Davis-Standard: Equipment Range Addresses Wood-Filled Demands

Davis-Standard supplies the industry’s most diverse line of processing equipment to the evolving wood-filled marketplace. Equipment options include single screw extruders, twin screw extruders and Davis-Standard’s unique Woodtruder®, each designed to accommodate various applications and budget requirements. Following is a summary of advantages and disadvantages of each technology.
Single Screw Extruder
Single screw extruders are best suited for profile applications using
pre-compounded pellets with dried fiber materials. Using 60 percent wood fiber and 40 percent polyethylene, the output for a 4 1/2-inch (115mm) extruder is around 800 to 900 pounds (360 to 400 kg) per hour while a 6-inch (150mm) extruder can achieve upward of 1,600 pounds (730 kg) per hour. Disadvantages of single screw extruders include a lower rate versus melt temperature, poor venting and the use of pre-compounded material which has one heat history.
Twin Screw Extruder
Four types of twin screw extruders can be used in wood-filled processes. These machines include co-rotating twin screw, counter rotating conical twin screw, counter rotating parallel twin screw and counter rotating twin screw using ancillary equipment.
A co-rotating twin screw extruder can process powder or pellet-fed polymers and wood fibers with up to 8 percent moisture content. This extruder provides excellent mixing and venting, but has poor pumping stability therefore requiring a screw or melt pump. A typical rate for processing 50 percent polypropylene and 50 percent wood fiber on a 47mm co-rotating twin is 300 pounds (135 kg) per hour at 300 RPM.
The counter rotating conical twin screw extruder is designed with a larger feed diameter for processing low bulk density materials such as dried wood and powdered polymers. This machine offers good mixing and pumping capabilities at lower melt temperatures. However, venting is limited and using pellets can be problematic. Typical output rates for a 61mm conical are around 300 to 400 pounds (140 to 180 kg) per hour.
Like the conical twin, the parallel counter rotating twin screw extruder has good mixing and pumping characteristics at reduced melt temperatures, but limited venting and difficulty processing pellets. A crammer feeder is recommended with this machine. Typical output rates are 500 to 600 pounds (225 to 270 kg) per hour for a 94mm extruder, 900 to 1,000 pounds (410 to 450 kg) per hour on a 114 mm machine and 1,600 to 1,800 pounds (730 to 820 kg) per hour using a 140mm extruder.
Ancillary equipment such as a pre-heater and crammer feeder can be used on conical and parallel twin machines for added flexibility. A pre-heater is required when processing wood fibers with moisture content above 1 percent or when using pellets. The pre-heater, located above the extruder, warms the material to around 200 degrees Fahrenheit and meters it directly into the extruder using intermeshing co-rotating screws. A crammer or force feeder is needed when feeding low bulk density wood flour/polymer blends. The feeder increases efficiency by utilizing an auger to force materials into the feed zone of the extruder screws.

Woodtruder
The Woodtruder™ is another Davis-Standard option for processing wood-filled products. This machine utilizes a single screw side injection extruder to melt the polymer and a parallel twin screw extruder for drying, mixing and pumping. There is no pre-drying or wood blending required and nearly all polymer forms can be used. A gravimetric feeder provides input consistency and moisture is removed via an atmospheric and vacuum venting system. This extruder is very efficient at pumping and mixing at low melt temperatures and processors have the option of using wet wood. Disadvantages include an output rate sensitive to wood type and moisture content as well as difficulty with evolved moisture. Typical processing rates are 500 to 600 pounds (225 to 270 kg) per hour for a 94mm Woodtruder, 900 to 1,000 pounds (410 to 450 kg) per hour on a 114mm machine and 1,500 pounds (680 kg) per hour on a 140mm model.
For more information about Davis-Standard’s wood-filled processing capabilities, contact Wendell Whipple at [email protected]

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