MIXING DEVICE FOR PRODUCING A PLASTIC MELT LOADED WITH A PROPELLANT

Abstract

A mixing device for producing a plastic melt loaded with propellant, from which a foamed plastic part is producible in a molding machine, includes a mixing chamber, at least one propellant inlet device for feeding propellant into the mixing chamber, an inlet channel for plastic melt, and an output channel for dispensing plastic melt loaded with propellant. A first gas-tight shutoff device is provided for closing the inlet channel, and a second gas-tight shutoff device is provided for closing the output channel.

Claims

1. A mixing device for producing a plastic melt loaded with propellant, from which a foamed plastic part is producible in a molding machine, in particular an injection molding machine, the mixing device comprising: a mixing chamber, at least one propellant inlet device for feeding propellant into the mixing chamber, an inlet channel for plastic melt, an output channel for dispensing plastic melt loaded with propellant, a first gas-tight shutoff device configured to close the inlet channel, and a second gas-tight shutoff device configured to close the output channel.

2. The mixing device according to claim 1, wherein the mixing chamber has a static mixer, wherein the static mixer is formed by a plurality of stationary mixing elements mounted on a wall of the mixing chamber.

3. The mixing device according to claim 1, further comprising a temperature control device for the mixing chamber, which preferably at least partially surrounds the mixing chamber.

4. The mixing device according to claim 1, wherein the propellant is a gas, preferably nitrogen.

5. The mixing device according to claim 1, further comprising a control or regulating unit for controlling or regulating the mixing device.

6. The mixing device according to claim 5, wherein the first shutoff device and/or the second shutoff device are/is switchable between a closed position and an open position by the control or regulating unit, preferably depending on a pressure measured in the mixing chamber.

7. The mixing device according to claim 5, wherein the propellant inlet device is controllable by the control or regulating unit.

8. The mixing device according to claim 1, wherein the propellant inlet device has a piston mechanism for the, preferably forced, high-pressure injection of the propellant.

9. An injection device for a molding machine comprising the mixing device according to claim 1.

10. The injection device according to claim 9, further comprising a plasticizing aggregate for melting a plastic starting product, in particular in granular form, for melting plastic and for conveying the plastic melt in the injection direction into the inlet channel of the mixing device.

11. The injection device according to claim 10, wherein the plasticizing aggregate has a plasticizing cylinder, a rotatable plasticizing screw, which is preferably linearly displaceable, and a drive device for the plasticizing screw.

12. A molding machine for producing a foamed plastic part, comprising the injection device according to claim 9.

13. The molding machine according to claim 12, further comprising a clamping unit, wherein the clamping unit comprises a stationary mold mounting plate, a movable mold mounting plate, a drive device for the movable mold mounting plate, and a molding tool clamped to the mold mounting plates.

14. The molding machine according to claim 13, wherein the second gas-tight shutoff device is formed by at least one shut-off nozzle of the molding tool.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Further details and advantages of the present invention will be explained in more detail below based on the figure description with reference to the embodiments shown in the drawings, in which:

[0034] FIG. 1 schematically shows a molding machine,

[0035] FIG. 2 schematically shows a part of a plasticizing aggregate and the mixing device,

[0036] FIG. 3 shows the region of the propellant inlet device of the mixing device,

[0037] FIG. 4 shows several propellant inlet devices arranged around the mixing chamber,

[0038] FIG. 5 is a schematic longitudinal section view through a mixing chamber with several mandrels of a propellant inlet device,

[0039] FIG. 6 is a suitable cross-section view to FIG. 5, and

[0040] FIG. 7 schematically shows a part of a plasticizing aggregate and the mixing device with a propellant inlet device in the form of a piston injector.

DETAILED DESCRIPTION OF THE INVENTION

[0041] FIG. 1 schematically shows a molding machine 2. In the left area the clamping unit 17 is visible, which has a movable mold mounting plate 19 and a fixed mold mounting plate 18. The movable mold mounting plate 19 is drivable via a drive device 20. This drive device 20 can, for example, be designed in the form of a toggle lever mechanism or a spindle drive. The molding tool 21 is mounted on the mold mounting plates 18, 19. When closed, at least one cavity K is formed in the molding tool 21. At least one injection channel 22 leads into this cavity K. In the right area of FIG. 1 an injection device 12 is schematically shown. This injection device 12 has a plasticizing aggregate 13 and the mixing device 1. Plastic melt is injected into the cavity K by the plasticizing aggregate 13 via the mixing device 1 and the injection channel 22. After the injected plastic melt has solidified, the molding tool 21 is opened, and the resulting molding part is ejected.

[0042] FIG. 2 shows a part of the injection device 12 with several details. In the right area, a part of a plasticizing aggregate 13 is shown. This plasticizing aggregate 13 consists of the plasticizing cylinder 14 and the plasticizing screw 15. The plasticizing screw 15 is driven by a schematically shown drive device 16 (e.g. an electric motor). In front of the plasticizing screw 14 there is the screw vestibule 23.

[0043] The mixing device 1 is shown on the left. This mixing device 1 has the mixing chamber 3. This mixing chamber 3 is formed in a mixing cylinder 24. A plurality of mixing elements 9 are attached to the wall of the mixing cylinder 24. These mixing elements 9 form a so-called static mixer. Between the mixing chamber 3 and the plasticizing aggregate 13, the mixing device 1 has the first gas-tight shutoff device 7. This first shutoff device 7 makes it possible to shut off the inlet channel 5 shown in dotted lines, which connects the plasticizing aggregate 13 with the mixing chamber 3. With the first shutoff device 7, the inlet channel 5 is switchable between an open position and a closed position. After the mixing chamber 3, the mixing device 1 has the second gas-tight shutoff device 8. This second shutoff device 8 is arranged in injection direction E after the mixing chamber 3, and in front of the schematically shown molding tool 21. In this molding tool 21, injection channel 22 is shown schematically in form of several hot runners. The output channel 6 is closable by the second shutoff device 8. This output channel 6 connects the mixing chamber 3 with the injection channel 22. The output channel 6 is switchable between an open position and a closed position by the second shutoff device 8. The first shutoff device 7 and the second shutoff device 8 can be designed different from each other oras shownidentical to each other.

[0044] The mixing device 1 also has a propellant inlet device 4. This propellant inlet device 4 is connected to the mixing chamber 3 via an appropriate shut-off valve 28. The propellant inlet device 4 is supplied with propellant T via a propellant source 30. Nitrogen is preferably used as propellant T.

[0045] The mixing chamber 3 can be tempered via a temperature control device 10. This temperature control device 10 can (as schematically shown) be designed in the form of heating bands, which are arranged around the mixing cylinder 24. However, the temperature control device 10 can also have at least one tempering conduct in the mixing cylinder 24.

[0046] In the mixing chamber 3, a sensor 25 can also be arranged to measure the pressure P in the mixing chamber 3. This sensor 25 is in a signal connection with the schematically shown control or regulating unit 11. This control or regulating unit 11 can operate the two shutoff devices 7, 8 via corresponding signals. The propellant inlet device 4 can also be controlled via this control or regulating unit 11. It is also quite possible that the drive device 16 of the plasticizing screw 15 is controlled or regulated via this control or regulating unit 11.

[0047] FIG. 3 shows the area of the propellant inlet device 4 of the mixing device 1 in more detail. This propellant inlet device 4 has an injection mandrel 26, which is connected to the other components of the inlet device 4 via an inlet channel 27 formed in the mixing cylinder 24. In particular, this inlet device 4 can also have a schematically shown, switchable inlet valve 28. The inlet mandrel 26 is arranged in the mixing chamber 3. The inlet mandrel 26, preferably tubular, has a plurality of inlet holes 29. These inlet holes 29 point in injection direction E. Via these inlet holes 29 the propellant T is fed into the plastic melt in mixing chamber 3. This regular arrangement of the plurality of inlet holes 29 ensures homogenous dispersion of the propellant T in the plastic melt.

[0048] In general, it is sufficient if a single propellant inlet device 4 is provided. However, in order to achieve a better dispersion, it is also possibleas shown in FIG. 4to provide several propellant inlet devices 4, each of which is connected to the mixing chamber 3 via an inlet channel 27. In the case shown, the four propellant inlet devices 4 are arranged at constant distances from each other around the mixing chamber 3.

[0049] According to FIG. 5, another variant of the propellant inlet device 4 is shown. Hereby several inlet mandrels 26 are arranged in the mixing chamber 3. Each of these inlet mandrels 26 again has a plurality of inlet holes 29.

[0050] FIG. 6 shows a corresponding cross-section through the mixing chamber 3. It can be seen that a total of four inlet mandrels 26 are arranged in the mixing chamber 3. Two inlet mandrels 26 run horizontally, while the other two run vertically. These inlet mandrels 26 can be connected to a single propellant inlet device 4, or they can be connected to one propellant inlet device 4 each, as shown in FIG. 4. This results in a homogeneous gas feed into the entire mixing chamber 3.

[0051] FIG. 7 shows for the most part the same structure as FIG. 2, which is why regarding the contents, it can be referred to the description of FIG. 2. The difference in FIG. 7 is that the propellant inlet device 4 has a piston mechanism or piston injectorprecisely a piston-cylinder unit. The inlet valve 28 is retained. In this piston mechanism, the desired amount of propellant is metered under moderate pressure conditions (in the range of 20 bar to keep the piston volume compact) before being pressed into the mixing chamber 3. This allows the advantage that the gas metering can be done very easily, and above all in a reproducible way. The propellant (gas) is then pressed into the melt. With this arrangement, no pressure control is necessary (the piston pressure merely has to be higher than the pressure in the mixing element, which in any case is guaranteed by sufficient hydraulic pressure of the piston). By pressing the entire gas volume into the piston mechanism, very precise metering is above all possible, which does not depend on pressure differences that are difficult to regulate. The injection is carried out as follows: [0052] according to the first process (see also above) while the plastic melt is at rest in the mixing chamber 3. This is the case after injection. The cylinder-sided shutoff device 7 is hereby either already closed, or it may possibly already be metered in parallel. [0053] according to the second process (see also above) during injection of the polymer-gas mixture into the cavity K. For this, the gas pressure in the cylinder of the piston mechanism must be above the injection pressure (up to 2000 bar). This must be ensured by the appropriate design of the piston mechanism of the propellant inlet device 4. The advantage of the second method is the additional axial dispersion of the propellant T.

[0054] The advantages of the mixing device according to the invention compared to the so far known gas loading systems are as follows: [0055] The use of an expensive MuCell aggregate (or other special screw geometries are not required). This means that the plasticizing process can be carried out with conventional 3-zone screws. [0056] Metering problems that occur during gas loading in the cylinder no longer play a role. [0057] Downtimes are no longer a problem. [0058] Gas is added at constant pressure conditions according to the first process sequence or by forced pressure conditions according to the second process sequence. Additional advantages of forced gas injection are exact additional metering, no need for pressure measurement or pressure control, and a simple and compact design. [0059] The danger of gas escaping via the inlet is eliminated by the first (cylinder-sided) shutoff device. [0060] The possible creation of a gas lake is prevented by the first (cylinder-sided) shutoff device.

[0061] It should generally be noted that the mixing chamber 3 is designed in such a way that it can have several shot volumes. During the first process sequence, it should also be noted that the gas can be fed in over the entire height of the mixing chamber 3. For this purpose a perforated mandrel (FIGS. 3, 5 and 6) or several injection points are distributed over the entire circumference (FIG. 4). The shutoff device can be in the form of shut-off nozzles. In particular, shut-off nozzles with pin shut-offs can be used instead of needle shut-offs. This results in a more compact design.

LIST OF REFERENCE SIGNS

[0062] 1 mixing device [0063] 2 molding machine [0064] 3 mixing chamber [0065] 4 propellant inlet device [0066] 5 inlet channel [0067] 6 output channel [0068] 7 first gas-tight shutoff device [0069] 8 second gas-tight shutoff device [0070] 9 mixing elements [0071] 10 temperature control device [0072] 11 control or regulating unit [0073] 12 injection device [0074] 13 plasticizing aggregate [0075] 14 plasticizing cylinder [0076] 15 plasticizing screw [0077] 16 drive device for the plasticizing screw [0078] 17 clamping unit [0079] 18 stationary mold mounting plate [0080] 19 movable mold mounting plate [0081] 20 drive device for the movable mold mounting plate [0082] 21 molding tool [0083] 22 injection channel [0084] 23 screw vestibule [0085] 24 mixing cylinder [0086] 25 sensor [0087] 26 inlet mandrel [0088] 27 inlet channel [0089] 28 inlet valve [0090] 29 inlet holes [0091] 30 propellant source [0092] T propellant [0093] P pressure [0094] E injection direction [0095] K cavity