Method for regulating an extrusion device and extrusion device using said method

Abstract

Method for regulating an extrusion device for the manufacture of a continuous strip of profiled product (P) made of an elastomeric compound, comprising a screw rotationally driven in a fixed barrel the outlet of which comprises an extrusion die and at least one rotationally driven cylindrical roller, the rotational speed of the screw (.sub.screw) being directly connected to the rotational speed of the roller (.sub.roller) by a proportionality coefficient, including: for a given elastomeric compound and a given profiled product (P), determining a setpoint value Ks of the proportionality coefficient, measuring the thickness (e) of the profiled product (P) leaving the extrusion die at each instant and correcting the value of the proportionality coefficient to Ks, correcting the rotational speed of the screw taking account of the corrected value Ks of the proportionality coefficient, such that .sub.screw=Ks*.sub.roller The roller speed is adjusted as a function of the corrected value Ks of the proportionality coefficient.

Claims

1. A method for regulating an extrusion device adapted for the manufacture of a continuous strip of profiled product (P) made of an elastomeric compound, comprising a screw rotationally driven in a fixed barrel the outlet of which comprises an extrusion die and at least one rotationally driven cylindrical roller, the rotational speed of the screw ( screw) being directly connected to the rotational speed of the roller ( roller) by a proportionality coefficient, the regulating method comprising: for a given elastomeric compound and a given profiled product (P), determining a setpoint value Ks of the proportionality coefficient, and the setpoint values for the roller speed roller up to a maximum setpoint roller speed roller max for which the maximum rotational speed of the screw is screw max=Ks* roller max, for the given elastomeric compound and the given extrusion device, determining a slip limit speed as being a mean rotational speed for the screw whereby the screw turns without delivering the profiled product (P), measuring the thickness (e) of the profiled product (P) directly as the profiled product (P) leaves the extrusion die is measured at each instant, correcting the setpoint value of the coefficient Ks as a function of the measured value of the thickness (e) to a corrected value Ks so as to bring the measured value back to a setpoint value (e setpoint) of the thickness of the profiled element (P), correcting the rotational speed of the screw taking account of the new corrected value Ks of the proportionality coefficient, such that screw=Ks* roller wherein the roller speed is adjusted as a function of the corrected value Ks of the proportionality coefficient, the roller speed is lowered when the rotational speed of the screw screw exceeds the slip limit speed, and the roller screw speed is maintained if the corrected screw speed is below the slip limit speed.

2. The regulating method according to claim 1, wherein when the screw speed does not exceed the slip limit speed, the roller speed increases with the reduction in the corrected value Ks of the proportionality coefficient.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will be better understood from the remainder of the description which relies on the following figures:

(2) FIG. 1 schematically depicts an extrusion device of the invention and the regulation control loops used for implementing the method according to an embodiment of the invention;

(3) FIG. 2 illustrates the change in the proportionality coefficient as a function of the measured thickness of the profiled product for an elastomeric compound;

(4) FIG. 3 illustrates various curves indicative of the slip limit speed for various elastomeric compounds;

(5) FIG. 4 graphically depicts the application of the regulation method of an embodiment of the invention to an elastomeric compound.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(6) The extrusion means depicted in FIG. 1 comprise a screw 2 rotationally driven by a motor (not depicted) in a cylindrical barrel 1. Downstream of the screw, the extrusion die 5 is formed by the gap between a profiled blade 4 and a roller 3 likewise rotationally driven. In one alternative form of the invention, the extrusion die 5 is formed between two rotary rollers, each one driven in rotation about its longitudinal axis which is perpendicular to the direction in which the profiled product leaves.

(7) The roller 3 collaborates directly with the receiving surface of a drum 6 on which the product P is intended to be wound. In this configuration, the roller 3 also acts as a roller for applying the strip of extruded product onto the receiving surface. This arrangement proves to be particularly advantageous when, as in the example illustrated in FIG. 1, the desire is to wind a narrow strip of elastomeric compound of constant section by filament winding so as to reconstruct a final section on a receiving surface such as a tire building drum.

(8) In order to be able to place the product P directly onto the receiving surface of the drum 6, there needs to be a possibility of varying the mass flow rate leaving the extruder in proportion to the exit speed, so that the cross section of the extruded profiled element remains constant. The exit speed may therefore be modified at will from a zero value to a nominal value without adversely affecting the geometry of the product.

(9) In order to achieve that, the device comprises speed measuring means, such as optical encoders, which are associated with the roller 3 and with the screw 2 and a first regulation control loop 8 regulating the speed of the screw 2. Thus, the optical encoder associated with the roller 3 determines at each instant the rotational speed of the roller .sub.roller and transmits this information to the command controlling the screw speed, so that at any instant the rotational speed of the screw .sub.screw is equal to the product of the rotational speed of the roller .sub.roller times a proportionality coefficient K, such that .sub.screw=K*.sub.roller.

(10) The setpoint value Ks of the proportionality coefficient K is determined experimentally for each elastomeric compound used with a specific extrusion device. To do that, a roller speed is selected and the screw speed needed in order to obtain a profiled element of the desired cross section is determined. It is found that this proportionality coefficient is substantially constant over the normal operating range of the extrusion device.

(11) However, variations in the proportionality coefficient may be observed as a function of the throughput flow, because of variations in the pressure and temperature of the compound in the cavity upstream of the extrusion die. These variations can be assessed during the phase of learning and experimental determination of the proportionality coefficient.

(12) In order to correct these linearity errors, a second regulation control loop 9 based on the measurement of the thickness e of the profiled element P directly as it leaves the extrusion die using a thickness measurement sensor 7, for example of the contactless type such as measurement means using a laser beam, is introduced. The measured value of the thickness e is input into a thickness regulator 10 which also receives the setpoint values of the thickness e.sub.setpoint originating from the memory-storage means 11. The thickness regulator 10 calculates a correction coefficient cor for correcting the proportionality coefficient. Thus, the setpoint value of the proportionality coefficient Ks is modified to a corrected value Ks so as to bring the measured value of the thickness e back to a setpoint value e.sub.setpoint of the thickness of the profiled element P.

(13) The new value of the proportionality coefficient Ks is input into the first regulation control loop 8 that regulates crew speed so that, using the corrected proportionality coefficient value Ks obtained from the relationship Ks=Ks*cor, the speed can be obtained using the relationship .sub.screw=Ks*.sub.roller or .sub.screw=Ks*cor*.sub.roller.

(14) The monitoring of the difference between the measured thickness e and the setpoint thickness e.sub.setpoint is performed by a conventional PID-type regulator.

(15) This regulation makes it possible to obtain a profiled product P of constant cross section leaving the extruder during continuous production. However, in a restart after a temporary production stoppage (which means a stoppage without emptying the barrel 1 of the device), the temperature of the elastomeric compound inside the extrusion device increases notably at the entry to the screw and the efficiency of the extrusion device drops. What happens is that it has been established that the optimum throughput per screw revolution is obtained when the elastomeric compound at the entry to the screw is cold (around 20 C.) and that at the exit of the screw is hot (around 110 C.) in relation to the temperature of the mechanical elements. As a result, during an extruder stoppage, the elastomeric compound at the entry heats up through prolonged contact with the hotter mechanical elements whereas the temperature of the elastomeric compound towards the exit drops upon contact with mechanical elements which are cooling.

(16) The screw efficiency also drops when the feed rate of the extruder device drops or when the elastomeric compound inside the device becomes more fluid.

(17) In order to correct for this drop in efficiency, the extrusion device of the invention comprises a third regulation control loop 12 that allows the laying speed, which is the reference speed of the roller, to be adapted as a function of the screw efficiency.

(18) Because screw efficiency is expressed as the throughput of elastomer per screw revolution, for one and the same screw, the ratio between the roller speed and the screw speed is indicative of the inverse of the efficiency of the extruder screw. Thus, in order to maintain a constant cross section, as the screw efficiency drops, the command controlling the speed of the screw demands an accelerating of the screw in order to keep the throughput constant. Accelerating the screw under working conditions in which this screw has low efficiency, would lead to an increase in the temperature of the compound and, thereby, cause the screw to run away and to slip.

(19) In order to avoid runaway of the screw on the occasion of a transient drop in its throughput per screw revolution, the solution proposed by the invention is to adapt the roller speed according to the change in the proportionality coefficient K. Thus, when the screw 2 receives a command to accelerate, the solution is to reduce the speed of the roller 3. As a result, the speed of the screw 2 will drop because it is directly connected to the speed of the roller 3 by the proportionality coefficient.

(20) In order to do that, the change in slip limit speed of the screw as a function of the roller speed and of the maximum screw speed acceptable for a given elastomeric compound is determined. This change in slip limit speed is taken into consideration by the roller speed control means which supply a reference value to the means that control the speed of the roller 3, in the way that will be explained on the basis of the example which follows.

(21) FIG. 2 illustrates an example of the way in which the proportionality coefficient K changes as a function of the measured thickness of the profiled product. Thus, the proportionality coefficient varies between a setpoint value Ks=1 (when the thickness is constant and equal to the setpoint value) and a corrected value Ks (which takes into account the drop in thickness which is a consequence of the drop in throughput). In the example depicted in FIG. 2, the corrected values Ks are Ks1=1.5, Ks2=2 and Ks3=3. The corrected value Ks is applied to the relationship linking roller speed to screw speed so as to bring the thickness back to its setpoint value. It will be noted in FIG. 2, that, for certain roller speed values, the screw speed may reach high values depending on the Ks value applied.

(22) The invention proposes a third regulation control loop 12 comprising means 14 of controlling the roller speed as a function of the value of the corrected proportionality coefficient Ks calculated by the second regulation control loop 9 so that the screw speed does not exceed the slip limit speed.

(23) FIG. 3 illustrates, using exponential curves, the change in screw slip limit speed for 3 elastomeric compounds and one and the same extrusion screw. The change in limit speed is defined in the example depicted using an evolution law the equation of which is

(24) $\begin{array}{cc}{}_{\mathrm{screw}}=a\sqrt[n]{\mathrm{roller}}& \left(1\right)\end{array}$
where a=.sub.screw max/.sub.roller max.sup.1/n, and n is a constant associated with a given elastomeric compound.
In the example depicted, the first curve .sub.p1 is plotted for a constant n=2 and a coefficient a=7, the second curve .sub.p2 is plotted for a constant n=4 and a coefficient a=19, and the third curve .sub.p3 is plotted for a constant n=5 and a coefficient a=23. The constant n is dependent on the type of elastomeric compound used and is preferably comprised between 2 and 6. Thus it is found that the higher the constant n, the smaller the limitation applied to the laying speed.

(25) It is also known that, according to the regulation control loops of the device of the invention:
.sub.screw=Ks*.sub.roller(2)

(26) Substituting the screw speed of equation (2) into equation (1) gives the calculated roller speed:
.sub.roller law=(.sub.screw max.sup.n/n-1/.sub.roller max.sup.1/n-n)*1/Ks.sup.n/n-1

(27) The roller speed .sub.roller law can thus be calculated as a function of the corrected proportionality coefficient Ks. This calculated value of the roller speed is analysed by the control means 14 which compare it against the setpoint speed .sub.roller determined for a pre-established recipe. Such a recipe is created for a given elastomeric compound and a given setpoint thickness and is stored in the memory 13 of the third regulation control loop 12 of the device. The values that characterize a recipe are: the roller setpoint speed, the roller maximum speed, the screw maximum speed, the setpoint value Ks of the proportionality coefficient and the constant n. If the calculated value of the roller speed .sub.roller law is lower than the setpoint roller speed defined in the recipe, then the reference roller speed value chosen by the control means is the calculated value of the roller speed .sub.roller law. In the opposite case, the reference roller speed value is that defined in the recipe.

(28) This regulating method may also be explained with reference to FIG. 4 in which the straight lines representing the proportionality coefficients of FIG. 2 and a screw slip limit speed curve illustrated in FIG. 3 have been superposed. The reference speed of the roller is the point of intersection between the curve .sub.p1 and the reference straight line of the coefficient Ks. Thus, when the corrected screw speed is on the straight line Ks.sub.3, but above the curve .sub.p1, the roller speed at the point A is chosen. For a roller speed corresponding to the point A, the extrusion device may produce a profiled element P of the correct thickness, but at a laying speed that is lower than that of the recipe. When the operating regime of the screw improves, i.e. when the hot mixture has been discharged, the throughput per screw revolution increases and the efficiency improves, which means that the regulation control loop commands a smaller correction to the proportionality coefficient Ks and the roller speed can increase up to values corresponding to the point B, then to the point C, up to the point F which corresponds to the maximum values for the screw and roller speeds.

(29) Thus, using the regulating method and the device of an embodiment of the invention, automatic adjustment of the roller speed (or of the laying speed) is obtained for an extrusion device screw speed that avoids slippage and supplies a profiled product of the correct dimensions and correct physico-chemical properties, for optimal productivity of the device.

(30) Other alternative forms and modes of embodiment of the invention may be conceived of without departing from the scope of the claims thereof.