Apparatus for injection molding of plastic materials

Apparatus for injection molding of plastic materials comprising a mold including at least one plate, a hot runner distributor of the fluid plastic material, at least one injector and an actuator for controlling the opening and the closing of the injector, supported by the distributor and whose cooling is carried out by means of thermal exchange contact with the plate. Provided for the cooling of the jack actuator provided for is at least one cover made of thermally conductive material at least partly surrounding the actuator in an axially slidable manner and it is maintained in thermal exchange contact with the plate by means of a magnetic or an electro-magnetic force.

FIELD OF THE INVENTION

The present invention refers to apparatus for injection molding of plastic materials of the type comprising a mold having a cavity and including at least one plate, a hot runner distributor of the fluid plastic material, at least one injector for the introduction of the plastic material from the distributor into the mold cavity, and an actuator for controlling the opening and closing of the injector.

More in particular, the invention refers to molding apparatus thus made wherein the actuator which actuates the injector, typically provided with a valve pin axially displaceable between a full closing position and a maximum opening position of the injector, consists in a fluid jack or in an electric motor supported by the distributor.

STATE OF THE PRIOR ART

During the operation of the molding apparatus the actuator, whether pneumatic or hydraulic or electric, is subjected to heating by means of the heat of the distributor, and it is conventionally cooled by means of an autonomous cooling hydraulic circuit. In some applications, even characterised by high temperatures of the mold, there arises the desire that the actuator, particularly in the case of the fluid jack, be without a cooling circuit so as to simplify the molding apparatus further. The heat of the hydraulic jack must however be dissipated, and solutions in which the cooling of the hydraulic jack occurs by means of thermal exchange with the aforementioned at least one mold plate were proposed for this purpose. As concerns this, it should be observed that the expression “mold plate” is used to indicate, in the description and in the claims that follow, the so-called mold clamping plate, as a whole or in relation with a component thereof in that it can consist of several adjacent plates, the die i.e. the fixed part of the mold, a plate of the distributor, as well as the plane of the press of the injection apparatus.

Solutions in which the cooling of the fluid jack occurs by means of the thermal exchange with a metal plate of the apparatus are for example disclosed by documents WO-2009/052611 on behalf of Mold Masters and documents WO-2011/119791 and US-2014041192, U.S. Pat. Nos. 8,349,244, 8,562,336, 8,728,378, EP-2550144, EP-2631059, WO-2015/183332, U.S. Pat. No. 9,682,504, US-2017/0246784, EP-3240666, US-2016/0361856, WO-2016/153608, WO-2016/153632, WO-2016/153703, WO-2016/153704 and WO-2016/153705 on behalf of Synventive Molding Solutions.

Provided in all these prior art solutions is an elastic thrust member, typically consisting of or including a metal spring, interposed between the cylinder of the hydraulic jack and the plate with the aim of guaranteeing the thermal exchange contact irrespective of the thermal dilation phenomena of the hot runner which supports the hydraulic jack directly or indirectly. The use of plastic thrust members is also considered necessary by the prior art so as not to have to demand high precision machining of the mold plate/s against which the cylinder of the actuator must be kept at thermal exchange contact.

The use of such elastic members entails construction complications and thus considerable costs, and the thermal exchange surface between the hydraulic jack and the plate made available by such elastic members is generally limited and thus scarcely efficient.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the aforementioned drawbacks and provide an apparatus for injection molding of plastic materials of the type defined above that allows to obtain an efficient thermal exchange cooling between the actuator and the mold plate in a more simple, economic and functional manner.

With the aim of attaining subject object, the main and peculiar characteristic of the invention resides in the fact that for the cooling of the actuator provided for is at least one cover made of thermally conductive material that at least partly surrounds the body of the actuator in an axially slidable manner and is kept in thermal exchange contact with the said at least one plate by a magnetic or an electro-magnetic force.

When the actuator consists of a fluid jack, i.e. a hydraulic or a pneumatic jack, such cover has a planar radial wall faced to the cylinder of the fluid jack and defining a flat surface at contact with said at least one plate. Such radial wall may be restrained against the plate by a magnetic or electro-magnetic attractive or repulsive force, by a pressurised fluid or by a pneumatic or hydraulic thrust, or still by an electrical drive force, or the combination thereof.

A radial gap is provided between the cover and the body of the jack, and such a gap may be conveniently maintained permanently: to this effect the body and the cover are made of materials having different coefficients of linear thermal deformation.

Guide means such as axial studs or screws are conveniently provided between the body of the jack and said cover.

The invention applies both to the case where the actuator is directly fixed to the hot runner, and in such case the aforementioned at least one plate may consist in the mold clamping plate or in the hot runner plate, or it can be indirectly fixed to the hot runner, laterally thereto, and in such case the aforementioned at least one plate consists of the die plate or hot runner plate. Alternatively, the aforementioned at least one plate may consist in an element of the press of the molding apparatus.

DETAILED DESCRIPTION OF THE INVENTION

An apparatus for injection molding of plastic materials conventionally comprises a press (not illustrated) for sending the fluid plastic material to a distributor or hot runner1from which the plastic material is injected into the mold cavity (not illustrated) through one or more injectors. Each injector comprises a displaceable valve pin2, relatively to a mold gate, between a full closing position and a maximum opening position. The displacement of the valve pin2, and thus the opening and closing of the injector, occur through an actuator which, in the case of the present invention, consists of a fluid jack indicated in its entirety with3, pneumatic or hydraulic. Alternatively, the actuator could consist in an electric motor.

In the embodiment which is now being disclosed with reference toFIGS.1-11the hydraulic jack3comprises a hollow cylinder4and a plunger20directly or indirectly connected to which is the valve pin2for example as shown inFIGS.4,6e7. The cylinder4is at least partly, and preferably almost entirely, surrounded by a cover7which shall be further disclosed in better detail.

It is to be pointed out that the term “cylinder” as used herein is intended to designate a body having not only a circular cross section but also any other shape falling within the geometrical definition of that term.

Accordingly, the cylinder4could have an elliptical or quadrangular or polygonal cross section, even in order to increase the amount of thermal exchange between the cylinder4of the jack3and the cover7and thus the amount of cooling of the jack3.

The cylinder4of the hydraulic jack3is supported by the distributor1: in the case of the embodiments represented inFIGS.1to8the cylinder4is directly fixed onto the distributor1, by means of a fixing unit generally indicated with5. Alternatively, the cylinder4of the hydraulic jack3may be indirectly fixed to the distributor1, for instance over or alongside the latter, as observable hereinafter with reference toFIGS.9and10.

The cylinder4is without an autonomous cooling circuit that conventionally uses a cooling liquid, and the cooling thereof occurs by means of thermal exchange contact with a plate of the molding apparatus. Such plate may include any mold plate and in particular, like in the case ofFIGS.1to8, consists in the clamping plate indicated with6. Alternatively, this could be a component of such plate or of the hot runner plate (not represented) or of the die (not represented) of the mold in the case where the hydraulic jack is arranged laterally to the hot runner1, as observable in the case ofFIGS.9and10.

It should be observed that the clamping plate6or other plates (not shown) can be provided with a liquid cooling circuit, indicated with12, or it can be without one.

In order to guarantee the thermal exchange contact between the cylinder4of the hydraulic jack3and the clamping plate6the invention provides for, in place of a thrust spring member like in the case of the prior art, the cover7which is made of thermally conductive material and at least partly, and preferably for a substantial and almost full perimeter width, surrounds the cylinder4. This cover7is axially slidable relative to the outer surface of the cylinder4for a stroke of limited degree by means of one or more C-shaped lateral bands8engaged—at one end—with the cylinder4and—at the opposite end—an axial clearance in a groove9of the cover7, as better observable inFIG.8.

A radial gap G is provided between the cylinder4and the cover7.

Preferably the cylinder4and the cover7are made of materials having different coefficients of linear thermal deformation. Namely, the coefficient of linear thermal deformation of the cylinder4is lower than the coefficient of linear thermal deformation of the cover7: this ensures that the radial gap G may be permanent, i.e. it may be maintained constantly during operation of the apparatus. For instance, the cylinder4is conveniently made of cast iron and the cover7is made of aluminium.

The minimum amount of the radial gap G can be conveniently in the range of at least 0.01 mm. under cold conditions, i.e. when the apparatus is not operating, and in the range of at least 0.05 mm. under hot conditions, i.e. when the apparatus is operating.

The cover7may have a generally L-shaped or C-shaped section with an end radial wall10facing the bottom plate11of the cylinder4and defining a flat surface at planar contact with the clamping plate6. The lateral wall of the cover7, indicated with22, is in sliding thermal exchange contact with the cylinder4of the hydraulic jack3.

The bottom plate11may consist in an element separated from the body of the cylinder4, or it can be made of a single piece with the cylinder4.

In order to constantly maintain the wall10of the cover7in thermal exchange contact with the clamping plate6, the invention provides for different alternative solutions among which the preferred one, represented inFIGS.2to5, applies a magnetic or an electro-magnetic attractive force. To this end, incorporated in the radial wall10are permanent magnets facing the clamping plate6and cooperating therewith. It should be observed that the arrangement could be inverted, or there could be provided for the incorporation of the magnets13in the clamping plate6.

As observable hereinafter, the magnetic force may also be a repulsive force instead of an attractive force.

FIGS.6and7show two variants in which the thermal exchange contact between the cover7and the clamping plate6is obtained by means of a pressurised fluid. The solution represented inFIG.6provides for a pneumatic thrust which occurs by means of a pressurised gas supplied, through an inlet14, into a sealed chamber15formed between the bottom plate11of the cylinder4and the wall10of the cover7.

In the case ofFIG.7the cover7is restrained against the clamping plate6by a hydraulic thrust conveniently carried out by the hydraulic fluid for actuating the hydraulic jack3: to this end, the sealed chamber15is placed in communication with the internal of the cylinder4through one or more gates16formed in the bottom plate11.

The same arrangement ofFIG.7can be provided for in the case where the hydraulic jack3is actuated pneumatically instead of hydraulically.

According to a further alternative solution not represented in the drawings, the thermal exchange contact between the cover7and the clamping plate6can be obtained by means of an electrical drive force, or through an electric actuator configured for pushing, by means of a transmission known to a man skilled in the art, the transversal wall10against the plate6.

It should be observed that the various alternatives described above could be combined with each other: so that the pneumatic thrust, the hydraulic thrust and the electrical drive force could be combined with each other and/or with the magnetic attraction.

As previously mentioned, the fluid jack3can be fixed, instead of directly onto the hot runner1, laterally thereto for example as represented inFIGS.9and10. In this case, the hydraulic jack3with the body7is carried by a support or plate17through a mechanical transmission (for instance including a swinging lever or a gear train) for the actuation of the valve pin2of the injector (not visible in these figures). The radial wall10of the cover7is in this case kept at thermal exchange contact, by means of one or more of the previously described alternatives, using a mold plate different from the clamping plate6and for example consisting in the die plate or in the hot runner plate or an auxiliary plate, not represented in the drawings since well known to a man skilled in the art.

According to a further alternative not represented in the drawings, the plate against which the cover7is in thermal exchange contact can consist in the plane or another element—magnetic or non-magnetic—of the molding apparatus and the relative press to which the mold is associated.

In the further variant of the invention schematically represented inFIG.10A, besides the cover7—kept in exchange contact using the cylinder4of the hydraulic jack3on the one side and using the plate6on the other side, provided for is an auxiliary element23made of thermally conductive material kept at front thermal exchange contact against the bottom plate of the cylinder4and arranged at thermal exchange slidable lateral contact with the lateral wall22of the cover7. Thus, the heat dissipation of the hydraulic jack3occurs in an even more efficient manner.

The auxiliary element23is kept at constant contact with the bottom plate11for example by means of the attraction force obtained by the permanent magnets24. Furthermore, there can be further provided for permanent magnets25,26respectively carried by the auxiliary element23and by the plate6and configured so as to obtain a magnetic repulsive force suitable to press the radial wall10of the cover7against the plate6.

The magnets26can also be directly applied to the bottom plate11of the cylinder4should the auxiliary element23not be present, and the magnetic repulsive force can also be combined with a pressurised fluid thrust and/or with an electrical drive force, as described previously.

It should be observed that the thermal exchange contact between the cover7and the plate6can be obtained not only by the radial wall10but also by the lateral wall22. This lateral contact can be obtained directly, and in this case the lateral wall22will be at least partly inserted into a recess or complementary seat of the plate6or of an element of the mold, or indirectly by means of an intermediate element fixed to the plate.

In light of the above it will be clear that the invention is capable of ensuring, during the operation of the molding apparatus, an efficient heat dissipation of the jack3in a constructively easy and economic manner.

Additional embodiments of the fluid jack and cover assembly are depicted inFIGS.11to16in connection with a hydraulic jack, and inFIGS.17to22in connection with a pneumatic jack.

The radial gap between the cylinder4and the cover7is better seen and referenced as G inFIGS.14and20, respectively, and the constant presence of such a gap G is also ensured by axial guide means provided between the cylinder4and the cover7. In the example ofFIGS.15and16these guide means comprise four studs30fitted within retaining bushes31of the hydraulic cylinder4, while in the example ofFIGS.17,21and22these guide means comprise a single retaining screw32provided centrally between the planar wall10of the cover7and the top wall33of the pneumatic cylinder4.

Obviously, the construction details and the embodiments may widely vary with respect to what has been described and illustrated, without departing from the scope of protection of the invention as described in the claims that follow. Thus, even if the above description is contemplating the body of the fluid jack and the cover being slidable relative to each other parallelly to the injector, said body and said cover could be instead slidable relative to each other transversely to the injector.