Method for the process management of a mold-filling process of an injection molding machine

Abstract

The invention relates to a method for filling a mold cavity of a molding tool in a volumetrically correct manner. A molded part/volume equivalence is ascertained during a learning phase, and production injection-molding cycles are influenced during a production phase such that the molded part/volume equivalence ascertained during the learning phase is also satisfied during the production injection-molding cycle.

Claims

1. A method for the volumetrically correct filling of a cavity of a mold with a melt of a material to be processed in an injection-molding process, said method comprising a learning phase and a production phase: performing said learning phase comprising the steps of: providing an injection molding machine equipped with a mold and set up for producing a usable part in a cavity of the mold; carrying out at least one learning injection-molding cycle to obtain the usable part and recording a pressure curve correlating to a mass pressure curve; determining a viscosity index, which characterizes a melt of the learning injection-molding cycle, during an injection phase of the learning injection-molding cycle or during a plasticization phase preceding the learning injection-molding cycle; determining a filling index as an index for the volumetrically correct filling of the cavity of the usable part in the learning injection-molding cycle, wherein the following equation applies: $\mathrm{FIL}=L_{t\left(s=\mathrm{CPi}\right)}^{t(s=C0\mathrm{Pd}}\mathrm{PLMass}\left(t\right)dt$ wherein FIL is the filling index, COPL is a changeover point in the learning injection-molding cycle, (s=COPL) is a screw position at the changeover point in the learning injection-molding cycle, (s=CPL) is a screw position at the start of integration for determining the filling index, t(s=COPL) is the time at the screw position of the changeover point in the learning injection-molding cycle, PLMass(t) is the pressure curve, and t(s=CPL) is the time at which the screw position has reached a position at which a predefined pressure PLMass(t)=PcP is applied, or has reached a position at which the filling of the cavity begins, wherein the following equation applies:
s=CPL>s=COPL; and forming a molded-part volume equivalent MPVeq=FILNIL, wherein VIL is the viscosity index, performing said production phase comprising the steps of: carrying out a multiplicity of production injection-molding cycles using the mold; recording at least one pressure curve correlating to the mass pressure curve during a current cycle of the multiplicity of production injection-molding cycles; determining a viscosity index, which characterizes the melt of the current production injection-molding cycle, during the injection phase of the current production injection-molding cycle or during a plasticization phase preceding the current production injection-molding cycle, after determining the viscosity index, calculating a required filling index for the current production injection-molding cycle in accordance with the equation:
FIp=MPVeq*VIp and adjusting a changeover point of the current production injection-molding cycle and/or an injection rate profile in such a manner during a remaining injection phase (EP) of the current production injection-molding cycle that the following equation applies: $\mathrm{Flp}=J_{t\left(s=\mathrm{CPp}\right)}^{t\left(s=\mathrm{COPp}\right)}\mathrm{PPMass}\left(t\right)dt=\mathrm{MPVeq}.Math.\mathrm{Vlp}$ wherein COPp is the changeover point in the production injection-molding cycle, (s=COPp) is a screw position at the changeover point in the production injection-molding cycle, (s=CPp) is a screw position at the start of integration for determining the filling index, FIp is the required filling index, PPMass(t) is the at least one pressure curve, VIp is the viscosity index.

2. The method of claim 1, wherein the determination of the viscosity index, which characterizes the melt of the learning injection-molding cycle, is implemented during the injection phase of the learning injection-molding cycle in accordance with the following equation:
VIL=FzeL*K1NM1 with $F_{\mathrm{ZEL}}=f_{\mathrm{It}\left(s={\mathrm{MI}}_{\mathrm{Pos}1}\right)}^{t\left(s={\mathrm{MI}}_{\mathrm{Po}\&z}\right)}{p}_{\mathrm{LMass}}\left(t\right)dt$ wherein (s=MIp051) and (s=MIPos2) are different screw positions during the injection phase and t(s=MIPos1) and t(s=MIPos2) are different times for the corresponding screw positions (s=MIPos1, s=MIPos2) and the following equation applies:
MIPos1>MIPos2>COPL, wherein VMi is an average value of a screw speed between the different screw positions, and K.sub.1 is a correction constant for scaling.

3. The method of claim 2, wherein one of the different screw positions is located sufficiently far upstream of the changeover point when the viscosity index in the production phase is determined in the injection phase of the injection-molding cycles, so that after determining the viscosity index, it is possible during the remainder of the injection phase up to the changeover point of the current production injection-molding cycle to influence a height of the filling index by shifting the changeover point or by adjusting the speed profile, and the following applies:
MIPos2>COPL+VMi*tRz+ASmax, wherein tRz is a calculation time for determining the filling index, and Asmax is a maximum expected local displacement of the changeover point in the learning phase compared to the changeover point in the production phase.

4. The method of claim 2, further comprising shifting a measurement interval in accordance with the equation MI=MI.sub.Pos1MI.sub.Pos2 during determination of the viscosity index in the learning phase or in the production phase as a function of a closing behavior of a non-return valve.

5. The method of claim 4, wherein the measurement interval is shifted to larger screw positions, when a predetermined reference pressure is locally passed through earlier in the production phase than in the learning phase in correspondence with:
s(P.sub.RefP)>s(p.sub.RefL), wherein s(p.sub.RefP) is the screw position at the reference pressure in the production phase, and s(p.sub.RefL) is the screw position at a reference pressure in the learning phase.

6. The method of claim 4, wherein the measurement interval shifted to smaller screw positions, when a predetermined reference pressure is locally passed through later in the production phase than in the learning phase in correspondence with:
s(p.sub.RefP)<s(P.sub.RefL), wherein s(p.sub.RefP) is the screw position at the reference pressure in the production phase, and s(p.sub.RefL) is the screw position at a reference pressure in the learning phase.

7. The method of claim 5, wherein the reference pressure is chosen in such a manner that it is smaller than a pressure applied at the screw position in the injection phase.

8. The method of claim 6, wherein the reference pressure is chosen in such a manner that it is smaller than a pressure applied at the screw position in the injection phase.

9. The method of claim 1, wherein the determination of the viscosity index, which characterizes the melt of the learning injection-molding cycle, is implemented during the plasticization phase preceding the learning injection-molding cycle in accordance with the following equation:
VILFzPlastL*K2/IMM with $F_{\mathrm{ZPlastL}}{}_{t\left(s={\mathrm{MM}}_{\mathrm{Pos}1}\right)}^{t\left(s={\mathrm{MM}}_{\mathrm{Pos}2}\right)}{M}_L\left(t\right)dt$ wherein (s=MMPos1) and (s=MMPos2) are different screw positions, t(s=MMPos1) and t(s=MMPos2) are different times for the corresponding screw positions (s=MMPos1, s=MMPos2) during the plasticization phase and the following applies:
MMPos1<MMPos2 and
IMM=MMPosrMMPos1, wherein ML(t) is a drive moment of a plasticization screw, and K.sub.2 is a correction constant for scaling.

10. The method of claim 1, wherein the determination of the viscosity index, which characterizes the melt of the current production injection-molding cycle, is implemented during the injection phase of the current production injection-molding cycle in accordance with the following equation:
VIp=FzEP*K1NM1 with $\mathrm{FzEP}=f_{\mathrm{It}\left(S={\mathrm{MI}}_{\mathrm{Poss}}\right)}^{t\left(S={\mathrm{Ml}}_{\mathrm{Pos},}\right)}{p}_{\mathrm{PMass}}\left(t\right)dt$ wherein (s=MIPos1) and (s=MIPos2) are different screw positions, t(s=MIPos1) and t(s=MIPos2) are the times for the corresponding screw positions (s=MIPos1, s=MIPos2) during the injection phase and the following applies:
Mlpos1>MfPos2>COPp, wherein VM.sub.1 is an average value of a screw speed between the different screw positions, and K.sub.1 is the correction constant for scaling.

11. The method of claim 10, wherein one of the different screw positions is located sufficiently far upstream of the changeover point when the viscosity index in the production phase is determined in the injection phase of the injection-molding cycles, so that after determining the viscosity index, it is possible during the remainder of the injection phase up to the changeover point of the current production injection-molding cycle to influence a height of the filling index by shifting the changeover point or by adjusting the speed profile, and the following applies:
MIpos2>COPL+VMr*tRz+Asmax, wherein tRz is a calculation time for determining the filling index, and Asmax is a maximum expected local displacement of the changeover point in the learning phase compared to the changeover point in the production phase.

12. The method of claim 1, wherein the determination of the viscosity index, which characterizes the melt of the current production injection-molding cycle, is implemented during the plasticization phase preceding the current production injection-molding cycle in accordance with the following equation: VIp=FzP1astP*K2/IMM with $F_{\mathrm{ZPlastP}}=L_{t(s,;\mathrm{MMPos}:\mathrm{il}}^{t\left(s={\mathrm{MM}}_{\mathrm{Post}}\right)}\mathrm{Mp}\left(t\right)\mathrm{dt}$ wherein (s=MMPos1) and (s=MMPos2) are different screw positions during the plasticization phase, t(s=MMPos1) and t(s=MMPos2) are different times for the corresponding screw positions (s=MMPos1, s=MMPos2) and the following applies:
MMPos1<MMPos2 and
IMM=MMPosrMMPos1, wherein Mp(t) is a drive moment of a plasticization screw, and K2 is the correction constant.

13. The method of claim 1, wherein the pressure curve in the learning phase and the pressure curve in the production phase, which correlate to the mass pressure curve, are an injection pressure curve, a hydraulic pressure curve, a cavity internal pressure curve or a mass pressure curve or are determined from a motor torque of an injection motor.

14. The method of claim 1, wherein in a holding-pressure phase of the current production injection-molding cycle, a holding pressure is changed with respect to a pre-set holding pressure, wherein the following equation applies:
PNP=PN*(1+K3*(VIpVIL)NIL), wherein PNP is the holding pressure, PN is the pre-set holding pressure, K3 is a correction constant, VIp is the viscosity index of the current production injection-molding cycle, and VIL is the viscosity index of the learning injection-molding cycle.

15. The method of claim 14, wherein the injection-molding cycle is implemented during the injection phase in the learning phase and the production phase up to the changeover points, respectively, in a position-regulated manner with regards to the screw positions or in a position-regulated and pressure-limited manner and after the changeover points takes place up to the end of the holding-pressure phase in a pressure-regulated manner.

16. The method of claim 1, wherein the screw positions at the start of integration for determining the filling index in the learning phase and the production phase are determined from a fixedly predetermined value or is a screw position, at which a non-return valve is closed.

17. The method of claim 1, further comprising adjusting the viscosity index in the production phase as a function of a melting temperature, a back pressure, or a plasticization speed.

18. The method of claim 17, wherein the viscosity index in the production phase is adjusted as a function of a cylinder temperature.

19. The method of claim 1, further comprising executing the learning phase for determining the molded-part volume equivalent on a first injection molding machine, and executing the production phase with a second injection molding machine, after a mold change from the first injection molding machine to the second injection molding machine and a value at least for the molded-part volume equivalent is carried over to a control of the second injection molding machine.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) In the following, the invention is explained in more detail by way of example on the basis of the drawing. In the figures:

(2) FIG. 1: schematically shows a graph of a learning injection-molding cycle for determining a molded-part volume equivalent MPV.sub.eq;

(3) FIG. 2: schematically shows the graph according to FIG. 1, without the cross-hatched area, which represents the filling index FI.sub.L, on the basis of which, a sensible possibility is explained, of how integration limits t(s=MI.sub.Pos1) and t(s=MI.sub.Pos2) can be determined;

(4) FIG. 3: schematically shows a graph, on the basis of which, a second possibility for determining the viscosity index VI.sub.L or VI.sub.P is explained;

(5) FIG. 4: schematically shows a graph, which shows a characteristic pressure curve p.sub.PMass(t) for a more viscous material compared to the learning injection-molding cycle, wherein a flow number F.sub.ZEP of the material and the required filling index FI.sub.P are illustrated cross-hatched;

(6) FIG. 5: shows the graph according to FIG. 4, wherein the flow number F.sub.ZEP of a less viscous material compared to the learning process and the required filling index FI.sub.P thereof are drawn in cross-hatched;

(7) FIG. 6: shows a graph, on the basis of which, a displacement of a measurement interval MI is explained;

(8) FIG. 7: schematically shows a graph over a multiplicity of production cycles, which shows the dependence of the molded-part weight of the viscosity index VI.sub.P of the material used in the case of a method according to the prior art without regulation according to the invention and in the case of a method according to the invention;

(9) FIG. 8: schematically shows a graph over a multiplicity of production cycles, from which it emerges, how in the case of a changing viscosity index VI.sub.P of the material, the changeover position COP.sub.P changes in the method according to the invention and remains constant in a conventional method according to the prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(10) A learning phase of the method according to the invention (FIG. 1) assumes that an injection machine, which is equipped with a mold, is provided and the injection molding machine is set up for producing a good part in a cavity of the mold.

(11) For better understanding of the following graphs, it is emphasized that the screw position s(t) in FIGS. 1, 2 decreases from an initial position s.sub.A up to the screw position at the changeover point COP.sub.L or COP.sub.P, i.e. at s=COP.sub.L or s=COP.sub.P.

(12) While a learning injection-molding cycle for obtaining a good part is carried out, a pressure curve p.sub.LMass(t) is recorded, which correlates to the mass pressure curve of the learning injection-molding cycle. The recording of this pressure curve takes place over the time t. In addition to this pressure curve p.sub.LMass(t), the screw position s(t) and the screw speed v(t) are drawn in by means of the dashed line in FIG. 1. Characteristic times t(s=CP.sub.L), t(s=MI.sub.Pos1), t(s=MI.sub.Pos2) and t(s=COP.sub.L) are drawn in on the time axis. The area below the curve p.sub.LMass(t) inside the limits of t(s=MI.sub.Pos1) and t(s=MI.sub.Pos2) represents a flow number F.sub.ZEL of the present melt and is determined by

(13) $F_{\mathrm{ZEL}}={}_{t\left(s={\mathrm{MI}}_{\mathrm{Pos}1}\right)}^{t\left(s={\mathrm{MI}}_{\mathrm{Pos}2}\right)}{p}_{\mathrm{LMass}}\left(t\right)dt$

(14) An average value of the screw speed v(t) is formed between the integration limits t(s=MI.sub.Pos1) and t(s=MI.sub.Pos2). The average value is labelled with V.sub.MI. The flow number F.sub.ZEL normalized with the average value V.sub.MI and if necessary multiplied with a correction constant K.sub.1 for scaling gives the viscosity index VI.sub.L, which represents the characteristic of the melt of the learning injection-molding cycle, determined in the injection phase EL.

(15) A filling index FI.sub.L is determined as an index in the learning injection-molding cycle, wherein the filling index FI.sub.L corresponds to the area below the curve p.sub.LMass(t) in the limits from t(s=CP.sub.L) to t(s=COP.sub.L) and is determined by means of the integral

(16) ${\mathrm{FI}}_L={}_{t\left(s={\mathrm{CP}}_L\right)}^{t\left(s={\mathrm{COP}}_L\right)}{p}_{\mathrm{LMass}}\left(t\right)dt$

(17) The upper integration limit t(s=COP.sub.L) in this case is the position, pre-set in the learning injection-molding cycle, of the changeover point COP.sub.L, to which the corresponding time value t(s=COP.sub.L) upon reaching the screw position s corresponds. In this case, it is assumed according to the invention that during the injection phase EL, the mold filling at the changeover point COP.sub.L is finished. A further mold filling during the holding-pressure phase NP, which is subsequent to the injection phase EP, is disregarded here. The lower integration limit t(s=CP.sub.L) from which the integration for determining the filling index FI.sub.L takes place, is determined in such a manner in this case that at the start of integration t(s=CP.sub.L), an effective filling of the cavity of the mold begins or has already begun. This is the case in particular if a non-return valve, which may be present, is securely closed. Because the determination of the accurate closing time of the non-return valve is technically complicated or is only possible imprecisely using simple technical means, a predetermined pressure value p.sub.CP can alternatively be chosen, at which, according to experience, the effective filling of the cavity has begun, that is to say a closing of the non-return valve has already taken place. A pressure value p.sub.CP=p.sub.LMass(t(s=CP.sub.L)) of this type is chosen expediently with regards to its size in such a manner that this pressure value is smaller than the pressure value p.sub.LMass(t) at time t(s=MI.sub.Pos1).

(18) During the good-part cycle, the two above-described integrals are recorded and the values of the viscosity index VI.sub.L and the filling index FI.sub.L determined here are subsequently placed in a relationship to one another, wherein this relationship FI.sub.L/VI.sub.L forms the molded-part volume equivalent MPV.sub.eq.

(19) In the following, a possibility for determining the integration limits t(s=MI.sub.Pos1) and t(s=MI.sub.Pos2) is explained by way of example on the basis of FIG. 2. The integration start t(s=MI.sub.Pos1) must be determined in a suitable manner so that the viscosity index VI.sub.L actually constitutes a measure for the characteristic of the melt in the learning injection-molding cycle. To this end, s=MI.sub.Pos1 must in any case be larger than s=MI.sub.Pos2. The integration start t(s=MI.sub.Pos1) should be chosen to be as small as possible, i.e. s=MI.sub.Pos1 should be chosen to be as large as possible, so that the integration interval between t(s=MI.sub.Pos1) and t(s=MI.sub.Pos2) becomes as large as possible and thus the determination of the viscosity index VI.sub.L becomes as accurate as possible. On the other hand, it should not fall below a minimum value t(s=MI.sub.Pos1), so that start-up and acceleration processes of the screw and compression effects and transient phenomena inside the melt do not negatively influence the viscosity index VI.sub.L. In order to overcome this conflict of objectives, determining the value s=MI.sub.Pos1 as follows has proven successful.

(20) As soon as the screw has reached the speed v(t) set in the control of the injection molding machine in a first stage of the set speed profile, this screw position s=x.sub.v is saved. A maximum compression path x.sub.vComp is deducted from this position s=x.sub.v as a safety distance. The safety distance x.sub.vComp is chosen in such a manner in this case that transient phenomena or compression processes inside the melt are eased safely. From this position it is satisfactorily ensured that the flow number F.sub.ZEL can be determined with sufficient accuracy. Thus, the first integration limit results when determining the viscosity index for t(s=MI.sub.Pos1)=t(s=x.sub.vx.sub.vComp).

(21) To sensibly obtain the upper integration limit t(s=MI.sub.Pos2) when determining the viscosity index VI.sub.L, it is necessary to determine the screw position s=MI.sub.Pos2 in a suitable manner. A suitable method for this initially proceeds from the position s=COP.sub.L of the changeover point COP.sub.L in the learning injection-molding cycle. In this case, the position s=COP.sub.L is smaller than the position s=MI.sub.Pos2. The invention is based inter alia on still having a sufficiently large remainder of the injection phase EP available in a production cycle, after the determination of the viscosity index VI.sub.P, in order to still have sufficient influence on the filling index FI.sub.P of the same injection phase EP as a function of the viscosity index VI.sub.P determined in the injection phase EP. In this case, one requires a certain time starting from the finishing of the integral for determining the viscosity index VI.sub.P, in order to calculate the required filling index FI.sub.P. This calculation time t.sub.RZ lasts a few milliseconds and, together with the path of the screw traveled in this time, gives a certain calculation path s=v.sub.MI*t.sub.RZ.

(22) Furthermore, according to the invention, the adjustment of the filling index FI.sub.P is implemented inter alia by means of a displacement of the changeover point COP.sub.P to larger or smaller screw positions s. One such maximum possible displacement of the changeover point COP.sub.P to larger screw positions s is labelled with s.sub.max, so that it has proven expedient to choose the screw position s=MI.sub.Pos2 of the upper integration limit for s=MI.sub.Pos2>COP.sub.L+V.sub.MI*t.sub.RZ+s.sub.max.

(23) This integration span, determined once in the learning injection-molding cycle during the injection phase EL, between the starting point t(s=MI.sub.Pos1) and the end point t(s=MI.sub.Pos2) is termed the measurement interval MI=MI.sub.Pos1MI.sub.Pos2 with respect to the associated screw positions s. This measurement interval MI is then retained in terms of the size thereof for the subsequent production injection-molding cycles.

(24) One alternative for determining the viscosity index VI.sub.L in the learning injection-molding cycle or analogously to the viscosity index VI.sub.P in the production injection-molding cycle is explained on the basis of FIG. 3. A typical moment curve M.sub.L(t) of a plasticization screw in the learning injection-molding cycle is illustrated in FIG. 3. One such typical curve also arises in the production injection-molding cycle as M.sub.P(t). The curves M.sub.L(t) and M.sub.P(t) in this case show a torque curve of a plasticization screw during a plasticization phase PP. It has been established that the plasticization phase PP is also suitable in order to determine a viscosity index VI.sub.L or VI.sub.P of the melt. To this end, a flow number F.sub.ZPlastL is formed as an integral over the drive moment M.sub.L(t) as a function of time in the limits of t(s=MM.sub.Pos1) to t(s=MM.sub.Pos2). This flow number F.sub.ZPlastL is normalized over the length I.sub.MM=MM.sub.Pos2MM.sub.Pos1 and, if necessary, multiplied by a correction constant K.sub.2 for scaling. In this case, the integration limits MM.sub.Pos1 and MM.sub.Pos2 are set in such a manner that on the one hand, a sufficiently large distance prevails between these positions in order to determine the viscosity index VI.sub.L; VI.sub.P with sufficient accuracy. On the other hand, the positions MM.sub.Pos1 and MM.sub.Pos2 should be sufficiently far away from transient or detuning phenomena when starting up and when braking the plasticization screw. An integration range, determined once in the learning phase, between the positions MM.sub.Pos1 and MM.sub.Pos2 or the associated times t(s=MM.sub.Pos1) and t(s=MM.sub.Pos2) is also retained in the later production injection-molding cycles. The plasticization phase PP in which the melt is prepared for a subsequent injection phase EP is for example considered as a relevant plasticization phase PP. The value for the molded-part volume equivalent MPV.sub.eq can in turn be determined according to the invention according to the equation MPV.sub.eq=FI.sub.L/VI.sub.L using the viscosity index VI.sub.L and the filling index FI.sub.L determined in the subsequent injection phase EL.

(25) It is only mentioned for the sake of clarity that, for the case that the viscosity index VI.sub.L is determined in the learning injection-molding cycle during the injection phase EL, as is illustrated in FIG. 2, the viscosity index VI.sub.P is of course likewise determined in the injection phase EP of the production injection-molding cycle in the subsequent production injection-molding cycles. If the viscosity index VI.sub.L is determined in the learning injection-molding cycle during the plasticization phase PP, the viscosity index VI.sub.P is also likewise determined during the plasticization phase PP in the subsequent production injection-molding cycles.

(26) During the learning phase, i.e. during the production of at least one good part, the following listed values were therefore learned on the basis of the good-part injection-molding cycle: a) The value for the molded-part volume equivalent MPV.sub.eq, b) The value for the measurement interval MI=MI.sub.Pos1MI.sub.Pos2, within which the flow number F.sub.ZEL was determined. The size of the measurement interval MI is also used as a basis for the subsequent production cycles. c) Furthermore, the pressure value p.sub.CP determined in the learning phase is likewise carried over into the production phase. Analogously to the learning phase, the time t(s=CP.sub.P) at which the pressure curve p.sub.PMass(t) passes through the predetermined or determined pressure value p.sub.CP is used in the production phase as the lower integration limit in the production phase during the determination of the filling index FI.sub.P. d) For the case that the viscosity index VI.sub.L was determined during the injection phase EL, the values of the screw position s=MI.sub.Pos1 and s=MI.sub.Pos2 are additionally carried over and if necessary adjusted with regards to the absolute values thereof, as is explained below on the basis of FIG. 6. e) For the case that the viscosity index VI.sub.L was determined during the plasticization phase PL, the values of the screw positions s=MM.sub.Pos1 and s=MM.sub.Pos2 are carried over. f) As long as constants K.sub.11 and K.sub.21 were used in the learning phase, these constants K.sub.1 and K.sub.2 are also carried over into the production phase.

(27) The production phase of the method according to the invention is explained in the following on the basis of FIGS. 4 to 6. FIG. 4 shows a graph, in which the mass pressure p.sub.PMass(t) is entered over time t. A solid line shows the mass pressure curve p.sub.PMass(t) of a material in the production injection-molding cycle. Furthermore, the mass pressure curve p.sub.LMass(t), as was recorded in the learning injection-molding cycle, is illustrated dotted for comparison. Furthermore, the injection phase EP and a portion of the holding-pressure phase NP are illustrated. The level of the pressure curve p.sub.PMass(t) is clearly increased within the integration limits t(s=MI.sub.Pos1) and t(s=MI.sub.Pos2) compared to the pressure curve p.sub.LMass(t) within these limits. Therefore, a larger value for the flow number F.sub.ZEP results therefrom, if the integral is determined over the pressure curve p.sub.PMass(t) within the limits t(s=MI.sub.Pos1) and t(s=MI.sub.Pos2). The viscosity index VI.sub.P of the material can be determined therefrom analogously to the learning phase using the constant K.sub.1 and the average speed V.sub.MI. From the value of the viscosity index VI.sub.P of the material, which is determined at time t(s=MI.sub.Pos2) at the latest, it is then possible to determine FI.sub.P=MPV.sub.eq*VI.sub.P using the equation, which value the filling index FI.sub.P for the material must achieve, in order to achieve a volumetrically correct mold filling and therefore also to obtain a good part using the material of the production injection-molding cycle, which has a different viscosity index VI.sub.P from the material from the learning phase. This succeeds in that the integral ongoing temporally from the time t(s=CP.sub.P) for determining the filling index FI.sub.P of the material is determined in an ongoing manner and as soon as the viscosity index VI.sub.P is known, the required filling index FI.sub.P is determined. If the ongoing integral for determining the current filling index FI.sub.P reaches the value of the required filling index FI.sub.P, then the changeover to the holding-pressure phase NP takes place.

(28) In the case of a more viscous material, the changeover point COP.sub.P is situated e.g. temporally after the changeover point t(s=COP.sub.L). The invention makes it possible to also maintain the value MPV.sub.eq, which was determined in the learning injection-molding cycle, in the production injection-molding cycle in the case of a material which has a different material quality compared to the material, which was used in the learning process and therefore to achieve a volumetrically correct filling of the cavity and thus to obtain a good part. A further improvement of the quality of the parts can be achieved in spite of fluctuating melt quality, i.e. in spite of fluctuating viscosity index VI.sub.P with respect to the viscosity index VI.sub.L determined in the learning process, if a holding pressure p.sub.NP in the production phase is adjusted with respect to a pre-set holding pressure p.sub.N, which may be e.g. holding pressure run in the learning phase. In this case, it has proven successful to adjust the holding pressure p.sub.NP in the production phase according to the formula p.sub.NP=p.sub.N*(1+K.sub.3(VI.sub.PVI.sub.L)/VI.sub.L, where K.sub.3 is a correction constant. The correction constant K.sub.3 can in this case map workpiece properties of the molded part to be produced. Thus, for example, the correction constant K.sub.3 can for example be applied somewhat smaller in the case of a particularly thin-walled molded part than in the case of a thicker walled molded part. This is because in the case of a thin-walled molded part, the mold filling is less effective in the holding-pressure phase than in the case of a thicker walled molded part.

(29) FIG. 5 shows a production injection-molding cycle, in which a pressure curve p.sub.PMass(t) runs at a lower level compared to a pressure curve p.sub.LMass(t). This means, in the case of otherwise identical boundary conditions, that the material has a low viscosity or a lower viscosity index VI.sub.P than the material which was used in the learning injection-molding cycle during the learning phase. A holding-pressure level p.sub.NP is lowered for the material of the production injection-molding cycle compared to the holding-pressure level of the learning injection-molding cycle or a pre-set holding pressure p.sub.N. The time t(s=COP.sub.P) is displaced to be earlier compared to the time t(s=COP.sub.L), which means a displacement of the changeover point COP.sub.P to a larger screw position s for the changeover point (s=COP.sub.P) of the material in the production injection-molding cycle.

(30) Due to certain effects, e.g. due to a changing closing behavior of a non-return valve, it may occur that a reference pressure value p.sub.Ref, e.g. the pressure value p.sub.CP is passed through temporally earlier at a time t(s=CP) (cf. FIG. 6). The lower integration limit for the determination of the filling index FI.sub.P is thereby changed, so that a miscalculation of the viscosity index VI.sub.P and therefore of the required filling index FI.sub.P would result if the integration limits t(s=MI.sub.Pos1) and t(s=MI.sub.Pos2) are maintained. This may result in production of a scrap part. To prevent this, it is expedient in such a case, in which passing through the reference pressure value p.sub.Ref is displaced by a period of time t in the direction of early or the direction of late, to also correspondingly displace the integration limits t(s=MI.sub.Pos1) and t(s=MI.sub.Pos1) towards early or late by the time period t. Alternatively, the measurement interval MI can also correspondingly be displaced to larger or smaller screw positions, wherein the size of the measurement interval MI preferably remains the same.

(31) In FIG. 6, this time displacement by t is shown qualitatively by way of the example of a material which is otherwise constant with regards to its viscosity.

(32) The positive mode of action of the method according to the invention becomes clear on the basis of FIG. 7. In a first curve (empty squares), FIG. 7 shows the course of the viscosity index VI.sub.P characterizing the melt over a multiplicity of production cycles. In the illustration according to the example, the viscosity index VI.sub.P initially increases sharply from the 17th cycle, in order to then approach a higher limit value. A curve of this type for example corresponds to a cooling of the melt, from which a higher viscosity index VI.sub.P results.

(33) In conventional process management, illustrated in a second curve of FIG. 7 (empty circles), a change of this type of the viscosity index VI.sub.P with increasing viscosity index VI.sub.P has a clearly falling molded-part weight as a consequence. This means that the volumetric filling was not satisfactory and sink marks or under-filling may occur in the case of falling molded-part weights of this type. Scrap parts are therefore created thereby.

(34) The course of the molded-part weight when the method according to the invention is applied is illustrated in a third curve of FIG. 7 (empty triangles). It becomes clear that in spite of increasing viscosity index VI.sub.P from the 17th cycle, the method according to the invention is able to keep the molded-part weight virtually constant in spite of changing melt properties. Although, starting from the 17th cycle, the melt characteristic changes considerably with regards to the viscosity index VI.sub.P thereof, the method according to the invention is able to keep the molded-part weights virtually constant and therefore to ensure a volumetrically correct filling of the cavity, which leads to good parts.

(35) In FIG. 8, it is shown how changing a viscosity index VI.sub.P over a multiplicity of production cycles has an effect on the changeover position s=COP.sub.P in conventional process management and in process management according to the invention. In a first curve (empty rectangles), the course of the viscosity index VI.sub.P is illustrated over a multiplicity of cycles. As already explained in FIG. 7, the viscosity index VI.sub.P increases sharply from the 17th cycle and approaches a higher level up to the 35th cycle. In conventional process management (empty circles), the changeover point COP.sub.P is not influenced. The changeover position s=COP.sub.P remains virtually constant during all 35 cycles. If the method according to the invention is applied, it becomes clear on the basis of a third curve (empty triangles), that the changeover position s=COP.sub.P is displaced to lower screw positions in a manner correlating to the increase of the viscosity index VI.sub.P and remains virtually constant starting approximately from the 26th cycle.

(36) The method according to the invention is suitable for application on electro- and hydromechanical injection molding machines of all sizes. In particular, it is easily possible, e.g. in the context of programming the operating software of an injection molding machine, to integrate the method according to the invention in new machines. Furthermore, the method according to the invention is based on measured values, e.g. pressure measurements during the injection and/or holding-pressure phase, travel measurements of the screw during the injection phase, travel measurements and torque measurements of a plasticization screw during a plasticization phase and the like, which are usually already measured in the case of injection molding machines, so that no additional measurement sensors or the like have to be attached for the method according to the invention. In this respect, the method according to the invention is also exceptionally suitable as a retrofit solution for pre-existing injection molding machines.

(37) Injection molding machines, which are operated using the method according to the invention, are able to automatically compensate negative effects of batch fluctuations on molded-part quality for example. In any case, negative effects on the molded-part quality when restarting the machines, e.g. in the event of faults or after a certain stoppage, are compensated automatically by means of the state-dependent process management according to the invention. The machine operator has to intervene in the production process less often, in order for example to manually adjust a parameter of the injection molding machine. The quality differences of the individual molded parts are reduced to a minimum, even in the case of changing production and/or environmental conditions.

(38) Depending on the material properties, for example the material moisture, the material composition (batch fluctuations) and the influence thereof on the operation of an injection molded machine, e.g. the influence thereof on the closing behavior of a non-return valve, can be corrected automatically by the method according to the invention without the intervention of a machine operator. As a result, over-injection or also underfilling of the cavities inter alia is prevented during the production of the molded parts. Considerable cost savings can be achieved as a result. The process reliability and the degree of automation can be increased.

(39) External influences, such a e.g. fluctuating environmental temperatures in a shop, in which the injection molding machine is installed, can also be compensated using the method according to the invention. Fluctuating environmental temperatures, which can be set for example by means of different solar irradiation or by means of a different number of injection molding machines or plants, which are operated in the shop, lead in the case of fixedly pre-set settings to minimal viscosity fluctuations in the melt to be processed. Viscosity fluctuations of this type have a negative effect on the molded-part quality. Change of the melt characteristic of this type, particularly of the viscosity, can be detected using the method according to the invention and in spite of that a reliable and complete filling of the cavity of the mold can be ensured by means of a changed process management.

LIST OF REFERENCE SIGNS

(40) p.sub.LMass(t) Pressure curve in the learning injection-molding cycle VI.sub.L Viscosity index in the learning injection-molding cycle EL Injection phase of the learning injection-molding cycle PL Plasticization phase of the learning injection-molding cycle FI.sub.L Filling index of the learning injection-molding cycle s Screw position t(s) Time at which a certain screw position s is reached COP.sub.L Changeover point s=COP.sub.L Screw position at changeover point s=CP.sub.L Screw position at the start of the integration for determining the filling index FI.sub.L MPV.sub.eq Molded-part volume equivalent p.sub.PMass(t) Pressure curve of a pressure correlating to the mass pressure curve during the production phase VI.sub.P Viscosity index during the production injection-molding cycle EP Injection phase of the production cycle PP Plasticization phase of the production injection-molding cycle FI.sub.P Filling index of the production injection-molding cycle MEP Machine setting parameter F.sub.ZEL Flow number determined during the injection phase in the learning injection-molding cycle K.sub.1 Correction constant V.sub.MI Average value of a screw speed v(t) between the screw positions MI.sub.Pos1 and MI.sub.Pos2 F.sub.ZPlastL Flow number of a melt determined during a plasticization phase PL of the learning injection-molding cycle MM.sub.Pos1 and MM.sub.Pos2 Screw positions s during the plasticization phase PL I.sub.MM Measurement interval during the plasticization phase PP M.sub.L(t) Drive moment during the learning injection-molding cycle K.sub.2 Correction constant MI Measurement interval during an injection phase EP; EL F.sub.ZEP Flow number determined during an injection phase in the production injection-molding cycle F.sub.ZPlastP Flow number determined during a plasticization phase PP in the production injection-molding cycle M.sub.P(t) Moment curve of a drive moment of a plasticization screw during the production cycle p.sub.NP Adjusted holding pressure p.sub.N Pre-set holding pressure K.sub.3 Correction constant t.sub.RZ Calculation time s.sub.max Maximum displacement of the changeover point p.sub.CP Pre-set pressure value at the screw position s=CP.sub.L or s=CP.sub.P p.sub.Ref Reference pressure p.sub.RefP Reference pressure in the production cycle p.sub.RefL Reference pressure in the learning cycle