Evaluation of heat transfer at the cavity-polymer interface in microinjection moulding based on experimental and simulation study

Maksims Babenko,John Sweeney,Petko Petkov,Franck Lacan,Samuel Bigot,Ben Whiteside

doi:10.1016/j.applthermaleng.2017.11.022

Maksims Babenko, John Sweeney + Show 4 more

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https://doi.org/10.1016/j.applthermaleng.2017.11.022

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Abstract

In polymer melt processing, the heat transfer coefficient (HTC) determines the heat flux across the interface of the polymer melt and the mould wall. The HTC is a dominant parameter in cooling simulations especially for microinjection moulding, where the high surface to volume ratio of the part results in very rapid cooling. Moreover, the cooling rate can have a significant influence on internal structure, morphology and resulting physical properties. HTC values are therefore important and yet are not well quantified. To measure HTC in micromoulding, we have developed an experimental setup consisting of a special mould, and an ultra-high speed thermal camera in combination with a range of windows. The windows were laser machined on their inside surfaces to produce a range of surface topographies. Cooling curves were obtained for two materials at different processing conditions, the processing variables explored being melt and mould temperature, injection speed, packing pressure and surface topography. The finite element package Moldflow was used to simulate the experiments and to find the HTC values that best fitted the cooling curves, so that HTC is known as a function of the process variables explored. These results are presented and statistically analysed. An increase in HTC from the standard value of 2500 W/m2 C to values in the region 7700 W/m2 C was required to accurately model the observations.

Highlights

Microinjection moulding is a leading technology for manufacturing polymer micro components in large quantities at a relatively low cost
Can influence the component form, surface properties, internal morphology, residual stresses and the dependent physical properties. This heat transfer is governed by the thermal contact resistance (TCR) or thermal contact conductance (TCC), and is affected by the area of the contacting surfaces, the temperature of the polymer and mould, the pressure applied and the surface topography
Heat still flows from the core to the skin, but once heat transfer coefficient (HTC) is reduced from 5000 W/m2 C to 1250 W/m2 C, not as much of the heat leaves the polymer, meaning it accumulates at the skin and generates a reheat of the surface

Summary

Introduction

Microinjection moulding is a leading technology for manufacturing polymer micro components in large quantities at a relatively low cost. During the microinjection moulding cycle the polymer undergoes a complex process where it is heated to its melt temperature, injected at high velocity and high pressure into a cavity, where it cools down and solidifies into a final product. The heat transfer between the tool surface and the polymer melt has a significant influence on the filling and cooling behaviour. Can influence the component form, surface properties, internal morphology, residual stresses and the dependent physical properties. This heat transfer is governed by the thermal contact resistance (TCR) or thermal contact conductance (TCC), (which is the inverse of the TCR) and is affected by the area of the contacting surfaces, the temperature of the polymer and mould, the pressure applied and the surface topography. Available simulation software products use the term heat transfer coefficient (HTC) which is the same as the TCC

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