Canadian Plastics

Sticker shock subsides for servos

Accuracy and reduced maintenance are the widely touted benefits of servo drives, which are becoming much more common in downstream extrusion equipment."We're finding that servo drives are becoming the...

January 1, 2001   By Cindy Macdonald, associate editor



Accuracy and reduced maintenance are the widely touted benefits of servo drives, which are becoming much more common in downstream extrusion equipment.

“We’re finding that servo drives are becoming the norm, not the exception,” reports Ernie Preiato, Conair’s vice-president of extrusion. “Servos used to produce sticker shock but they’ve come way down in price, and the performance has improved.”

For pullers, cutters and travelling saws, you’ll see the benefit in improved length tolerances, he adds.

The Model 4-26 high-speed servo-driven puller from Conair allows small tubing to be produced at speeds up to 2000 ft./min. Mechanical innovations avoid problems like belt overheating and excessive wear which often plague high-speed pullers. In this model, high-speed servo motors are matched with a flat belt and pulley system for smooth, precise operation.

Servo drives and digital speed control are also available on Custom Downstream Systems’ (CDS) cleated haul-offs. The units have pneumatically controlled clamping pressure with balance control of the conveyor assembly. Quick change cleats enable the use of dedicated shapes to minimize distortion of the tube or profile.

Barry Reynolds, vice-president sales and marketing, predicts the industry will see more use of servo motors and PLCs, as well as increased integration of downstream equipment into the main extruder control.

G.F. Goodman has introduced the “Simple Servo” on-demand rotary cutter featuring velocity servo controls. According to Goodman, velocity controls offer higher accuracy and easier operator control at lower cost than microprocessor-controlled servo cutters. Duke Davis, Goodman vice-president, says PLCs are more costly and delicate than velocity controls, and require software and programming, which the Goodman controls do not.

Davis also notes that velocity controls offer a simpler interface. The operator simply enters the cut length into a counter.

“Rotary cutting is a basic function that doesn’t require the broad capabilities of a front-end microporcessor,” says Davis.

Wayne Machine uses two servo drives for its Servo-trac puller — one for each belt. This eliminates the need for chains, gears and shafts that may build up inertia and have backlash that can negatively affect tooling tolerances, says the company. Because of their accuracy and precision, Wayne recommends servo pullers for exacting medical tubing applications. A less expensive servo option uses a single, large servo drive and a shaft/gear drive mechanical system.

In Wayne’s integrated Servo-trac puller/ cutters, a third servo drive is used for the cutter. These are synchronized by the digital control system.

The special needs of lumber

A number of manufacturers have developed equipment optimized for larger profiles such as lumber for fencing and decking applications, and wood/plastic composites. This market is characterized by heavy profiles and a need for speed.

CDS offers high-volume spray units and calibration tables up to 42 ft. long for this market. Spray tanks from CDS are constructed of stainless steel. Each compartment contains a temperature indicator, stainless steel spray bars, product support hold-down rollers and standpipes for flood level control. Each compartment is operable in flood or spray mode. A full recirculation system is optional.

Conair’s high-intensity spray cooling tank provides the company’s fastest cooling option for heavy walled profiles such as composite lumber, fencing and pipe. The secret to the MSB-HI-3-24-9 tank’s effectiveness is a uniform, high-intensity water spray that optimizes evaporative cooling to quickly release heat that can get trapped in thick profiles.

“For the wood composite market, we had to develop technology that would achieve the throughput they wanted,” explains Conair’s Preiato. “The high intensity spray tank does just that. Wood composites don’t use vacuum sizing so it is critical to freeze off the extrudate with a high volume of water.”

Unlike a more conventional system, which can lose more than half of initial pump pressure to filters, heat exchangers and plumbing, the Conair system loses just five to 10 percent. A unique tank design turns the entire tank base into a reservoir, reducing the amount of plumbing. Two large turbo filters, oversize heat exchangers and a bypass system combine to reduce pressure drop throughout the system.

The high-intensity tank helps extruders reach throughput rates as high as 850 lb./hr. with a 24 ft. tank.

Nutter Machine Co. Inc. has added the SNP 17 model to its line of profile saws to accommodate larger profiles such as plastic lumber. A rolling cover contains a full 30 in. of table travel within the frame. A 5 hp saw arbor motor provides the power needed for quick, clean cuts. Electronic length measurement is standard.

Tighter control for sizing and calibration

A new generation of vacuum sizing equipment from G.F. Goodman features a closed recirculation loop and variable speed pump that automatically maintains vacuum setpoints over time with no drift.

Sales manager Gary Grossman says the closed-loop system eliminates the need for operator monitoring of vacuum levels and manual readjustment.

Each vacuum chamber is fitted with a pressure transducer linked to a controller that operates the variable speed water pump. This prevents pressure spikes, cavitation and wide pressure fluctuations.

In the first vacuum tank chamber, an adjustable spray ring coil encircles the calibrator head die plates to provide additional heat dissipation during the most critical phase of the sizing process.

For larger sized tubing, Harrel offers the VC-108 cylindrical vacuum tank. The cylindrical shape is said to eliminate “washboarding” or the flexing of sides that can occur in larger rectangular tanks. All Harrel tanks use closed-loop PID controls, precision electronic sensing of vacuum, and venturis and gear pumps driven by Harrel digital drives.

American Maplan showed at NPE 2000 a profile extrusion line with a new system consisting of a short dry calibrating section with several downstream calibrating tanks. The tanks can be full, spray or flooded baths. American Maplan’s dry calibrating systems are joined by tongue and groove, so maintenance is quick and easy, even during machine operation. The calibrating unit’s base plate is fitted with integrated supply for vacuum and water with central connections which lead to extremely short change-over times when fitting new calibration systems.


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