Stefan Kremling, Thomas Hochrein, Peter Heidemeyer
Papers # 2016 Berlin
In plastics industry, the use of terahertz technology opens up opportunities for novel non-destructive and contactless testing applications. For example, the thickness of individual layers of a foamed multilayered plastic pipe can be monitored contactless and continuously during the extrusion process. Furthermore, internal defects and foreign material inclusions can also be detected. Some materials and additives exhibit characteristic fingerprints in the terahertz spectral range allowing identification.
Electromagnetic radiation with terahertz (THz) frequencies exhibit unique properties for the inspection of dielectric materials such as paper, ceramics or plastics. The majority of thermoplastic materials are highly transparent and only slightly absorbing within this spectral range. Thus, THz radiation can pass through and is well suited for nondestructive testing (NDT) applications to detect internal defects as well as for layer thickness determination. Further, the electromagnetic character allows an examination of foamed materials and does not require a coupling medium. THz technology is a novel promising technique which can make a considerably contribution in the field of NDT of plastics.
THz radiation can be generated in different ways: Either applying optical technologies utilizing one or more lasers or high frequency electronic microwave systems. THz time domain spectroscopy (TDS) systems emitting short THz pulses in the picosecond (ps) range by exciting a semiconductor switch with ultra-short laser pulses. Time of flight measurements of this short THz pulses allows very precise thickness measurements of single layers down to a few ten microns if there is a noticeable difference in refractive index between adjacent layers. This can be used even for inline monitoring in plastic pipe extrusion for an efficient production process with minimal material consumption. These systems can also enable broadband spectral information about the investigated material allowing the determination of e. g. filler content or the mixing ratio of a polymer blend. On the other side the operation principle of most allelectronic THz systems is based on frequency modulated continuous wave (FMCW) radars. Such systems emit chirped electromagnetic radiation in the lower THz range with reduced bandwidth but high output power density. The all-electronic technique allows very fast measurement rates and compact devices. These systems can also be used for inline thickness control of thick pipes, but they are also suited for imaging applications to detect internal defects.
