Experimental study and numerical simulation of preform or sheet exposed to infrared radiative heating

doi:10.1016/S0924-0136(01)00882-2

Journal of Materials Processing Technology

Volume 119, Issues 1–3, 20 December 2001, Pages 90-97

https://doi.org/10.1016/S0924-0136(01)00882-2 Get rights and content

Abstract

Thermoplastic processing like the injection stretch blow moulding and thermoforming processes provide the heating stage with infrared oven. This is a critical stage of the process, as the final part thickness is strongly dependent on the preform or sheet temperature distribution prior to forming. Optimisation of the infrared oven is therefore necessary. Experiments have been conducted in order to characterise the heat source of the infrared emitter and the interaction between the heaters and a semi-transparent PET sheet. An 880 LW AGEMA infrared camera has been used to determine the surface distribution of the transmitted heat flux by measuring the temperature distribution on the surface of the thermoplastic sheet. In addition, numerical simulations of the temperature distribution using control-volume method have been carried out and compared with experimental data.

Introduction

In both the injection stretch blow moulding and thermoforming processes, a tube- or sheet-shaped thermoplastic preform (PET, PVC, PP, etc.) must be heated before forming. In order to increase the temperature of the preform above the glass transition, infrared ovens are used [1]. Infrared heating permits a rapid heating with heat flux (proportional at the fourth power of the source temperature) occurring even on the inner surface of the preform far from the incident radiative source.

The final thickness distribution of the part is dependent on the initial temperature distribution inside the preform. It is very important to investigate parameters which determine the heating stage. They depend both on the geometrical and spectral interactions between lamps and irradiated plastic materials.

As a result of these interactions, the ratio between electric power and absorbed energy in the PET tubular preform is approximately 20%. Optimisation of the infrared oven is then necessary to allow for the control of temperature distribution and also minimisation of the energy costs. In this way, the following methodology has been applied on the heating of PET sheets:

•
characterisation of infrared heaters;
•
measurement of the surface temperature distribution;
•
development of a three-dimensional control-volume model of the heating stage.

Section snippets

Characterisation of the heat source

The spectral properties of the infrared heaters have been already determined in the previous papers [2], [3]. The infrared lamps used are sketched in Fig. 1. They are composed of a coiled tungsten filament, contained in a quartz tubular enclosure and a reflector in order to increase the heat flux efficiency to the product.

The lamps are called L300 and L400 with a nominal electric power of 300 and 400 W, respectively. The L300 is used with a cylindrical reflector made of polished aluminium. The

Experimental procedure

Fig. 5 shows the experimental set-up which has been developed in order to measure the surface temperature evolution when a PET sheet is heated using the infrared lamp.

An 880 LW AGEMA infrared camera (8–12 μm bandwidth) is used to evaluate in a remote sensing, the spatial and transient temperature distributions. The surface dimension of the sheet is $20 cm ×20 cm$ and the thickness is 1.5 mm. The distance between heaters and sheets is 5 cm. The frequency of analysis is 25 frames/s and the device is

Temperatures

Measurements have been processed for 0.7 and 1.5 mm PET sheet thicknesses. Table 1, Table 2 show a very low average transmittivity on this spectral band. The average emissivity is then calculated from the reflectivity measurement.

Taking into account these characteristics, temperatures are measured during heating and cooling stages from the back and front surfaces of the sheet. It takes 3 s to bring the front face in front of the camera during which a cooling stage due to natural convection occurs.

Control-volume formulation

A three-dimensional control-volume model has been developed for computing the radiative heat transfer during the infrared heating stage. The sheet-shaped domain sketched in Fig. 10 is discretised into cubic elements called control volumes [9]. The temperature balance (Eq. (1)) including radiative transfer (thermal power absorbed by the polymeric sheet from the infrared heaters) is integrated over each control volume and over the time from t to t+Δt. V=ΔxΔyΔz is the control volume, $q_{c} →$ the

Conclusion

Preliminary investigations of the infrared heater properties (spatial and spectral) fitted with diffuse reflectors allow to consider them as an isotropic radiative source. The interaction between these lamps and PET sheet has been observed using an infrared camera. The temperature distribution from the back to the front face of the sheet has allowed an estimation of temperature gradient when the heating is switched off. The heating stage of the PET sheet, submitted to an industrial infrared

Acknowledgements

This work is supported by Perrier-Vittel Company. Special thanks goes to G. Denis, A. Contal and J.-P. Arcens.

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