Note: Descriptions are shown in the official language in which they were submitted.
1~'7~61 24377
CONTROL SYSTEM FOR DYNAMIC EXTRUDER
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This invention relates to extrusion of thermoplastics. In one
aspect the invention relates to a control system for preventing da~age to
an extrusion die due to excessive pressure.
Extrusion is a frequently u6ed technique for producing elongated
profile shapes. It is often used, for ex~mple, in the production Or pipe
or tubing or other cross-sectional conrigurations from thermoplA~tic
materials such as nylon, polyvinyl acetate, polyvinyl chloride and poly- -
olefins, for example, polyethylene, polypropylene, and copolDmers of these
or other polyolefins or mixtures of tuo or more materials. The extrusion
procedure include~ plasticizing, forming a~d setting.
Dy~amic extrusion is a proces~ wherein a pla~tic mass is forced,
by means of a reciprocating piston, through a cool zone, then into a heated
zone to melt the plastic mass, then to a mixing section and a long land die,
in which the shape takes form and is cooled to a solid or near solid state
prior to leaving the die.
In the event the plastic mass is not heated to its proper ex-
trusion temperature, as uhere the feed rate is too high, where one or more
heaters fail to function properly or during startup, the de~ign stress of
the die can be exceeded, ~ith resultant rupture of the die. The problem is
e~pecially acute during startup when the material in the heated zone has
previously been allc~ed to cool and solidify. The extruder operator can,
of course, closely monitor extrusion pressure in the heated zone during
startup; however, the pressure buildup in the extruder can be 80 sudden that
the operator cannot react in time to prevent rupture of the die. During
operation of the extruder, pre6sure ca~ al60 increa6e too fast for the
operator to react in time to prevent die rupture.
It is ~n obJect of this invention to provide a control system for
a dynamic extruder.
It is another ob~ect to provide a novel dynamic extrusion
apparatus.
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It is 8 further ob~ect to provide a method for protecting the
long land die of a dynamic extrusion apparatus from rupture due to excessive
pressure.
These and other ob~ectR of the invention will become apparent to
one skilled in the art from the following detailed description, the appended
claims and the acccmpanying drawing wherein:
FIGURE 1 is a diagrammatic illustration of an extrusion apparatus
which can be used in conJunction with the control system of this invention;
and
FIGURE 2 is a cross-sectional view of the extruder showing the
control system Or this invention.
In accordance with the present inventio~ there is provided a co~-
trol system for a dynamic extruder which protects the extrusion die from
rupture due to excessive pressure ~ithin the extruder apparatus. There is
*urther provided a dynamic extrusion apparatus incorporating the control
system Or the present invention and a method for preventing rupture Or the
long land die of a dynamic extrusion apparatus due to excessive pressure of
the plastic melt within the apparatus.
Specifically, the control system Or the present invention ccmprises
a signal means adapted to provide a signal PM in accordance ~ith the melt
pressure o~ the plastic mass in the plasticizing zone Or a dynamic extruder
and a signal recei-ring and control means in communication with the signal
means uherein the signal PM is compared with at least one set point. When
sign&l PM i8 outside the allowable range determlned by the set point or set
points, the control means i8 adapted to stop the feeding means Or the es-
truder apparatus, thus stopping the rloN Or solid plastic material to the
*eed section Or the extruder apparatus. Interruption Or the feed to the
extruder permits the extrusion pressure to decrease to a safe range.
In a rirst embodiment of this invention the control means i~
adapted to compare signal PM to a set point PH, the value Or set point PH
being lower than the design stress Or the extruder die and to stop the ~eed-
ing means when PM is e~u~l to or greater than PH. Thus, in accordance with
the present invention, when the melt pressure of the plastic mass within
the plastici~i~g zone is equal to or exceeds a predetermined value, the
means supplying solid plastic material to the extruder is stopped, thus
stopping the feed to the extruder.
In a second embodiment of this invention, the control means is
further adapted to ccmpare the signal PM with a second set point PL, the
value of PL being lower than the normal operating pressure of the apparatus,
and to stop the feeding means when PM is equal to or less than PL. In this
embodiment, material feed to the extruder is stopped when the signal PM is
equal to or less than set point PL, as would be the situation should the
pressure-responsive element fail to function properly.
In another embodiment of this invention, a means for averaging
signal PM is disposed between the pressure-respsnsive element and the con-
trol means. ~ue to the nature of the dynamic extruder, pressure upon the
plastic mass is alternately applied and relaxed as the piston reciprocates.
Thus during the forward stroke of the piston, the pressure of the plastic
mass is greater than the pressure during the piston's backstroke. Without
such averaging means, set point PH must be empirically determined, by in-
corporating such averaging means, set point PH can be set in accordance with
the design stress of the die.
In yet another embodiment of this inve~tion the control means is
further adapted to compare the sienal PM with a set point Ph, which is
lower than set point PH, and greater than the normal operating pressure of
the apparatus and to activate an alarm means when PM is equal to or greater
than set point Ph but leas than set point PH, thus providing a warning that
PM is approaching PH.
In a further e~bodiment, the control means is &dapted to compare
the signal PM with a set point Pl, which is higher than set point PL and
less than the normal operating pressure of the apparatus, and to activate
an alarm means when PM is equal to or less than set point Pl, but greater
than PL, thus providing a warning that PM is approaching PL.
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The extrusion apparatus comprises first, second, third and fourth
zones as hereinafter described, feeding means for feedine a solid, extrudable
plastic material to the extrusion apparatus and reciprocating means to force
the plastic material into and through the extrusion apparatus. In the first
zone a plastic melt is formed into a desired cross-sectional area and main-
tained in the desired shape until solidified. This first zone is generally
a long land die having a la~d length of 10 to 100 times the thickness of the
profile.
In the second zone, the plastic melt is plasticated, i.e., the
melt is thoroughly admixed or kneaded in preparation for extrusion. In the
third zone, the plastic material is heated to form a plastic melt, and the
fourth zone comprises the feed input section of the extruder apparatus.
In associ~tion with the second zone, there is generally a flow
control means to ensure that material flow is unidirectional within the
extruder.
In association with the first zone are means to provide a smoothly
decreasing temperature gradient along the first zone. The plastic mass is
in the melted state as it enters the first zone asd must be solidified prior
to removal from the first zone.
Heating means are provided in association with the third zone to
heat the plastic material to a melted state.
Plastic material is supplied to the first zone by a feeding means
which can be a vibratory feeder, a screw feeder or other means known in the
art.
Referring now to FIGURE 1, the extrusion apparatus comprises a
long land die 2, a feed hopper 4, a plasticizing and flo~ control means 6,
a plunger 8 attached to crosshead 10, a connecting rod 12, and a fly ~heel
14, driven by motor 16 by means of belt 18. The entire extrusion apparatus
is su~ported on a base 20. A supply hopper 22 and feeder 24 supply pellets
of extrudable thermoplastic to feed hopper ~, the plastic being supplied to
hopper 22 from a suitable source, such as indicated diagrammatically by line
~L0tj7~ L
26. The extrudate is illustrated by the elongated portion 28. The diagram-
matic representation 30 represents other suitable apparatus such as water
coolers, brakes, supports, etc., which are supplied as desired.
As illustrated in FIGURE 2, the plastici~ing and ~low control
means 6 comprises a body 32, a flow control mandrel 34, and a plasticizing
mandrel 36. It is seen that m~ndrels 34 and 36, together with body 32, de-
fine an annular pa~sage 38. The plasticizing and flow control means further
comprises a transition mandrel 40 which serves to adapt annular passage 38
to the annular passage 39 defined by die 2. There are illustrated a plural-
ity of heaters 42 on the exterior of body 32. Heaters can also be provided
in the interiors of mandrels 34 and 36.
The flow of the plastic mass through that portion of the apparatus
illustrated in FIGURE 2 is from left to right. Shown on the exterior of die
2 are means 44 for providing a smoothly decreasing temperature gradient along
the die to solidi~y the melt prior to removing the shaped article from the
die.
The control system shown in FIGURE 2 comprises a signal means 48
in the annular passage 38 adapted to provide a signal PM in accordance with
the melt pressure of the plastic mass in the plasticizing zone of the ex-
truder, and signal receiving and con~rol means 50 in communication with
signal means 48 in further communication with feeder 24. Control means 50
is adapted to compare signal PM with a set point PH and to stop feeder 24
when PM i8 equal to or greater than PH.
Control means 50 can also be adapted to compare signal PM with
a set point PL and to stop feeder 24 when PM is equal to or less than PL.
Control means 50 can further be adapted to compare signal PM to set points
Ph and Pl and to activate an alarm means when PM i8 outside the range of P
to Ph, i.e. Pl ~ PM ~ Ph, thus providing a warning that PM is approaching
PL or Px~
~he set points PH and Ph are determined in accordance ~ith the
design stress of the long land die. Since it is important that the design
stress not be exceeded, PH is generally set lower than the design stress in
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order to provide a safety factor. For example, in a typicAl operation of
a dynamic extruder wherein the long land die has a design stre~s of 5,000
psi, the melt pressure of the plastic mass is in the approximate range of
1,000 to 1,500 psi. Control means 50 is set to provide an alarm signal when
the melt pressure is about 3,000 psi and to shut off the supply of pla3tic
to the extruder when the melt pressure is in the approximate range of 3,500
to 4,000 psi. Thus, set point P~ is proportional to the up~er range of
3,500 to 4,000 psi and set point Ph is proportional to 3,000 p8i.
The low set point PL and low alarm set point Pl are set below the
normal operating melt pressure. These settings are not considered to be
critical. Their functions are to indicate failure of the pressure-re~ponsive
element and/or loss of power to the pressure-responsive element.
Pressure-responsive element 48 should be located so that it will
be responsive to the pres3ure of the plastic melt. In this regard, it i3
preferred that the element 48 be located in communication with annular
passage 38 at or near the zone defined by the plasticizing mandrel 36. Two
possible locations, 49a and 49b, are shown in FIGURE 2. Of the two, loca-
tion 49a is presently preferred since the melt pressure at this point is
more representative of the pressure at the inlet end of long land die 2.
In the embodiment illustrated in FIGURE 2, the control system is
electro-pneumatic. Air pressure, from a source not shown, is ~upplied
through conduit 52 to signal means 48 which provides a signal PM relative
to the melt pressure within annular passage 38, through conduit 54, valve
56 and surge tank 58 to control means 50. Valve 56 and surge tank 58 are
provided in conduit 54 to smooth out, i.e., to average, the signal PM.
Within control means 50 signal PM is compared to set points PH and PL.
Line 60 represents one of the electric power leads to feeding means 24.
Where signal PM is ou~side the range PL to PH, electric power to feeder 24
via line 60 is interrupted in control means 50 and the supply of plastic to
feed hopper 4 i8 halted.
In one instance5 feeder 24 was a Syntron Vibratory Feeder Model
F-T01, signal means 48 was a Rosemont Pressure Transmitter Model 1401A-3D2
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with a receiver gauge 0-10,000 psi range, signal receiving and control means
50 was a Rosemont Model 1403-1 adJustable pressure switch with two sets of
contacts individually adJustable to a pneumatic signal of from 0-30 psi. One
set of points was set to close, sounding an alarm when the pneumatic ~ignal
PM indicated the pressure had reached 3000 p8i, and the second set of points
was set to open when the pneumatic signal indicated the pressure had reached
3,500, interrupting the power supply to the Syntron Vibratory Feeder.
Although not used in the specific instance previously described,
an ad~ustable pressure switch for the low alarm set point Pl and l~w set
point PL can be a separate control with one set of points set to close,
sounding an alarm when the pneumatic signal PM indicates the pressure has
dropped to 500 psi, and the second set of points set to open when the pres-
sure has dropped further to 200 psi, interrupting the power supply to the
vibrator feeder. The ad~ustable pressure switch for the low pressure alarm
and set point could be combined with the high pressure alarm and set point
if desired.
While the control system of this invention has been described in
terms of an electro-pneumatic system, it will be appreciated that all
electric and all pneumatic or hydraulic systems can also be employed. Other
features which c~n be employed in the control system of this invention such
as manual override for startup, alarm warning systems, visual indication of
melt pressure and the like will be apparent to those skilled in the art.
Reasonable variations and modifications of this invention can be
made or followed, in view of the foregoing disclosure, without departing
from the spirit or scope thereof.
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