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Home >> Technical Articles >>The Function of Extrusion Equipment and Energy Control During Processing
The Function of Extrusion Equipment and Energy Control During Processing
Time: 2009-02-20
Wood-plastic composites (WPC) are the combination of wood fibers, most commonly wood flour waste and a thermoplastic such as high density polyethylene, polypropylene or PVC. The polymer content by weight is generally 40-50%. WPCs are extruded into profiles mainly for the building and construction industry as an alternative to wood and treated wood. Consumers are choosing WPC over wood, even though it costs more, because of lower maintenance requirements and aesthetic appeal. WPC does not require painting, resists warping and termite attack, and is generally resistant to moisture-related issues such as mold development. Wood-plastic composites can be competitive to natural wood with proper selection of extrusion equipment and subsequent control of energy during processing.
 
WPC business depends on cost-factor: the cost of the end product should not be more than natural wood. Energy costs, at 3.75 times the annual extrusion investment cost, are a significant part of the WPC product-making it important to reduce the energy cost in production. It is necessary to get the right match of the product and equipment to minimize energy consumption. Energy costs can become a known variable and a basis for managing internal demand. The extrusion machines should be energy efficient. The motors should be maintenance free. The advantages of these motors lie in their high degree of efficiency and their optimal behavior under partial load. Increased energy efficiency is also ensured by barrel insulation, which minimizes radiation loss, and screw geometry that optimizes the energy input into the extruder so that a lower torque is sufficient to reach the same throughput. Active screw root tempering has deliberately been dispensed with and replaced by the IntraCool system, which re-distributes the energy input within the plasticizing unit so that very little external cooling is required.
 
A European survey of plastics processors showed that pipe and profile producers consume about 1,500 watt-hours/kg of electricity, while most polymers only require up to 250 watt-hours/kg, revealing a large efficiency potential. As per Omnexus, energy costs analysis comprises:
 
• Measuring energy consumption of the extruder and the complete plant.
• Evaluating energy flow throughout the production plant.
• Analyzing energy load and consumption.
• Evaluating energy cost and possible savings by product, extruder and extrusion line.
• Evaluating the cost of the energy being consumed.
 
After performing these five steps, set cost reduction targets, organize actions to be taken, and put in place a permanent energy measurement system. Once consumption patterns of each area are known, costs can in many cases be reduced by load management measures. One should always match orders/dimensions to the right extrusion lines finding savings potentials and weak links in the production. The extruders should be able to transmit heat from the screw to the material very efficiently. Conical twin screws create the compression and low shear-mixing capability needed. The tapered screw design allows a large feed zone for light bulk density materials. Natural compression during processing results in wood flour being "wetted out" and polymers being properly mixed prior to devolatilization. The deep flights of the screw technology optimize mixing while reducing shear forces. A tungsten clad conical barrel is used for highly abrasive WPC formulations.
 
The storage conditions need to be carefully controlled. Then the wood flour or other natural fiber is conveyed from storage, warmed, mixed, and dried. This treatment removes a large part of the moisture. Any residual moisture evaporates during processing in the 36D counter-rotating, double-venting extruder. Pre-drying and inline venting can provide energy efficiency. Energy efficient drying is accomplished by vacuum venting that draws away volatiles.
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