FR3128661A1 - HEATING ELEMENT FOR A PLASTIC MATERIAL INJECTION NOZZLE AND NOZZLE PROVIDED WITH SUCH A HEATING ELEMENT - Google Patents

Abstract

The invention relates to a heating element (1) for a nozzle (2) for injecting plastic material, adapted to be wound around said injection nozzle, comprising a first heating resistor (11) arranged in a first region (R1 ) intended to be located around a front zone (Z1) of the nozzle and a second heating resistor (12) arranged in a second region (R2) intended to be located around a rear zone (Z2) of the nozzle, separate from the front zone (Z1). The invention also relates to a nozzle (2) for injecting plastic material, comprising: - a body (20) defining a channel for the passage of a plastic material in the molten state, and - said heating element (1) wrapped around the body (20) such that the first region (R1) of the heating element surrounds the front zone (Z1) of the nozzle and the second region (R2) of the heating element surrounds the rear zone ( Z2) of the nozzle. Figure for abstract: Fig 6

Description

HEATING ELEMENT FOR A PLASTIC MATERIAL INJECTION NOZZLE AND NOZZLE PROVIDED WITH SUCH A HEATING ELEMENT

The invention relates to a heating element for a plastic material injection nozzle, as well as an injection nozzle provided with such a heating element.

STATE OF THE ART

Hot runner injection molding is a popular technique for molding plastic material in different applications. In this type of molding, the feed channel of the plastic material in the molten state in the molding cavity is heated to a temperature higher than the melting temperature of the plastic material.

For this purpose, said channel is arranged in an injection nozzle provided with heating means.

However, there are thermal losses along the path of the plastic material in the molten state between the inlet and the outlet of the nozzle. In particular, in the rear area of the nozzle, which includes the inlet, mainly heat losses by convection occur. In the front area of the nozzle, which includes the outlet orifice, mainly conductive heat losses occur between the plastic and the nozzle material.

These thermal losses are responsible for a decrease in the temperature of the plastic material in the molten state between the inlet orifice and the outlet orifice of the nozzle.

To homogenize the temperature along the nozzle, it is known to use heating collars, which are mounted around the nozzle. These collars are in the form of tubes with an axial slot and comprising, on either side of the slot, a screw fixing flange. In the open position, thanks to the slot, the collar is wide enough to be fitted onto the nozzle; the slot is then closed by screwing the flange in order to apply the collar in intimate contact with the nozzle. The collar incorporates a heating resistor which, when powered by an electric current, is heated by the Joule effect and allows the nozzle to be heated by conduction.

However, in particular for nozzles of small dimensions, for example having a diameter of less than 20 mm, such collars are difficult to handle and to apply in intimate contact with the nozzle, due to the stiffness of the material. This results in uneven heating of the nozzle.

Moreover, such collars increase the size of the nozzle, which penalizes its installation in a plastic injection mold.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to design a heating element for a plastic material injection nozzle which makes it possible to heat more homogeneously the plastic material in the molten state which circulates inside the channel of the nozzle.

To this end, the invention proposes a heating element for a plastic material injection nozzle, adapted to be wound around said injection nozzle, comprising a first heating resistor arranged in a first region intended to be located around a front zone of the nozzle and a second heating resistor arranged in a second region intended to be located around a rear zone of the nozzle, separate from the front zone.

The size and heat output of each heating resistor can be adjusted independently of each other to provide even heating of the plastic material within the nozzle.

In a particularly advantageous manner, each heating resistor is connected to two respective electrically conductive wires, and the heating element further comprises a sealed termination located on the side of the second heating resistor opposite to the first heating resistor and maintaining said electrically conductive wires.

Preferably, the heating element further comprises a sheath in which the first and the second heating resistors are arranged.

In certain embodiments, said sheath is made of stainless steel.

Each heating resistor can be wound in the form of a plurality of turns.

In certain embodiments, each heating resistor has two parallel sets of turns, the first heating resistor extending in an inclined plane with respect to the second heating resistor.

In other embodiments, the turns of the first and of the second heating resistor are coaxial.

Another aspect of the invention relates to a plastic material injection nozzle comprising:

- a body defining a channel for the passage of a plastic material in the molten state, between a rear zone comprising an inlet of the body and a front zone comprising an outlet of the body adapted to open into a cavity of molding a plastic injection mold, and

- a heating element as described above, wrapped around the body such that the first region of the heating element surrounds the front zone of the nozzle and the second region of the heating element surrounds the rear zone of the nozzle.

Particularly advantageously, the heating element is wound with a smaller pitch in the front area than in the rear area.

Particularly preferably, the nozzle comprises a peripheral groove arranged in an outer wall of the body, and the heating element is embedded in said groove.

The groove is advantageously arranged in the wall of the body in a helical shape, the pitch of which may be different between the front zone and the rear zone of the nozzle.

In certain embodiments, the nozzle also comprises two temperature sensors arranged in the body, respectively in the front zone and in the rear zone of the nozzle.

According to particularly advantageous applications, the body of the nozzle has an outside diameter of less than 20 mm.

Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which:

- there is an overall view with a partial section of the heating element according to the invention;

- there is a perspective view of the arrangement of the two heating resistors according to a first embodiment of the invention;

- there is a perspective view of the arrangement of the two heating resistors according to a second embodiment of the invention;

- there is an overall view of the heating element once assembled on the nozzle (the nozzle not being shown);

- there is a view of the nozzle before the installation of the heating element;

- there is a nozzle view of the equipped with the heating element and temperature sensors.

DETAILED DESCRIPTION OF EMBODIMENTS

In a manner known per se, an injection nozzle comprises a body defining a channel for the passage of a plastic material in the molten state. Said channel extends between an inlet orifice and an outlet orifice of the body, said outlet orifice being adapted to open into a molding cavity of a plastic injection mold. The outlet orifice is located in a zone called the front zone of the nozzle, while the inlet orifice is located in a rear zone of the nozzle, the front and the rear being defined with respect to the footprint molding, in a direction opposite to the direction of flow of the plastic material.

The invention proposes a heating element adapted to be wound around an injection nozzle.

Said heating element comprises two separate heating resistors, each powered electrically by two respective electrically conductive wires. Each heating resistor can for example be formed from a wire of a nickel-chromium alloy, typically with 80% nickel and 20% chromium. Power wires can be made of nickel and coated with an electrically insulating layer, such as polytetrafluoroethylene (PTFE).

Said heating resistors can advantageously have different calorific powers, in order to best adjust the temperature of the plastic material flowing in the channel of the nozzle.

Said heating resistors are arranged in separate regions of the heating element, so as to avoid or compensate in a localized manner the thermal losses of the plastic material circulating in the nozzle between the rear zone and the front zone.

More specifically, when the heating element is wrapped around the nozzle, a first heating resistor is arranged around the front zone of the nozzle and a second heating resistor is arranged around the rear zone of the nozzle.

Insofar as the plastic material arriving in the front zone of the nozzle has undergone more thermal losses than in the rear zone, the calorific power of the first heating resistor can be greater than that of the second heating resistor. However, in other embodiments, the heat output of the first resistance heater may be lower than or the same as that of the second resistance heater.

Furthermore, in addition to or as an alternative to this configuration of the calorific powers of the heating resistors, the heating element can be wound with a smaller pitch in the front zone of the nozzle than in the rear zone. For example, the heating element can be wound by forming very close turns, even contiguous, in the front zone of the nozzle, and by forming more distant turns in the rear zone of the nozzle.

The temperature of the plastic material flowing through the nozzle can be measured using one or more temperature sensors, such as thermocouples. Preferably, at least one temperature sensor is used arranged to measure the temperature of the nozzle or of the plastic material in the front zone of the nozzle, and one temperature sensor arranged to measure the temperature of the nozzle or of the plastic material in the rear area of the nozzle.

The temperature sensors are connected to a control unit of an injection system. The data provided by said sensors can be used by the control unit to adjust the calorific power of each of the two heating resistors in order to obtain the desired temperature for the plastic material in the rear zone and in the front zone of the nozzle.

To facilitate handling of the heating element, the heating resistors are arranged inside a sheath, in two different regions of the sheath: the first heating resistor is arranged in a first region of the sheath intended to be placed around of the front zone of the nozzle, and the second heating resistor is arranged in a second region, separate (or remote) from the first region, intended to be placed around the rear zone of the nozzle. The power supply wires for the two heating resistors are also arranged inside the sheath. The sheath has a front end (on the side of the first heating resistance) which is hermetically closed, and an open rear end, through which the power supply wires of the heating resistances emerge.

Thus, the heating element comprises two heating regions which appear arranged "in series" insofar as they extend successively in the direction of the length of the heating element, but whose heating resistors are wired in parallel thanks to separate power leads.

The sheath may have a rectangular or round section or any other appropriate shape.

The sheath can be made of stainless steel or any other material suitable for conducting heat.

To facilitate the installation and maintenance of the heating element around the nozzle, the outer wall of the nozzle is advantageously provided with a groove adapted to receive the sheath in intimate contact with the body of the nozzle. Such a groove can in particular be obtained by machining the body of the nozzle. The dimensions of the groove are preferably chosen to allow embedding of the sheath in the groove, without requiring additional fastening means.

Thus, a single groove allows the placement of the two heating regions, which simplifies the design of the nozzle and in particular the machining operations necessary for the placement of the heating element.

Additionally, the heating element is flush with the outer surface of the nozzle, so it does not increase the overall footprint of the nozzle.

However, other methods of fixing the heating element around the nozzle remain possible, including in the absence of the aforementioned groove, for example by gluing or any other suitable means.

Although two heating resistors are described in this text, it goes without saying that three or more heating resistors could be arranged according to the principle described above, but such a more complex arrangement could be done to the detriment of the compactness of the element heating.

There illustrates one embodiment of such a heating element.

The heating element 1 comprises a sheath 10 inside which are arranged a first heating resistor 11, located in a first region R1 of the heating element, and a second heating resistor (not visible on the but shown in Figures 2 and 3), located in a second region R2 of the heating element. The regions R1 and R2 are distinct from each other, that is to say that the heating resistors do not overlap. The R1 and R2 regions can be contiguous or distant from each other.

On the , the heating element is shown straight, but it is then intended to be wound around an injection nozzle, as will be described in detail below (see in particular the ).

The length of the first region R1 and the second region R2 is chosen so that, when the heating element is wrapped, the first region R1 is located around the front area of the nozzle and the second region R2 is wrapped around the rear area of the nozzle.

The supply wires 11a, 11b, 12a, 12b of each heating resistor extend into the sheath 10 from which they emerge at one end located on the side of the second region R2, in a sealed termination 13.

Figures 2 and 3 illustrate two possible but non-limiting conformations of heating resistors (the sheath has not been shown to facilitate the visualization of these conformations).

In the embodiment illustrated in the , the first heating resistor 11 is in the form of two windings 111, 112 of a resistive wire, for example of an 80/20 nickel-chromium alloy as mentioned above. Said windings 111, 112 extend parallel to each other, along two axes which define a first plane.

The second heating resistor 12 is also in the form of two windings 121, 122 of a resistive wire, for example of an 80/20 nickel-chromium alloy. Said windings 121, 122 extend parallel to each other, along two axes which define a second plane.

Preferably, the second plane is inclined with respect to the first plane, for example by an angle of 90°. This arrangement makes it possible to distribute the supply wires 11a, 11b, 12a, 12b evenly within the sheath.

In the embodiment illustrated in the , the first heating resistor 11 is in the form of a single winding of a resistive wire, for example of an 80/20 nickel-chromium alloy as mentioned above.

The second heating resistor 12 is also in the form of a single winding of a resistive wire, for example of an 80/20 nickel-chromium alloy as mentioned above.

Preferably, these two windings are coaxial, which makes it possible to promote the compactness of the assembly of the two heating resistors within the sheath.

There is an overview of the heating element in an assembly configuration on the nozzle (the nozzle has not been shown to facilitate the visualization of this configuration).

The first region R1 of the heating element 1 is wound according to a first pitch d1. In this first region, the coils of the heating element are contiguous or separated by a small interval. The distance between the first and the last turn of the winding in region R1 is chosen to be substantially equal to the length of the front zone of the nozzle.

The second region R2 of the heating element 1 is wound according to a second pitch d2, greater than the first pitch d1. In this second region, the coils of the heating element are therefore separated by a greater interval than in the first region. The distance between the first and the last turn of the winding in region R2 is chosen to be substantially equal to the length of the rear zone of the nozzle.

The power supply wires of the two heating resistors are inserted into a sealed termination 13, the ends of said wires coming out of said sealed termination 13 to be able to be connected to a source of electric current (not shown).

There illustrates an embodiment of an injection nozzle adapted to receive the heating element of the .

The body 20 of the nozzle 2 comprises a channel extending between an inlet 202 and an outlet 201. The direction of circulation of the plastic material in the molten state inside the channel is represented by the arrow. The front area of the nozzle is designated Z1 while the rear area is designated Z2.

The outer wall of the body 20 comprises a groove 21 having a helical shape adapted to receive the heating element in the conformation illustrated in the .

The body 20 also advantageously comprises two orifices 30, 40 each adapted to receive a temperature sensor intended to measure the temperature of the plastic material in the molten state or of the nozzle itself, respectively in the front zone and in the rear area.

There illustrates the heating element 1 of the assembled on nozzle 2 of the , as well as two temperature sensors, 3, 4, for example thermocouples, arranged in the orifices 30, 40.

In a particularly advantageous manner, the heating element 1 is flush with the outer surface of the body 2, so that it in no way affects the overall size of the nozzle.

In addition, the embedding of the heating element in the groove makes it possible to ensure intimate contact with the body, whatever the diameter of the body, which makes it possible to minimize heat dissipation at the interface between these two components.

Claims (13)

Heating element (1) for a nozzle (2) for injecting plastic material, adapted to be wound around said injection nozzle, comprising a first heating resistor (11) arranged in a first region (R1) intended to be located around a front zone (Z1) of the nozzle and a second heating resistor (12) arranged in a second region (R2) intended to be located around a rear zone (Z2) of the nozzle, distinct from the front zone (Z1). Heating element according to Claim 1, in which each heating resistance (11, 12) is connected to two respective electrically conductive wires (11a, 11b; 12a, 12b), the heating element further comprising a sealed termination (13) located side of the second heating resistor opposite to the first heating resistor and maintaining said electrically conductive wires. Heating element according to one of Claims 1 to 2, further comprising a sheath (10) in which the first and the second heating resistors are arranged. A heating element according to claim 3, wherein the sheath (10) is of stainless steel. Heating element according to one of Claims 1 to 4, in which each heating resistor (11, 12) is wound in the form of a plurality of turns. Heating element according to Claim 5, in which each heating resistor (11, 12) has two parallel sets of turns (111, 112; 121, 122), the first heating resistor extending in an inclined plane with respect to the second resistor heating. Heating element according to Claim 5, in which the turns of the first and of the second heating resistance (11, 12) are coaxial. Nozzle (2) for injection of plastic material, comprising:
- a body (20) defining a channel for the passage of a plastic material in the molten state, between a rear zone comprising an inlet orifice (202) of the body and a front zone comprising an outlet orifice (201) of the body adapted to open into a molding cavity of a plastic material injection mold, and
- a heating element (1) according to one of claims 1 to 7 wound around the body (20) such that the first region (R1) of the heating element surrounds the front zone (Z1) of the nozzle and that the second region (R2) of the heating element surrounds the rear zone (Z2) of the nozzle. Injection nozzle according to Claim 8, in which the heating element (1) is wound with a smaller pitch in the front region (d1) than in the rear region (d2). Injection nozzle according to one of Claims 8 to 9, comprising a peripheral groove (21) arranged in an outer wall of the body (20), the heating element (1) being embedded in the said groove (21). Injection nozzle according to Claim 10, in which the groove (21) is arranged in the wall of the body in a helical shape. Injection nozzle according to one of Claims 8 to 11, further comprising two temperature sensors (3, 4) arranged in the body (20), respectively in the front zone (Z1) and in the rear zone (Z2) of the nozzle. Injection nozzle according to one of Claims 8 to 12, in which the body has an outside diameter of less than 20 mm.