If you want ultra-sensitive removal of tramp metal from plastics at very high throughput rates, you are fighting the laws of physics. A simple fact of nature is that the level of sensitivity of a metal detector is inversely related to the material throughput rate going by it, says Rick Vavra, materials and logistics manager at GE Plastics in Mt. Vernon Ind.
This truth put Vavra in a tough spot. The Mt. Vernon facility is where GE produces Valox PBT resins and compounds and Xenoy PBT/PC alloys. These compounded resins are often used for electrical connectors, which are extremely metal sensitive. "Metal contamination is hard to detect in final applications, so it is crucial to catch metal before a part is made, assembled, tested, or in use," says Vavra.
GE runs several dozen compounding lines at Mt. Vernon on a 24-hour, 7-day basis, producing pellets at an average rate of about 4000 lb/hr (16,000 standard-size pellets/lb, meaning 64 million pellets/hr) At that rate, he says, "we need to detect pencil-point sizes of ferrous and nonferrous metals."
If that weren't a tall enough order, metal detection at Mt. Vernon takes place in a grueling environment. The pellets come through the detector at temperatures above 160 F. The flow of the plastic can create a static charge. Also, the plant can be humid, as GE uses water instead of air to clean out the lines during material changeovers.
At GE's processing rates, existing metal detectors could sense metal particles as small as 0.7 mm, but GE wanted higher sensitivity. Metal detectors designed for the pharmaceutical industry were more sensitive, but they operated at slower processing rates.
GE initiated joint development programs with suppliers of metal detectors to find one that could meet GE's criteria of high rates and high sensitivity. GE ultimately selected Safeline Inc. of Tampa, Fla. Collaboration between the two firms resulted in an evolutionary enhancement of Safeline's GF-65 gravity-feed metal detector, says Tony Ross, commercial manager at Safeline. The advancements made to the system were introduced commercially last year as options on the equipment.
The systems now in place at Mt. Vernon can detect ferrous particles as small as 0.3 mm and nonferrous and stainless-steel bits as small as 0.5 mm, says Ross. The units are capable of such sensitivity at throughputs up to 6000 lb/hr. "There is no other technology that we know of in the world that can measure down to that level of sensitivity at that throughput," says Vavra. The sensitivity of the detector is adjustable to accommodate use of metal-containing additives, such as iron oxide pigment.
The unit uses a three-coil system, with a transmitter coil in the middle and two receiver coils on either side, to generate an electromagnetic field. Safeline reconfigured the head in order to give it sensitivity comparable to pharmaceutical-type units. The reconfigured head permits high accuracy even with hot pellets.
In order to handle static charges, Safeline developed a conductive sensing tube that dissipates the charge just before the material reaches the sensor. The issue of moisture was addressed with an aperture membrane that creates a water-tight environment for the sensor and control board, protecting the unit from water and condensation. The system can now withstand direct high-pressure washdowns.