JP3044400B2 - Hollow injection molding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hollow injection molding method in which a molten resin is press-fitted into a mold cavity and then pressurized gas is further press-fitted into the mold cavity to form a hollow molded product. More specifically, hollow injection molding in which molten resin is injected from a single injection nozzle into a plurality of gates simultaneously to press the molten resin from each gate into a mold cavity and then pressurized gas into the mold cavity from the same gate. About the method.

[0002]

2. Description of the Related Art Conventionally, after a molten resin is press-fitted into a mold cavity from a gate by using an injection nozzle having a built-in gas nozzle, a pressurized gas is further press-fitted into the mold cavity from the same gate to form a hollow molded product. A molding method is known (JP-B-57-14968).

On the other hand, when molding a large molded product, a plurality of gates are provided in one large mold cavity, and molten resin is supplied to these gates from one injection nozzle at the same time. In addition, it has been practiced to easily fill the molten resin and shorten the press-in time of the molten resin. The same applies to the case where a plurality of molded products are molded at the same time. That is, a gate is provided in each of a plurality of mold cavities, and the molten resin is simultaneously supplied from one injection nozzle to these gates, thereby simultaneously injecting the molten resin into each mold cavity.

In injection molding using such a plurality of gates, a large amount of molten resin is discharged from a gate having a relatively large mold cavity or a gate opened to a relatively large mold cavity. A large gate to be opened and a gate opened to a relatively small volume cavity or a relatively small volume cavity,
It is a small gate from which a small amount of molten resin is discharged. The large gate usually has a large diameter so that the runner and the gate itself can easily flow the molten resin.
The small gate usually has a small diameter for the runner and the gate itself.

To explain further, conventionally, in order to prevent sink marks in injection molding using a mold having a plurality of gates, a molten resin is injected into a mold cavity to fill the cavity, and then cooled while further applying an injection pressure. It is done by that. In this case, the mold cavity filled with a large amount of molten resin is
Since sink marks are likely to occur, it is necessary to maintain a high molten resin pressure during cooling. On the other hand, since the mold cavity portion filled with only a small amount of the molten resin is unlikely to cause sink marks, it is not necessary to maintain a very high molten resin pressure during cooling. The large gate and the small gate are arranged as described above in order to maintain the molten resin pressure at the time of cooling according to the molten resin filling amount. Also, it takes longer to cool the mold cavity where a large amount of molten resin is press-fitted than the mold cavity where a small amount of molten resin is press-fitted. Is easily added, so a large-diameter large gate is provided in such a mold cavity,
In the mold cavity portion into which a small amount of the molten resin is press-fitted, the cooling is completed in a relatively short time, so that a small gate is used.

[0006]

By the way, in the injection molding using a plurality of gates from the above-mentioned conventional technology,
It is conceivable that a molten resin is simultaneously injected from one injection nozzle through these gates, and then a pressurized gas is sent to the same gate from a gas nozzle built in the injection nozzle to form a hollow molded article.

However, when the above-described molding is performed, a required amount of the pressurized gas cannot be press-fitted into the mold cavity from each gate when pressurized gas is press-fitted. There is a problem that a hollow portion having a size cannot be formed.

To explain this problem in more detail, if the pressurized gas is distributed and supplied to a plurality of gates at the same time, the distribution of the pressurized gas is not properly performed. Occurs.

When the present inventor observed a molded product in which pressurized gas injection was inappropriate, the larger the gate designed to discharge a larger amount of molten resin, the more pressurized gas was injected. It has been found that a small gate designed to discharge a small amount of molten resin is easily injected with a pressurized gas. That is, a hollow portion is hardly formed at a position where a large amount of molten resin is press-fitted, so that the formation of the hollow portion completely prevents sink marks.

[0010] However, the large gate usually has a large diameter so that the runner and the gate itself can easily flow the molten resin, and the small gate has a thinner runner and the gate itself. It is usually the diameter.
This means that the large gate has a smaller flow resistance than the small gate and the fluid easily flows, which is inconsistent with the actual state observed in the defective molded article.

According to the present invention, molten resin is supplied from a single injection nozzle to a plurality of gates at the same time, whereby molten resin is pressed from each gate into a mold cavity, and then pressurized gas is supplied from the same gate into a mold cavity. In this case, it is an object of the present invention to investigate the cause of an inappropriate distribution amount of the pressurized gas, eliminate the cause, and surely prevent sink marks by forming a hollow portion.

[0012]

For this purpose, according to the invention of claim 1, molten resin is injected from one injection nozzle into a mold cavity simultaneously through a plurality of gates, and then from a gas nozzle built in the injection nozzle. In a hollow injection molding method in which a pressurized gas is injected into a mold cavity through a plurality of gates, the injection of the molten resin into the mold cavity is performed by setting the molten resin pressure immediately before each gate to be substantially equal. It is.

[0013]

First, the cause of the inappropriate distribution of the pressurized gas will be described.

In the conventional mold having a plurality of gates, as described above, the runner and the gate press a required amount of molten resin from each gate into the mold cavity,
That is, it is designed mainly to supply a large amount of molten resin to the large gate and supply a small amount of molten resin to the small gate.

The reason why it is difficult to pressurize the pressurized gas from the large gate in such a mold is that the pressure of the molten resin immediately before the large gate is high at the time of pressurizing the molten resin, and the pressurized gas is hardly penetrated. It is because it becomes.

That is, the runner leading to the large gate has a large diameter in order to supply a large amount of molten resin, so that the molten resin easily enters, and the flow resistance reaching the large gate is small. Tends to be high. On the other hand, the runner leading to the small gate has a small diameter because it only needs to supply a small amount of molten resin, and the flow resistance reaching the small gate is large, so the molten resin pressure immediately before the small gate is also low. Cheap.

When the pressurized gas is supplied in a state where there is such a pressure difference of the molten resin after the injection of the molten resin, the pressurized gas is added in that the gas does not receive the flow resistance as much as the molten resin. It flows mainly to the small gate immediately before the low molten resin pressure, and hardly flows to the large gate immediately before the high molten resin pressure.

Therefore, in the first aspect of the present invention, the molten resin is injected into the mold cavity by making the pressure of the molten resin immediately before each gate substantially equal to each other. It is to prevent that. Also, the flow resistance when the pressurized gas passes through the runner or the gate is negligibly small compared to the flow resistance when the molten resin passes, so that the molten resin pressure immediately before each gate is almost equal. For example, even if the diameters of the gates and runners are different, pressurized gas can be simultaneously injected from the gates.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, the present invention is characterized in that the molten resin is injected into the mold cavity by making the molten resin pressure immediately before each gate substantially equal. A specific method for performing the injection with the molten resin pressure substantially constant will be described below with reference to the drawings.

In the injection molding which is the object of the present invention, a molten resin is supplied to a plurality of gates 1 and 1 at the same time to form a mold cavity 2.
1 and 2, and 5 by injection molding
6 and the case where the mold 3 having two or more mold cavities as shown in FIGS. 3 and 4, and FIGS. 7 and 8 is used. There is.

First, the mold 3 shown in FIGS. 1 and 2 will be described.

As shown, the mold cavity 2 has a rectangular shape having a uniform thickness, and gates 1 and 1 are provided at symmetrical positions.

Runners 5 and 5 branched from a sprue 6 connected to the resin pressure inlet 4 are connected to the gates 1 and 1, respectively. The resin pressure inlet 4 is an opening for pressing an injection nozzle (not shown) to feed the molten resin to the mold 3, and is usually an opening of a sprue 6 opened on the outer surface of the mold 3. is there.

Runners 5, 5 branched from sprue 6
Are equal in diameter, and both gates 1, 1 to which the runners 5, 5 are connected are also equal in diameter.

The mold 3 shown in FIG. 3 and FIG.
Two disk-shaped mold cavities 2 of equal diameter and thickness,
The gates 1 provided at the center of each of the mold cavities 2 have the same diameter.

Runners 5 and 5 branched from a sprue 6 connected to the resin pressure inlet 4 are connected to the gates 1 and 1, respectively. Both runners 5 and 5 have the same diameter and the same length.

The mold 3 shown in FIGS. 1 and 2 described above.
The case where hollow injection molding is performed using the mold 3 shown in FIGS. 3 and 4 will be described.

First, in both molds 3, the runners 5 and 5 have the same diameter and the gates 1 and 1 have the same diameter, so that when the molten resin is injected, the gates 1 and 1 are almost uniformly melted. Resin is distributed. Therefore, when the molten resin is injected, the pressure of the molten resin immediately before both gates 1 and 1 is maintained at substantially the same pressure. If the pressure of the molten resin immediately before the two gates 1 and 1 is almost equal, the pressurized gas is supplied to the two gates 1 and 1 in a substantially equal distribution when the pressurized gas is injected thereafter. Pressurized gas can always be injected into the mold cavity 2 from both gates 1 and 1.

The injection of the molten resin into the mold cavity 2 may be a full shot that completely fills the mold cavity 2 or a short shot that stops the injection before the mold cavity 2 is completely filled. In some cases, pressurized gas is injected while being injected.

When pressurizing the pressurized gas after the full shot, the pressurized gas is pressurized into the mold cavity 2 in accordance with the shrinkage of the molten resin in the mold cavity 2 accompanying cooling. Further, when pressurized gas is injected after the short shot, the molten resin in the mold cavity 2 is pushed out in a balloon shape with the pressurized gas injection, so that a hollow portion is formed.

Next, the case where the mold 3 shown in FIGS. 5 and 6 and FIGS. 7 and 8 is used will be described.

These molds 3 have a difference in the volume of the mold cavity 2. With respect to the molds having one mold cavity 2, the volume of the mold cavity 2 is not uniform and has a difference of magnitude. For those having two or more mold cavities 2, there is a difference in the size of the volume between the mold cavities 2.

In this specification, a gate opened to a relatively large volume portion of the mold cavity 2 or a mold cavity 2 having a relatively large volume is called a wide area corresponding gate 1a, and a relatively small volume of the mold cavity 2 is defined. The gate opened to the mold cavity 2 having a part or a relatively small volume is referred to as a narrow area corresponding gate 1b. The runner connected to the wide area corresponding gate 1a is called a wide area corresponding runner 5a, and the runner connected to the narrow area corresponding gate 1b is called a narrow area corresponding runner 5b.

First, the mold 3 shown in FIGS. 5 and 6 will be described.

As shown in the figure, the mold cavity 2 has a trapezoidal shape at right angles to the plane, and has two wide-area corresponding gates 1a on its left side and one narrow-area corresponding gate 1b on its right side.
Is provided. That is, the two wide area corresponding gates 1a on the left side in the figure are provided in a relatively wide portion of the mold cavity 2, and the one narrow area corresponding gate 1b on the right side has a relatively narrow width of the mold cavity 2. It is provided in the part.

Each wide area gate 1a has a resin pressure inlet 4
The wide area corresponding runners 5a branched from the sprue 6 are connected to each other, and the narrow area corresponding runners 5b similarly branched are connected to the narrow area corresponding gates 1b.

The wide-area and narrow-area runners 5a and 5b branched from the sprue 6 are all of equal diameter and the same length, and the wide-area and narrow-area corresponding gates 1a and 1b to which the wide-area and narrow-area runners 5a and 5b are connected. Are all of the same diameter.

The mold 3 shown in FIG. 7 and FIG.
It has two disk-shaped mold cavities 2 and 2 having different diameters, two wide-area gates 1a provided in the large mold cavity 2 on the left side, and the small mold cavity 2 on the right side.
Are all equal in diameter.

Each wide area gate 1a has a resin pressure inlet 4
The wide area corresponding runners 5a branched from the sprue 6 are connected to each other, and the narrow area corresponding runners 5b similarly branched are connected to the narrow area corresponding gates 1b. The runners 5a and 5b corresponding to the wide area and the narrow area all have the same diameter and the same length.

The mold 3 shown in FIG. 5 and FIG.
The case where hollow injection molding is performed using the mold 3 shown in FIGS. 7 and 8 will be described.

First, each of the wide area and narrow area runners 5a,
5b is an equal-diameter, equal-length gate for each wide area and narrow area.
Since a and 1b have the same diameter, when the molten resin is injected, the molten resin is almost uniformly distributed to all the wide-area and narrow-area corresponding gates 1a and 1b. Therefore, when the molten resin is injected, the pressure of the molten resin immediately before each of the wide-area and narrow-area corresponding gates 1a and 1b is maintained at substantially the same pressure. Each wide area and narrow area corresponding gate 1a,
If the pressure of the molten resin immediately before 1b is substantially the same, the pressurized gas is supplied to the wide-area and narrow-area corresponding gates 1a and 1b in a substantially even manner when the pressurized gas is injected thereafter. Therefore, the pressurized gas can be forced into the mold cavity 2 from each of the wide-area and narrow-area corresponding gates 1a and 1b.

In this embodiment, the number of the wide area corresponding gates 1a is increased by two wide area corresponding gates 1a and one narrow area corresponding gate 1b, but the present invention is not limited to this. Can be the same number (single or plural).

However, it is preferable that the number of the wide area corresponding gates 1a is larger than the number of the narrow area corresponding gates 1b. In this manner, it is possible to adjust the filling of the molten resin from the wide area corresponding gate 1a and the filling of the molten resin from the narrow area corresponding gate 1b almost at the same time, so that the molding can be performed efficiently. .

The injection of the molten resin may be a full shot or a short shot. However, when the wide area corresponding gate 1a and the narrow area corresponding gate 1b are the same number, the inner volume of the mold cavity 2 filled with the molten resin by the wide area corresponding gate 1a and the mold cavity 2 filled with the molten resin by the narrow area corresponding gate 1b. If there is a large gap between the internal volumes, the portion of the mold cavity 2 filled with the molten resin by the narrow area corresponding gate 1b2 is filled first, and the portion of the mold cavity 2 to be filled with the molten resin is not filled by the wide area corresponding gate 1a. The pressurized gas press-fitted from the wide area corresponding gate 1a side
The molten resin may break through toward the unfilled portion of the molten resin on the side of the wide area corresponding gate 1a. In such a case, it is preferable to use a full shot.

When the above-mentioned full shot is performed, the narrow area corresponding gate 1b completes the filling of the molten resin first, so that
Thereafter, until the filling of the molten resin from the wide area corresponding gate 1a is completed, the molten resin pressure immediately before the narrow area corresponding gate 1b is slightly increased. However, when the filling of the molten resin into the entire mold cavity 2 is completed, the wide area and narrow area corresponding gates 1a, 1b
Since the pressure of the molten resin just before is balanced and almost equal,
The subsequent press-in of the pressurized gas is not hindered, and the pressurized gas according to the amount of contraction accompanying cooling of the molten resin in the mold cavity 2 is press-injected.

[0046]

The present invention is as described above. Since a pressurized gas can be simultaneously injected into the mold cavity from each gate, a hollow portion can be formed at a required position of a molded product, and the hollow portion can be formed. The formation of sinks due to the formation can be ensured.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an example of a mold used in the present invention.

FIG. 2 is a cross-sectional view of the mold of FIG.

FIG. 3 is a longitudinal sectional view showing a second example of a mold used in the present invention.

FIG. 4 is a cross-sectional view of the mold of FIG.

FIG. 5 is a longitudinal sectional view showing a third example of a mold used in the present invention.

FIG. 6 is a cross-sectional view of the mold of FIG.

FIG. 7 is a longitudinal sectional view showing a fourth example of a mold used in the present invention.

FIG. 8 is a cross-sectional view of the mold of FIG. 7;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gate 1a Gate corresponding to wide area 1b Gate corresponding to narrow area 2 Mold cavity 3 Mold 4 Resin pressure inlet 5 Runner 5a Runner corresponding to wide area 5b Runner corresponding to narrow area

Continuation of the front page (56) References JP-A-4-62122 (JP, A) JP-A-2-108510 (JP, A) JP-A-60-63120 (JP, A) JP-A-3-71817 (JP) JP-A-50-71756 (JP, A) JP-A-3-124416 (JP, A) JP-A-58-118736 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB Name) B29C 45/00-45/84

Claims (3)

(57) [Claims] 1. An injection nozzle injects molten resin into a mold cavity through a plurality of gates at the same time, and then pressurized gas is injected into the mold cavity through a plurality of gates from a gas nozzle built in the injection nozzle. A hollow injection molding method for press-fitting, wherein the molten resin is injected into the mold cavity by substantially equalizing the molten resin pressure immediately before each gate. 2. The hollow injection method according to claim 1, wherein each runner branched from the sprue to each gate uses a mold having the same diameter and length. 3. A hollow injection mold according to claim 2, wherein a mold having a difference in the space width of the mold cavity, wherein the number of gates corresponding to a wide area is larger than the number of gates corresponding to a narrow area. Molding method.