Faculty Publications
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Item Thermal Resistance at the Polymer/Mold Interface in Injection Molding(Springer, 2022) Kamala Nathan, D.K.; Prabhu, K.N.In injection molding, the thermomechanical condition of the solidifying part inside the cavity determines the morphology developed during cooling and thus the final properties of the component. This condition is significantly affected by the thermal contact resistance (TCR) at the polymer/mold interface. TCR is one of the most significant heat transfer characteristics that affect the quality of injection-molded components. TCR values significantly influence the simulated temperature distribution of the solidifying part inside the cavity. Using incorrect TCR values affect the accuracy of the simulated results leading to defects in the molded components. Further, the overall heat transfer during injection molding is influenced by the coolant characteristics and the thermophysical properties of the mold material. This paper gives an insight into the role of thermal transport phenomenon in the injection molding process, and particularly the importance of TCR during simulation of injection molding. © 2021, The Indian Institute of Metals - IIM.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 Effect of Mold Contour on Interfacial Heat Transfer During Solidification of AlSi11Cu3Fe Alloy (ADC-12)(Springer Science and Business Media Deutschland GmbH, 2024) Kamala Nathan, D.K.; Prabhu, K.N.The present work investigated the effect of flat, concave, and convex mold contours on heat transfer during the solidification of an aluminum AlSi11Cu3Fe (ADC-12) alloy. Experiments were designed with copper/steel cylindrical and flat molds to study the effect of convex and flat casting/mold interface on heat transfer. To examine the effect of a concave and flat interfaces, an experimental setup consisting of a cylindrical and square bar chill was fabricated. Casting/mold (chill) interfacial heat flux was estimated by solving an inverse heat conduction problem (IHCP). The temperatures measured at locations inside the mold/chill were used as input to the inverse solver. It was observed that the flat contour yielded higher heat flux than a convex contour for both copper and steel molds. Although the volume to surface area (V/A) ratio for castings solidified against a flat and convex interface are the same, the larger mold volume associated with the flat interface yielded higher heat flux transients. Experiments involving chills suggested that the flat interface resulted in higher heat transfer when the (V/A) ratio for the chill was the same. To study and compare the combined effect of mold material and contour on heat transfer during casting solidification, the molds must have the same volumetric thermal effusivity per unit surface area available for heat transfer. © American Foundry Society 2023.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 Wettability of polyethylene terephthalate melt on steel substrates and the effect of cooling rate on polymer amorphicity(John Wiley and Sons Inc, 2024) Kamala Nathan, D.K.; Prabhu, K.N.The effect of surface roughness and temperature on wettability of polyethylene terephthalate (PET) melt on steel substrates is assessed. A plot of contact angle, ?, Vs. time, ?, revealed the significant effect of temperature on wetting compared to surface roughness. The surface free energy of the steel substrate decreased by about 22% with increase in surface roughness from 0.21 to 3.8 ?m. The measured surface tension of the polymer melt was 31.36 mN m?1 at 260°C. The variation in the cooling rate of the solidifying molten PET resulted in varying degree of polymer opacity. The opacity variations were quantified as a function of pixel intensity. The opacity of the solidified PET melt drops sharply declines at lower cooling rates before stabilizing at higher rates. No significant difference in the mean pixel intensity is observed between PET melts solidified at different substrate temperatures. Three distinct regimes in PET melt solidification are identified: (i) increased crystallization propensity at lower cooling rates (<8°C min?1), (ii) dominance of amorphous characteristics at higher rates (>199°C min?1), and (iii) a transitional regime at intermediate cooling rates. © 2024 Wiley Periodicals LLC.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.