METHOD FOR OPERATING AN INJECTION-MOULDING MACHINE, IN PARTICULAR WITH RESPECT TO IMPROVED CONSTANT MOULD FILLING, AND INJECTION-MOULDING MACHINE FOR CARRYING OUT THE METHOD

Abstract

A method for operating an injection-molding machine, including the steps: a) in a current injection-molding cycle, after an accepted-part reference injection-molding cycle learned in a learning phase: detecting a compound pressure change k.sub.1 relative to an accepted-part reference compound pressure p.sub.masse, ref during at least part of an injection phase of the current injection-molding cycle by measuring a current compound pressure p.sub.masse, act and comparing the current compound pressure p with the accepted-part reference compound pressure p.sub.masse, ref, b) determining a target mold internal pressure curve p.sub.wkz,soll (1) for a holding-pressure phase of the current injection-molding cycle, wherein for this purpose a mold internal pressure curve p.sub.wkz,ref (t) of the accepted-part reference injection-molding cycle is adjusted at least in dependence on the compound pressure change k.sub.1 detected in step a), and c) traveling the holding-pressure curve p.sub.masse, Hld, act (t) of the current injection-molding cycle in such a way that an actual mold internal pressure curve pwkz,act (0 of the current injection-molding cycle runs at least more closely along the target mold internal pressure curve p.sub.wkz,soll (t) than a mold internal pressure curve p.sub.wkz,ref(t) that is unchanged in comparison with the accepted-part reference cycle.

Claims

1. Method for operating an injection-moulding machine comprising the steps: a) in a current injection-moulding cycle, after an accepted-part reference injection-moulding cycle learned in a learning phase: detecting a compound pressure change k.sub.1 with respect to an accepted-part reference compound pressure p.sub.masse,ref during at least a part of an injection phase of the current injection-moulding cycle by measuring a current compound pressure p.sub.masse,act and comparing the current compound pressure p.sub.masse,act with the accepted-part reference compound pressure p.sub.masse,ref, b) determining a target mould internal pressure curve p.sub.wkz,soll(t) for a holding-pressure phase of the current injection-moulding cycle, wherein for this purpose a mould internal pressure curve p.sub.wkz,ref(t) of the accepted-part reference injection-moulding cycle is adjusted at least depending on the compound pressure change k.sub.1 detected in step a) and c) travelling the holding-pressure curve p.sub.masse,Hld,act(t) of the current injection-moulding cycle in such a manner that an actual mould internal pressure curve p.sub.wkz,act(t) of the current injection-moulding cycle runs at least more closely along the target mould internal pressure curve p.sub.wkz,soll(t) than a mould internal pressure curve p.sub.wkz,ref(t) that is unchanged with respect to the accepted-part reference cycle.

2. Method according to claim 1, wherein before, during or after step b), the method comprises the following steps: b1) determining a target switch-over mould internal pressure p.sub.wkz,umschalt,soll for the current injection-moulding cycle at least depending on the compound pressure change k1 detected in step a) b2) switching over upon reaching the target switch-over mould internal pressure p.sub.wkz,umschalt,soll determined in step b).

3. Method according to claim 1, wherein the determination of the target mould internal pressure curve p.sub.wkz,soll(t) is additionally determined depending on a pressure transmission characteristic between a mould internal pressure p.sub.wkz and a corresponding compound pressure p.sub.masse and/or using a material-specific factor k.sub.mat.

4. Method according to claim 1, wherein in the accepted-part reference injection-moulding cycle and in the current injection-moulding cycle a compound pressure integral during injection is determined as measured variable which correlates with a processing viscosity of the melt, according to the formula:
W.sub.z,act/ref=∫.sub.t.sub.0.sup.t.sup.1p.sub.masse,act/ref(t)dt, wherein t.sub.0 is a time of beginning the injection phase t.sub.inj,start or a time after closure of a back flow valve and t.sub.1 is a time of the switch-over t.sub.inj,umschalt or a time which lies after to but before t.sub.inj,umschalt.

5. Method according to claim 1, wherein in the accepted-part reference injection-moulding cycle and/or in the current injection-moulding cycle additionally at least one of the following quantities is determined and recorded if necessary: maximum mould internal pressure p.sub.wkz,max as well as the appurtenant time t.sub.wkz,max, a drop of the mould internal pressure after reaching the maximum (m.sub.1) a mould internal pressure average p.sub.wkz,avg and/or a compound pressure average p.sub.masse,avg in the injection phase and/or a mould internal pressure average p.sub.wkz,Hld,avg and/or a compound pressure average p.sub.masse,Hld,avg in the holding-pressure phase, a mould internal pressure area p.sub.a,wkz in the holding-pressure phase, as pressure integral according to the following formula
∫.sub.t.sub.2.sup.t.sup.3p.sub.wkz,Hld(t) a compound pressure area p.sub.a,masse in the holding-pressure phase, as pressure integral according to the following formula
∫.sub.t.sub.2.sup.t.sup.3p.sub.masse,Hld(t) wherein: p.sub.wk,Hld(t) is the mould internal pressure curve in the holding-pressure phase, p.sub.masse,Hld(t) is the compound pressure curve in the holding-pressure phase, p.sub.wkz,avg is an average of the mould internal pressure during the injection phase, p.sub.masse,avg is an average of the compound pressure during the injection phase, p.sub.wkz,Hld,avg is an average of the mould internal pressure during the holding-pressure phase, p.sub.masse,Hld,avg is an average of the compound pressure in the holding-pressure phase, t.sub.2 is a time at or after the switch-over point, is being the switch-over point and t.sub.3 is a time after t.sub.2, e.g. the time of the end of the holding-pressure phase t.sub.Hld,End or a time before the end of the holding-pressure phase, being the time t.sub.wkz,max of the maximum mould internal pressure of the respective cycle.

6. Method according to claim 1, wherein a ratio of the pressure integral during injection in the current injection-moulding cycle and the pressure integral during injection of the accepted-part reference injection-moulding cycle is determined as the compound pressure change k.sub.1, according to the formula
k.sub.1=W.sub.z,act/W.sub.z,ref or a ratio of the average of the compound pressure during injection in the current injection-moulding cycle and the average of the compound pressure during injection of the accepted-part reference injection-moulding cycle is determined as the compound pressure change k1, according to the formula
k.sub.1=p.sub.masse,Hld,avg/p.sub.masse,ref,avg or a ratio of one or more pressure individual values during injection in the current injection-moulding cycle and during injection of the accepted-part reference injection-moulding cycle is determined as the compound pressure change k1, according to the formula
k.sub.1=p.sub.masse,act/p.sub.masse,ref.

7. Method according to claim 1, wherein a dependence (k.sub.2) between mould internal pressure p.sub.wkz and compound pressure p.sub.masse during the holding-pressure phase in the accepted-part reference injection-moulding cycle and in the current injection-moulding cycle is determined as a pressure transmission characteristic, according to at least one of the formulae $a)k_2=\frac{{\int }_{t2}^{t3}{p}_{wkz,\mathrm{Hld}}\left(t\right)}{{\int }_{t2}^{t3}{p}_{\mathrm{m\alpha sse},\mathrm{Hld}}\left(t\right)},b)k_2=\frac{p_{\mathrm{wkz},\mathrm{Hld},\mathrm{avg}}}{p_{\mathrm{m\alpha sse},\mathrm{Hld},\mathrm{avg}}}c)k_2=\frac{p_{\mathrm{wkz},\mathrm{Hld}}}{p_{masse,\mathrm{Hld}}}$ wherein: p.sub.masse,Hld(t) is the compound pressure curve during the holding-pressure phase p.sub.wkz,Hld(t) is the mould internal pressure curve during the holding-pressure phase, t.sub.2 is a time at or after the switch-over point, is being the switch-over time t.sub.inj/umschalt and t.sub.3 is the time of the end of the holding-pressure phase t.sub.Hld,End or a time before the end of the holding-pressure phase but after t.sub.2.

8. Method according to claim 1, wherein the temporal mould internal pressure curve p.sub.wkz,ref(t) of the accepted-part reference injection-moulding cycle and/or of the current injection-moulding cycle p.sub.wkz,act(t) within the injection and/or the holding-pressure phase is recorded by means of at least one mould internal pressure sensor.

9. Method according to claim 1, wherein the target mould internal pressure curve p.sub.wkz,soll(t) of the current injection-moulding cycle is determined according to the formula $p_{\mathrm{wkz},\mathrm{soll}}\left(t\right)=p_{\mathrm{wkz},\mathrm{ref}}\left(t\right).Math.\frac{k_{\mathrm{mat}}}{k_1.Math.k_2}$ wherein k.sub.mat are one or more empirically determined material-specific factors and k.sub.1, k.sub.2 are factors which take into account at least the flow behaviour of the melt and/or a mould-specific dependence between compound pressure p.sub.masse and resulting mould internal pressure p.sub.wkz.

10. Method according to claim 1, wherein the dependence k.sub.2 is calculated from a pressure transmission function f(p.sub.masse,Hld,ref,i) recorded in the accepted-part reference injection-moulding cycle, wherein
f(p.sub.masse,Hld,ref,i)=∫.sub.t.sub.2.sup.t.sup.3p.sub.wkz,ref,i(t).

11. Method according to claim 1, wherein in an injection mould which has a plurality of mould internal pressure sensors, the method is carried out in parallel for one or more of these mould internal pressure sensors.

12. Method according to claim 1, wherein during the holding-pressure phase of the current injection-moulding cycle, the actual mould internal pressure w.sub.wkz,act obtained from the calculated holding-pressure change is measured and compared with the target mould internal pressure p.sub.wkz,soll.

13. Method according to claim 1, wherein the method is applied for holding-pressure phases with several profile stages.

14. Method according to claim 1, wherein order to determine a current switch-over point in the current injection-moulding cycle, the switch-over mould internal pressure p.sub.wkz,umschalt,ref of the accepted-part reference injection-moulding cycle is determined/read out at the switch-over point of the accepted part reference injection-moulding cycle, then a target switch-over mould internal pressure p.sub.wkz,umschalt,soll corresponding to the switch-over mould internal pressure p.sub.wkz,umschalt,ref of the accepted-part reference injection-moulding cycle is determined on the target mould internal pressure curve p.sub.wkz,soll(t) of the current injection-moulding cycle and as soon as p.sub.wkz,act≥p.sub.wkz,umschalt,soll in the current injection-moulding cycle, a switch-over takes place from the injection phase into the holding-pressure phase.

15. Method according to claim 1, wherein in order to reach the target mould internal pressure curve p.sub.wkz,soll(t) in the current injection-moulding cycle, the target holding-pressure curve p.sub.masse,Hld,soll(t) required for this in the current injection-moulding cycle is calculated according to one of the following formulae $\begin{array}{cc}a)p_{\mathrm{masse},\mathrm{Hld},\mathrm{soll}}\left(t\right)=\frac{p_{\mathrm{wkz},\mathrm{soll}}\left(t\right)}{{\int }_{t2}^{t3}{p}_{\mathrm{wkz},\mathrm{ref}}\left(t\right)}.Math.{\int }_{t2}^{t3}{p}_{\mathrm{masse},\mathrm{Hld},\mathrm{ref}}\left(t\right)=p_{\mathrm{wkz},\mathrm{soll}}\left(t\right).Math.k_2^{-1}\mathrm{or}b)p_{\mathrm{masse},\mathrm{Hld},\mathrm{soll}}\left(t\right)=\frac{p_{\mathrm{wkz},\mathrm{soll}}\left(t\right)}{p_{\mathrm{wkz},\mathrm{ref},\mathrm{avg}}}.Math.p_{\mathrm{masse},\mathrm{Hld},\mathrm{ref},\mathrm{avg}}=p_{\mathrm{wkz},\mathrm{soll}}\left(t\right).Math.k_2^{-1}\mathrm{or}c)p_{\mathrm{masse},\mathrm{Hld},\mathrm{soll}}\left(t\right)=\frac{p_{\mathrm{wkz},\mathrm{soll}}\left(t\right)}{p_{\mathrm{wkz},\mathrm{ref}}}.Math.p_{\mathrm{masse},\mathrm{Hld},\mathrm{ref}}=p_{\mathrm{wkz},\mathrm{soll}}\left(t\right).Math.k_2^{-1}& \end{array}$ wherein p.sub.masse,Hld,ref is a holding-pressure parameter, p.sub.masse,Hld,ref(t) is the holding-pressure curve, p.sub.masse,Hld,ref,avg is an average of the holding pressure and p.sub.wkz,ref,avg is an average of the mould internal pressure of the accepted-part reference injection-moulding cycle.

16. Method according to claim 1, wherein during the holding-pressure phase of the current injection-moulding cycle, the actual mould internal pressure curve p.sub.wkz,act(t) of the current injection-moulding cycle is compared with the target mould internal pressure curve p.sub.wkz,soll(t) and the holding-pressure curve p.sub.masse,Hld,act(t) is adjusted iteratively during the current injection-moulding cycle or in a subsequent injection-moulding cycle such that any deviation is compensated over several cycles.

17. Method according to claim 1, wherein the target holding-pressure curve p.sub.masse,Hld,soll(t) of the current injection-moulding cycle is calculated according to the formula $p_{\mathrm{masse},\mathrm{Hld},\mathrm{soll}}\left(t\right)=\frac{p_{\mathrm{wkz},\mathrm{soll}}\left(t\right)}{{\int }_{t2}^{t3}{p}_{\mathrm{wkz},\mathrm{ref}}\left(t\right)}.Math.{\int }_{t2}^{t3}{p}_{\mathrm{masse},\mathrm{Hld},\mathrm{ref}}\left(t\right)+\left(p_{\mathrm{wkz},\mathrm{soll}}\left(t\right)-p_{\mathrm{wkz},\mathrm{act}}\left(t\right)\right).Math.k_3$ wherein k.sub.3 is a control factor.

18. Injection-moulding machine which is adapted and configured to carry out the method according to claim 1.

Description

[0115] In the following, the invention is explained in detail as an example with reference to the figures. In the figures:

[0116] FIG. 1: shows an exemplary diagram of a compressed injection-mouldable compound at the switch-over point for a lower-viscosity (“thinner liquid”) plastic melt (MFI 11) and a higher-viscosity (“more viscous liquid”) plastic melt (MFI 6);

[0117] FIG. 2: shows schematically a compound pressure and mould internal pressure curve of two cycles with the same process adjustments but injection-mouldable compounds (materials) having different viscosities (MFI values);

[0118] FIG. 3: shows compound pressure and mould internal pressure curves for two melts having different viscosity (polypropylene MFI 6) and polypropylene MFI 11) with particular illustration of a measurement range in which the compound pressure is measured in the injection phase and thus the flowability (viscosity) of the plastic compound can be evaluated or is evaluated;

[0119] FIG. 4: shows a reference mould internal pressure from an accepted-part reference injection-moulding cycle and a calculated target mould internal pressure curve which is obtained for a higher-viscosity material;

[0120] FIG. 5: shows two exemplary compound pressure curves for PP MFI 6 and PP MFI 11 during the injection phase and their mould internal pressure curves in the holding pressure phase with particular illustration of a changed switch-over point and a changed holding-pressure level.

[0121] FIG. 6: shows a compound pressure and mould internal pressure curve of a low-viscosity or higher-viscosity plastic melt (PP MFI 11 and PP MFI 6) with measurement and control variable required for the inventive method;

[0122] FIG. 7: shows a flow diagram of the method according to the invention.

[0123] Fundamental relationships between pressure, viscosity and filling volume of a cavity in the injection moulding process will now be explained hereinafter with reference to FIGS. 1 and 2.

[0124] FIG. 1 shows highly schematically a screw 1 in a screw cylinder 2 of a plastic injection moulding machine (not shown). In the upper diagram in FIG. 1 a polypropylene material (PP material) having a melt flow index (melt flow index MFI) of 6 is located in a screw pre-chamber 3 and shown schematically in a cavity 4. This constitutes a relatively highly viscous (thick liquid) material in the melt state. In the example of FIG. 1 shown below, a polypropylene which however has a lower viscosity (thinner liquid) in the melt and has a melt flow index MFI of 11 is likewise located in the screw pre-chamber 3 and in the cavity 4. It is clear that in the same screw positions X the cavity 4 in the example case of polypropylene with MFI=11 contains more melt volume (filling volume). In the case of the higher-viscosity material (polypropylene MFI 6) an increased pressure requirement is required to bring the higher-viscosity melt into the cavity 4. In this respect at a higher pressure level the melt of the PP MFI 6 material is more strongly compressed with the result that a smaller filling volume arrives in the cavity 4 than in the case of a lower-viscosity PP MFI 11 material. When applied to a real injection-moulding process this has the result that the cavity of a mould with otherwise the same parameters is less completely filled in an injection-moulding process with higher-viscosity material than with a lower-viscosity material.

[0125] Likewise, a change in the compound pressure curve or the mould internal pressure curve results from the viscosity difference between two materials with otherwise constant injection speed and geometry etc. Compound pressure curve or compound pressure are understood according to the invention as pressures or pressure curves which are formed in a screw pre-chamber 3 during a process. Such compound pressures or such compound pressure curves are temporally assigned hereinafter to an injection phase of an injection moulding process. The terms “mould internal pressure” and “mould internal pressure curve” relate to pressures or pressure curves measured in a cavity or generally in an interior of a mould. Temporally the terms “mould internal pressure” and “mould internal pressure curve” within the framework of this description predominantly relate to the holding-pressure phase of an injection-moulding process. The term “holding pressure/holding-pressure curve is fundamentally understood, unless specified otherwise, as the compound pressure during a holding-pressure phase, i.e. that pressure of the melt that is located in the screw pre-chamber 3.

[0126] By reference to FIG. 2 it is clear that a compound pressure curve of the lower-viscosity melt material PP MFI runs below the compound pressure or the compound pressure curve of a higher-viscosity melt material of PP MFI 6 material during the entire injection phase of the depicted injection-moulding process. Conversely in the holding pressure phase, a mould internal pressure curve of the thinner-liquid (lower viscosity) material PP MFI 11 is arranged above the mould internal pressure or the mould internal pressure curve of a melt of a higher-viscosity material MFI 6 during the entire holding-pressure phase. In particular, it is striking that at a switch-over point which is shown by a vertical dashed line in FIG. 2, the mould internal pressure with the lower-viscosity material PP MFI 11 increases significantly more substantially than with the higher-viscosity material PP MFI 6. These significant differences in the pressure curves result in different mould fillings and as a result, extremely different component qualities. The aim is to optimize the moulded part filling and ensure a process-reliable complete filling of the moulds and thus a constant accepted-part volume independently of the viscosity.

[0127] In the following, it is mentioned, for example, that the determined pressure curves, i.e. the compound pressure (compound pressure curve) and the mould internal pressure (mould internal pressure curve) of the lower-viscosity polypropylene (PP MFI 11) is used as reference curve R for the further explanation, from which inventive adjustments are accomplished for example. Thus, assuming that the curves of the lower-viscosity polypropylene (PP MFI 11) are learnt as reference curves in a reference cycle, the viscosity increases due to a batch change to a higher-viscosity polypropylene similarly to the curve of PP MFI 6.

[0128] FIG. 3 shows by means of the dashed line for a PP MFI 6 (higher-viscosity polypropylene) the curve of the compound pressure p.sub.masse during the injection and holding-pressure phase. In addition, the diagram according to FIG. 3 shows by means of the dark-dotted line the compound pressure curve n masse of a PP MFI 11 (lower-viscosity polypropylene melt) during the injection and the holding-pressure phase.

[0129] A narrow hatched area between time limits t.sub.0 and t.sub.1 represents a compound pressure area p.sub.a,masse below the compound pressure curve of the PP MFI 6. This compound pressure area corresponds to the injection work W.sub.z,ref in the limits between t.sub.0 and t.sub.1, wherein the lower time limit to lies after closure of a back flow valve and the upper time limit t.sub.1 still lies before a switch-over point.

[0130] Within the same limits t.sub.0 and t.sub.1 a cross-hatched area in FIG. 3 shows a compound pressure area p.sub.a,masse of the PP MFI 11, i.e. of the thinner-liquid polypropylene melt. This area corresponds to the injection work W.sub.z,act and within the limits t.sub.0 to t.sub.1 is smaller than the injection work W.sub.z,ref of the thicker-liquid (higher-viscosity) PP MFI 6 melt.

[0131] In the further course, the compound pressure curve of the two PP melts MFI 6 and MFI 11 during the holding-pressure phase shows the same pressure behaviour (prior art). In the lower diagram range this results in differently running mould internal pressures. The light-dotted line represents the mould internal pressure curve p.sub.wkz(t) of the lower-viscosity PP melt (MFI 11). This curve lies at a higher level compared with a mould internal pressure curve of a PP MFI 6 melt which is thicker-liquid and shown by a dark-dashed line.

[0132] In order to determine a compound pressure change k1 during the injection phase on transition from a first melt to a second melt having different viscosity, expediently a quotient of the injection work W.sub.z,act of a current injection-moulding cycle and the injection work W.sub.z,ref of an accepted-part reference injection-moulding cycle is now formed. In the present example, for example, the compound pressure curve of the PP MFI 11 melt should be regarded as the reference compound pressure curve p.sub.masse,ref and the compound pressure curve p.sub.masse,act of the PP MFI 6 melt should be regarded as the compound pressure curve p.sub.masse,act of the current injection moulding cycle.

[0133] According to the invention, now at least taking into account the compound pressure change k.sub.1, preferably additionally taking into account a pressure transmission characteristic k.sub.2 and a material-dependent factor k.sub.mat, a conversion is made from the mould internal pressure of the accepted-part reference injection-moulding cycle (here: mould internal pressure of the PP MFI 11 melt) to a target mould internal pressure curve p.sub.wkz,soll(t) for the thick-liquid PP MFI 6 melt to be processed in the current injection-moulding cycle.

[0134] In order to determine the pressure transmission characteristic of the mould used, the factor k.sub.2 is determined from a ratio of the pressure integrals over the mould internal pressure curve during at least a part of the holding-pressure phase p.sub.wkz,Hld(t) and over the compound pressure p.sub.masse,Hld (t) within the limits t.sub.2 to t.sub.3. Here t.sub.2 is a time at or after the switch-over point. t.sub.3 is a time after t.sub.2 and before the end of the holding-pressure phase. Preferably here a time t.sub.wkz,max of the maximum mould internal pressure p.sub.wkz,masse is used for t.sub.3.

[0135] Alternatively the factor k.sub.2 can also be formed as the quotient of average values of the mould internal pressure p.sub.wkz,Hld,avg and the mould pressure p.sub.masse,Hld,avg during the holding-pressure phase.

[0136] Further alternatively individual pressure values of these curves can also be used. A material-specific factor k.sub.mat is determined empirically and introduces material-specific property changes of a melt of the current injection-moulding cycle with respect to the melt of the accepted-part reference injection-moulding cycle.

[0137] From this the following calculation can then be made according to the formula

[00005]$p_{\mathrm{wkz},\mathrm{soll}}\left(t\right)=p_{\mathrm{wkz},\mathrm{ref}}\left(t\right).Math.\frac{k_{\mathrm{mat}}}{k_1.Math.k_2}$

[0138] Such a target mould pressure curve tends to lie above a mould internal pressure curve when the holding pressure remains unchanged during a transition from a lower-viscosity melt as reference melt to a thicker-liquid (higher-viscosity) melt of a current injection-moulding cycle.

[0139] After obtaining the target mould internal pressure curve p.sub.wkz,soll(t), a holding-pressure curve p.sub.masse,Hld,act (t) corresponding to the target mould internal pressure curve p.sub.wkz,soll(t) can now be calculated by means of the pressure transmission characteristic k.sub.2. For this purpose the corresponding target holding pressure can be calculated with low computing expenditure by dividing the target mould internal pressure curve by the factor k.sub.2. This applies both to the use of pressure integrals over a sub-region of the mould internal pressure and the compound pressure and also to the average values thereof or even individual values. This then results in a holding-pressure curve p.sub.masse,Hld,act(t) to be travelled in the holding pressure phase of the current injection-moulding cycle which in the example described lies above the holding pressure which fluctuates when the mould internal pressure is unchanged (cf. FIG. 4 for the determination of the target mould internal pressure curve p.sub.wkz,soll(t)).

[0140] The previously described process can also be seen in FIG. 5. In addition, it can be identified from FIG. 5 that a switch-over point of the accepted-part reference injection-moulding cycle (t.sub.umschalt,ref) during a transition from the accepted-part reference injection-moulding cycle (PP MFI 11 melt) to the current injection-moulding cycle (PP MFI 6 melt) is shifted backwards in time to a time t.sub.umschalt,act. In order to determine the switch-over time of the current injection-moulding cycle t.sub.umschalt,act, in the accepted-part reference injection-moulding cycle the mould internal pressure p.sub.wkz,umschalt,ref at the switch-over point is selected as starting point. This value for the mould internal pressure in the accepted-part reference injection-moulding cycle at the switch-over point is converted in the manner according to the invention to a target mould internal pressure p.sub.wkz,umschalt,soll and upon reaching this mould internal pressure p.sub.wkz,umschalt,soll in the current injection-moulding cycle switch-over takes place in the current injection-moulding cycle. This then yields the switch-over time .sub.tumschalt,act. The holding-pressure curve p.sub.masse,Hld,act (t) after the switch-over is then travelled in such a manner that the resulting actual mould internal pressure curve p.sub.wkz,act (t) of the current injection-moulding cycle follows as closely as possible, ideally precisely the target mould internal pressure curve p.sub.wkz,soll(t).

[0141] In the event that such a precise following of the target mould internal pressure curve p.sub.wkz,soll(t) is not entirely achieved, this is optionally iteratively approximated in following cycles.

[0142] This procedure and also the previously described procedure for determining the switch-over point in the current injection-moulding cycle starting from the switch-over point of the accepted-part reference injection-moulding cycle can also be seen from FIG. 6. The indicated pressure curves over time in FIG. 6 are drawn somewhat further spaced apart with regard to their scaling in order to illustrate that according to the invention the aim is specifically not pursued to keep the mould internal pressure as identical as possible by suitable holding-pressure adjustment although different melts with different material properties are used but specifically a melt-specific adjustment also takes place taking into account pressure transmission characteristics of the mould with regard to the mould internal pressure and in order to achieve this target mould internal pressure determined independently of melt and/or pressure transmission, a holding-pressure adjustment aimed at this takes place in the method according to the invention.

[0143] The method according to the invention is designed for use on electro- and hydromechanical injection-moulding machines of all sizes. Thus, it is possible to use the method in all new machines and as a retrofit. A prerequisite is at least one mould internal pressure sensor integrated in the machine.

[0144] Injection-moulding machines operated using the method are able to automatically compensate for negative effects of, for example, batch fluctuations on the moulded part quality. Likewise negative effects on the moulded part quality upon restarting the machines (after faults or down time) are automatically compensated by calculating the optimal target mould internal pressure curve or characteristics derived therefrom. The machine operator needs to intervene in the production process less frequently in order, for example, to manually track adjustment parameters. The quality differences of the individual moulded parts are reduced to a minimum even under changing production conditions. Cost savings due to automation and process reliability are the direct consequence.

[0145] As a result of a targeted evaluation of the parameters at the machine, for example, faults in the mould or the mould temperature control can also be identified. Furthermore, it is possible to change the mould between two injection-moulding machines without a tedious start-up and adjustment process being required to set a robust process point. The moulded-part quality can be reproduced independently of machine, personnel or raw material without expensive follow-up checks being required.

[0146] The invention is explained in detail hereinafter as an example by reference to a flow diagram according to FIG. 7.

[0147] There exists a stored compound pressure curve p.sub.masse,ref (t) which originates from an accepted-part reference injection-moulding cycle. Furthermore, a current compound pressure curve p.sub.masse,act (t) is recorded, for example, measured during the injection phase of an actual injection-moulding cycle.

[0148] The reference compound pressure curve p.sub.masse,ref (t) and the current compound pressure curve p.sub.masse,act (t) are compared. If the actual compound pressure curve p.sub.masse,act (t) differs from the reference compound pressure curve p.sub.masse,ref(t) or the difference is below a certain threshold value, the injection-moulding cycle is not modified with regard to its further course. If the difference between the reference compound pressure curve p.sub.masse,ref (t) and the actual compound pressure curve p.sub.masse,act (t) exceeds a certain limit, a target mould internal pressure curve p.sub.wkz,soll(t) is initially calculated depending on the quotient. This target mould internal pressure curve p.sub.wkz,soll(t) should be achieved as accurately as possible in the holding-pressure phase of the current injection-moulding cycle. Furthermore, preferably as a second measure a switch-over mould internal pressure p.sub.wkz,umschalt is calculated, which also depends on the magnitude of the quotient of the reference compound pressure curve p.sub.masse,ref (t) and the actual compound pressure curve p.sub.masse,act (t) in the injection phase. After these calculations, the holding pressure p.sub.masse,Hld is adjusted in such a manner that the target mould internal pressure curve p.sub.wkz,soll(t) is travelled as accurately as possible. The switch-over point is also adapted so that switch-over takes place at the calculated switch-over mould internal pressure p.sub.wkz,umschalt. The actual mould internal pressure curve p.sub.masse,act (t) during the holding-pressure phase is monitored (measured). If the actual mould internal pressure curve p.sub.masse,act (t) differs from the target mould internal pressure curve p.sub.wkz,soll(t) or falls below a certain limit, no further adjustment of the holding pressure p.sub.masse,Hld is accomplished in the next cycle, that is the next cycle is travelled with the same mould internal pressure curve as the current cycle. If the difference exceeds a certain limit, the holding pressure in the next cycle is iteratively adjusted so that the target mould internal pressure curve p.sub.wkz,soll(t) is reached at the latest as far as possible after a few cycles.

REFERENCE LIST

[0149] 1 Screw [0150] 2 Cylinder [0151] 3 Screw pre-chamber [0152] 4 Cavity [0153] t.sub.0,t.sub.1 Time points/integration limits [0154] EP Injection phase [0155] MFI 11 Low-viscosity plastic melt [0156] MFI 6 High-viscosity plastic melt [0157] PP MFI 11 Low-viscosity polypropylene [0158] p.sub.wkz,ref (t) Mould internal pressure curve in accepted-part reference injection-moulding cycle [0159] p.sub.wkz,ref,max Maximum mould internal pressure in reference cycle [0160] t.sub.wkz,ref,max Time of p.sub.wkz,ref,max [0161] p.sub.a Mould internal pressure area [0162] k.sub.1 Correction factor [0163] W.sub.z Injection work [0164] p.sub.wkz Mould internal pressure [0165] t.sub.xfr,ref Switch-over point (reference cycle) [0166] p.sub.masse,umschalt,ref Switch-over pressure (reference cycle) [0167] p.sub.wkz,act (t) Actual mould internal pressure curve [0168] p.sub.masse,Hld,ref Holding pressure level (reference cycle) [0169] p.sub.wkz,soll(t) Target mould internal pressure curve [0170] p.sub.masse,act Compound pressure curve of the current injection-moulding cycle [0171] p.sub.masse,ref (t) Reference compound pressure curve//compound pressure curve of the accepted-part reference injection-moulding cycle [0172] p.sub.masse,Hld,act (t) Holding pressure curve of the current injection-moulding cycle [0173] p.sub.wkz,act Actual mould internal pressure curve of the current injection-moulding cycle [0174] p.sub.wkz,umschalt,act Switch-over mould internal pressure of the current injection-moulding cycle [0175] p.sub.masse Compound pressure [0176] t.sub.inj,start Time of the beginning of injection [0177] t.sub.inj,umschalt Time of the switchover [0178] p.sub.masse,Hld Compound pressure during the holding-pressure phase [0179] p.sub.wkz,Hld Mould internal pressure during the holding-pressure phase [0180] t.sub.umschalt Time of the switch-over [0181] t.sub.Hld,end Time of the end of the holding-pressure phase [0182] p.sub.wkz,max Maximum mould internal pressure [0183] t.sub.wkz,max Time of the maximum mould internal pressure [0184] p.sub.a,wkz Mould internal pressure area [0185] p.sub.a,masse Compound pressure area [0186] k.sub.mat Material-specific factor [0187] k.sub.i (k.sub.1, k.sub.2) Process-specific factors [0188] p.sub.wkz,ref,umschalt Mould internal pressure of the earlier injection-moulding cycle or of the accepted-part reference injection-moulding cycle at the switch-over point