by Doug Brewster, product manager-conveying, and Vincent Carpentieri, product manager-size reduction
Dust is a perennial problem in plastics processing plants and, today, when scrap is likely to be recycled in-house, the issue can be especially frustrating. The process of granulating material and then conveying it to storage or back to a molding machine or extruder presents many opportunities for dust creation and then for it to escape into the plant environment. There it becomes a housekeeping problem, can cause personnel-safety issues and even can affect the quality of finished products. So, how can you prevent the formation of dust and fines, or at least minimize the problems it can cause?
In granulation, maintenance – or the lack thereof – is probably the most common factor. Granulators sometimes seem like outcasts in the auxiliary equipment world. They often are the last piece of equipment to be considered for purchase in a plastics processing operation and, more times than not, the first to be ignored when it comes to proper care. If you seem to be generating more dust and fines than seems reasonable, this is probably the reason why.
The first question to ask is whether you have the right granulator. Different cutting chamber designs and rotor configurations are available to handle different kinds of scrap. Choosing the right granulator for a given application is the first step toward minimizing dust and fines. It is always a good idea to consult with a competent granulator and solicit a recommendation based on the following:
- the size and shape of the scrap that will be processed,
- the material(s) being processed,
- the volume,
- how it will be fed to the granulator and
- what you will do with the regrind.
Once the right granulator has been selected, maintenance becomes the critical factor in the effort to produce high-quality granulate with minimal dust and fines.
The first rule is to keep the knife blades sharp. All blades will go dull eventually, but if regrind contains more dust and fines than normal, chances are this is why. In fact, dull knife blades are far and away the most common granulator problem. How fast will blades become dull? That depends on how much the granulator processes and what kind of material is being ground. Any glass-filled material will be harder on blades than acrylic or polyethylene. And, a machine that runs more or less constantly will have blades requiring sharpening long before a granulator that is run only a few hours each day. The worst case, of course, would be if a wrench or a bolt should happen to fall into the feed hopper. This can ruin blades in an instant.
Check the blades regularly – both rotor blades and bed knives. A quick visual inspection is all it takes. If this practice is built into a regular preventive maintenance schedule, it soon will become clear how often the knives need to be sharpened.
If the blades are dull, there are two options: replace or resharpen. Most suppliers provide either sharpening or replacement programs (or both), which should be clearly outlined in your operations manual.
Also, if the blades are dull and especially if they’ve gone unattended for long periods of time, the screen and screen cradle may have suffered as well. Take a look at the screen holes: If they are beginning to appear pear-shaped, it’s probably time for a replacement.
If the blades are sharp and everything else appears to be in good repair, but regrind quality is not what it should be, chances are the all-important knife gap is out of tolerance. The gap refers to the space between the rotating knives and the fixed bed knives – normally between 0.20 and 0.30 mm. Using a wrench, feeler gauge and a pair of gloves, adjusting the gap is a relatively simple process. Gap adjustment should be on every preventive maintenance schedule.
Tips for conveying
Whereas maintenance is probably the most critical issue with granulators, equipment selection, system design and operation are more important in conveying. Following are some tips that can help.
If regrind – or any material for that matter – is conveyed using a conventional blower system, the impact of impeller blades could create additional fines. Consequently, it always is better to move material using a vacuum or negative-pressure system (in which the material never touches the impeller) with an oversized blower and large diameter conveying lines.
A side benefit of a negative pressure system is that, even if there is a leak in the conveying tubing, dust cannot escape because the negative pressure is pulling air into the system and cant blow dust out. Pressure conveying, on the other hand, would tend to push air and any suspended dust out into the plant environment.
In a vacuum-conveying system (see illustration on page 42), it still is important to keep the circuit closed to the degree possible. When a vacuum receiver is above a machine hopper, it usually is mounted on a lid that covers the hopper around the receiver outlet, keeping dust inside the hopper. Quite often, however, when a receiver is loading a bin, Gaylord or drum for temporary storage, the space above is left open, and dust stirred up as the material falls into the container can easily escape. Various plastic or fabric covers are available for exactly these situations. They fit snugly around the box/bin and the receiver discharge to prevent dust from escaping.
Inevitably, however, some dust will remain in the receiver, usually clinging to the screen at the top that prevents pellet carryover as conveying air is pulled toward the vacuum pump. The simplest solution is a valve that can be opened at the end of each load cycle. This breaks the vacuum seal, allowing air to rush into the receiver and dislodge the dust. Since the receiver is only returning to normal pressure, the chance of dust escaping is lower.
Finally, at the end of the conveying line and just before the air enters the vacuum pump, it should pass through a central dust collector that separates dust from conveying air so it can drop into a collection bin. The dust in the collection bin can be emptied manually or automatically at the end of each vacuum cycle. In the auto-dump mode, the dust collector is mounted on an elevated frame so the dust can fall into a drum or Gaylord for later disposal. However – as when a receiver dumps material – dust can escape if the container is not covered. Here, too, some kind of cover should enclose the discharge port of the dust collector and the container into which the dust falls.
The prior recommendations are aimed at preventing dust from escaping into the processing plant and/or preventing damage to vacuum pumps. Recently, however, new products have been introduced to keep dust from getting into the processing machine, where the difference in melting characteristics between it and pellets can result in black spots and other product defects.
Unlike conventional, high-speed or dilute-phase conveying, the new product offering moves pellets at slow speeds in compact slugs of material in a process known as dense-phase conveying. Material speeds can range from as slow as 230ft/min (70m/min) up to 1,200ft/min (366m/min). Compare that to conventional high-speed conveying, where 4,500ft/min (1,372m/min) is a typical low speed, and speeds up to 6,000ft/minute (1,829m/min) are not uncommon.
The system uses many of the same components as a conventional high-speed system. Some special valves and other components are required, but since it uses standard deep-vacuum pumps, conveying tubes and material receivers, the system can be easily retrofitted to existing systems. In customer testing, it was found that the amount of dust generated was negligible, even though conveying throughput rates were unchanged compared to the customer’s high-speed system.
To remove dust at the machine feed throat, a compact machine-mounted unit is available that uses the Venturi effect to maximize dust removal while minimizing carryover of good product. Inside the unit, the compressed air is split into two air streams for optimal removal of dust, fines and streamers, and an ionizer is used to release the electrostatic bond between the pellets and the dust. The cleaning air blows the removed dust and fines into two mini-cyclones, which separate the materials from the air. The separated contaminants are collected in an integral dust container. The entire unit mounts on the machine between the hopper and machine throat, adding only about 10 inches to the height of the equipment.
Clearly, it is possible to minimize the amount of dust and fines created when grinding and conveying plastic materials. However, even in systems that are designed, maintained and operated to the highest standards, some dust can be created and escape. By paying close attention to the details and problem areas mentioned in this article – and enlisting the help of an auxiliary equipment supplier – processors can operate (mostly) dust-free.
Doug Brewster is conveying product manager for the Conair Group and the primary developer of the R-PRO slow-speed conveying system. He joined Conair in 1987 as a systems engineer. Vincent Carpentieri (firstname.lastname@example.org) joined Conair in February 2014 as size-reduction sales manager. Prior to that, he spent 25 years with Cumberland Engineering, working first in assembly, then technical service and application engineering. The Conair R-PRO and Deduster C50 are new Conair product offerings referenced within the article. For more information, visit www.conairgroup.com.