New Gravimetric Extrusion Control System from Conair Auto-Adjusts Process for Consistent Weight/Length; Suits Both Mono-layer and Co-Extrusion Applications

The new Conair TrueWeigh™ gravimetric control system allows extrusion processors to put their equipment on “cruise control.” It tracks material being fed to the extruder and automatically adjusts screw rpm and the haul-off device to maintain a consistent product output regardless of changes in raw materials, ambient conditions or processing variables. The new system, part of an expanded range of Conair products for the extrusion market, offers functionality that suits both mono-layer and co-extrusion applications in film, sheet, extrusion coating, pipe, profile, tubing, and wire & cable extrusion.

“Without some kind of adaptive control over screw speed, the actual throughput of an extrusion line is constantly fluctuating,” explains Don Macnamara, General Manager, Conair Extrusion. “Varying process conditions, increased backpressure as a screen pack begins to blind, material bulk density (which can vary by up to 6%), and even operator skill sets can reduce extruder throughput and negatively impact productivity and profitability. This easy-to-use extrusion control system gives processors complete visibility and control over throughput and even finished product dimensions.”

In co-extrusion applications, the Conair TrueWeigh system can coordinate the operation of up to 12 extruders, governing not only total product throughput and dimensions, but also the relative weight of different layers.

HOPPER WITH LOAD CELL
Mounted directly on the extruder feed throat, the TrueWeigh hopper is a critical component in the Conair extrusion control system. It incorporates a highly accurate load cell that constantly measures the loss-in-weight of material as it flows out of the hopper and into the extruder. These readings are sent instantaneously to the TrueWeigh control unit, which adjusts extruder screw speed and/or haul-off to maintain a consistent throughput over time.
Processors realize increased yield through precise, consistent control over extruder output and weight per length, since the operator no longer has to use trial and error to maintain consistent rate and product thickness. In addition, product quality can be improved with precise layer-ratio control in co-extruded film or sheet, and though better dimensional stability in tubing, pipe and profile wall thickness. The control identifies problems and alarms before the product goes out of tolerance.

Conair is introducing three different hopper sizes to handle material throughput rates as low as 10 lb/hr (4.5 kg/hr) or as high as 5000 lb/hr (2275 kg/hr). The hoppers are designed with a 60-degree bottom cone that ensures first-in/first-out mass flow, which helps prevent separation of blended materials. The steep-wall hopper accommodates any free-flowing material. A clear polycarbonate cleanout door or sight glass allows for visual inspection of the hopper contents.

The hopper is also designed so that a Conair TrueBlend gravimetric blender can be mounted above it to deliver accurately blended materials to the extruder without separation. The biggest hopper can support blenders as large as a TB900 (maximum throughput 3500 lb/hr or 1588 kg/hr) with all associated loaders and feeders. Larger blenders would require stand or other support.

ACCURATE CONTROL
The heart of the new Conair extruder control system is the TrueWeigh controller, which adjusts the extruder screw rpm to maintain the specified weight vs. time (weight/hr) throughput. This allows the processor to run at a minimum thickness (±0.5% of setpoint) and still make acceptable product. If the controller is connected to a puller, winder or other haul-off device, it can also control product weight per length (weight/ft), adjusting extruder screw speed and haul-off speed to keep to within ±0.5% of the weight per length setpoint. Either extruder output or the haul-off system can be designated as the primary variable for weight-per-length control. When the secondary variable is changed – to slow the line to allow for changing finished-product rolls, for instance – the controlled variable will follow automatically.

The TrueWeigh controller can store the set-up parameters for up to 250 named recipes, allowing for faster startups with reduced waste. Layer weights and thicknesses can be adjusted “on the fly.” A resin-naming capability aids in inventory management. Users can track total consumption of specific resins across different formulas. Up to 500 resin names can be stored.

The PLC-based control features a 5.7-inch LCD touch-screen operator interface with color graphics. A series of simple questions about the application leads the operator through the quick and easy set-up process. User-friendly screens are configurable to match the needs of specific extrusion processes. The control also features a unique and easy startup sequence that uses three touch screen commands to cycle through drool, ramp up and auto-run. Users can monitor all control setpoints, haul-off control, current and historical alarms, and complete material inventory and usage data.

Advanced programming allows the TrueWeigh system to ramp extruder and haul-off drives synchronously. This allows the system to make smooth transitions from one line output setting to a new target output while maintaining product quality. Optional motorized potentiometers can replace manual controls for extruder and haul-off speed. This ensures a smooth “bumpless” transfer from manual control during start-up to automatic control during continuous production. The TrueWeigh controller also provides a computer-controlled manual mode that allows operators to set extruder and haul-off speeds digitally.

Remote interface capabilities allow the system to be integrated into a complete extrusion line or plant supervisor control system. Communication may be accomplished through either serial or Ethernet links. Optional interface protocols, including Ethernet IP, Modbus TCP, Modbus serial, Profibus, Allen-Bradley DH+, Canbus and DeviceNet, are available. The TrueWeigh system can be monitored and/or controlled remotely via these interfaces.

TOTAL CONTROL OF COEXTRUSION LINES
In multilayer extrusion, when two or more extruders feed materials to a common die and haul-off, the TrueWeigh controller gathers loss-in-weight data from each of the weigh hoppers on the individual extruders. Input screens allow operators to enter percentages for each of the different layers and then the control adjusts extruder speeds to maintain both the total material throughput and the correct percentage for each layer. This sort of coordination can be critical when one of the layers contains high levels of regrind, which can vary widely in bulk density. TrueWeigh can control up to 12 extruders in a coextrusion line.

As in mono-layer extrusion, the control can also govern haul-off speed to maintain finished product dimensions. Multiple extruder drives and the haul-off drive can all be ramped up and down synchronously.

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