Effect of Heat Transfer and Cooling Behavior on Opacity of Injection Molded Polyethylene Terephthalate (PET)

Abstract

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.