CA2167782A1 - Extrusion surge controller and method - Google Patents

Abstract

An apparatus is provided for extruding a polymer which is subject to periodic surges. The apparatus includes a barrel with a upstream portion for receiving a polymer, a downstream portion and a discharge port near the downstream portion. The apparatus also includes a shaft mounted for rotational movement about a longitudinal axis within the barrel as well as drive means connected to rotate the shaft. The apparatus has a surge suppressor for urging polymer upstream toward the discharge port while permitting a portion of polymer to flow downstream into the surge suppressor. A sensor is positioned within the apparatus to detect surges in the polymer and a controller is connected to the sensor and to the drive means for adjusting shaft rotation in response to polymer surges.

Description

21~g~

E~'r~ 10~ ~l,~GE CO~OLLE~ AND MET~OD
lhi~ applieation is a contint~ on-in~part of ~pplicatio~ Sc~ial No. 0~ 2505909 filed May 315 1~4.

;l~k~oPlld of the In~ on This illvention pro~ides aII ~p~rd~ or re~uc~ or elimina~ pressure and ~ow rate surges i~ polymer e~trusion mA~.hines. It aLso rel~tes tn a me~od of e~trudin~ wi~
a substantiahy constan~ polymer pressure. I~ pa~icular, tbis iIlvention provides an e~tmsion apparatu~ and ~ethod util~ing a surge s~ r~ r incorporat~d into the e~t~ude~
screw.

~;eld of the In~ention Surges within p~ly~er e~ ders are recog~ed as a ma50r ~r~blem faced ~y dle e~t~usio~ industry. Surges are output var~ationS from an ext~uder screw correspo~ding to ~ariations in po]$~me~ pressure aIld chaDges i~ poly~er flo~ r~te. ~ccord~ngly, sur~es are nearly synon~mous in the extrusion indus~y with yres~ule and Q~w variations. Put simply, sur~es ~e like waves wherein m~um output and pressure ~ccur at t~e top of ~he w~ve an~ mi~mum out~ut and pressu~e occur at the ~ottom of the waYe. W~e~ a ~have-lilce surge arriYes at ~e discharge end of ~e ext~usio~ ~crew. t~2ere will be a O ~orrespnr~din~ sulge in discharg~ p~essure and f~ow ra~. Accc rd}ngly. ~ ~ taneous pressure or flow rate su~ge will produce an inst~nt~n~ous sur~e at t~e e~tr~sio~ die.
P~essure a~d flow variations at the extrusi~n die are know to resuit in d~me~sio~l a~iations in the e~ctruded p~oduct ~uch dime~siorLal va~ia~i~>ns create severe problems, e~pecially when it is desi~ed or !l~cess~ry to e~ude nlbe, rod or ot~er s~pes ha~ng ~16~78~

ught tol~ra~lces. ~im~nsio~al t~ariatior~ may result in ~.he extrusion of large ~l~nt~
of e~pe~si~e m~terials ~n~o usele~s products ~Ioreove:r, pre~su~e a~d flow cha~ges at thç
extrusion die cause d ~enslonal variati~ along the length of an extrusion. Inspection o~ one portion of ~e ex~u~ed ~ro~uct may result in different results from vther po~tions, 5 reducillg predictabili~y. Dimensional and other va~i~eio~ resu]ti~g ~om p~lymer s lrging ultimatel~ resu'~ ateri~3 ~cte, product reje~tion, and other i~e~fictencies.
Var,ous ~empts have bee~ r~ade to efficie~tly a~d effectively control v~tion~
in extru~er outpu~. For exsmple, val~e control of e~truder out~ut was co~sidered i Pattersoll et a~, The Dyn~nzc ~eh~u~our of l~xtruders, SPE ~N~C, pp. 483487 (1978).
lC~ A~so~ ntrol of melt tempera~re and pressure by continuol}sly varying screw speed cou~led wi~ infrequent vanado~s i~ die resist~nce was co~side~ed i~ ~'a~aby et al, Deve~opme~t of Computer Co~rol St~egies for Pla~'c F,xtTuders, Polym. En~ Sci., ~lolume 15, ~o. 8, pp. 594-~5 (1~75).
Pr~ssu~e controilers have bee~ used in ~O~JUIlCtiO~ wid;l ~ea~ pumps to compensat~
15 f~r e~usio~ pressure flUCtU;ltiGIlS. ~r pumlps are somet~mes positioned be~een an extruder ba,rel and an extruder discharge. Such pllmps generally have preci~e outputs as well as a fixed resistance at a p~ticular speed. However, increas~d e~truder out~ut has ?oeen known to ~e~erate very hi~h pressure between ~e extruder ~nd ge~r pump, s~rne~i~es c~usi~ the pu~p gears tO se~ze. By use of a pres~ure controller, e~ der 20 serew spe~l may ~e re~uced L~l ~n atternpt to col~1pcl~ate ~or pressure L~crease~
raised to compe~sate f~r pres~ur~ reductic~lls. ~xampl~s of sllch pressure co~trol1ers include those provide~ ~y Dyr~isco, Sharon, MA (Model r~lo. 660~ and Barber Colem~, Lo~es Park~ IL (~I~del ~o. g40).

213i7~ ~

P~essure c~ntrollers frequently ~tilize a pressure sensor positio~ed Ile~r the gea~
pump Con~ollers ar~ ge~erally ad~usted to i~,nore sb~rt-~.er~ p~essur~ fluctu~iorLs and to correct ext~uder screw speed in res~ sc to long-~en~ ~r~s~lre trehds.
Pressure controller~ u~ed ill conjuncti~n wi~h gear pumps ~>metime c~use uns~ble5 pressures when they ~tte~pt to correct for smal~ or sudde~ pressure fluctuation. There is in~erently so m e del~y betwe~ll the sensing of a pressure fluctuation a1ld ~he actual screw-speed chaun~e i~itia~.ed by the contr~ller d~e to i~stnuction delay auod sys~e m i ~ r~
(i.e., inhererlt inertia of the drive motor, dli~e belts, ~d~ ission ~nd other system com,r,onents). Delay causes the con~ er instruction to be out of phase with the ~ressure ~ucbuation and ~herefore m ~kes Lhe pressure flu~-tu~tio~ w orse.
Pressure controlle~ insta~ityis agy~a~at ~ by ~he c ~ o~c alud ILnpredictl~le llanure of extrlsi~n pressure fluct~ations. Snn~Il press~lre uncreases or decreases often do n~t indicatç contimlin~ pressure increa~es ~r decrea~es. P~i~s~ure fluctuations ~e often quite short in duration.
i5 Lee, in U.S. Patent No. 4,118,163, also recog~izod diff1culties ~ collt~ he w~i~rrnity of pu~ ing zone pre~sure ~d attempt~ ~ minimi7.e pressure and flow sur~es ~t the d~scharge end of an extr~er. Lee ~ovided a complicat~d screw extruder apparatus haY~n~ two ~eparate pul~lp~ zones~ a first zo~e fnr feeding pl~stic from a hopper and a secolld zol~e for ~llp~;g plastic back ~ow~d the fLrst zone ~3d out a later~l 20 exit orifice. The first and second zones were connected by a central bore formod i~ ~be second zone of t~e screw which communicated wit3~ rst 20ne through radial passageways. The ~ee extrude~ ~as an e~Fen~ e de~-ice requir~ng a ~ ly spe~ial~ed extruder screw.
Thes~ attempts failed to pro~ide a practical and effec~e apparan~s or method for25 reduc~r.~ pol~ner surging. ,4c~ord~ngl~, ~ere is a great ar~ thus fa~ unsatisfied dema~d 216~

f~r a p~actic~l apparatus and me~cd for r~ducing or eli~nin~tin~ surgin~ of poly~e~
ex~ ion processes.

Obj~cts of tl~e I~e~li4~l It is an object of this ~rr~re~tiQn to provide atl apparatus which oYerc~mes thepr~blerns assoeiated wi~ co~entio~a~ e~tmde~s.
It is another obje~t of the inYen~on to pro~ride a~ app~rdnls f~r re~ J~ o~
elimin~tin~3 su~ges of polymer dllring ex~usi~n processes.
It is another ob~ect of t~e invention tu pro~ide a surge supp~essing ex~der hA~ving 10 ~ e and ~elia~le means for redueing or elimin~tin~ su~g~s bef~re pressurized polymer ~eaches the e~usion die.
It is still another ~)bject of the tnve~tion to proYide a method for reducing ore]imin~e~ surges ~cnown to occu~ in con~Jenuor!al e~ usion processes.
It is a fi~rther objee~ (~f the invention to pro~ide a ~e~od for r~ducing or 15 eiimin~tin~ pol~mer sur~ing bv providin~ a m~ifi~ ~x~si~ screw together ~th a pressure controller.
Other }~por~ant o~ects of the isn~ention will become ~pare~t to orle of sl~ll ~n this ~rt i~ view of the fo'lowing descriptio~s, th~ ap~ended figures aIId ~ claims.

Sllmn~qry of the Inventi~n ~his invention pr~vides ~n extruder ha~ing a barrel and ~ sh~Pt moun~ for rota~ion wilhin ~le barrel. The sba~ has a cnn~eying screw flight for conveyi~ dmelling pol~ner pelle~ oduc~d into t.he ~ar~el throu~h a hoI~per A LuctcrL~g s~xew fltght meter~ ~e melted polymer ~ld deliv~rs the polymer to aII e~t~usion die. A surge ~uyyr~ss~ng screw tli~t on the shaft do~vnstre~ of ~e die u~ges polymer toward the die 2i6~7~2 ~hile per~.itting a pO~tiOIl of the polvmer to ~ow i~to o} pa~t t~e surge s-lppr~ss~ screw fli~ht. .~ ~eal is optionally pr~Yided near the surge suppressiIlg screw flight to prevent fu~e~ polymer f~ow.
The surge supp~ssor absorbs ins~antall~ous ~res~ure and flow increases. The S surge suppressor also c~mpensates for inst~n~neous pressur~ ~d flow drops.
According~y, t~e surge suppressor d~mpens p~e~sure ~nd fl~w surges to n~ n a substantially ~ iform pressure at ~e extrusion die.
The surge sup~ressor is optionally used in connection with a sclew-speed controller. The surge sllppressor pe~ts improYed coI~oller pelformance with minim~l co~t~o~ler adjus~en~.
This in~ ention ~Iso pr~vides a me~hod for reducin~ or elimiIl~tin~ ly~e~ preS~UI
and flow sur~æ~ durirlg e~trusion pr~esses. ~9 Ch~ft ~f ~rl ext~uder ~s provided wi~ a su~ge suppressi~g screw fli~ht. The surge ~ul~rcssing ~crew flight ~ener~tes a pressure less th~n or e~ual to ~e extru~r r~eteri~g s~rew a~d a portion o~ the p~ u, ~zed polymer t5 flow~ into the surge sl~l~p~ssing screw flight. A se~l is optionally pro~lded downstream of ~e ~urge supp~essing screw flight to pre~,~ent f~ eI polymer fl~wing.

Br;ef ~escription of t~ Drawin~
Figs. la ar;d lb are side ~iews of con~entior$1 e~tlusioD ~crew~.
2CI ~i~. 2 is a side view of a ~,e~nent o~ a cou~e~ti~al e~trusioll scr~w witbi~ a barrel.
Figs. 3~ ~nd 3b are side views of tuo fon~s ~f extrusion scre~s embodying fea~es of this in en~ion.
~ig. 4~ arld 4b are side ie~s of ~vo more forrns of extn~sion scre~s embodyi~, 2~ fean~res of ~lis in~enti~.

_5_ 21-1~7~2 ~ig. S is a side view of ye~ ot~er foIm of extnlsioIl screw embudying fea~es of this in~ention.
Fig. 6 is a ~ide Yiew of s~il~ another fo~n of ex~sion sc~ew embodying fe~tures uf this ~n~enti~ ~.
S Fig. 7 is ~ schematic vjew of an e~usion system embodyi~ fieatures of this invention.

~et~ed ~cr~,p~.o~of the Invention Tbe ~i~OWiII~ deseription is intended to refer to the specific em~odiments of ~etO inYention illustrated in the dr;lWUlg5. This description is not intended to define or limit ~he $~ope of the i~enti~, which is def~L1led separat~ly Ln the cla~ that follow. ~lso, the drawings refeIred t~ throu~hout ~e following descriptio~ are ~ot to s~ale and are not inten~ o r~ect actual dLmensions o~ proportions.
Pig5. 1~ and lb are pro~ded to iL~ustrate fean~res f~u~d in conventional e~rusion 15 sc~e~s utilLzed m conveJItional extrude~s. ~c e~;~usion screws shown in Figs. l~ a~d lb both have an upstream portion to ~e right and a downstream portion tO the left Referring to F.g. la, al~ extrusion screw S is dr n ~rom a drive end DR located at the upstr~m er~d of ~x~rusion screw S. 3ust dow~stream of drive end l)R~ h~pper pellets ~IP are introduced i~lto the ex~uder ba~rel ~not sbow~) within wb~ich extrusion 20 ~crew S is rotational~y mounted. Hopper pel~et$ HP ~re eo~veyed downstream and melted int~ mo]ten polymer in conveying ~nd meltin~ zone CM. Me~ted polymer is then metered in ~etering zo~e .~1. sometun~s re~err~d to ~s a pumping zone, locatell downstrea~ of conveying ~nd me~ting zone CM. Melted polymer is the~ disc~a~ged to an ext~usion die thrw~h an ~ial dischar~e D.

21677~ -Sur~es in press~zed melt~ ymer ~cur within meteriag ~one ~ he of pressu~e sur~es and flow surges. Such su~ges a~e caused, as desc~ibed a~uve, by extrusiorl scre~ rot~ion .speed variation~, variations in polymer temper~ure! v~iation~
in polymer supply~ ~d other cvmtnoniy e~countered p~rameter changes. Such ~ur~esc-ommonly result i~ pressure ~d flow rate ~urges at ~e extrusion die.
Re~erring tc, Fig. lb, ext~usion sc~ew ~S is driven .~rom a drive e~ at the downstre~ d of e~ ion screw S. Hopper pellets ~P are m~oduced into the barrel ~not sh~ nd are con~eyed alld me}~ ve~ ~d m~lting zone CM at the ups~eam portivr~ of el~sio~ scre v S. Melt~d polymer is ~eterecl iD metering zone just upstream of a radial extrusion die discha~ge D.
~n order to pre~ent pressunzed potymer from flowtng dow~ eam of ~ usion die dischar~e D and into a tra~mission meeh~llism ~not shu~vn; at drive end DR, 2 dynami~
seal DS is pro~/ided downs~eam o~ extrusiorl die discharge D. Seals similar to dy~nic seal DS~ also known in the i~dus~y as seal screws or vlscous seals, ~e co~ly use~
on ~ear pumps and on some d~um extruders. Dy~mic se~ls are also used in internal ~xers and vertical si~gle screw e~truders tO ~eep polymer melt away fr~m critical parts o~ proces~ machi~ery.
h~ost ex~uders dri~!e the extrusion screw ~rom the end opposite t~e ~xtrusion die.
In ~ther words~ the e;~trusion screw begins at the ~a~s~i~sio~ æt an upst~eam p~rtion and ~0 terminates in a point at the opposite~ down~tream end (see Fig. la). An ex~ple of sueh an extrusion s~rew ~as illus~ted ~y Adderley. ~r., i~ U.S. Patent No. 4~ ,451.
O~he~ extruders drive the e~trusion ~crew ~rom its dowDstream por~ion and l~e an ~xtrusion die ~ischarge be~-een ~e upstream and dowr~stre~m e~ds of ~he screw. TlLis tvpe of e~trusion screw S is sh~wn in ~i~. lb. An example of such a~ ex~der was also illustrated b~ Li et âl.~ in U.S Patent ~o. 4,69~724(). I~ c seals ha~e l~een used ill 216~78~

such e~truders to pre~ent melt~d polymer f~om ~nterir,g and fouling tr~nsmissionm~ch~sms attach~d tu ~rive ~e screu Referrillg t~ Fi~. ~b, dy~c s~l D~ preYerts polymer flow past extrusion die discharge D to drive ~nd DR. ~ccardingly. d~mamie seal DS is desi~ned to maxi~e pressur~ so as ta generate pressure greater than t~at developed i~ metenng zone ~.
l~ecause the pitch of the screw ~n dyna~c seal DS is opposite that ~f metering ~ e M
an~ ~he screw is desi~ned to gelle~ate maximum pressu~e, dyIlamic seal DS pumps melted poIym~r back up~l,ealll to ~e extrusio;l die a~ æals a~inst downstream polymer ~ow.
A variety c,f extrusio~ screws, ~ y of which included dyn~mic or ~iscous seals.
were d~,closed in the foll~w.ng pate~ Geier et ~., U.S. Pater~ ~o. 3,023.455; ~ tT~
et al.~ ~r.s. Patent No. 3!632~25~ atinen, U.S. ~ate~t ~'~. 3,7g7~550; Okada et al., ~J.S. Patent ~o. 3,802.~70; Shinm~to~ U.S. Pa~n~ No. 3~g~4?~417 Markei et ah, ~.S.
Patellt ~o. 4.~9~187; }~o~ssow, U.S. Pat~nt l~lo. 4,730,935; Sh~g,enji et al.. U.S.
Patent ~. 4~76~,676; Pena, ~T.S. Pate~t ~~To, 4!966,539; and Klein, U.S. Patent No.
lS 5,10~,286. ~he extrusion screw ~hown i~ U.S. Pate~t No. 3, 924,841, ir.corporated he~ein by referenee, has a re~erse thread ~rtion which sen~es to force ~ack the molten res~ to~ard the mixing zorle to prevent polymer le~ ge p~t the ~xtruder screw sha~k.
Fig ~ illustr~tes stluctural ~leme~ts of conventiona~ ~xtrusion screws. E~n~sionscrew S has a flight F heli~ally arrang~d at a h~l~ a~gle H. Pli~ht F is also known a~
'O a thread or screw. Flig~t F has a fli~ht wid~h ~W. The space be~ween adja~ent fl~hts de~mes a ch~nnel C between e~rusion screw S and an extmder barrel B. C~hannel C h~
a chamlei depth CD ~sometimes ~ow~ ~ ~hread d~pth) ~nd a eha~ne~ ~id~l CW.
Extrusio~ screw S ~as a sha~ ameter 1~ and ~x~usio~ sc~ew S is sized to fit wi~in barrel B h~YLrlg ~ ba~rel ~iameter Db.

~16~82 Rota~ion of ex-rusion screw S sl~n i~ Fig. 2 conveys polymer (n~t shown) in ch~nnel C in a downstream direc~ll. For e~ample~ rotating extrusion sere~ S in aco~nte~Ioçkwise direetion f~om the righ~-h~d side of F~g. 2 con~eys polymer towar~
L~e lef~-hand side ~f ~ig. 2.
Extm~io;l screws c~ ~f course be p~vid~d with a w~dc varie~n,r of dimensiotls, configurat~ns and shap~s. Meyer~ in IJ.S. Patent No. ~,21~.374, illu~trate~ a v~riety of extru~ion screw shapes.
Figs. 3a. 3b. ~a, 4b, 5 and & illustrate se-~eral embo~unen~s of the ext~u~ion su~ge suppres~or ~ccording t~ this inventlon This inventio~ is ~ot, however, l~ted t~ the 19 em~imen-~s illus~ated i~ the ~lgures, but instead is d~fi~ed separately i~ the appe~d~d cl~ilns.
Re~erring to ~ig. 3a, an extrusio~ scre~v is provided ~ith a met~ering zone M
do~ïlst~eam fro~ a con~eying and ~el~in~ ~one (not shown~ into which hopper pe~lets are introdu, e~ o~nstream of mete~ing zone ~ is ~ radia~ly extending 2xt~usi~n die i 5 discharge D. Fareher do~l~s~ie~n from extrusion dte d~scharge D is a su~ge suppressor SS fo~ suppressing mclted po~ymer pre~su~e and flow rate surges. ~urge suppre~sor ~;S
i~ ~s embod~eI2t gener3tes a pol~er pressure less ~ ~at gellerat~d in meter.ng zo~e ~, thereby allo~ ing some melte~ polymer to flow dow~stre~m t:l~rou~h surge supp~ssor SS. Sur~e suppressor SS is in dle T-or~fl of a screw flight havin~ a direction opposite ~at in metenng zo~e ~I. Acc~rdi~giY, sur~e suppressor SS pum~s ~ sub~ tial p~rtion of mel.ed polyn~r back ~owa~d meter~g zone M and extrusion die dis~harge ~. Details of prefer~e~ surge suppressor SS are provi~ed below.
A dy~,ic seal DS is provided on ex~sio~ scre~- S downstream from surge supplessvr SS. Dyn~lnic seal DS se~ls against downstream flow of the meited polymer ~5 that p~ Ps th~ough sur~e supp~essor SS. ~yn~ic ~ )S is formed from a helical g ~1~778'~

groo~,-e eu. in~o extru~ion screw S i~ a d~ection opposite to the ~crew flig~ts in meten~g ~o~e M Dyna~ic seal DS ge~erates a hi~h polymer pressure .gre~ter than th~t genelated ill metering zo~e M.
To gcnerate high polyl;ne~ pressure"ly~ ic seal DS is provided with a small heli~
angle H ~Fig. 2~ a s~allow chanr~l dep~h ~D ard!or ~ ~rrow cha~el width CW.
Dyna~ie seal DS is prefera~ly ~o~med ~ith a sm~l axial lengt~ t~ pen~t a shoIterextnision sc~ew S. D~ic SP~ll DS is pro~ided with a helix angle H not e~cee~
abo~lt h~.lf that ~f ~te~in~ zone M. Channel depth C~ in dynamic seal DS does not ex~e~d about half that of mete~ wne ,~. ~Is~, the axial length of ~y~amic seal DS
10 is l~ss than or equal tc~ about 25% that ot` meteri~g ~one L~i. Finally~ chanJlel wid~ CW
in dyn~mic sea~ D~ d~s not exeee~ abou~ 10% of the screw diamete~. ~ dynamic seal DS s~ designed generates a ~res~-e mtl¢h greater ~an m~tering z~ne ~ and preYents flow of me~ted pol~mer to the d~ive end DR of e~trusion s~re~ S ~nd i~to the screw drive mecha~ism ~n~t sho~n) attached to d~ive end DR.
lj ~e~errin~ to Fig. 3b, an extrusioll screw S is s~ilar tO t~t sho~n i~ Fig. 3~
except t~at ex~usio~ screw S is dri-~en f~om a drive end D~ at the upstream end of ex~usiol~ screw S ~rive end D~ Is proYid~d upsL~-eal~l ~om a con~eying and meiting zone CM irl~o whieh hopper pe~le~ (not show~) a~e Ln~roduced. D~wns~ea~ ~om conveying and meltlng 20ne C~ is a met~ zone M for l~lete~ g melted polymer and 20 dsli~eIing th~ polymer to a ra~ia31y exten~ing extrusi~n dii~ discharge D. I)ow~stream from ext~usi~ die dis harge D is a sur~e suppresso~ SS sixnilar ~o that descri~ed with reference tu Fi~. 3a As i~ Fig. 3a, ~e surge s~ppressing scre~ em~odlment s~own in Fig 3b h~ a dynamic seal ~S loeat~d dou,lslrta~ from surge suppres~or SS. Dynamic seal DS has a s~ucture similar to that des~ with refere~ce to Fig. 3a. l:tynamic seal 25 ~S prevents dowrls~am flow of melted polymer tha~ fl~ws thr~gh s~:rge suppressor SS.

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Accordingly dyr~nic se~l DS p~vents flow of pressurized melted polyme~ downstream into do~nstream po~tions of barrel B.
~ i~s. ~a and 3b both illustra~ embodiments h~v~ng a sur~e suppressor permitting dov~ tream fl9w of s~me polymer ~ith a dynamic s~a~ which seals agains~ far~er 5 downstre~ f~ow. Fig. 3a shows suc~ ~n embod~me~t driven from a d~w~stream end of dle ex~usion scre~-. Fig. 3b shows a~ embodilne~l ~herein the e%tmsion sc~w is dri fro~ its upst~e~l end.
Refemn~ ~ Fig. 4a, anc,~cr e~tlusion screw ~nbodiment is p~o~ided ~ h a meter~ ~one M downstream fro~ a conveying a~d melting ~one ~not s~ow~ vhich 10 hopper pe!lets ~re introduc~. r~feterlng zon~ ~I ~eters melted polyme~ and delivers it ~o a r~diall~ extending e~trusi~n die discharge D Downs~e~ from extrusion die dischar~e D is a surge ~upp~ssor SS sinul~ to those s~own in Figs. 3a and 3b. Surge ~u~pres~or SS pumps ~ portion of the ~e~ted polymer ~ac~ u~stream toward m¢te~ng7.0Ile ~ and e~usion die disch~rge D. Surge suppre~sor SS ~Iso ~ downstream 15 flow of a po~tiv~. of melted po1ymer The po~tion of melted polymer that flo-vs downstre~m p~st su~ge suppressor SS
exits ba~ l B thrO~!gh a radia~ly extelulin~ po~ymer discharge po~ 10. Radial discharge port 10 may 31so take the form of a bleed hole for ~h~ escape of small amounts of melted po1ymer. I~adial po~ymer discharge port 10 optio~ y le~ds to a læ,~kiclion valve (not ~0 shQw~ r any other known means of r~stric~in~ melted pGly~e~ flow. Oisch~rge of t~e pO~iOIl of pOIy~lET th~t passes downst~ea~l ~2rough surge suppressor ss and out por~ 10 pre-~nts foulin~ of ~nsmission mecl~isms at a driv~ en~ at ~he downstream end of e~rusion screw S.
Referrino ~o FJg. 4b~ an extrusion sc~w S similar to that sho~n in Fig~ 4a is 25 sho-4n! differing ma~nly Ln that extrusion scre~ S in Fig. 4b is driven from a drive end 21~77~7.

DR at tke upstream e~ of extlusion scre~ S. Down~t~e~m from dr~e e~}d DR ~ a con~eying ~nd melting 7C)I~e C~ itlto which h~pper pe;let~ are intro~uced. Melted polymer is rnetered in a metering ~o~e M downstream from conveyin~ d m~ltin~ zone CM for deli-~ery ~o a r~dially extend~ng exlrusion die discharge I~. n~wn~tream from 5 ex~usic~n die disc~arge D is a sur~e suppressor SS similar t~ that shown irl Fig. 4a.
The po~tion of melted polymer ~at fl~ws dow~stream tl~ou~ a~d past surge suppres~ior S~ exits barrel B through a1l a~ial polymer di~ch~rge 20. Axial polymer discharge port ~0 ~ptionall~ ter~ n~es at a rest~ic~on v~lve or other knowll rest~ictio~
device. Port 20 ~ay also be referred to as a ble~d ~ole Aceordingly, ~e s~lall p~rtion 10 of melted polymer th;~t flows dow~s~eam fro~ surge su~ressor SS e~ e extruderthrough port 2û wh~ie the majori~ of melted polsrme~ e~its extlusion die dise~arge D
upst~earn from surge suppressor SS~
Figs. 4a ~nd 4b bo~ show extrusion screw e~b~diments w~erein a surge s~l~ressor which pennits do~ passage of me~ted pol~mer is provide~ i~
15 con~c~n with a bleed hole and op~ior~l res~riction deYice. Fig. 4a shows a~
embodiment h~vin~ ~ surge suppressor comb~ed ~ith a radially extending bl~d hoIe.
Fig~ 4b sh~ws aIl embodimen~ ~ving a surge suppresso~ co~bined with an ~xi~lly exte~ing loleed hoie.
~efe~nng to Fig. S, yet anoLher eml~odinnent of an extrusion surge suppressor 20 aecording to this in~,entiGn is illustrated. This embodimen~ p~ovi~les an e~trusion screw S ~vin~ a me~ering ~one M downst~eam from a conveying a~d melting z~ne (not shown~
which supplies m- lted polymer~ tering zo~e ~I mete~s ~nd delivers pressunzed polymer to a r~ extending e~trusion d~e discharge D. Downstream from extru~ion die discllarge D is a su~ge suppress~r SS. Surg~ ~u~prtssor SS pumps som~ mel~d poIyn1er ~acl~ upstream toward metertng zone M and extmsion die discharge D ~hile 216~78'~

permitting a portion nf melted polymer tO flow fartbe~ downs~eam through surge suppr~ssor SS.
A~ O-r~ng 30 is captu~d within an U-r~ ~roo~e 4~ fonned in ba~l B. O-ring 30 prev~nts dow~slr~l, ~ow uf ~hc mel~ed polymer th~t passes throug~ surge suppresso~
5 SS. O-ring 30 is preferabJy formed from any known elastomeric mate~ial Wl~tever material 1S selected, howeYer, O-r~ng 30 should be ~pab~e of withstandin~ ~ eleva~d ter~peratures tn~intained du~ing extmsion processes.
O~ g 30 provides ~ circu~ft~ ial se~l against an oute~most surface of O~
groo~e 40 a~d a circumfere~tial seal against ~e surface of extn~sion screw S. These 10 seals pr~id~d by O-ri~g 30 preve~t passage of melt~d p~ymer t~ e drive end DR of e~trusion screw S! ~nereby preveMiIlg fouling ~f ally transmiss~on mecnanism attached tO drive end DR.
T~e e~trusi~n sur~e suppressor embodiment show~ irl Fig. 5 ha~ a surge suppressor in combi~tion ~ith a~ O-ring seal which seals-off the polyme~ ~at flows 15 through the surge suppressor. It is of course colltemplated ~ ugh no~ sho~n) th~lt the sc~ew in Fig. S could also be driven from a dri~e end lo~ted at ~e upstream end of th~
sc~e~- It is also contempl~ed ~t any o~er known mechanic~l se~l device subsLituted for O-ring 30 ~d O-r~ng groo-e 40.
Refemlng to Fig. 6, an extrusio~ scre~ S is ~gai~ provided with a ~eter~ zolle M which re~eives melted poiymer from a co~ve~ing and melti~g ~one (not sho~vn~
Meter~rg 7,0ne M mçte~ and deli~ers pressuri~d a~ ~el~ed polymer to r~dially extendL~g extn~sion die dischar~e ~. Downstream fro~ ex~usiun die discharge ~ ~
provided ~ ~urge suppressor SS simil~r to ~at shown i~ Fig. 5. Sur~e suppressor SS
purnps a por~ion of melte~ poly~er ~a~k towar~s meteri~g zone M and out extru~ion d~e 21S~2 discharge D. Anotber portion of tbe melted polymer flow~s downstre~n through ~t least a portion of surge suppresso~ SS.
A coolant rese~oir 50 is pr~t ided Wi~ill barre~ B at ~ posi~ion which ~ref~rably overlaps with surge ~uppressol SS on extrusion screw S. Coolant is circul~ted ~rough S coolant reservoir 50 to cool the melted poly~er in a portion of su~ge suppre$sor SS. As the melted poly~er is cooled, it tends to pre-~nt f~ e~ dounstre~n flow. ~cordin~ly, ~e portion of melted polymer w~ieh llo~s downsL~eam throu~h a portion of ~urge suppr~ssor SS is se~led against flowi~g ~ar~er downstream, thereby preve~ng fou~of screw transmissi~n rnech~nisms attaehe~ ~t dri~e end DR.
The enlb~diment show~ in Fig. 6 illustrates the ~omb~nation of a sur~e suppressor ~ith polymer cool~ng t~ redllce or elimi~te polyme~ sur~g ~lvhile pre~enting polymer leakage. It is of ~urse contemplated t~at dme end I)~ could 31so be located at ~e upstl~n e~d of extn~sion scre~ S. It is als~ contemplated ~hat cool~nt ~ese~oi~ ca~ be s~ uled for a:ly ~nown cooling means, includulg but not limited t~ a coil~d coolant 15 flow passage or e~en c~ ection cooli~g iTuluced by air flow around or th~ough ~e extnude~ barre]. .~lso~ coolant reservoir ~0 or any oth¢~ ooiing mea~ ~an be positio~d to coinci~e with surge sllppr~ssor SS, carl overl~p with sur~ç sup~ressor SS
or c~n be positioned do~nstream of surge suppre$~o~ SS.
Operation of the extrusion surge suppressor a¢c~rding to ~his invention wil~ now20 be described with ,eference t~ FIgs. 2 and 3a. In essence~ the surge sup~ssor portion of the e~tru~ion screw provides a unifolm output pressu~e at the ex~usion die byabs~bin~ surges i~ polymer pressl~re and flow rate. I~ conventi~nal e~ ders, s~ges ~re known to occu~ wllen the extrude~ speed is incre~sed or when oth~r extmsio~ls ~arameters such a~ t~mperatu~e are va~i~d. Suc~ chan~es result in ~ c~uatio~s in out~put.
2~ ~ccordin~ly, in convention~l ~x~uders. any cha~ge in ex~ er ~utput is t~ansmitted -1~

216~78~

directly tO d~e extruder die, thereby causin~ t~e se-re~e disad~antages descri~ed ~bo~e.
The extr~sion su~ge suppressor of ~li5 inve~tion ~ n)~t~s the peak~ a~d valleys of w~e-like surges to provi~e a~ output tbat is uniform. ~ore specifi~ally, ~e surge suppre~s~r acts to store s~rgin~ pol~ner assoeiated with pressure ~ flo~ rate ir~creases so that e~ccess polyn~e~ does not ~a~el directly from the metering wne to the ext~usion die. Accordin~ly~ ~e surge suppressor absorbs the sur~e while preYen~ing ~ansmission of the surge direc~iy to the ext~us;on ~ie. Wher~ the o~er band, the surge repre~ents a pres~ure dro~ or ~ow re~uctio~, ~he s.l~ge suppresso~ gives up some of i~ stored polymer to ~ e~trLsio~ ~ie discharge to e-ren-~ut the die out~ut.
1~ The surge sllppressor is p~efer~bly forrned ~ith a ler.~h sufficie~t to allow mo~en polymer pressure ge;~eration approaching ~at of the meterir~ e. The surge suppressor~s ability to generate pressure increases vvith length. ~s polymer enters the sur~e suppressor ~pushed i~to the surge s~ppresso~ by pressure gene~a~ed in the metering zone~, Lhe pressure in the surge ~p~ssor ayproaches ~e pres~ure ~ the meteri~g zo~e.
15 ~ccordingly, a sur~e of molten polymer flows ~to the surge ~upp(~ssor before it reaches the discha~ge die.
It is believed ~t the func~ion of the ~ur~e suppressor a~d me~od according tO ~lIS
inventio~ is founde~ uFon ~ln~lmnent~ls of p~lymer flow. I~ a steady~ state ~e drag flo~
of po~ymer in the s~rge supp~essor rel~tes to the pressure fl~w of the polymer accor~ng ~0 ~o the followin~ e~a~ons:
'~2(~w?~cD~y~b (1) (CW)(CD)3P (2) z wherein ~uanti~y ~13 is polymer dra~ flow ~n the surge ~uppr~ssor a~ qua~tity ~2) is 5 polvmer pressure flow. CW and CD are defin~d i~ Fig. 2. P is dæ pressure develc~ped `- 216778.~

in ~e screw meterirl~ zone, z is the helic~l len~th o~er wh~ch pressure P is de~elop~d in tbe surge suppressor, a~d Y* is thc relative Yeloci~ between ~e extruder screw and extrud~r ~arrel. II1 steady state operatio~ of the sur~e suppressor, dr~g flow and pressure flow are ~ppro~imately equal:
'~2~C-V~(CD)v5t, - (CW)~CD)3P
~2~z ~3) Aec~rding to relstio~ship (3~, there is an illcr~ase in flow into ~e surge sllppress~r wherlever an ins~ntaneou~ pressure increas~ occur~ at dle surge suppressor entrance.
This ~w incre~se causes aIl increase in fille~ length z. If the pressute incr~ase is mainr~iner~ for suf~lcietll t~me, a new e~ilibrium will be reached ~i~ t~e new filled gtll 2 un~i~ dra~ flou is again propo~ti~nal tO pressure flow.
If molten polyme~ is pres-lmed to be i~c~ ssible, ~r~ n~us pressure s~ge will be accomp3~ied by an i~se~nLane~us flow rate incre~se. Such an i~ t~n~o~s fl~w rate surge is absorbed 'oy the surge s,lp~l~ssor of d~is inve~io~ The in~t~al flow surge into the surge suppre~sor is large a~d then gr~du211y tapers. Accord~ngly, ~niaal t~o~ i~crease ineo the surge ~-lpp~ssor immediately r~uees flow L~to the die, thereby reducing o~ltpUt vanations at the ext~usion die In other words~ a step change in p~essu~e or flow rate ill ~e metering zone will not produee a step ehange in the disc.harge pressu~e or flow rate at the ext~s~o~ die ~vhen a surge suppressor acco~ing to Ihis ~nve~io used.
If the durati~ surge is very short (less ~a~ five seconds, for example), flow int~ the surg~ suppressor ~nd pressure in the e~tnu~ion di~ will still be buildir~ ore the sur~e ends. Accor~ing1y. pressure ~d nt~w will start t~ reduce e~-en be~ore ~ new steady st~te is reach~d and the a,nplitude of the pressu~e and flow surge is dramatically reduced or elimillated.

~778~

It has been disco~e~ed that it is easiest to s~ppress su~ges whe} t~e time requir~d ~o~ flow into the surge ~u~pressor to re~ equilibriu~ is l~llger than t~e ~me dura~ion of the sur~e (~ ,rge~. For e~mpl~ surge~ ~tri~ a dura~ion as l~ng as ~D hQur ~arl be ~illlit~d by presslIre feedback ~ontl-~l. Acc~rdingly, it is most preferabl~ to design S the surge suppres~or accordi~g to t~s in~-ention ~or redi.lc~i~n o~ s~or~-term su~ges lasti~g only a few secmlds.
Surge suppressorC accordi~g tO dlis inventio~ are preferabIy dcsi~ to ~n~e pol~er volumetnc c~pacity so as to abso~ ~arger pressure and volullle~ ic ~urges. ~his is prefe~ab~y acc~mplished by adjusting the flights in the su~ge suppresso~ ;,.,i,.in~
10 chan~el ~idth CW. chanIl~l depth C~, and helix arJgle H (Fig. ~).
Surge suppresso~s according lo ~is inve~tion ger~erate less p~essure than dynalnic seals, such as the one described above with referellce to ~;ig. 3a, ~ h are int~ d to seal agaiIlst pol~mer flow. As compared ~ d~c seals, th~ surge suppressor o~ Ubis tion wil] }lave a ~reater ~elix angle H (Fig. ~), a greate~ channei depth CD a~d/or 15 a wider eh~el ~ridth CW.
Wid~r char;ne1s provide ~ncreased polymer sto~a~e capacitv du~ing a surge.
Inc~eased polymer storage capacity al~o ~llo~ s ~e surge suppresso~ to pump back i~o the extrusîo;~. ~ie a greater ~olume of polymer dur~g a pressure or flow ~ate drop.
~not~er ad~ f wider surge suppres~or cha~nels is that the o~ve~all a~ial le~th of 20 ~he surge supp~essor ean be made smallel (bec~use fewer cb~e1s a~ required~ a~
allo~s t~e ma~ufaeture of ~ small ex~uder. Accordi~gly, t~e channe~ width of ~e pref~rred ~urge supF~re~sor embodiment ~s ~reater than about 10% of the screw di~mete~.
~e~erri~, tc Fig. 2. channel width CW in ehe sur~e suppre~csor is preferably ~reater t~
a~out 10~ of barrel diaIneter D~.

216778'~

The most preferable surge suppressor according to his j~rention ge~e~tes lowe~
p~css~res ~a~ that ~enerated ill the metering zo~e, Lower p~essure gene~ation m~int~ s conti~uous flow of ~olterl pol~mer downstream th~ g~ ~e surge s~ ressor. Such conti~luous ~ow r~p!enishes mol~n po~ymer in ~he surge ~uppressor, t~ereby pr~ve~ing S polymer de~,rad~tion a~d b~. As des~ribed above, the flow of molten polymer downstreaIrl from the surge suppress~r can be stopped wi~h ~ny seali~g meshod ~ 5imply allowed ~o tlow ~rom the extruder barrel. To ae~ieve these be~efits, the pJ~fel-led sulge suppres~or embodiment gene~aees a pressu~e less ~a~ the meter~ zone a~d, most pre~erably, wiIl onIy be c~p~ble of genera~ing a pressure up to appro~rnate~y 9~ % nf t~e 10 p~essure ~enerated .n the meterin8 ~one.
A surg~ suppresso~ ~cording to this in~en~io~ is prefera~ly formed uith a helix angle H (Pig. 2) larger ~a~ that of ~he dynamic seal i~ Fig. 3~, or grea~er than about half ~l~t of the mete~ing zon~ screw flight. ~Ielix ~ngle H of ~he surge ~uypressor is most pleferably slightly larger than ~he helix angle H in the meteri~g zone. W~ere ~e other 15 surge s~lpp~ssor d~mensioTls coincide w;~ those in the meteri~g zonc, the preferr~d s~l~ge suppressor embodi~ent has ~ ~eli~ a~le H about lQ% lar~er ~an the metering zone.Such a he3i~ angl~ h~s bee~ discoYer~ to ~er~erate a ~x~u~n p~essure of about 90~;
the pressu~e ~e~erated in ~e meter~ng sectio~ if the other dimensi~n~ are ~he ~me.
A s~g~ suppress~;>r accord~n~ to this inver~tion pr~r~ly ha~ a cb~el dep~ Cr) 20 larger than that ~f the dvnamic seal in Fig. 3a. or ~reater ~han ~out half that of the metenn~ zone. If he~ix angle ~ were ~he same as in met~ring zo~e ~, d~e ~,I:~St preferred surge ~uppres~or would ha-re a chan~l depth CD ab~ut twl~e that in themeterin~ ~ne Such a cha~e1 deplh CD has been discove~ tu generate about Z5 $~0 the pressu~e ~enera~ed in the metenng zoD~e.

2~6~

The ~xial length of the sur~e suppIessor accordl~g to this illve~tion is ~-el~ldbty ~reater ~an thal of the dynamic se~l of Fig. 3a, or greater thall abou~ 2~a that of the metenn~ zorR. Hnwever, axial le~th of the surge suppressor is alsv prefer~bly less ~han th~t of the ~etering zone. If ch~nnel depth C~I) an~ beli~ an~le H 4ere bo~ the same 5 in ~he s~ e suppressor as in the meterin~ zone, ~educing the su~g~ suppre~sor length to about ~5 % of the nletering zone le~ ould ge~erate a~out 95 ~ of the metering zolle p~essure.
Of course, ~ny c~mbina~on of ch~nnPl depth C~, channel width CW, surge suppressc,~ length and hel;,x angle ~ can be use~ so l~n~ as ~ su~ge suppressor re~
10 its surge suppress~ng fimction. Howeve~, it is most preferable that t~e ~tge su~pressor is not des~gned to gene~ate more tha~ ~bout 95 % Q~ ~e metering ~,ne pre~sure.
In any embodiment, ~e sur~ge sup~,~sso~ ~ccording to this i~veIItion provides signific~ntbenefits. The surge supp~es~or dr~nlaticall~ reduces ~r elin~ Ps pressure and volumetric surges c~n~non~y kllown to occur in conventional e~t~uders. The surge15 suppressor prevents these sur~es from traIl~fe~r~g directly to the extrusion die, t}2ereby redu~in~ ~r elimin~in~ v3~iatio~s in e~c~uder p~odue.t d~ne~sions a~ qualit~.
~ ese su}pn~ significa~t bene~ts ~re co~e~red without dis~dv~ntage. The continuous flo~ ~f molten polvrner ~hrough the sl~ge suppressor pre~en~ degradatlon or ~uru~ng of the polymer. Also, tJ~ surge s~ppressc~r c~n be provided without requiIing 20 a si~niflcant irlc~ease in extr~der ~en~,th. Accordillgly, the surge sup~re~sc~r a~d method according to this inventi~n provides a simple., effec~ve and prachcal solu~ion to the longsta~ding proble~ of surging in extruders.
It h~s als~ beer. diseo~iered that surprising he~efi~s ~re obtai~ hen ~ s~rge sup~ressor accord~g tO thiS inventio~ is u~ed in combination ~i~ ~ p~ess~re contr~ller S designe~ to ~ary screw spe~d to c~mpe~sate fc~ p~essure v~riat;ons sensed ~ear an -lg-~1~778~

ex~ruder gear pump, such as t~hose d~cribed he~e~n under "Field of ~e I~ve~ion. " T~e surge suppressor has ~ discoY~red So irnp~ve pressure ~ oller pe~fo~a~ce.
Furtbe;~more~ it has been dis~overe~l that ~se of a sur~e supp~ess~r acco~ing tothis i~Yention rrla}~e~ it possible tO util~ze a pressu~e controller to reduce s~gi~g in 5 e~truders ~rithout ~e~r pumps. It is speculated ~at this s~rprisi~ kenefit results frvm seYeral ~actors. The surge shppressor reduces or el ates short dura~on pressure sllrges that often cause ~ pressu~e controll~r to over~on~p~sate. Also, it iS speculated d~t surge suppressc~rs perform dynamically and react ~nediately to pressure surges as ~ey occur, no matter what their cause ~y be, to help tbe ~o~ollel reg~late pressure.
10 Sur~e ~uppr~sso~s are also p~esumed to c~ampen ove~-compen~tin~ responses of the pressure control~er.
rhe following ~3~ample il~us~ates specific beI:I~fitS of the i~ventio~. This E~ple is not intellded to defiIle or limit ~he ~ope o~ the invention, which i~ de~ned sepa~ately i~ ~e claim~ f~ w.
15EY~P1e An exper~ment wa~ per~ormed U~iD~ the e~ ude~ shown schert~ lly in Fig. 7.
A pressure cor~troller (Model No. 6G0, suppliod by Dynisco) ~?tili7ing a simple tach feedba~ D~ motor co~rol was c~nnecte~ to the drive end I~R ~f extrusion screw S
having a dy~amic seal r~s and a su~e suppressor SS. The presstlr~ controller was ~Iso 20~o~e~.ted to ~ pressure sensor PS mou~t~d in ~e barrel B of the e~truder ne~r the discharge D where pressures ~re most unstable. A pressure ~order was ~lso ~on~e~ted to monitor aIld rec~d pressure within the barrel. The extruder did not include a gear pump.
The barrel pressure wa~ mc nitored fc~r a period of one houl without operation of 2Sthe pressure contr~l1er. A ~rro~ pressure range of 1,350 to 1,450 psi was recorded -2~

--` 2~6~7~

during ~a~ one-hour period. The bar~el pre~sure was subst~nti~tly evenly divide~ about a ~id-point pressure of 1,400 psi. Occasiunal pr~ssure spikes ~Yere aIso eve~ly divided about this 1,400 psi rrlid-point, Includ~ng five s~rt peaks to 1,4S0 psi a~d six short tr~ugl~ to 1,350 psi.
The pres~ure controller was ~en actua~ed to vaJy extIusion scre~ speed in respo~e to press-lre Yariations A pressure ran~e of 1,350 to 1,400 was recorded.f~ceordingly, actu~orl of the pressure c~ntroller reduced ~e ~el pressure range from IdO psi to 50 psi. Tbe pressure in the barrel mostly resided betwee~ 1,350 psi and 1,380 psi. There were several short-teTm peaks raising ~e pressure to 1,400 psi, but ~kere 1~ were IlO trou~hs.
'rhe ~ecord of ba~rel pres~,~re duri~g pressu~e c~troller act,latioII also inl1ic~
inte~als wherei~ thç pressu~ ge was ro~l~ced ~ 2~ psi. It was also noted that intervals ~cluded highly stable ba~rel pr~ssures including only sho~ spikes"
devi~tl~g su~sta~ntia11y e~ally oll both sides of the st~ble pressure.
lt is anticipated that t~ese ~urprising res~llts could even be ~p~oved by optim the interactive set points of ~ pressu~e con~o~ler. ~Iso! the pre~ cont~lle~
perf~ ance ~ould be furthe~ imp~oved by using n~ore accur~te and faste~ t~es of motor co~t~o} ~or the e~truder.
Many r~odi~1cations t~ ~he surye suppressor ~mbodiments ~escribed herein can be ~0 made without departing ~rom ~e spirit and sc~pe of this inYention. For example, instead of forn~ing helical grc~ves in the e~ sic;~ screw to produce the surge suppressor, a su~ge suppressin~ uit can be produced by creau~g helieal ~r~o~es in the cylindrical housing cr barrel. Also, the surge suppressor is optiona~1y a s~pa~a~ely dnven comp~ne~t. Ie is Lhe relative mot~on ~ een the sh3ft and the barrel that creates the ~5 important pumpin~ effect of the surge supp~essor.

2~ 6778i~

~ l~o, mor~ than one ble~ hole can be pro~ided for ~e embodiments shown in Figs. 4~ and 4b and aIly kn~u~ c~IIventio~al seal or ~est~iction device ~ be used i~
~onjunction wi~ the ~urge ~uy~reS~o~ iD any er~bodiment. Of course, multiple se~s ca~
be use~ in coIr~bination if desir~ble or necessary, ~:or e%ar~ple, a dynamic seal can be S used ~n conju,rle~ion ~ith ~olym~ cooling.
lt is also con~emplat~ tha~ e(l holes 10 and 20 ~Fi~s. 4a and 4b, respectively~
may ~lso coFnmunica~e with ~ flexible mem~raIIe coY~rin~ the ho1e or a pist~n illserted into the hole as oppose~ to other for~s o~ restricti~ll devi~e. Such a ~le~ble memb~alle or piston could pro~ide some additioIlal surge reduc~ion and/ol sealin~ c~pability.
!0 The screw dLme~sio;ls a~ conf~guration ran be ~ari~d in any way in any conn~ ioll in the surge suppress~r so lon~ as the sur~e suppressi~g ~unction is mqinl~ined. It is also contemplated that the cha~el ~vidth CW, cha~el depth CD and he1ix ~ng1e H may vary o-er the surge supp~sso~'s length. Altho~gh the shaft diameter Ds is prefera~oly constant throughout the surge suppressor. sh~ dian~eter Ds ~ . 2) may 15 optio,~lly be tapered.
The su~ge suppressor acco~di~g to this irne~tion is opti~nally used in combin~t~on with an ext~uder c:ontroller in a Y~riety of ways to st~ilize extrusion pressure arld reduce or eli~nate preC~ure surges. It is conternplated that any kno~ ~pe of e~truder con~oller can be used in conjunctic~n wi~ a surge suppre~sor. ~lso, ~ pressure-type ex~ruder controller op~ionally m~nit~s pres~ure ~ia a se~or mounted in ~e ba~el~ the transfer pipe ahead of ~he die, or at an~ vther d~ired location. r~e e~truder controller option~l1y :~nonitors poly~er pressure, ext~uded pr~duc~ di~ensions, pol~mer flow r~
or a~y other para~et~r relate~ t~ ~xtrusion pres~re.

~67~8~

~ any o~r ~odifica~;ons wil1~ ,arellt t~ tho~e Gf skill in the ex~usion a~t.Such modificatic-n~ are wi~in the seope of this ~nventi~n, wh~ch is defned i~ ~efollowing claims.

Claims (6)

What is claimed is:

1. An apparatus for extruding a polymer which is subject to periodic surges, said apparatus comprising:
a barrel having an upstream portion for receiving said polymer, a downstream portion and a discharge port proximal said downstream portion;
a shaft mounted for rotational movement about a longitudinal axis within said barrel;
drive means connected for rotating said shaft;
a surge suppressor including a screw flight on said shaft and located downstreamof said discharge port for urging said polymer upstream toward said discharge port while permitting a portion of said polymer to flow in a downstream direction into said surge suppressor.
a sensor positioned to detect said surges of said polymer; and a controller connected to said sensor and to said drive means for adjusting saidrotational movement of said shaft in response to said surges of said polymer.

2. The apparatus described in Claim 1, wherein said shaft has a conveying screw flight and a metering screw flight arranged for conveying and metering said polymer downstream through said barrel, wherein said surges are developed within said barrel.

3. The apparatus described in Claim 1, further comprising sealing means located downstream of said surge suppressor.

4. The apparatus described in Claim 1, wherein said sensor monitors polymer pressure to detect said surges.

5. The apparatus described in Claim 4, wherein said sensor monitors said polymer pressure in said barrel.

6. A method for extruding a polymer using an extruder having a barrel and a shaft rotationally mounted within said barrel, said method comprising the steps of:
conveying polymer through said barrel with a conveying screw flight;
metering said polymer in said barrel with a metering screw flight, thereby generating a metering pressure and a metering flow rate;
suppressing surges in said metering pressure or said metering flow rate by generating a suppressing pressure less than or equal to said metering pressure with a reverse screw flight;
monitoring said metering pressure or said metering flow rate; and controlling said metering pressure or said metering flow rate by adjusting the speed of said shaft to compensate for said surges in said metering pressure or said metering flow rate.