oss-in-weight feeders arethe most important pieceof auxiliary equipment forcompounding extruders.Feeding errors can not only pro-duce off-spec product and down-time but can also cause machinery

damage and personal injury.If anything, the importance

of feeders has grown in the lastfew years. During this period, twin-screw compounding machinery withhigher speed and torque has madeit possible to increase productivityby over 100%. These higher-speed machines (over 600 rpm)need loss-in-weight feederswith much better short-term accuracy than in thepast. Disturbances in feed-ing—for instance, duringfeeder refills—that are tol-erable at 400 rpm may not beacceptable at 600 to 1000 rpm.Plastic in a twin-screw com-

pounder run-ning at 1000rpm has a resi-

dence time ofonly about 1 to 1.5 sec

between entering the feedthroat and starting to melt.That’s too short a time for a

feeder to correct any fluctuation,thus leading to the possibility of atorque surge that could shut down

the machine. (Such occurrences havenot been uncommon.)

Feeder manufacturers haveimproved their products to meet

the accuracy needs of high-speedcompounders. But it’s impor-

tant that plant engineeringand operations staff under-stand this and other issueswhen they select and operate

a loss-in-weight feeder system.The “system” concept is critical,since processors need both goodindividual feeders and a good

Myth #1:Relax, everything’s fine

because your feedercontrol says it is.

By Steven Jackson of Coperion, Clem Iannello of PolyOne, and Fred Burbank of

ILLU

STR

ATIO

NS

BY

DEN

NIS

DIT

TRIC

HREPRINTED FROM:

A GARDNER PUBLICATION / JULY 2002WWW.PLASTICSTECHNOLOGY.COM

-1-

overall feeding system, from the refillmechanism to the extruder, if the com-pounding line is to operate efficiently.

For proper selection and operation ofa loss-in-weight feed system, processorsmust avoid some prevalent myths aboutfeeders that can lead to endlessheadaches and lost productivity:

A ctually, multi-flight, single- screwfeeders have been shown to feedjust as accurately over a 1- to 2-sec

time frame. Twin-screw feeders are ver-satile and convey free-flowing powderswell, but they have limitations. Shear-sensitive materials can bind up in twin-screws causing erratic flow, material

10MYTH #Twin-screw feeders arealways more accuratethan single screws.

Compounding Engineering Solutions

Feeder controls don’t display real-time loss-in-weight data. They average feed-rate dataover time. The longer this interval is, the more accurate the feeder will look, by averagingout short-term fluctuations.

ABOUT LOSS-IN-WEIGHT FEEDERSFOR COMPOUNDING EXTRUDERS

-2-

TOP 10 MYTHS

agglomeration in the screw, and possi-ble screw damage. Small, improperlyconfigured twin-screw feeders may jamwith pellets or hard granular feed-stocks. And replacement screws for atwin-screw feeder cost 50% to 100%more than one for a single-screw feeder.

R unning a test is a great way toevaluate feeder performance andthe overall capability of the ven-

dor. But just any test won’t do. Thetest should have a written plan basedon your specific objectives and shouldbe run under conditions as close to thoseof your production as possible. Thetest should use the same feedstock youuse, and the feedstock should be con-veyed the same way. Powder that is air-conveyed in dilute phase, for example,flows differently from powder that’smore gently handled, which can haveup to 33% higher bulk density.

Be sure to evaluate feeder perfor-mance during multiple refills. Also don’tbe afraid to give the feeder a whackwhile it is running to see how it behavesduring unplanned disturbances.

Most feeding problems start with

the refill system, so the test shouldmimic the proposed refill system too.Visiting your vendor lets you learntheir rules of thumb for equipment set-up, such as how many refills theyexpect per hour or at what hopper levelthey expect to start refills. Also, evalu-ate how often you will need to clean andreconfigure the feeders. Certain pig-ments and additives require hoppersand feeders made of special plastics orhighly polished steel in order to be eas-ily cleanable.

W hen installing a feeder, look atthe environment in which theequipment will operate. Vibra-

tion must be minimized. Vent lines forthe refill bins and the feeder (essentialto prevent choking the system) must nottouch equipment on the feeder scale.

You can also be sure someone willdrop or ram a feeder sometime duringa start-up, when there tends to be lotsof activity around a machine. So youmust investigate the durability andreplacement cost of scale mechanisms.Some are more expensive and harderto get than others.

Y our instrumentation and controlengineers and process engineersshould be involved in the selec-

tion and planning of the feeder controlsystem, but your lead operators mustplay part in the detailed design. Engi-neers often want all the bells andwhistles on a control system. That’s finein principle, but the actual operatorsshould make sure that the screens theywill have to work with are clear and log-ical. Thought must also be given toappropriate security levels for screens(usually three levels: operator, plantmaintenance, and engineering). Oth-erwise your feed system is likely to havea slow start-up because it will be moredifficult to debug.

N ewer scale electronics and bettercontrol algorithms make vibra-tory feeders every bit as accurate

as screw or belt feeders. For productsthat are very sensitive to overhandling,like chopped glass fibers, the vibratinghopper, sometimes combined with aslow-turning single screw, is a viablesolution. The accuracy of a vibratingfeeder handling chopped glass at 330 to760 lb/hr for a 35% glass-filled nylon 66compound was recently shown to havea standard deviation of 0.015% over 22test runs.

L iquids differ from solids in thattheir density is constant at a con-stant temperature and they rarely

vary in flowability. A positive-displace-ment pump can feed almost any liquid

9MYTH #You should perform somekind of test on a feeder

before you buy it.

8MYTH #Where you install the

feeder on your compounderisn’t that important.

7MYTH #It’s an engineer’s, not anoperator’s, job to specifythe feeder control system.

6MYTH #Vibrating loss-in-weightfeeders are complicatedand not accurate enough.

Myth #5:Sure, loss-in-weight feedersare a little more complexthan volumetrics, but they’re the only way tofeed liquids accurately.

5MYTH #Only a loss-in-weight mechanism can feedliquids accurately.

-3-

with a viscosity of 0.5 to 100,000 cps at±1% accuracy. Piston or diaphragmpumps work in the 0.5- to 100-cpsrange. Gear pumps can handle higherviscosities up to 100,000 cps.

If you need to monitor a liquid feedrate, a Coriolis-type meter, which mea-sures flow disturbance, or an integrat-ing scale on the feed reservoir may beused to indicate real-time feed rate andto alarm when it’s outside the targetrange. But the feedback loop for liquidrate control can often cause more prob-lems than it solves.

T he importance of solid technicalsupport within the vendor’s repnetwork cannot be overesti-

mated. With the need to limit down-time, it is essential that you have agood local rep, who can serve yourinterests with the feeder manufac-turer. Most feeder manufacturers dealwith general equipment representa-tives that sell a variety of equipment.If your local equipment rep is some-one who gives you his home phonenumber and has good contacts atvendor headquarters when you needa favor, stick with that individual. Heshould also have enough technicalsavvy that he can help out in yourplant if your lead maintenance manneeds a hand.

T ypical side feeders have severalitems not found on regular hop-per feeders for solids, including

wear-resistant construction, relativelyhigh-torque screws, fully intermeshingscrews, and metal rather than nylongears for torque transmission. In somecases, solids (hopper) feeders used asside feeders have been destroyed afteronly three to four days on stream. If thefeeder salesman says his solids feeder

will function as a side feeder, ask for awritten guarantee on its performanceand service life.

A volumetric feeder, well-matchedto the material, could performas well as, or better than, a poorly

matched gravimetric one. Gravimetricsare just a mechanism to control andmonitor the feeder. They cannot over-come a feeder design that doesn’t con-sistently move material from the feedhopper to the conveying screw or belt.

Every loss-in-weight feeder shouldhave the ability to run in volumetricmode to let you track actual weight lossover time. When evaluating differentfeeding systems—for example, a vibra-tory feeder vs. a screw feeder—exam-ine the accuracy of each feeder in volu-metric mode to see the inherent accu-racy of the feeding mechanism,separate from the ability of the gravi-metric controls to dampen system insta-bilities. Controls can be tweaked inthe field to improve performance to acertain extent, but the wrong feederdesign for the material will still requiremodification or possibly even replace-ment with a different model.

K eep in mind how feeder rate iscalculated. The displayed rate isan average of the loss in weight

of the hopper over a certain time span.The longer this time span, the more thedisplayed value tends to smooth overany short-term fluctuations. If your lineis running at over 600 rpm, you can’tignore short-term fluctuations, even ifyour data-collection device does.

The easiest way to see if the feederis on the money is to look at the DriveCommand, which shows motor rpm for

a screw feeder or belt speed for a beltfeeder. If the Drive Command is fluc-tuating ±5% and the display says thefeed rate is ±1%, the algorithm for cal-culating the feed rate is hiding short-term fluctuations.

—Edited by Jan H. Schut

4MYTH #It’s the feeder brandthat matters, not the

local rep.

3MYTH #Using a hopper feeder

instead of a side feeder cansave you money.

2MYTH #A volumetric feeder isn’tnearly as accurate as a

loss-in-weight one.

Myth #7:It’s the engineers’ job to specify the bells and whistleson a new feeder control system. If they’re happy with it,machine operators will learn to love it, too.

Steven Jackson is manager of process engi-neering at Coperion Corp., Ramsey, N.J.,which builds twin-screw extruders andkneaders and designs compounding systems.Tel: (201) 327-6300, www.coperion.com

Clem Iannello is a senior process engi-neer at PolyOne Corp. in Cleveland, one of theworld’s largest compounders. Tel: (800) 866-POLYONE, www.polyone.com

Fred Burbank is chief operating officer atCompounding Engineering Solutions Inc., atoll compounder in Clifton, N.J. Tel: (973) 340-4000, www.compes.com

1MYTH #If the feeder display says the

feeder is running at ±1%accuracy, everything’s okay.

PT

And now for the mostcommon myth of all . . .

Reprinted from the July 2002 Plastics Technology Magazine and Copyrighted ©2002 by Gardner Publications, Inc., 6915 ValleyAve., Cincinnati, Ohio 45244-3029.

-4-