METHOD FOR ADJUSTING THE CLOSING FORCE OF A MOLD OF A PLASTICS PROCESSING MACHINE, IN PARTICULAR AN INJECTION MOLDING MACHINE

Abstract

A method for adjusting the closing force of a mold of a plastics processing machine. In order to work with an optimized closing force, the method comprises the steps of: a) in a first production cycle: Closing the mold with a nominal initial closing force and recording the mold deformation caused thereby and calculating the deformation work introduced by the mold deformation; b) in a subsequent further production cycle: Closing the mold with a reduced closing force and recording the resulting mold deformation and calculating the deformation work introduced by the mold deformation; c) Recording the determined deformation work versus the closing force, carrying out a linear extrapolation of the course of the deformation work versus the closing force and determining a reduced closing force which is at a predetermined percentage value of one of the previously determined deformation work; d) Carrying out the subsequent further production cycle with the determined reduced closing force.

Claims

1. A method for adjusting the closing force of a mold of a plastics processing machine, in particular an injection molding machine, wherein the mold has a spring constant, so that a mold deformation results when the mold is subjected to the closing force, wherein the method comprises the following steps: a) in a first production cycle: Closing the mold with a nominal initial closing force and recording the mold deformation caused by this and calculating the deformation work introduced by the mold deformation; b) in a subsequent further production cycle: Closing the mold with a closing force which is reduced relative to the nominal initial closing force by a predetermined force difference and recording the mold deformation caused thereby and calculating the deformation work introduced by the mold deformation; c) Recording the determined deformation works versus the closing force, performing a linear extrapolation of the course of the deformation works versus the closing force, and determining a reduced closing force which is at a predetermined percentage value of one of the previously determined deformation works, wherein the predetermined percentage value of the deformation work is at most 20% of the previously determined deformation work; d) Execution of the subsequent further production cycle with the determined reduced clamping force.

2. The method according to claim 1, wherein after step b) and before step c) the following step is performed: b1) in a subsequent further production cycle following step b): Closing the mold with a closing force which is reduced by a predetermined force difference compared with the closing force during the execution of step b) and recording the mold deformation caused thereby and calculating the deformation work introduced by the mold deformation, wherein the further determined deformation work is taken into account in the linear extrapolation of the course of the deformation work versus the closing force when carrying out step c).

3. The method according to claim 2, wherein after step b1) and before step c) the following step is carried out: b2) in a subsequent further production cycle following step b1): Closing the mold with a closing force which is reduced by a further predetermined force difference compared with the closing force during the execution of step b1) and recording the mold deformation caused thereby and calculating the deformation work introduced by the mold deformation, wherein the further determined deformation work is taken into account in the linear extrapolation of the course of the deformation work versus the closing force when carrying out step c).

4. The method according to claim 2, wherein the linear extrapolation is performed by a straight line determined by linear regression of the values of the deformation works versus the closing force.

5. The method according to claim 1, wherein the force difference is between 25 kN and 75 kN, preferably between 40 kN and 60 kN.

6. The method according to claim 1, wherein it is carried out in an injection molding machine.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0056] In the drawing:

[0057] FIG. 1 shows schematically for three successive injection molding cycles the deformation work of the mold with decreasing clamping forces, wherein the deformation work being plotted as an energy difference over the clamping force of the mold,

[0058] FIG. 2 shows schematically, on the basis of the representation according to FIG. 1, a regression line which was laid through the determined three values for the deformation work and its intersection with a predetermined percentage value of the deformation work in order to find a reduced closing force,

[0059] FIG. 3 shows schematically in the representation according to FIG. 2 an improved regression line, where a fourth value for the deformation work has been taken into account,

[0060] FIG. 4 shows the data acquisition procedure for an embodiment of the proposed method,

[0061] FIG. 5 shows for an embodiment of the proposed method the finding of an optimal closing force and

[0062] FIG. 6 shows, for an embodiment of the proposed method, the iterative reduction of the closing force to find its optimal value.

DETAILED DESCRIPTION OF THE INVENTION

[0063] FIG. 1 illustrates the beginning of the proposed method for finding an optimum closing force of the mold of an injection molding machine. In the coordinate system shown here, the closing force F of the mold is plotted on the abscissa and the energy difference ΔW according to the above formula is plotted on the ordinate.

[0064] According to this, a nominal initial state with a nominal clamping force F.sub.1 first results in a value of ΔW.sub.1 for the energy difference. In the embodiment, a clamping force of 1,800 kN was selected as the initial state; this value can correspond in particular to the maximum clamping force of the mold. A first production cycle was carried out with this clamping force, i.e. the production of an injection molded part.

[0065] It should be mentioned here that the described process can of course also be used after a number of molded parts have already been produced. In this respect, the term first production cycle should be understood to mean that it is the first cycle with which the proposed process starts.

[0066] In a subsequent working cycle, in particular in the immediately following working cycle, a reduced closing force F.sub.2 is then used. The closing force is therefore reduced by an amount ΔF, which in the embodiment is 50 kN. Now, in the same way, a value ΔW.sub.2 is obtained for the deformation work introduced into the mold (differential work).

[0067] In a further subsequent working cycle, which follows on in particular from the second working cycle described, the closing force F.sub.3 is reduced again. Again, this is reduced by an amount ΔF, which in this case is also 50 kN. This results in an analogous value ΔW.sub.3 for the deformation work introduced into the mold.

[0068] After recording the three values determined in the present embodiment, an evaluation is carried out, which is illustrated in FIG. 2:

[0069] The determined deformation works ΔW.sub.1, ΔW.sub.2 and ΔW.sub.3 are plotted in the said and displayed closing force—energy difference diagram and a linear extrapolation of the course of the deformation work over the closing force is carried out. For this purpose, a linear regression is preferably carried out, i.e. a “compensation line” is determined through the three recorded values of ΔW. This procedure is sufficiently well known as such, so that it need not be explained further here.

[0070] According to the linear regression, the energy difference ΔW is proportional to the closing force F, i.e.

ΔW˜F.sub.C

[0071] respectively

ΔW=α.Math.F.sub.C+β

[0072] wherein the coefficients α and β are determined by the linear regression. After determining the mentioned coefficients, the closing force in the embodiment is reduced to a value of 20% of the last determined value for the energy difference ΔW.sub.3, i.e. the compensation line through the points ΔW.sub.1, ΔW.sub.2 and ΔW.sub.3 is intersected with a line parallel to the abscissa, which is at the level of 20% of the value of ΔW.sub.3. The reduced closing force found here is denoted by F.sub.C in FIG. 2. Mathematically expressed, the following applies to F.sub.C

[00003]$F_C=\frac{0,2.Math.\Delta W_3-\beta }{\alpha }$

[0073] A new injection molding cycle is then started with the reduced clamping force F.sub.C found in this way. In this cycle, too, the mold deformation caused can be recorded and the deformation work ΔW.sub.4 introduced by the mold deformation can be calculated. This is illustrated in FIG. 3.

[0074] As can be seen from FIG. 3, a linear regression is now carried out again and the four values ΔW.sub.1, ΔW.sub.2, ΔW.sub.3 and ΔW.sub.4 now obtained are taken into account. In FIG. 3, an arrow indicates that the previously determined straight line is now shifted somewhat and thus a new, improved value for the reduced closing force F.sub.C can be determined. The basis for finding the reduced closing force F.sub.C is again the intersection of the compensation line with the parallel to the abscissa, which is at 20% of the value of ΔW.sub.3.

[0075] An improved value for the reduced closing force results from this because the first linear extrapolation was still too inaccurate due to the large prediction span (based on the values ΔW.sub.1, ΔW.sub.2 and ΔW.sub.3); the result can therefore be improved if the linear regression is repeated including the value ΔW.sub.4.

[0076] In the embodiment example, the reduced closing force was determined on the basis of a value of 20% of the value of the energy difference ΔW.sub.3. In general, an even lower value than 20% should be aimed for, ideally 5% of the energy difference of ΔW.sub.3.

[0077] However, in the range between 20% and 5% of the energy difference ΔW.sub.3, it can easily happen that the energy difference becomes negative, resulting in a detrimental uncontrolled opening of the mold during the injection molding process.

[0078] Therefore, preferably in this area, the step size for further reduction of the closing force is adjusted iteratively on the basis of the procedure described below. In this way, rapid changes are detected and taken into account.

[0079] The entire algorithm provided for this purpose is shown in FIGS. 4, 5 and 6. FIG. 4 shows the data acquisition process, FIG. 5 the search for an optimum reduced clamping force, and FIG. 6 the procedure for iteratively reducing the clamping force.

[0080] For the iterative reduction of the closing force, FIG. 6 shows that when this partial algorithm is called (see process step “C” in the lower left corner of FIG. 5), three linear models are created (see step “SM21” in FIG. 6).

[0081] The first model determines a regression line through the last three data point pairs of ΔW and F.sub.C. The result is the coefficients α.sub.S1 (slope) and β.sub.S1 (intercept) for the regression line as explained above.

[0082] The second model calculates the regression line through the last and penultimate pair of data points of ΔW and F.sub.C. The result is the coefficients α.sub.S2 (slope) and β.sub.S2 (intercept) for the regression line.

[0083] The third model puts a regression line through the second to last and third to last pair of data points of ΔW and F.sub.C. The result is the coefficients α.sub.S2 (slope) and β.sub.S2 (intercept) for the regression line.

[0084] For the further calculation some limit values and constants are necessary. These values can be set on the machine. The values given in the sequence diagram (see FIG. 6, step “SM22”) represent standard values. The step size ΔF.sub.Smin determines the smallest possible change in the clamping force. The step size ΔF.sub.Smax determines the largest possible change in the closing force. The attenuation factor τ and the maximum deviation Φ serve as factors for the exponential function described later. The threshold value for the relative deviation Φ indicates at which value of the relative deviation the sequence “C” (according to FIG. 6) should be aborted.

[0085] Once all three models are created, a weighted slope α.sub.Sw is calculated. This is the sum of 70% of the slope from model 2 and 30% of the slope from model 3 (see step “SM23” in FIG. 6). The relative deviation μ (see step “SM24” in FIG. 6) results from the relation

[00004]$\mu =\frac{{\alpha }_{\mathrm{Sw}}}{{\alpha }_{S1}}-1$

[0086] If the relative deviation μ is greater than the threshold value Φ, it is assumed that a rapid change occurs. The sequence “C” (according to FIG. 6) is aborted, the penultimate closing force is set. If the relative deviation is smaller, the step size adjustment (see step “SM26” in FIG. 6) is performed by the equation

[00005]$\Delta F=\left(\Delta F_{\mathrm{Smax}}-\Delta F_{\mathrm{Smin}}\right).Math.\exp \left(\frac{-\tau .Math.\mu }{\phi }\right)+\Delta F_{\mathrm{Smin}}$

[0087] The new closing force is adjusted by the step size. The sequence is called up as often as necessary until the 5% mark of the differential work of ΔW.sub.3 has been reached.

[0088] The iterative reduction is repeated a maximum of ten times. After finding the optimum closing force, a certain value (ΔW.sub.REF) is obtained. This value serves as a reference for the subsequent process cycles. If an impermissible deviation of the differential work is detected in one of the subsequent cycles, the closing force is adjusted with a cycle delay via the linear approximation in order to return to the level of ΔW.sub.REF.