Method, Apparatus and Press for Injection Moulding of Plastic Material

Abstract

A method, apparatus and press for the injection moulding of plastic material by a plurality of injectors connected to a distributor and provided with respective obturators movable between a closing condition and an opening condition in a controlled fashion. The injection pressure in the distributor is maintained substantially constant by an actuator which actuates a screw of the injection press, from a start of a filling step up to an end of a packing step of the plastic material into a cavity of a mould. A moulding cycle is entirely controlled by each injector, by managing the position/velocity/acceleration of the respective obturator, without affecting the other injectors.

Claims

1. A method for injection moulding of plastic material, comprising: introducing the plastic material into a cavity of a mould in a filling step followed by packing the plastic material introduced into the mould (M) during a subsequent packing step, wherein the filling step is performed by plurality of injectors connected to a hot runner or distributor and provided with respective obturators movable between a closing condition and an opening condition in a controlled fashion with respect to at least one among position, velocity and acceleration; supplying the plastic material to the distributor by an injection press comprising a screw axially displaceable by an actuator, wherein the plastic material supplied to the distributor is at an injection pressure; and maintaining the injection pressure supplied to the distributor substantially constant by said actuator from a start of the filling step up to an end of the plastic material packing step.

2. The method according to claim 1, wherein said distributor is configured so that said injection pressure at an inlet of said injectors is substantially unvaried.

3. The method according to claim 1, wherein said filling and packing steps are managed exclusively by controlling at least one among position, velocity and acceleration of the obturator of each injector.

4. The method according to claim 1, wherein said obturators are actuated by respective electric motors.

5. The method according to claim 1, wherein said actuator of the injection press consists of a hydraulic cylinder whose piston moves said screw, and wherein said injection pressure is maintained substantially constant by the displacement of said piston.

6. The method according to claim 5, wherein said injection pressure is detected at an outlet of said screw upstream of an inlet of said distributor, and said piston is displaced so as to advance said screw when a decrease of said injection pressure is detected.

7. The method according to claim 5, wherein the hydraulic pressure within said cylinder or within a hydraulic circuit thereof is detected and said piston is displaced so as to advance said screw upon revealing a decrease of said hydraulic pressure.

8. The method according to claim 1, wherein said substantially constant injection pressure is a maximum pressure.

9. The method according to claim 1, wherein the method is implemented for joint-moulding components of a same family, having different shapes and/or sizes and/or volumes, with a multi-cavity mould and respective injectors associated to said cavities.

10. The method according to claim 1, wherein the mould is equipped with a sensor arrangement designed to detect given moulding conditions and there is provided a control unit configured to compare the detected given moulding conditions with one or more reference values for a moulding cycle, wherein the control unit is further designed to provide commands to one or more injectors so as to bring values detected by the sensors close to the one or more reference values.

11. The method according to claim 10, wherein the sensor arrangement includes pressure sensors facing the cavity of the mould.

12. An injection moulding apparatus configured to carry out the method according to claim 1.

13. An injection press comprising the screw rotatable and axially displaceable by the actuator of claim 1, wherein said actuator is controlled so as to implement the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The invention will now be described in detail with reference to the attached drawings, provided purely by way of non-limiting example, wherein:

[0036] FIG. 1 is a cross-sectional schematic view of an example of a sequential injection moulding apparatus,

[0037] FIGS. 2 to 8 are charts and diagrams regarding the state of the art described previously,

[0038] FIGS. 9 to 13 are similar representative diagrams of the invention,

[0039] FIG. 14 shows a variant of FIG. 1,

[0040] FIG. 15 is a diagram similar to FIG. 11 referring to the variant of FIG. 14,

[0041] FIG. 16 shows a further variant of FIG. 1, and

[0042] FIG. 17 schematically shows an example of the injection press according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0043] The diagram of FIG. 17 exemplifies the injection press P of the apparatus according to the invention. It comprises, in a known manner, a plastification and supply screw V movable in a hot chamber R in which the granules of the thermoplastic polymer to be moulded are supplied by means of a hopper T. The screw V is driven in rotation by an electric motor M and it is connected to the piston B of a hydraulic cylinder H which controls the translation thereof in the hot chamber R. The cylinder H is connected to a hydraulic cylinder including, in a known manner, a pump K and solenoid valves not illustrated controlled by an electronic unit E. The pump K may be associated to a VFD (variable-frequency-drive) control for modulating the rotation velocity.

[0044] The outflow nozzle of the press P, indicated with U, supplies the fluidified plastic material to the distributor or hot runner D of FIG. 1: to this end, the advancement of the screw V is controlled by the electronic unit E which receivesin inputthe signals of a position sensor S3 of the screw V, as well as the signals of a pressure sensor S4 located at the end of the nozzle U of the press P immediately upstream of the inlet of the distributor D and/or of a pressure sensor S5 provided for in the hydraulic circuit of the cylinder H. When the sensor S4 and/or the sensor S5 detect a pressure drop, the electronic unit E guides the hydraulic circuit so as to control the advancementby means of the piston Bof the screw V, which translates up to the pre-established position, detected by the position sensor S3, with velocity/acceleration controlled.

[0045] Thus, the injection press P is configured, according to the peculiar characteristic of the invention, to provide an almost constant injection pressure during the entire injection moulding process, from the start of the introduction step up to the end of the packing step.

[0046] In this manner, the distributor D is kept at constant pressure (and temperature) and each injector I can be managed separately without affecting the others in any manner whatsoever, as if it were the only injection point of the system. The electronic controlcarried out in a per se known mannerof the position and/or of the velocity of opening/closing and/or of the acceleration of the obturator of each injector enables to adjust the injection pressure and/or flow rate of the plastic material sequentially introduced into the cavity of the mould by each nozzle. The position of the obturator of each injector and its condition (partial/full opening, closure) does not affect the other injectors in any manner whatsoever, irrespective of their number, position and condition.

[0047] The press P, in particular through variations in the position and velocity of the injection screw V, automatically reacts to the opening of the injectors I so as to always guarantee the constant pressure condition in the distributor D. The constant injection pressure is preferably the peak pressure achieved should the moulding of the component be carried out by means of conventional methods.

[0048] Considering the same shape of the nozzle and of the obturator of each injector, the position (and velocity/acceleration) of the obturator is varied to adjust the flow rate and pressure constantly flowing out from the single nozzle during the entire cycle. Actually, interventions are made on head losses between the nozzle and the obturator, in particular between the tip and the obturator.

[0049] FIG. 9 is a diagram showing the development of the injection pressure during a moulding cycle carried out according to the invention, typically with injectors whose obturators are actuated by electronic control electric motors: as observable, the injection pressure remains constant, at the value corresponding to the hydraulic pressure set by means of the injection pressure P and represented in the diagram of FIG. 10. Such hydraulic pressure, except for slight pressure drop due to the sequential openings of the injectors only, is also substantially constant, contrary to what is represented in FIG. 4 with reference to the state of the art. Comparing the diagrams of FIGS. 4 and 10 it is overtly clear that thanks to the invention, the passage between the filling step and the packing step can no longer be identified: the management of the packing pressure, actually the management of the entire process, actually goes from the press P to the single injectors I, operating on the opening/closing of the respective obturators.

[0050] FIGS. 11 and 12 are diagrams similar to FIGS. 5 and 6 which show the development of the pressure in the cavity C of the mould M recorded by the sensors S1 and S2 of FIG. 1. It is clear that the increase of pressure in the cavity is almost linear and low up to stabilising at a packing value, while it used to be sudden up to a higher peak value, before dropping to the packing value, in the conventional cases.

[0051] The development according to the invention implies an optimal control of the moulding process, carried out by controlling the obturators of the injectors I1-I5. Furthermore, the pressure difference in the cavity at the end of the packing (.sub.pS1-S2), previously considerable, is now almost absent, meaning that the moulded article is homogeneous.

[0052] The chart of FIG. 12, regarding the opening of the obturators of the injectors I1-I5, also shows that the central injector I5 is conveniently initially opened at a height proximal to the closure height and held in such position to partially discharge the injection pressure which entirely pushes thereon, thus avoiding the formation of localised defects.

[0053] The chart of FIG. 13 showsindicatively with a dashed and double dotted linethe reaction of the press P, in form of the position of the injection screw V, following openings of the various injectors I1-I5 with the aim of guaranteeing constant pressure in the distributor.

[0054] FIG. 14, similar to FIG. 1, exemplifies an alternative embodiment of the invention applied to a family mould, i.e. for the co-moulding of components of a same family, having different shapes and/or dimensions and/or volumes, with a multi-cavity mould C1, C2, C3 and respective injectors I1-I5 associated to such cavities. In this case, it is clear that the invention enables managing different packing pressures/flow rates in the single cavities C1-C3, though maintaining the same pressure in the distributor D.

[0055] For example, the sensor S2 is combined with a cavity C3 of smaller dimensions with respect to the cavity C2 to which the sensor S1 is associated, and thus requires a lower packing pressure, as represented in the diagram of FIG. 15, by means of the control of the relative injector I5.

[0056] According to another advantageous variant, the invention may provide for combination with a closed-loop management software which links the displacement of the obturators of the various injectors to corresponding sensors. For example, let us consider pressure sensors S positioned in the cavity C of the mould M in proximity of an equal number of injection points, as schematised in FIG. 16. In this case, each sensor S sendsto an electronic control unit La respective signal regarding the moulding condition, which is analysed by a special software. If the target condition is not achieved, the software sends control signals to one or more obturators to automatically adjust the relative position/velocity/acceleration. In absence of mutual effect between the injectors, thanks to the invention, the closed-loop software will be simplified. [0057] Thus, the advantages arising from the invention can summarised as follows: [0058] the moulding cycle is entirely controlled by each injector, by managing the position/velocity/acceleration of the respective obturator, without affecting the control of the pressure and/or flow rate of the other injectors: there is no mutual effect between the injectors, [0059] the moulding process is separated from the press, except for the initial setting of the injection pressure, then the entire process is controlled by directly managing the obturators of the single injectors, [0060] the transfer of a mould from one press to another is definitely much easier, given that it requires configuring much fewer parameters (shorter times), [0061] a system like FLEXflow, which enables managing the position/velocity/acceleration of the single obturators, is exploited even more efficiently, [0062] the flow simulations (e.g. filling the cavity of the mould) are much more reliable if there is no mutual effect between the injectors, [0063] in the case of family moulding the management of various cavities is simplified, [0064] in the case of a closed-loop management software, it will be simplified. [0065] Obviously, the details and embodiments of the moulding apparatus and the injection press according to the invention may widely vary with respect to what has been described and illustrated by way of example, without departing from the scope of protection of the present invention as defined in the claims that follow.