Lubrication oils are commonly used to ensure that the surface of sheet steel behaves consistently during stamping and punching operations. Without a thin layer of lubricant on the steel, the steel may tear, heat up too much, or cause the punch press to jam. However, to prevent unplanned line stoppages, lubricants are often applied in excess, reducing profitability, creating slip hazards in the coil storage areas and generating an additional waste stream to manage. Each of these scenarios can lead to production delays. Online measurement of oil coatings helps metals component manufacturers eliminate costly delays in their process, save raw materials, reduce re-work, and improve product quality.
Traditional oil measurement methods require cutting samples, punching out coupons of exact diameters, and precisely weighing the sample before and after stripping the oil from the surface. While generally accepted, this process reduces yield by taking from the finished coil and happens after the components have been made, too late to make any changes.
An online infrared (IR) light sensor is ideal for measuring hydrocarbon-based lubricants and offers a better solution to ensure that the critical oil layer is thick enough to guarantee consistent production, but not excessively thick to cause waste or pooling in the dies. By positioning an IR light source and detector on the same side of the coated product, a system of optics can be used to measure the intensity of a specific wavelength of reflected light relative to a reference wavelength. By comparing the ratio of the two intensities, a relative measurement of the coating thickness can be made. In general terms, the thicker the coating, the more absorption will occur at the measured wavelength in relation to the reference wavelength and the larger the observed ratio.
A sensor that measures a wide spectrum of IR light is the most appropriate for those production situations that involve multiple coating/substrate compositions. When configured to measure wavelengths associated with an entire range of hydrocarbon bonds, an IR sensor can accurately measure coating weights for nearly every type of coating. Additionally, by using optical light, the sensor is immune to background interference due to air temperature changes between sensor and strip.
By synchronizing source lamp emissions with detector sampling, the sensor will eliminate background light variations and sheet flutter.
The spectrum-based detector, as compared to a two-wavelength IR approach, can measure the coating without the influence of naturally occurring localized variations in zinc coatings, for example. As the light intensity varies across the spangle, the two-wavelength IR sensor would falsely interpret that as a coating change, whereas the spectrum-based detector monitors the influence on the entire spectrum in one single measurement location and is thereby immune to any variation.
The infrared spectrum technology described here addresses the typical influences present in a rolling mill and provides reliable measurements for oil coating thickness in real time. This method results in significant benefits associated with improved coating uniformity, reduced re-work, and elimination of delays while destructive tests are made by sheet suppliers.
(Content from this post originally appeared in the February, 2016 issue of Steel Times International)