Method of determining a solution state of a gas

Abstract

A method of determining a solution state of a gas in a plastic melt used in a plastic shaping method includes: (vi) providing the plastic melt together with the gas in a chamber, (vii) altering—in particular reducing—a volume of the chamber to alter a pressure of the plastic melt together with the gas—in particular increased from a first pressure value to a second pressure value, (viii) introducing the plastic melt into a shaping cavity, (ix) computing at least one compression parameter characteristic of the compression behaviour of the plastic melt, in particular a modulus of compression, from the first pressure value and the second pressure value, and (x) determining from the at least one compression parameter whether the gas is substantially completely dissolved in the plastic melt and/or a solubility limit of the gas in the plastic melt is determined from the at least one compression parameter.

Claims

1. A method of determining a solution state of a gas in a plastic melt used in a plastic shaping method, wherein following method steps (i) to (iv) are carried out: (i) the plastic melt is provided together with the gas in a chamber, (ii) by altering a volume of the chamber a pressure of the plastic melt together with the gas is altered from a first pressure value to a second pressure value, (iii) the plastic melt is introduced into a shaping cavity, (iv) at least one compression parameter characteristic of the compression behaviour of the plastic melt is computed from the first pressure value and the second pressure value, wherein in addition (v) from the at least one compression parameter it is determined whether the gas is substantially completely dissolved in the plastic melt and/or a solubility limit of the gas in the plastic melt is determined from the at least one compression parameter.

2. The method according to claim 1, wherein the provision of the plastic melt together with the gas is carried out by producing the plastic melt and then introducing the gas.

3. The method according to claim 2, wherein an injection unit with a plasticising screw arranged in a plasticising cylinder is used, wherein the plasticising screw is rotatably moved for the plasticising operation and is axially moved for the injection operation.

4. The method according to claim 2, wherein the provision of the plastic melt together with the gas is carried out by producing the plastic melt using an injection unit.

5. The method according to claim 1, wherein the reduction in the volume of the chamber in accordance with method step (ii) is carried out as part of an introduction operation in accordance with method step (iii).

6. The method according to claim 1, wherein the volume of the plastic melt together with the gas is so greatly reduced in accordance with method step (ii) that the second pressure value is above those pressures which otherwise occur during the injection method in the plastic melt together with the gas, or that the first pressure value is above those pressures which otherwise occur during the injection method in the plastic melt together with the gas.

7. The method according to claim 1, wherein the chamber is delimited at the shaping cavity side by a shut-off device.

8. The method according to claim 7, wherein the shut-off device is in the form of a needle closure nozzle and/or that the chamber at its side remote from the shaping cavity is delimited by a plasticising screw or an injection piston.

9. The method according to claim 1, wherein a screw pre-chamber in a plasticising cylinder is used as the chamber.

10. The method according to claim 1, wherein a step for determining the first pressure value and the second pressure value is carried out by means of a pressure sensor at the chamber and/or the step of determining the first pressure value and the second pressure value is carried out indirectly.

11. The method according to claim 1, wherein a step of determining a change in volume (ΔV) of the chamber is carried out by means of a step of determining a plasticising screw travel.

12. The method according to claim 1, wherein the reduction in the volume of the chamber in accordance with method step (ii) is carried out in pressure-regulated mode.

13. The method according to claim 12, wherein a pressure-regulated axial movement of a plasticising screw and/or an injection piston is used.

14. The method according to claim 1, wherein a temperature of the plastic melt is open-loop or closed-loop controlled, wherein a target temperature for open-loop or closed-loop control is kept substantially constant during execution of method step (ii).

15. The method according to claim 1, wherein, in the course of method step (ii), the plastic melt is kept at the second pressure value until substantially an equilibrium state occurs.

16. The method according to claim 1, wherein the gas used is an inert gas.

17. The method according to claim 16, wherein molecular nitrogen or carbon dioxide is used.

18. The method according to claim 1, wherein method steps (i) to (v) are carried out a plurality of times.

19. The method according to claim 18, wherein the compression parameters determined for different amounts of injected gas are fitted in the course of a curve fit to a parametrised curve generally describing the relationship between the injected amount of gas and the compression parameter and that the solubility limit is determined as or from at least one fit parameter generated in that curve fit.

20. The method according to claim 19, wherein the parameterised curve is given by the following equation: $K=\frac{K_0-\mathrm{kc}+P_u{d}^{\frac{c}{e}}}{{\left(1+{.Math.\lambda c.Math.}^a\right)}^{\frac{1-n}{a}}}$ wherein K.sub.0 denotes the modulus of compression of the plastic melt without gas loading, k the initial increase below the solubility limit, c the gas concentration, P.sub.u the unit pressure, d, n, a and e scaling factors as well as λ the inverse solubility limit.

21. The method according to claim 18, wherein starting from a compression parameter which corresponds to a smallest amount of injected gas a linear relationship is determined between a partial amount of the compression parameter which is determined and that the solubility limit is determined as the smallest of those amount of the injected gas, in respect of which a deviation from the linear relationship of more than a previously established limit value occurs.

22. The method according to claim 18, further comprising a first procedure for introducing the gas into the plastic melt, that method steps (i) to (iv) for determining the compression parameter are carried out with a second procedure for introducing the gas into the plastic melt, and that amounts of introduced gas into the plastic melt are compared by means of the first procedure and the second procedure on the basis of the determined compression parameters.

23. The method according to claim 18, wherein different amounts of the gas are introduced into the plastic melt and compression parameters are calculated for the different amounts of the injected gas.

24. The method according to claim 1, wherein the operation of determining the solubility limit of the gas is carried out automatically by a machine control of the shaping machine.

25. The method according to claim 24, wherein the operation of determining the solubility limit of the gas, and therefrom a target amount for the injection of the gas into the plastic melt, is carried out automatically by a machine control of the shaping machine.

26. The method according to claim 1, wherein the solubility limit is determined in dependence on a process parameter.

27. The method according to claim 26, wherein the process parameter is a temperature and/or the pressure of the plastic melt and the process parameter is optimised utilising the solubility limit.

28. A use of a shaping machine in the method according to claim 1.

29. The method according to claim 1, wherein the altering in method step (ii) is reducing the volume of the chamber which results in the pressure of the plastic melt together with the gas being increased from a first pressure value to a second pressure value, and the at least one compression parameter characteristic of the compression behaviour of the plastic melt in method step (iv) is a modulus of compression.

30. A non-transitory computer-readable storage medium storing a program for determining a solution state of a gas in a plastic melt used in a plastic shaping method comprising commands which cause a computer when executing the program to carry out the following steps: outputting at least one first actuation signal to a shaping machine for providing the plastic melt together with the gas in a chamber, outputting at least one second actuation signal to the shaping machine for altering a volume of the chamber, whereby a pressure of the plastic melt together with the gas is altered from a first pressure value to a second pressure value, outputting at least one third actuation signal to the shaping machine for introducing the plastic melt into a shaping cavity, computing at least one compression parameter characteristic of the compression behaviour of the plastic melt from the first pressure value and the second pressure value, and determining from the at least one compression parameter whether the gas is substantially completely dissolved in the plastic melt and/or determining a solubility limit of the gas in the plastic melt from the at least one compression parameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and details of the invention will be apparent from the Figures and the related specific description. In the Figures:

(2) FIG. 1 shows an embodiment of an injection moulding machine for carrying out a method according to the invention,

(3) FIG. 2 shows a further embodiment of an injection moulding machine for carrying out a method according to the invention,

(4) FIG. 3 shows a diagram to illustrate the relationship between the modulus of compression and the gas loading of a plastic melt, and

(5) FIG. 4 shows a diagram to illustrate a curve fit for determining the solubility limit.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1 shows a shaping machine 1—in this case an injection moulding machine. It has an injection unit 3 for producing a plastic melt 2 by plasticisation of a plastic (generally in the form of granular material).

(7) For that plasticisation operation a plasticising screw 7 is arranged in a plasticising cylinder 6. The plastic is fused by rotation of the plasticising screw 7 (shearing heat) and heating of the plasticising cylinder 6, and it is then in the form of a plastic melt 2 in the screw pre-chamber in the plasticising cylinder 6. That operation of producing the plastic melt is also referred to as “metering”.

(8) The plasticising screw 7 can also be moved axially. In particular the plastic melt 2 can be injected into the purely diagrammatically illustrated shaping cavity 5 by an advance movement of the plasticising screw 7.

(9) The drive 10 for the rotating and axial movement of the plasticising screw 7 and a machine control 11 are also only diagrammatically shown.

(10) A gas injector 12 is provided for introducing the gas into the plastic melt.

(11) In this embodiment the gas injector 12 is arranged in overlapping relationship with a mixing portion of the plasticising screw 7.

(12) Arranged between the plasticising cylinder 6 and the shaping cavity 5 are a measurement flange 13 and a shut-off device 8 which for example can be in the form of a needle closure nozzle.

(13) The measurement flange 13 serves for the connection of a pressure sensor 9 for detecting the pressure in the plastic melt 2. The pressure sensor 9 however could also be arranged elsewhere, for example directly in the plasticising cylinder 6. Finally the pressure of the plastic melt 2 can also be measured indirectly, for example as a hydraulic pressure in a hydraulic cylinder driving the screw advance movement (as part of the drive 10) or as torque of an electric machine driving the screw advance movement (being part of the drive 10). The measurement flange 13 is therefore to be considered as optional for the structure shown in FIG. 1.

(14) The shut-off device 8 serves for shutting off the flow of plastic melt 2 into the shaping cavity 5. In that way the screw pre-chamber can form the chamber 4 in which the plastic melt 2 can be enclosed. The volume of the chamber 4 formed in that way can be altered by axial movement of the plasticising screw 6.

(15) The ultrasound sensor 14 is also purely optional, which by detection of any bubble formation in the plastic melt 2 can serve to verify the information acquired according to the invention about the solution state of the gas in the plastic melt 2.

(16) The execution of a method according to the invention using the injection moulding machine shown in FIG. 1 will now be described.

(17) After the metering operation the gas-loaded plastic melt is under a dynamic pressure (first pressure value) in the chamber 4. Thereupon the injection operation is initiated by the advance movement of the plasticising screw 7 wherein the shut-off device 8 firstly still remains closed. The screw advance movement is continued in pressure-regulated fashion until a predetermined increased dynamic pressure (second pressure value) is reached. The change in volume in the course of method step (ii) can be detected by detection of the travel covered by the plasticising screw 7 from a value which corresponds to the volume of the chamber 4 prior to the reduction thereof to a further value which corresponds to the volume of the chamber 4 after the reduction. The predetermined dynamic pressure (second pressure value) can in that case be held over a period of time in order to ensure that an equilibrium state has occurred.

(18) The shut-off device 8 can then be opened and the process of injecting the plastic melt into the shaping cavity 5 can be continued. Naturally it is possible—but not obligatory—for the dynamic pressure to be reduced by moving the plasticising screw 7 back prior to opening the shut-off device 8.

(19) Independently of the further plastic shaping process it is possible from the detected screw travel against the known diameter of the plasticising cylinder 6 to calculate the change in volume ΔV of the chamber 4 and from the first pressure value and the second pressure value which were detected it is possible to calculate the change in pressure Δp. In a similar manner a starting volume V.sub.0 of the chamber 4 can be determined prior to beginning the reduction in the volume of the chamber 4. From those data it is possible to calculate the modulus of compression K, defined as

(20) $K=-V_0\frac{\Delta p}{\Delta V}.$

(21) With the known relationship between the modulus of compression K and the solution state of the gas used in the plastic melt 2, by virtue of the calculated modulus of compression K it is possible to infer whether the gas is substantially completely dissolved in the plastic melt or—expressed in other terms—there is an upper or lower limit for the solubility limit. In that way it is also possible to carry out automatic monitoring of the plastic shaping method in regard to solution of the gas in the plastic melt 2.

(22) If there is no such relationship then the above-described method can be carried out a plurality of time with injection of different amounts of the gas into the plastic melt. For that purpose attention is directed to FIG. 3 and the specific description relating thereto.

(23) FIG. 2 shows an alternative embodiment for the chamber 4. In this case there is a separate component (which can preferably be in the form of a mixing device). A static mixing element is additionally provided in that separate component.

(24) In this embodiment the gas injector 12 injects the gas directly into the chamber 4 of the separate component.

(25) For carrying out the method according to the invention a shut-off device 8 near the shaping cavity can be closed. During compression of the plastic melt 2, that is to say during the reduction in the volume of the chamber 4, that shut-off device 8 disposed between the separate component and the plasticising cylinder 6 remains open. The plasticising screw 7 then compresses the plastic melt 2 in the above-described fashion. Then therefore the screw pre-chamber is also part of the chamber 4.

(26) A pressure sensor 9 is not illustrated in the structure shown in FIG. 2, but can naturally be provided in the described forms or on the separate component, or the pressure in the plastic melt 2 can be measured indirectly.

(27) FIG. 3 shows an actually measured relationship between the amount of gas introduced into the plastic melt 2 (gas loading or gas content) and the modulus of compression K, wherein the values of the modulus of compression were ascertained by carrying out a method according to the invention. In this case the modulus of compression K at a gas loading 0 is standardised to 1. The gas loading is specified relative to the amount of the plastic melt.

(28) The dependency which can be seen in respect of the modulus of compression K on the gas loading represents a characteristic configuration, as occurs in particular when other process parameters (temperature, dynamic pressure and so forth) are kept constant.

(29) The solubility limit is clearly marked by a vertical line. With lower levels of gas loading there is a linear relationship between gas loading and modulus of compression K. Above the solubility limit there is a relatively sharp drop in the modulus of compression K which is in fact caused by the higher compressibility of the small bubbles which form due to incomplete solution of the gas in the plastic melt 2.

(30) If the relationship shown in FIG. 3 is known the plastic shaping method can be monitored in regard to gas solution by reviewing the modulus of compression K.

(31) If the relationship is not known it can be afforded by carrying out the method according to the invention a plurality of times. That affords the possibility of optimisation (real) of the plastic shaping method in respect of the amount of gas.

(32) In addition the solubility limit can be automatically determined by for example determining (fitting) the linear relationship for the lower gas loading value and by ascertaining where a deviation goes beyond a certain previously established limit value.

(33) That criterion can also be used in order to abort in automated fashion a measurement series for determining the solubility limit.

(34) That finally also permits automatic adjustment (in particular optimisation) of the plastic shaping method, for example by the machine control 11 establishing a target value for the amount of gas loading automatically at or in the proximity of the solubility limits—preferably beneath it by a predetermined value.

(35) For that purpose and generally for carrying out the method according to the invention the machine control 11 can be supplied with signals from the pressure sensor 9—insofar as that is used separately.

(36) FIG. 4 shows a chart which is similar to FIG. 3, wherein a respective modulus of compression was determined for three different first pressures (80 bars, 140 bars and 200 bars) for different gas loadings (amounts of introduced gas).

(37) The illustrated data points can also be in the form of mean values of determined values for the compression parameter in the case of a plurality of compression tests carried out under identical conditions (equal settings).

(38) The Figure also illustrates a respective curve fit for the three measurement series, in the course of which the parameters of the following equation were ascertained (K-equation):

(39) $K=\frac{K_0-\mathrm{kc}+P_u{d}^{\frac{c}{e}}}{{\left(1+{.Math.\lambda c.Math.}^a\right)}^{\frac{1-n}{a}}}$

(40) In that respect the following parameters are used:

(41) K.sub.0 modulus of compression of the plastic melt without gas loading [bar]

(42) k initial increase below the solubility limit [bar/%]

(43) c gas concentration [%]

(44) P.sub.u unit pressure [1 bar]

(45) d, n, a dimension-less scaling factors [−]

(46) e scaling factor [%]

(47) λ inverse solubility limit [1/%]

(48) The following table lists the values of the fit parameters ascertained in the course of the curve fit together with their units:

(49) TABLE-US-00001 Parameter Unit By fitting ascertained values K.sub.0 bar 6786.77 k bar/% 0.21 d — 0.0161 n — 0.00 a — 9.11 λ.sub.80 1/% 1.18 f % 0.0325 K.sub.0 bar 6757.06 k bar/% 300.29 d — 1.8666 n — 0.40 a — 152.63 λ.sub.140 1/% 0.69 f % 0.1894 K.sub.0 bar 6820.27 k bar/% 337.04 d — 1.1462 n — 0.39 a — 350.00 λ.sub.200 1/% 0.43 f % 0.0588

(50) As already mentioned detected values for introduced amounts of gas are often not comparable between different procedures for introducing the gas. The present invention makes it possible to compare those amounts of gas by determining the characteristic compression parameter—in particular the modulus of compression—in the further gas loading processes which are to be called upon for the comparison.

LIST OF REFERENCES

(51) 1 shaping machine 2 plastic melt 3 injection unit 4 chamber 5 shaping cavity 6 plasticising cylinder 7 plasticising screw 8 shut-off device 9 pressure sensor 10 drive 11 machine control 12 gas injector 13 measurement flange 14 ultrasonic sensor