WO2023094220A1

WO2023094220A1 - Method for maintaining a process window of a plastic extrusion facility

Info

Publication number: WO2023094220A1
Application number: PCT/EP2022/082000
Authority: WO
Inventors: Frank Golla; Joseph Kastner; Martin Schwanck
Original assignee: REHAU Industries SE & Co. KG
Priority date: 2021-11-23
Filing date: 2022-11-15
Publication date: 2023-06-01
Also published as: DE102021130574A1

Abstract

The invention relates to a method for maintaining a process window of a plastic extrusion facility (100) having at least one electrically driven extruder screw (3), the extruder screw (3) being mounted in an extruder screw housing (1) which can be externally heated by means of at least one heating device (10) and/or externally cooled by means of at least one cooling device (20), the electrical power of the heating device (QH) and/or of the cooling device (QK) being adjusted, in particular controlled in a closed-loop manner, by means of at least one temperature sensor (300) on the plastic extrusion facility (100), and the electrical drive power (PS) of the extruder screw (3) being continuously determined and processed as a process variable. According to the invention, in addition to the drive power (PS) of the extruder screw (3), the electrical power of the heating device (Q̇H) and/or of the cooling device (Q̇K) is also continuously determined and processed as a process variable. Depending on the ratio (S) of these two measured variables of heating or cooling power (Q̇H, Q̇K) and drive power (PS) to each other, the overall toughness of the plastic mass (50) located in the extruder screw housing (1) is altered, or alternatively the extrusion process is stopped, in particular for the purpose of replacing the extruder screw (3) and/or the extruder screw housing (1).

Description

Procedure for maintaining a process window of a plastic extrusion line

The invention relates to a method for maintaining a process window of a plastics extrusion system with at least one electrically driven extruder screw, the extruder screw being mounted in an extruder screw housing which can be heated on the outside by means of at least one heating device and/or can be cooled on the outside by means of at least one cooling device, the electrical Power of the heating device and / or the cooling device via at least one temperature sensor on the plastic extrusion system is set, in particular regulated and the electrical drive power of the extruder screw is continuously determined and processed as a process variable.

A process window describes the limits within which a process must run in order to guarantee a product quality that conforms to specifications. When manufacturing a complex window frame profile with a large number of hollow chambers by means of an extrusion process, the degree of preparation of the melt and the wall thickness of the profile, for example, are of essential importance in order to ensure sufficient mechanical stability of the profile on the one hand and, on the other hand, the lowest possible stability for cost reasons Having to use a lot of plastic material. The process window for maintaining the corresponding wall thicknesses (or more generally of parameters of the extrusion profile to be produced) within tolerance limits can include a wide variety of process parameters measured during the process, e.g. temperatures, pressures, discharge speeds, etc..

In this context, it is known in the prior art to use the specific drive power requirement as a parameter for maintaining the process window. This is the ratio of the drive power of the extruder screw to the extruded quantity of extrudate, eg measured in kWh / kg. Also known as a parameter is the relative heating power, ie, for example, the switch-on time of heaters and fans for external temperature control of the extruder screw in relation to the process duration. The interpretation of this However, the aforementioned parameters are difficult due to their multidimensional dependency. A lack of conclusions leads to incorrectly adjusted processes and thus to permanent additional costs, which result, for example, from increased waste values, process units that wear out more quickly (e.g. extruder screw), etc. Furthermore, this also means that process developments are lengthy and correspondingly expensive.

The invention is based on the object of specifying a method for maintaining a process window of a plastics extrusion plant, which method can be carried out effectively and at the same time inexpensively.

Based on a method with the features described above, this object is achieved according to the invention in that, in addition to the drive power of the extruder screw, the electrical power of the heating device and/or the cooling device is also continuously determined and processed as a process variable and that depending on the ratio of these two process variables heating - or cooling power and drive power to each other changes the overall viscosity of the plastic mass located in the extruder screw housing or alternatively the extrusion process is stopped, in particular for the purpose of replacing the extruder screw and/or the extruder screw housing. The process variable processing is expediently carried out by means of electronic data processing.

In addition to the operating parameters (temperature, pressure, etc.), the overall toughness depends, among other things, on the rheological properties of the plastic melt or the plastic particles, the degree of preparation, the mixing quality and of course the amount of plastic. It represents the total resistance which counteracts the rotational movement of the extruder screw(s) due to the entire plastic mass contained in the extruder screw housing. Due to the rotation of the at least one extruder screw, plastic is conveyed in the direction of the shaping nozzle of the extrusion system (ie the extrusion tool) and finally pressed through this nozzle in order to produce a corresponding profile. This rotation results in shearing of the plastic particles, which generates frictional heat (dissipation) and significantly promotes the melting of the plastic particles in the extruder screw housing. If necessary, this melting process is supported by external heating of the extruder screw housing. If too much frictional heat is released as a result of the shearing described above, the (usually cylindrical or conical) extruder screw housing cooling devices arranged on the outside have a cooling effect. In many cases, the type and amount of external thermal energy used (ie the extent of heating or cooling) determine the quality of the melt at the die outlet and thus the properties of the extruded semi-finished product. For example, in a process to be cooled with a cooling device from the outside, it can be concluded from comparatively less frictional heat to be dissipated (cooling device(s) runs/run less frequently) that the homogeneity of the melt decreases due to reduced shear forces. The teaching according to the invention is now based on the consideration of considering the overall viscosity of the plastic mass (ie both particles and melt) located in the extruder screw housing as the central control variable for maintaining the process window. In this case, this toughness does not have to be recorded directly as a measured variable. Rather, what is essential to the invention is that when a change is desired, measures are taken that go in the right direction. If, for example, with a constant drive power of the extruder screw and based on an external heating output that is too low, it is determined that too much dissipation (i.e. too much heat development) takes place in the housing, the said overall viscosity is too high. In this case, measures must be taken to reduce this overall toughness. This can be, for example, a reduced supply of plastic particles, which gradually leads to a reduction in the plastic mass in the extruder screw housing and thus lowers the overall toughness. Another possible measure in this case is the increased supply of lubricants to the plastic to be processed in order to reduce the overall toughness. Of course, other formulation changes with regard to the plastic to be processed in the extrusion process can also be made in order to reduce toughness. Further measures can be, for example, a change in the target temperatures along the extruder screw housing, with an increase in the target temperatures reducing the overall viscosity of the melt or a reduction in the target temperatures increasing the overall viscosity of the melt. If, on the other hand, the heating output is too high, there is not enough friction within the extruder screw housing and countermeasures must be taken accordingly (e.g. increasing the plastic supply, reducing lubricants, etc.). The inclusion of the drive power for the extruder screw in the considerations leads to a standardization with regard to the production speed. Furthermore, absolute deviations with regard to the heating power at high production speeds (i.e. high drive power of the extruder screw) have less of an effect on the process regulation or control than is the case with low drive powers. The heating or Cooling power in relation to the drive power of the extruder screw is used as the central process variable for the control and regulation process to maintain the process window.

Overall, the teaching according to the invention is characterized by cost-effective process management, since reduced reject and return quantities and reduced wear of screws and housings occur. Further advantages are a faster and thus more cost-effective process development and setting, a more flexible use of recipes as well as higher possible process speeds due to very precise and fast process regulation or control. Moreover, the teaching according to the invention can generally be implemented without the installation of additional measuring devices, resulting in additional cost advantages.

Furthermore, the method according to the invention is also suitable for identifying unsuitable screw geometries or generally unfavorable operating conditions, which can occur in particular if there are excessive signs of wear with regard to the extruder screw and/or the extruder screw housing. Here, it is generally not possible to adhere to the process window or to manufacture products that meet the specifications. In this case, it makes sense to stop the extrusion process in order to keep the amount of rejects as small as possible or to prevent damage to the system / to keep it as small as possible. If, for example, the ratio of heating or cooling power to drive power exceeds or falls below a specific limit value, the extrusion system is shut down for the above reasons to be on the safe side.

According to a preferred embodiment of the invention, if the heating output is too high or the cooling output is too low in relation to the drive power, the supply of plastic particles is increased in order to increase the degree of filling of the extruder screw housing with plastic mass and thus the overall toughness and/or the recipe of the plastic to be processed is so changed so that the viscosity of the resulting plastic melt is increased and/or the target temperatures along the extruder screw housing are reduced. The plastic particles can be plastic granulate and/or plastic powder, which is suitable for processing in an extrusion process. If necessary, the plastic particles can comprise a blend of several different polymers and, if necessary, processing aids can be added to the plastic particles (e.g. lubricants) may be added. On the other hand, if the heating output is too low or the cooling output is too high in relation to the drive output, the supply of plastic particles is preferably reduced in order to reduce the filling level of the extruder screw housing with plastic mass and thus the overall toughness and/or the recipe of the plastic to be processed is changed in such a way that the viscosity of the resulting plastic melt is reduced and/or the target temperatures along the extruder screw housing are increased.

It is within the scope of the invention that two counter-rotating extruder screws are mounted axis-parallel in the extruder screw housing. The plastics extrusion system expediently has at least one gravimetric and/or volumetric dosing device for the supply of plastics particles. In this way, the filling level of the extruder screw housing with plastic and thus the overall viscosity of the plastic melt in the extruder screw housing can be adjusted very easily. This also enables manual and/or preferably automated adjustment in the event of changing boundary conditions. This can be both raw material and / or recipe fluctuations as well as changes associated with the wear and tear of the processing unit.

The extruder screw housing can have several, preferably four to ten, control zones along the extruder screw, in which the external supply of heat and external cooling can each be adjusted separately. At least one separate heating device and/or cooling device is preferably provided in each control zone. As a result, a targeted and locally different heating or cooling of the extruder screw housing is possible. In this case, at least one temperature measurement is expediently carried out in each of these control zones, with the power of the heating or cooling device located in a control zone being at least set, in particular also controlled, via a temperature measurement in the same control zone. It is also conceivable that temperature measurements from other control zones are used to set the heating or cooling capacity in a specific control zone (e.g. in a control zone arranged downstream). External cooling of the extruder screw housing is usually carried out using at least one electric fan. The toughness of the plastic melt is expediently increased or reduced by changing the proportion of processing aids (eg lubricants) in the formulation of the plastic to be processed. A change in formulation with the aim of increasing or reducing the overall toughness is also generally within the scope of the invention. For example, when using different plastics as raw material, the ratio of these plastics to one another can be changed (if the specification of the extrudate to be produced can still be met), if this is beneficial to the desired change in overall toughness. Of course, a combination of toughness-increasing or toughness-reducing measures is fundamentally within the scope of the invention.

In principle, the teaching according to the invention can be used for all types of extrusion processes and correspondingly produced extrusion products. The teaching is particularly suitable for the production of window or door profiles, e.g. made of rigid PVC, for the production of pipes made of e.g. PE (especially cross-linked polyethylene) or PP and for the production of edgebands for the furniture industry, e.g. made of PP, PMMA, ABS, PET or even PVC.

Of course, it is also fundamentally within the scope of the invention to influence the overall toughness successively in different directions during the course of the production process, for example by initially lowering it using suitable measures and then increasing it again in order to continuously maintain the process window.

The invention is explained in detail below with reference to a drawing that merely represents an exemplary embodiment:

Fig. 1 is a schematic representation of a plastic extrusion plant in which the method according to the invention is applied and

2 shows a further exemplary embodiment in which the teaching according to the invention is applied.

The figures schematically show a method according to the invention for maintaining a process window of a plastics extrusion system 100. According to FIG. Extrusion system 100 has an extruder screw housing 1, a single extruder screw 3 mounted axially rotatably in the extruder screw housing 1 and driven by an electric motor 2 via a gear (not shown) for melting and conveying plastic granules 4, and an extrusion tool 5 arranged downstream of the extruder screw 3. Depending on the extruded product 70 to be produced (eg a window or door profile for a building, a pipe or an edging strip for the furniture industry), pressures of eg 10 to 1500 bar and temperatures of eg 60 to 300° C. are used. The length of the extruder screw 3 is generally 25 to 40 times the screw diameter. The extruder screw 3 can be designed as a so-called high-speed and is operated at speeds of up to 1500 revolutions per minute. The speed Ni of the extruder screw 3 can vary depending on the product production (e.g. fast extrusion at high speed when producing extrusion profiles with simple geometries, e.g. edge bands or hoses, slow extrusion at low speed when producing complex geometries, such as window profiles with a large number of Hollow chambers) or recipe of the plastic granules to be processed 4 can be adjusted differently. Polypropylene plastic granules, for example, can be used as the material 4 to be extruded. However, other materials such as polyvinyl chloride (PVC), polyethylene (PE), polyamide (PA) or polymethyl methacrylate (PMMA) can also be used.

When determining a suitable process window for carrying out the extrusion process, tests have shown that maintaining a specific temperature or a specific temperature profile or temperature field inside and/or on the outside of the extruder screw housing 1 (e.g. on the extruder screw 3, in the melt stream etc.) is a prerequisite for the production of an extrusion profile 70 that meets specifications, for example a frame profile for a building window, within tolerances that have also been determined by testing. The temperature itself is kept correspondingly constant by means of a corresponding supply of heat or, if necessary, removal of heat by the heating or cooling devices. In order to reliably maintain the corresponding process window (which ensures production that conforms to specifications) over the long term, this alone is not sufficient to keep the temperature or the temperature profile constant. This is where the teaching of the invention comes into play. According to the invention, it was recognized that the heating or cooling power(s) QH, Qidm required to maintain the temperature or the temperature profile/temperature field is in relation to the drive power Ps of the extruder screw 3, ie the quotient S = QH / PS or S = QK / PS also within a certain value interval [Smin; Smax] must move in order to be able to constantly produce extruded products 70 that meet specifications. If the process is outside of this value interval [Smin; Smax], countermeasures must be taken to maintain the process window. If, for example, in a process that requires additional heating power Q, this is too small, that is to say the quotient S is below the lower limit Smin, this indicates that the frictional heat development within the extruder screw housing 1 is too high. Consequently, the overall viscosity in the housing 1 must be reduced, e.g. by reducing the supply of plastic granulate 4 and/or increasing the amount of lubricant and/or increasing the target temperatures along the extruder screw housing 1 a dissipation of excess thermal energy Q _K requires (see Fig. 2), this thermal energy to be dissipated is too low in terms of its amount, this indicates insufficient dissipation processes within the extruder screw housing, i.e. the overall viscosity must be increased in this case, e.g. by increasing the quantity of plastic supplied and/or also reducing the quantity of lubricant and/or by reducing the target temperatures along the extruder screw housing 1.

Consequently, FIGS. 1 and 2 each show methods according to the invention for maintaining a process window of the plastics extrusion system 100, which are based on the above considerations. The extruder screw housing 1 in FIG. 1 is designed to be heatable on the outside with six heating devices 10, for example in the form of electric heating jackets.

The heating jackets 10 surround the circumference of individual axial sections of the extruder screw housing 1 and are arranged one behind the other along the housing 1 in the production direction x. In Fig. 2, on the other hand, the extruder screw housing 1 is designed with five cooling devices 20 in the form of fans (indicated by schematic propellers) so that it can be cooled on the outside, with several fans 20 being provided along the outer circumference of the housing 1 in order to bring about uniform cooling over the circumference of the housing can. It can also be seen that the extruder screw housing 1 is equipped with a plurality of temperature sensors 300 in each of the two exemplary embodiments. In FIGS. 1 and 2, the sensors 300 are each located in the wall of the extruder screw housing 1 and are arranged along the housing 1 in the production direction x. However, other arrangements of the sensors 300 are also possible and possibly expediently, for example directly on the inside and/or outside of the housing wall, it being self-evident that different arrangements can be combined with one another in a housing 1 . The arrangement of the temperature sensors 300 depends in particular on which temperature profiles or temperature fields are ultimately particularly sensitive with regard to compliance with the process window. The temperatures measured by the sensors 300 should now be kept at values that are constant over time in order to permanently maintain the process window (within tolerance limits, of course). In the exemplary embodiments, this takes place via corresponding temperature controls, the electrical outputs Q _H I -Qne of the heating jackets 10 in FIG. 1 and the electrical outputs Q _K I -C of the blowers 20 in FIG. 2 serving as manipulated variables for the controls. Furthermore, the electrical drive power P _s of the extruder screw(s) 3 consumed by the electric motor 2 is continuously determined and processed as a process variable. It is now essential to the invention that, in addition to the drive power P _s of the extruder screw(s) 3, the electrical power Q _H I -Qneder of the heating jackets 10 or the electrical power QKI - QKS of the fan 20 are continuously determined and processed as process variables. Depending on the previously defined ratio S of these two process variables heating power QH to drive power Ps or cooling power to QK TO drive power Ps to one another, the overall viscosity of the plastic mass 50 in the extruder screw housing 1 is changed. In the exemplary embodiments, the individual heating outputs Q _HI - QH6 or cooling outputs QKI - ÜKs are each added up and the quotient S is formed accordingly using the total outputs QH.GBS or QK.GBS. Determining the quotients from one or more heating outputs QHI -H6 or cooling outputs QKI-KS and the drive power Ps also provides information on the extent to which the intervention in the process must take place.

For example, if the heating power Q,Ges or the cooling power QK.GBS is too low in relation to the drive power _Ps , the supply of plastic particles 4 is increased in order to increase the degree of filling of the extruder screw housing 1 with plastic mass 50 and thus the overall toughness. Alternatively or additionally, the formulation of the plastic to be processed can also be changed in such a way that the toughness of the resulting plastic melt is increased, for example by reducing the amount of lubricant added. Alternatively or additionally, the target temperatures along the extruder screw housing 1 can be reduced. On the other hand, if the heating output QH.GBS or the cooling output QK.GBS is too high in relation to the drive power Ps, the supply of plastic particles 4 is reduced in order to increase the filling level of the extruder screw housing 1 with plastic compound 50 and thus the overall viscosity of the plastic compound 50 located in the extruder screw housing 1 to reduce. Alternatively or additionally, the formulation of the plastic to be processed can be changed in such a way that the toughness of the resulting plastic melt is reduced, for example by increasing the supply of lubricant. Alternatively or additionally, the target temperatures along the extruder screw housing 1 can be increased.

If the quotient S shows values that are too far away from the process window, the system recognizes that production that conforms to specifications cannot be achieved with control engineering measures and shuts down the extrusion process. This can be the case, for example, if the extruder screw 3 and/or the extruder screw housing 1 is excessively worn. 1 and 2 it can also be seen that the plastic extrusion system 100 each has a gravimetric or alternatively volumetric dosing device 200 for the supply of plastic parti no 4 . Overall, it can also be seen that the extruder screw housing 1 has a plurality of control zones 40 along the extruder screw 3 (six in FIG. 1 and five in FIG. 2), in which the external supply of heat and external cooling can each be adjusted separately. For this purpose, a heating device 10 or a cooling device 20 is provided in each control zone 40 .

In the exemplary embodiment according to FIG. 2, the plastics extrusion system 100 is equipped with a twin screw 3′, ie there are two extruder screws 3 arranged parallel to one another in a common extruder screw housing 1 and expediently intermeshing, which in turn are generally connected via a gear 60 from a common Motor 2 are driven at the same speed N ₂ . In this case, the housing 1 has, viewed in cross section, a correspondingly 8-shaped free cross section.

It goes without saying that it is also within the scope of the invention for cooling to take place in at least one control zone along an extruder screw housing, while heating is required in at least one other control zone. In this case, it makes sense to take the individual heating and cooling outputs into account when determining the total heating or cooling output. Also one in at least one Control zone temporarily alternating heating and cooling is within the scope of the invention. This is expedient in particular if the heat of dissipation occurring in the extruder screw housing approximately covers the heat requirement which is required for extrusion to specification.

patent claims

Claims

Method for maintaining a process window of a plastics extrusion plant (100) with at least one electrically driven extruder screw (3), wherein the extruder screw (3) is mounted in an extruder screw housing (1) which can be heated on the outside by means of at least one heating device (10) and/or or can be cooled on the outside by means of at least one cooling device (20), the electrical output of the heating device (QH) and/or the cooling device (QK) being adjusted, in particular regulated, via at least one temperature sensor (300) on the plastics extrusion system (100) and wherein the electrical drive power (Ps) of the extruder screw (3) is continuously determined and processed as a process variable, characterized in that in addition to the drive power (Ps) of the extruder screw (3), the electrical power of the heating device (Q ) and/or the cooling device ( Q _K ) is continuously determined and processed as a process variable and that depending on the ratio (S) of these two process variables heating or cooling capacity (Q , QK) and drive power (Ps) to one another, the overall viscosity of the plastic mass (50 ) is changed or alternatively the extrusion process is stopped, in particular for the purpose of replacing the extruder screw (3) and/or the extruder screw housing (1). Method according to Claim 1, characterized in that a) if the heating output (QH) or cooling output (QK) is too high in relation to the drive power (Ps), the supply of plastic particles (4) is increased in order to fill the extruder screw housing (1st ) with plastic compound (50) and thus to increase the overall toughness and/or to change the formulation of the plastic to be processed in such a way that the toughness of the resulting plastic melt is increased; or b) if the heating output (Q ) or the cooling output (QK) is too low in relation to the drive power (P _s ), the supply of plastic particles (4) is reduced by the filling level of the extruder screw housing (1) with plastic mass (50) and thus to reduce the overall toughness and/or the formulation of the plastic to be processed is changed in such a way that the toughness of the resulting plastic melt is reduced. Method according to Claim 1 or 2, characterized in that two counter-rotating extruder screws (3') are mounted axis-parallel in the extruder screw housing (1). Method according to one of Claims 1 to 3, characterized in that the plastics extrusion plant (100) has at least one gravimetric and/or volumetric metering device (200) for the supply of plastics particles (4). Method according to one of Claims 1 to 4, characterized in that the extruder screw housing (1) has several, preferably four to ten, control zones (40) along the extruder screw (3), in which the external supply of heat and external cooling are each set separately can be. Method according to Claim 5, characterized in that at least one separate heating device (10) and/or cooling device (20) is provided in each control zone (40). Method according to one of Claims 1 to 6, characterized in that the increase or reduction in the overall toughness takes place by changing the proportion of processing aids in the formulation of the plastic to be processed. Method according to one of Claims 1 to 7, characterized in that the external cooling is carried out using at least one electric fan (20).