Plastics Engineering Magazine
Terahertz waves have been exciting researchers in the National Aeronautics and Space Administration and teaching hospitals for 50 years. NASA uses them to measure remnants of energy from the Big Bang and to test materials in the space program. Hospital labs are trying to use them to identify skin cancer cells in minutes instead of days. But practical industrial applications were nearly non-existent until six years ago.
One of the first industrial uses of pulsed terahertz waves was in plastics, so used because of the wave’s unique ability to penetrate opaque, shiny, even black plastic, and “see” multiple layers.
The first plastics application was for in-line control of layer thickness in co-extruded polyolefin roofing, installed in November 2011. Later applications include wall thickness measuring in PVC foam core pipe; diameter and thickness measuring in dual-wall corrugated pipe; and control of two densities in coextruded polystyrene foam sheet. None of these could be measured and controlled by existing devices like nuclear, X-ray, infrared, or ultrasound measurements. Nuclear and X-ray gauges are widely used to calculate total wall thickness but have worker safety issues. Infrared spectroscopy can identify polymers and verify the presence of a barrier layer in co-extrusion but can’t measure opaque plastic or differentiate layers of the same material. Ultrasonic waves can verify total pipe wall thickness in a water tank but not if walls have air in them like foam core PVC pipe or dual-wall corrugated. Foam density can be measured by combining nuclear and “laser shadow” gauges but not as precisely as with a single terahertz gauge.
Terahertz—and similar millimeter—wave systems to control thickness, density, and dimensions in plastic extrusion have been a well-kept secret. There are now nearly 60 commercial installations in plastic sheet, film, pipe, and lamination, mostly in the U.S. and Europe. All but one, however, are confidential. The application of the technology for plastics has only been briefly mentioned at major plastics trade shows over the past five years. The technology itself has been written up almost exclusively in photonics and electronics journals. Read More.