Patent Application
20230302701
The present invention relates to a method for controlling a fluidity index of a molten resin.
In an injection molding machine, a resin as a molding material, fed into a heating barrel, is plasticized by rotating a screw. The molten resin is conveyed to an area in front of the screw and metered there while moving the screw backward. In an injection step, the screw is moved forward to fill the molten resin into a mold.
How to handle a flowing molten resin is of essential significance in injection molding. Therefore, in order to obtain a high-quality molded product, it is important to detect and assess the fluidity of a molten resin. The fluidity of a molten resin is generally expressed in terms of the viscosity.
To measure the viscosity of a molten resin in a heating barrel is difficult as compared to the measurement of the temperature or the pressure; therefore, the measurement of the viscosity has often not been performed. However, these days there are improved techniques for measuring the viscosity of a molten resin.
For example, patent document 1 describes a technique which involves injecting a molten resin when a nozzle is not in touch with a mold in a step different from a molding step, and calculating the viscosity of the molten resin from the injection pressure.
Patent document 2 describes a technique which involves determining the pressure of a molten resin and the flow rate of the resin in a resin flow path in every injection operation to calculate the viscosity of the resin. This technique enables real-time online measurement of the viscosity of the molten resin during molding.
Patent document 3 describes a technique which involves measuring the pressure of a molten resin at the front end of a nozzle in an injecting step, and calculating the viscosity of the molten resin based on the pressure.
The method disclosed in patent document 1, which involves purging a molten resin from the nozzle when it is separated from the mold in order to measure the viscosity of the molten resin in a step different from a molding step, has the following problem: The purging operation needs to be repeated a plurality of times in order to obtain a reliable viscosity value, resulting in disposal of a large amount of the resin. In the case of determining the viscosity of a molten resin in an injection step as described in patent documents 2 and 3, the viscosity of the molten resin cannot be determined unless it is injected into a mold
In addition, the fluidity of a resin cannot be detected during successive molding operations. In order to solve this problem, the applicant has proposed a method for measuring a fluidity index of a molten resin (Japanese Patent Application No. 2019-95406).
On the other hand, the properties of a resin material, which is provided in the form of pellets by a manufacturer, may vary among different production lots. Thus, the fluidity of a molten resin may vary among different lots of the same product and, for some reason, the variation can be considerably large.
The present invention has been made in view of the above problems in the prior art. It is therefore an object of the present invention to provide a method and an apparatus for controlling a fluidity index of a molten resin, which can detect the fluidity of a molten resin even during successive molding operations and, in addition, can control the fluidity within a target range.
In order to achieve the object, the present invention, in one embodiment, provides a method for controlling a fluidity index of a molten resin in an injection molding machine which injects the molten resin in a heating barrel from a nozzle into a mold by means of a screw moving forward in the heating barrel, the method comprising: assuming that a narrow flow path, formed in a flow path for the molten resin, is a capillary or an orifice, and measuring, based on the amount of a metered molten resin and the back pressure applied to the screw during a metering step, a fluidity index which indicates the fluidity of the metered molten resin; and feeding back the measured fluidity index value and comparing it with a target value, and controlling the back pressure or the rotating speed of the screw so as to eliminate a deviation between the target value and the measured value.
Embodiments of a molten resin fluidity index control method according to the present invention will now be described with reference to the attached drawings.
In
The screw 24 is slidably and rotatably housed in the heating barrel 22. The base end of the screw 24 is coupled to a pulley 25 of a rotary drive mechanism. The rotary drive mechanism is configured to transmit the rotation of a screw rotating motor 26 to the pulley 25 via a transmission belt 27. A load cell 30 is provided behind a bearing 28 that supports the pulley 25. The load cell 30 is a load measuring device for measuring an axial load applied to the screw 24.
The screw 24 is configured to axially move back and forth in the heating barrel 22 by means of a back-and-forth movement mechanism 32. The back-and-forth movement mechanism 32 includes a pulley 33, to which the rotation of a not-shown back-and-forth movement motor is transmitted via a belt, a nut portion 35, a ball screw 36, a bearing 37 that supports the ball screw 36, etc.
Referring to
As shown in
The check ring 60 is disposed between the rear end surface 63 of the screw tip 61 and a seat 65 provided at the front end of the screw 24. A second flow path 66, in which a molten resin flows and which communicates with the first flow path 64, is formed between the inner peripheral surface of the check ring 60 and the peripheral surface of the small-diameter shaft 62.
The flow of a molten resin upon its metering is shown by the dotted arrows in
When injecting the molten resin, the rear end surface of the check ring 60 is pressed against the seat 65, whereby the narrow flow path 68 is closed; therefore, backward flow of the molten resin is prevented.
In the molten resin fluidity index control method of this embodiment, a fluidity index of a molten resin is calculated using the narrow flow path 68, which is formed behind the check ring 60 during a metering step, and the second flow path 66. Before describing the index control method, the capillary rheometer method, which is a common fluid viscosity test method, will be described with reference to
In
The capillary rheometer method comprises forcing a molten resin in the cylinder 70 out of the capillary 72 with the piston 71 moving at a constant speed, measuring the load applied to the moving piston 71 with a load cell 73, and calculating the viscosity of the fluid using the following formulae (1) to (4). The viscosity is finally calculated by the formula (4).
Q=Aν (1)
γ=32Q/πD3 (2)
τ=pD/4L (3)
η=τ/γ (4)
where Q: flow rate of molten resin (mm3/s)
Assume that referring to
In the capillary rheometer method, the molten resin is forced out through the capillary 72, which is a narrowed flow path, by pressure from the piston 71. In the metering step, the molten resin is forced out through the narrow flow path 68 by pressure from the screw 24. Thus, the two methods have a commonality in that a resin is forced out of a narrow flow path by applying pressure to the resin.
Though there are differences in shape and size between the piston 71 and the screw 24 and between the capillary 72 and the narrow flow path 68, the two methods are conceptually the same in the use of a narrowed flow path, which is essential for the measurement of the fluidity of a molten resin. In this embodiment, the narrow flow path 68 is assumed to be equivalent to the capillary 72.
In the metering step illustrated in
The “flow rate of molten resin” corresponds to the amount of the metered resin per unit time. In this embodiment, the backward movement speed of the screw 24 is detected and, based on the backward movement distance per unit time of the screw 24, the diameter of the screw 24, the inner diameter of the heating barrel 22, etc., the volume between the screw 24 and the heating barrel 22 is calculated to determine the amount of the metered resin per unit time.
The back pressure applied to the screw 24 can be detected by the load cell 30.
In the metering step, the backward movement speed of the screw 24 is controlled such that the back pressure is kept constant. The backward movement speed is not constant in a strict sense; an average speed throughout the metering step or the average of several measured speeds may be taken as the backward movement speed.
Such assumed correspondence relationships necessitate a modification of the formulae (2) and (3); an appropriate change of the coefficient may be made in advance. A value obtained from the modified formula (4) is not an absolute viscosity value strictly in accordance with the capillary rheometer method; however, the value obtained is practically sufficient as an index used to relatively assess the fluidity of a molten resin.
The fluidity index of a molten resin can be measured with a fluidity index measuring section 82 by the above-described measuring method based on measurement values from a flow rate measuring section 81 for measuring the amount of a metered resin based on the backward movement speed of the screw 24, and from the load cell 30 for detecting a back pressure applied to the screw 24. The measured fluidity index is fed back to the control system and compared to a target command value from a molding condition command section 83. The controller 80 then controls the back pressure or the rotating speed of the screw 24 so as to eliminate a deviation between the target command value and a detection value.
The operation of the above-described fluidity index control apparatus will now be described in relation to successive molding operations of the injection molding machine.
As used herein, “successive molding operations” refer to a repetition of a molding cycle, including the steps of mold closing, mold clamping, metering, injection, pressure holding, mold opening, and molded product removal, performed in a successive manner over a long period of time while the nozzle of the injection apparatus is kept in touch with a mold. However, the nozzle 21 may sometimes move backward, e.g. on completion of cooling, during one cycle.
In the metering step of each molding cycle, the amount of a metered resin is measured based on the backward movement speed of the screw 24 and, in addition, the back pressure applied to the screw 24 is detected. This enables the fluidity index measuring section 82 to measure a fluidity index value by the above-described quasi-capillary rheometer method. Therefore, it becomes possible to assess, based on the measured index value, the fluidity properties of a metered molten resin online during successive molding operations.
In a metering step during successive molding operations, a target value has been set for a fluidity index of a molten resin. In particular, molding is performed in advance using a real machine, and a desirable value for a molded product is set as the target value.
However, in actual successive molding operations, an error from the target value is produced in a fluidity index value of a molten resin due to different lots, different moisture contents, different compositions, etc. of the raw material resin.
In view of this, in this embodiment feedback control is performed so that the measured fluidity index value of a molten resin becomes equal to the target value.
In the initial stage of a metering step, due to a response delay, the measured fluidity index value gradually approaches the target value. There may be a compositional variation between lots of the raw material resin. Due to the variation, the measured fluidity index value can exceed the upper limit of the target fluidity index range. The measured value that has been fed back is compared to the target value, and the back pressure applied to the screw 24 is adjusted so as to eliminate a deviation between the target value and the measured value. Consequently, the fluidity index value returns to the target range. While the fluidity index then makes a repetitive change in a vibrating manner, the fluidity index value is generally held within the target range. In this manner, the fluidity index of a molten resin is maintained within the intended target range in a metering step during successive molding operations. This enables stable molding of good products by successive molding operations.
In the first embodiment described above, a fluidity index of a molten resin in a metering step is controlled using one target value as a reference. In a second embodiment, on the other hand, a fluidity index is controlled using different target values set for a plurality of regions of a metered resin in the heating barrel 22.
In this embodiment, the metered resin is divided, for example, into three regions A, B and C for which different fluidity index target values are set. The frond end-side region A has a low target value, the rear end-side region C has a high target value, and the middle region B has an intermediate target value.
While the screw 24 is moving through the zone 1, a measured fluidity index value that has been fed back is compared to the target value, and the back pressure applied to the screw 24 is adjusted so as to eliminate a deviation between the target value and a measured value, whereby the fluidity index is maintained within the target range I. Similarly, feedback control of the fluidity index is performed for each of the zones 2 and 3, so that the fluidity index is maintained within the target range II or III.
According to the second embodiment described above, it becomes possible to meter a molten resin, whose fluidity properties differ between the regions A, B and C, as shown in
(Variation) While a fluidity index is calculated according to the quasi-capillary rheometer method in the above-described embodiment, it is also possible to measure a fluidity index by a quasi-MFR (melt flow rate) method during metering of a molten resin, assuming that the narrow flow path 68 is an orifice.
Referring to
The metering step is performed under control so that a constant back pressure is continually applied to the screw 24; therefore, the amount of a metered resin per 10 minutes is to be calculated. The amount of a metered resin per 10 minutes can be calculated from the amount of the metered resin per unit time, determined in the above-described embodiment.
As with the measurement of a fluidity index by the quasi-capillary rheometer method and the feedback of the measured value, performed in the above-described embodiments, the measurement of a fluidity index by the quasi-MFR method and the feedback of the measured value makes it possible to control the fluidity index within a target range.
In the above-described embodiments, the present invention is applied to injection molding using a thermoplastic resin as a molding material. The present invention can also be applied to injection molding using a thermosetting resin, etc. An injection apparatus for a thermosetting resin, etc. cannot use any check ring at the front end of a screw. In the injection apparatus, a thermosetting resin flows through a narrow path separated by a flight and formed on the peripheral surface of the screw. Therefore, as in the case of a thermoplastic resin, it is possible to calculate a fluidity index in a metering step.
While the back pressure of the screw is used as a manipulating variable for controlling a fluidity index in the above-described embodiments, it is also possible to use the rotating speed of the screw or a combination of the back pressure of the screw with the rotating speed of the screw or with the heating temperature of a barrel heater.
While preferred embodiments of the molten resin fluidity index control method according to the present invention have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the present invention. Indeed, the novel apparatus, method and system described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes may be made to the embodiments described above without departing from the spirit of the present invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-195388 | Oct 2019 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/039491 | 10/21/2020 | WO |
