Detection of metal impurities
in PVC compound on conveyor
Customer / Field / Equipment manufacturer
A smooth plastic compound is transported on a 1500 mm large conveyor. Before material is fed into the transformation machines, possible metal impurities must be detected and removed. Metal objects can hide anywhere in the compound over the entire conveyor width. They can lie on the bottom as well as suspended in the compound, which is 7 mm high. Conveyor itself is 4 mm thick and sensors can be arranged at 3 mm distance under the conveyor. Consequently, sensors must achieve at least 15 mm real switching distance, and detect metal impurities as small as a M4 steel nut.
Metal impurities must be removed from the plastic compound so as to prevent possible damage of the rolls of the processing machine.
Previous solution / Competitor product
Multiple Allen Bradley sensors in 80x80 casing with 65 mm nominal switching distance were spaced under the conveyor. A certain distance from each other was observed in order to prevent mutual interference. This caused large detection gaps which customer could not accept.
An optical device could not fit this application, as impurities can be also inside the compound, not just on its surface.
20 inductive proximity switches of the ProxiPlus series IPK 080 were arranged in two rows on an aluminium plate. Devices feature identical mechanical, electrical and sensing properties, but work with different internal frequency.
In this application spacing of 45 mm between two sensors on the same row was considered. This would not result in any detection gap, as the gap is covered by the sensor in the second row.
To identify a model with frequency offset, “F” is added in the type name (F1 or F2).
Sensors of the ProxiPlus series were chosen because of their extended switching range which best recognize very small objects.
Advantages compared to previous solution or competition
PROXITRON sensors with offset frequency can be mounted close to each other without mutual interference. This provides for full coverage of the conveyor width (no detection gap).
As individual switching signals are available, customer gets the additional information on where the metal impurity is located over the conveyor.
A possible sensor failure will not impair functioning of the whole system. The replacement of a defective sensor is very easy to perform.
A metal searching system is by far more expensive and does not permit to locate object position.
20 pieces IPK 080 sensors, standard and with offset frequency, arranged in two rows:
First row: S, F1, F2, S, F1, F2, S, F1, F2, S
Second row: F2, F1, S, F2, F1, S, F2, F1, S, F2
S= IPK 080.38 GS4 (Standard version)
F1 = IPK 080.38 GS4 F1 (frequency offset F1)
F2 = IPK 080.38 GS4 F2 (frequency offset F2)