglas Interna- tional unit, which continues to produce Plexiglas acrylic sheet and resins. The North American base for Bayer's sheet business is at the former Sheffield Plastics in Sheffield, Mass. t TIFGmFMEQMBTUJDT DPN 50 60 70

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