RAW MATERIALS HANDLING SYSTEMS: A TROUBLESHOOTING GUIDE
By Mark Stephen, Managing Editor
A ccording to Murphy's Law, whatever can go wrong, will. This probably sums up the way many plastics processors feel about their raw materials handling systems. There are, after all, so many different...
By Mark Stephen, Managing Editor
According to Murphy’s Law, whatever can go wrong, will. This probably sums up the way many plastics processors feel about their raw materials handling systems. There are, after all, so many different raw materials handling components to go wrong. Adding to the difficulty, processors, under mounting pressure to manufacture more parts more quickly, may not always give their systems enough close attention…until a problem arises.
Here are solutions to some of the most common problems, provided by several leading raw materials handling equipment suppliers.
PROBLEM: MATERIAL IS NOT DRYING CAUSE:
Tom Martin, head service technician for Motan Inc., highlighted several leading sources of this problem. “First, the customer might not be allowing the material to be in the drying bin for the recommended residence time,” he said. “In addition, filters and breather screens on the dryer may not be maintained at the proper intervals; and processors may also be neglecting to change the desiccant material in their dryers at the proper intervals.”
The solution is very simple, suppliers say: follow the directions and maintenance as dictated by operating manuals, particularly as regards regular changing of desiccant.
Also, if the material being dried is very hygroscopic, processors need to use a closed-loop conveying system that doesn’t introduce ambient air. “Typically, if a processor is running a closed-loop conveying system properly, they shouldn’t encounter any problems in drying material,” said Joe Corturillo, engineering manager at Wittmann Canada Inc. “And as long as the dryer is sized properly for throughputs, there shouldn’t be any issues in conveying the dried resins, either.”
PROBLEM: DROP IN THROUGHPUT 1ST CAUSE:
Pelletized materials require sufficient air to be transported, and there are several common reasons for airflow to weaken, resulting in a drop in throughput. “Clogged filters in vacuum receivers, vacuum takeoff boxes, and vacuum pumps are the most frequent causes of this,” said Novatec Inc.’s Les Mischaud, installation manager, and John Kraft, marketing specialist.
Mischaud and Kraft recommend inspecting and cleaning filters on a regular basis. “Also, don’t forget to empty the catch pans under cyclone filters if you have them,” they said. “If the pans fill up, dust can be pulled back through the system to clog the filters again.”
Another reason for a decrease in throughput is a vacuum leak — evidenced by sluggish conveying, longer fill times and pellets on the floor — which diminishes performance because the system is sucking air rather than material. “Vacuum leaks are one of the biggest causes of decreased throughput,” said Brian Davis, general manager at Maguire Canada Inc., which also distributes Novatec products.
“Other sources of vacuum leak include a break in a coupling, faulty solenoids or bad cylinders, or a dust cap that has come off,” said Jordan Lauder, a service technician at Wittmann Canada Inc. “It’s also possible for an abrasive material, such as a glass-filled nylon, to wear away at the inside of a conveying line, causing leaks and failure.”
Again, the solution seems obvious: locate and eliminate vacuum and material line leaks. “It’s a tedious problem to solve, though, because you have to break the system down and check each machine for any type of leaks, cracks, or anything else that’s allowing excess air into the system,” said Maguire’s Brian Davis. “The best way of doing this is to develop a methodology, involving breaking the line down into segments, and then testing each segment for conveying strength.”
If a processor is drying and then conveying resin, they may also want to think about installing either aluminum flex or stainless steel flex hoses, according to Wittmann Canada’s Jordan Lauder. “Standard polyvinyl chloride flex hoses aren’t constructed for these temperatures, and may melt and collapse,” he said.
And if a processor is conveying an abrasive material, they may want to consider investing in stainless steel elbows or ceramic line bends that will protect the material line, Maguire’s Brian Davis advised.
This last point is particularly important, according to Gord Stowar, technical sales at Wittmann Canada, and highlights a larger issue regarding material handling systems. “When installing a system, it’s not enough to know simple conveying distances,” he said. “Processors also have to be aware of changes in direction and elevation, and share that information with suppliers.”
PROBLEM: GETTING RAW (VIRGIN) MATERIAL INTO FILTER CANISTERS CAUSE:
Overfilling the receiver, often as the result of fill times that are too long, typically causes this problem.
“Each receiver needs to be adjusted individually based on the size of the unit,” said Motan’s Tom Martin. “The purge time is the biggest factor; the customer wants as much material per pull as possible, but neglects to count the purge cycle which clears the line of all material into the equation.”
PROBLEM: CAPACITY LOSS 1ST CAUSE:
“Adding any new equipment — dryers, blenders, loaders, etc. — to a conveying system will have an adverse affect, because the system was originally designed and sized to operate without that additional capacity,” said Wittmann Canada’s Joe Corturillo.
“The problem can be detected by knowing your system and finding out where the capacity loss is occurring,” Corturillo continued. “The best solution is to bring your supplier into the equation when making changes or adding new equipment, so the system will be properly sized to handle the additional capacity.”
This problem typically originates when a processor doesn’t have the right information about their vacuum pump, especially relating to pounds per hour. “Processors that purchase equipment at machinery auctions may wind up acquiring an undersized pump if they’re not careful,” said Maguire’s Brian Davis. “Some processors also add new machinery and then try to cut corners by using an existing pump that’s too small.”
Ensure that a vacuum pump is sized for your location. “Power systems operating at 50 Hz rather than 60 Hz can cause the pump to turn more slowly, and a processor will need a bigger pump or one that turns at higher rpm to compensate,” said Conair’s Doug Scott. “And don’t assume that a system spec’d for one city will operate the same way in another. Thinner altitudes, for example, mean the pump will be moving less mass of air with each revolution.”
PROBLEM: LACK OF MATERIAL CAUSE:
Material is not reaching the vacuum receivers beyond a particular station in a common-line system. Common causes include plugged material lines, or something as simple as an empty Gaylord or surge bin.
“Decrease the load time; or if the amount of material loaded in a single shot needs to be increased, add a spacer below the check valve or switch to a larger receiver,” said Novatec’s Les Mischaud and John Kraft.
Plugged material lines are typically related to a lack of transport air, they continued. “With virgin pellets, simply remove the material line from the material source and the plug should break up. If this works, slightly increase the air flow by adjusting the air mixing valve to avoid a repeat of the problem.” Regrind material can often pack the lines hard enough to require manual removal with a rod. “Sufficient transport air is the best preventative,” they advised.
Also, make sure employees regularly check material levels. “This sounds like a simple thing, but having employees check material levels helps avoid costly downtime,” said Conair’s Doug Scott.
At approximately $2,000, automatic tilt tables are an inexpensive way of ensuring consistent material flow from a Gaylord, said Wittmann Canada’s Gord Stowar. “Tilt tables are also ideal for helping improve material flow if the material is clumping.”