Method and apparatus for injection molding of plastic materials

Abstract

Method for injecting plastic material into a cavity by means of a molding device, the molding device comprising at least one injector, the injector comprising a needle valve displaceable in a controllable manner with respect to its position and speed between a fully closed position and a maximum open position, and vice versa. In the opening displacement from the fully closed position to the maximum open position, the needle valve initially moves at a first speed and subsequently moves at a second speed: the first speed being the maximum opening displacement speed of the needle valve.

Description

[0001] This application is a divisional application of the Chinese patent application with the application date of December 11, 2015 (the priority date is December 11, 2014), the invention name of which is “Method and apparatus for injection molding of plastic materials” and the application number is 201511036296.3. Technical Field

[0002] The present invention relates to injection molding of plastic materials and more particularly to a device for injection molding under pressure by means of a dispenser containing fluid plastic material, the dispenser being connected to at least one syringe comprising a needle valve displaceable between a completely closed position and a maximum open position, and vice versa. Background Art

[0003] Typically, these injection molding methods include a step of filling the cavity with plastic material, followed by moving the needle valve from a closed position to a maximum open position, followed by a step of compressing the plastic material injected into the cavity under pressure, wherein the needle valve is held in the maximum open position. The needle valve is then moved from the maximum open position to a fully closed position, and the molded detail is removed from the mold after a waiting period, allowing the plastic material to solidify.

[0004] The displacement of the needle valve or each syringe is usually performed by a fluid actuator. Recently it has been proposed that the needle valve is operated in a controlled manner by rotating an electronic actuator via an electronic system.

[0005] The use of electronic actuators to operate the needle valves of each injector in a molding device is described, for example, in JP-06114887, US Pat. No. 7,121,820, and EP-2679374 (on behalf of the applicant). Compared to fluidic actuators, electronic actuators are easier to control, using electronic systems that operate according to process parameters, detected by specific sensors and / or specific algorithms. This allows for effective control of the position of the needle valve between a closed position and an open position, thereby varying the flow of injected plastic material during the molding cycle, as described, for example, in US Pat. No. 6,294,122, and varying the displacement speed of the needle valve during its displacement from the closed position to the open position. Thus, WO-2012 / 074879 and WO-2012 / 087491 disclose controlling an electronic actuator to continuously move the needle valve from the closed position to the open position, initially at an initial speed and subsequently at one or more speeds exceeding the initial speed. Such control is performed over time or over the space covered by the needle valve from its closed position.

[0006] This type of control is difficult for real process conditions, ie a series of widely varying parameters, such as the pressure of the plastic material fed to the injector and the operating conditions and physical state of the plastic material.

[0007] In particular, injection pressure in the distributor is greatest just before the needle valve, and thus the mold's injection gate, opens. Under these conditions, the significant displacement required to open the needle valve can cause surface defects in the molded part near the gate due to the rapid and concentrated release of injection pressure. These aesthetic defects, corresponding to the flow lines of the injected plastic material, can sometimes be noticeable and unacceptable.

[0008] Other non-negligible aesthetic defects may occur due to possible leakage of plastic material towards the gate at the end of the needle valve closing step.

[0009] Therefore, more precise control is particularly desirable for injection molding, which requires high-quality details, both from a mechanical and aesthetic perspective. Summary of the Invention

[0010] The purpose of the present invention is to provide an effective technical solution to the aforementioned technical problems by controlling the injection process in a manner that allows limiting or completely eliminating the defects of the molded parts as described above, by acting on the position and speed of the needle valve of each molding device injector through a new and novel control of the various electronic actuators throughout the opening and closing cycle.

[0011] With the aim of achieving this object, the present invention aims to provide an injection molding method of the type defined in the introductory part, wherein, during the opening displacement from the completely closed position to the maximum open position, the needle valve is initially moved at a first speed and subsequently at a second speed, its main feature being the fact that the first speed is higher than the at least one second speed.

[0012] Due to this technical solution concept, a partial vacuum of the molding device distributor occurs at the beginning of the injection cycle, as a result of which the surface quality of the molded part can be significantly improved.

[0013] The first speed is conveniently the highest opening displacement speed of the needle valve, at which it moves from the fully closed position to the partially open position, wherein the needle valve is temporarily stopped for a determined time, so that its displacement towards the maximum open position is discontinuous.

[0014] According to another aspect, the method according to the invention is further characterized in that the needle valve, when displaced from the maximum open position toward the fully closed position, is temporarily stopped for a determined time period in a partially closed intermediate position, whereby its displacement toward the fully closed position is discontinuous.

[0015] Another unique feature of the present invention lies in the fact that at the end of said determined stop time in the partially closed intermediate position, a torque predetermined as a function of process parameters is applied to the electronic actuator in order to displace the needle valve from the intermediate partially closed position to the fully closed position.

[0016] This feature allows obtaining a “forced closure” of the gate at the end of the injection cycle, which allows ensuring the elimination of leaks of plastic material and thus further advantageously affects the aesthetic quality of the molded part.

[0017] The invention also relates to a device for carrying out the injection molding method. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will now be described in detail, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0019] Figure 1 To show a partial schematic and axial sectional view of a part of an injection molding device shown in a first step of the method according to the invention,

[0020] Figures 2 to 8 To illustrate the other steps of the method according to the present invention Figure 1 Similar diagrams,

[0021] Figure 9 To express that according to Figures 1 to 8 is a graphical representation of the position of the needle valve as a function of time during the steps of the method of the invention shown in, and

[0022] Figure 10 To show that Figure 9 Graph of needle valve velocity as a function of time relative to needle valve position shown in FIG. DETAILED DESCRIPTION

[0023] In a conventional manner and on behalf of the applicant as described in the document EP-2679374 mentioned above, Figure 1 Schematically shows part of an apparatus for injection molding plastic material, wherein an element or hot chamber for distributing molten plastic material supplies the plastic material to one or more injectors, each of which comprises a nozzle generally indicated at 1, which communicates with a cavity 2 of a mold 3 via an injection gate 4. The flow of the plastic material through the gate 4 is controlled by a needle valve 5 which can be moved along the nozzle 1 by an actuator (not shown). Figure 1 The fully closed forward position shown in Figure 5 axially displaced between the maximum open retracted position shown in .

[0024] The needle valve 5 is able to accurately vary the flow rate of the plastic material passing through the gate and into the cavity 2 of the mold 3 between a zero value (fully closed position) and a maximum value (maximum open position), and vice versa.

[0025] In the case of the example described, the tip of the needle valve 5 cooperating with the gate 4 is indicated at 6 and is cylindrical: alternatively, it could be frustoconical.

[0026] The actuator that controls the displacement of the needle valve 5 is an electric actuator, and more specifically a rotary electric motor. It is not shown for the sake of simplicity and is, for example, of the type described and illustrated in the aforementioned document EP-2 679 374 in the name of the applicant, in which the shaft of the rotary electric motor drives the needle valve 5 via a transmission comprising a screw and nut assembly and an oscillating lever.

[0027] The electric motor is in turn operatively connected to a programmable electronic control unit and is configured to drive the needle valve 5 in a controlled manner, depending on its position and its displacement speed. This control can be performed as a function of operating parameters of the molding device and / or pre-set parameters, such as the pressure of the plastic material injected into the cavity 2 of the mold 3.

[0028] Below is a description of the opening cycle (upstream) and closing cycle (downstream) of the needle valve 5 carried out according to the method of the invention, which allows to ensure an optimized surface quality of the molded part.

[0029] Filling Steps

[0030] The injection pressure in the nozzle 1 is maximum just before the gate 4 is opened. Under these conditions, due to the rapid and concentrated release of the injection pressure, a significant displacement of the opening needle valve 5 can cause surface defects on the molded part near the gate 4. In particular, in this case, flow lines of the injected material can be observed on the molded part.

[0031] The concept underlying the present invention is to limit or completely eliminate this type of drawback by precisely controlling the position and speed of the needle valve 5 through an electronic actuator and an associated electronic control unit. In particular, the present invention provides for performing the initial partial decompression of the nozzle 1 and the distributor or hot chamber of the molding device by a unique method as follows:

[0032] -See Figure 1 , the needle valve 5 is initially in its tip 6 (cylindrical or frustoconical as described) in the fully closed position A of the gate 4. The injection pressure in the hot chamber and acting in the nozzle 1 of the injector is maximum;

[0033] - The needle valve 5 is initially moved at the maximum speed observed during the entire opening step, up to a first position B which partially opens the gate 4 and is appropriately set in the electronic control unit. Position B is such that a minimum gap is left for the material to flow outside the gate 4 and start filling the cavity ( Figure 2 ). However, such minimum clearance should not cause material degradation;

[0034] - Position B is thus maintained while the flowing material fills the cavity 2 of the mold to such an extent that it is possible to correctly read the injection pressure with the aid of a dedicated sensor. The pressure measurement can be performed using a sensor placed in the cavity, or indirectly by reading the torque of an electronic motor, as described in Italian patent application T02014A000701 on behalf of the applicant, which was not published at the time of filing the present application. The needle valve 5 remains fixed in position B until the injection pressure reaches a value predetermined by the electronic control unit (point B', Figure 3 ). Thus, the high initial pressure is released in a controlled and non-abrupt manner, limiting or eliminating corresponding surface defects of the molded article near the gate 4.

[0035] Alternatively, in the electronic control unit, position B is maintained for a preset time period, for example obtained as a result of a dynamic process simulation or subsequently attempted in a step of starting the forming device, evaluating each time the surface obtained;

[0036] - When the desired pressure is reached, or at the end of a preset time period, the needle valve 5 moves further upwards to a partially open position C ( Figure 4 ). However, the opening speed from B' and C is always lower than the initial part AB;

[0037] - The needle valve 5 moves further upstream until it reaches the maximum opening position Z ( Figure 5 ), does not necessarily coincide with the upper cut-off center of the needle valve stroke (100%). The opening speed of portion CZ depends on the optimal cycle set in the electronic control unit and can be insignificantly higher, lower, or the same (constant speed) relative to the previous portion B'-C. In any case, it will always be lower than the initial portion AB.

[0038] It is possible to provide further positions D, E, F, etc. between position C and position Z, identifying corresponding sections CD, DE, etc. The speed profiles drawn by the individual sections B'-C, CD, DE, EF, etc. between B' and Z may be of different shapes (not necessarily a constant or single speed increase) as a function of the optimal cycle programmed in the electronic control unit. The speed of each individual section contained between B' and Z will always be clearly lower than that of the first section AB.

[0039] After reaching the maximum opening position Z, the cycle continues with a step of holding the plastic material in the cavity 2 of the mould 3. The position Z can remain fixed (zero speed) for a predetermined period of time between the end of the filling step and the start of the holding step.

[0040] Pressure holding step

[0041] During the holding step, the torque applied by the electronic motor varies rapidly and automatically, adjusting the closing cycle of the preset needle valve 5 as a function of the observed ambient conditions (such as pressure). At the end of the cycle, the needle valve 5 should close the gate, thus ensuring that there is no leakage.

[0042] For this purpose, the invention provides for applying a torque value preset in the electronic control unit as a function of process conditions (e.g. material, pressure) during the final part of the needle valve closing stroke, which allows obtaining a "forced" mechanical closure of the gate 4 and guarantees the absence of leakage.

[0043] The closing cycle according to the invention is performed as follows:

[0044] - From the maximum open position Z, the needle valve 5 moves downstream towards the gate 4 through one or more intermediate positions Y, X, W, etc. ( Figure 6 ). For each individual section ZY, YX, XW, etc. that may exist, the observable closed speed curve can be variously formed as a period set in the electronic control unit (it does not necessarily have to be a constant or single speed increase, but there may be, for example, zero speed sections);

[0045] - The needle valve 5 continues to move downwards until it reaches a partially closed position N near the gate 4 (the distance may be, for example, 1 mm). Such a position may be maintained for a short time period (point N', Figure 7 );

[0046] - From position N', a torque preset as a function of process parameters (e.g. material, pressure) is applied to the needle valve 5, which, thanks to this, reaches the fully closed position A of the gate 4, ensuring the absence of leaks and completing the holding step with a suitable safety margin ( Figure 8 ). Typically, the closing speed of the last part will be quite slow.

[0047] Figure 9 and 10 The position and velocity curves during the substantially fully open and closed cycles of a needle valve 5 actuated according to the method of the invention described previously are shown, respectively, and the main unique features of controlling the position and velocity of the needle valve 5 are summarized as follows:

[0048] - opening in section AB at the maximum speed observed in the opening step;

[0049] - Maintaining position B (zero speed) until a preset pressure is reached in the mold cavity (point B'), or until the end of a preset time period;

[0050] - The speed profiles between B' and Z can be different, but the opening speed of the individual sections is always lower than the speed of the initial section AB;

[0051] - Maintaining position N (zero speed) near the gate for a preset short period of time before applying a preset "forced" closing torque (point N') determined as a function of the process parameters;

[0052] - Closure N'-A due to application of a preset torque;

[0053] - Moving the needle valve for discontinuous, ie discontinuous, opening and closing.

[0054] Finally, it should be observed that, although the present invention has been described with reference to a single syringe, it is particularly advantageous to use multiple syringes for cascade or sequential injections.

Claims

1. A device for injecting a molding plastic material in a cavity of a mold to produce a molded article during an injection cycle, the device comprising at least one injector supplied with a fluid plastic material under pressure and comprising a needle valve, the needle valve being displaceable between a fully closed position and a maximum open position, and vice versa, wherein the needle valve is driven by an electronic actuator connected to an electronic control unit, the electronic control unit controlling the position and speed of the needle valve during an opening displacement of the needle valve, wherein the needle valve during the opening displacement from the fully closed position to the maximum open position initially moves at a first speed and then moves at at least one second speed, and wherein the electronic control unit is configured such that the first speed is higher than each second speed and the first speed is the highest maximum speed of the opening displacement of the needle valve, the opening displacement of the needle valve being sufficient to generate a partial vacuum of a distributor of the molding device at the beginning of an injection cycle, thereby improving the surface quality of the molded article, wherein The electronic control unit is further configured to displace the needle valve from the fully closed position to the partially open position at the first speed, wherein the electronic control unit is further configured to temporarily stop the needle valve at the partially open position for a determined period of time.

2. The device according to claim 1, characterized in that The electronic control unit is also configured such that both the opening displacement and the closing displacement of the needle valve are discontinuous, including at least one time interval in which the speed of the needle valve is zero.

3. The device according to claim 1, characterized in that The electronic control unit is further configured to displace the needle valve from a maximum open position to an intermediate partially closed position, in which intermediate partially closed position a torque is applied to the electronic actuator, which is predetermined as a function of process parameters, in order to displace the needle valve to the fully closed position.

4. The device according to claim 1, characterized in that In the partially open position, the distal end of the needle valve is near the entrance of the gate of the mold and partially blocks the plastic material from flowing into the gate.

5. The device according to claim 1, characterized in that The needle valve has a cylindrical or frustoconical tip.

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