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Extruder control
Copyright
© IFAC PRP 4 Automation,
Session 4
Ghent, Belgium 1980
EXTRUDER CONTROL
s. Dormeier Fachbere':ch Elektrotechnik-Elektronik, Universitiit- Gasamthochschule, Paderborn, Federal Republic of Germany
Abstract. Methods of regulating extruders, or extruder plants an extrusion blow moulding unit has been chosen as an example of this, are discussed. First the state of technology in the sphere of extruder regulation in current industrial practice is dealt with. The increasing requirements in quality and profitability make an improvement in control strategies necessary. This in turn necessitates the application of new measuring methods and an analytical description of the extrusion process. The new insight into the extrusion process thus gained can normally only be put into practice on the basis of computer controlled regulation methods, microelectronics supports this trend with increasingly efficient and inexpensive microcomputers. The application of a computer based control strategy is demonstrated through the example of an extrusion blow moulding unit. Keywords. Extruder control; temperature control; process model; microcomputer; extrusion blow moulding process; adaptive systems. INTRODUCTION
These processing stages can normally be ascribed to individual sections of an The increasing use of extruded products extrusion plant (Fig. 8). In the extruin technical spheres and the resultant der, as seen from the hopper to the increase in required standards have a screw tip, the melt is produced; in the growing influence on the development extruder die which, in many cases, can be considered as a continuation of the and application of measuring and conextruder, the forming out and possibly trol systems which make possible a calibration, for example, for flexible reaction to changed process situations and enviro~tal influences. - blow moulding, - film or sheet extrusion and In the extrusion process the most important factors are the level and con- pipe extrusion stancy of the throughput and the m?inare carried out. tenance of a prescribed product quality. In this respect "quality" means a At the third stage the extruded matericombination of properties which are strongly dependent on where and how the al is cooled, sometimes in a plant section separate from the extruder. All particular extruded product is to be used later. However, a factor common to these sections have an effect on the quality, but in the last two sections all elements of quality is that confaults from the first section can be sistency can only be achieved by an corrected to some extent. The throughappropriate control of the most imporput and hence the profitability are tant process variables. largely determined by the first two phases. The extrusion process can be divided into three stages in the flow direction Optimal open and closed loop control in of the extruded material: the extrusion must therefore take into - production of a homogeneous melt account the entire plant. of a particular through?ut and state of melt, The following deals with some control - forming out and possibly calibraproblems in extruders and - here in the tion, case of extrusion blow moulding - complete extrusion plants. In the future, - cooling. the technical realisation of these
551
s.
552
Dormeier
control concepts will be based almost exclusively on the application of microprocessors.
volume due to the volume shrinkage connected with the melting, - flat out discharging zone (metering-/homogenizing zone).
SOME REMARKS ON THE EXTRUDER AND EXTRUSION PROCESS
In conventional extrusion the metering zone determines the conveying rate, this generally leads to low throughputs Extruder /1/. In practice measurements almost always show that the pressure profile As a conveying machine the extruder has a maximum ("pressure peak") at the today belongs almost exclusively to the family of the screw machines. There beginning of the metering zone. The metering zone then produces no more are two basic types of plastics extrupressure, but is a pressure consumer. der: the single and the twin screw The conveying in the feeding zone of a extruder. In this paper only the single conventional extruder depends on the screw extruder is taken into considefriction forces between the solid ration since this is the most frematerial and the cylinder as well as quently used form of plasticizing exscrew surfaces. At this point, the truder. The principle of the single friction coefficient steel/stock plays screw plasticizing extruder is shown an important role. The conveying is in Fig. 1. often insufficient as the ducts are often not filled completely. This can be a reason for throughput pulsations. Compulsively conveying extruder hea ter band
barrel
-hopper
In contrast to the conventional extruder, the feeding zone becomes det~rmi nant. Essentially higher and quasl pulsation free throughputs are the result. The process steps here are as follows /1/: --;tock temperature L - _ - - - - - - - -_ _~ pressure reduction gear throughput barrel temperature of zone 2 viscosity
- feeding, - conveying, compacting (pressure built up), - conveying of a compacted solid block,
Fig. 1. The principle of an extruder. The screw rotates in a heated cylindrical barrel with a die at the outlet end. Solid polymer of a granulated or powdery kind is added to the feed hopper and from there transported by the screw to the die. As a consequence of shearing in the screw and heat transfer through the barrel, the polymer is melted. This melt is extruded through the shaped die at high pressures. The plasticizing is achieved by convective heat transfer over cylinder and screw and by friction in the melt. At present two extruder designs can be found for the single-screw extruders: - conventional extruder, - compulsively conveying extruder. Conventional extruder The screw is divided into three zones corresponding to the process steps: - deep cut feeding zone, - transition zone with decreasing
- preheating, preplasticizing, - rest plasticizing, - homogenizing. Good trickling behaviour of the material to be extruded and a suitable feed opening are a necessary but not sufficient condition for high throughputs of an extruder as the friction conditions also determines the conveying behaviour of the material .. I~ conveying auxiliaries such as addltlonal screw systems or stuffing cylinders are disregarded, one can say that excentrical, inclined feed openings are optimal for a feed design /2/. Whereas in the conventional extruder a force (friction) based connection in the smooth feeding zone is obtained as a result of the friction ratio stock/ cylinder/screw, in the compulsively conveying extruder grooves in the feeding zone lead to a shape-based connection between material and cylinder wall, since the conical bush geometry contributes to a strong compacting of the material. The grooves cause a more effective
Extruder Control
"holding" of the compacted material block against rotation with the screw than more frictional effects, so that the material is approximately axially conveyed like a fixed nut on a rotating spindle.
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a screw speed range controlled in the limits of 3:1 up to 6:1. The following drive types are normally applied /3/: a) drives with mechanical speed control b) drives with electric speed control
The extruder as a multi-variable system The extruder can be considered as a multi-variable system. The controlled variables are the throughput, and those which represent the state of melt, such as stock temperature, pressure and viscosity. As step variables one may consider, for example, the heat introduced into or extracted from the cylinder, the friction energy, or the screw speed, while fluctuations in the electric power or environmental temperature together with the chemical and physical material properties may operate as disturbance variables. These relationships are represented in Fig. 2.
process parameters
disturbance variables -
step variables
'I
~
con trolled variables
ex truder 1
controtlers
l
set POints
-
- d.c. shunt wound electric motor with thyristor control - Ward-Leonard set and d.c. shunt wound motor - three phase a.c. shunt wound motor with pole changing c) drives with hydraulic speed control With a d.c. shunt wound motor a continuous and exact screw speed adjusting is possible (fluctuations ~ 1 percent). Torque fluctuations have a slight dependence on the screw speed. With a current limitation one can ensure that the torque does not exceed prescribed limits. For a long time three phase ~.c. shunt wound motors were of great lmportance as separate drives for extruders before thyristors came on to the market. The speed can be adjusted to an exactitude of 2 to 3 percent. An on-off controller with internal dynamic feedback is normally used as a regulator. Nowaday a.c. drives are only used on small extruders. They are only economic up to a motor capacity of 60 - 75 kW /1,3/.
Fig. 2. Multivariable extrusion process block diagram. Screw temperature control Because of the construction difficulties involved in such a control, it is only carried out in large extruders. The advantages of screw temperature control are to be seen in the reduction of the heating up period and in the possibility of regulation of the stock temperature profile during production. CONTROL STRATEGIES The radial temperature profile of the melt can be reduced by means of screw The simplest, and therefore most comcooling. monly applied method of extruder conFor heating and cooling, oil is used as trol, is the simplification of the a tempering medium, but if the screw problem so that only the easily measured variables such as screw speed and is only to be cooled during production water is used as a cooling agent. ' barrel wall temperatures have to be taken into account, or rather are conBarrel wall temperature control trolled by means of single loop control equipment. Improvements in measuring As shown in Fig. 1 the extruder barrel techniques and process know how obis divided into several heating zones, tained from steady state and dynamic process models make it possible to re- normally 3 to 6. As a result of the heat energy contained in the transporalize more effective control strateted material and the conductivity of gies such as cascade loops or adaptive the barrel vlall, the heating zones are systems. thermically connected. In practice, there is a single loop control for each Screw speed control heating zone. Because of the extreme Extruders with only one screw speed are low pass character of the controlled exceptional (compounding-, granulating elements with time constants in the units). Controllable drives mostly have order of ten to thirty minutes, on-off
The disturbance variables not only add to the influence of the step variables but also, to some extent, affect the relationship between step and controlled variables.
s.
554
Dormeier
controllers are mostly used in industrial practice. These controllers, which are equipped with internal dynamic feed-backs make it possible to set up combinations of PO and PlO controllers /4,5/, which ensure that the temperature course doesn't overshoot when a set point change is made. This effect is achieved by a sort of I-channel disconnection in the case of significant deviations, so that a reset wind up is avoided. An on-off controller with a relay output as adjusting element costs about a third of the price of a comparitive steady controller. New developments in controllers aim towards the application of microprocessors so that multiloop, cascade or adaptive systems can be set up. A further advantage with digital controllers is the long term stability of the control setting and the increased possibility of exact reproduction. For example, an investigation of a sheet extrusion line has shown that a temperature drift of 1 0 C in a heating zone near the die can bring out a local change of thickness of the sheet up to 3.6 % depending on the material type and throughput /6/. Investigations have been carried out on digital barrel wall and stock temperature control systems /7-11/. The modified PlO and a compensation algorithm were employed. The PlO algorithm can be derived by setting up the difference equation and using numerical integration. The recursive form is then: y (k) = Y(k - 1) + do xd (k ) + d
1
x
d
(k - 1) + d
2
x
d
(k -
2)
(1)
where:
Fig. 3 demonstrates courses obtained by a step set-point change.
200
b
tolerance :!0,5
J
195
~190 ~
OJ c......
~
CJ
185
c......
OJ
~
OJ ....... Qj
~ 180 -Cl
o
5
10
15
20 min
25
time t
Fig. 3., Closed loop response to a step set-point change. Curve a: PlO with "I-channel" disconnection Curve b: PlO with optimal constants for process disturbance Curve c: Compensation-algorithm for a PT -T element 1 t Curve d: PlO with saturation of the adjusting element.
The control results demonstrate clearly that the use of disturbance and set y = output signal (step variable) point optimal controller parameters leads to fluctuations in the courses x = input signal (error signal) d of the process variables, which means d ,d ,d = Coefficients which depend that the current practice of steady o 1 2 on magnitude ratio, incontroller tuning for both types of tegral time constant, de- operation is no longer appropriate for rivative time constant future OOC systems. and sampling time The theoretically calculated settling In order to restrain the algorithm from time for the compensation algorithm producing an output signal beyond the could not be realized because of the saturation of the adjusting element non-linearities of process (changing because of the time lag, absolute high of operating point, material and enand low limits of the output value are vironmental conditions). considered. Using the digital simulation of the whole control loop for A comparison of the control results optimal controller adjustment this achieved with various PlO controllers limitation can be allowed for. at step set point changes is shown in Fig. 4.
Extruder Control
555
time have proved favourable for this purpose /12-14/. ~
critical feature of the thermocouples that they protrude into the melt so that there is a certain danger of damage if insufficiently plasticised material flows past them in the initial phase or the stock temperature drops below a critical level during production resulting in increased viscosity. The positive features are the simple hardware structure and the low financial costs. 1S
QJ
c....
~ 185OC.......----II--I~----+-------+----+------I CJ c....
~ .....
Smin
10min
15min time
20min
2Smin
Ultrasonic temperature measurement involves no melt contact. The measuring of the travel time of ultrasonic pulses traversing the polymer melt in the flow channel of the extruder gives a reliable indication of the effective temperature of the flowing melt . Fig. 5 shows the principles of arrangement for ultrasonic travel time measurement /13/. Increasing distance between molecules causes a lower sonic velocity i.e. an increasing travel time.Thus the travel time of an ultrasonic pulse will increase approximately in proportion to a rise in temperature and to decreasing pressure.
Fig. 4. Comparison of closed loop responses. Curve a: Continiuous PlO controller, Curve b: Best result achieved with the compensation algorithm, Curve c: Switching PlO controller, Curve d: Modified PlO algorithm. The transistor from conventional switching controllers to the digital kind is justified, not so much by improved control results in barrel wall temperature control loops as by economic considerations combined with digital date processing. Stock temperature control The methods of temperature control considered so far are based on the assumption that a suitable temperature profile along the barrel wall together with the friction energy contributed by the rotating screw will lead to suitable stock temperature for the processing operation. This assumption is in the main correct for the steady state operating point but not for the unforseen effects of disturbances or changes in operating points so that the operators have to carry out adjustment~ To overcome this uncertainty in the process control it is necessary to measure the stock temperature in front of the screw tip. Measurement by means of thermocouples or ultrasonic travel
Fig. 5. Measurement of ultrasonic travel time at the end of the barrel. At a given pressure the travel time of an ultrasonic pulse traversing the cylinder or flow channel is influenced almost linearly by the temperature distribution between ultrasonic transmitter and receiver and therefore gives an indication of the specific heat transport to the die. Industrial application is to be found in the PVC pipe extrusion /14/. If the stock temperature is measured by one of these methods a cascade loop as shown in Fig. 6 can be set up and either all or only the most effective barrel temperature controllers can be employed as auxiliary controllers; ~M here represents the stock temperatu:e, ~Z3 ~he wall temperature of the thlrd heatlng zone, P Z 3 the heating
s.
556
Dormeier
power and K3 the magnitude ratio. The same principles apply for the other heating zones.
screw tip has been developed to measure the torque at the screw tip depending on the viscosity. The correlation between viscosity and torque for several materials has been proved. The torque-measuring signal can be taken as the reference input, whereas the temperatures of the barrel zones will serve as subsequent values for a cascade loop control.
cJ'Mset
Fig. 6. Cascade loop for a stock temperature control. A control strategy of this kind was employed with extruders on the basis of conventional controller (PlD and PD controllers) in the early seventies /15,16/. These systems did not spread to any extent because of the relatively complicated operating skills required of the processing staff. Further control systems were developed on the basis of the 8-bit microprocessors /7,8,10/. The operation can be simplified considerably by means of appropriate software, so that in future the large scale industrial use of digital stock temperature control systems is to be expected. Melt pressure control In many cases the registration of melt pressure is indispensable, because pressure changes cause throughput fluctuations and so disturb the processing operation. Both mechanical and electrical systems are used to measure pressure. Fig. 7 /3/ shows the design principles of a melt pressure control.
Fig. 7. Set-up of a pressure control. Equations and models for the control of the extrusion process The control strategies discussed so far are classic 'feed back' systems because corrective actions are only made after deviations have occurred. The growing demands made on the quality of extruder products led to the consideration of a mathematical description of the extrusion process with the aim of optimising plants and production process. The extrusion process is controlled by physical equations governing the conservation of energy, momentum and mass. Because of the complex interactions between the various polymer properties and process variables a dynamical modelling of the extrusion process is very difficult and a continuous measurement of the polymere properties during a dynamic situation is not possible.
Viscosity control Previous research works on The flow behaviour of the plastified the extrusion process modelling have polymer is characterised by the visbrought out steady state models which cosity as a function of shear-rate and are used for screw design and set up for temperature. Because of the difficulthe extruder /2,15,18,19/. A simple ties involved in measuring the state of steady state equation which describes melt directly by means of viscosity the influence of the barrel zone temwhile an extruder is in operation, it perature ~zi' the screw speed n s and is only possible to keep a check on the the throughput m on the average stock barrel wall temperatures or, if more temperature near the screw tip was used is needed, on the stock temperature for the setting up of a 45 mm diameterand pressure as indicators of the state extruder in /20/: of melt. One continuous measurement of u viscosity throughout the extrusion ~S = L: K. ~Z' + K I • i=1 1 1 (2 ) process is mentioned in /17/. A special
Extruder Control
with Ki , K , K : Coefficients I II n: Number of heating zones Range of validity for the extruder used: 180 0 C ~,J:. ~ 50 0 C Zl 15 min- 1 ~ n ~ 50 min- 1 s ~
6.9 kg/h
m
~
22,9 kg/h
An extension of this steady state equation to the dynamic sphere, provided there are constant material parameters, was carried out by MeiBner /15/. A comparison of the measured and calculated frequency responses shows a notably high conformity for the frequencies w ~ 0.01s-1. White and Scott /21/ determined the transfer functions relating screw speed to throughput experimentally by using the stepresponse techniques. They used step changes of the screw speed of about 20 rpm. In view of the non-linearity of the process, these steps are too high. Tadmor and Lipschitz /22/ presented a model which is based on a one-dimensioned parallel plate steady state momentum equation. Lumped rates of change of momentum effects are taken into consideration too. Kochar and Parnaby /23/ built up a model on the basis of stochastic identification. They developed the discrete transfer functions for the interaction between screw speed pertubations as input signal and melt pressure and temperature. The input signal has a PRBS-characteristic. The output signals ~M and p are both assumed to be contaminated by noise originating in random changes in other variables, including polymer parameters and measurement errors. This noise is assumed to be independent of the screw speed. This assumption can be made if one overlooks the interactions between screw speed pertubation dissipation and material properties. The model form reads /23/:
t9'"M (k)
Z -dJ--- BJ-( Z -1 )
C+,( Z-1 )
• e (k)
+
with: e: sequence of independent random variables (0,1), z-dp, z-d~: represent transport delays, PRBS: Pseudo Random Binary Sequence.o The model parameter values were chosen by step wise hill climbing adjustment in order to minimise a loss function which is proportional to the square of the difference between the model output and the actual process response. This discrete model was applied on a 38 mm screw diameter laboratory extruder and it was seen that the calculated model output variables conformed closely to those measured. The screw speed variations came to ±4 rpm and the resultant temperature variations approx. ± 2 0 C /23/. PROCESS CONTROL IN THE EXTRUSION BLOW ~·10ULDING In general the extruder is only one part of a plant producing films, sheets, mouldings, pipes etc. Hence the processing stages subsequent to the extrusion must be integrated into the control system, as is shown in the example of extrusion blow moulding. Remarks on the extrusion blow moulding process The continuous extrusion blow moulding process is characterised by the following four processing phases (Fig.8):
111 moulding and cooling in blow station IV demoulding and ~
11
I I moulding of par/son by extrusion II graspmg and cutting off of par/son
(3)
(Z-1) •
PH(k)
A
z-dp
(Z-1)
• ns(k)
p -1
+ Ct? ( Z D
p
)
(Z -1 )
• e (k)
~
.
,blow station
""AJ 111
~ -G- ~ ~IV
I
moulding of parison by extrusion,
11
grasping and cutting off of parison,
III
moulding and cooling in blow station,
IV
demoulding and cutting off of waste.
D'1J-( Z-1 ) B
cutting off of waste
Fig. 8. Course taken by extrusion blow moulding process.
1 · n (k) s A-",( Z - )
=
557
(4)
Process analysis /24/ has revealed that the demoulding temperature influences shrinkage, being a moulding
s.
558
Dormeier
quality. The term shrinkage is used to describe the accuracy to size of a moulding. The demoulding temperature is understood as the stock temperature of a moulding on an arbitrary point of moulding while the blow mould is opened. It is a value not to be measured directly, but to be computed by means of a system of equations /25/. Using these equations it is possible to determlne the cycle time (step variable) at a given demoulding temperature (target value). The cycle time tz consists of the blow time tB and the machine time tM. The machine time is known through motions, such as lift mould/lower mould etc. In /25/, for the process model an implied equation of following kind: t z - A exp[A + A exp(A - A t )] 1 2 5 Z 4 3
+ A - t M 6
=
f(t } z
(5)
was set up.
mould and have to be removed manually. This interruption leads to additional loss of material and time /9/. Despite variable production conditions, due to disturbing influences or to a change in the point of operation, parison length is to be kept constant. Bottom flash length or flash weight provides a measurement of optimum parison length. In the case of the given bottle (0,7 1) the target for process control should be a maximum flash weight of 3 - 3.5 g, not to be exceeded if possible, with a minimum flash weight of 1 g (weld line). With a preset cycle time, parison length depends upon parison output velocity from nozzle. Output velocity may be adjusted via screw speed. The velocity pattern is non-linear over the length of the parison. It is determined largely by screw speed , melt temperature and the size of nozzle opening. Parison velocity is measured by photo cells LS1 and LS2 (Fig. 9).
This equation is valid for tB ~ Tcand it proves to converge and have a zero point which can be determined iteratively in the process model(T : crysC tallisation time). Control strategy Because of the process model the strategy can only be realized by computer application. Control of parison length and the constant demoulding temperature desired are to some extent contradictory requirements, since the irregular parison velocity, in particular during startup, means that cycle time cannot be kept constant - as required for optimum parison length. A certain tolerance must therefore be allowed for the demoulding temperature. Observation of a pre-set parison length is very important for the economic production of hollow articles. A prerequisite for constant length is that the continuous parison extrusion process should be coordinated in time with the noncontinuous production of finished parts. Once an article has been demoulded, the parison for the next finished part must have achieved optimum length so that it may be transferred directly to the mould without additional waiting time. If the parison is too long, this leads to high material wastage and if wall thickness regulation is used then this causes displacement of thick and thin points along the longitudinal axis. If the parison is too short, this not only gives rise to material loss but often causes a stoppage in the automatic operation cycle when the remains of the parison stick to the inside of the blow
-,--
cuf- off /dev,ce -
~---.
partson procesSing ngfh
'r
315mm
300
.L-rzszsJ+-
3~rzszsJ+--_ 85 _.L-rzszsJ+-
Fig. 9. Arrangement of photo cells to control parison length. The measuring result has to be adjusted with a correction factor to give mean overall velocity from the mean parison velocity between LS1 and LS2. Photo cell LS3 monitors mould closing time during production to ensure that the mould is not closed prematurely. During the start-up phase LS3 is also used to measure mean parison velocity. Photo cells LS4 and LS5 monitor flash length. Parison length control based solely on determination of flash length has the disadvantage that when deviation does occur, correction by means of a change in screw rotational velocity only becomes effective as from the following parison. It would seem more appropriate to measure parison length before the end of the blow process using LS1 and
Extruder Control
559
The change in cycle time after the LS2. When the velocity pattern of the 25th and 55th cycle was effected parison is known, then the mean velocity measured at a specific point can rapidly with a simultaneous change be extrapolated to give the mean resi- in parison velocity. A velocity reduction takes longer than an increase. dual velocity of the parison between the point of measurement and optimum This is due to the fact that pressure processing length. Given a constant reduction in the screw chamber takes cycle time, parison length can be cal- place more slowly than pressure buildculated in advance. If this does not up. tally with the target value then cycle time can be lengthened or shortened to CONCLUSION give a favourable parison length within As a result of the constantly growing the same cycle. The basic operating mode of a computer demands of quality and profitability on the extrusion process, it has becontrol /9/ may be seen from Fig. 10. The measurements were recorded during come necessary to look for possibilities of improving the supervision and the start-up phase. Target cycle time control of extruders and extruder is not taken into account during the first four cycles. Cooling is effected plants. The digital computer, particularly in the form of the process for a blow time calculated by using a correction factor K2, since the paricomputer has here proved to be an son velocity does not correspond to its effective aid to the automation of target value. K2 is then corrected from the extrusion process for the following one cycle to the next and flash weight reasons: sinks accordingly from 4.8g to 3.5g. - computer controlled methods of For the fifth cycle the parison veloci~ measuring can register process corresponds to its target value. Hence, in order to observe cycle time, cooling variables such as viscosity, stock follows for the target blow time. Subtemperature etc; up to now this sequent monitoring of the flash shows has been possible with highly too great a flash length, whereupon a complex procedures. new velocity target value is calculated. This is then repeated until after the - a computer permits the introduc10th cycle when a favourable processing tion of special process models length has been achieved and cycle time into the process control concept is respected simultaneously (Fig. 10). and also means that an increasing number of theoretical methods may be used to solve automation 20 r-o-------,--I-----r------o-rl-----,\ tasks. mm 0°000°°0°00000000°000% °ocx>ooOoOoOGo c:: ~-s- 0 ° I REFERENCES ~ ~ 16 o~------+---------------r-~ t:::1~ ~
Q.
~
:
72
l--
-l------+-----~-I
c:: C
CJ a ClJ-
000%000
~ ~
/1/
Menges, G.: Kunststofftechnologie I. Vorlesung an der RWTH Aachen, WS 1976/77.
/2/
Kosel, U.M.: Optimierungsrechnungen beim Extrudieren. Dissertation an der RWTH Aachen, 1971 .
/3/
Fischer, P.: Stand der Regelungsund Steuerungstechnik bei Extrudern und Extrusionsanlagen dargestellt am Beispiel einer Tafelanlage. VDI Tagungsbuch "Rechnergesteuerte Extrusion?", VDI Verlag 1976, S. 27-48.
/4/
Merkel, J.: Das PDPI-Reglerprinzip. Elektronik 5(1973), S. 165-168.
/5/
Martens, J., Schweinebraten, M.: Temperaturregelung mit Zweipunktreglern und deren schaltungstechnische Realisierung. Regelungstechnische Praxis, 9(1978), S. 262-266.
ooooo~ooooooooooooCX>o I
39
s 29 24 79
r-------~----;_-------:-_____"ll ---~----
I
I
- fOOOOOOOOOOOOOOOOOOOOOOOOjO - - - - - ~o - - -----
14 {oooooooooooooo=oooooooo
-----
i I
~-
i oooo
Fig. 10. Flash weight pattern with adaptive parison length control.
560
s.
Dormeier
/6/
Michaeli, W.: Zur Analyse des Flachfolien- und Tafelextrusionsprozesses. Dissertation an der RWTH Aachen, 1975.
/18/ Maddock, G.H.: Extruder scale up by computer. Polymer Engineering and Science, 12(1974) .
/7/
Ramm, F.: Von der ProzeBanalyse zur Rechnersteuerung. 10. Kunststofftechnisches Kolloquium 1980, Aachen, S. 72-77.
/19/ Tadrnor, Z., Klein, I.: Engineering principles of plasticating extrusion. Van Nostrand-Reinhold, New York (1970).
/8/
Schwab, E.: Mikrorechnereinsatz zur ProzeBflihrung beim Extrusionsblasformen. Kunststoffberater, 7(1979). S. 324-327.
/9/
Dormeier, S.: Ein Beitrag zur /21/ White, D.H., Schott, N.R.: Dynamic Automatisierung des Extrutesting of plastics extrusion sionsblasformens. Dissertasystems. Proc. 30th ANTEC of tion an der RWTH Aachen, 1977. Society of Plastics Engineer~ Chicago (1972), S. 797-801. Junk, P.B., Brinkmann, N.: Der Mikrorechner an der Kunst/22/ Tadmor, Z., Lipschitz, S.D., stoff-Blasmaschine. MitteiLavie, R.: Dynamical model of lung der Firma Voith-Fischer, a plasticating extruder. Polymer Enging Sci. 2(1974), Lohmar. S. 112-119. PleBmann, K.W.: Mikroprozessoren. Vorlesung an der RWTH Aachen, /23/ Kochar, A.K., Parnaby, J.: DynamiWS 1976/77. cal modelling and control of plastics extrusion process. van Leeuwen, J.: Fundamental meas~ Automatica, Vol. 13(1977), rement and control in the exS. 177-183. trusion process. Plastics & Polymers, 6(1974) S. 104-113. /24/ Kulik, M.: Ein Beitrag zur Analyse des kontinuierlichen ExtruHerbertz, T.J.M.: Better control sionsblasformens, Dissertatmn of extrusion by ultrasonic RWTH Aachen, 1974. measurement. 3rd International IFAC Conference on Instm- /25/ Dormeier, S.: Development of a mentation and Automation in process computer control for the PRP-Industries. Brussels, an extrusion blow moulding Conference Proceedings 1, unit. Proc. 1,3rd IFAC ConS. 349-351. ference on Instrumentation and Automation in the Paper, Schiller, A.D.: Ultrasonic process Rubber and Plastics Industries control in PVC pipe extrusion Brussels, 1976, S. 261-267. Proceedings 37th ANTEC, Society of Plastics Enginee~, New Orleans, 1979, s. 378-38~
/10/
/11/
/12/
/13/
/14/
/15/ MeiBner, M.: Regelungstechnische Untersuchungen an Kunststoffextrudern. Dissertation an der RWTH Aachen, 1971. /16/ Melchior, F.: Kaskadenregelung an Kunststoffverarbeitungsmaschinen. Kunststoffe 9(1971), s. 609-612. /17/ Revesz, H., Hubeny, H.: Continuous measurement and control of viscosity throughout the extrusion process. Proc. 1,3rd IFAC Conference on Instrumentation and Automation in the Paper, Rubber and Plastics Industries, Brussels, 1976, S. 69-76.
/20/ Dormeier, S.: Aufbau von ProzeBrechnerprogrammen zur Steuerung von Extrusionsanlagen. VDI Tagungshandbuch "Rechnergesteuerte Extrusion"? VDIVerlag 1976, S. 109-130.
© IFAC PRP 4 Automation,
Session 4
Ghent, Belgium 1980
EXTRUDER CONTROL
s. Dormeier Fachbere':ch Elektrotechnik-Elektronik, Universitiit- Gasamthochschule, Paderborn, Federal Republic of Germany
Abstract. Methods of regulating extruders, or extruder plants an extrusion blow moulding unit has been chosen as an example of this, are discussed. First the state of technology in the sphere of extruder regulation in current industrial practice is dealt with. The increasing requirements in quality and profitability make an improvement in control strategies necessary. This in turn necessitates the application of new measuring methods and an analytical description of the extrusion process. The new insight into the extrusion process thus gained can normally only be put into practice on the basis of computer controlled regulation methods, microelectronics supports this trend with increasingly efficient and inexpensive microcomputers. The application of a computer based control strategy is demonstrated through the example of an extrusion blow moulding unit. Keywords. Extruder control; temperature control; process model; microcomputer; extrusion blow moulding process; adaptive systems. INTRODUCTION
These processing stages can normally be ascribed to individual sections of an The increasing use of extruded products extrusion plant (Fig. 8). In the extruin technical spheres and the resultant der, as seen from the hopper to the increase in required standards have a screw tip, the melt is produced; in the growing influence on the development extruder die which, in many cases, can be considered as a continuation of the and application of measuring and conextruder, the forming out and possibly trol systems which make possible a calibration, for example, for flexible reaction to changed process situations and enviro~tal influences. - blow moulding, - film or sheet extrusion and In the extrusion process the most important factors are the level and con- pipe extrusion stancy of the throughput and the m?inare carried out. tenance of a prescribed product quality. In this respect "quality" means a At the third stage the extruded matericombination of properties which are strongly dependent on where and how the al is cooled, sometimes in a plant section separate from the extruder. All particular extruded product is to be used later. However, a factor common to these sections have an effect on the quality, but in the last two sections all elements of quality is that confaults from the first section can be sistency can only be achieved by an corrected to some extent. The throughappropriate control of the most imporput and hence the profitability are tant process variables. largely determined by the first two phases. The extrusion process can be divided into three stages in the flow direction Optimal open and closed loop control in of the extruded material: the extrusion must therefore take into - production of a homogeneous melt account the entire plant. of a particular through?ut and state of melt, The following deals with some control - forming out and possibly calibraproblems in extruders and - here in the tion, case of extrusion blow moulding - complete extrusion plants. In the future, - cooling. the technical realisation of these
551
s.
552
Dormeier
control concepts will be based almost exclusively on the application of microprocessors.
volume due to the volume shrinkage connected with the melting, - flat out discharging zone (metering-/homogenizing zone).
SOME REMARKS ON THE EXTRUDER AND EXTRUSION PROCESS
In conventional extrusion the metering zone determines the conveying rate, this generally leads to low throughputs Extruder /1/. In practice measurements almost always show that the pressure profile As a conveying machine the extruder has a maximum ("pressure peak") at the today belongs almost exclusively to the family of the screw machines. There beginning of the metering zone. The metering zone then produces no more are two basic types of plastics extrupressure, but is a pressure consumer. der: the single and the twin screw The conveying in the feeding zone of a extruder. In this paper only the single conventional extruder depends on the screw extruder is taken into considefriction forces between the solid ration since this is the most frematerial and the cylinder as well as quently used form of plasticizing exscrew surfaces. At this point, the truder. The principle of the single friction coefficient steel/stock plays screw plasticizing extruder is shown an important role. The conveying is in Fig. 1. often insufficient as the ducts are often not filled completely. This can be a reason for throughput pulsations. Compulsively conveying extruder hea ter band
barrel
-hopper
In contrast to the conventional extruder, the feeding zone becomes det~rmi nant. Essentially higher and quasl pulsation free throughputs are the result. The process steps here are as follows /1/: --;tock temperature L - _ - - - - - - - -_ _~ pressure reduction gear throughput barrel temperature of zone 2 viscosity
- feeding, - conveying, compacting (pressure built up), - conveying of a compacted solid block,
Fig. 1. The principle of an extruder. The screw rotates in a heated cylindrical barrel with a die at the outlet end. Solid polymer of a granulated or powdery kind is added to the feed hopper and from there transported by the screw to the die. As a consequence of shearing in the screw and heat transfer through the barrel, the polymer is melted. This melt is extruded through the shaped die at high pressures. The plasticizing is achieved by convective heat transfer over cylinder and screw and by friction in the melt. At present two extruder designs can be found for the single-screw extruders: - conventional extruder, - compulsively conveying extruder. Conventional extruder The screw is divided into three zones corresponding to the process steps: - deep cut feeding zone, - transition zone with decreasing
- preheating, preplasticizing, - rest plasticizing, - homogenizing. Good trickling behaviour of the material to be extruded and a suitable feed opening are a necessary but not sufficient condition for high throughputs of an extruder as the friction conditions also determines the conveying behaviour of the material .. I~ conveying auxiliaries such as addltlonal screw systems or stuffing cylinders are disregarded, one can say that excentrical, inclined feed openings are optimal for a feed design /2/. Whereas in the conventional extruder a force (friction) based connection in the smooth feeding zone is obtained as a result of the friction ratio stock/ cylinder/screw, in the compulsively conveying extruder grooves in the feeding zone lead to a shape-based connection between material and cylinder wall, since the conical bush geometry contributes to a strong compacting of the material. The grooves cause a more effective
Extruder Control
"holding" of the compacted material block against rotation with the screw than more frictional effects, so that the material is approximately axially conveyed like a fixed nut on a rotating spindle.
553
a screw speed range controlled in the limits of 3:1 up to 6:1. The following drive types are normally applied /3/: a) drives with mechanical speed control b) drives with electric speed control
The extruder as a multi-variable system The extruder can be considered as a multi-variable system. The controlled variables are the throughput, and those which represent the state of melt, such as stock temperature, pressure and viscosity. As step variables one may consider, for example, the heat introduced into or extracted from the cylinder, the friction energy, or the screw speed, while fluctuations in the electric power or environmental temperature together with the chemical and physical material properties may operate as disturbance variables. These relationships are represented in Fig. 2.
process parameters
disturbance variables -
step variables
'I
~
con trolled variables
ex truder 1
controtlers
l
set POints
-
- d.c. shunt wound electric motor with thyristor control - Ward-Leonard set and d.c. shunt wound motor - three phase a.c. shunt wound motor with pole changing c) drives with hydraulic speed control With a d.c. shunt wound motor a continuous and exact screw speed adjusting is possible (fluctuations ~ 1 percent). Torque fluctuations have a slight dependence on the screw speed. With a current limitation one can ensure that the torque does not exceed prescribed limits. For a long time three phase ~.c. shunt wound motors were of great lmportance as separate drives for extruders before thyristors came on to the market. The speed can be adjusted to an exactitude of 2 to 3 percent. An on-off controller with internal dynamic feedback is normally used as a regulator. Nowaday a.c. drives are only used on small extruders. They are only economic up to a motor capacity of 60 - 75 kW /1,3/.
Fig. 2. Multivariable extrusion process block diagram. Screw temperature control Because of the construction difficulties involved in such a control, it is only carried out in large extruders. The advantages of screw temperature control are to be seen in the reduction of the heating up period and in the possibility of regulation of the stock temperature profile during production. CONTROL STRATEGIES The radial temperature profile of the melt can be reduced by means of screw The simplest, and therefore most comcooling. monly applied method of extruder conFor heating and cooling, oil is used as trol, is the simplification of the a tempering medium, but if the screw problem so that only the easily measured variables such as screw speed and is only to be cooled during production water is used as a cooling agent. ' barrel wall temperatures have to be taken into account, or rather are conBarrel wall temperature control trolled by means of single loop control equipment. Improvements in measuring As shown in Fig. 1 the extruder barrel techniques and process know how obis divided into several heating zones, tained from steady state and dynamic process models make it possible to re- normally 3 to 6. As a result of the heat energy contained in the transporalize more effective control strateted material and the conductivity of gies such as cascade loops or adaptive the barrel vlall, the heating zones are systems. thermically connected. In practice, there is a single loop control for each Screw speed control heating zone. Because of the extreme Extruders with only one screw speed are low pass character of the controlled exceptional (compounding-, granulating elements with time constants in the units). Controllable drives mostly have order of ten to thirty minutes, on-off
The disturbance variables not only add to the influence of the step variables but also, to some extent, affect the relationship between step and controlled variables.
s.
554
Dormeier
controllers are mostly used in industrial practice. These controllers, which are equipped with internal dynamic feed-backs make it possible to set up combinations of PO and PlO controllers /4,5/, which ensure that the temperature course doesn't overshoot when a set point change is made. This effect is achieved by a sort of I-channel disconnection in the case of significant deviations, so that a reset wind up is avoided. An on-off controller with a relay output as adjusting element costs about a third of the price of a comparitive steady controller. New developments in controllers aim towards the application of microprocessors so that multiloop, cascade or adaptive systems can be set up. A further advantage with digital controllers is the long term stability of the control setting and the increased possibility of exact reproduction. For example, an investigation of a sheet extrusion line has shown that a temperature drift of 1 0 C in a heating zone near the die can bring out a local change of thickness of the sheet up to 3.6 % depending on the material type and throughput /6/. Investigations have been carried out on digital barrel wall and stock temperature control systems /7-11/. The modified PlO and a compensation algorithm were employed. The PlO algorithm can be derived by setting up the difference equation and using numerical integration. The recursive form is then: y (k) = Y(k - 1) + do xd (k ) + d
1
x
d
(k - 1) + d
2
x
d
(k -
2)
(1)
where:
Fig. 3 demonstrates courses obtained by a step set-point change.
200
b
tolerance :!0,5
J
195
~190 ~
OJ c......
~
CJ
185
c......
OJ
~
OJ ....... Qj
~ 180 -Cl
o
5
10
15
20 min
25
time t
Fig. 3., Closed loop response to a step set-point change. Curve a: PlO with "I-channel" disconnection Curve b: PlO with optimal constants for process disturbance Curve c: Compensation-algorithm for a PT -T element 1 t Curve d: PlO with saturation of the adjusting element.
The control results demonstrate clearly that the use of disturbance and set y = output signal (step variable) point optimal controller parameters leads to fluctuations in the courses x = input signal (error signal) d of the process variables, which means d ,d ,d = Coefficients which depend that the current practice of steady o 1 2 on magnitude ratio, incontroller tuning for both types of tegral time constant, de- operation is no longer appropriate for rivative time constant future OOC systems. and sampling time The theoretically calculated settling In order to restrain the algorithm from time for the compensation algorithm producing an output signal beyond the could not be realized because of the saturation of the adjusting element non-linearities of process (changing because of the time lag, absolute high of operating point, material and enand low limits of the output value are vironmental conditions). considered. Using the digital simulation of the whole control loop for A comparison of the control results optimal controller adjustment this achieved with various PlO controllers limitation can be allowed for. at step set point changes is shown in Fig. 4.
Extruder Control
555
time have proved favourable for this purpose /12-14/. ~
critical feature of the thermocouples that they protrude into the melt so that there is a certain danger of damage if insufficiently plasticised material flows past them in the initial phase or the stock temperature drops below a critical level during production resulting in increased viscosity. The positive features are the simple hardware structure and the low financial costs. 1S
QJ
c....
~ 185OC.......----II--I~----+-------+----+------I CJ c....
~ .....
Smin
10min
15min time
20min
2Smin
Ultrasonic temperature measurement involves no melt contact. The measuring of the travel time of ultrasonic pulses traversing the polymer melt in the flow channel of the extruder gives a reliable indication of the effective temperature of the flowing melt . Fig. 5 shows the principles of arrangement for ultrasonic travel time measurement /13/. Increasing distance between molecules causes a lower sonic velocity i.e. an increasing travel time.Thus the travel time of an ultrasonic pulse will increase approximately in proportion to a rise in temperature and to decreasing pressure.
Fig. 4. Comparison of closed loop responses. Curve a: Continiuous PlO controller, Curve b: Best result achieved with the compensation algorithm, Curve c: Switching PlO controller, Curve d: Modified PlO algorithm. The transistor from conventional switching controllers to the digital kind is justified, not so much by improved control results in barrel wall temperature control loops as by economic considerations combined with digital date processing. Stock temperature control The methods of temperature control considered so far are based on the assumption that a suitable temperature profile along the barrel wall together with the friction energy contributed by the rotating screw will lead to suitable stock temperature for the processing operation. This assumption is in the main correct for the steady state operating point but not for the unforseen effects of disturbances or changes in operating points so that the operators have to carry out adjustment~ To overcome this uncertainty in the process control it is necessary to measure the stock temperature in front of the screw tip. Measurement by means of thermocouples or ultrasonic travel
Fig. 5. Measurement of ultrasonic travel time at the end of the barrel. At a given pressure the travel time of an ultrasonic pulse traversing the cylinder or flow channel is influenced almost linearly by the temperature distribution between ultrasonic transmitter and receiver and therefore gives an indication of the specific heat transport to the die. Industrial application is to be found in the PVC pipe extrusion /14/. If the stock temperature is measured by one of these methods a cascade loop as shown in Fig. 6 can be set up and either all or only the most effective barrel temperature controllers can be employed as auxiliary controllers; ~M here represents the stock temperatu:e, ~Z3 ~he wall temperature of the thlrd heatlng zone, P Z 3 the heating
s.
556
Dormeier
power and K3 the magnitude ratio. The same principles apply for the other heating zones.
screw tip has been developed to measure the torque at the screw tip depending on the viscosity. The correlation between viscosity and torque for several materials has been proved. The torque-measuring signal can be taken as the reference input, whereas the temperatures of the barrel zones will serve as subsequent values for a cascade loop control.
cJ'Mset
Fig. 6. Cascade loop for a stock temperature control. A control strategy of this kind was employed with extruders on the basis of conventional controller (PlD and PD controllers) in the early seventies /15,16/. These systems did not spread to any extent because of the relatively complicated operating skills required of the processing staff. Further control systems were developed on the basis of the 8-bit microprocessors /7,8,10/. The operation can be simplified considerably by means of appropriate software, so that in future the large scale industrial use of digital stock temperature control systems is to be expected. Melt pressure control In many cases the registration of melt pressure is indispensable, because pressure changes cause throughput fluctuations and so disturb the processing operation. Both mechanical and electrical systems are used to measure pressure. Fig. 7 /3/ shows the design principles of a melt pressure control.
Fig. 7. Set-up of a pressure control. Equations and models for the control of the extrusion process The control strategies discussed so far are classic 'feed back' systems because corrective actions are only made after deviations have occurred. The growing demands made on the quality of extruder products led to the consideration of a mathematical description of the extrusion process with the aim of optimising plants and production process. The extrusion process is controlled by physical equations governing the conservation of energy, momentum and mass. Because of the complex interactions between the various polymer properties and process variables a dynamical modelling of the extrusion process is very difficult and a continuous measurement of the polymere properties during a dynamic situation is not possible.
Viscosity control Previous research works on The flow behaviour of the plastified the extrusion process modelling have polymer is characterised by the visbrought out steady state models which cosity as a function of shear-rate and are used for screw design and set up for temperature. Because of the difficulthe extruder /2,15,18,19/. A simple ties involved in measuring the state of steady state equation which describes melt directly by means of viscosity the influence of the barrel zone temwhile an extruder is in operation, it perature ~zi' the screw speed n s and is only possible to keep a check on the the throughput m on the average stock barrel wall temperatures or, if more temperature near the screw tip was used is needed, on the stock temperature for the setting up of a 45 mm diameterand pressure as indicators of the state extruder in /20/: of melt. One continuous measurement of u viscosity throughout the extrusion ~S = L: K. ~Z' + K I • i=1 1 1 (2 ) process is mentioned in /17/. A special
Extruder Control
with Ki , K , K : Coefficients I II n: Number of heating zones Range of validity for the extruder used: 180 0 C ~,J:. ~ 50 0 C Zl 15 min- 1 ~ n ~ 50 min- 1 s ~
6.9 kg/h
m
~
22,9 kg/h
An extension of this steady state equation to the dynamic sphere, provided there are constant material parameters, was carried out by MeiBner /15/. A comparison of the measured and calculated frequency responses shows a notably high conformity for the frequencies w ~ 0.01s-1. White and Scott /21/ determined the transfer functions relating screw speed to throughput experimentally by using the stepresponse techniques. They used step changes of the screw speed of about 20 rpm. In view of the non-linearity of the process, these steps are too high. Tadmor and Lipschitz /22/ presented a model which is based on a one-dimensioned parallel plate steady state momentum equation. Lumped rates of change of momentum effects are taken into consideration too. Kochar and Parnaby /23/ built up a model on the basis of stochastic identification. They developed the discrete transfer functions for the interaction between screw speed pertubations as input signal and melt pressure and temperature. The input signal has a PRBS-characteristic. The output signals ~M and p are both assumed to be contaminated by noise originating in random changes in other variables, including polymer parameters and measurement errors. This noise is assumed to be independent of the screw speed. This assumption can be made if one overlooks the interactions between screw speed pertubation dissipation and material properties. The model form reads /23/:
t9'"M (k)
Z -dJ--- BJ-( Z -1 )
C+,( Z-1 )
• e (k)
+
with: e: sequence of independent random variables (0,1), z-dp, z-d~: represent transport delays, PRBS: Pseudo Random Binary Sequence.o The model parameter values were chosen by step wise hill climbing adjustment in order to minimise a loss function which is proportional to the square of the difference between the model output and the actual process response. This discrete model was applied on a 38 mm screw diameter laboratory extruder and it was seen that the calculated model output variables conformed closely to those measured. The screw speed variations came to ±4 rpm and the resultant temperature variations approx. ± 2 0 C /23/. PROCESS CONTROL IN THE EXTRUSION BLOW ~·10ULDING In general the extruder is only one part of a plant producing films, sheets, mouldings, pipes etc. Hence the processing stages subsequent to the extrusion must be integrated into the control system, as is shown in the example of extrusion blow moulding. Remarks on the extrusion blow moulding process The continuous extrusion blow moulding process is characterised by the following four processing phases (Fig.8):
111 moulding and cooling in blow station IV demoulding and ~
11
I I moulding of par/son by extrusion II graspmg and cutting off of par/son
(3)
(Z-1) •
PH(k)
A
z-dp
(Z-1)
• ns(k)
p -1
+ Ct? ( Z D
p
)
(Z -1 )
• e (k)
~
.
,blow station
""AJ 111
~ -G- ~ ~IV
I
moulding of parison by extrusion,
11
grasping and cutting off of parison,
III
moulding and cooling in blow station,
IV
demoulding and cutting off of waste.
D'1J-( Z-1 ) B
cutting off of waste
Fig. 8. Course taken by extrusion blow moulding process.
1 · n (k) s A-",( Z - )
=
557
(4)
Process analysis /24/ has revealed that the demoulding temperature influences shrinkage, being a moulding
s.
558
Dormeier
quality. The term shrinkage is used to describe the accuracy to size of a moulding. The demoulding temperature is understood as the stock temperature of a moulding on an arbitrary point of moulding while the blow mould is opened. It is a value not to be measured directly, but to be computed by means of a system of equations /25/. Using these equations it is possible to determlne the cycle time (step variable) at a given demoulding temperature (target value). The cycle time tz consists of the blow time tB and the machine time tM. The machine time is known through motions, such as lift mould/lower mould etc. In /25/, for the process model an implied equation of following kind: t z - A exp[A + A exp(A - A t )] 1 2 5 Z 4 3
+ A - t M 6
=
f(t } z
(5)
was set up.
mould and have to be removed manually. This interruption leads to additional loss of material and time /9/. Despite variable production conditions, due to disturbing influences or to a change in the point of operation, parison length is to be kept constant. Bottom flash length or flash weight provides a measurement of optimum parison length. In the case of the given bottle (0,7 1) the target for process control should be a maximum flash weight of 3 - 3.5 g, not to be exceeded if possible, with a minimum flash weight of 1 g (weld line). With a preset cycle time, parison length depends upon parison output velocity from nozzle. Output velocity may be adjusted via screw speed. The velocity pattern is non-linear over the length of the parison. It is determined largely by screw speed , melt temperature and the size of nozzle opening. Parison velocity is measured by photo cells LS1 and LS2 (Fig. 9).
This equation is valid for tB ~ Tcand it proves to converge and have a zero point which can be determined iteratively in the process model(T : crysC tallisation time). Control strategy Because of the process model the strategy can only be realized by computer application. Control of parison length and the constant demoulding temperature desired are to some extent contradictory requirements, since the irregular parison velocity, in particular during startup, means that cycle time cannot be kept constant - as required for optimum parison length. A certain tolerance must therefore be allowed for the demoulding temperature. Observation of a pre-set parison length is very important for the economic production of hollow articles. A prerequisite for constant length is that the continuous parison extrusion process should be coordinated in time with the noncontinuous production of finished parts. Once an article has been demoulded, the parison for the next finished part must have achieved optimum length so that it may be transferred directly to the mould without additional waiting time. If the parison is too long, this leads to high material wastage and if wall thickness regulation is used then this causes displacement of thick and thin points along the longitudinal axis. If the parison is too short, this not only gives rise to material loss but often causes a stoppage in the automatic operation cycle when the remains of the parison stick to the inside of the blow
-,--
cuf- off /dev,ce -
~---.
partson procesSing ngfh
'r
315mm
300
.L-rzszsJ+-
3~rzszsJ+--_ 85 _.L-rzszsJ+-
Fig. 9. Arrangement of photo cells to control parison length. The measuring result has to be adjusted with a correction factor to give mean overall velocity from the mean parison velocity between LS1 and LS2. Photo cell LS3 monitors mould closing time during production to ensure that the mould is not closed prematurely. During the start-up phase LS3 is also used to measure mean parison velocity. Photo cells LS4 and LS5 monitor flash length. Parison length control based solely on determination of flash length has the disadvantage that when deviation does occur, correction by means of a change in screw rotational velocity only becomes effective as from the following parison. It would seem more appropriate to measure parison length before the end of the blow process using LS1 and
Extruder Control
559
The change in cycle time after the LS2. When the velocity pattern of the 25th and 55th cycle was effected parison is known, then the mean velocity measured at a specific point can rapidly with a simultaneous change be extrapolated to give the mean resi- in parison velocity. A velocity reduction takes longer than an increase. dual velocity of the parison between the point of measurement and optimum This is due to the fact that pressure processing length. Given a constant reduction in the screw chamber takes cycle time, parison length can be cal- place more slowly than pressure buildculated in advance. If this does not up. tally with the target value then cycle time can be lengthened or shortened to CONCLUSION give a favourable parison length within As a result of the constantly growing the same cycle. The basic operating mode of a computer demands of quality and profitability on the extrusion process, it has becontrol /9/ may be seen from Fig. 10. The measurements were recorded during come necessary to look for possibilities of improving the supervision and the start-up phase. Target cycle time control of extruders and extruder is not taken into account during the first four cycles. Cooling is effected plants. The digital computer, particularly in the form of the process for a blow time calculated by using a correction factor K2, since the paricomputer has here proved to be an son velocity does not correspond to its effective aid to the automation of target value. K2 is then corrected from the extrusion process for the following one cycle to the next and flash weight reasons: sinks accordingly from 4.8g to 3.5g. - computer controlled methods of For the fifth cycle the parison veloci~ measuring can register process corresponds to its target value. Hence, in order to observe cycle time, cooling variables such as viscosity, stock follows for the target blow time. Subtemperature etc; up to now this sequent monitoring of the flash shows has been possible with highly too great a flash length, whereupon a complex procedures. new velocity target value is calculated. This is then repeated until after the - a computer permits the introduc10th cycle when a favourable processing tion of special process models length has been achieved and cycle time into the process control concept is respected simultaneously (Fig. 10). and also means that an increasing number of theoretical methods may be used to solve automation 20 r-o-------,--I-----r------o-rl-----,\ tasks. mm 0°000°°0°00000000°000% °ocx>ooOoOoOGo c:: ~-s- 0 ° I REFERENCES ~ ~ 16 o~------+---------------r-~ t:::1~ ~
Q.
~
:
72
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CJ a ClJ-
000%000
~ ~
/1/
Menges, G.: Kunststofftechnologie I. Vorlesung an der RWTH Aachen, WS 1976/77.
/2/
Kosel, U.M.: Optimierungsrechnungen beim Extrudieren. Dissertation an der RWTH Aachen, 1971 .
/3/
Fischer, P.: Stand der Regelungsund Steuerungstechnik bei Extrudern und Extrusionsanlagen dargestellt am Beispiel einer Tafelanlage. VDI Tagungsbuch "Rechnergesteuerte Extrusion?", VDI Verlag 1976, S. 27-48.
/4/
Merkel, J.: Das PDPI-Reglerprinzip. Elektronik 5(1973), S. 165-168.
/5/
Martens, J., Schweinebraten, M.: Temperaturregelung mit Zweipunktreglern und deren schaltungstechnische Realisierung. Regelungstechnische Praxis, 9(1978), S. 262-266.
ooooo~ooooooooooooCX>o I
39
s 29 24 79
r-------~----;_-------:-_____"ll ---~----
I
I
- fOOOOOOOOOOOOOOOOOOOOOOOOjO - - - - - ~o - - -----
14 {oooooooooooooo=oooooooo
-----
i I
~-
i oooo
Fig. 10. Flash weight pattern with adaptive parison length control.
560
s.
Dormeier
/6/
Michaeli, W.: Zur Analyse des Flachfolien- und Tafelextrusionsprozesses. Dissertation an der RWTH Aachen, 1975.
/18/ Maddock, G.H.: Extruder scale up by computer. Polymer Engineering and Science, 12(1974) .
/7/
Ramm, F.: Von der ProzeBanalyse zur Rechnersteuerung. 10. Kunststofftechnisches Kolloquium 1980, Aachen, S. 72-77.
/19/ Tadrnor, Z., Klein, I.: Engineering principles of plasticating extrusion. Van Nostrand-Reinhold, New York (1970).
/8/
Schwab, E.: Mikrorechnereinsatz zur ProzeBflihrung beim Extrusionsblasformen. Kunststoffberater, 7(1979). S. 324-327.
/9/
Dormeier, S.: Ein Beitrag zur /21/ White, D.H., Schott, N.R.: Dynamic Automatisierung des Extrutesting of plastics extrusion sionsblasformens. Dissertasystems. Proc. 30th ANTEC of tion an der RWTH Aachen, 1977. Society of Plastics Engineer~ Chicago (1972), S. 797-801. Junk, P.B., Brinkmann, N.: Der Mikrorechner an der Kunst/22/ Tadmor, Z., Lipschitz, S.D., stoff-Blasmaschine. MitteiLavie, R.: Dynamical model of lung der Firma Voith-Fischer, a plasticating extruder. Polymer Enging Sci. 2(1974), Lohmar. S. 112-119. PleBmann, K.W.: Mikroprozessoren. Vorlesung an der RWTH Aachen, /23/ Kochar, A.K., Parnaby, J.: DynamiWS 1976/77. cal modelling and control of plastics extrusion process. van Leeuwen, J.: Fundamental meas~ Automatica, Vol. 13(1977), rement and control in the exS. 177-183. trusion process. Plastics & Polymers, 6(1974) S. 104-113. /24/ Kulik, M.: Ein Beitrag zur Analyse des kontinuierlichen ExtruHerbertz, T.J.M.: Better control sionsblasformens, Dissertatmn of extrusion by ultrasonic RWTH Aachen, 1974. measurement. 3rd International IFAC Conference on Instm- /25/ Dormeier, S.: Development of a mentation and Automation in process computer control for the PRP-Industries. Brussels, an extrusion blow moulding Conference Proceedings 1, unit. Proc. 1,3rd IFAC ConS. 349-351. ference on Instrumentation and Automation in the Paper, Schiller, A.D.: Ultrasonic process Rubber and Plastics Industries control in PVC pipe extrusion Brussels, 1976, S. 261-267. Proceedings 37th ANTEC, Society of Plastics Enginee~, New Orleans, 1979, s. 378-38~
/10/
/11/
/12/
/13/
/14/
/15/ MeiBner, M.: Regelungstechnische Untersuchungen an Kunststoffextrudern. Dissertation an der RWTH Aachen, 1971. /16/ Melchior, F.: Kaskadenregelung an Kunststoffverarbeitungsmaschinen. Kunststoffe 9(1971), s. 609-612. /17/ Revesz, H., Hubeny, H.: Continuous measurement and control of viscosity throughout the extrusion process. Proc. 1,3rd IFAC Conference on Instrumentation and Automation in the Paper, Rubber and Plastics Industries, Brussels, 1976, S. 69-76.
/20/ Dormeier, S.: Aufbau von ProzeBrechnerprogrammen zur Steuerung von Extrusionsanlagen. VDI Tagungshandbuch "Rechnergesteuerte Extrusion"? VDIVerlag 1976, S. 109-130.