Pipe and Profile Extrusion: Extruders chasing higher throughput
The drive for higher line speeds continues in extrusion of profiles for fencing, decking and windows. In both North America and Europe, dual downstream systems have emerged as one route to optimize ex...
The drive for higher line speeds continues in extrusion of profiles for fencing, decking and windows. In both North America and Europe, dual downstream systems have emerged as one route to optimize extruder capacity and improve efficiency and throughput.
In a dual downstream set-up, sometimes called multiple strand extrusion, the melt flow is split in the die area to form two profiles, each running on a separate line of downstream equipment. It is beneficial in situations where extruder capacity exceeds cooling capacity.
“It is used a lot by high-volume extruders in the fencing and decking market, because they are competing against commodity lumber, and any increase in throughput is desirable,” explains Kurt Waldhauer, vice-president of sales and marketing for extrusion system manufacturer American Maplan.
Typically, a co-extrusion dual downstream system consists of one main extruder for the substrate and two extruders for capstock layers. The flow is generally split after the die, so one capstock extruder is necessary for each profile.
American Maplan and Davis-Standard are both introducing innovations to this process.
American Maplan has developed a hybrid profile extrusion system with a unique co-extrusion feed block design. The tooling evenly and efficiently splits the flow from the co-extruder, eliminating the need for a second co-extruder, reports Waldhauer.
Maplan also incorporates an innovative X-Y axis wall thickness control. “If you take a 5 in. x 5 in. profile, for example,” says Waldhauer,” there’s plenty of opportunity for the pin to get off-centre. Our system allows you to adjust the position of the pin on the fly.”
Davis-Standard is working with a European tooling and downstream supplier, a+g Extrusion Technology GmbH, to offer a system optimized for producing two profiles simultaneously.
During K 2001, the dual downstream line will produce a typical U.S. window profile and a European-style window frame concurrently. The line uses a Davis-Standard GP 94, 28:1 parallel twin screw extruder with a 100 hp AC motor. In this configuration it has a throughput of 1000 lb./hr. of PVC.
The downstream equipment is set up in two independent tracks and controlled at the die exit in front of the calibration table. The dual haul-off uses separate drive systems for the upper and lower chain carriages controlled by a digital synchronization system with automatic torque drives.
ALL COMPONENTS CONTRIBUTE TO RAISING THROUGHPUT
A multi-supplier demonstration last spring achieved speeds about 50 percent faster than industry norms for a fence rail profile. The record-setting demo line used an Atlas 114 parallel twin screw extruder from ExtrusionTek Milacron to produce substrate and a Milacron E55 conical twin to pump the UV stabilized capstock. These were paired with a die and calibrator from Kansas American Tooling and downstream equipment from Conair. The line achieved speeds of 25 ft./min.
Extruders of fencing, and similar products such as decking and window components, face two challenges to profitability, according to Kenton Gearhart, president of Kansas American Tooling. They need to keep material costs down and extrusion rates up, he says.
The tooling demonstrated at Conair’s event in April addresses both these concerns. The coextrusion die is designed to apply a very uniform UV-stabilized capstock layer on top of a PVC substrate. KAT uses a system that introduces the capstock material just one inch before the profile exits the die. Thickness tolerances can be held to +/- 0.002 in., allowing producers to reduce capstock material consumption by as much as one-third, reports KAT.
To address cooling limitations, KAT drilled air holes in the mandrels that form the hollow sections of the profile, venting them out through the spider mandrel supports and finally through bolt holes in the die housing.
KAT used six calibration sections, roughly twice the number that would normally be used, in order to maximize heat removal and allow higher throughput rates. The calibrators are mounted on their own support stands between the die and the vacuum/cooling tank, allowing the entire tank to be used for final sizing and cooling.
Conair contributed its high-intensity spray cooling tank to the record-setting line. The high-intensity tank can significantly increase extrusion rates and takes up less floor space than conventional cooling tanks. It is just 24 ft. long, but provides cooling equivalent to that of a 40-ft. conventional tank.
LONGER SCREWS IMPROVE OUTPUT
Longer screw length is one contributor to the increased output of Davis-Standard’s Gemini parallel twin screw extruders. Available in 94 mm and 114 mm 28:1 L/D models, the GP extruders offers substantially higher output compared with other models in their class. They can process increased amounts of resin per screw revolution because of longer screws and a higher torque capacity.
For pipe producers, Krauss-Maffei’s new twin-screw extruders for PVC pipe promise up to 17 percent increase in output. The new machines have 36:1 L/D ratios.
Their design incorporates two venting zones. According to Krauss-Maffei, removing air from the material at an early stage of the process results in far better heat transfer. The ratio of mechanical energy to heat energy applied to the melt is thus significantly reduced. This innovation reduces wear on the screws and barrel, and is particularly effective when processing formulations with high filler content.
THE SPECIAL NEEDS OF WOOD
For wood/plastic composites, ExtrusionTek Milacron offers what it calls the category’s highest output extruder — the CM92 V-MEDS.
V-MEDS stands for Material Enhancement Delivery System with Vertical Integrated Feed Option (otherwise known as a crammer). The MEDS preheats premixed wood fibre plastic compound and drives off residual moisture.
The CM92’s large-diameter screws (184 mm diameter tapering to 92 mm) provide a large area in the feed zone for better filling with low-density materials.
Davis-Standard’s Woodtruder for wood/ plastic composites uses two extruders — one twin-screw to convey fibre at low rpm and low shear, and a single or twin injection extruder which melts and injects plastic compound into the main screw extruder. This set-up allows plastication to be completed separately from the fibre. The main extruder thoroughly compounds the fibre/plastic composite and has vacuum venting to remove volatiles or moisture.