Home > MacroLetter > 13.1 - Autumn 2009 > Blown Film Gauge Sensors & Scanners

Blown Film Gauge Sensors & Scanners

Continued from MacroLetter - Autumn 2009.

Sensing Devices

There are a number of sensing technologies currently employed in blown film gauge control systems.  Common  types used include:  capacitance sensors, gamma backscatter sensors, and infrared sensors.

Capacitance Sensors

Capacitance sensors are relatively inexpensive sensors that are capable of measuring a wide range of polymers accurately, including opaque polymers; yet they are used mainly for PE and PP.  They are single-sided sensors.  They are sensitive to temperature and moisture, incapable of reading certain combinations of polymers correctly (such as Nylon/PE, EVOH/PE) and cannot distinguish different polymer layers.

Gamma Backscatter Sensors

These sensors use a radioactive source to measure film thickness.  They are relatively inexpensive, accurate and different radioactive sources can be used to read the thickness of a wide range of polymers.  The sensor's isotopes generally have very long half-lives,
meaning that the sensor remains stable and accurate without the need to recalibrate.  These sensors are also insensitive to air temperature fluct-uations.  The main disadvantage of this type of sensor is its radioactive source; some countries do not allow the use of these sensors, others require certification and safety training.  Like capacitance sensors, radioactive sensors cannot distinguish different polymers in a multilayer film.

Infrared Sensors

Infrared (IR) sensors are two-sided sensors capable of reading the thickness of barrier polymers, such as Nylon, EVOH and EVA.  This can prove beneficial to maintaining tight tolerances on those expensive barrier layers. The main disadvantage of infrared sensing is its difficulty measuring black polymers.  

Within the infrared group of sensors are enhanced technologies that feature the ability to measure the thickness of individual materials within a multilayered film. These advanced technologies allow the processor to keep tight tabs on the use of specific resins in their films, such as the costly barrier resins used in food packaging.    


Aside from the sensors discussed above, there are a variety of other sensing technologies available, such as beta, X-Ray, and infrared interferometry sensors, all of which have their own advantages. To discuss these technologies, please contact us.

Scanning Devices

In addition to having choice when it comes to the type of sensing device to use, there are also different ways to position the sensing device.  Because the sensor has to traverse the width of the film in order to gather complete thickness readings it must be mounted to a scanning device.  There are limitations to where it can be positioned on a production line.  Most commonly, the sensor is positioned at the perimeter of the bubble shortly after it is formed; however, technology also exists to position the sensor around the flattened tube, after the bubble has been collapsed.

Rotating Scanning Systems

Traditionally, blown film scanners are set up to oscillate or rotate around the rising bubble, taking measurements of film thickness as the bubble passes by on its way up to the primary nip.  These systems have been shown in commercial production to generate consistently reliable results.  

These systems require the use of a rotating scanner to carry the sensor around the perimeter of the bubble in a controlled manner.  Unless twisting occurs in the bubble or a rotating die is being used, the thickness data collected can be directly mapped to the die/air ring position directly below the point where the reading was taken.  The use of a rotating scanner requires that the sensor be one-sided, such as capacitance and gamma backscatter sensors.  Two sided sensors, such as infrared sensors, cannot be used with a rotating scanner setup.

Flat Scanning Systems

Unlike traditional blown film gauge scanning setups that read film thickness by rotating a scanner around the bubble's perimeter, flat-based scanning systems read film thickness after the bubble has been collapsed.  The main limitation to this configuration is that the positions of any gauge irregularities are not as easily traced back to the position on the die lip responsible for the anomalies because of the randomizing that occurs between the die and the scanner.  Sophisticated software is required to take into account the rotation of the tower's randomizer and extrapolate the precise location at either the die or air ring where the film passed through.  

A considerable benefit of the flat scanning setup is that it can be equipped with highly advanced IR technology capable of measuring individual material thickness within  multilayer structures.  

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