Pump for conveying molten masses of polymers and elastomers

DESCRIPTION

The present invention refers to a gear pump designed to be positioned upstream of an extrusion head to convey molten masses of polymers and or elastomers and in particular highly degradable plastic materials, such as PVC and the like.

Said pumps serve to compensate for the unevenness of flow and of the vibrations caused on the material by the extrusion screw of an extruder, so as to have a constant flow of molten material coming from the pump, to obtain a high quality finished product that complies with strict tolerances.

Pumps for non-degradable extruded plastic materials, that is for products that do not give rise to problems of degradation in the event of stagnation of the material inside the pump, are known to the art. Said types of pumps are volumetric pumps that provide control of flow speed and pressure, so as to ensure a constant product output volume.

Said known pumps generally comprise a pump body consisting of a single body, substantially parallelepiped, having an inlet duct for entry of the material and an outlet duct for discharge of the material. The inlet and outlet ducts communicate with a central seat or chamber to house a pair of gears for conveying the material from the inlet duct to the outlet duct of the pump.

The central gear seat is transversally open to the outside to allow insertion of the gears. Consequently, cylindrical supports or bearings which rotatably support the shafts of the gears are mounted in the pump body to support the gears.

Between each bearing and its seat formed in the pump body, sharp edges are formed wherein the material conveyed by the pump infiltrates. In said sharp edges, the material stagnates and undergoes combustion. As a result, the material that passes through the pump carries with it the dirt and burnt matter coming from the stagnating material.

Said drawback is accentuated by the fact that the pump gears are generally self-lubricated with the same material handled by the pump which passes through the gaps between the gear shafts and the bearings which support said shafts.

The foregoing proves a serious drawback, especially in the case of highly degradable materials, such as PVC. In fact in the finished product leaving the pump, a dark stripe can be noted due to contact with the stagnating material in the pump. Said dark stripe is unacceptable, for example, for transparent plastic materials. Furthermore, said dark stripe sets off a rapid degradation of all material in close contact therewith, with the result that, after a short time, the finished product becomes unusable.

For this reason degradable plastic materials such as PVC are extruded directly, without the use of pumps at the outlet from the extrusion screw and upstream of the extrusion head. All this to the detriment of quality and accuracy of tolerances of the finished product.

The object of the present invention is to overcome the drawbacks of the prior art, providing a pump for conveying molten masses of polymers and elastomers that is able to provide an end product of high quality that complies with narrow tolerances.

Another object of the present invention is to provide a pump for conveying molten masses of polymers and elastomers that is structurally simple, and thus cheap and easy to make.

Advantageous embodiments of the invention are apparent from the dependent claims. The pump for conveying molten masses of polymers and/or elastomers according to the invention comprises a pump body within which are formed an inlet duct for entry of the material, an outlet duct for discharge of the material and a central seat communicating with the inlet and outlet ducts and intended to house at least one gear mounted on a motorised shaft for conveying material from the inlet duct toward the outlet duct.

The main feature of the pump according to the invention is represented by the fact that the pump body consists of two parts that can be assembled together. Each part of the pump body comprises at least one through hole communicating with the central chamber. In this manner at least one gear shaft is rotatably supported directly inside the pair of opposite facing holes of the two parts of the pump body.

With this solution, use of bearings or cylindrical supports mounted in the pump body to rotatably support the gear shafts is avoided. Consequently, in the pump according to the invention sharp edges between the gear shaft supports and the pump body, in which material stagnates with consequent deterioration of the quality of the end product leaving the extrusion head and downstream of the pump, are avoided.

It is evident that the pump according to the invention can be used in particular for highly degradable polymers, such as PVC (Poly Vinyl Chloride), obtaining an end product that has good quality and narrow tolerance characteristics.

Further characteristics of the invention will be made clearer by the detailed description that follows, referring to a purely exemplary and therefore non-limiting embodiment thereof, illustrated in the enclosed drawings, in which:

FIG. 1

is an exploded perspective view of a pump for conveying molten masses of polymers and elastomers according to the invention;

FIG. 2

is an exploded perspective view of the pump of

FIG. 1

, seen from another point of view;

FIG. 3

is a perspective view of the pump of

FIG. 1

, assembled;

FIG. 4

is a perspective view of the pump of

FIG. 3

, sectioned along the sectional plane IV—IV of

FIG. 3

, in which the pump gears have not been sectioned;

FIG. 5

is a cross sectional view of a gear of the pump according to the invention, illustrating the gear tooth profile.

The pump according to the invention, designated as whole with reference numeral 1, is described with the aid of the figures.

The pump

1

comprises a substantially parallelepiped pump body consisting of two shell halves

2

,

2

′, substantially identical and able to be assembled with each other. For greater clarity, the first shell half (that is, the one on the left side with reference to the figures) will be denoted hereunder by

2

and the second shell half (that is, the one on the right side with reference to the figures) will be denoted by

2

′. Furthermore, the direction of entry/discharge of the material into/from of the pump

1

will be indicated with an axial direction.

The first shell half

2

has transverse through holes

3

peripherally to receive screw or nut means

4

that engage in threaded holes

5

provided in the shell half

2

′. In this manner the two shell halves

2

,

2

′ can be assembled together.

The two shell halves

2

,

2

′ present axially an inlet aperture

7

and an outlet aperture

8

, substantially semicylindrical. In this manner, when the two shell halves

2

,

2

′ are coupled together, they define a cylindrical inlet duct

7

for entry of the material and a cylindrical outlet duct

8

for discharge of the material. The inlet and outlet apertures

7

and

8

have rounded inner profiles

12

to avoid the presence of sharp edges wherein the processed material could stagnate.

On the outer surface of each shell half, around the inlet aperture

7

and the outlet aperture

8

there is defined a semitoroidal flange

9

with holes

10

for fixing the pump

1

. The two flanges

9

of the two shell halves

2

,

2

′ assembled together form a toroidal flange around the respective inlet and outlet ducts

7

,

8

.

The inlet and outlet apertures

7

,

8

of each shell half

2

,

2

′ communicate with a central seat

11

having a substantially elliptical shape seen in axial section. Between the inlet and outlet openings

7

and

8

and the central seat

11

there are formed rounded connections

13

to avoid sharp edges in which material could stagnate.

In each central seat

11

of each shell half two through holes

14

are formed having a rounded peripheral connection

17

with the central seat

11

to avoid sharp edges in which material could stagnate. The transverse through holes

14

receive a pair of gears

15

and

16

, wherein

15

is the driver gear connected to a motor unit and

16

is the driven gear.

Each gear

15

,

16

comprises a substantially cylindrical shaft

18

whereon a toothing

19

in the form of a sprocket wheel or pinion is provided. The toothing

19

of the driver gear

15

and the driven gear

16

engage each other into the central seats

11

of the shell halves

2

,

2

′.

The shaft

18

of each gear

15

,

16

is rotatably supported directly in the respective holes

14

of the shell halves

2

,

2

′, without the need to provide further supports or bearings mounted in the pump body which give rise to sharp edges in which the material stagnates.

One end

39

of the shaft

18

of the driver gear

15

protrudes outward from the first shell half

2

to be connected to a drive shaft or to a transmission of a motor (not shown in the figures) which gives the driver gear

15

the rotatory motion around its own axis. As a result, the driver gear

15

sets the driven gear

16

in rotation and, through the toothing

19

of the gears, the molten plastic material entering the inlet duct

7

is distributed peripherally in the central seat

11

and leaves from the outlet duct

8

according to the speed of rotation of the gears

15

and

16

.

For this purpose, the speed of rotation of the driver gear

15

determines a uniformity of the flow leaving the volumetric pump

1

, irrespective of the feeding speed of the material inside the extruder upstream of the pump

1

. In this manner a uniformity of the flow leaving the pump

1

is ensured, with the result of an end product having good characteristics of quality and tolerance.

As shown in

FIG. 5

, each gear

15

,

16

has a special toothing

19

without sharp edges to ensure the smooth flow of the material and prevent it from stagnating between the edges of the toothing. Each tooth

40

of the toothing

19

has side profiles

41

substantially rounded on the flanks. Furthermore the root fillet

42

between one tooth

40

and the next is substantially rounded.

The other end of the shaft

18

of the driver gear

15

has a substantially conical or frusto conical shape

20

that protrudes outward from the second shell half

2

′ to engage, in a rotating conical coupling relationship, in a cap

21

having a conical or frusto conical cavity, complementary to the frusto conical end

20

of the shaft of the driver gear

15

. The frusto conical cap

21

has a flange

22

with through holes to receive screw means for fixing to the shell half

2

′ around the hole

14

.

The shaft

18

of the driven gear

16

has two ends

20

having a substantially frusto conical shape like the end of the driver gear

15

. The ends

20

of the driven gear

16

protrude outward from the first and second shell half

2

,

2

′, respectively, to engage in respective frusto conical caps

21

substantially the same as the cap of the driver gear

15

.

Thus both the driver gear

15

and the driven gear

16

, besides being rotatably supported in the holes

14

of respective shell halves

2

,

2

′, are centred, in a rotatable conical coupling relationship, in the respective frusto conical caps

21

disposed outside the pump body.

The gears

15

and

16

are self-lubricated by means of the plastic material that flows in the gaps left between the outer surface of the shafts

18

and the respective holes

14

of the shell halves

2

and

2

′ and in the gaps left between the outer surface of the conical ends

20

of the shafts

18

and the conical caps

21

. For this purpose the end of the conical caps

21

has an outwardly open hole

23

, through which the plastic material used for lubrication can exit. In this manner, the material used for self-lubrication can be recycled and re-used later, avoiding putting it immediately into the flow of material that passes though the inlet and outlet ducts

7

,

8

of the pump

1

.

The conical caps

21

can advantageously be adjusted in their axial movement so as to adjust the clearance of coupling with the conical end

20

of the gears, to adjust the flow of self-lubrication material to a minimum flow depending on the operating pressure. In the outer surface of each shell half

2

,

2

′, peripherally with respect to the holes

14

, a pair of semi-annular seats

24

are formed, substantially with an opposite facing U shape. Inside the U-shaped seats

24

there are formed a plurality of transverse through holes

25

which are arranged peripherally around the central seat

11

, which receives the pinions

19

of the gears

15

and

16

.

The holes

25

serve for the passage of a cooling and heat-regulating fluid to regulate the temperature of the material in the central seat

11

wherein the gear pinions

19

acts. The heat-regulating fluid can be, for example, water or oil, according to the process temperature at which the pump must work.

The heat-regulating seats

24

are tightly closed by means of substantially U-shaped closing covers

26

complementary to the seats

24

. The closing covers

26

are applied from the outside of the pump body and are provided with special sealing means

30

, such as gaskets.

As shown in

FIG. 2

, in the inner surface of the second shell half

2

′ around the heat regulating holes

25

, there are disposed sealing gaskets

27

to ensure tightness on coupling of the two shell halves

2

,

2

′.

The heat regulating seats

24

communicate with the respective heat regulating ducts

28

formed in the outer edges of the shell halves

2

,

2

′ which have advantageously been rounded off. In this manner there are eight inlet/outlet ducts

28

for the heat-regulating fluid, so as to have cross-flows according to heat-regulation requirements. In this manner perfect regulation of the process temperature can be ensured.

Obviously, as an alternative or in addition to said heat-regulation system, the shafts

18

of the gears can have an axial duct through which the cooling fluid flows.

In the present embodiment of the invention a pump body consisting of two substantially symmetrical shell halves

2

,

2

′ has been illustrated. However, a pump body consisting of two different parts could also be provided, that is to say a main body in which are formed the inlet and outlet ducts

7

and

8

for the material, the central seat

11

of the gear toothing

19

, which remains outwardly open, and the holes

14

for supporting the shafts

18

of the gears, and a cover, in which only the holes

14

for supporting the shafts of the gears are integrated, which covers only the central seat

11

of the gears.

Numerous changes and modifications of detail within the reach of a person skilled in the art can be made to the present embodiment of the invention, without thereby departing from the scope of the invention as set forth in the enclosed claims.