Gravimetric feeding plus profile control yields ultimate blown film uniformity.

One way to reduce double-digit thickness variation on a blown-film line that has either a rotating die or oscillating hauloff is to combine a gravimetric control system with a profile-control system. Together, they combat machine-direction and transverse-direction gauge variation, respectively. But up to now, you've probably had to combine a gravimetric blender system from one supplier with a profile-control device from another source. Such systems may require recalibration when you run either a different material or bubble diameter. And if the profile measurement is taken off line or downstream from the film bubble, it may take 30 minutes or more to determine whether process changes are necessary.

Now there's a single-source, modular package that's said to be less expensive than previous systems. Further, it picks up gauge variations close to the die and also self-calibrates automatically. Plast-Control Inc., Newburyport, Mass., claims to be the first to offer a full package of profile measurement, profile control, and gravimetric extrusion control that's available for all makes of blown film lines. Plast-Control is introducing to the U.S. its Pro-Con profile-control system for all types of PE. According to national sales manager Mark Anderson, Pro-Con can reduce certain causes of gauge variance by up to 50% and it can be tied smoothly into the company's gravimetric extrusion-control system. In Europe, more than 140 Pro-Con systems have been installed over the past six years, but the first U.S. installation occurred in 1996. The Pro-Con system can be seen at next month's NPE show.

GAUGE IS A MOVING TARGET

Anderson says it is not uncommon to see film gauge vary by [+ or -]18% in systems that have a rotating die or hauloff and no gravimetric control. About two-thirds of this variation is in the machine direction and one-third in the transverse direction. About half the MD variation can be eliminated with a gravimetric control system and the remainder can be minimized with a melt pump. Cross-profile gauge variability can be addressed only with a profile-control system.

PLUG-IN CONTROL

Pro-Con consists of a closed-loop computer control, a newly designed contact capacitance gauge called the C-head, and a new segmented air ring. The C-head traverses 360 [degrees] around the bubble, collecting at least one measurement per degree of circumference. After three full cycles, the C-head backs off of the bubble, recalibrates itself to zero, then it moves back into contact with the bubble.

Profile control is provided by the segmented air ring, which adjusts the volume of air at a particular segment around the bubble. Actual film thickness is calculated from the average deviation of the bubble profile and the average weight per length of film calculated by the gravimetric control system.

Pro-Con can be tied into a segmented air ring or die-heating controls supplied by other OEM's. Anderson says the Pro-Con system can be used without gravimetric extrusion control and will yield the same improvement in profile uniformity, but readings will be expressed as a percentage deviation rather than absolute thickness.

The C-head and computer for profile measurement cost $25,000-$55,000. Adding the segmented air ring raises the tab to $120,000-$160,000. And with gravimetric extrusion control, the total rises to $200,000-$280,000.

Rex-Rosenlew International, a maker of coextruded LLDPE and LDPE shipping sacks, put the complete Pro-Con system with gravimetric control on one of three blown film lines installed at its new Longview, Texas, plant. The other two lines have just profile measurement and gravimetric control.

The company has cut cross-direction gauge variation to [+ or -]596 on the line with Pro-Con control, versus [+ or -] 1096 on the other lines. Pro-Con also has enabled Rex-Rosenlew to tighten the gauge tolerance band by 10% and cut time from start-up to good product by 25%.