A ray tracing method to simulate the infrared heating of semi-transparent thermoplastics

Abstract

This paper focuses on RAYHEAT, a software based on the ray tracing method, developed in order to simulate the InfraRed (IR) heating of semi-transparent polymers. In this study, RAYHEAT is used to simulate the IR heating step of a PET preform for the Stretch-Blow Moulding (SBM) process. The general principle of the method is to discretize, into a set of rays, the radiative heat flux emitted by halogen lamps, then to follow these rays inside the oven while they are not fully absorbed. The ray tracer computes the optical path of each ray - accounting for specular or diffuse reflections, refractions, etc… - from its emission point, and throughout the preform thickness. PET is assumed to behave like a non-scattering cold medium. Thus, the radiative heat flux absorption inside the preform is computed according to the Beer-Lambert law. Finally, the distribution of the radiative source term is calculated in the preform. In a second step, the radiative source term is applied as an input data in the commercial finite element software ABAQUS®, in order to calculate the 3D temperature distribution in the preform. The source term is assumed to be time dependent in order to account for the preform movement throughout the IR oven. This method provides relatively small computation times, while keeping the memory requirements down to a minimum. Numerical results have been compared with temperature measurements performed on an in-lab IR oven. The model simulates suitably the infrared heating stage, and provides accurate predictions of the temperature distribution in the preform. The relative error between the temperature calculated by RAYHEAT, and the measured temperature, is less than 5%.