Faculty Publications
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Item Polymer/mold interfacial heat transfer during injection molding(John Wiley and Sons Inc, 2024) Kamala Nathan, D.K.; Prabhu, K.N.An experimental injection molding setup was designed and fabricated. The purpose of the setup is to cast polymer components and estimate the polymer/mold interfacial heat flux transients during injection molding. The mold plate is instrumented with K-type thermocouples to record its thermal history continuously during the cyclic process. Experiments were performed at a melt injection temperature of 280°C. Velocity and shear rate profiles were determined to assess the flow behavior of the melt. The spatiotemporal heat flux transients at the interface and the mold surface temperature were estimated using measured temperature data inside the mold as input to an inverse heat conduction problem. The estimated boundary heat flux transients were used to numerically simulate the polymer melt's cooling behavior. From the estimated heat flux and surface temperatures, heat transfer coefficients (HTC) were determined. The peak value of the HTC was 5775 W/m2K and occurred at a mold surface temperature of 35.7°C and polymer surface temperature of 47.4°C. The evolution of the air gap at the interface was quantified using an exponential fit. The estimated air gap width corresponding to peak HTC was about 4 μm and increased to about 100 μm towards the end of the solidification. While the peak heat flux is associated with the start of the formation of polymer skin on the mold surface, the peak HTC corresponds to the onset of nucleation of the air gap or a nonconforming contact. Highlights: An experimental setup to study heat transfer during injection molding. Spatiotemporal heat flux transients (q) were estimated during injection molding. Polymer temperatures were simulated using q, and HTC was determined. The peak HTC indicated the onset of nucleation of an air gap. Evolution of air gap at the interface was modeled using an exponential fit. © 2023 Society of Plastics Engineers.Item Heat Transfer During Solidification of Polyethylene Terephthalate (PET) in Injection Molding(Springer, 2024) Kamala Nathan, D.K.; Prabhu, K.N.In injection molding, heat transfer at the polymer/mold interface during solidification of the polymer significantly affects the cooling rate, microstructure, and hence the product quality. An accurate estimation of the boundary heat flux transients is essential for the successful simulation of polymer solidification, which can aid in predicting and preventing potential defects that may arise from improper filling and cooling. Simulation studies also help in optimizing the cycle time with different process parameters. In the present work, a pneumatically-operated injection molding machine capable of producing a single component in one cycle was designed and fabricated in-house to estimate the heat flux transients at the polymer/mold interface. The mold used for solidification of the polymer was made from tool steel (P20) with a simple rectangular cavity. The mold was instrumented with thermocouples across the thickness to record its thermal history during injection molding. The polymer/mold interfacial heat flux transients were estimated by solving an inverse heat conduction problem (IHCP). The temperature measured at locations beneath the cavity surface inside the mold was used as an input to the inverse solver. Altering the melt injection and mold temperatures showed negligible effects on heat flux transients at the polymer/mold interface. The estimated solidification time for the polymer sample was about 2 s. © The Indian Institute of Metals - IIM 2024.Item Effect of Heat Transfer and Cooling Behavior on Opacity of Injection Molded Polyethylene Terephthalate (PET)(John Wiley and Sons Inc, 2025) Kamala Nathan, D.K.; Prabhu, K.N.A hand-operated injection molding machine was designed to investigate the effect of cooling behavior on the opacity of Polyethylene Terephthalate (PET). The study examined the effect of sample thickness, ranging from 0.5 to 3 mm, on the % transmission of molded samples. At a melt injection temperature of 280°C, reductions in % transmission for UV (365 nm), visible (560 nm), and IR (950 nm) regions were 43.8%, 19.9%, and 20%, respectively, in the steel mold. At 260°C, the corresponding reductions were higher, at 86.7%, 63.3%, and 40%. Copper and stainless steel molds were used to assess the effect of mold material on cooling behavior and heat flux transients. A faster heat extraction obtained with the copper mold resulted in a higher peak heat flux (135.2 kW m?2) than that for the stainless steel (55.3 kW m?2). Rapid cooling of the samples in the copper mold with a solidification time of 2.2 s resulted in high transmission values. In contrast, the longer solidification time of 4.4 s in the stainless steel mold promoted crystallization, significantly reducing the transparency of the molded components across all wavelength regions. No significant difference exists between the heat flux transients estimated at different melt injection temperatures. The study suggested that variation in the cooling rate during polymer processing significantly affects the transparency of the polymer. © 2025 Wiley Periodicals LLC.