A non-invasive experimental approach for surface temperature measurements on semi-crystalline thermoplastics

Abstract

Most of the thermoforming processes of thermoplastic polymers and their composites are performed adopting a combined heating and forming stages at which a precursor is heated prior to the forming. This step is done in order to improve formability by softening the thermoplastic polymer. Due to low thermal conductivity and semi-transparency of polymers, infrared (IR) heating is widely used for thermoforming of such materials. Predictive radiation heat transfer models for temperature distributions are therefore critical for optimizations of thermoforming process. One of the key challenges is to build a predictive model including the physical background of radiation heat transfer phenomenon in semi-crystalline thermoplastics as their microcrystalline structure introduces an optically heterogeneous medium. In addition, the accuracy of a predictive model is required to be validated experimentally where IR thermography is one of the suitable methods for such a validation as it provides a non-invasive, full-field surface temperature measurement. Although IR cameras provide a non-invasive measurement, a key issue for obtaining a reliable measurement depends on the optical characteristics of a heated material and the operating spectral band of IR camera. It is desired that the surface of a material to be measured has a spectral band where the material behaves opaque and an employed IR camera operates in the corresponding band. In this study, the optical characteristics of the PO-based polymer are discussed and, an experimental approach is proposed in order to measure the surface temperature of the PO-based polymer via IR thermography. The preliminary analyses showed that IR thermographic measurements may not be simply performed on PO-based polymers and require a correction method as their semi-transparent medium introduce a challenge to obtain reliable surface temperature measurements.

A combined experimental and numerical approach for radiation heat transfer in semi-crystalline thermoplastics

“…Thanks to this step, the sensitivity of both the employed IR cameras was analyzed at these preferably selected wavelengths. Initial analyses showed that the sensitivity of SC325 was too low for both the narrow bands, whereas enough sensitivity was recorded by the cooled LW IR camera only for the optical filter which has a spectrum between 7.5 and 7.7 μm [16,39]. It should be mentioned here that the operating spectral range of the employed LW IR camera is between 8 and 12 μm [21] indicating that spectral sensitivity of the IR detector is nominal in the corresponding range, whereas the detector is still sensitive at the wavelengths beyond, but close to, this range.…”

“…This calibration step is explained further in Ref. [39]. The question arises here about the sensitivity of the optical filter mounted IR camera in case of receiving the emitted energy from a real body, unlike a black body.…”

“…Apart from the sensitivity of the IR camera, the accuracy of the surface temperature measurements on PE was analyzed in terms of the D p of PE in the spectral range where IR thermography was achieved. In our previous study [39], the D p of PE was calculated considering the spectral band of the employed NBP optical filter, between 7.5 and 7.7 μm. Unlike to PET, it was found that its D p is around 350 μm indicating that the absorbed and therefore emitted radiation may not be considered on the surface of PE but, in a range close to surface.…”