JPH09277299A - Injection molding of resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain the generation of warp of a molded form effectively even when the resin molded form of a complicated configuration is molded through injection molding. SOLUTION: Molten resin is injected into the cavity 1 of a molding tool 2 consisting of split molds and, thereafter, minute mold opening of the molding tool 2 is effected during the injection of pressure gas into the molten resin fed in the cavity. The resin pressure in the cavity 1 is reduced by the minute mold opening and, therefore, sufficient amount of pressure gas can be injected into the molten resin and the generation of warp of the molded form and the like can be restrained effectively.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a resin injection molding method for molding a resin by injection molding.

[0002]

2. Description of the Related Art For example, injection molding of a thermoplastic resin is performed by injecting a molten resin into a cavity through a sprue, a runner and a gate of a molding die by an injection cylinder and then cooling and solidifying the molten resin in the cavity. Done. Then, in order to compensate for the volume contraction accompanied by the cooling and solidification of the molten resin injected and filled in the cavity, after the molten resin is completely filled in the cavity, pressure is continuously applied to the molten resin in the injection cylinder, Back pressure sprue,
It is common to carry out a holding pressure step of acting on the molten resin in the cavity via the runner and the gate.

However, in the above holding pressure step of applying a back pressure to the molten resin in the cavity by using the molten resin as a medium, the temperature and viscosity of the resin are not uniform in each part of the sprue, runner, gate and cavity, and the molten resin is not uniform. Since the pressure loss due to the high viscosity is large, and the viscosity increases as the molten resin cools, it is impossible to uniformly transmit the pressure throughout the cavity. Therefore, when molding a molded product having a complicated shape, there is a problem that warpage of the molded product occurs.

In view of this, Japanese Patent Laid-Open No. 64-63122 discloses a holding pressure process by injecting and filling a molten resin into a cavity and then applying back pressure to the molten resin in the cavity using a pressurized gas fluid as a medium. An injection molding method of a thermoplastic resin by a so-called gas injection method is disclosed. When back pressure is applied using a pressurized gas fluid as a medium, the pressure drop due to the viscosity of the fluid is smaller than when a back pressure is applied using a molten resin as a medium, so the gas fluid injected into the molten resin melts. The resin flows uniformly in each part of the resin, and has an effect of pressing the molten resin in the cavity against the mold surface with a uniform pressure, and as a result, it is possible to effectively suppress warpage of the molded product.

On the other hand, in the injection molding of a foamed resin in which a foaming agent is mixed with a molten resin, the molten resin mixed with the foaming agent is injected into the cavity of the mold by a short shot to cool and solidify the molten resin in the cavity. A foamed resin molded product is molded by foaming.

[0006]

By the way, in the case of the injection molding using the above-mentioned gas injection, the injection pressure in the injection process is usually about 50 MPa, whereas the gas injection pressure in the gas injection is 20 MPa.
It is about MPa. Therefore, it is difficult to inject the gas fluid into the molten resin in the cavity immediately after the injection process is completed. Since the resin pressure in the cavity gradually decreases with the lapse of time after the completion of the injection process, it is conceivable to perform the gas injection after the resin pressure in the cavity has sufficiently decreased after the completion of the injection process. But,
The resin pressure in the cavity and the resin viscosity are in a contradictory relationship, and the resin pressure decreases with the passage of time, while the resin viscosity increases. Here, considering the ease of injecting the gas fluid into the molten resin, the injection amount of the gas fluid decreases as the resin viscosity increases. For this reason, after the injection process is completed, when the resin pressure in the cavity is reduced to such an extent that a sufficient gas fluid can be injected, the increase in the resin viscosity hinders the injection of the gas fluid. Therefore, it becomes difficult to inject a sufficient gas fluid into the molten resin after the completion of the injection process, and even in the injection molding using the above-mentioned gas injection, the reduction of warpage in the molded product was not sufficient.

Also in the injection molding of a foamed resin in which a foaming agent is mixed with a molten resin, the resin pressure in the cavity has a great influence on the foamability, and if the resin pressure is high, the foaming will not be sufficient. In addition, the molten resin injected into the cavity,
The resin pressure varies depending on the location in the cavity. Therefore, especially when molding a foamed resin molded product with a complicated shape,
There is a problem that it is difficult to uniformly foam, and warpage of the molded product occurs due to the denseness of air bubbles.

The present invention has been made in view of the above circumstances, and it is possible to effectively suppress warpage of a molded product even when a resin molded product having a complicated shape is molded by injection molding. It is a technical problem to be solved to provide a resin injection molding method.

[0009]

In the resin injection molding method of the first invention for solving the above problems, a molten resin is injected into a cavity of a mold comprising a split mold, and then the molten resin is injected into the cavity. A resin injection molding method for cooling and solidifying to form a resin molded product, comprising a resin injection step of injecting and filling the molten resin in the cavity, and injecting a pressurized gas into the molten resin filled in the cavity. And a gas injection step for performing the gas injection step, and the mold is finely opened during the gas injection step.

In the resin injection molding method according to the first aspect of the present invention, in the resin injection molding method by so-called gas injection method, in which the molten resin is injected and filled into the cavity and then the pressurized gas is injected into the molten resin, The mold, which is a split mold, is opened minutely while injecting a pressurized gas.
Due to this minute mold opening, the pressure inside the cavity is reduced, and the resin can be expanded by the amount of the mold opening, so that the resin pressure inside the cavity is also reduced accordingly. Therefore, after the completion of the filling of the molten resin into the cavity and before the resin viscosity becomes too high, the above-mentioned micro mold opening can be performed to inject a sufficient amount of pressurized gas into the molten resin. Therefore, even when injection molding a molded product with a complicated shape,
It is possible to extremely effectively suppress the occurrence of warpage of the molded product.

The fine mold opening is performed in a state where the surface portion of the molten resin in contact with the cavity surface is solidified to some extent in order to prevent the molten resin from flowing out of the mold. The mechanism of warpage of a molded product and the mechanism of suppressing warpage by injection of pressurized gas are as follows. That is, the molten resin injected and filled in the cavity begins to solidify from the surface portion in contact with the cavity surface of the molding die. Then, when the molten resin in the central portion of the molded product is solidified, the resin in the central portion is contracted, and the previously solidified surface portion is pulled toward the central portion by the contracting force, so that the molded product is warped. Therefore, by injecting a pressurized gas into the molten resin and replenishing the compressed portion of the resin in the central portion with the pressurized gas, the surface portion is pulled toward the central portion side and injected into the central portion. It is possible to counter the pressurized gas and suppress the generation of warpage of the molded product.

In the resin injection molding method of the second aspect of the present invention which solves the above problems, a molten resin is injected into a cavity of a mold composed of split molds, and then the molten resin is cooled and solidified in the cavity to mold the resin. A resin injection molding method for molding a product, comprising a resin injection step of injecting the molten resin having a foaming agent added into the cavity, and foaming the molten resin having the foaming agent added in the cavity. And a step of micro-opening the molding die during the foaming step.

In the resin injection molding method according to the second aspect of the present invention, in the so-called foam injection molding method in which the molten resin added with the foaming agent is injected into the cavity and then the resin is foamed in the cavity, while the molten resin is foamed. A mold made of split molds is opened by a minute mold. This minute mold opening lowers the cavity internal pressure and allows the resin to expand by the amount of the mold opening, so that the resin pressure inside the cavity also decreases and the foaming of the molten resin is promoted. Therefore, even a molded product having a complicated shape can be uniformly foamed in the entire molded product, and the warp of the molded product due to the denseness of the bubbles can be effectively suppressed.

The mechanism by which warpage of a molded article is generated in foam injection molding and the mechanism by which the generation of warpage is suppressed by opening a minute mold are as follows. That is, as described above, the foaming property of the foamed resin greatly affects the resin pressure, and the molten resin injected into the cavity has a different resin pressure depending on the location in the cavity. Therefore, if the resin pressure in the cavity is too high, it is unavoidable that the resin is less likely to foam in the portion of the cavity where the resin pressure is high. Then, the shrinkage rate of the resin is higher in the portion where the foaming rate per unit volume is lower than in the portion where the foaming rate is high. Therefore, the shrinkage rate of the resin becomes uneven depending on the location, and warpage of the molded product occurs. Therefore, if the resin pressure in the cavity is slightly reduced during resin foaming and the resin pressure in the cavity is made relatively lower than that before the resin is opened, it is possible to suppress the deterioration of the foamability in the portion where the resin pressure is high in the cavity. As a result, even in a portion where the resin pressure is high in the cavity, sufficient foaming can be achieved to the same extent as in a portion where the resin pressure is low. Therefore, the foaming rate and thus the shrinkage rate of the resin become uniform, so that the warpage of the molded product can be suppressed.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be specifically described below with reference to the drawings. (Embodiment 1) In the injection molding apparatus according to the present embodiment, as shown in the schematic sectional view of FIG. 1, an injection molding die 2 forming a cavity 1 having a predetermined shape is composed of a stationary die 3 and a hydraulic cylinder 4. It is composed of a movable die 5 which can be moved linearly. A resin supply passage 6 for supplying a molten resin into the cavity 1 and a gas supply passage 7 for supplying a pressurized gas into the cavity 1 are formed in the fixed mold 3 and the movable mold 5. . Then, the molten resin injected from the injection cylinder 8 at a predetermined injection pressure is supplied into the cavity 1 through the resin supply passage 5, and the pressurized gas supply device 9 is supplied.
The pressurized gas injected at a predetermined pressure from the gas supply passage 6
It is designed to be supplied into the molten resin in the cavity 1 via. Also, the hydraulic cylinder 4 and the injection cylinder 8
The pressurized gas supply device 9 can be controlled by the control device 10.

Using the above injection molding apparatus, nylon 66 containing 33 wt% of glass fiber as a synthetic resin was injection molded as shown below. The signals shown in the flowchart of FIG. 3 are sent from the control device 10 to the hydraulic cylinder 4, the injection cylinder 8 and the pressurized gas supply device 9, respectively. The injection cylinder 8 is controlled by the control device 10.
Then, the molten resin was injected and filled into the cavity 1 at an injection pressure of about 80 to 100 MPa (90 MPa in the present embodiment) (state of FIG. 1). The cavity 1 after the filling is completed
The resin pressure inside is 50 to 60 MPa (50 in the present embodiment.
MPa). The resin temperature at the time of injection is 295 ° C.

1 to 5 seconds (1.5 seconds later in the present embodiment) from the start of injection, the control device 10 sends a signal to start the injection of the pressurized gas to the pressurized gas supply device 9, and the pressurized gas is supplied. 15-25M in the molten resin in the cavity 1 from the supply device 9
Nitrogen gas was injected at a gas pressure of about Pa (20 MPa in this embodiment). The resin pressure inside the cavity 1 at the start of injecting the pressurized gas is about 45 MPa, which is about 25 MPa higher than the resin pressure inside the cavity 1 after opening the micro mold.

After a lapse of 6.5 to 11.5 seconds (6.5 seconds in this embodiment) from the start of the injection, the control unit 10 sends a signal for opening the mold to the hydraulic cylinder 4 to activate the hydraulic cylinder 4. Then, the movable mold 5 was moved to perform a minute mold opening (state of FIG. 2). The minute mold opening is performed at a timing such that the surface portion of the molten resin in contact with the cavity surface is solidified to the extent that it can be expanded, and the viscosity of the molten resin in the central portion that is not solidified is not too high. Further, the minute mold opening amount (δt) was set to 0.2 mm, and the resin pressure in the cavity 1 was lowered to about 15 MPa due to this minute mold opening.

15 to 30 seconds (26.5 seconds after in the present embodiment) from the start of the injection, the control device 10 sends a signal to the pressurized gas supply device 9 that the injection of the pressurized gas is completed, and the pressurized gas is supplied. The supply of the pressurized gas from the supply device 9 to the cavity 1 was stopped. At this point, the resin in the cavity 1 is almost cooled and solidified. And, like normal injection molding,
After waiting for the resin in the cavity 1 to be completely cooled and solidified, the mold 1 was opened and the injection molding was completed.

As described above, in the resin injection molding method of this embodiment, after the injection and filling of the molten resin into the cavity 1 is completed, a pressurized gas is injected into the molten resin injected and filled in the cavity 1 and the pressure is applied. During the gas injection, the molding die 2 is finely opened before the resin viscosity becomes too high. Due to this minute mold opening, as shown in FIG. 4, the resin pressure in the cavity 1 drops sharply. Since the pressurized gas is injected at a pressure higher than the resin pressure after opening the minute mold, a sufficient amount of the pressurized gas can be injected into the molten resin in the cavity 1. Even when a molded product having a complicated shape is injection-molded with gas, it is possible to extremely effectively suppress warpage of the molded product.

In the above embodiment, the case where nylon 66 is used as the synthetic resin and nitrogen is used as the pressurized gas has been described, but the types of the synthetic resin and the pressurized gas are not particularly limited, and injection molding is possible. A gas inert to the synthetic resin and the synthetic resin used can be appropriately used. Further, it is preferable that the amount δ of the minute mold opening is 0.1 to 1 mm. If it is smaller than 0.1 mm, the pressure inside the cavity, and by extension, the resin pressure, will not drop sufficiently.
If it is larger, the precision of the molded product will be reduced.

Further, the pressure of the pressurized gas to be injected needs to be at least higher than the resin pressure in the cavity 1 after opening the micro mold. Specifically, the pressure of the pressurized gas to be injected can be about 15 to 25 MPa as described above, but this pressure is 0 to 10 MPa higher than the resin pressure in the cavity 1 after opening the micro mold. . If the pressure of the pressurized gas is lower than 15 MPa, warpage of the molded product cannot be sufficiently suppressed. on the other hand,
The upper limit of the pressure of the pressurized gas can be appropriately set within a range that does not exceed the mold clamping force, but the current mechanical capacity is about 25 MPa.

As described above, the fine mold opening solidifies to such an extent that the surface portion of the molten resin in contact with the cavity surface can expand, and the viscosity of the molten resin in the unsolidified central portion does not become too high. It is carried out at a timing, but specifically, it is preferably carried out 6.5 to 11.5 seconds after the start of injection. (Embodiment 2) The injection molding apparatus according to the present embodiment is shown in FIG.
As shown in the schematic cross-sectional view, the configuration is the same as that of the injection molding apparatus according to the first embodiment except that the gas supply passage 7 and the pressurized gas supply apparatus 9 are not provided.

Using the above-mentioned injection molding apparatus, as shown below, foaming injection molding was performed by adding sodium bicarbonate as a foaming agent to nylon 66 containing 33 wt% of glass fiber as a synthetic resin. Under the control of the control device 10, the molten resin containing the foaming agent was injected and filled into the cavity 1 in a short shot from the injection cylinder 8 at an injection pressure of about 80 to 100 MPa (90 MPa in the present embodiment) (state of FIG. 5). The resin pressure in the cavity 1 after the completion of filling is 50 to 60 MPa (50 M in this embodiment).
Pa). The resin temperature at the time of injection is 295 ° C.

Then, the surface of the molten resin in contact with the cavity surface was solidified and the molten resin was foamed in the cavity 1 (state of FIG. 2). 6.5 ~ from the start of injection
After 11.5 seconds (6.5 seconds in the present embodiment), the control device 10 sends a signal for opening the minute mold to the hydraulic cylinder 4,
The movable die 5 was moved by the operation of the hydraulic cylinder 4 to perform a minute die opening (state of FIG. 3). The micro-mold opening is performed in a state in which the surface portion of the molten resin in contact with the cavity surface is solidified to the extent that it can expand. In addition, the minute mold opening (δ
t) was 0.2 mm, and the resin pressure in the cavity 1 was lowered to about 15 MPa due to this minute mold opening.

Then, as in ordinary foam injection molding, after waiting for the resin in the cavity 1 to completely foam and cool and solidify, the mold 1 was opened and the foam injection molding was completed.
As described above, in the resin injection molding method of the present embodiment, after the injection of the molten resin into the cavity 1 is completed, the molding die 2 is opened minutely during the foaming of the molten resin, and the resin pressure in the cavity 1 is reduced. Therefore, the foaming of the molten resin is promoted, and even in the case of a molded product having a complicated shape, it is possible to uniformly foam the entire molded product and effectively suppress the warpage of the molded product due to the denseness of air bubbles. be able to.

Also in this embodiment, it is preferable that the minute die opening δ amount is 0.1 to 1 mm. 0.1m
If it is smaller than m, the pressure inside the cavity and, by extension, the resin pressure will not be lowered sufficiently, while if it is larger than 1 mm, the precision of the molded product will decrease. Further, the micro-mold opening is performed at the timing when the surface portion in contact with the cavity surface is solidified to the extent that it can be expanded as described above. Specifically, 6.5 to 1 from the start of injection.
It is preferable to carry out after 1.5 seconds.

[0028]

As described above in detail, in the resin injection molding method of the first aspect of the present invention, in the resin injection molding method by the so-called gas injection method, molding is performed by the split mold while injecting the pressurized gas into the molten resin. By opening the mold minutely, it is possible to inject a sufficient amount of pressurized gas into the molten resin due to the decrease in resin pressure in the cavity. However, it is possible to extremely effectively suppress the warpage of the molded product.

In the resin injection molding method of the second invention,
In the so-called foam injection molding method, by opening the molding die consisting of the split molds while foaming the molten resin, the resin pressure in the cavity is lowered and the foaming of the molten resin is promoted. Even
It is possible to uniformly foam the entire molded product, and it is possible to effectively suppress warpage of the molded product due to the denseness of air bubbles.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing an overall configuration of a resin injection molding device according to a first embodiment.

FIG. 2 is a cross-sectional view showing a state after the micro mold is opened according to the first embodiment.

FIG. 3 is a flowchart of a resin injection molding method of the first embodiment.

FIG. 4 is a diagram illustrating how the resin pressure and the resin viscosity change after the start of injection according to the first embodiment.

FIG. 5 is a sectional view schematically showing an overall configuration of a resin injection molding device according to a second embodiment.

FIG. 6 is a cross-sectional view showing a state in which foaming of the molten resin proceeds and the surface of the molten resin is solidified according to the second embodiment.

FIG. 7 is a cross-sectional view showing a state after the micro mold is opened according to the second embodiment.

[Explanation of symbols]

1 is a cavity, 2 is a mold, 3 is a fixed mold, 4 is a hydraulic cylinder, 5 is a movable mold, 8 is an injection cylinder, 9 is a pressurized gas supply device, and 10 is a control device.

Claims (2)

[Claims] 1. A resin injection molding method for molding a resin molded article by injecting a molten resin into a cavity of a molding die comprising a split mold and then cooling and solidifying the molten resin in the cavity. It comprises a resin injection step of injecting and filling the molten resin inside and a gas injection step of injecting a pressurized gas into the molten resin filled in the cavity. A resin injection molding method characterized by opening. 2. A resin injection molding method for molding a resin molded article by injecting a molten resin into a cavity of a mold comprising a split mold, and then cooling and solidifying the molten resin in the cavity. A resin injection step of injecting the molten resin having a foaming agent added therein, and a foaming step of foaming the molten resin added with the foaming agent in the cavity, wherein the molding die is formed during the foaming step. A resin injection molding method characterized by opening a micro mold.