Canadian Plastics

Scrap Shouldn’t Go to Waste

You've tweaked your process, invested in new, accurate auxiliary equipment, and documented your quality procedures. What's left in the pursuit of lower cost and higher productivity? Your regrind.

October 1, 2004   By Cindy Macdonald, associate editor



You’ve tweaked your process, invested in new, accurate auxiliary equipment, and documented your quality procedures. What’s left in the pursuit of lower cost and higher productivity? Your regrind.

All your scrap plastic has value. It should be turned into good quality, uncontaminated regrind, and either reused or sold.

“Granulation is a component of good material management,” says Jack Bowne, vice-president sales and marketing with Hosokawa Polymer Systems. “The granulator is always the last item anyone thinks of when outfitting a plant because it doesn’t make money. But with proper material management, it saves a bundle of money.”

OPTIMIZING REGRIND QUALITY

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Proper material management of regrind begins at its point of origin: the granulator.

“The selection of a granulator is very job-specific. That’s a factor that people overlook,” says Bowne. Whether you’re feeding butter tubs loose or butter tubs stacked makes a difference when sizing and specifying your machine.

Operator education can also be a factor in regrind quality. Operators must be aware that overfeeding or underfeeding can be detrimental to the quality of regind produced, and to the efficiency and longevity of the granulator.

Proper evacuation is also important, says Bowne. “If a granulator is not evacuated properly, it can create more dust and fines because material stays in the chamber and gets pulverized.”

To improve regrind quality, Gloucester-CMG offers an affordable dust and fines separator that can be integrated with the company’s beside-the-press granulators. “Fines separation is becoming more and more popular, especially when companies have critical parts,” says Bob Barski, North American sales manager.

The fines separator works in conjunction with an auger discharge system. As a unit, the granulator and fines separator are portable, quiet and clean.

Tria also offers a compact, low-cost dust removal system for granulators of up to 300 lb./hr. throughput. In this system, a fan pulls the material from the granulator’s discharge area and conveys it to a centrifugal dust collector (cyclone), where an initial air-material separation takes place. At the separator discharge Tria’s LP10 system further separates the fine particles. Tria states that the system is suitable for materials such as PET, PC and PMMA.

Similarly, Rapid Granulator’s TRACS (Transport, recovery and air cleaning system) is a modular, compact, closed system that not only gets rid of dust and fines in recycled granulate, but also recycles and filters air.

Once your granulating system is in place, it is very important to monitor the equipment and the material to ensure that proactive maintenance is scheduled, says Len Tompkins, general manager, Hamilton Avtec. Screens, knives and blades should be checked for wear, and metal detectors or conveying systems should periodically have the set-up and calibration checked. “This will allow for a longer-lived, more productive piece of equipment and higher quality yield.”

FINES, BEGONE

Reduction of dust and fines seems to be the current buzz in granulator design.

Design changes to Foremost Machine Builders’ popular HD line of granulators have reduced the amount of dust and fines generated by the grinding process. Called the C series, the redesigned granulators have a new knife-to-knife cutting geometry combined with a new screen arrangement, which has greatly reduced the amount of fines generated by the cutting action. As well, the rotary knives are now significantly narrower than the cutting chamber to provide clearance to the sideplates. This means the knives do not drag product against the chamber wall, producing dust.

The C series is available as a retrofit to existing machines, or as a new unit.

In search of balance between regrind quality and throughput, Rotogran has developed a variable speed granulator that can operate at speeds as low as 50 rpm. “It has all the same features as our standard Rotogran granulators, but at low speeds it is quieter and makes fewer fines,” explains Mike Cyr.

Using an inverter duty motor and variable frequency drive, Rotogran’s variable speed granulator maintains its torque even at low speeds. Speed is easily adjusted by a knob on the control cabinet.

A byproduct of this development, says Cyr, is that it “has allowed us to experiment with what speed is best for certain plastics.”

Granutec’s large-throat, multi-tasking granulator system achieves maximum throughput with minimal operator assistance. The TFG2060 unit is suitable for both front and rear feed roll devices and accepts sheets up to 1 in. thick. The system is designed with an amp feedback system to prevent overfeeding of material.

For maximum ease of cleaning and maintenance, a power-assisted tilting hopper and swing down cradle provide complete access to the cutting chamber, knives and lift-out screens.

Infeed height is approximately 60 in., which eliminates the need for pits or platforms. Throat size is 20 in. x 60 in.; horsepower ranges from 75 to 125. Throughput can be as high as 3000 lb./hr.

For high-capacity central grinding, Cumberland’s B Series will withstand years of rugged use. The granulators use bolted and dowelled heavy steel construction, and three-knife, high-shear rotors. Machined interior surfaces allow easy clean out.

THE UNIQUE NEEDS OF FILM

Shred-Tech has developed a line of hydraulically-driven shredding systems for the conditioning of plastic film for use as extruder feedstock. The heart of the system is Shred-Tech’s two-shaft, low-speed, high-torque shear shredder.

The system provides a constant flow of size-controlled shredded film without the use of screens or shaft reversal cycles.

The closed-loop hydraulic drives used in these systems are ideal as they provide infinitely variable speed to keep the shredder running as long as possible. Another benefit is that the electric motor does not have to be continually starting and stopping with the shredder; the hydraulic pump simply shifts into the neutral position. This drastically lengthens the electric motor and contactor life and reduces electricity costs.

Hamilton Avtec’s AR series granulators are used as part of a complete automatic scrap recovery system for inline recycling of edge and bleed trims, roll scrap and loose scrap. There are six model sizes with different horsepower ratings.

The system operates by feeding the scrap through a dancer assembly to assure that no pull pressure is exerted on the process machine. The scrap is then fed into knurled feed rollers that provide a constant feed rate to the granulator. An open cutting rotor design provides air flow for cooling.

LOW SPEED, LOW THROUGHPUT

Low speed, screenless granulators are much more widely available now. The advantages of this type of granulator are reduced dust, improved consistency of granulate, lower power consumption and less noise than conventional high-speed granulators.

Glen Billinger, president of Plastics Machinery Inc., sells Moditec screenless granulators in Canada. He explains that a conventional granulator relies on horsepower and inertia to do the cutting. Therefore it needs high speed to overcome inertia on the moving metal parts. A screenless granulator relies on torque to do the cutting. It doesn’t require inertia or high horsepower, and can therefore run at a lower speed.

“Screenless granulators are not the answer to everything, but they are the answer to a lot of granulating problems,” says Billinger. Screenless units are limited in their throughput, however. According to Billinger, 300 lb./hr. is about the upper limit. As well, these units cannot handle film, extremely thin sheet, or low durometer products.

Christian Weiss, Wittmann product manager with Nucon Wittmann agrees that screenless granulators are generally best suited for low-throughput applications. However, he points out that the larger screenless granulators do offer versatility. They are quiet enough to handle small parts or sprues and runners in a beside-the-press function, but also have the capacity to handle occasional large sc
rap.

Wittmann has added an optional third shaft with breaker knives to its SJD Series low speed, high torque screenless granulators. These units are suitable for bulky parts such as nested runners, automotive bumpers and instrument panels. In some applications, the third shaft, which reduces parts before they reach the cutting chamber, will permit the use of a smaller overall granulator with lower capital cost.

Moditec’s Goliath Twin uses two rotors and anywhere from five to nine pre-cutters to regrind bulky parts. An optional rotating “masher” can be used to pull bulky parts into the cutting chamber and push the parts toward the cutting chamber. Chamber size ranges from 223 x 176 mm to 223 x 457 mm.

PINCH YOUR PENNIES WISELY

Although many processors aspire to cut costs to improve their competitiveness, Bob Barski of Gloucester-CMG urges buyers not to be short-sighted. “I don’t believe the way [to cut costs] is to buy cheap machines. You have to consider the value of quality machinery and how it affects your bottom line.”

Downtime, regrind quality and efficiency are all less easily calculated than sticker price, but in the long run, they will be equally important to your business.

***

THE SYSTEMS APPROACH

“Systems are the biggest thing happening today in granulators,” says Jack Bowne, vice-president sales and marketing with Hosokawa Polymer Systems.

The system shown at left was designed by Hosokawa Polymer Systems for a blow molded automotive part. The bellows-type part is molded with a dome on each end which is cut off while it is still within the machine. The molder used to collect the domes manually and return them to a central granulator. The company ran multiple materials, and all were black, so the potential for cross-contamination was high. The objective of installing this automated closed loop system was to eliminate the need for an operator to handle the scrap, and to eliminate material contamination.

The domes are now sucked up a tube by a venturi, and then blown onward to the granulator by a blower. The granulator gets a constant feed of parts. Regrind is evacuated by a blower/cyclone and conveyed to a receiving vessel. From there it can be picked up by a vacuum loader, and returned to the molding process if desired.

“The entire system is soundproofed, and runs at 78 dB,” explains Bowne.


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