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Figure 2 is a viscosity vs. shear-rate graph. The pink region represents the typical region of melt index shear rates, from very low ranges up to somewhere around 200 sec"'; while the green region reflects the real world of injection molding. As you can see, these areas don't even overlap. Shear rates in gates will commonly be over 40,000 sec ', runners over 10,000 sec ', and cavities from hundreds to tens of thousands of sec'.
Figure 3 contrasts the viscosity/ shear rate behavior of two 15% glass-filled PBTs from different suppliers, both having the same MFI of 10. When you look at the curves, you can see that the behavior is somewhat different. In fact, their relative viscosities cross at somewhere around 300 sec ' shear rates. So,one of them has a higher viscosity at low shear rates, and the same material has a lower viscosity at high shear rates. Now, let's contrast two PBTs that have different MFIs (Fig. 4). PBT #1 has a MFI of 8. PBT #2 has a MFI of 3.6. PBT #1 is represented by the light purple line. At low shear rates it appears to be the lower viscosity material, which would agree with the fact that it has a higher MFI. But notice that it crosses over somewhere around 200 sec"', where at the higher shear rates it becomes the higher-viscosity material. These examples help illustrate that using the extrusion-based MFI as an indicator of how plastic materials will behave during injection molding can be very deceptive and can easily cause one to make the wrong decisions.
The third method for evaluating how a plastic melt will flow in a mold is injection molding simulation. Although this is not a classic means of characterizing a plastic's melt behavior, it does provide a way to evaluate how it will now in a mold. However, this method is entirely dependent on other mate- rial characterization methods, including capillary rheometer, thermal conductivity, specific heat, and melt density. The simulation software is designed to utilize the extrusion-based rheometer data by coupling mathematical modeling of non-Newtonian fluid flow (based on the rheology data developed from the rheometer) with thermal solutions (frictional heating as well as heat loss to the mold) and phase changes (frozen-layer development).
As a new, patent-pending method, the Therma-flo Moldometer is the first material characterization method developed specifically for evaluating the injection moldability of a plastic melt. Its ideal application is to provide a relative comparison of how various polymers will flow in an injection mold. The method fully maps a material's injection moldability through various geometries and processes under highly controlled conditions. This is quite different from previous methods, which only provide general flow characteristics and totally omit evaluating the thermal exchange that happens between the melt and the mold during mold filling.
First, Therma-flo captures both the rheological and thermodynamic characteristics of a melt as experienced during injection molding. Additionally, it evaluates the impact that flow rates and geometries have under these non-isothermal conditions. The method includes cooling the flow channels as in actual injection molding. This allow for the normal thermal exchange between melt and mold and the formation of a frozen layer. Both of these factors are significant aspects of injection moldability and are entirely missed by traditional extrusion test methods. Additionally, Therma-fio uses realistic injection flow rates, melt temperatures, flow-channel temperatures, and flow-channel geometries based on those found during normalinjection molding. The new Therma-flo method and apparatus currently conducts up to 150 different tests to characterize the injection moldability of a polymer melt. The method is part of a fully integrated and semi-automated test cell that includes the moldometer tooling, a high-performance injection molding machine with specially developed controls, and high speed data-collection system. A Sodick Piastech machine is used to provide excellent melt preparation and extreme precision during injection. This two-stage machine uses a fixed screw for plastication and a separate plunger for injection. This new integrated methodology provides highly controlled shear and thermal melt history seen during nor- mal injection molding.
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