Pressure‐dependent heat transfer coefficient measurement for thermoplastic melts

Abstract

A common calculation error of thermal processes in injection molding simulation programs is caused by the inadequate description of the heat transfer between the polymer melt and the wall of the mold. No correct description of the effect of pressure on the heat transfer coefficient is available for the numerical calculations of injection molding yet. During the injection molding cycle, the pressure of the melt can vary from atmospheric pressure to thousands of bars. When such a high pressure is applied, the heat transfer coefficient can change several orders of magnitude. To calculate heat removal accurately, we developed a novel measuring method and based on the measurement data, we created a model that describes the heat transfer coefficient as a function of pressure and temperature. Calculated maximal heat transfer coefficients vary between 250 and 800 W/(mK) at the pressure of 500 bar. The pressure dependence of the heat transfer coefficient can be described with a sigmoid function. The heat transfer coefficient increases as the temperature difference between mold and melt increases and as pressure increases. We determined measurement error and found it to be less than 5%.

Mold filling behaviour of LPIM feedstocks using numerical simulations and real-scale injections

“…the first crystallisation peak representing the temperature at which the melt-to-solid transition occurs [35]. Based on a value proposed by Ilinca et al [36], and to better reflect the heat transfer occurring between the mold interface and such feedstocks, the mold-melt heat transfer coefficient (HTC) for the filling phase was set to 100,000 W/m 2 ·°C as MIM feedstocks exhibit much higher thermal conductivities as compared to polymers using a default value of about 5000 W/m 2 ·°C [37,38]. The numerically obtained flow pattern and pressure were validated with real-scale injections performed with the laboratory injection press described in [29] and equipped with the instrumented molds illustrated in Figure 5(a–c).…”

Meta‐analysis of thermal contact resistance in injection molding: A comprehensive literature review and multivariate modeling

“…30 Zink and Kovacs used a sigmoidal function to describe the pressure dependence of the heat transfer coefficient during filling and packing, demonstrating that HTC increased significantly for higher pressure. 6 Mold topography is also an essential factor affecting the TCR. 31 Liu and Gehde measured HTC for varying surface roughness values produced by electro-discharge machining, showing that higher Ra (i.e., arithmetic average profile roughness) values lead to higher HTC.…”

“…For IM, the cavity wall thickness affects the polymer pressure, and a smaller wall thickness can promote a higher polymer/mold contact area 30 . Zink and Kovacs used a sigmoidal function to describe the pressure dependence of the heat transfer coefficient during filling and packing, demonstrating that HTC increased significantly for higher pressure 6 …”

“…5 TCR can result in temperature gradients across the mold surface, leading to uneven cooling and warpage of the molded parts. 4 The surfaces are in contact at the polymer/mold interface, and so TCR has been reported to be influenced by various factors, including the contact pressure, 6 surface roughness, 7,8 and thermal conductivity of the materials involved. 9,10 Over the last three decades, TCR in injection molding has been a relevant research topic due to its impact on the quality and consistency of molded parts through its effect on polymer cooling.…”

“…TCR can result in temperature gradients across the mold surface, leading to uneven cooling and warpage of the molded parts 4 . The surfaces are in contact at the polymer/mold interface, and so TCR has been reported to be influenced by various factors, including the contact pressure, 6 surface roughness, 7,8 and thermal conductivity of the materials involved 9,10 …”