IT201700006608A1 - INSULATED SCREW PLASTICIZATION GROUP - Google Patents

Description

DESCRIPTION

The subject of the present invention is a screw plasticizing assembly equipped with an insulation system.

A screw plasticizing group usually consists of a cylinder, a screw that can be rotated inside the cylinder, heating means, typically electrical resistances distributed along the cylinder, and sensors for measuring the cylinder temperature. The heating means are activated so that the molten plastic material is produced inside the cylinder, which, by the action of the screw, is then delivered out of the cylinder, towards an injection molding machine (press) or towards a machine of extrusion.

The resistances are fed in such a way that a predetermined temperature value (set-point temperature) is reached in the relative section of the cylinder, so as to provide the molten plastic mass in the optimal temperature conditions for subsequent processing.

Such plasticizing groups have the drawback of using large amounts of electricity to heat the resistances.

The object of the present invention is to provide a plasticizing unit equipped with an insulation system capable of overcoming these drawbacks. This object is achieved by a plasticizing unit in accordance with claim 1. The dependent claims identify further embodiments of the invention.

The characteristics and advantages of the plasticizing unit according to the present invention will be evident from the description given below, given by way of non-limiting example, in accordance with the attached tables, in which:

Figure 1 shows a schematic view of a plasticizing assembly according to the present invention, according to an embodiment;

- figure 2 represents a graph showing an example of interpolation of cylinder set-point temperature values and detected temperatures.

In accordance with the attached tables, a plasticizing unit 1 is suitable for transforming a plastic material in a solid state, usually in granules, into a molten plastic mass and for its delivery to a machine, typically an injection machine (press ) or an extrusion machine.

The plasticizing unit 1 comprises a cylinder 2 having extension along a main axis Z, between a rear area 4 and a front area 6.

The cylinder 2 consists of an annular side wall 2a having a predetermined radial dimension, which defines its thickness.

The rear area 4 has an inlet opening 8 for the introduction of the solid plastic material, at room temperature or preheated, usually in the form of granules, through a loading hopper 10. In the front area 6, however, the cylinder 2 has a front opening 12.

According to an embodiment (not shown), the side wall 2a of the cylinder 2 has a variable thickness, preferably increasing from the rear area 4 towards the front area 6.

For example, the thickness is increasing while remaining constant at intervals along the cylinder, or increasing continuously.

The plasticizing unit also includes a head 14 applied to the front opening 12 of the cylinder 2 and, preferably, a nozzle 15, applied frontally to the head 14, for engagement with the processing machine.

The head 14 can be disassembled from the cylinder 6 and the nozzle 15 can be disassembled from the head 14, so as to be replaced according to the needs dictated by the processing machine.

The plasticising unit 1 also comprises an extrusion screw 20, inserted inside the cylinder 2 and arranged along said main axis Z. The screw 20 is provided with a single-start or multi-start thread (called "barrier screw") . These constructive characteristics have the purpose of improving the melting process of the plastic material.

The screw 20 has, in correspondence with the rear area 4 of the cylinder 2, a connection section 24 for the operational connection with handling means (not shown) which allow it to rotate around the main axis Z.

According to an embodiment, said movement means also allow the translation of the screw 20 back and forth along said main axis Z.

In particular, in the application of the plasticizing unit 1 to an injection machine, during normal operation the screw 20 is initially arranged in an advanced limit position and the handling means maintain a rotation while the solid plastic material is fed through the opening. 8 of the rear area 4.

As the plastic material advances along the main axis Z towards the front area 6, pushed by the thread 22 of the screw 20, it undergoes heating and passes to the state of molten plastic mass.

At the same time, the handling means move the screw back, so that the molten plastic mass accumulates in the front area 6 of the cylinder 2, near the front opening 12 of this.

When the molten plastic mass reaches a predetermined volume, corresponding to a predetermined retraction of the screw 20 to a retracted limit position, the handling means rapidly advance the screw 20 towards the advanced limit position, so that the molten plastic mass is pushed out through the front opening 12.

In the application of the plasticizing unit 1 to an extrusion machine, the handling means allow only the rotation of the screw 20 and the molten plastic mass is delivered continuously externally, pushed by the thread of the screw.

In any case, the molten plastic mass passes through the head 14 and the nozzle 15 of the plasticizing unit 1 to be delivered to the machine.

To provide for the heating of the plastic material, the plasticizing unit 1 comprises heating means 30 comprising a plurality of electrical resistances distributed axially along the cylinder 2.

For example, said resistors are arranged on the external surface of the side wall 2a of the cylinder 2 or inserted in said side wall. Furthermore, the plasticizing unit 1 comprises a unit management system 40 comprising a plurality of cylinder temperature sensors 42i axially distributed along the cylinder 2.

For example, each cylinder temperature sensor 42i (i = 1… n) is inserted in the side wall 2a of cylinder 2.

For example, for the sake of clarity, it is assumed that cylinder 2 is axially divided into imaginary sections Si (i = 1 ... n), in which the first section S1 is close to the anterior zone 6 and the last section Sn is close to the rear area 4.

In this example, the resistance Ri (i = 1… n) operates for each section and the cylinder temperature Ti is detected by the cylinder temperature sensor 42i. Furthermore, for each section Si of the cylinder, it is desired to reach the cylinder set-point temperature T <*> i.

Furthermore, preferably, the group management system 40 comprises at least one melt temperature sensor 44 applied to the head 14 or, according to a further embodiment (not shown), to the nozzle 15.

For example, said melt temperature sensor 44 passes through the side wall of the head 14 and emerges in the central duct from which the outgoing molten plastic mass passes, so as to directly measure the melt temperature Tmf of the melted plastic mass. For the outgoing melt, a melt set point temperature T <*> mf is desired.

The diagram in Figure 2 shows an example of the detected cylinder temperatures Ti and cylinder setpoint temperatures T <*> i.

Preferably, the group management system 40 further comprises an electronic processing device 50, for example an electronic control unit, a PLC or a microprocessor.

Said processing device 50 comprises a temperature processing device 52, operatively connected with the melt temperature sensor 44, preferably with the cylinder temperature sensors 42i, and, directly or indirectly, with the heating means (electrical resistances Ri) of the group lamination 1.

The temperature processing device 52 is configured, for example programmed by means of software, to process the signal coming from the cylinder temperature sensors 42i and from the molten mass temperature sensor 44 and adequately command the heating means, according to a control logic, so that the melt temperature Tmf is equal to the melt temperature set-point T <*> mf or contained in a predefined environment around it.

For example, according to a control logic, the cylinder set-point temperature values T <*> i are interpolated by means of a predefined interpolation function F. For example, the interpolation function F is a polynomial of degree n + 1.

The detected melt temperature value Tmf is fed back and subtracted from the desired melt temperature set-point T <*> mf and the cylinder temperature values Ti detected are fed back and subtracted from the respective cylinder set-point temperature values T <* > i.

The differences T <*> mf-Tmf and T <*> i-Ti are processed by a regulator device 52 'of the temperature processing device 52, for example a regulator device type PID (proportional, integral, derivative), which moves towards curve F up or down in a controlled manner, so that, for each section Si of cylinder 2, new set-point values are calculated for the cylinder set-point temperatures and consequently the heating means, i.e. the respective resistors Ri.

The procedure is repeated until the melt temperature difference T <*> mf-Tmf is zeroed or until a difference less than a predefined value is obtained.

Furthermore, according to a further control logic, function F is modified over time, for example when the factors that influence the process intervene only punctually, bringing only one or only some of the cylinder temperatures Ti detected beyond the established environment.

In this case, the control logic, through the regulator device 52 ', varies the form of the function, setting a new F1 function.

The plasticizing unit 1 according to the invention is usually inserted in an apparatus (not shown), which includes an injection machine or an extrusion machine.

The machine is equipped with a machine management system 60 and, according to an embodiment of the invention, the group management system 40 is indirectly connected to the heating means (resistances Ri) through the machine management system 60.

According to the invention, the plasticization unit 1 comprises insulation means 100 applied to the cylinder 2 to limit heat dispersion.

Said insulating means comprise an annular cover 102, axially continuous or divided into portions, arranged around the side wall 2a of the cylinder 2 (and around the heating means).

Said cover 102 comprises for example a plurality of insulating elements, applicable to the cylinder, so as to wrap the resistances, for example by means of a belt, for example with Velcro closure, or by means of spring hooks.

According to a preferred embodiment, said insulation elements comprise a fabric core impregnated with a polymer, preferably a synthetic fluoropolymer, for example Teflon®, and an insulation jacket, for example made of ceramic fibers.

According to a further embodiment, the group management system 40 comprises a pressure sensor 70 operatively connected to a pressure processing device 54 of the processing device 50, directly or indirectly connected to the means for moving the screw to control them.

In particular, said embodiment finds application in the case in which the plasticizing unit is applied to an injection machine and the handling means are therefore suitable for translating the screw along the main axis Z.

In normal operation, the advancement translation of the screw from the retracted limit position to the advanced limit position occurs in two phases:

- in a first phase or filling phase, the handling means impose a practically constant speed of advancement on the screw; during this phase, the molten material comes out of the nozzle and fills the cavity of the mold of the press;

- in a second step or compression step, the handling means impose a predefined thrust on the screw so as to compress the molten plastic mass in the cavity; in this phase, the advancement speed is practically zero, while, by virtue of the thrust exerted by the screw, the molten mass fills the cavity and becomes compact.

According to an embodiment of the invention, the pressure sensor 70 detects the pressure in the head 14 or in the nozzle 15 and transmits the detected values to the pressure processing device 54 which controls the actuation of the movement means so as to pass from the filling phase to the compression phase when the detected pressure exceeds a predefined threshold value p <*>.

Innovatively, the plasticizing unit according to the present invention overcomes the drawbacks mentioned with reference to the known art, by virtue of the aforementioned insulation means.

Furthermore, advantageously, the feedback of the melt set-point temperature allows to obtain an optimal melt, even in particular environmental conditions or in the presence of worn parts.

According to a further advantageous aspect, the cylinder structure is optimized in order to limit thermal inertia.

According to an even further advantageous aspect, the plasticizing unit with pressure sensor allows to overcome some drawbacks of the known art such as excess burrs in the molded article or incomplete filling of the mold cavity.

It is clear that a person skilled in the art, in order to meet contingent needs, could make changes to the plasticizing unit described above, all contained within the scope of protection as defined by the following claims.

Claims (16)

CLAIMS 1. Plasticization unit (1) for plastic materials which can be associated with an injection machine or an extrusion machine, comprising: - a cylinder (2) extending along a main axis (Z) between a rear area (4), which has an inlet opening (8) for feeding solid plastic material, and a front area (6), provided with a front opening (12); - an extrusion screw (20) inserted in the cylinder (2) along the main axis (Z), with one or more starts, which can be operated in rotation; - electric heating means comprising electric resistances (Ri) engaged with the cylinder (2); - insulation means (100) applied to the cylinder (2), around the electrical resistances (Ri), to limit thermal dispersion. 2. Plasticizing assembly according to claim 1, wherein the insulation means comprise an annular cover (102), axially continuous or divided into portions, arranged around the side wall (2a) of the cylinder (2), surrounding said electrical resistances ( Re). 3. Plasticizing assembly according to claim 2, wherein the cover (102) comprises a plurality of insulating elements, applicable to the cylinder (2), so as to wrap the resistors. 4. Plasticizing assembly according to claim 3, in which said insulating elements are applied to the cylinder (2) by means of a belt, for example with Velcro closure, or by means of spring hooks. 5. Plasticization group according to claim 3 or 4, in which said insulating elements comprise a fabric core impregnated with a polymer, preferably a synthetic fluoropolymer, for example Teflon®. 6. Plasticization group according to any one of claims 3 to 5, in which said insulation elements include a core and an insulation jacket, for example made of ceramic fibers, which wraps the core. A plasticizing unit according to any one of the preceding claims, further comprising: - a head (14) removably applied to the front opening (12) of the cylinder (2) and, preferably, a nozzle (15) applied in a removable manner to the head (14); - a group management system (40) comprising: i) a plurality of cylinder temperature sensors (42i) axially distributed along the cylinder (2) from the rear region (4) to the front region (6) to detect the cylinder temperature (Ti) of sections (Si) of the cylinder (2); ii) a melt temperature sensor (44) applied to the head (14) or to the nozzle (15) to detect the temperature of the molten plastic mass intended for dispensing; iii) an electronic temperature processing device (52) operatively connected with the cylinder temperature sensors (42i), with the melt temperature sensor (Tmf) and, directly or indirectly, with said heating means, to control said heating means in to obtain a desired melt temperature set-point (T <*> mf) for the plastic melt. 8. Plasticising unit according to claim 7, in which the temperature processing device (52) is configured so as to implement a predefined control logic, according to which, defined the cylinder set-point temperature values (T <*> i) desired for each section (Si): - the cylinder set-point temperature values (T <*> i) are interpolated by means of a predefined interpolation function (F); - the detected melt temperature value (Tmf) is fed back and subtracted from the desired melt temperature set-point (T <*> mf) and the cylinder temperature values (Ti) detected are fed back and subtracted from the respective temperature values cylinder set-point (T <*> i); - the differences (T <*> mf-Tmf; T <*> i-Ti) are processed by a regulating device (52 ') of the temperature processing device (52) to move up or down so controlled curve (F), so that, for each section (Si) of the cylinder (2), new set-point values are calculated for the cylinder set-point temperatures and consequently the heating means are activated. 9. Plasticizing unit according to claim 8, wherein, according to said control logic - the procedure is repeated until the difference is zeroed (T <*> mf-Tmf) or until a difference less than a predefined value is obtained. 10. Plasticising group according to claim 8 or 9, in which the regulator device (52 ') is of the PID type. 11. Plasticizing unit according to any one of claims 8 to 10, in which, according to said control logic, the shape of the function (F) is modified over time on the basis of the feedback of the detected cylinder temperatures. 12. Plastification unit according to any one of the preceding claims, in which the cylinder (2) consists of an annular side wall (2a) having a predefined radial dimension which defines its thickness and said thickness is variable, for example increasing from the area rear (4) towards the front (6). 13. Plastification unit according to claim 12, in which the thickness is increasing while remaining constant at intervals along the cylinder, or increasing continuously. Plastification unit according to any one of the preceding claims, wherein said management system (40) comprises a pressure sensor (70) applied to the head (14) or to the nozzle (15), operatively connected to an electronic device of pressure processing (54). 15. Plasticizing unit according to any one of the preceding claims, comprising movement means suitable for translating the screw back and forth along said main axis (Z). 16. Plasticizing unit according to claims 14 and 15, in which the pressure processing device (54) is operatively connected to the handling means to control, during the advancement of the screw, the passage from a filling phase to a filling phase. compression, when a predefined pressure threshold value (p <*>) is exceeded.