CA1067661A - Control system for dynamic extruder - Google Patents

Abstract

CONTROL SYSTEM FOR DYNAMIC EXTRUDER

Abstract of the Disclosure A control system for a dynamic extruder wherein the pressure of the plastic melt is monitored and compared to a set point. When the melt pressure is equal to or greater than the set point, the supply of plastic to the extruder is halted, thereby preventing any increase in melt pressure and thereby precluding rupture of the extrusion die due to excess pressure.

Description

1~'7~61 24377 CONTROL SYSTEM FOR DYNAMIC EXTRUDER
_ 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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~0~7661 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.

~67G61 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 ~7~6~

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 ~0~76~1 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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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A dynamic extrusion apparatus comprising (1) a housing and a die means having a mandrel therein and defining first, second, third, and fourth zones wherein said first zone is where a plastic melt is formed into a desired cross-sectional shape and maintained in said shape until solidified;
said second zone is in operable communication with said first zone and is where said plastic melt is plasticated; said third zone is in operable communication with said second zone and is where a plastic material is heated to form said plastic melt; and said fourth zone is in operable communication with said third zone and is where said plastic material is introduced into said apparatus; (2) heating means in association with said third zone for heating said plastic material to a melted state; (3) means providing a smoothly decreasing temperature gradient along said first zone; (4) a reciprocating means communicating with said fourth zone to force said plastic material through said third and second zones into said first zone; (5) feeding means for feeding said plastic material to said fourth zone; (6) signal means in operable communication with said second zone to measure pressure of plastic melt in said second zone and provide a signal PM; (7) signal receiving and control means in communication with said signal means adapted to receive said signal PM and to compare said signal to a set point PH, the value of set point PH being lower than the design stress of said first zone, said means being in further communication with said feeding means and adapted to stop said feeding means when said signal PM is equal to or greater than set point PH, thereby stopping the flow of said plastic material to said fourth zone.

2. The apparatus of claim 1 wherein said control means is further adapted to compare said signal PM to a set point PL, the value of said set point PL being lower than the normal operating pressure of said apparatus, and adapted to stop said feeding means when said signal PM is equal to or less than said set point PL.

3. The apparatus of claim 2 wherein said control means is further adapted to compare said signal PM to a set point Ph, the value of said set point Ph being less than said set point PH and greater than the normal operating pressure of said apparatus and adapted to activate an alarm means when said signal PM is equal to or greater than said set point Ph.

4. The apparatus of claim 3 wherein said control means is further adapted to compare said signal PM to a set point P1, the value of said set point P1 being greater than said set point PL and less than the normal operating pressure of said apparatus and adapted to activate an alarm means when said signal PM is equal to or less than said set point P1.