No-hassle shredding and granulating: Maximizing regrind and saving money
Many plastics processors approach granulating and shredding half-heartedly, either by failing to purchase the right systems, or by misusing the equipment they have....
Many plastics processors approach granulating and shredding half-heartedly, either by failing to purchase the right systems, or by misusing the equipment they have.
But an effective size reduction system, with the right equipment and smart controls can be an effective way to save money on materials.
Skimping on the price, and not buying the best system up front, is one of the biggest errors plastics processors can make, said Bruno Casciato, president of Royal Ecoproducts (Concord, ON), a division of Royal Group Technologies Ltd. Buying the best system means you experience less downtime and spend less time maintaining machines, he explained.
Jack Bowne, vice-president of sales and marketing at Hosokawa Polymer Systems (Berlin, CT), agreed that companies are generally too cost-conservative when purchasing size reduction systems.
“People don’t like to spend money on a product that doesn’t [overtly] make money for them,” he explained. “Except that it does make money for them, but they can be too blind to see it.”
As a result, many companies just throw out their scrap, or try to make do with the granulators they have, regardless of what the machines were designed to grind. For grinding 60lbs of scrap every three minutes in a granulator designed to handle 50lbs every three minutes will cause problems, Bowne said. Or they will grind a more abrasive material, or a differently shaped part, without considering the consequences.
To avoid making this mistake, processors need to consider their future needs as well as their current ones. As a rule of thumb, it better for processors to over-specify size and horsepower when purchasing a granulator, said Mike Verner, granulator product manager at Conair (Pittsburgh, PA). “The feed hopper and cutting chamber should be sized to handle the largest possible part without pre-cutting,” he said.
For large granulators, especially those that will be operating at capacity, or near capacity, on an ongoing basis, throughput is an important consideration, he added. This will also affect the horsepower of the motor, rotor design, the number of rotating and bed knives, rotor speed, screen size and regrind removal.
The crux of the issue is that size reduction systems need to be tailored for the unique needs of the user, said Mike Cyr, vice-president of sales at Rotogran International.
One of Royal Ecoproducts’ granulators was a custom-made granulator from Rotogran. Royal provided the design and in the end, it didn’t cost that much more than purchasing a standard granulator out-of-the-box, Casciato said. Royal Ecoproducts recycles scrap generally made from a mixture of polypropylene (PP) and polyethylene (PE), he said. The facility also employs shredders — of its own design — to break down plastic waste, such as bales, before the scrap is fed into their granulators.
Of course most processors don’t have the wherewithal to provide a vendor with their own blueprints for a granulator, but they can take the time to think about their unique requirements.
MYTHS AND MISTAKES
A common myth is that the type of resin you’re grinding will determine what type of granulator you need, Rotogran’s Cyr said.
“When in fact, it’s the physical, size, shape and throughput necessary that will dictate what size granulator you will buy,” he explained. “And it’s ultimately dictated by the application: Is it beside the press or central?”
However, the type of resin does plays a part when grinding abrasive material like glass-filled nylon, or anticipating grinding such materials in the future, he noted.
In those cases, Cyr recommends a granulator that is protected against abrasion with a coating, or with removable inserts that can be replaced once they wear out.
When grinding low melt temperature resins, lower rpm granulators are recommended, he said.
Christian Weiss, Wittmann product manager at Nucon Wittmann Inc. agreed.
If you’re grinding abrasive, brittle material that you don’t want exposed to high temperatures, then low-speed granulators are an option, he said. Low-speed, screenless granulators produce fewer fines and eliminate longs.
“With low-speed granulators, the blade size on the roller cutter determines the regrind size, Weiss added.
With high-speed granulators, screen size determines the regrind size, and it should be sized so that no runners pass through, creating longs, he said.
Choosing the right rotor — open or closed — also affects regrind quality.
“The reason why you’d want a closed rotor is because you’re looking for something that will take small bites out of a totally solid mass,” Cyr said. “However, closed rotors have a big disadvantage of running hot, causing fines and angel hair within your regrind.”
And for grinding film, Royal Ecoproducts’ Casciato recommends using a closed rotor because open rotors will get easily clogged up.
But if you use a shredder before granulating, Casciato finds it more effective to use a granulator with an open rotor.
Additionally, the physical size of the part or scrap will determine the configuration of the cutting chamber, Verner said.
“Tangential feed chambers have rotors that are positioned at an offset from the feed opening so that bottles, TV cabinets and other large bulky parts meet the rotor knives on the downward portion of their cutting circle,” he said. “This allows the knives to take a bigger bite when the part first enters the chamber.”
For smaller, heavier-walled parts, Verner recommends a straight, drop-through design.
Before purchasing a granulator, it helps to provide samples of the parts and scraps you will be regrinding to the vendor. That way, they can test a variety of configurations before recommending the right machine.
But just as important, or more important than the granulators and shredders themselves, are the systems that are built around it.
Bowne said he also couldn’t stress the importance enough of effectively handling material to prevent contamination.
Ipex Inc. (Toronto, ON) is running a small, beside-the-press granulator from Nucon Wittmann, to grind sprues from injection molded pipe fittings. Right now, the granulator is being fed manually, but soon the company will be adding a sprue picker robot to increase productivity.
For this application, IPEX chose a dedicated system because the material it is using is expensive — about $15/kg — so the firm wanted to eliminate the possibility of contamination, explained Rod Huels, technical manager at IPEX’s Toronto location. About 15% of the material ends up as scrap in the form of sprues and runners.
After IPEX determined it wanted a beside the press system, it contacted Nucon Wittmann and gave them the specifications of the material, the size of the part and the throughput it needed. Based on this information, Nucon Wittmann was able to recommend an ideal granulator. The granulator has an open rotor, with a tangential feed chamber so the small pipe-fittings can be efficiently cut by the scissor-type knives. It also has an optional flywheel for added inertia.
One factor IPEX had to consider when implementing the system, was ensuring the regrind was mixed to a proper ratio with the virgin material. If the virgin material runs out, the parts would be processed simply with regrind, and as a result, wouldn’t pass IPEX’s quality control.
To solve the problem, IPEX put a sensor in the storage bin containing the virgin material, so when levels get low, the whole system automatically shuts down.
FEEDING THE GRANULATOR
Another aspect of the system is feeding the granulator, Verner said.
“(For manual feeding) Hoppers that open to the front of the unit for easy feeding are designed to eliminate flyback and prevent operators from reaching the cutting chamber when the unit is operating,” Conair’s Verner noted.
In closed loop systems employing a robot to drop scrap, such as sprues, straight i
nto the granulator, hoppers are open at the top.
On a central system, Rotogran’s Cyr advises moving the material to the granulator on an incline conveyer, passing the material through a tunnel-type metal detector that will pick up ferrous metals, non-ferrous metals and stainless steel.
Royal Ecoproducts, for example, uses a conveyer to feed its shredder, which is capable of grinding up metal. This comes in handy when the firm receives bales held together by wire, Casciato said.
But the metal has to be separated from the shredded material before it is fed into the granulator. At Royal, on the conveyer between the shredder and the granulator, the shredded material passes through two metal detectors, Casciato said, a magnetic system which will automatically remove ferrous material and an eddy current system, which will automatically remove the non-ferrous material.
Conveyers can also be setup with smart controls that will stop automatically if metal is detected, so the operator can reverse the conveyer, manually remove the metal and restart it, Cyr said.
EVACUATING THE GRINDER
Grinding is the easy part, but removing the regrind from the evacuation chamber is key to a successful size reduction system.
“If a large volume of material is being processed, automatic evacuation of regrind will improve grinder efficiency and reduce dependence on manual labour,” Verner said.
“Proper evacuation of a granulator is just as important as the size reduction itself,” Rotogran’s Cyr said. “You want to draw air through the cutting chamber to keep things cool and assist in the regrind making its way through the screen.”
Using a vacuum loader to evacuate a granulator on a closed loop system is a common mistake, he said. Vacuum loaders don’t allow airflow through the granulator, which can negatively affect the quality of the regrind because it gets hot, and produces more fines and angel hair.
On a closed loop system, Cyr recommends evacuating a granulator using a blower/cyclone system, which will drop the regrind into a surge bin. Then it can then be picked up with a vacuum loader and fed back into the system.
Bowne agreed. Good airflow keeps the regrind cool, preventing melting and the production of fines. However, if the material is going directly into a storage container, a vacuum loader is perfectly acceptable for evacuating the granulator.
Size reducing isn’t hard. However, choosing the right equipment and implementing the right system is the only way to make money off scrap. Otherwise, it’s just garbage.
FIVE MAINTENANCE TIPS FOR BETTER QUALITY REGRIND
* Devise a maintenance schedule and stick to it
* Regularly sharpen knives; check and replace dull or worn blades regularly
* Stock spare components such as knives and screens
* On a shredder, regularly rotate the knives
* Ensure employees are properly trained to use the equipment
THE ABC’s OF SHREDDING: A UNIQUE APPROACH TO SIZE REDUCTION
Although shredders are slower and noisier than granulators, one plant manager said if he knew five years ago what he knows now, then he would never have bought a granulator.
ABC Plastics Inc. (Brampton, ON) is running 70 blow molding lines for plastic automotive parts, mainly running boards and cargo management for the Chevrolet Equinox, said Mahyar Nikseresht, plant manager. Most of the large parts ABC processes are made of either a proprietary ABC glass-filled nylon or GE Advanced Material Inc.’s Cycoloy resin, a polycarbonate(PC)/acrylonitrile-butadiene-styrene copolymer (ABS) blend.
The result of grinding large parts and purge made of gritty, tough plastics, means ABC’s granulators were wearing out fast.
“We were changing the blades every two weeks on our grinders but only every two months on the shredder,” Nikseresht said.
And even though shredders cost about $10,000 more than a granulator, Nikseresht said it’s worth it because he can save more money using a shredder on maintenance alone.
The plant uses three, 75-hpw RG52 shredders from Vecoplan LLC. Nikseresht bought the first one about a year-and-a-half ago and the other two within the past year. He’s also recently purchased a fourth, which will soon be in production.
“I’m very pleased with them because they are very low maintenance,” he said.
“I can put all kinds of purge material in them, which we used to throw away. Now we reuse it.”
Prior to buying the shredders, the plant disposed of about 30,000lbs of purge every five weeks, and now it only throws away about 9,000lbs of material, he said. The material ABC discards is unusable because it has been contaminated, he explained.
“For any business that is generating purge, the material cannot be put in a grinder unless they chop it up first with a bandsaw,” he said.
Nikseresht also speculated that processors of small components would make do with just a shredder. “If they go with a shredder, then even for a small part, they can combine a few lines together because small parts are not heavy.”
The ABC plant Nikseresht manages has a materials handling room, where shredders and granulators work with specific production lines during the week. The scrap is moved to the materials handling room on a conveyer where it is fed into the machines and reduced in size. Right now, each of the three shredders services its own line.
Prior to the plant’s expansion in 2004, some lines were beside the press granulators, but now it’s been all moved into materials handling room, Nikseresht said.
On the weekends, when the plant isn’t in production, the shredders are used to size reduce purge from production lines with granulators. It takes about an hour to clean the shredder between sizing reducing different materials, Nikseresht said.
The drawback with shredders is less throughput, Nikseresht said. With grinders, the plant can get up to 1,200lbs/hr, but only 600 to 800lbs/hr with the shredders. However, Nikseresht said it’s not a problem at his plant because they couldn’t exceed that anyway.