Infrared sensor takes thermal portraits of blown-film bubbles.

Blown-film temperature monitoring does not have to end at the die--not when you can create a thermal snapshot of the bubble itself.

One processor, Viskase Film Co. of Aurora, Ohio, recently applied this downstream monitoring method to PVC packaging film, using the supplemental temperature data to help fine-tune its process automatically. And Dow Plastics of Midland, Mich., presented a paper at the SPE ANTEC in San Francisco last month detailing the use of noncontact infrared temperature sensing and bubble-deformation analysis as a way to predict the height of the bubble's crystallization and "freeze" lines. These lines, in turn, correlate with key fabrication variables--such as blow-up ratio, frost-line height, die gap, and output rate--and ultimately influence film properties, according to Thomas Butler, the senior development associate who authored the paper. "The crystallization line and freeze line are invisible to the eye; infrared sensing is the only way to see them," Butler notes.

Infrared sensing works by translating the amount of infrared radiation that is emitted by the warm bubble into temperature readings. Using i-r sensing to collect bubble-temperature measurements may still be rare, but there are a few converts to this technique. Butler reports that a handful of Dow customers, for example, now do some i-r temperature sensing. "They all view the method as proprietary," Butler adds. Ircon, Inc. of Niles, Ill., which supplies the i-r instrument Dow uses, says its sensors are being used in several blown film operations.

Hints on the successful implementation of i-r sensing come from Viskase, which uses the Thermalert i-r sensing system from Raytek of Santa Cruz, Calif., to collect real-time temperature measurements from four PVC blown-film lines. "We're using the system as a temperature monitor to make sure that what comes out of the machine is what we really want," explains project engineer Robert Drake. "If not, a correction is made."

Viskase positions its i-r temperature sensors above the die and just below the frost line. So far, its attention to bubble temperature has paid off in greater consistency of key physical properties. The PVC film's extensibility, for instance, has become more stable through bubble-temperature monitoring, says Drake.

Though Viskase uses the on-line temperature sensors on four lines, its plans call for extending use of the technique throughout the plant--the size of which Viskase would not disclose. In the meantime, plant personnel can point hand-held Raytek i-r sensors at the bubbles that do not yet have on-line monitors installed.

HOW IT'S USED

After much development work, Viskase recently started to use the on-line system to provide real-time automatic adjustments to processing parameters. First, the company had to run many hours of trials to gather empirical information that correlated bubble temperature to process set-points. "The bubble temperature is not the same as the die or extruder temperature, but it can be correlated," says Drake. "We just kept zeroing in until we knew how."

Once collected, the temperature data are displayed graphically on the internally developed user interface for Viskase's CIM system. The temperature information is also stored for SQC documentation purposes.

Since there are several types of i-r sensors on the market, Dow's Butler lays out two criteria for picking an appropriate device: The spot size must be narrow and its wavelength must fall within a range suitable for thin films. Butler specifies a wavelength of about 3.4 microns and a focal spot of 0.26-in. diam. at 13 in. away from the sensor.