Using Non-Lubricated Squeeze Flow to Obtain Empirical Parameters for Modeling the Injection Molding of Long-Fiber Composites
Lambert, Gregory Michael
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The design of fiber-reinforced thermoplastic (FRT) parts is hindered by the determination of the various empirical parameters associated with the fiber orientation models. A method for obtaining these parameters independent of processing doesn't exist. The work presented here continues efforts to develop a rheological test that can obtain robust orientation model parameters, either by fitting directly to orientation data or by fitting to stress-growth data. First, orientation evolution in a 10 wt% long-glass-fiber-reinforced polypropylene during two homogeneous flows (startup of shear and planar extension) was compared. This comparison had not been performed in the literature previously, and revealed that fiber orientation is significantly faster during planar extension. This contradicts a long-held assumption in the field that orientation dynamics were independent of the type of flow. In other words, shear and extension were assumed to have equal influence on the orientation dynamics. A non-lubricated squeeze flow test was subsequently implemented on 30 wt% short-glass-fiber-reinforced polypropylene. An analytical solution was developed for the Newtonian case along the lateral centerline of the sample to demonstrate that the flow is indeed a superposition of shear and extension. Furthermore, an existing fiber orientation model was fit to the gap-wise orientation profile, demonstrating that NLSF can, in principle, be used to obtain fiber orientation model parameters. Finally, model parameters obtained for the same FRT by fitting to orientation data from startup of steady shear are shown to be inadequate in predicting the gap-wise orientation profile from NLSF. This work is rounded out with a comparison of the fiber orientation dynamics during startup of shear and non-lubricated squeeze flow using a long-fiber-reinforced polypropylene. Three fiber concentrations (30, 40, and 50 wt%) were used to gauge the influence of fiber concentration on the orientation dynamics. The results suggest that the initial fiber orientation state (initially perpendicular to the flow direction and in the plane parallel to the sample thickness) and the fiber concentration interact to slow down the fiber orientation dynamics during startup of shear when compared to the dynamics starting from a planar random initial state, particularly for the 40 and 50 wt% samples. However, the orientation dynamics during non-lubricated squeeze flow for the same material and initial orientation state were not influenced by fiber concentration. Existing orientation models do not account for the initial-state-dependence and concentration-dependence in a rigorous way. Instead, different fitting parameters must be used for different initial states and concentrations, which suggests that the orientation models do not accurately capture the underlying physics of fiber orientation in FRTs.
General Audience Abstract
In order to keep pace with government fuel economy legislation, the automotive and aerospace industries have adopted a strategy they call “lightweighting”. This refers to decreasing the overall weight of a car, truck, or plane by replacing dense materials with less-dense substitutes. For example, a steel engine bracket in a car could be replaced with a high-temperature plastic reinforced with carbon fiber. This composite material will be lighter in weight than the comparable steel component, but maintains its structural integrity. Thermoplastics reinforced with some kind of fiber, typically carbon or glass, have proven to be extremely useful in meeting the demands of lightweighting. Thermoplastics are materials that can be melted from a feedstock (typically pellets), reshaped in the melted state through use of a mold, and then cooled to a solid state, and some common commodity-grade thermoplastics include polypropylene (used for Ziploc bags) and polyamides (commonly called Nylon and used in clothing). Although these commodity applications are not known for their strength, the fiber reinforcement in the automotive applications significantly improves the structural integrity of the thermoplastics. The ability to melt and reshape thermoplastics make them incredibly useful for highthroughput processes such as injection molding. Injection molding takes the pellets and conveys them through a heated barrel using a rotating screw. The melted thermoplastic gathers at the tip of the barrel, and when a set volume is gathered, the screw is rammed forward to inject the thermoplastic into a closed mold of the desired shape. This process typically takes between 30-60 seconds per injection. This rate of production is crucial for the automotive industry, as manufacturers need to put out thousands of parts in a short period of time. The improvement to mechanical properties of the thermoplastics is strongly influenced by the orientation of the reinforcing fibers. Although design equations connecting the part’s mechanical properties to the orientation of the fibers do exist, they require knowledge of the orientation of the fibers throughout the part. Fibers in injection-molded parts have an extremely complicated orientation v state. Measuring the orientation state at each point would be too laborious, so empirical models tying the flow of the thermoplastic through the mold to the evolving orientation state of the fibers have been developed to predict the orientation state in the final part. These predictions can be used in lieu of direct measurements in the part design equations. However, the orientation models rely on empirical fitting parameters which must be obtained before injection molding simulations are performed. There is currently no standard test for obtaining these parameters, nor is there a standardized look-up table. The work presented in this dissertation continues efforts to establish such a test using simple flows in a laboratory setting, independent of injection molding. Previous work focused exclusively on using shearing flow (e.g. pressure-driven flow found in injection molding) to obtain these parameters. However, when these parameters were used in simulations of injection molding, the agreement between measured and predicted fiber orientation was mediocre. The work here demonstrates that another type of flow, namely extensional flow, must also be considered, as it has a non-negligible influence on fiber orientation. this is crucial to injection molding, as injection molding flows have elements of both shearing and extensional flow. The first major contribution from this dissertation demonstrates that extensional flow (e.g. stretching a film) has a much stronger influence than shearing flow, even at the same overall rate of deformation. The second major contribution used a combination shear/extensional flow to demonstrate that the empirical model parameters, thought to be characteristic of the composite, are actually strongly influenced by the type of flow experienced by the sample, and that no single set of model parameters can fit the full orientation state. The final major contribution extends the previous case to long-fiber reinforcement at multiple fiber concentrations which are of industrial interest. This finds the same results, that the model parameters are dependent on the type of flow experienced by the sample. The flow-dependence of the parameters is a crucial point to address in future work, as the flows found in injection molding contain both shearing and extensional flow. By further developing this flow-type dependence, future injection molding simulations should become more accurate, and this will make computer-aided injection-molded part design much more efficient.
- Doctoral Dissertations