O-Frame Scanner overcomes limitations of oscillating capacitance gauge and other sensors in the measurement and gauge control of barrier films.
O-Frame Scanner overcomes limitations of oscillating capacitance gauge and other sensors in the measurement and gauge control of barrier films. One way of maintaining blown film quality when barrier layers are coextruded is with real-time gauge control, which is most effective when thickness data are quickly transmitted to a process controller. Many blown film lines utilize oscillating capaci-tance gauges that when positioned near the film bubble, misread the gauge of com-mon barrier resins like nylon and EVOH as a result of process heat and its effect on the dielectric constants of these barrier layers. The precision of gamma nuclear sen-sors, an alternative to oscillating capacitance gauges, is affected by limitations in positioning of the scanning head as it rotates around the bubble. This is due to gamma based sensors' inherent sensitivity to pass line distance and bubble flutter. Gloucester Engineering Co. Inc., Massachusetts, has developed an innovative scanner for barrier film that is accurate and capable of rapid data transmission to a process controller. Called the O-Frame Scanner, it utilizes a fixed beta gauge for noncontact measurement of multilayer film that is effective with all barrier resins, regardless of film temperature, and is capable of providing real-time control through the company's patented Extrol™ process controller. The O-Frame Scanner mounts between the oscillating hauloff and secondary nip, measuring the collapsed layflat. Its beta gauge is enclosed within a rectangular frame (the O-Frame) through which film passes prior to separation. The sensor accurately scans both sides of the film and begins transmitting data after only a 90degree rotation of the oscillating hauloff. Most conventional fixed-gauge sensors need a full 360-degree rotation of the oscillating unit before they can calculate film thickness and transmit data, a process that can take as much as 20 minutes, says Carl Johnson, president of Gloucester Engineering. The O-Frame Scanner is thus not only accurate, but begins relaying process control data significantly faster than other sensors, in as little as 5 to 6 minutes, Johnson notes, well before the hauloff completes its first revolution. The scanner utilizes sophisticated signal-processing algorithms that permit Gloucester's proprietary Extrol process controller to mathematically separate the thick-ness profile of each side of the film and determine gauge accuracy. Based on the data it receives, the Extrol controller adjusts process parameters to achieve or main-tain specified film gauge, and drastically reduces film thickness variation when util-ized with gravimetrics and Automatic Profile Control. The O-Frame Scanner is normally configured to measure film thickness but can be piggybacked to different sensors like infrared or nuclear which supply data on, and segregate, barrier-layer thickness. The scanner's noncontact operation makes it ideal for use in applications that are sensitive to surface finish of the film. The O-Frame Scanner eliminates the issue of changing dielectric when using capaci-tance gauges “on the bubble”. When compared to gamma sensors, beta has the ad-ditional advantages of a stronger signal to noise ratio, faster time constant, and en-hanced source life, all contributing to a more accurate and stable measurement over time. The O-Frame Scanner is priced competitively with oscillating capacitance gauges, Johnson says. It is an option on new blown film lines from Gloucester Engineering, and can be retrofit to Gloucester lines that have an Extrol 6032 process controller.