DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a mixer-extruder for rub ber, silicone, plastic and thermoplastic materials, or other ma terials, more specifically, to a mixer-extruder provided with means for temporarily adjusting the compounding chamber.

TECHNICAL BACKGROUND

It is well known that in some types of screw extruders for synthetic materials, notably materials having a good degree of elasticity such as rubber, elastomers, plastic and / or thermo plastic materials, or silicone, feeding to the extrusion screw is carried out through a loading hopper open to the upper side.

A commercially available extrusion machine which proved to be particularly effective is the one described in WO 2005/039847 in the name of the same Applicant, herein incorpored as a refer ence, where a compounding function has been provided in an ex truder of the twin-screw dump extruder type.

More precisely, referring to Fig. 1 illustrating, partially in section, such a prior art machine, a compounding chamber 1 accommodates a pair of extrusion conical screws 2 which push the material towards an extrusion die 3, which can be closed by a blind flange or closing cover 4. The compounding chamber 1 is provided with an inlet la which is open to a feed hopper (not shown) . The closing cover 4 occludes the extrusion die 3 and forces the material to recirculate into the compounding chamber 1, causing a compounding by the two conical screws. The material is actually driven forwardly, i.e. towards extrusion die 3, by the pair of counter-rotating screws, and then forced to go back when the die is occluded by the cover 4. In certain applica tions, the rotation of the two screws may be also temporarily reversed, to improve the effect of compounding.

The cover 4 can be opened or easily removed, so as to keep the extrusion die 3 occluded during the compounding step, until desired, and then opened to allow the extruded material to out- flow at the end of mixing.

When the extrusion die 3 of the dump extruder is closed by the cover 4, the material which is introduced by the hopper is forced to recirculate on itself inside the chamber 1, being thus homogeneously compounded.

Notably, in operation, with the blind flange 4 arranged in a closed position, the load to be compounded - e.g. a compound already pre-mixed in another machine, or bulk ingredients such as rubbers, fillers, accelerators, etc. - is loaded into the feed hopper and fed into the compounding chamber 1 while the mo tor keeps in counter-rotation the conical screws, which push the material to be compounded towards the die. However, the closing cover 4 does not allow the mixture to outflow from the chamber 1, and forces the mixture to recirculate back to the chamber. An increasing pressure gradient is thus determined between a low- pressure material feeding area and a high-pressure "ducted" area in the proximity of the blind flange 4. The recirculation be tween areas having variable pressure of the various basic compo nents allows a quick and effective compounding of the mixture.

The two counter-rotating conical screws are accommodated partly within the large chamber 1, where the material is loaded, and partly in converging and intersecting conical channels, wherein the peripheral profile of the screw threads runs adja cent to the channel surface. The material is thus forced to fol low the vane profile of the screw to a progressively narrower volume, increasing the pressure. In order to achieve a recircu lation of the material (when the cover 4 is closed) , at least in the most advanced area of the conical channels, the peripheral wall is provided with an enlargement, such that the screw vane is at a certain distance from the wall. Typically, the enlarge ment is provided at the upper part of the accommodating chan nels. Thanks to the presence of this enlargement of the channel for accommodating the extrusion screws, the material is allowed to circulate backwards, when the cover is closed, returning to a low-pressure area of the chamber 1, and thus resulting in an ef fective compounding. Although this extruder is fully functional, the applicant has found that there is room for improvement. Indeed, it has been noted that the material to be compounded is freely disposed within the mixing chamber 1 based on the local pressures, and this causes possible stagnation or delays in mixing, and some times an excessive increase in the temperature and pressure of the material. Therefore, it would be desirable to better control the process during the compounding step.

In the prior art, some singular configurations of extruders have already been proposed, wherein mobile elements are used to control at least the input of the material into the extruder screws. For example, DE102007033355 discloses an extruder pro vided with a material thrusting plate, which operates in the in let hopper in the extruder. By contrast, US2016/0361697 relates to a mixer, wherein a removable closing element, which opens and closes the material mixing chamber, is provided. However, none of these prior art systems is able to control, during the pro cess, the pressure and temperature conditions of the material being compounded, adapting both to the compounding conditions and to the extrusion conditions.

PROBLEM AND SOLUTION

Therefore, the underlying problem of the invention is to supply an extrusion and compounding machine of a rubber-based or silicone-based material, as described above, which overcomes the aforementioned drawbacks and which allows greater control on the mixing and the temperature/pressure of the material, as well as on compounding times during processing.

This object is achieved through the features defined in es sential terms in the appended claims.

Notably, according to a first aspect, there is provided an apparatus for the mixing and extrusion of plastic materials, for example rubber-based and silicone-based materials, comprising a dump extruder body wherein two conical screws converging towards an extrusion die suitable for being occluded by closing means are mounted, the conical screws being accommodated within a low- pressure compounding chamber and within a pair of converging conical channels also defining a high-pressure chamber, said low-pressure chamber corresponding to an upstream area with re spect to an extrusion direction, wherein

in said high-pressure chamber and/or in said low-pressure chamber a presser body, forming a movable wall suitable for causing a controlled change in the volume of the respective chamber, is provided.

According to a preferred aspect, at least said high- pressure chamber is provided with a presser body controlled in position .

According to another aspect, the presser body of the low- pressure chamber is arranged so as to occlude a plastic material loading opening, such as an inlet hopper of the dump extruder body .

Typically, the presser body is mounted so as to be slidably movable but can also be mounted so as to have one or more de grees of rotation. The presser body is movable between a lifted (or home) position and a lowered (or operating) position, pref erably under the action of a controlled actuator. According to a variant, the actuator determines a constant position of said presser during mixing.

According to a preferred aspect, the presser body has a movable wall shaped as two intersecting conical surface por tions, so as to adapt and follow the profile of the vanes of the two screws.

It is understood that the presser body is displaced in po sition at least during a compounding step, wherein said closing means prevent the plastic material from passing through said ex trusion die.

The closing means may also be in the form of an extraction gear pump which is stopped and occludes the outlet of said die. BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention are anyhow more evident from the following detailed description of some preferred embodiments, given by mere way of non-limiting example and illustrated in the accompanying drawings, wherein: Fig. 1, as already mentioned, is a side elevation, partial ly sectional, view of a prior art extruder/mixer system;

Fig. 2A is a schematic enlarged view of a detail of the single compounding chamber of Fig. 1, but relating to an extrud er system according to a first embodiment of the invention;

Fig. 2B is a view similar to the one in Fig. 2A, but relat ing to a variant of the first embodiment;

Fig. 3A is a view similar to the one in Fig. 2A, but relat ing to a second preferred embodiment of the invention;

Fig. 3B is a cross-sectional view, taken along the line III-III of Fig. 3A;

Fig. 4A is a view similar to Fig. 3A in an enlarging step of the recirculation chamber;

Fig. 4B is a cross-sectional view, taken along the line IV- IV of Fig. 4A; and

Fig. 5 is a view similar to the one in Fig. 2A, but relat ing to a third embodiment .

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An extrusion and mixing machine consists of the basic ele ments shown in Fig. 1 and already described in the introductory section with reference to the Document WO 2005/039847 incorpo rated herein as a reference.

Notably, a "dump extruder" machine has a machine body de fining a compounding chamber 1 which accommodates a pair of con ical extrusion screws 2, arranged with intersecting spiral vanes spirals and axes converging towards an extrusion die 3.

The extrusion die 3 may be occluded by suitable closing means 4 (schematically illustrated in figures), such as a remov able cover, or simply by an extraction gear pump which is stopped and occludes the outlet of the die.

The compounding chamber 1 has an upper inlet la from which the material, such as rubber-based or silicone-based material, to be processed is introduced.

In the front part of the chamber 1, in the proximity of the extrusion die 3, the pair of conical screws 2 is accommodated within converging conical channels with inner walls almost touching the vane edges of the conical screws. The sectional profile of such conical channels is therefore shaped as two in tersecting circles. In at least one area of such sectional pro file, the inner wall of the channels has an enlargement, thus detaching from the vanes of the conical screws 2 and defining a material recirculating chamber. Typically, such enlargement is provided in the upper part of the conical screws 2 (see area 2' in Fig. 4B) .

Referring now to Figs. 2A and 2B, it should be noted that, according to a first embodiment of the invention, the inlet la of the compounding chamber 1 is provided with an occluding body 5, hereinafter referred to as presser, which is mounted so as to be lifted and lowered.

Notably, according to a first variant, the presser 5 is mounted so as to be linearly slidable between a lifted position and a position in contact with the material inside the chamber 1 (as in Fig. 2A) ; in this case, the presser 5 is essentially made similarly to a piston, which occludes the inlet la and is mova ble therein substantially according to an axis substantially perpendicular to the lying plane of the pair of screws 2, so as to be an element for variably closing the chamber 1.

According to a second variant, a presser 5' is mounted piv oting about a hinge 5a, between an upwardly rotated position and a downwardly rotated position (as in Fig. 2B) , wherein it is in contact with the material inside the chamber.

The presser 5 or 5' is mounted with a certain clearance with respect to the inlet la, so it is not necessary to seal the entrance to the chamber 1, and is intended to rest on the mate rial which is compounded inside the chamber, for imparting lo cally - in the low-pressure area of chamber 1 - a pressure to the material itself. In other words, the presser 5 or 5' is in tended to reduce the volume of the chamber 1 in a controlled way, so as to be able to regulate the pressure and temperature of the material as desired in a controlled way.

The localized pressure action by the presser 5 exerts an advantageous effect on the material being compounded by the pair of conical screws. Notably, the material on its way back from the high-pressure area to the low-pressure area, is kept inside the vanes of the conical screws by the presser, which improves the mixing effect.

The pressure in this low-pressure area of the chamber 1 al lows to better control the amount of reflux of the material, so as to favour and accelerate the homogenization effect of the ma terial .

For this purpose, the presser can be made either simply in the form of a heavy body mounted movable - as seen above - in such a way that the same pressure (due to its own weight under the effect of gravity) is always applied to the material, or in the form of a movable piston under the action of a controlled actuator (not shown) , such as a hydraulic, electric or pneumatic jack .

In the second case, the actuator may be controlled so as to exert a constant force - and thus produce an effect similar to that of a weight - or so as to keep the piston in a fixed posi tion, thereby actually setting a desired volume where the pres sure may vary cyclically. Furthermore, the position and pressing force of the presser 5, 5' may vary and be set during the com pounding and extrusion cycle.

The presser 5 does not necessarily have a circular cross- sectional shape, but may take any other shape, e.g. a square, rectangular, triangular or circle sector section. Obviously, it is required only that it has a shape able to fit the contour of the inlet la.

In the two variants of the first embodiment of Figs. 2A and 2B, the presser 5, 5' is applied in the so-called low-pressure area of the chamber 1, i.e. the area immediately next to the loading inlet la.

By contrast, in a preferred embodiment (Figs. 3A-4B) , a presser 6 operates in the high-pressure area of the extrusion chamber 1, i.e. in the area where the two conical screws are ac commodated within the respective conical channels. In the high- pressure chamber, pressure is normally in the range from 5 to 200 bar. In this case, the inlet la of the hopper could be opened or closed by a simple cover defining a constant volume, and the pressure control on the compounded material is exerted by the presser 6 only.

Notably, as well represented by the comparison of Figs. 3B and 4B, in this preferred embodiment, the presser 6 constitutes a movable wall of an enlargement of the conical channels which defines a high-pressure compounding chamber 2' . By displacing the presser 6, it is therefore possible to control the change in volume of the high-pressure compounding chamber 2' , thus deter mining a change in pressure and, indirectly, in temperature of the material being processed.

In this case, since the presser 6 operates in the high- pressure area, where the wall of the conical channels is ajacent to the vanes edges of the screws, the presser 6 defines a mova ble wall which is shaped as two intersecting conical surface portions, which constitutes indeed a part of the respective con ical channels (as can be clearly seen in Fig. 4B) .

Since the size of the high-pressure area is also far lower than that of the low-pressure area, it is certainly preferable that the presser 6 is controlled by an actuator which, while acting on a smaller presser, is able to exert a sufficient over all pressure on the material. Moreover, the changes in volume in this area, i.e. on the high-pressure compounding chamber, prove to be very effective, since they cause a high percentage change, and therefore have a significant impact on the behaviour of the material .

According to a third embodiment, there are provided both a presser 5, 5' in the low-pressure area, and a controlled presser 6 in the high-pressure area. The control action is preferably carried out on both pressers, so as to have the greatest inter vention flexibility.

As can be understood, by using mobile pressers 5, 5' and / or 6 which vary the volume of the high-pressure chamber 2 and / or of the low-pressure chamber 1, it is possible to place an ad ditional control element, which allows to regulate optimally the degree of mixing and the process speed / performance.

It has also been found that it is convenient to control the intervention of the pressers even during the extrusion step, i.e. when the closing cover 4 is opened and the material is ex- truded through the die 3. In this step, it is therefore possible to better regulate the material outflow pressure and an improved emptying of the compounding chamber, avoiding air inclusions in the material, if any.

It is understood, however, that the invention is not to be considered as limited to the particular arrangements illustrated above, which represent only some exemplary implementations thereof, but different variants are possible, all within the reach of a person skilled in the art, without departing from the scope of the invention itself, as defined by the following claims .