ROTATIONAL MOULDING PROCESS AND RELATED MOULD FOR ROTATIONAL MOULDING COMPRISING A VENTING DEVICE

Abstract

Rotational moulding process for producing an article (300), the process comprising: providing a mould (100) comprising: a main body (99) which defines an inner cavity (30) having a conformation surface (31) counter-shaped to the article (300), and a venting device (1) fixed to the main body (99) and comprising: an inner chamber (2) in air communication with the inner cavity (30) through a first mouth (3) and with an outer environment (35) through at least one second mouth (4); a shutter (5) movable between an open position, in which let free the first mouth (3) for putting the inner cavity (30) in air communication with the outer environment (35), and a close position, in which closes the first mouth (3) for preventing the air communication; an actuator (6) for alternately moving the shutter (5) between the open position and the close position, the process further comprising: inserting a fluid form polymeric material into the inner cavity (30); closing the main body (99) and rotating the mould (100); while keeping the mould (100) in rotation and with the shutter (5) in the close position, heating the main body (99) until reaching a first temperature; while keeping the mould (100) in rotation, heating the main body (99) until reaching a second temperature higher than the first temperature; cooling the main body (99) and extracting the article (300), and wherein, during the heating of the main body (99) until the reaching of the first temperature, the process further comprises subtracting heat from the shutter (5) for maintaining the shutter (5) at a respective temperature lower than the first temperature.

Claims

1. A rotational moulding process for producing an article, the process comprising: providing a mould comprising: a main body which defines an inner cavity having a conformation surface counter-shaped to said article, and a venting device fixed to said main body and comprising: an inner chamber in air communication with said inner cavity (30) through a first mouth and with an outer environment through at least one second mouth; a shutter movable between an open position, in which let free said first mouth for putting said inner cavity in air communication with the outer environment, and a close position, in which closes said first mouth for preventing said air communication; an actuator for alternately moving said shutter between the open position and the close position, the process further comprising: inserting a fluid form polymeric material into said inner cavity; closing said main body and rotating said mould; while keeping the mould in rotation and with said shutter in the close position, heating said main body until reaching a first temperature; while keeping the mould in rotation, heating said main body until reaching a second temperature higher than said first temperature; cooling said main body and extracting said article, and wherein, during said heating said main body until reaching the first temperature, the process further comprises subtracting heat from said shutter for maintaining said shutter at a respective temperature lower than said first temperature.

2. The process according to claim 1, wherein said inner chamber is in air communication with the outer environment also through a third mouth arranged at opposite side of said inner chamber with respect to said shutter, wherein said subtracting heat from said shutter comprises generating an air flow which from the outer environment reaches a surface of said shutter facing away from said inner cavity, and then returns to the outer environment, and wherein said air flow goes from said third mouth to said surface of said shutter, then to said inner chamber and finally to said outer environment through said at least one second mouth.

3. The process according to claim 1, wherein said venting device comprises: a respective main body which defines said inner chamber; and a cooling conduit which passes through said respective main body, said cooling conduit not being in air communication with said inner chamber, wherein, during said heating said main body until reaching the second temperature and during said cooling said main body, the process comprises cooling said respective main body of the venting device for maintaining said respective main body at a respective temperature lower than said first temperature through a further air flow which from the outer environment passes through said cooling conduit for returning to the outer environment.

4. The process according to claim 1, wherein said first temperature is greater than or equal to 90? C., preferably greater than or equal to 110? C., and/or less than or equal to 160? C., preferably less than or equal to 140? C., wherein said second temperature is greater than or equal to 150? C., preferably greater than or equal to 170? C., and/or less than or equal to 250? C., preferably less than or equal to 230? C., and wherein a difference between said first temperature and said respective temperature of the shutter is greater than or equal to 10? C., preferably greater than or equal to 30? C.

5. The process according to claim 1, wherein during said heating said main body until reaching said second temperature and during said cooling said main body, said shutter is in the open position.

6. The process according to claim 1, wherein said mould comprises a further venting device fixed to said main body and comprising: a respective inner chamber in air communication with said inner cavity through a respective first mouth and with an outer environment through at least one respective second mouth; a respective shutter movable between a respective open position, in which let free said respective first mouth for putting said inner cavity in air communication with the outer environment, and a respective close position, in which closes said respective first mouth for preventing said air communication; a respective actuator for alternately moving said respective shutter between the respective open position and the respective close position; a further main body which defines said respective inner chamber; and a respective cooling conduit that passes through said further main body, said respective cooling conduit not being in air communication with said respective inner chamber, and wherein, during said cooling the main body, the process comprises: cooling said further main body of the further venting device 1 through a respective air flow that from the outer environment passes through said respective cooling conduit for returning to the outer environment; interrupting said cooling of said respective main body of the venting device.

7. The process according to claim 6, comprising, during said heating said main body until reaching said first temperature and said second temperature, operating said further venting device as said venting device is operated, and, during said cooling said main body: keeping said shutter in the open position and said respective shutter in the respective open position; generating a still further air flow which from the outer environment reaches said inner cavity through said venting device, and then returns to the outer environment through said further venting device.

8. A mould for rotational moulding comprising: a main body which defines an inner cavity having a conformation surface counter-shaped to said article, and a venting device fixed to said main body and comprising: an inner chamber in air communication with said inner cavity through a first mouth and with an outer environment through at least one second mouth; a shutter movable between an open position, in which let free said first mouth for putting said inner cavity in air communication with the outer environment, and a close position, in which closes said first mouth for preventing said air communication; an actuator for alternately moving said shutter between the open position and the close position.

9. The mould according to claim 8, wherein said venting device comprises: a respective main body which defines said inner chamber; a cooling conduit which passes through said respective main body, said cooling conduit not being in air communication with said inner chamber, and wherein said inner chamber is in air communication with the outer environment also through a third mouth arranged at opposite side of said inner chamber with respect to said shutter, wherein said venting device comprises a control rod which mechanically connects said shutter to said actuator, said control rod comprising an inner channel which puts in fluid communication said third mouth with a surface of the shutter facing away from said inner cavity and with said inner chamber.

10. A machine for rotational moulding comprising a mould according to claim 8 and rotating means of the mould.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] FIG. 1 shows in a purely schematic and partial way a top view of a machine comprising a mould for rotational moulding according to an embodiment of the present invention;

[0073] FIGS. 2-3 show a schematic and partial section along the line 200 of the mould of FIG. 1 with the shutter of the venting device respectively in the close and open position;

[0074] FIG. 4 shows a schematic and partial section along the line 201 of the mould of FIG. 1;

[0075] FIG. 5 schematically and partially shows a side section of a machine comprising a mould for rotational moulding according to a further embodiment of the present invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

[0076] The features and advantages of the present invention will be further clarified by the following detailed description of some embodiments, presented by way of non-limiting example of the present invention, with reference to the attached figures.

[0077] In the figures with the reference number 100 it is indicated a machine for rotational moulding comprising a mould 100 for rotational moulding. The machine 100 comprises rotating means of the mould 100, not shown nor described as, for example, of known type.

[0078] With reference to FIG. 1, a top view of the mould 100 is exemplarily shown in which the section line 200, along which the sections shown in FIGS. 2-3 are obtained, and the section line 201, along which the section shown in FIG. 4 is obtained, are highlighted.

[0079] The mould 100 comprises a main body 99 (for example a metal shell, e.g., made of aluminum, formed by two half-moulds) which defines an inner cavity 30 having a conformation surface 31 counter-shaped to an article 300 to be produced (schematically shown in FIG. 5). The main body 99 (only partially shown) can have any shape.

[0080] The mould 100 comprises at least one venting device 1 fixed to the main body 99, for example by fastening means such as screws or bolts (as schematically shown in FIGS. 2-4).

[0081] The venting device 1 comprises an inner chamber 2 in air communication with the inner cavity 30 through a first mouth 3 and with an outer environment 35 through at least one second mouth 4. For example, the venting device can comprise from one to eight second mouths. Exemplarily the venting device 1 shown in the attached figures comprises four second mouths (two of which are visible in FIGS. 2-3) arranged at the end of four conduits 12 arranged in a radial pattern.

[0082] The venting device 1 comprises a shutter 5 movable between a close position (schematically shown in FIG. 2) in which it closes the first mouth 3 to prevent an air communication between the inner cavity 30 and the outer environment 35, and an open position (shown schematically in FIG. 3) in which let free the first mouth 3 to allow the aforesaid air communication (e.g., through the air path created by: cavity 30, inner chamber 2, conduits 12 and second mouths 4, as schematically represented by the arrows in FIG. 3).

[0083] Exemplarily the shutter 5 has a conical conformation tapered towards the outside of the mould 100. The tapered conical conformation is advantageous for a simple closure of the first mouth 3.

[0084] The venting device 1 also comprises an actuator 6 (for example of pneumatic type) for alternately moving the shutter 5 between the open position and the close position. Exemplarily the actuator 6 is a cylinder that comprises a hollow chamber and a piston movable inside the hollow chamber due to the pressure exerted by a fluid (e.g., air, or water or oil) present inside the hollow chamber. The machine 100, as schematically shown in FIG. 1, comprises a control system of the actuator 6 comprising: two fittings 70 and 70 connected to a compressor 71 for example through a respective physical tube, schematically indicated by the lines P and P. In this way, it is possible to move the piston inside the hollow chamber at both ends of the hollow chamber, so as to alternately move the shutter 5 between the open and close positions.

[0085] Alternatively, the actuator 6 can be of the electric type, for example a linear electric actuator.

[0086] Exemplarily the inner chamber 2 is in air communication with the outer environment 35 also through a third mouth 8 exemplarily arranged on the opposite side of the inner chamber 2 with respect to the shutter 5. Exemplarily the machine 100 comprises a system for introducing air which comprises (as schematically shown in FIG. 1) a fitting 80 at the third mouth 8 and connected to a pump 81, for example by a physical tube schematically indicated by the line A.

[0087] Exemplarily the venting device 1 also comprises a control rod 7 which mechanically connects the shutter 5 to the actuator 6 and which exemplarily comprises an inner channel 9 which puts in fluid communication the third mouth 8 with a surface of the shutter 5 facing towards the inner chamber 2, and (through at least one opening 20) with the inner chamber 2 itself.

[0088] Exemplarily the venting device 100 comprises a respective main body 11 which defines the inner chamber 2. Exemplarily the venting device 1 also comprises a cooling conduit which passes through the respective main body 11. This cooling conduit 10 is not in air communication with the inner chamber 2.

[0089] The machine 100 exemplarily comprises an air-cooling system which exemplarily comprises (as schematically shown in FIG. 1) three fittings 90a, 90b and 90c and an outlet mouth 92. The fitting 90a is connected to a pump 91 through for example a physical tube, schematically indicated by the line R, and it is connected to the fitting 90b by the cooling conduit 10. The fitting 90b is in turn connected (for example by a U-shaped tube, schematically indicated by the line R) to the fitting 90c which is in turn connected to the outlet mouth 92, for example by a conduit (not shown) equal to the cooling conduit 10.

[0090] In the following, it is disclosed an example of a rotational moulding process implementable with the above-described mould 100.

[0091] Firstly, the process comprises the insertion of a polymeric material, for example polyethylene (alternatively or in addition, one or more of: polypropylene, polyamide and polycarbonate can be used), in fluid form (for example in powder form) in the inner cavity 30. Subsequently, the main body 99 is closed (e.g., the two half-moulds are fixed by suitable fastening means) and the mould 100 is rotated, for example about two (or three) rotation axes perpendicular to each other.

[0092] At this point, with the mould 100 in rotation, the main body 99 is heated up for example by an oven, or by electrical resistances arranged on the outer surface of the main body 99, or by a heat-transfer fluid which flows in suitable channels (not shown) made in the walls of the main body 99.

[0093] The heating of the main body 99 can be ideally divided into two subsequent phases, identifiable for the reaching of a determined condition, typically a predetermined temperature value. In these two subsequent phases, the shutter 5 of the venting device 1 is in the two positions shown with reference to FIGS. 2-3 and as described below. In the first heating phase, the main body 99 is heated until the reaching of a first temperature, exemplarily equal to 120? C. In this first heating phase, the shutter 5 is kept in the close position (shown in FIG. 2) in which it obstructs the first mouth 3, and it is provided to subtract heat from the shutter 5 to keep it at a respective temperature, which can have a drift but remains lower than the first temperature. For example, the respective temperature of the shutter 5 oscillates between 40? C. and 50? C. during the first heating phase of the main body 99.

[0094] Exemplarily the heat subtraction from the shutter 5 comprises generating an air flow (e.g., generated by the pump 81) which from the outer environment 35 reaches the surface of the shutter 5 facing the inner chamber 2 (i.e., the surface opposite to the inner cavity 30), and then returns to the outer environment 35. Exemplarily the air flow (schematically represented by the arrows in FIG. 2) follows the air path formed by: third mouth 8, inner channel 9, opening 20, inner chamber 2, conduits 12 and second mouths 4.

[0095] At the reaching of the first temperature (for example detectable by a suitable thermal probes), the second heating phase of the main body 99 starts until the reaching of a second temperature, exemplarily equal to 190? C. During this second phase, the aforesaid air flow is interrupted and the shutter 5 is moved to the open position (shown in FIG. 3) in which allows the air communication between the inner cavity 30 and the inner chamber 2 (as schematically shown by the arrows in FIG. 3).

[0096] Exemplarily, during the second heating phase of the main body 99, the respective main body 11 of the venting device 1 is cooled to be kept at a respective temperature lower than the first temperature (for example a temperature of about 60-70? C.). The respective main body 11 is exemplarily cooled by a further air flow (generated by the pump 91 shown in FIG. 1) which from the outer environment 35 (e.g., through the fitting 90a) passes through the cooling conduit 10 to return to the outer environment (e.g., through the air line formed by the fittings 90b, 90c, outlet mouth 92 and respective connection lines).

[0097] At the reaching of the second temperature, the cooling of the main body 99 begins (e.g., by fans external to the mould, not shown) during which the shutter 5 is kept in the open position and preferably the further air flow is kept active to cool the respective main body 11 of the venting device 1. After the cooling is complete, it is possible to extract the formed article 300.

[0098] In one embodiment shown with reference to FIG. 5, the mould 100 comprises a further venting device 101 fixed to the main body 99.

[0099] Exemplarily the further venting device 101 has all the features described above for the venting device 1, exemplarily the venting devices 1 and 101 are equal.

[0100] In this embodiment, the rotational moulding process also comprises to operate also the further venting device 101 in a completely similar way to what is described above for the venting device 1 during the two heating phases of the main body 99.

[0101] During the cooling of the main body 99, the shutter of both the venting devices 1 and 101 is kept in the open position, and a still further air flow (schematically shown by the arrows in FIG. 5) is generated (e.g., by a pump 95) which from the outer environment goes in the inner cavity 30 through the venting device 1, and then returns to the outer environment through the further venting device 101. In one embodiment, the pump 96 is able to generate both an overpressure for generating the air flow useful for subtracting heat from the shutter of the venting device 101 during the first heating phase of the main body 99 (see above), and a depression to favour the suction of the air from the inner cavity 30 during the cooling of the main body 99.

[0102] Exemplarily the still further air flow follows the following air path: from the pump 95, it reaches (e.g., through a tube) the third mouth of the venting device 1, then, through the inner chamber of the venting device 1, it reaches the inner cavity 30, and finally, through the inner chamber and the third mouth of the venting device 101, it reaches the pump 96 (e.g., through a suitable tube).

[0103] Alternatively, the still further air flow can flow outside through the second mouths of the venting device 101, instead of through the third mouth of the venting device 101. Exemplarily, during the cooling of the main body 99, the air flow used to cool the respective main body of the venting device 1 is interrupted, and only the air flow used to cool the further main body of the venting device 101 is kept active.