JPH04201423A - Manufacture of hollow formed product - Google Patents

Abstract

PURPOSE:To prevent pressurizing gas from forcibly entering basic thick part by a method wherein the cross-sectional area of a cavity between some part of thick-walled part to be hollowed and its extension part is made to be smaller at the hollowing of at least said part, of the thick-walled part performed by pouring the pressurizing fluid into the molten resin in the cavity. CONSTITUTION:Partitions 5 are provided between the thick-walled part 4 of a mold and their extension parts and made preferably to be forward and backward movable to the direction of a cavity. In addition, counter-pressure is preferably applied by charging pressurizing gas in the cavity before the injection of molten resin. After the injection of the predetermined amount of resin, the partitions 5 are advanced in order to make the cross-sectional area of the cavity smaller. Next, a needle 6 is advanced so as to pour pressurized gas in order to produce a hollow space in the molten resin and, at the same time, to push the molten resin towards the gap at the tip of the cavity, resulting in moving the molten resin so as to come into close contact with the mold. During this time, the counter pressure is released. Next, when the partitions 5 are retreated, the molten resin advances as the partitions 5 retreat, developing no defective part at the sites of the partitions 5 of a formed article.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for making hollow molded objects from resin.

[Conventional technology]

After or during introduction of the molten resin into the mold cavity, a pressurized fluid is introduced into the molten resin within the mold cavity;
In particular, there are many known methods for making hollow objects by injecting pressurized gas and releasing the pressure after the resin has solidified (for example, Japanese Patent Application Laid-Open No. 60-24913). By making the molded object hollow, The molded body can be made lightweight, and the amount of resin used can be reduced.

As shown in Figure 4, the mold cavity is constructed so that a part of the molded body is thick, and pressurized gas is injected into the molten resin in this thick part. It is also known to proceed so as to create a hollow space. Such a flow path for pressurized gas is called a gas channel, and the thick wall portion is 2 to 5 times as thick as the basic wall thickness. The thick interior also serves to increase the stiffness of the compact.

[Problem to be solved by the invention]

However, even if the pressurized gas flow path is defined as described above, in reality, as shown in Figure 5 (partial cross-sectional view of the molded product in Figure 4), the basic wall thickness outside the gas channel is There was a tendency for gas to enter the area.

Conversely, pressurized gas often does not enter a portion of the gas channel portion. In other words, the distribution of hollow parts in a molded product may differ considerably from that intended at the time of product design, and furthermore, this difference is not constant from mold to mold. As a result, the physical properties, dimensional accuracy, and stability of the molded product deteriorate, and productivity also decreases.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks, and to provide a method in which a pressurized fluid flows only through pre-designed channels, thereby producing a molded body having a designed hollow portion.

[Means to solve the problem]

The present invention includes introducing a molten resin into a mold cavity and making at least a part of a thick walled part of a molded article hollow by injecting a pressurized fluid into the molten resin in the mold cavity. In the method for manufacturing a hollow object, the cross-sectional area of the cavity is narrowed between at least a part of the thick walled part to be hollow and its extension part, thereby preventing pressurized fluid from entering the extension part. This method is characterized by the following.

The pressurized fluid is preferably a pressurized gas, hereinafter simply referred to as pressurized gas.

The present invention will be described with reference to the drawings.

FIG. 1 is a conceptual diagram of a cross section of a mold of an injection molding machine used in the present invention. 1 is an upper mold, 2 is a lower mold, and 3 is a cavity. In a preferred embodiment, the cavity is filled with a pressurized gas, preferably air or nitrogen, to provide a counterpressure prior to injecting the molten resin into the cavity.

Next, molten resin is injected into the cavity. The amount of molten resin is a predetermined amount insufficient to fill the cavity,
It depends on the desired hollow volume fraction. The volume percentage of hollow parts in hollow products is, for example, 10 to 70%, especially 30 to 70%.
It is 60%.

A partition 5 that can move forward and backward in the direction of the cavity is provided between the thick part 4 and its outer extension part in the mold. After injecting (or while injecting) a predetermined amount of molten resin as described above, the partition 5 is moved forward to narrow the cavity cross-sectional area.

This state is shown in FIG. How narrow the cavity cross-sectional area should be depends on factors such as the thickness of the basic wall thickness of the outer extension, the complexity of the overall shape of the molded product, the pressure of the pressurized gas, and the flow viscosity of the resin. 10-90% of the thickness of the thick part
, preferably 30 to 70% of the cross-sectional area should remain. If a cross-sectional area smaller than this is left, there is a risk that the flow of molten resin through the cross-section will be blocked when pressurized gas is injected, which will be described later, and a void will remain at the tip of the cavity.

On the other hand, if a cross-sectional area larger than the above is left, the effect of the present invention will be small.

A pressurized gas (eg, air or nitrogen) is then injected directly into the molten resin in the thick section. To do this, the mold is provided with a needle 6 whose tip is located in the cavity as shown in FIG. The needle 6 may be fixed or may be movable back and forth. FIG. 2 shows a mode in which the needle 6 is retracted before injecting the molten resin. After injecting the molten resin, the needle 6 is advanced so that the tip of the needle reaches the thick inside. Pressurized gas is injected through the needle 6. As pressurized gas is injected, a hollow is formed in the molten resin, and as the hollow becomes larger, the molten resin is simultaneously pushed toward the gap at the tip of the cavity and moves tightly to the mold surface. . In addition, when the counter pressure is applied to the cavity according to the above-mentioned preferred embodiment, the counter pressure at the cavity tip is released while the molten resin is flowing toward the cavity tip.

Then, the partition 5 is moved back and returned to its original position as shown in FIG. At this time, the gas in the hollow of the thick wall part is still under high pressure and presses the molten resin toward the mold surface, so that as the partition 5 retreats, the molten resin advances.

Therefore, in the finished molded product, there is no recess at the partition 5.

The resin is then solidified and the pressure of the pressurized gas inside the hollow is reduced to atmospheric pressure. For this purpose, the pressurized gas is discharged into the atmosphere through the needle 6 or is withdrawn.

In the above, a mode in which each operation is performed sequentially has been described, but each operation may partially overlap.

In particular, the injection of pressurized gas may be started during the advancement of the partition 5, and the injection of the pressurized gas may begin slightly before the end of the injection of pressurized gas into the partition 5.
may be moved back.

The above-mentioned partition 5 can be provided in all of the thick-walled portions, but may be provided in a part of the thick-walled portions in order to avoid complicating the mold structure. The location where the partition 5 should be provided is a location where the pressurized gas can easily penetrate into the basic wall thickness, and although this depends on the structure of the molded product, it is generally close to the pressurized gas injection port, that is, the needle 6. This is a thick part. The same applies when there are multiple pressurized gas injection ports. Although it is generally preferred that the partition 5 be provided directly adjacent to the thick portion, it may also be provided a short distance away from the thick portion, for example 1 to 5 mm, depending on the shape of the thick portion.

As a mechanism for moving the partition 5 forward and backward, the partition 5 can be connected to a rod driven by hydraulic pressure or pneumatic pressure, and the partition 5 can be driven at an appropriate timing.

Furthermore, the partition 5 may be fixed to the mold.

This aspect is shown in FIG. In this case, if the cross-sectional area remaining at the partition 5 is too small, problems may arise in terms of the strength of the molded product, or the flow of the molten resin will be restricted when pressurized gas is injected, causing the resin to reach the tip of the cavity. The problem may arise that it does not. The cross-sectional area can be determined as appropriate depending on the shape of the molded product, the required strength, etc. In general, the basic thick part 5
It is 0-70%. This embodiment can also be carried out in the same manner as the mold shown in FIG. 1 above, except that the partitions are not moved forward or backward.

〔Effect of the invention〕

By providing the partition 5 in the present invention, pressurized gas is prevented from entering the basic thick part.

Therefore, a hollow portion can be created in the molded product as designed with good reproducibility.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a mold implementing the present invention. The numbers in the figure indicate the following. 1 and 2 = Upper mold and lower mold 3: Cavity 4: Thick walled part 5: Partition 6; The needle for pressurized gas in Fig. 1 is
It is a partially enlarged view of the figure, showing a state in which resin is injected. FIG. 3 is a schematic cross-sectional view of a mold for implementing the present invention, in a different aspect from FIG. 1. The numbers in the figure indicate the same as above. FIG. 4 is a sketch showing the appearance of the molded product of the present invention (and the conventional method). FIG. 5 is a cross-sectional view of a part of a molded product made by a conventional method. Applicant 2 Japan GE Plastics Co., Ltd. II1
Figure 2 Figure 3

Claims (1)

[Claims] 1. Molten resin is introduced into a mold cavity, and a pressurized fluid is injected into the molten resin in the mold cavity to make at least a part of the thick part of the molded body hollow. In the method for manufacturing a hollow object, the cross-sectional area of the cavity is narrowed between at least a portion of the thick walled portion to be hollow and its outer extension, so that pressurized fluid enters the outer extension. A method characterized by preventing. 2. In the part where the cavity cross-sectional area is narrowed,
Claim 1: A mold surface constituting the cavity is capable of moving forward and backward toward the cavity, and the cross-sectional area of the cavity is narrowed by advancing the mold surface before or while injecting the pressurized fluid. The method described in section. 3. After injection of the pressurized fluid is completed, the mold surface is retreated, and at this time, the molten resin flows to follow the retreating mold surface, creating a hollow molded product with no recesses on the surface of the molded product. A method according to claim 2 for obtaining. 4. The method according to claim 1, wherein a mold is used in which the mold surface protrudes toward the cavity at the portion where the cavity cross-sectional area is narrowed.