Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 ~rhe present in~ention i5 directed to methods and
2 apparatus for forming pellets of thermoplastic materials
3 such as polystyrene, polyethylene, polyvinylchloride, and
4 polypropylene, among others. The pelletizing oE such ma-
terials with incorpora-ted additives i5 common because the
B pellets may be conveniently measured, packed, shipped,
7 stored and used by the end user.
~ While many pelletizing systems find the thermo-
plastic material, in a flowable state, being expressed under
11 pre~sure through orifices in a stationary die, as by a screw
12 extruder, the useful e~ficiency (hydraulic power/total power
13 supplied) of such systems is quite low.
14
To overcome -the foregoing, and other pro~lems
1~ associated with the stationary die type extrusion, centri-
17 fugal extruders have been developed in which thermoplastic
18 material in a ~lowable state is fed into the interior of a
19 hollow rotor to be centrifugally expressed through oriices
in the rim of the rotor ~see, for example, U~S. patents Nos.
21 3,358,323; 3,424,832; and 3,483,281).
2~
3 The present invention is especially directed to
improvements in pelletizers: of the ~entrifugal -type wherein
the centrifugal head developed in the rotary polymer melt
~6 extruder is used to generate the pressure required ~or ex
a7 trusion and the extrllder is self-emptying~ In such pelleti-
28 ~ers there is no need to further heat -the polymer because o~
insufficient pressure development (advantageous with th~rmally
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1 sensitive polymers~ and an eddy current heating sys-tem can
X be efficiently and economically used to compensate for heat
3 lost by the spinning rotor to -the surroundings.
A prime object of the present invention is to
provide certain novel improvements in methods of centri-
7 fugally pelletizing, and centrifugal pelletizing systems,which greatly enhance the practicality of such systems
g and will hereinaf-ter be described in detail and claimed.
lû
11 In accordance with the disclosure which follows,
12 a hollow rotor having radiall~ directed ori~ices in its peri-
13 pheral rim portion is mounted for rotation within a housing
1~ about a horizontal axis~ A sleeve projects coaxially from one
side of the rotor outwardly through -the adjacen-t side of the
; 1~ housing and a feed tube may be inserted, with clearance to
17 provide for spillover relief, into the sleeve to feed thermo-
18 plastic material in a flowable state into the interior of the
19 rotor. An endless strand severing device is mounted upon a
bracket supported on the housing and includes an endless se-
2~ vering band trained about pulleys on the bracket. The bracket
22 is positioned so that one of a pair of severing bands extends
23 parallel to the rotor axis across the path of strands ex-
pressed from the rotor orifices to cut or se~er pellets
j 25 from the distal ends of the strands. The band is driven in
2~ continuous movement to equalize wear on the band and to
maintain that portion of the band contacting the strands at a
g~ relatively cool temperature.
-- 2 --
1 Pellets severed by the band move from the sever-
2 ing location in a path -tangential to -the rotor into a receiv-
3 ing duct. A water spray arrangement is located at the en~
trance to the duct to cool the pellets en-tering the duct,
thereby minimizing agglomera-tion of the pelle-ts. The bracket
~ carrying the severing band is mounted for both radial and
7 circumferential adjustment rela-tive to the rotor axis~
g The shape and uniformity of the pellets is re-
gulated by selecting the radial diskance fxom the rotor rim
11 at which the severing portion of the band engages the strands.
12
13 One of the prime objects o~ the invention is to
14 provide an energy efficient system of the type described in
which there is a control of air flow, coolan-t flow, and
1 1~ pellet tra~ectory to minimize heat losses.
17
18 A further object of the inven-tion is to provide
19 a system which operates with minimum opera-ting energy con-
sumption.
2~
~2 other objects and fea-tures o:E the invention will
æ3 become apparent by reference to the following specifica-tion
and to the drawings.
~6 Figure 1 is an end eleva-tional view/ with certain
27 parts broken away, omitted, or shown in sec-tion, of a cen-
~8 trifugal pelletizer embodying the presen-t inven-tion;
~9
Figure 2 is a side elevational view with cer~ain
-- 3 --
1 par-ts broken away, omitted, or shown in section, of the
~ pelletizer of Figure l;
. 3
4 Figure 3 is an enlarged detail end view, wi th
cextain parts shown .in broken lines, more particularly
~ .illustrating the severing knife assembly;
8Fi~ure 4 is a detai.l side elevational view of the
seve.ring means illustrated in Figure 3;
11~ Figure 5 is an enlarged detail perspective view
~2 showing blade support mechanism;
13
14 Figu~s 6a and 6b are schematic diagrams dis-
closing two types of severing operations; and
:i 16
17 Figure 7 is a graphic representation of pellet
18 size distri.bution under certain operating conditions.
19
~, 20 Referring first particularly to Figures 1 and 2,
Zl a centrifugal pelletizer embodying the presen-t invention in-
22 cludes a housing designated generally 10, within which a cooled
23 rotor shaf-t 12 is rota-tably supported, as hy bearings 14.
~4 A drive motor 16 is mounted within the housing and is
drivably coupled to dri~e ro-tor 12, as wi~h a bel-t and pulley
2B coupling designated generally 18.
2~
~8 ~ hollow rotor 20 is fixedly moun-ted upon one end
~ of shaft 12 to rotate with the shaft and a plurali-ty of
1 radially dixec-ted strand expressing orifices 22 open rom
2 the interior of the rotor through the rotor rim. In the
3 embodiment shown in Figure 1, two rows of orifices 22 are
shown.
At the side of the rotor opposite its coupling to
7 shaft 12, a hollow sleeve 24 extends-axially outwardly from
the rotor beyond the adjacent side wall 26 of housing 10,
~ sleeve 24 passing.freely through an opening 2a in side wall
! 10 26. Rotor sleeve 24 is formed with a central opening 30
11 through which a stationary feed tube 32 may be inserted with
12 clearance to feed thermoplastic material a-t a predetermined
13 velocity (feed rate) into the hollow interior of rotor 20
1~ opposite a pin-like diverter and impeller 31 having a flared
annular surface 31a and a fron-t face 31b broken by communicatinc
I 1~ slots 31c extending radially at 90 in-tervals to form impellers
17 which help to accelera-te the flow from the O r.p.m. at which
18 is received from tube 32 to the speed of rotation o-E rotor
19 20. It is believed apparent that, upon rota-tion of rotor 20,
thermoplas-tic ma-terial, heated to a 1Owable state and fed from
21 feed tube 32 into the interior of rotor 20, will be direc-ted
22 and impelled radially and -then centrifugally expressed from
23 the interior of ro-tor 20 through orifices 22 so that strands
~4 S ~Figures 6a and.6b) of thermoplastic material will be
expressed xadially outwardly from the xo-tor periphery. The
26 strands are converted into pelle-ts by a severing device,
a7 designated generally 34 and most clearly shown in Figures 3
~8 and 4. A like annularly flared surface 30a on the rotor
~ front wall leads to -the space 30 which, by permik-ting overflow
1 to exi-t from the housing wall 26, avoids a polymer overflow
buildup between the rotor and housing which would bo-th increase
3 power consumption and tend to cause a ~amp-up to occur.
As best seen in Figure 4, severing device 34
~ preferably includes two severing mechanisms designated gen-
7 erally 36a and 36b, which are identical except for being
mirror image arrangements. The two mechanisms 36a and 36b
g are mounted upon a common carriage 38. Carriage 38 carries
four support rollers 40 which support the carriage upon con-
11 fined tracks 42a provided upon an axially extending annular
~2 shoulder 42 on housing 10 whose surface is coaxial with the
13 rotor axis. A pin 44 fixedly mounted on housing 10 projects
14 through a slot 46 in carriage 38 to define the end limits
of circumferential movement of carriage 38 relative -to the
1~ housing 10 which permit selective location of either sever-
17 ing device 36a, or severing device 36b, at a selected cir-
1~ cumfexential position relative to the rotor. Suitable set
19 screw locking means 84, or the like, may be employed to
releasably affix carriage 38 at the selected circumferential
21 pOsition.
22
~3 As previously stated, severing devices 36a and 36b
24 are of similar constructionr thus only severing device 36a
will be described in detail, it being understood the de-
~6 scription i5 equally applicable to severing device 36b
~7 (primed numberals being used to designate like parts). Se-
~8 vering device 36a includes a main frame or bracket 48 upon
which a pair of pulleys 50 and 52 are mounted. Pulley S0
~0
1 is a driven pulley and is mounted upon the shaft 54 of a
2 drive motor 56 whose housing is fixedly mounted upon bracke-t
3 48.
S Pul~ey 52 is mounted for free rotation about a
- ~ pin 58 carried on the lower end of an arm 60 pivotally mounted
7 upon bracket 48 as at 62. A spring tension device, designated
generally 64, biases arm 60 in a counterclockwise direction
0 about pivot 62 as viewed in Figure 3 to apply tension to an
endless severing band 66 (which may he constructed of th.in
11 spring steel in the range 0.1 to 0.5 mm in thickness), trained
12 about pulleys 50 and 52. the device 64 includes a handle 63
13 pivoted at 63a and connected at 63b to an arm 63c which attaches
14 to spring 63d, spring 63d secur.ing at an opposite end to arm
60. As best seen in Figure 3, the lower run of endless band
1~ ~6 extends parallel to the a~is of rotor 20 and is located to
17 extend transversely acros~ -the path of movement of strands S
18 expressed from the rotor oriEices.
~9
Brac]cet 48 may be bodily adjusted radially of ~he
21 rotor axis -to vary the spacing between the rotor rim and the
22 lower or severlng run of severing band 66. The adjus-tment is
23 accomplished ~y structure which includes a mounting lug 68 a-t
2~ the left-hand end of bracke-t 48 as viewed in E'igure 3, which
is threaded to receive an adjustment screw 70 whose lower end
26 bears upon a pivot pin 72 mounted upon carriage 38. At either
27 side of mounting lug 68~ a pair of side pla-tes 74 are pivotally
~ mounted upon pin 72. Elongate slot 76 in each side plate 74
29 slidably receives a pair of locking screws 78 likewise
1 threadably receivea in lug 68. Wi-th screws 78 loosened,
2 rotation of screw 70 -thus raises or lowers lug 68 relative
3 to pivot 72, and the lug 68, and hence bracket 4B can be
4 locked in a selected posi~ion of vertical adjustmen-t b~
5 then tightening down the lock screws 78.
The pivo-t 72 serves as a pivotal mounting for the
8 entire bracket 48 so that the bracke-t may be pivoted in a
9 counterclockwise direction from the position shown in Figure
3 to move severing band 66 of the severing device, clear of
11 the path of movement of strands expressed by the rotor. By
12 employing two sevexing devices 36a and 36b as shown in Figure
13 4, one of the two devices 36a o-r 36b may be pivo-ted to an in-
14 operative position so that its blade may be replaced, for
e~ample, while the other of the two severing devices is in
1~ its opcrative position. This axrangement enables blades to
17 be replaced w.ithout requiring shut-down of the operation.
18 Either cutter can be swung into position while the o-ther i.s in
19 opera-tion, thereby pe~mit-ting an interchange without in
terrupting polymer Elow. The bracket 48 may be maintained
21 in the opera-tive posit.ion shown in Figure 3 by the engagement
22 be-tween the lower surface 80 of side plates 74 with a fla-t
23 surface 82 on carriage 38. IIowever, it may be pxeferable to
24 provide further support for the can-tilevered end of the
bracket 48 which is swung to the operative position by means
26 of a spring biased snap clamp device designated generally
~7 84 (Figure 3).
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2~ TO assis-t in maintaining the operating porti.on of
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1 severing blade 66 in accurate allgnmen-t and to guard against
twisting of the blade, blade guides 86 (Figure 3) are Eixedly
, 3 mounted on bracket 48. The guides 86 are formed with blade-
; ~ xeceiving slots 88 (best seen in Figure 5) which slidably re-
ceive the blade to brace the blade against the impact it re-
ceives when it is contacted by the rapidly rotating strands
; 7 and to maintain the blade against -twisting. The guides 86 are
; 8 located on opposite sides oE the paths of movement of the set
;l 9 or sets of strands expressed from the embodirnent of the rotor
disclosed. In the case where rotors of grea-ter axial thick-
11 ness and grea-ter numbers of sets o-E orifices are employed,
12 additional blade guides are employed so that -the unbraced
13 por-tion of the blade would span no more than two sets of
~ 14 orifices.
';i 15
16 While movemen-t of -the blade ~6, by operation of
17 motor 56, will normally provide for adequate cooling of the
18 renewable flexible blade with many polymer melts by continu-
19 ously changing the poin-t of engagement be-tween -the blade and
0 strands so that the blade is kept cool and sharp, addi-tional
21 blade cooling can be economically and efficiently provided
22 by means of passages 90 in blade guides 86 (on blade guide
23 holder 86a), which may be employed to supply coolant contin-
; 24 ously to the top of the blade (remo-te from the die orifices
22 so as not to chill them). The blade guides 86 not only
26 commence the quenching operation with some rnore difficult to
27 handle polymers (thus increasing the polymer range which can
~8 be handled), but also reduce -the unsupported blade span -~o
minimi~e vibration.
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1 Referring now particularly -to Figures 1 and 2, it
2 is seen that housing 10 includes rotor housing side plates
: 3 26 and 92, which are spaced from rotor 20 with hu-t a slight
axial clearance. As best seen in Figure 2, however, the cir-
cumferen-tial portion 94 of housing`l0 is spaced a subs-tantial
~ distance from the periphery of rotor 20 to allow for a sub-
7 stantial radial growth and projection of strands from the
8 rotor orifices and provide a chamber C between wall 94 and
~ the rotor. The rotor housing is formed with an opening ~t
96 to permit movement of the bands G6 into and out of opera-
11 -tive relationship with the rotor. Mounted on the outer side
12 of rotor housing side wall 26 to receive and confine thermo-
13 plastic material which may flow -through the clearance between
lg rotor sleeve 24 and feed tube 32 upon over-filling of the
rotor, i5 a spillover housing 98. The sleeve 24 thus prevents
any over-flow from ge-tting into the relatively confined space
17 between the rotor and the adjacent side wall 26 of the housiny.
18 The lower end oE spillover housing 98 is shown as open, but
19 could be connected to a collection duct.
21 In order to make up heat loss which the closed con
22 s-truction keeps to a minimum and maintain the temperature of
æ~ material within rotor 20, magnetic heaters such as 100 may be
~4 mounted upon the rotor housing to function in a manner simi-
lar to that described in patent No. 3,483,231
26
a7 On initial start-up of the device, ~ome -time i.s re-
~8 quired to achieve steady sta-te tempera-ture and other operating
; a~ conditions, and, during this period of time, material eY~truded
from rotor 20 is diverted by means of a knife-gate 102, pivo-t-
-- 10 --
~L~9~
1 ally mounted upon the housing as at 104, see Figure 2, adja-
cent a housing outle~. in the form of a scrap chute 106. At
3 this t.ime, the band blade mechanisms 36a and 36b axe both pi-
voted to up posi-tion, and it is knife-gate 102 which per~orms
the strand severing unction and diverts the material cut off
to chute 106. ~uring star-t-up, and shut down -too, (the rotor
7 is self-emptying of material at the end of a run), the material
8 ex-truded fro~ the ro-tor orifices is shaved off by pivoting
9 diverte.r knife 102 into contact with the rotor periphery, so
~ that the shaved-off strands are discharged through tube 106.
11 The knife 102 is in the Eorm of a hollow housing and includes
i2 walls 102a and 102b with closed ends 102c provi.ding a chamber -
13 102d to which a temperature controlling liquid (normally a
14 coolant) medium may be circulated via inlet and ou-tlet hoses
103. Plate 102a has a trailing rear end 102e carried by an
1~ actuating handle device 103a which is manipulated to move the
lq kniEe 102 to selected positions and releasably main-tain them
18 there. During normal operation, the gate 102 is pivoted to
19 a position where it closes the entry -to diver-te.r chute 106 so
no air ~and therefore heat) is lost ou-t this opening. In
21 some instances, during operation, ga-te 102 may be pivoted to
~2 an .intermediate position to divert very fine hair-like plas-
23 tic material (known in~the trade as "anyel hairl') which is
in extruding operations formed and carried by the air curren-t
generated around to this point, to thus permit its removal
~6 ou-t duct 106. The pin 104 which is fixed to gate 102 is
27 journaled in a bearing 104a supported on the housing 10 and
~8 a readily releasable set or lock screw, or handle operated
2~ cam lock device, 105 can be used to secure the ga-te-knife 102
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~36~
in selected position at or between the extreme positions
illustrated in Figure 2.
4 As described above, during operation, only one of
the two severin~ devices 36a, 36b will be located in an
~ operative position, wi-th the severing run of its associated
7 severing band 66 located at a selected radial distance from
8 the rotor periphery to sever pellets from the radially
: g projecting strands carried past the band by the rotating
rotor 20. Pellets severed by the severing device continue
11 in straight line moti.on in a pa-th that is generally tangen-
12 tial to ro-tor 20 and pass in-to the entrance of a collect-
13 ing duct 108 of retangular cross sec-tion affixed to hous-
14 ing 10. Coolant spray manifolds 110, have ports direct.ing
spray curtains Cl and C2 of wate~ or another suitable me-
1~ dium into tube 108 (and away from rotor 20~, along -the
17 Ealse top 108a and false bottom 108b provided therein. These
18 sprays and the resultant coolant which fills duct 108 a
19 short distance downstream from the entrance of tube 108
quench the pellets to prevent or minimize agglomeration of
21 the pellets. The air passages 108c formed between the false
X~ top 108a and the duc-t 108 and the false bottom 108b and the
23 duct provide chambers for air induced into the duc-t by and
24 with the coolant to disengage and flow in a reverse direction
: 25 back to the mouth of duct 108. In -this way additional aix is
~ prevented from being induced into the system by the air
27 current inducing flow of the sprays Cl and C2 acting to pull
28 other air into the duct 108.
2~
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1 In Figures 6a and 6b two types of cutting action
2 of the apparatus are schematically illustrated. The differ-
3 ent cutting ac-tions depend primarily upon the physical
properties and characteristics of the material being handled.
~ In Figure 6a, there is shown a schematic represen-
- 7 tation of what might be termed the generalized theory of
8 operation of the device. Rotor 20 is rotating in the di-
~ rection indica-ted by arrow A, and cutter band 66 is posi-
tioned at a radial dis-tance X from the ro-tor periphery.
11 Cylindrical s-trands of -thermoplastic material S expressed
12 from the rotor orifices are carried by the rotation of
13 rotor 20 successively into engagement with the severing
14 blade 66 which severs a length of the ma-terial at the dis~
tal end of the strand S to form cylindrical pellets P, the
16 severed pellets having suEficien-t momentum to continue to
17 travel in straight line motion (-tangen-tially) through the
18 air currents in chamber C between rotor 20 and housing
19 peripheral wall 94, on through the coolant curtain, and
on into the pellet duct 108 (not shown in Figure 6a). The
21 peripheral positioning of -the blade severing via pla-tEorm
a2 carrier 38 is also important since the exact angle of
23 trajectory is influenced by the angle of the cu-t-ting edge.
a4 With the cooled moving blade the pelle-ts moreover, will
uniformly fly off in a straight line (do not stick), more
~6 suited to quenching in the water spray curtain and pelle-t/
27 water slurry formed in duct 108. The severed strands Sl and
28 S~ of Figure 6a immediately begin to grow or extend radially
2~ as the centrifugal force on the flowable thermoplastic ma-terial
- 13 -
. ` !,'' !
l within rotor 20 continues to express material through the ori-
2 fices so that, at the conclusion of one revolution, the strand
3 Sl has radially gxown to the length of the strand S as it again
4 approaches severing blade 66. Where this generalized situa-
tion exists, pellet size may be maintained subs-tantially
~ unifc,rm and controlled simply by appropriately selecting the
7 blade spacing X with respec-t to s-teady state operation of the
8 device.
However, experimental work has indicated tha-t, in
ll the case of highly visco-elas-tic polymers, such as low den-
12 sity polyethylene, pellet uniformity is not so simply
13 achieved. Upon severing of the pellet, the removal of
l4 the mass oE the pellet from the outer end of the strand
causes the uncu-t portion of the strand -to snap back,as
l~ indicated in Figure 6b. The amount by which the visco-
17 elastic material snaps back or foreshortens, upon the sever-
18 ing o~ a length from its distal end, is generally proportional
l9 to the mass of the material severed. Thus, a greater snap-
back or retraction of the unsevered leng-th of -the strand will
21 occur when a xelatively long portion is severed from i-ts
22 distal end. The magni-tude of the "snap-back" may be such
23 that the s-~rand does not grow back to its Eormer length,
24 during the next revolution to the severing blade. In this
case, when the foreshor-tened strand arrives at the blade,
~6 a smaller length of material will be cut from its distal
~7 end by the blade and the "snap-back" will not be as great
~8 as in -the previous case. Thus, during the next revolu-tion,
the s-trand will grow to a length greater than in the first
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1 case and again a longer leng~h of stxand will be severed.
~ Thus~ pelle-ts of short and long leng-ths will alternately
3 be severed on alterna-te successive revolutions of the rotor.
This phenomenon is illustrated graphically in
Figure 7 in which the size or mass distribution of pellets
7 for three difEerent knife spacings are plotted. The curve
8 Xl represents a first knife spacing and shows a relatively
9 narrow dis-tribution of pellets which represents only a
slight non-uniformity of pellet mass Erom a desired mass M.
11
12 Curve X2 shows the mass distribution which occurs
13 at a slightly greater knife spacing X2 in which the pheno-
14 menon described above is evident. It will be noted that
the mass distribution is much wider than in the case of
1~ knife spacing Xl, and that the distribu-tion curve has two
1~ separate peaks indicative of the severing of alternate
18 short and long lengths from a given strand on successive
19 revolutions because of the "snap-back" characteristic de-
~,o scribed above.
21
~2 Curve X3 represents a situation in which the
23 length of the strand which is severed causes a "snap-back"
a~ of the unsevered portion of the strand oE a magnitude such
that during the completed revolution af-ter the severiny,
26 the s-trand does not grow to a leng-th sufficient -to engage
27 the severing blade a-t the conclusion of tha-t revolution~
~8 At this point, the dis-tribution becomes narrow ayain, but
the distribution curve peaks a-t a pellet mass which is
~ ~ 9 ~
1 twice that attained for a knife spacing represented by
2 the curve Xl. In this instance, a given strand is being
cut onl~ once for each two complete revolutions of the
rotor.
Depending upon the visco-elastic properties of
7 the material, it thus becomes possible to provide unifoxm
B small pellets, or uniform twice as large pellets, a-t the
9 same feed rate, rotor speed and other operating conditions,
merel~ by changing the kni~e position from a spacing X
11 (which creates pellets Xl), to a greater spacing which
12 cuts the strand uniformly once each two revolutions (and
13 creates pellets X3 of twice the mass). Selected bimodal
14 distributions are also possibleO
1~ While one embodiment o~ the invention has been
17 described in detail, it will be apparen-t ~o those skilled
18 in the art that the disclosed embodiment may be modified.
19 Therefore, the foregoing description is to be considered
~0 exemplary rather than limiting, and the true scope of the
21 invention is that defined in the following claims.
~2
23
26
27
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