New gravimetric extrusion control gives truer grip on film gauge.

Gravimetric feeding systems that control weight per length of blown film have long been "blind" to one factor that can distort the results. Because these systems measure line speed at the nip rolls, they cannot account for film shrinking or stretching between the nip and the winder. A new winder-speed sensing option for one gravimetric system is said to make it the first to "see" past this traditional blind spot.

Another new option can coordinate the speed of a liquid additive pump to the extruder's output. Both options are available on the GDC Gravimetric Dosing Controller from Plast-Control Inc., Newburyport, Mass.

NO SLIP BETWEEN WINDER & NIP

Shrinkage as the film cools between the nip rolls and winder can result in film that is heavier and thicker than the target. Likewise, stretching can occur with a change in winder tension or take-off speed, resulting in a thinner and lighter film than intended.

Film makers typically calculate the percentage of shrink or stretch and then adjust the line at the next run. However, changes in plant temperature, line speed, extruder conditions, or film-thickness target can alter the degree of shrinkage or stretching from one run to the next.

A lot of production can be wasted while the operator tries to adjust for these changes. "It can take hours to see an adjustment because the operator has to wait until the end of the next roll to see if the changes were good," says Mark Anderson, national sales manager.

Despite efforts to estimate shrink or stretch, film-line operators typically end up with roll weight and film thickness that differ [+ or -]2% from the target, Anderson says. He adds it's not unusual for roll weight to vary 5-10% from target when not using weight/length control.

Plast-Control's new option for its GDC system automatically calculates film shrink or stretch on-line. The option reportedly can keep roll weight to [+ or -]0.5% of target. It eliminates guesswork while adjusting automatically for changes in plant environment or line conditions.

The GDC system measures the loss in weight of material in the hopper continuously and then adjusts the line speed or screw speed. On a coextrusion line, the GDC maintains a constant line speed and also adjusts each individual screw speed to give the proper weight control for each layer and the overall film.

A new algorithm for the GDC system imports speed data from encoders on the winder and nip rolls and calculates shrink or stretch based on the difference between winder and nip speeds. The GDC adjusts haul-off speed or extruder rpm so that the material on the roll is at the desired weight regardless of changes in production rate.

The GDC can be programmed to suspend winder readings during roll changes. Reports can be generated showing the true weight of the film and its individual layers, something you won't get from a winder's length count, Anderson notes. The GDC can calculate average film thickness using product density, line speed, and film width.

CONTROLLING LIQUID PUMPS

What's said to be another first for gravimetric extrusion control is a second algorithm for the GDC that can control a liquid additive pump. Such a pump normally bypasses the gravimetric feeding control. The new algorithm matches the amount of liquid pumped per stroke or rpm to a set percentage of the line's overall throughput - even if that throughput changes. An additional safety feature stops the pump when the extruder is not running.