Setting a Baseline
If extrusion processing is a highway, baselining your system to establish the expected throughput is the best way to kick the tires before hitting the road. If you don't, you might wind up driving blind.
April 1, 2013 by Mark Stephen, editor
Taking measurements to establish baseline data for an extruder system sounds pretty dull. Dull, but important. Simply put, when a new single screw or twin screw extruder is installed, or a new screw mounted in an existing machine, it’s vital to know what the expected throughput rate is going to be. If you don’t, the job of setting realistic expectations for its performance becomes next to impossible. Want to get inspired about it? Equate the process with opening up your new car on a straight, lonely stretch of highway to find out what it can really do.
But there’s a difference. Dropping the pedal on that car is pretty straight forward: floor it and hang on. But baselining your extruder is an intricate process, and you’d better understand the ins-and-outs if you want to reap the benefits.
ZONING IT IN
Just as you’d check with your car dealer for important details about what’s under the hood, it’s a good idea to bring the extrusion machine vendor into the baselining process at the outset. “When purchasing a new extruder or a new screw, ask the vendor what theoretical throughput rate you should expect to get, what temperature profile they would recommend, and also about any metallurgical characteristics that might be unique to that model and that could affect performance,” said John Effmann, director of sales and marketing for Entek Extruders. “We can also help with potential changes — barrel lengthening or shortening in a twin screw extruder, for example — that might be necessary if the processor has a machine bought on the used market that they want to convert and baseline for a different type of extrusion process.”
And when replacing a screw, take a few extra minutes to inspect the alignment of the extruder. “You don’t want to put a new screw into a machine that’s out of alignment,” said John Christiano, vice president of technology for Davis-Standard LLC. “This is also a good time to perform a general maintenance check of the used extruder, changing oil filters if necessary, calibrating pressure transducers and thermocouples, and ensuring that barrel zones are working properly.” In the past, when using a single-stage screw, most extrusion operators would use either a “flat” barrel temperature profile or an “increasing ramp” profile. “These temperature profiles would work, but they weren’t optimal,” said Timothy Womer, president of consulting firm TWWomer & Associates LLC. And now? “Using today’s barrier screw technology, much more sophisticated barrel temperature profiles must be used to optimize extruder operating conditions,” he said. Many experts recommend using a “hump” temperature profile, in which the first barrel zone is set at the normal zone 1 setting, but zone 2 is set as much as 75°F to 100°F higher; the remaining zones decrease in temperature uniformly to the point where the last barrel zone is set approximately 10°F below the desired melt temperature.
An important detail? “The melt temperature should always be measured using a handheld pyrometer for the best accuracy,” Womer said. “Using immersion probes in a flow adapter, or infrared guns, will often produce false readings due to their inherent limitations.”
Also, get familiar with the polymer that’s going to be used. “Understand the resin before it’s processed. Know the resin brand, the part number, the lot number, and any rheological information that you can obtain,” said Mike Puhalla, general manager of the extrusion business unit of Milacron Plastics Technologies. “This can save troubleshooting time down the road if the processor changes resin suppliers and suddenly starts to see process drift.”
This also means understanding how using regrind can affect the process. “Adding regrind to the feedstock drops the throughput rate in proportion to the percentage difference in the overall bulk density of the feedstock,” Timothy Womer said. “So, if 100% pellets are being fed into the extruder with a bulk density of 32 lbs per cubic foot and the blend of pellets and regrind weighs only 29 lbs per cubic foot, the actual throughput rate could drop by approximately 10% if all other conditions are the same.”
One last thing: don’t forget to look downstream. “The processor is not going to obtain the maximum production rates from their extruder during baselining without having adequate downstream cooling and handling equipment in the extrusion system,” said Antonio Pecora, vice president of Custom Downstream Systems Inc.
GATHERING THE DATA
With the preliminaries out of the way — the new extruder or screw is installed, and the hopper has been filled with the resin for which the screw was designed — it’s time to determine the baseline capacity as throughput rate in pounds per hour.
“Begin with a series of rate checks, typically carried out at 20%, 40%, 60%, 80%, and 100% of full screw speed — or 25%, 50%, 75%, and 100%, depending on the extruder size, how much resin you can afford to sacrifice, and the time allowable,” Timothy Womer said. “At each screw speed, three two-minute weigh samples should be taken.” On larger extruders — 4.5 inches and above — the sampling time can be reduced to one minute, the experts say, in order to conserve material. And you shouldn’t worry about damaging the extrusion system by running at full screw speed. “If extrusion screws are designed properly, they typically will be able to withstand the maximum torque available from the extruder and the resin being processed,” said Matt Sieverding, general manager of extrusion technology for Krauss-Maffei Berstorff in the U.S.
Next, weigh each sample on a set of digital scales that are accurate to within at least one decimal place. A common mistake? “Too many processors weight samples with a pallet scale, which only measures to within plus or minus 0.5 lbs,” said Timothy Womer. “When this inaccuracy is carried over into the throughput calculation stage, it can produce an inaccurate result.”
With the throughput rate data-gathering method clear, the other information needed to baseline the extruder includes screw speed, sample rate, melt temperature, drive motor amp (or per cent of load), head pressure, and the barrel zone settings. “All of this can easily be captured in a simple spreadsheet,” Womer said. “To make it simpler, use ‘S’ and ‘A’ on your table to distinguish between the ‘Set’ and ‘Actual’ barrel temperature settings.”
REAPING THE BENEFITS
With the baseline data taken, simply store the information in a handy place — not a difficult task with modern extrusion control systems. ”Today’s controllers have recipe storage capabilities that allow the processor to easily save and recall baseline information,” said Mike Puhalla. “The goals are not only to lock in the baseline parameters and set performance expectations, but also to trend a variety of process variables such as temperature profiles, screw speed, and discharge pressure.”
A big benefit of that trending information — gathered by periodic repetition of the same test baseline method — is to avoid downtime, or even prolonged shutdown, in the future by scheduling preventive maintenance. “We have several wood-plastic composite extrusion customers that can predict, based on their baseline information and upon how many hours their machines have run, exactly when they’re going to need a screw rebuild or a barrel replacement; the wear can be calculated that precisely,” said John Effma
nn. “And while not every extrusion application is this wear-specific, a good many are.”
And if the throughput rate — and thus the wear — is tracked on a regular basis, capital expenditures for a new screw or barrel can also be planned in advance instead of biting you on the backside out of the clear blue.
In the end, the benefits of baselining seem well worth the time, effort, and — possibly — the hassle up front. “Taking the time when a new extruder is installed, or a new screw mounted in an existing machine, followed up with an hour of time during a shutdown to gather data every six months and then tabulating all of the information could have a significant affect on a company’s overall profit at the end of the year,” said Timothy Womer.
It certainly makes better economic sense than the gift you’ll get from Officer Friendly when he catches you conducting that speed test on the highway.
Custom Downstream Systems Inc. (Montreal);
Davis-Standard LLC (Pawcatuck, Conn.);
Auxiplast Inc. (Ste-Julie, Que.);
Milacron LLC — Cincinnati Milacron Extrusion Systems
(Batavia, Ohio); www.plastics.milacron.com; 513-536-3320
Entek Extruders/Entek Manufacturing Inc. (Lebanon, Ohio);
Krauss-Maffei Corporation (Florence, Ky.);
TWWomer & Associates LLC (Edinburg, Pa.);