JPH09193187A - Method for molding hollow product and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to uniformly mold the thickness of a hollow product by increasing the flowing resistance of the resin extruded by the movement of a floating core to a sub-cavity after the start of moving the core, and decelerating the flowing velocity. SOLUTION: A main molding is molded by a main cavity 1, and a sub- molding is molded by a sub-cavity 7. The sub-molding is used for a product or reuse. Pressurized fluid is press injected from a pressure pump 5 in the state that the cavity 1 is fully filled with a molten resin, a floating core 4 is moved, and the resin is extruded from a communication port 6 to a primary sub-cavity 7a. The extruded resin is fed to a secondary sub-cavity 7b to become a sub- molding. The opening area of the connecting port 8 communicating with the cavities 7a, 7b is reduced smaller than that of the port 6. Thus, the resin flowing velocity to the cavity 7 is decelerated, and the moving speed of the core 4 is suppressed to obtain a molding having uniform thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for molding a hollow body such as a pipe.

[0002]

2. Description of the Related Art Conventionally, as a method and apparatus for integrally molding a pipe with a synthetic resin, a floating core having a diameter corresponding to the inner diameter of the pipe and a pressure port for pressurizing a pressurized fluid are provided at one end of a main cavity. Is provided,
Use a mold in which a sub-cavity is provided at the other end of the main cavity through a communication port, fill the main cavity with molten resin, and then pressurize a pressurized fluid from a pressure port to connect the floating core. It is known that the hollow pipe is integrally molded by moving it to the mouth side to form a hollow portion in the resin in the main cavity and extruding the surplus resin from the communication port to the sub cavity. 4-2084
No. 25 gazette: To facilitate understanding, some member names are described in a unified manner with the present invention).

The above-mentioned conventional method and apparatus have a great feature in that the hollow portion is formed by mainly using injection molding and pressurizing the pressurized fluid together with moving the floating core. It has an advantage that a pipe in good condition and having high dimensional accuracy can be obtained. Further, for example, it is easy to mold with a reinforced resin having a reinforced fiber such as glass fiber, carbon fiber, and metal fiber, and the bending is not limited to a straight pipe as long as the bending of the floating core is possible. It also has the advantage of being able to manufacture pipes.

[0004]

However, in the conventional method and apparatus described above, the floating core is moved by pressurizing the pressurized fluid at a high pressure all at once, so that the floating core is instantaneously pressed by the pressurizing fluid. Move to the communication port side. As a result, the floating core does not necessarily pass through the center of the main cavity, but passes through a biased position, which causes a large variation in the thickness distribution of the obtained hollow body. In particular, in the case where the hollow body to be formed is a bent pipe or the like, the floating core has a strong tendency to pass near the inner circumference of the bent portion, and the fluctuation of the wall thickness distribution becomes remarkable.

On the other hand, it is difficult to control the moving speed of the floating core by the pressure of the pressurized fluid or the speed of press-fitting because it is difficult to move the floating core if the low-pressure pressurized fluid is slowly pressed.

Further, the resin extruded into the sub-cavity by the conventional method and apparatus is taken out as a block-shaped or rod-shaped lump. The resin extruded into the sub-cavity is preferably crushed and used for recycling in order to eliminate waste of material, but the lump resin is difficult to crush,
It is an obstacle to recycling.

The present invention has been made in view of the above-mentioned conventional problems, and it is an object of the present invention to provide a hollow body having a uniform wall thickness distribution. The present invention also provides a second
In addition, the purpose is to facilitate the recycling of the resin extruded into the sub-cavity.

[0008]

In order to achieve the above first object, in the invention of claim 1, a pressurizing port having a floating core is provided at one end, and an openable and closable communication port is provided at the other end. After injecting the molten resin into the main cavity where the sub-cavities communicate with each other, pressurizing fluid is pressed from the pressure port to move the floating core to the communication port side and push the resin from the communication port to the sub-cavity. In the hollow body molding method, the resistance of the resin extruded by the movement of the floating core into the sub-cavity is increased after the movement of the floating core is increased, and the flow velocity of the resin into the sub-cavity is reduced to move the floating core. It is what controls the speed.

In order to make it possible to easily carry out the method according to the first aspect of the present invention, in the third aspect of the present invention, a pressurizing port having a floating core is provided at one end, and the opening / closing is provided at the other end. It has a main cavity in which the sub-cavities communicate with each other through possible communication ports, and after injecting the molten resin into the main cavity, pressurizing fluid from the pressurizing port to move the floating core to the communication port side. Also, in the hollow body molding device for extruding resin from the communication port to the sub-cavity, the sub-cavity is composed of a primary sub-cavity connected to the communication port and a secondary sub-cavity connected to the primary sub-cavity via the connection port. The opening area of the connection port is made smaller than that of the opening.

In order to achieve the second object in addition to the first object, in the invention of claim 4, in the hollow body molding apparatus according to the invention of claim 3, a secondary sub-cavity is provided. It is branched into a plurality of thin plates or thin rods.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION First, a hollow body molding apparatus according to the present invention will be described with reference to FIGS.

Reference numeral 1 in the drawing denotes a main cavity, and the main cavity 1 shown has a shape along the outer shape of a main molded product 2 as shown in FIG. 8, and a molten resin is supplied from a gate 3. It is a thing.

At one end of the main cavity 1, a main molded product 2
The floating core 4 having a diameter corresponding to the inner diameter of the pipe (see FIG. 8) is provided, and the pressurizing fluid for moving the floating core 4 to the other end side of the main cavity 1 is pressed. Port 5 is provided. The pressurizing port 5 is connected to a pressurizing fluid system (not shown) for pressurizing and discharging the pressurizing fluid.

The floating core 4 is pressed by the pressurizing fluid which is press-fitted from the pressurizing port 5.
Is provided in the main cavity 1 with the back as
For example, in addition to metal such as brass, stainless steel, iron, and aluminum, resin may be used as long as it does not melt and deform significantly during molding. In particular, if it is made of resin, since it is light, it can be pressed and moved without increasing the pressure of the pressurized fluid so much. Further, as compared with the case of metal, the resin injected and contacting the floating core 4 is less likely to be rapidly cooled, so the main molded product 2 (see FIG. 8)
There is an advantage that the molding state of the inner surface of the pressure port 5 side is improved. The shape of the floating core 4 may be, for example, a conical shape, a shell shape, a hemispherical shape or the like as long as the maximum diameter corresponds to the inner diameter of the main molded product 2 (see FIG. 8), in addition to the spherical shape shown. it can.

A communication port 6 is provided on the other end side of the main cavity 1, and the main cavity 1 is connected through the communication port 6.
Are communicated with the sub-cavity 7. Although the communication port 6 has a size that allows the floating core 4 to pass therethrough,
It has a slightly constricted shape.

The sub-cavity 7 is composed of a primary sub-cavity 7a directly connected to the communication port 6 and a secondary sub-cavity 7b connected to the primary sub-cavity 7a via a connection port 8. According to the main molding apparatus, the main cavity 1 molds the main molded product 2 (see FIG. 8), and the sub cavity 7 molds the sub molded product 9 (see FIG. 8). The sub-molded product 9 can be separated from the main-molded product 2 and used as it is, but it is usually crushed and reused.

As will be described later, the primary sub-cavity 7a has a floating core 4 which is pushed and moved by a pressurizing fluid which is press-fitted from a pressurizing port in a state where the main cavity 1 is filled with molten resin, and a movement of the floating core 4. Accordingly, it has a volume capable of accommodating a part of the resin pushed out from the communication port 6. Specifically, the primary sub-cavity 7a
Where V _{1 is} the volume and V _c is the volume of the floating core 4, it is preferable to have a volume in the range of 10 V _c ≧ V ₁ ≧ 2V _c . Primary sub-cavity 7
If the volume of a is too smaller than this, the floating core 4 finally reaches the communication port 6 even if an attempt is made to move the floating core 4 by pressurizing a pressurized fluid while the main cavity 1 is filled with the molten resin. It becomes difficult to obtain the initial speed for That is, if the volume V _c of the primary cavity 7a is too small, unnecessary pressure loss tends to occur when the floating core 4 starts moving due to the fluid pressure.
If the volume V _c of the primary cavity 7a is too large,
The speed of the floating core 4 cannot be controlled enough,
It becomes difficult to prevent the variation of the thickness distribution of the obtained main molded product 2 (see FIG. 8).

The secondary sub-cavity 7b communicates with the primary sub-cavity 7a through a connection port 8. The opening area of the connection port 8 that communicates between the primary sub cavity 7a and the secondary sub cavity 7b is smaller than the opening area of the communication port 6 that communicates between the main cavity 1 and the primary sub cavity 7a. When the resin is further extruded from the communication port 6 after the primary sub-cavity 7a is filled with the resin, the resin filling the primary sub-cavity 7a is extruded to the secondary sub-cavity 7b. At this time, since the opening area of the connection port 8 is smaller than the opening area of the communication port 6 and the resin is less likely to enter the secondary sub-cavity 7b, the resistance of the resin to flow from the communication port 6 to the sub-cavity 7 is reduced. Becomes large and the inflow rate of the resin into the sub-cavity 7 decreases. Then, as a result, the moving speed of the floating core 4 is suppressed.

The volume of the secondary sub-cavity 7b is equal to the total volume of the primary sub-cavities 7a (the sub-cavity 7b).
The total volume) may be set to be equal to or larger than the total volume of the resin extruded from the communication port 6 and the floating core 4. In addition, the sub-cavity 7b
Is preferably branched into a plurality of thin plates or thin rods as illustrated. With such a sub-cavity 7b, the sub-molded product 9 shown in FIG. 8 has a shape branched into a plurality of thin plates or thin rods, and this sub-molded product 9 part is separated from the main molded product 2 part and reused. When
It has the advantage of being easily crushed and reused.

If the secondary sub-cavity 7b is branched into a plurality of thin plates or thin rods as shown in the drawing, the flow resistance of the resin in the secondary sub-cavity 7b can be increased, so that the primary sub-cavity is It becomes easy to make a difference in inflow resistance of the resin into the sub-cavity 7 before and after filling 7a, that is, a difference in the inflow speed of the resin into the sub-cavity 7. The cross-sectional shape of this branch part is circular, semicircular, rectangular, etc.
Although it can be arbitrarily set, the flow cross-sectional area ratio A of the primary sub-cavity 7a and the secondary sub-cavity 7b is 1/100 ≦ A ≦ 1 / in any cross section of the secondary sub-cavity 7b.
It is preferably 3. If the flow cross-sectional area ratio A is less than 1/100, it becomes difficult for the resin to flow from the primary sub-cavities 7a into the secondary sub-cavities 7b, and the movement of the floating core 4 is likely to be stopped. Further, if the flow cross-sectional area ratio A exceeds 1/3, the flow resistance of the resin in the secondary sub-cavity 7b cannot be increased so much, and each branch portion also becomes thick, and the secondary sub-cavity becomes thicker. The meaning of branching the cavity 7b into a plurality of thin plates or thin rods is lost.

The above flow cross-sectional area ratio A is obtained by (flow cross-sectional area of secondary sub-cavity 7b) / (flow cross-sectional area of primary sub-cavity 7a), and each flow cross-sectional area is the flow direction of resin. Cross-sectional area in the direction perpendicular to

A receiving shaft 10 which can be moved forward and backward toward the communication port 6 is inserted through almost the center of the primary sub-cavity 7a. This receiving shaft 10 is, as shown in FIG.
The forward end peripheral portion is pressed against the peripheral wall of the communication port 6 when moving forward to close the communication port 6, and as shown in FIG. 2, the communication port 6 is opened when moving backward. Moreover, the tip end of the receiving shaft 10 can mount the floating core 4 which moves to the primary sub-cavity 7a at the time of pressurization of the pressurized fluid. In this example, the communication port 6 is opened and closed by advancing and retracting the receiving shaft 10, but the opening and closing of the communication port 6 may be performed by another opening / closing means instead of the receiving shaft 10. .

Next, a molding procedure using the above molding apparatus will be described with reference to FIGS.

First, with the receiving shaft 10 advanced and the communication port 6 closed, molten resin is injected from the gate 3 into the main cavity 1 to fill the main cavity 1 with the resin as shown in FIG.

As the resin, a wide variety of thermoplastic resins used in general injection molding and extrusion molding can be used, and if necessary, a thermosetting resin can also be used. In addition, these resins may contain glass fiber, carbon fiber,
Reinforcing fibers such as metal fibers, various fillers, additives, colorants and the like can be added.

Injection of the molten resin is carried out by an injection machine as in the case of ordinary injection molding. The injection pressure is the same as in ordinary injection molding, and it varies depending on the type of resin used, the presence / absence of addition of reinforcing fibers, and the addition amount thereof, but is generally 50 to 200 k.
It is about g / cm ² G.

The molten resin is injected by the floating core 4
Is performed on the pressurizing port side. this is,
For example, the gate 3 (see FIG. 1) may be provided on the communication port 6 side of the floating core 4.

Next, as shown in FIG. 4, the receiving shaft 10 is retracted to the extent that the floating core 4 can be received in the primary sub-cavity 7a to open the communication port 6, and the pressurized fluid is supplied from the pressurizing port 5 to the pressurized fluid. Press fit.

As the pressurized fluid, a gas or liquid that does not react or be compatible with the resin used under the temperature and pressure of injection molding is used. Specifically, for example, nitrogen gas, carbon dioxide gas, air, glycerin, liquid paraffin and the like can be used, but an inert gas such as nitrogen gas is preferable.

When a gas such as nitrogen gas is used to pressurize the pressurized fluid, the pressurized gas stored in the accumulator tank is preliminarily boosted by a compressor and introduced to the pressurization port 5 through a pipe, or the compressor is compressed. It is possible to directly pressurize the pressurized gas to the pressurizing port 5 and sequentially increase the pressure. In the former case, the pressure of the pressurized gas supplied to the pressure port 5 varies depending on the type of resin used and the like, but is usually 50 to 30.
It is about 0 kg / cm ² G.

When the pressurized fluid is pressed in, the floating core 4 leaves the resin near the outer periphery of the main cavity 1 in which the cooling and solidification has started, and as shown in FIG. While pushing out to the primary sub-cavity 7a through the communication port 6, it moves to the primary sub-cavity 7a. After the floating core 4 has passed, the hollow portion 11 having a diameter substantially equal to the diameter of the floating core 4 is formed. Therefore, by selecting the diameter of the floating core 4, the diameter of the hollow portion 11 to be formed (the inner diameter of the hollow molded product) can be adjusted. Further, the resin around the hollow portion 11 is pressed against the peripheral wall surface of the main cavity 1 by the pressure of the pressurized fluid that has been press-fitted, and its shape is maintained.

As shown in FIG. 5, the receiving shaft 10 is further retracted, and the interior of the primary sub-cavity 7a is filled with resin.
When the floating core 4 further advances to push out the molten resin from the communication port 6, the resin in the primary sub-cavity 7a is pushed out from the connection port 8 into the secondary sub-cavity 7b.

Since the communication port 6 has a relatively large opening area and the flow cross-sectional area of the primary sub-cavity 7a is also relatively large, the resistance of the resin to flow into the sub-cavity 7 until the primary sub-cavity 7a is filled with the resin. Is small and the resin easily flows into the sub-cavity 7. Therefore, by the pressurization of the pressurized fluid, the floating core 4 starts to move smoothly and moves forward while pushing out the molten resin at a high speed. However, the opening area of the connection port 8 is smaller than that of the communication port 6, and the resin does not easily flow from the primary sub-cavity 7a to the secondary sub-cavity 7b, so that the primary sub-cavity 7a is filled. After that, the inflow resistance of the resin into the sub-cavity 7 increases, and the sub-cavity 7
The inflow speed of the resin into is suppressed. Along with this, the moving speed of the floating core 4 is suppressed, and the thickness distribution of the main molded product 2 (see FIG. 8) can be made uniform, and
The smoothness of the inner surface can also be improved. If the moving speed is suppressed before the floating core 4 reaches the curved portion, the wall thickness in the circumferential direction can be made uniform in the curved portion where the wall thickness distribution is apt to vary.

As described above, the secondary sub-cavity 7b is branched into a plurality of thin plates or thin rods, and the flow sectional area ratio A of the primary sub-cavity 7a and the secondary sub-cavity 7b is
By setting 1/100 ≦ A ≦ 1/3, it is possible to more reliably suppress the moving speed of the floating core 4. In addition, the volume V _{1 of the} primary sub-cavity 7a
Is 10 V with respect to the volume V _c of the floating core 4.
_When the range of _c ≧ V ₁ ≧ 2V _c is set, the speed of the floating core 4 can be sufficiently controlled while reliably obtaining the initial speed for the floating core 4 to finally reach the communication port 6. .

While controlling and suppressing the moving speed of the floating core 4 as described above, the floating core 4 is advanced until it enters the primary sub-cavity 7a, as shown in FIG. The floating core 4 is placed on the receiving shaft 10 in a state of being inserted into the primary sub-cavity 7a. Further, the volume of the sub-cavity 7 is set to be larger than the total volume of the resin to be extruded and the floating core 4, so that the hollow portion 11 is formed to the inside of the primary sub-cavity 7a as shown in FIG. If you put
The thickness of the portion molded in the primary sub-cavity 7a of the sub-molded product 9 (see FIG. 8) is also thin, which is preferable because it can be easily crushed.

After the resin in the mold is cooled, the pressurized fluid in the hollow portion 11 is discharged, the receiving shaft 10 is further retracted, the primary sub-cavity 7a is retracted, and the molded product is taken out from the mold. The discharge of the pressurized fluid can be performed by opening the pressure port 5 to the atmosphere when gas is used as the pressurized fluid, but is recovered in a recovery tank (not shown) and circulated. It is preferable.

The molded product to be taken out is as shown in FIG. 8, and comprises a main molded product 2 molded in the main cavity 1 and a sub molded product 9 molded in the sub cavity 7. Since a thin constricted portion 12 formed by the constricted communication port 6 is interposed between the main molded product 2 and the sub molded product 9, the main molded product 2 is separated from the sub molded product 9 through the constricted portion 12. The desired hollow molded body can be obtained by separating.
As described above, the floating core 4 is mounted on the receiving shaft 10 in a state of entering the sub-cavity 7 (primary sub-cavity 7a) and remains in the sub-molded product 9, which is advantageous. The main molded product 2, which is a pipe, has both ends opened from the beginning.

The case of molding a curved pipe has been described as an example, but the hollow molded product molded in the present invention may be a straight pipe or other molded product.

[0039]

【Example】

Example 1 Outer diameter 3 cm, inner diameter 2.4 cm, wall thickness 0.3 cm, total length 3
A 5 cm U-shaped pipe was molded using a molding apparatus and an injection molding machine (“BMT4000” manufactured by Battenfeld, Germany) as shown in FIGS. 1 and 2.

The diameter of the main cavity was 3 cm, the diameter of the communication port was 2.6 cm, the diameter of the primary sub-cavity was 3.4 cm, the length was 4 cm, and the volume V ₁ was about 37 cm ³ . The diameter of the connection port that connects the primary and secondary sub-cavities is 0.6 cm, and extends 10 mm from the connection port to the left and right of the primary and secondary cavities in the shape of a thin rod to the left and right, and further down to the left and right, 8 each. A secondary sub-cavity having a diameter of 0.6 cm and a length of 20 cm branched into a thin rod was provided. The volume of the secondary sub-cavity was about 130 cm ³ . As the floating core, a steel ball having a diameter of 2.4 cm (volume: about 7.3 cm ³ ) was used.

As a molding material, polyamide 66 containing 33% by weight of glass fiber (“Leona 1” manufactured by Asahi Kasei Corporation) was used.
300G ”), and injecting this at a resin temperature of 290 ° C. and an injection pressure of 90 kg / cm ² G.
After a second, the floating core is pressed at a pressure of 100 kg / cm ^2.
The nitrogen gas of G was moved by being pressed in from the pressure port.

The molded product was taken out 30 seconds after the pressurization of the pressurized gas. The obtained molded product was a U-shaped pipe having an outer diameter of 3 cm, an average inner diameter of 2.4 cm, an average wall thickness of 0.3 cm, and a total length of 35 cm. The variation in wall thickness of this pipe was within ± 30% even at the curved portion. The obtained pipe was loaded with an internal pressure of 3 kg / cm ² in an environment of 80 ° C. and a durability test was carried out for 2000 hours. As a result, no abnormality occurred and it was used for industrial purposes such as water distribution pipes and automobile cooling pipes. It had sufficient performance as a curved pipe. Further, the main part of the sub-molded product molded in the sub-cavity was a branched thin rod having a diameter of 0.6 cm, the load on the crusher was small, and it could be easily recycled.

Comparative Example 1 A sub-cavity having a diameter of 4 cm and a length of 13.3 cm was prepared.
A similar U-shaped pipe was molded in the same manner as in Example 1 except that the volume was about 167 cm ³ and only the primary sub-cavity was used. The obtained molded product has an outer diameter of 3 cm and an average inner diameter of 2.4 c.
It was a U-shaped pipe having m, an average wall thickness of 0.3 cm, and a total length of 35 cm. The thickness variation of this pipe is ± 35% at the curved part
That was all. 3k on the obtained pipe under the environment of 80 ℃
As a result of carrying out a durability test for 2000 hours while applying an internal pressure of g / cm ² , cracks were generated from the thin portion.

[0044]

EFFECTS OF THE INVENTION The present invention is as described above, and it is possible to easily form a hollow molded product having a substantially uniform wall thickness distribution, and the sub-cavities are a primary sub-cavity and a secondary sub-cavity. By dividing the secondary sub-cavity into a thin plate or a thin rod shape, the resin extruded into the sub-cavity can be reused easily.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an example of a hollow body molding device according to the present invention.

FIG. 2 is a view showing a state where a receiving shaft of the hollow body molding device shown in FIG. 1 is retracted.

FIG. 3 is a view showing a state where the main cavity of the hollow body molding device shown in FIG. 1 is filled with resin.

FIG. 4 is a diagram showing a process of moving a floating core toward a sub-cavity by pressurizing a pressurized fluid from a pressure port.

FIG. 5 is a view showing a process in which a pressurized fluid is press-fitted from a pressurizing port to move the floating core to the sub-cavity side.

FIG. 6 is a diagram showing a process of moving a floating core toward a sub-cavity by pressurizing a pressurized fluid from a pressure port.

FIG. 7 is a diagram showing a state in which a floating core has moved into a sub-cavity.

FIG. 8 is a view showing the obtained molded product.

[Explanation of symbols]

 1 Main Cavity 2 Main Molded Product 3 Gate 4 Floating Core 5 Pressurizing Port 6 Communication Port 7 Secondary Cavity 7a Primary Secondary Cavity 7b Secondary Secondary Cavity 8 Connection Port 9 Secondary Molded Product 10 Bearing Shaft 11 Hollow Part 12 Constricted Part

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication // B29L 22:00

Claims (6)

[Claims] 1. A molten resin is injected into a main cavity which has a pressurizing port having a floating core at one end and a sub-cavity communicates via an openable / closable communication port at the other end, and then the molten resin is injected from the pressurizing port. In a hollow body molding method in which a pressurized fluid is press-fitted and the resin is extruded from the communication port to the sub-cavity while moving the floating core to the communication port side,
It is possible to suppress the moving speed of the floating core by increasing the inflow resistance of the resin extruded by the movement of the floating core into the sub-cavity after starting the movement of the floating core, and decreasing the inflow speed of the resin into the sub-cavity. A method for forming a hollow body, which is characterized. 2. The floating core is moved to increase the resistance of the remaining resin to flow into the sub-cavity after the resin of 2 times or more and 10 times or less of the volume V _c of the floating core is extruded into the sub-cavity. A method for forming a hollow body, comprising: 3. A main cavity having a pressurizing port having a floating core at one end and a sub-cavity communicating at the other end with a communication port that can be opened and closed, and a molten resin is injected into the main cavity. After that, in the hollow body molding device that pressurizes the pressurized fluid from the pressure port to move the floating core to the communication port side and extrude the resin from the communication port to the sub cavity, the sub cavity is connected to the communication port. A hollow body molding device comprising a cavity and a secondary sub-cavity connected to the primary sub-cavity via a connection port, and having an opening area of the connection port smaller than that of the communication port. 4. The hollow body molding device according to claim 3, wherein the secondary sub-cavity is branched into a plurality of thin plates or thin rods. 5. The hollow body molding apparatus according to claim 4, wherein a flow cross-sectional area ratio A of the primary sub-cavities and the secondary sub-cavities is 1/100 ≦ A ≦ 1/3. 6. When the volume of the primary sub-cavity is V ₁ and the volume of the floating core is V _c , 10 V _c ≧ V ₁
6. The hollow body molding apparatus according to claim 3, wherein ≧ 2V _c .