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"Can I mold that part with this material in this machine?" It's perhaps the oldest question in injection molding, and now there's a way to
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answer it with more practically relevant data than ever before. Mold designers, part designers and processors commonly rely on melt flow index (MFI) data as an indicator of how a plastic material will process in an injection mold. This is often the only data available for a mold or part designer to make critical decisions such as estimation of whether a mold can be filled, selection of gating locations, sizing of runners and gates, or determining cavity wall thickness. The problem is that MFI data is generally a very poor indicator of how a melt will flow through a mold and often results in bad decisions. It is generally expected that a high-MFI melt should be easier to mold than one with a low MFI. But how often have you selected a material based on this type of data, only to find that it had little or no relationship to how the material actually processed? Unfortunately, one might find that an 8 MFI nylon fills a mold with less pressure than a 20 MFI nylon (opposite to what is indicated by the MFI data). Or one PBT fills a mold much easier than a second PBT with the same MFI. And a MFI of 12 means something completely different for a nylon than it does for a PC or a PMMA or a HDPE. Did you ever wonder why? In response to a growing need for plastic melt characterization for injection molding, Beaumont Technologies has devel- oped its patent-pending "Therma-flo" Moldometer. This new method characterizes the injection moldability of a plastic material through a broad range of mold geometries and pro- cesses using an actual injection molding machine. The new method significantly improves on the ability of a mold or part designer to make informed decisions when selecting or designing for a given plastic material. It also helps the injection mold- er to determine whether a particular machine will be capable of filling that mold.19687
During the plastic material selection process, one must consider both the application and the manufacturability requirements of a material. One of the more critical considerations when selecting a plastic material for manufacturability is trying to determine how it will flow in an injection mold. I refer to this as "injection moldability." Before delving into the various factors comprising injection moldability, let's review what happens as a plastic melt flows through the sprue, runners, gates, and cavities of an injection mold.
The injection mold-filling process is highly complex and often not fully understood. First, we are trying to deal with a heated non-Newtonian fluid (plastic melt) with a thermally sensitive viscosity flowing through relatively cold mold channels with varying cross sections. Second, the actual melt temperature of these thermally sensitive materials is dependent on a thermal balance between heat loss through conduction to the mold and heat gained from viscous dissipation (shear). And third, there are freezing flow boundaries (frozen layer), which cause the actual cross-sectional size of the melt-flow channels (sprue, runner, gates, and cavity) to be continually changing.
The challenge is exacerbated by the fact that plastic melts have a relatively high viscosity, which requires filling pressures that can exceed 30,000 psi. Higher and higher pressure limits of molds and machines are continually being challenged by part designers who seek to use higher-molecular-weight mate- rials for improved end-use performance and thinner cavity walls to reduce part cost and weight. Currently there are three primary methods for evaluating plastic flow behavior in a mold. Why aren't they sufficient to accomplish this task? Let's look more closely at each of these different techniques.
The first method is a capillary rheometer. This isothermal extrusion test is probably the most detailed means of characterizing a plastic melt. With this method, you can look at the viscosity versus shear rate of a non-Newtonian fluid while also looking at the effect of temperature on viscosity. A capillary rheometer consists of a heated chamber that is loaded with plastic pellets that are manually compacted and heat soaked until the material becomes liquid. A ram is then driven forward at controlled velocities utilizing a computer-controlled drive mechanism. This forces the melt through the heated chamber and down through a small capillary (commonly 1 mm diam.). Ram velocities are progressively increased or decreased to get multiple shear rates while pressure is normally measured just prior to the capillary with a direct melt-pressure transducer. From the ram displacement, the flow rate through the capillary can be determined.