Canadian Plastics

Blenders Cut Waste in Extrusion

The payoff for an extrusion yield control system can be swift, when you consider that highly accurate blenders with yield control functions can reduce start-up scrap, eliminate over-feeding of expensi...

September 1, 1999   By Cindy Macdonald, associate editor



The payoff for an extrusion yield control system can be swift, when you consider that highly accurate blenders with yield control functions can reduce start-up scrap, eliminate over-feeding of expensive raw materials and maintain tight control of weight-to-length specifications.

The Conair Group calculates that by cutting waste and eliminating overweight product, extruders can increase yield by three to four percent with its XB Extrusion Blender equipped with yield control on a mono-extrusion line. So a typical company with an extruder running 600 lb. of LDPE per hour, 6000 hours per year, could recover the cost of the XB blender in less than seven months.

For coextrusion film and sheet applications, extruder control by the blending system has the benefit of monitoring and adjusting each individual layer, rather than just adjusting overall gauge.

A multi-national manufacturer of film and sheet products recently installed the Extrusion Control Network from Maguire Products Inc. on a three-layer cast film line to evaluate any productivity gains resulting from the system. Consisting of a primary extruder and two satellites, the line produces 1.5 to 2.0 mm (60 and 80 mil) coextruded sheet with a 20/60/20 ratio of skin layers to core.

Prior to installation of the network, extruder rpm and downstream take-off speed were slaved to a thickness gauging system. The system was capable of maintaining a constant traverse profile of the coextruded web but was unable to maintain uniform layers.

“The blenders were the missing link in stabilizing the process because they are the only equipment capable of measuring and controlling the actual material flow for each extruder and its respective layer in the coextruded structure,” explains B. Patrick Smith, vice-president of marketing and sales at Maguire.

After installing the extrusion control network and slaving extruder rpm to the three gravimetric blenders, the sheet manufacturer was able to maintain a consistent flow of material through the die for each extruder, resulting in higher end-quality extruded sheet.

HOW THEY WORK

Conair’s new XB Extrusion Blender combines highly accurate gravimetric weigh blending technology with continuous mixing to deliver blended material with 0.1 percent accuracy. The blender can also be supplied with yield control functions that provide adaptive control of extruder or take-off speeds, increasing yield by up to four percent.

In mono-extrusion, the blender compares haul-off speed with the weight of material being fed by the blender to the extruder. If extruder throughput changes, due to a screen build-up or change in material bulk density, the control automatically adjusts screw speed to compensate.

According to Conair, conventional open-loop controls may allow swings in extruder throughput of four percent or more. The XB blender’s yield control keeps fluctuations within +/-0.5 percent.

In coextrusion, the LRC-8000 layer ratio control system, in conjunction with XB blenders, can monitor haul-off rates and adjust the speed of up to five extruders to ensure the relative amount of material in each layer remains constant.

Process Control Corp. offers extrusion control on its Guardian Series blenders. This cost-effective batch blender incorporates extruder drive control and line speed control into the blender package.

From the mixing chamber, the blend is released to the Gravitrol hopper. Based on true mass flow measured by this hopper, the blender calculates extruder usage and compares the actual rate to the desired rate. Changes to the extruder screw speed are made to maintain output constant to +/- 0.5 percent by weight.

For applications where control of weight-per-length is critical, the Guardian Series blenders offer a line speed control option. The blender now controls the line speed device as well as material percentages and extruder rate.

THE NETWORKING OPTION

Maguire has introduced a streamlined system for controlling extrusion output by networking gravimetric blenders. It provides greater control over monolayer and co-extruded product thickness than conventional yield control systems, yet costs 50 percent less, says the company.

Stephen Maguire, president, reports that in commercial tests the Maguire Extrusion Control Network has greatly improved the overall gauge uniformity and layer-to-layer material distribution, permitting substantial savings in raw material costs.

Quicker ramp-up to on-spec product and elimination of “heavy” product can also contribute to materials savings.

Suitable for film, sheet, profile, pipe, tubing or wire and cable, the system uses proprietary software embedded in Maguire weigh scale blenders. It provides extrusion control for single extruders, primary and satellite extruders in coextrusion lines, and multiple lines in one or several plants.

The Maguire extrusion control system continuously analyzes material flow data as it is retrieved in real time by one or more weigh scale blenders, and calculates the changes that would be required in extruder-screw rpm or line speed to keep throughput constant and maintain layer ratios.

“The Maguire Extrusion Control Network eliminates the redundant components found in conventional yield control systems, such as secondary hoppers that, in effect, reweigh blended material that has already been weighed in the primary gravimetric blender,” explains Smith. “These secondary hoppers often have their own controls, further complicating the process and adding unnecessary capital equipment cost.”

IMPROVED CONSISTENCY BENEFITS ALL PROCESSES

For molders and extruders, batch-to-batch consistency of the blender can be as important as long-term blender accuracy. Your blender may achieve its stated accuracy over an average of 20 to 30 batches, but products are not made from an average of batches. Product consistency depends on batch-to-batch consistency of blended material.

A recent research program at Process Control Corp. aimed to develop the best way to determine the accuracy of gravimetric weigh blenders. It demonstrated that processors can get more practical value from assessing batch-to-batch blend homogeneity than from the long-term blend accuracy that is typically cited by blender suppliers.

According to Process Control, blend accuracy is determined by measuring the amount of each material present in the resulting blend over a specified time period. This measure tells you little about dispensing errors within a single batch. Blend homogeneity is closely related to blend accuracy, but may relate better to product quality.

The company explains that a processing machine typically takes in blended materials in discrete bites as the extruder turns. In general, there is limited mixing from one flight to another, so the consistency of the end product is heavily influenced by blend variations over much shorter time periods than those typically used to test blend accuracy.

As a result of this research program, Process Control developed the Guardian Series of gravimetric weigh blenders with enhanced mixing to improve blend homogeneity.

Mould-tek’s GXB gravimetric weigh scale batch blenders use microprocessor-controlled RTLS (Real Time Live Scale) to deliver target weight consistently from batch-to-batch. The live scale weighs each ingredient as it is deposited into the scale hopper, so the dispensing of each ingredient is controlled real-time, by weight. More traditional technology, using the “hunt” or “snapshot” method, dispensing is based on timed opening and closing of the dispensing devices, explains Brian Da Silva, general manager of Mould-tek. Adjustments and corrections are made to subsequent batches. This can result in inconsistencies from batch-to-batch.

Mould-tek’s GXB dispenses each of up to eight ingredients within 1/10th of a percent accuracy. It dispenses major ingredients using pinch valves that are unaffected by abrasive, dusty or irregular materials.

K-Tron America suggests that its K2-G SmartFlow Feeding System can improve recipe accuracy and reduce material costs of multiple feed streams witho
ut mixing. K-Tron has determined in lab tests and field study that its continuous, loss-in-weight, multi-stream feeding provides greater ratio accuracy and repeatability than a batch blending system that requires mechanical mixing. K-Tron’s K2-G SmartFlow feeding system adapts the company’s high performance modular feeder design specifically for pellets and other free flowing materials. Feeder modules feature the unique K2 sphere that promotes improved mass flow. Each loss-in-weight feed module is individually weighed using an ultra-stable single point weighing system.

INCREASED THROUGHPUT ALSO BOOSTS EFFICIENCY

By applying new hardware and software advances, Maguire has raised by up to 200 percent the maximum throughput rates of its gravimetric blenders.

Maguire has accelerated the dispense, weigh, mix and discharge operations of each blending cycle without compromising blender accuracy. The improved productivity means processors can often reduce capital equipment costs by using smaller blenders.

Contributing to the higher throughput are enhanced control software, replacement of auger feeders by pneumatically-actuated vertical valves and pneumatic mixing chambers.CPL


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