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The science of extrusion

If Bob Bessemer had his druthers, extruders, particularly profile manufacturers, would increasingly treat their process more like injection molders, trading art and feel for science and hard numbers.

Tony Deligio

February 5, 2009

6 Min Read
The science of extrusion

If Bob Bessemer had his druthers, extruders, particularly profile manufacturers, would increasingly treat their process more like injection molders, trading art and feel for science and hard numbers.

“We’re trying to take the artistry and make it a science,” says Bessemer, product manager downstream extrusion at The Conair Group (Cranberry Township, PA). In particular, Bessemer wants shops to pay closer attention to temperatures, both in dry-calibration tools and water tanks, cooling only as much as needed for greater production and energy efficiency. “You want to have a system that is closed loop to control that temperature,” Bessemer says, “and that’s one of the big things where we’re trying to have extrusion catch up to injection molding. Let’s close a lot more of these loops.”

A built-in drive system trundles the cooling tank beyond the end of base in Conair’s new hybrid dry-table/cooling tank to expose a dry-tooling bed at the upstream end. Depending on how far the tank is retracted, that bed can be as short as 6 inches, as long as 4 feet, or any length in between.

For shops that are already meeting production quotas, or are generally change averse, it could be a tough sell to have technicians swap out the ball valves that are currently used to control water flow and temperature with thermocouples and temperature control units. “Extrusion tends to be an area that is very slow and resistant to change,” Bessemer says. “Change is difficult, that’s human nature, and if somebody can make something with their current technology, why should they change?”

For those who are open to change, Bessemer has several suggestions.

Extrusion optimized

Bessemer believes both pipe and profile extruders need to optimize their die draw-down ratio, or the ratio of the pin and the bushing, which describes the size of the die compared to the pipe to be extruded. In order to minimize changeover times, Bessemer says some extruders attempt to make two different-size pipes from one die set, basically playing with the gap between the die face and where the pipe enters the calibration tooling. Extruders need to weigh any downtime saving they might achieve (from not having to change the pin and bushing to change pipe sizes) against the potential for reduced throughput that results when there’s mismatch between the die set and final pipe diameter.

Elsewhere, Bessemer says some shops are considering cooling the inside of the pipe by sending chilled air through the die to increase heat-transfer rates and, thereby, throughput. Perhaps more important, however, is the adoption of advanced drive technologies for pullers, including servodrives, which already see use in Europe. Augmenting this would be the use of OD/ID (outer diameter/inner diameter) wall-gauging units to automatically measure dimensions inline and make closed-loop adjustments to the puller speed.

Extruders could also benefit by considering heat-transfer rates and cooling-water temperatures. “You don’t always need 50-55°F water to cool a part,” Bessemer advises. A point of diminishing returns can be reached where “you’re not really removing a dramatically higher amount of BTUs for that differential of water temperature.” Conair has seen this in wood-plastic composites, where water temperatures can be staged, and tower water, at 80-95°F, versus chiller water, can be almost as effective when the profile first emerges from the die. “The first 4 to 10 feet is where 70% of your heat comes out, so why put all that load on the chiller if you don’t have to?”

Another area to consider is the dry-calibration tool, which Bessemer believes has more similarities to an injection molding tool than differences. Many injection molding tools will have multiple temperature zones, using differential cooling to accommodate thick and thin sections and prevent flaws like warpage. “A calibration tool is very, very similar,” Bessemer explains, “it’s just that the plastic is continuously flowing through.” Instead of process technicians feeling the profile, or watching for things like chatter, a thermocouple within the tool would check the temperature and then a temperature control unit would adjust cooling water accordingly.

In spite of these efforts, Bessemer says adoption of new technologies remains slow. “We’ve been talking about [temperature control units for dry calibrators] for three to five years,” Bessemer says, “but in extrusion it seems that change can take a long time.” In any case, Bessemer remains committed to advancing the technology. “Let’s put our thinking caps on – there  are ways to improve this process and enhance profitability.”

Suppliers unite to optimize PEX pipe production

High temperature and chemical resistance, as well as flexibility, very high impact resistance at low temperatures, and strong creep resistance have pushed crosslinked polyethylene (PEX) pipes into greater use in a variety of industrial applications, including gas and water distribution systems.

At the end of 2008, a cooperative effort between six companies ranging from resin and additive suppliers to machinery manufacturers was launched, with the intention to boost process efficiency for PEX pipe. The result has been pipes with outside diameters of up to 32 mm produced at line speeds of up to 25 m/min, with low scrap rates and long, continuous production runs.

Crosslink Finland Oy, which produces infrared crosslinking units; Hans Weber Maschinenfabrik, which develops twin-screw extruders for PEX pipe; iNOEX, which supplies mixing, dosing, and control units; resin supplier Borealis, which supplies resin for PEX pipe; AkzoNobel, which provides peroxide; and Ciba (now under BASF), which provides the necessary additives, announced the project last October.

Borealis specifically developed two new grades in its BorPEX range of crosslinkable, high-molecular-weight, high-density polyethylene especially designed to suit the new production process. Typically PEX pipe manufacturers buy the PE, peroxide, and associated functional additives (antioxidants, UV stabilizers) separately and dry-mix them in a batch process. The iNOEX dosing and control equipment allows this to be done continuously inline, along with the extrusion and crosslinking steps.

The company’s Saveomat multicomponent dosing station allows homogenous mixing of liquid and solid materials. As part of this technology, an integrated nozzle system injects the peroxide during the mixing process to ensure the granulate is homogenously wetted.

Because of the mixing required, twin-screw extruders have become the preferred process method for producing PEX pipe, and Weber twin-screw extruders reportedly benefit the process by running at extremely low temperatures with short residence time, which helps avoid peroxide decomposition in the extruder. Weber also offers optimized screws with low friction and minimal dead zones, as well as specialized die-head geometry and die coatings.

Borealis BorPEX HE1878E PE-X grades come in powder and pellet form, with the powder form providing a higher melt-flow rate than traditional PEX resins. New for this process is BorPEX HE1878E-C2, which the company describes as a fully formulated compound in mini-pellet form that only requires peroxide to produce PEX pipe.

In the system, pipe is crosslinked immediately after exiting from the extruder head via high-power short-wavelength infrared radiation. The special infrared heating uses light wavelengths that penetrate the entire pipe wall at once. A special vacuum-calibration method calibrates the pipe to narrow tolerances. In the finished product, Ciba stabilizer additives protect the product in the field, while AkzoNobel organic peroxides promote crosslinking. [email protected]

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