DRYERS: The Necessary Nine
...tips for a better drying result, that is. We're not saying we've covered it all, but here's what you need to know as a bare minimum.
April 1, 2013 by Mark Stephen, editor
When it comes to drying polymers, figuring out the “why” is easy: to control viscosity and ensure maximum performance. It’s the “how” that can keep you up at night. Resin drying — especially involving hygroscopic materials that pull moisture from the air — is a complicated job, and it’s hard to suit the action to the word. Telling the guys on the shop floor to dry some PET at 250°F for three hours is one thing; having them do it correctly can be a whole ‘nother ball game.
The totality of information about polymer drying has filled a few books, at least. Looking for something less exhausting? Here are nine details you need to know.
CHECK YOUR DESICCANT
A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container. If you think it’s important for drying polymers, you’ve just passed the entrance exam for Plastics Processing 101.
But when was the last time you actually checked your desiccant? Do you know if it’s still active? And if not, why not? “Desiccants can become contaminated or ruined by off-gases from processing a variety of resins, but the number one factor that results in desiccant breakdown is plasticizer: it can weaken desiccant permanently and irreversibly,” said Mark Haynie, dryer sales product manager for Novatec Inc. “Having a good plasticizer removal system is important in preventing this.”
And if you dry cellulosics, TPUs, flexible PVCs, TPVs, and other soft-touch resins, check the desiccant’s condition every six months, because these materials emit volatiles that can ruin desiccant also. Under normal conditions, some dryer suppliers say, desiccant will last for about three years. A good rule of thumb? “Change it every 18 months,” Haynie said. “Desiccant isn’t cheap, but product recalls are much more expensive.”
COOL YOUR DESICCANT
Ever experienced resin granules melting together in your hopper to produce a single, Baby Huey-sized pellet? If so, don’t blame your material supplier. “Most of the time, melted granules are caused by the dryer bringing the newly regenerated desiccant onstream before it’s cooled down properly,” said John Bozzelli, founder of consulting firm Injection Molding Solutions. “With some dryers, you can get a slug of 350°F to 450°F air going into the drying hopper when the system switches desiccant beds.”
MEASURE YOUR DEWPOINT
A quick science lesson: dewpoint temperature is the temperature at which the air can no longer “hold” all of the water vapor which is mixed with it, causing some of the vapor to condense into liquid water. The dewpoint is always lower than (or equal to) the air temperature.
For the processor, measuring dewpoint of the drying air doesn’t tell you if the resin is dry, it just tells you whether the dryer has the ability to dry the resin. Some equipment suppliers recommend running the dryer at less than -25°F dewpoint. “A good dryer should run at -40°F dewpoint,” John Bozzelli said. Others aren’t convinced. “Dewpoint tends to be an overemphasized aspect of polymer drying,” said Joe Dziedzic, group leader, material handling for AEC Inc. “In most cases, -25°F dewpoint is good enough for effective drying. While it’s true that being able to go below -40°F is the sign of a quality dryer — and also a good marketing gimmick — it’s an incremental change that’s usually unnecessary, although more and more customers are insisting that their dryers have that capability.”
Using an extremely low dewpoint simply expends a lot more energy to dry a little faster, some dryer suppliers say, which should only matter to processors who are constantly changing resins. An alternative for the rest of you? “Use a larger hopper,” Mark Haynie said. “Since hoppers don’t require electrical power, increasing the size won’t add anything to your production costs.”
LOOK FOR LEAKS
Nothing can ruin an otherwise first-rate dryer faster than a leak (or a crushed hose). “In general, all leaks go from the process side to the regeneration side,” Mark Haynie said. “A leak causes a loss in process air flow and introduces wet return air into the regeneration, which makes the regeneration less effective and forces the regeneration heaters to work much harder.” The solution is about as simple as you’d think. “Check for leaks and crushed hoses,” John Bozzelli said. “Do it, and do it right the first time.”
CHECK REGENERATION HEATER ELEMENTS
Don’t ask us why, but this is a maintenance step that many processors seem to be ignoring. “It will astound you to discover how many regeneration heater elements aren’t functioning correctly in the typical plastics processing plant,” said John Bozzelli. “If they aren’t working properly, you won’t regenerate the desiccant properly, and it won’t dry your resin as it’s intended to.” Tip: put an “idiot light” on the control panel to tell you if the resistance of the elements in the regeneration heater is correct, Bozzelli said.
MAINTAIN PROPER AIRFLOW
Having adequate airflow through the hopper is a big deal in drying. But how do you maintain proper airflow over time, and how do you determine if the airflow is being compromised? “Measure the material temperature profile from the top of the hopper to the bottom; if the correct drying temperature profile exists in a properly sized hopper, then it’s safe to conclude that the airflow is sufficient and the material will be properly dried,” said Jamie Jameson, dryers product manager for The Conair Group. “If the temperature or volume of drying air begins to fall away from setpoint — if a heater blows out, for example, or an air duct becomes crimped — it will no longer be able to heat the full volume of material in the hopper.” Tip: get yourself a quality drying monitor that will recognize this condition and alert your operator, Jameson said.
TEST YOUR RESIN
Testing some dried resin for moisture content might just be the only sure way to know if you’re drying correctly. “There are a number of ways to measure moisture,” said John Bozzelli. “However you do it, make sure your method is measuring just moisture, and not all the volatiles driven off a sample of pellets.”
DON’T OVERDRY IT…
Overdrying resin generally lowers the material properties, and can stress the polymer to the point where the finished parts are brittle. It happens all too often, especially in the winter months — and you wouldn’t believe some of the tricks molders use in trying to remedy the problem. “I’ve seen processors who, knowing they were overdrying their nylon resins, tried to restore moisture by soaking the finished parts in tubs of water,” said Mark Haynie. No word on whether this works or not, but it sounds a lot more difficult than simply watching your process to avoid overdrying in the fist place.
…BUT DON’T UNDERDRY IT, EITHER
On the flip side of that coin — and usually a summertime phenomenon — are resins that are either underdried or allowed to regain too much moisture. “Once dry, most resins will pick up moisture in 15 to 30 minutes,” said John Bozzelli. “Therefore, don’t convey from a central hopper to the machine, or leave it in the machine’s hopper, without a blanket of dry air.”
Speaking of hoppers, moisture absorption — which can result in splayed parts — often results from residence in a poorly designed or poorly insulated hopper. “Resin flow in the hopper should be uniform, so that pellets don’t spend too much time along the wall instead of in the centre of a hopper,” said Joe Dziedzic. “Tall, skinny hoppers tend to offer better mass flow than hoppers that are short and wide; but regardless of which shape you have, the resin flow will be better if the hopper is kept full.”
Looking for a good rule of thumb to apply to any hopper? “If the hopper isn’t cool to the touch, it’s under-insulated,” said Mark Haynie.
There’s a lot that we’ve left uncovered, including the proper angle of the hopper bottom cone, which hoses to insulate, and where to vent and where not to vent the regeneration air. Your equipment supplier can help with those and many other questions — or you can read one of those books we mentioned earlier.
AEC Inc. (Schaumberg, Ill.); www.aecinternet.com; 847-273-7700
EquiPlas (Toronto); 416-407-5456
Auxiplast Inc. (Ste-Julie, Que.);
The Conair Group (Cranberry Township, Pa.);
Dier International Plastics Inc. (Unionville, Ont.);
Industries Laferriere (Mascouche, Que.);
Injection Molding Solutions (Midland, Mich.);
Maguire Canada/Novatec Inc. (Vaughan, Ont.);
Barway Plastic Equipment Inc. (Vaudreuil-Dorian, Que.);