Continuous Gravimetric Extrusion Control System

Coperion K-Tron's Continuous Gravimetric Extrusion Control package offers the ability to control extruder speed so that its output has a predictable and repeatable mass flow.

By accurately measuring material throughput, gravimetric extrusion control can compensate for material and process variations and provide more consistent end product quality.

This technology is most often used in conjunction with single screw, flood-fed extruders. The most common applications are: profile extrusion, cast sheet, blown film, cable coating, laminating, precision tubing, and pipe extrusion. Gravimetric extrusion control is virtually a necessity for multi-layer coextrusion systems.

The Coperion K-Tron package offers a fully integrated feeder and control solution with mass flow or weight per length control, allowing the processor to set the extruder speed from the Coperion K-Tron interface. The feeders serve a dual purpose, both gravimetrically controlling the recipe for the layer and acting as a mass flow sensor.

The level in the weigh hopper is held constant and the extruder mass flow is constantly computed and controlled from feeder mass flows and weigh hopper changes.

All auto/manual transitions are bump-less and the software provides monitoring and alarming on all parameters.

Benefits of Gravimetric Extrusion Control include:

  • exact control of product quality
  • significant savings in material costs,
  • highly accurate multi-layer coextruded films possible
  • longer production runs due to consistent extruder output
  • detection of problems before the product goes out of specification

Principle
Extruder throughput deviation can be detected independent of extruder speed if the material level above the extruder inlet can be measured with sufficient speed and precision. By utilizing a weigh-hopper with patented SFT technology, the relative weight of the material head above the extruder can be accurately measured.

Unlike level sensors, the weigh-hopper does not care that material is entering and exiting, that the material surface is not flat on top due to its angle of repose, that its dielectric constant changes with formulation and humidity, or if the material has an electrostatic charge.

The integrated control of the loss-in-weight feed system with a precision weigh-hopper provides the most accurate gravimetric extrusion control. The extruder head load is held constant, feeding is continuous and accurate, material ratios are always correct, material segregation is unlikely and extruder mass flow can be continuously measured and controlled.

Thanks to computer and networking technology data can also be more easily exchanged between multiple systems for both alarm and control purposes. Information such as extruder screw speed, die pressure, haul-off system speed and even gauge data can easily be made available to the gravimetric extrusion control system.

Components
A typical extrusion control package includes the following components:

  • K4G Feeding and Blending Station: The compact design of the K4G stand allows 1, 4 or 6 feeders to be grouped directly above the extruder inlet. A variety of feeders is available for use with the K4G stand.
  • Coperion K-Tron Weigh Hopper: The compact Coperion K-Tron weigh hopper is designed to fit neatly between the K4G stand and the extruder inlet. 10 dm3 (0.35 ft3) Weigh Hopper with 50mm (2 in) outlet for a max. rate of 2500 dm3/hr (88 ft3/hr) 20 dm3 (0.7 ft3) Weigh Hopper with 75 mm (3 in) outlet for a max. rate of 7500 dm3/hr (265 ft3/hr) * the rates are based on typical plastic pellets with a bulk density of 0.5 kg/dm3
  • Coperion K-Tron Control Module with Extrusion Control

Software: coordinates feeder and extruder output to provide optimal results.

Optional Line Controller
A choice of two user interfaces is available depending on the needs of the application.

  • K-Vision for a single line of up to 15 feeders plus extruder
  • Coperion K-Tron Smart Commander for up to 8 extruder lines and 30 feeders (less the number of extruders)

Accurate Layer Thickness Monitoring
Using gravimetric extrusion control, the individual layer thicknesses can be estimated and monitored with high precision. With calibration, the loss-in-weight feeder can estimate each material’s bulk density. To calculate the thickness contribution of each layer, the percentage of each ingredient in the layer is multiplied by its bulk density and then summed together to determine the relative density of the layer.

The relative density of all layers is then summed and the thickness contribution of each layer is the ratio of layer's relative density to the sum. If a beta gauge measurement is available, each layer’s thickness contribution can be expressed in the same units as the gauged thickness.

Using the recipe capability of the gravimetric extrusion control system, each layer’s formulation and thickness can be adjusted while still maintaining the overall thickness of the sheet.

Accurate Weight per Length Control
By measuring the take-off speed, the gravimetric extrusion control system can control the weight-per-length (kg/m) of the extruded product, which now becomes the units for the system set point. The gravimetric extrusion control system can be set up to automatically adjust either the extruder mass flow or the haul-off system speed as the primary control variable to maintain the weight-per-length set point. The operator can adjust the secondary variable and the controlled variable will follow. This is particularly convenient for applications where frequent line speed changes are required to change finished product spools or rolls.

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