DESCRIPTION

The present invention relates to an extrusion device for extrudable material.

Within the scope of the present description and the following claims:

- the term "extrudable material" is used to indicate a material intended to be inserted in the extrusion device to be extruded following a softening due to heating, therefore the term extrudable material is used to indicate a raw material in the solid state such as a thermoplastic polymer material (resin), a thermoplastic biopolymer (bioresin) and the like, and can also be considered a material to be used in the food industry (for example flours, food pastes, etc.), in the cosmetic industry (for example creams) or in other industrial sectors (for example for the production of synthetic fats).

- the term extrusion screws is used to indicate not only screws with one or more helical threads, but also similar elements provided with mixing elements not similar to threads but capable of fulfilling the same technical function as the screws.

As is known, an extrusion device, in particular an extrusion device provided with counter-rotating screws, is a machine which is widely used in processing plastics as well as in other industrial sectors such as the processing of foods or chemicals.

Generally an extrusion device comprises a pair of screws/augers (in the following of the description and of the claims referred to as "screws") which rotate in the opposite direction inside a cylindrical chamber specially shaped to accommodate said screws.

The aforesaid chamber of the extrusion device is suitably temperature-regulated in order to be able to transfer energy to the extrudable material being processed and cause the softening thereof, essentially until the extrudable material is softened or made fluid, in combination with a concomitant mechanical compression action due to the rotation of the screws and the effect of the threads on the softened and processed extrudable material.

Since the screws of the extrusion devices can have the respective threads partially interpenetrated or not interpenetrated, it should be noted that the transformation of the plastics which occurs in a counter-rotating extrusion device implies the softening of the processed material which loses its original shape and transforms into a continuous material in the softened state or in the fluid state. The initial shape of the processed extrudable material can be discrete (powder, regular granules or derived from grinding plastic objects) or continuous (continuous strips obtained from previous plastic transformation processes). In both cases, the apparent density of the extrudable material in input to the extruder is always lower relative to the actual density of the extrudable material processed due to the presence of gaps between the granules or between the flaps of the strips feeding the extruder.

Even while considering the reduction in density resulting from heating the plastic, the density of the softened continuous or fluid material at the output of the extrusion device is greater than the apparent density of the material in input. It follows that it would be suitable for the specific volumetric flow rate of the counter-rotating extrusion device to change between the material input zone (high volumetric flow rate) and the fluid or softened continuous material output zone (low volumetric flow rate).

The volumetric flow rate is also linked to the time spent by the extrudable material in the different zones of the extrusion device. It is convenient that such time spent is high in the initial part of the extruder to allow the softening of the extrudable material, while it is low in the end part to avoid excessive thermal stresses of the extrudable material in the softened state or in the fluid state.

In the extrusion devices with two interpenetrating counter-rotating screws, the above needs are met by changing the volume of the chambers defined by the screw threads and by the surface of the chamber housing the screws. This modification can be obtained according to two different modes to be used, if necessary also in combination with each other:

- modification of the geometry of the screw threads (pitch, thread thickness, inclination of the sides) or

- continuous change of the outer diameter of the chamber.

To compensate for the taper of the outer diameter of the screws and maintain the mutual meshing features of the threads, this second mode involves the use of two screws with axes which are not parallel but convergent, with consequent difficulties and construction problems relative to parallel screw extrusion devices.

In fact:

- an extrusion chamber suitable for housing tapered screws is difficult to produce;

- a gear reducer which reciprocates tapered screws must have output shafts with convergent axes, with consequent construction difficulties and

- taper angle of the screws limits the maximum length of the extrusion device.

In view of the foregoing, it is evident that the need for an extrusion device with counter-rotating interpenetrating screws is currently quite felt, so as to exploit the advantages deriving from the use of counter-rotating interpenetrating screws, without incurring the drawbacks and problems referred to above.

The problem underlying the present invention is that of devising and providing an extrusion device for extrudable material which has structural and functional features such as to meet the aforesaid need, while remedying the problems referred to with reference to the background art.

Such a problem is solved by an extrusion device for extrudable material according to claim 1.

The solution idea of the present invention is to provide an extrusion device with interpenetrating counter-rotating screws, in which the variation of the volumetric flow rate of the screws is ensured through discrete variations (stepwise) of the outer diameter of the screws. As a result, the inner diameter of the chamber housing the screws and the depth of the compartment obtained in the screws (thread depth) at these discrete variations in the outer diameter are varied.

At the aforesaid discrete variations of the outer diameter of the screws, passage sections are provided in the extrusion chamber between an upstream diameter and the subsequent diameter of limited length, as better described below, relative to the length of the extruder.

It should be noted here that extrusion devices with counter-rotating extrusion screws which have discrete variations in the outer diameter of the screws are found both in the literature and in practical applications, but this is achieved at the expense of the thread interpenetration feature, which is not substantially maintained/ensured for the entire length of the extrusion chamber, unlike what is allowed by the extrusion device according to the present invention thanks to the claimed geometry of the extrusion chamber.

Further features and advantages of the extrusion device for extrudable material according to the present invention will emerge from the description given below of a preferred embodiment thereof, given by way of non- limiting example, with reference to the accompanying figures.

In particular:

- figure 1 depicts a longitudinal-section plan view of an extrusion device for extrudable material according to the invention;

- figure 2 depicts a longitudinal section view of the single chamber of the extrusion device for extrudable material of figure 1 and

- figure 3 depicts a longitudinal view of the extrusion device for extrudable material of figure 1.

With reference to the accompanying figures, 1 generically denotes an extrusion device for extrudable material according to the invention, for example a thermoplastic polymer material or a thermoplastic biopolymer.

The extrusion device 1 comprises:

- a chamber 2 extending in a main axial direction X-X between a first and a second end and delimited by a temperature-controlled sidewall 3 for transferring energy to the extrudable material being processed in said chamber 2 and causing it to be softened at a material discharge opening;

- an inlet for introducing extrudable material to be processed into said chamber 2 located proximate to said first end of the chamber 2;

- an outlet 5 for discharging processed extrudable material from said chamber 2 located proximate to said second end;

- two separate extrusion screws 6 having a predetermined longitudinal axis Y-Y each rotatably supported to rotate in said chamber 2 about the respective longitudinal axis Y-Y;

- motor means for driving said extrusion screws 6 to rotate about the respective longitudinal axis Y-Y so that the rotation direction of an extrusion screw 6 is opposite to the rotation direction of the other extrusion screw, to cause the extrudable material to be continuously fed and compacted from said inlet to said outlet 5 of said chamber 2. In particular, in figures 1 and 3 the shank of the extrusion screws 6 intended to be engaged by the mechanical transmission means of the aforesaid motor means is indicated with 4.

Preferably, the aforesaid inlet of extrudable material to be processed in the chamber 2 is a radial opening provided in the side wall 3 delimiting the chamber 2, while

- said outlet 5 is an axial opening located at said second end of said chamber 2. According to the invention:

- the longitudinal axes Y-Y of the two extrusion screws 6 extend parallel to each other in the aforesaid axial direction X-X of the chamber 2;

- the chamber 2 comprises at least one connecting section 2 _c in which the diameter of the chamber itself has its internal dimension reduced without undercuts to prevent stagnation points from forming inside the chamber of the extrudable material processed in passing from a first section 2 _a to a second section 2 _b of said chamber 2 located relative to said connecting section 2 _c :

• upstream towards said first end of said chamber 2 and, respectively,

• downstream towards said second end of said chamber 2;

- the aforesaid first section 2 _a and the aforesaid second section 2 _b of the chamber 2 are cylindrical linear sections having substantially constant internal cross section;

- the inside radius R _a of the first section 2 _a of the chamber 2 is greater than the inside radius R _b of the second section 2 _b of the chamber 2;

- the aforesaid connecting section 2 _c extends over an axial section L _c of the chamber 2 whose length, as measured in the aforesaid axial direction X-X of the chamber 2, is less than or equal to three times the inside radius of the first section 2 _a of the chamber 2 upstream of the aforesaid connecting section 2 _c , therefore according to the relationship:

L _c ≤ 3xR _a;

- said extrusion screws 6 are spaced apart so that the crests of their respective helical threads will at least partially interpenetrate with each other, so as to cause at least partial overlapping therebetween, possibly also total overlapping therebetween, in the direction of the longitudinal axis Y-Y of the screws 6 both in said first section 2 _a and in said second section 2 _b of said chamber 2.

In essence, the two screws 6 are positioned parallel to each other so as to have the respective threads partially radially interpenetrating with each other.

With reference to the term "connecting section" used in the context of the present description and the subsequent claims, it should be specified that it indicates that the change in inner diameter of the chamber 2 between two cylindrical sections occurs with a reduction in internal size without undercuts to prevent stagnation points from forming inside the chamber of the extrudable material processed in passing from a larger inner diameter to a smaller inner diameter.

Preferably the aforesaid connecting section 2 _c extends over an axial section L _c of said chamber 2 whose length, as measured in the axial direction X-X of the chamber 2, is less than or equal to two times the inside radius R _a of said first linear cylindrical section 2 _a upstream of said connecting section 2 _C , therefore according to the relationship:

Lc ≤ 2xR _a .

Preferably, the aforesaid connecting section 2 _c extends over an axial section L _c of said chamber 2 whose minimum length, as measured in said axial direction X-X of the chamber 2, is greater than or equal to 0.1 times the inside radius R _a of said first linear cylindrical section 2 _a upstream of said connecting section 2 _c , therefore according to the relationship :

L _c ≥ 0.1xR _a .

More preferably, the aforesaid connecting section 2 _c extends over an axial section L _c of said chamber 2 whose minimum length, as measured in said axial direction X-X of the chamber 2, is less than or equal to 0.6 times the inside radius R _a of said first linear cylindrical section 2 _a upstream of said connecting section 2 _c , therefore according to the relationship:

L _c ≥ 0.6xR _a .

In accordance with the illustrated embodiment, the aforesaid connecting section 2 _c extends in a linear fashion between the inner wall of the aforesaid first linear cylindrical section 2 _a and the inner wall of said second linear cylindrical section 2 _b upstream of said connecting section 2 _c . Alternatively, it is possible to provide an extended connecting section 2 _c with a non-linear trend, for example with a curvilinear trend or other profile suitable for avoiding undercuts.

Preferably:

- the aforesaid first section 2 _a of the chamber 2 extends from said connecting section 2 _c of the chamber 2 to the aforesaid first end of the chamber 2 while

- the aforesaid second section 2 _b extends from the aforesaid connecting section 2 _c of the chamber 2 to the aforesaid second end of the chamber 2, in accordance with what is illustrated in figures 1 and 2.

In accordance with the illustrated embodiment, the chamber 2 provides a single connecting section 2 _c interposed between the two aforesaid cylindrical sections 2 _a and 2 _b having an inner wall with a constant diameter, so that the chamber 2 can be formed by two cylindrical axial portions 2 _a , 2 _b which are sealingly joined end-to-end with the aforesaid connecting section 2 _c . More preferably the aforesaid connecting section 2 _c is made in one piece with one of the aforesaid two cylindrical axial portions 2 _a , 2 _b , in the example of figures 1 and 2 with the linear cylindrical section 2 _b , so that the aforesaid diameter variation is of simple construction from the technological point of view.

However, it should be noted that, to meet specific technical requirements, it is possible to provide a chamber of the extrusion device having two or more connecting sections between cylindrical portions of the chamber having decreasing or increasing variations of the inside size of the chamber with respect to cylindrical portions of said chamber located downstream or upstream of said two or more connecting sections.

In particular, the aforesaid connecting sections between cylindrical portions of the chamber having decreasing or increasing variations of the inner size of the chamber, for example at degassing zones of the chamber, i.e., zones where vents are provided for the removal of volatile substances present in the material being processed.

In the case of interpenetrating and counter- rotating extrusion screws 6, the helical threads have opposed right-handed and left-handed trend, respectively.

In accordance with the embodiment illustrated in the figures, at the aforesaid first section 2 _a of the chamber 2 the two extrusion screws 6 comprise, starting from said first end of said chamber 6, a first axial section L ₁ having a predetermined pitch P _c and a subsequent second axial section L ₂ , more proximate to said connecting section 2 _c , having a pitch P ₂ shorter than the pitch P ₁ of said first section.

In order to meet specific technological needs, the aforesaid two extrusion screws can have along said direction of the longitudinal axis Y-Y:

- non-constant pitch, depth, thickness and/or shape and/or

- thread breaking points.

In accordance with the illustrated embodiment, at the aforesaid second section 2 _b of the chamber 2, the two extrusion screws 6 comprise a thread core having a diameter d _n increasing from said connecting section 2 _c towards said second end of said chamber 2.

Preferably, the extrusion device 1 comprises heating means 7 at least at the aforesaid second section 2 _b of the chamber 2 placed downstream of the connecting section 2, more preferably, according to what is illustrated, the extrusion device 1 comprises heating means 7 also at the part of the chamber 2 closest to said first end where the inlet of extrudable material to be processed is provided.

It should be noted that both the chamber 2 and the extrusion screws 6 can be made by interconnected portions, so as to allow the assembly and disassembly of the extrusion device 1 and to be of less complex construction.

As can be appreciated from what has been described, the extrusion device for extrudable material according to the present invention makes it possible to meet the aforesaid need and to overcome the drawbacks mentioned with reference to the prior art.

In particular, the extrusion device according to the invention allows to have the two screws of the extruder with the threads interpenetrating therebetween, in the sense previously described, substantially for the entire axial length of the extrusion chamber.

A further advantage of the extrusion device according to the invention lies in the fact that the chamber of the device, as well as the extrusion screws, can be made by assembling cylindrical sections having a less complex embodiment relative to the conical chambers of the extrusion devices in accordance with the prior art.

A further advantage of the extrusion device according to the invention is identified in the fact that the mechanical gear reducer of the motor means has parallel output shafts and is therefore a conventional gear reducer for extrusion devices with two counter- rotating screws.

A further advantage of the extrusion device according to the invention lies in the fact that the length of the extruder is not limited by the conicity of the screws as occurs in the devices of the prior art.

Obviously, a person skilled in the art may make numerous modifications and variations to the extrusion device for extrudable material described above so as to satisfy contingent and specific requirements, all of which are contained in the scope of protection of the invention as defined by the following claims.