JP2003048239A - Injection molding method, injection molding machine used for the same and screw of injection molding machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an injection foam molding method capable of sufficiently stirring and kneading a resin even if a screw is short, capable of certainly preventing the back flow of the molten resin to a plasticizing part by the resin pressure at the time of injection of a foaming agent impregnated molten resin in a weighing part and capable of also manufacturing a small-sized molded article, an injection molding machine used therein and the screw of the injection molding machine. SOLUTION: A high pressure resin forming part having a stirring blade structure, which makes the absolute value of a resin feed speed less than that of the resin feed speed of the plasticizing part, is provided between the plasticizing part and stirring part of the screw of the injection molding machine and the back flow of the foaming agent impregnated molten resin to the plasticizing part is prevented by the molten resin high pressure part formed by the high pressure resin forming part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection molding method, an injection foam molding method, an injection molding machine used in these molding methods, and a screw of the injection molding machine.

[0002]

2. Description of the Related Art A gas which is non-reactive with a resin such as carbon dioxide gas or nitrogen gas (hereinafter referred to as "non-reactive gas") which becomes a free-flowing agent and / or a foaming agent is melted before injection. As an injection molding method of a thermoplastic resin foam in which a resin is impregnated and the molten resin impregnated with a non-reactive gas is injected into a mold to foam, 1) a raw material resin in a solid state before being charged into a cylinder Known methods include a method of previously impregnating a non-reactive gas under high pressure, and a method of impregnating a molten resin melted in a cylinder with a non-reactive gas.

However, in the former method of impregnating a solid state raw material resin with a non-reactive gas, it is possible to obtain a thermoplastic resin foam, but for impregnating a non-reactive gas, for example, a molding machine is used. Since it is necessary to stop the supply of the resin to and to impregnate the gas in the pressure resistant chamber, it takes several tens of hours to reach the gas saturated impregnation state, which is difficult to carry out industrially.

On the other hand, in the latter method, the resin pellets or the like supplied from the raw material supply port into the cylinder are plasticized in the plasticizing part in the cylinder, and then the non-reactive gas such as carbon dioxide gas is blown in the stirring part. As a result, the foaming agent is impregnated into the molten resin while it is fed into the cylinder and the screw is retracted to measure the molten resin at the tip measuring portion of the cylinder. By the way, in the latter method, since the non-reactive gas is supplied to the high-pressure molten resin during measurement, it is difficult to inject the gas into the molten resin unless the pressure of the supply gas is higher than the molten resin pressure.

In view of this, in Japanese Patent Laid-Open No. 11-64130, a check ring for preventing backflow is attached to the screw main body between the plasticizing portion and the stirring portion of the screw, so that the non-reactive gas in the stirring portion is removed. The supplied high-pressure resin is prevented from flowing back to the plasticizing section side. However, as described above, if a check ring is provided between the plasticizing section and the stirring section, it will not be possible to stir the resin at the check ring section. Kneading may be insufficient.
Therefore, in order to sufficiently stir the resin, the length of the injection molding machine must be long, which causes a problem of high equipment cost.

Further, the screw having the structure in which the check ring is provided between the plasticizing section and the stirring section as described above has a screw member for the plasticizing section in which stirring blades are formed around the shaft and the stirring section. Screw member for external use, after the check ring is externally fitted to the rear end portion of the screw member for the stirring unit, a male screw unit extending along the central axis of the screw from the tip of the screw member for the plasticizing unit is added to the stirring unit. It is obtained by connecting by screwing into a screw hole provided along the central axis of the screw at the rear end of the screw member for use.

However, if the screw structure as described above is to be adopted in an injection molding machine having a small diameter such as 50φ or less, the male screw portion is forced to be small in diameter, and therefore the male screw portion has a resin pressure at the time of injection. May be damaged by. Therefore, with the structure in which the check ring is provided between the plasticizing section and the stirring section as described above, it is difficult to manufacture a small molded product.

[0008]

In view of such circumstances, the present invention is capable of sufficiently agitating and kneading a resin even when the screw length is short, and is effective for injection of molten fluid-impregnated molten resin in a measuring section. An injection foam molding method capable of reliably preventing the backflow of the molten resin to the plasticizing portion side by the resin pressure and also capable of manufacturing a small molded product, an injection molding machine used for this injection foam molding method, and this injection molding Intended to provide machine screw.

[0009]

In order to achieve such an object, an injection molding method according to claim 1 of the present invention (hereinafter referred to as "molding method of claim 1") is an injection molding machine. As a freezing agent and / or a foaming agent in the cylinder at the position facing the stirring part of the screw, the molten resin that has been plasticized and melted in the plasticizing part in the cylinder by the rotation of the screw A gas impregnating step of supplying a working gas and mixing and impregnating the gas with a molten resin in a cylinder, and measuring the obtained gas-impregnated molten resin, and then injecting the measuring resin into a mold cavity to form a molded article. And a high-pressure resin forming section having a molten resin pressure higher than the molten resin pressure of the stirring section is provided between the plasticizing section and the stirring section of the screw. To the side And to prevent the backflow of molten resin. An injection foam molding method according to claim 2 of the present invention (hereinafter, referred to as "molding method of claim 2") is the shaping method according to claim 1, in which between the plasticizing portion and the stirring portion of the screw, A high-pressure resin forming section having a stirring blade structure is provided in which the absolute value of the resin feed rate is smaller than the absolute value of the resin feed rate of the plasticizing section.

An injection foam molding method according to claim 3 of the present invention (hereinafter referred to as "molding method of claim 3") is the same as the molding method of claim 1 or 2, wherein gas is introduced into the screw rod. As a result, the gas was supplied to the inside of the cylinder through a gas supply passage opened to the stirring portion of the screw.

The injection foam molding method according to claim 4 of the present invention (hereinafter referred to as "molding method of claim 4") is the resin as gas in the molding method according to any one of claims 1 to 3. A non-reactive gas was used. Here, the non-reactive gas used is an organic or inorganic substance that is in a gaseous state at room temperature and atmospheric pressure, and does not adversely affect the resin or the like. Such a gas is not particularly limited as long as it satisfies the above conditions, and examples thereof include an inorganic gas and an organic gas (for example, CFC gas, low molecular weight hydrocarbon gas) and the like. Inorganic gas is preferred because it has a low adverse effect on the environment and does not require gas recovery, and carbon dioxide is preferred from the viewpoints of high solubility in resin and large decrease in melt viscosity of resin. The non-reactive gas may be used alone or in combination of two or more kinds.

An injection molding machine according to claim 5 of the present invention (hereinafter referred to as "injection molding machine according to claim 5") is a cylinder in which a screw melts resin supplied into the cylinder by rotation of the screw. The molten resin in the cylinder is provided with a plasticizing section having a stirring blade that conveys in the distal direction and a stirring section that further stirs and kneads the molten resin that has become molten in the plasticizing section, and faces the stirring section of the screw. An injection molding machine equipped with gas supply means for supplying a gas that acts as a freezing agent and / or a foaming agent to
A configuration in which the screw has a high-pressure resin forming section having a stirring blade structure between the plasticizing section and the stirring section, in which the absolute value of the resin feeding rate is smaller than the absolute value of the resin feeding rate of the plasticizing section. And

An injection molding machine according to claim 6 of the present invention (hereinafter referred to as "injection molding machine according to claim 6") is the same as the injection molding machine according to claim 5, in which the stirring blade of the high-pressure resin forming part is It was configured to have a reverse flight structure with respect to the stirring blade of the plasticizing part.

An injection molding machine according to claim 7 of the present invention (hereinafter referred to as "injection molding machine according to claim 7") is the same as the injection molding machine according to claim 5 or 6, The structure is such that it is formed into a dullage-type stirring blade structure.

An injection molding machine according to claim 8 of the present invention (hereinafter, referred to as "injection molding machine according to claim 8") comprises:
In the injection molding machine according to any one of claims 7 to 9, the gas supply means is provided with a gas supply passage that is opened from the gas inlet provided at the rear end of the screw through the inside of the screw to the wall surface of the stirring portion of the screw.

The screw according to claim 9 of the present invention (hereinafter referred to as "the screw of claim 9") is a cylinder tip direction in which the resin supplied into the cylinder by the rotation of the screw is melted by a stirring blade. Is provided between the plasticizing section and the stirring section, and the stirring section for stirring and kneading the molten resin melted in the plasticizing section by the stirring blade And a high-pressure resin forming section having a stirring blade structure that is smaller than the absolute value of the resin feeding speed of the section.

According to a tenth aspect of the present invention (hereinafter, referred to as "screw of the tenth aspect"), the screw of the ninth aspect is such that the inside of the screw rod extends from the gas inlet provided at the rear end of the screw. The gas supply passage opened to the wall surface of the stirring portion of the passing screw is provided.

In the present invention, the carrying direction and carrying speed of the stirring blade of the high-pressure resin forming section should be such that the molten resin in the stirring section can be prevented from flowing back to the plasticizing section between the plasticizing section and the stirring section. Although not particularly limited, it is 1.2 to 3 times that of the molten resin in the plasticizing portion, and 1.1 times that of the molten resin in the stirring portion.
It is preferable to set the pressure to be about 5 times higher.

The resin used in the injection foam molding method of the present invention is not particularly limited, but preferably has a melt tension suitable for foaming (specifically, 0.069 N = 7 g at 200 ° C.). , And the above), which have extensional viscosity characteristics, and include, for example, polyolefin resins such as polyethylene resins and polypropylene resins, polystyrene resins, acrylonitrile-butadiene-styrene copolymers (ABS resins), and poly resins. Vinyl chloride resins and the like can be mentioned, and these can be used alone or in combination of two or more kinds. Among these resins, it is particularly preferable to use polypropylene resin from the viewpoint of recyclability and physical properties. preferable.

The polypropylene resin referred to here is not limited to a homopolymer of polypropylene, but is a general polypropylene such as a random copolymer or a block copolymer with another copolymerizable monomer, or a metallocene obtained by using metallocene as a catalyst or the like. It also includes polypropylene, polypropylene having long-chain branching, and graft-polymerized other components, and these may be used alone or in combination of two or more.

The resin that can be used in the injection molding machine of the present invention is not particularly limited. For example, in addition to the resin used in the injection foam molding method of the present invention, a resin that is difficult to melt mold because of its high melt viscosity, Examples thereof include a resin which is easily decomposed by heat, a low-boiling-point additive or a difficult-molding resin containing an additive which is easily decomposed by heat.

Examples of the resin which is difficult to be melt-molded due to its high melt viscosity include resins for engineering plastics such as ultra high molecular weight polyethylene, ultra high polymerization degree polyvinyl chloride, polytetrafluoroethylene, polyimide and fluororesin. To be

Examples of the resin which is easily decomposed by heat include biodegradable resins such as polylactic acid and polyhydroxybutyrate, polyvinyl chloride having a high degree of chlorination, and polyacrylonitrile. In the injection molding machine according to claim 7 and the screw according to claim 9, the position of the gas supply port is not particularly limited as long as it is a stirring part, but the pressure of the molten resin on the rear end side of the screw front end melts at the measuring part. A position lower than the resin pressure, that is, a portion where the molten resin is not completely filled between the screw and the cylinder is preferable.

For example, in the agitator, by increasing the pitch of the front and rear screw flights and / or by decreasing the screw shaft diameter, the internal volume of the inter-flight space is larger than that of the front and rear inter-flight spaces. It is preferable to have a structure in which an expanded gas impregnation zone forming part that forms a large expanded gas impregnation zone is provided, and to open a gas supply port in this expanded gas impregnation zone forming part of the screw. It is more preferable to open the resin non-filled portion near the front flight of the impregnation zone so as to face it.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.
A detailed description will be given with reference to the drawings. 1 to 4 show a first embodiment of an injection molding machine according to the present invention.

As shown in FIGS. 1 to 3, the injection molding machine 1a is mainly composed of a cylinder 2, a screw 3a arranged inside the cylinder 2, and a seal box 4. The cylinder 2 is provided with a raw material resin supply port 25 through which a resin raw material is supplied, on the rear end side, and a shutoff valve 24 at the front end.

The screw 3a includes a plasticizing section 31, a high-pressure resin forming section 32, a stirring section 33, a gas supply passage 5, and a tip check ring 38. Plasticizing part 31
Is provided with a first stirring blade 35 on the peripheral surface of the screw rod 30, and from the hopper 13 to the raw material resin supply port 25.
The resin raw material supplied into the cylinder 2 through the above is agitated by the first agitating blade 35 as the screw rod 30 rotates, and is brought into a molten state while being conveyed toward the tip end of the cylinder 2 at a predetermined speed. ing.

The high-pressure resin forming section 32 has a so-called reverse flight shape in which the feeding direction is opposite to the first stirring blade 35 of the plasticizing section 31, and the absolute value of the conveying speed is the first stirring. A second stirring blade 36, which is slower than the blade 35, is provided on the peripheral surface of the screw rod 30. Stirrer 33
Is the third stirring blade 3 in the same transport direction as the first stirring blade 35.
7 is provided on the peripheral surface of the screw rod 30, and the expanded gas impregnation zone 11 in which the internal volume of the inter-flight space is large and the resin feeding speed is faster than that on the upstream side is the cylinder 2 and the stirring section 33. It is supposed to be formed between.

In FIG. 1, 12 is a measuring section, 13 is a resin supply hopper, 14 is a screw drive unit, and 15 is a connecting arm of the seal box 4.

As shown in FIGS. 2 to 4, the gas supply path 5 has one gas inlet port 51 opening to the outer wall surface of the screw outside the cylinder 2 and a gas opening to the outer wall surface of the stirring section 33. And a supply port 52. Seal box 4
Is fixed to the drive unit 14 of the injection molding machine 1 by the connecting arm 15 so that the position of the gas introduction port 51 does not shift with respect to the forward and backward movement and rotation of the screw 3a. Surrounding the part.

The seal box 4 has a gas inlet 5
1 is provided with a ring-shaped recessed groove 41, and the recessed groove 41 is elastically contacted with the peripheral surface of the screw rod 30 so as to sandwich the recessed groove 41 from both front and rear sides of the screw 3a to prevent gas leakage from the inside of the recessed groove 41. A pair of seal members 42 for preventing the above are provided.

Further, in the concave groove 41 of the seal box 4,
An injection port 43 for carbon dioxide gas as a gas injected from the gas injection device 9 is provided outside the seal box 4. In FIG. 1, 9 is a gas supply device.

Since the injection molding machine 1a is constructed as described above, when the raw material resin is supplied from the hopper 13 into the cylinder 2 through the raw material supply port, the raw material resin is first plasticized by the rotation of the screw 3a. While being melted and kneaded by the first stirring blade 35 of the portion 31, it is sent toward the tip of the cylinder 2.

The molten resin that has reached the high-pressure resin forming section 32 is further melted and kneaded by the second stirring blade 36. However, since the second stirring blade 36 has the reverse flight structure, the plasticizing section 31 is connected to the cylinder. The force acts in a direction that hinders the conveyance of the molten resin that is about to be sent in the direction of the tip of No. 2.
However, the transport speed by the first stirring blade 35 is the second
Since the absolute value of the conveying speed of the stirring blade 36 is large, the molten resin that has reached the high-pressure resin forming section 32 is decelerated to a high pressure state and sent to the stirring section 33. That is, a molten resin portion having a higher pressure than the molten resin of the plasticizing portion 31 is formed between the plasticizing portion 31 and the stirring portion 33.

Next, the molten resin sent to the stirring section 33 enters the expanded gas impregnation zone 11 in which the inner volume of the space between flights becomes large, and the third stirring blade 37 is in the same direction as the first stirring blade 35. Since the conveying speed is higher than that of the molten resin in the high-pressure resin forming section, the resin pressure becomes lower than that in the high-pressure resin forming section 32. Then, the resin pressure is lowered and the resin is conveyed to the screw tip direction while being stirred. In the impregnation zone 11, the molten resin is agitated and impregnated in the molten resin to form a foaming agent-impregnated molten resin, which is sent to the measuring section 12.

That is, the carbon dioxide gas that has flowed out of the gas injection device 9 enters the groove 41 of the seal box 4 through the gas pipe 91 and the gas inlet 43, and from the gas inlet 51 that opens at a position facing the groove 41. It enters into the gas supply path 5 and is supplied to the expanded gas impregnation zone 11 from the gas supply port 52, and is impregnated with the molten resin filled in the expanded gas impregnation zone 11.

Then, as the gas-impregnated molten resin is fed to the measuring section 12, the screw 3a is gradually retracted in accordance with the amount of the resin fed, as shown in FIG. At, a predetermined amount of gas-impregnated molten resin is measured.

In this way, the gas-impregnated molten resin whose metering has been completed is injected into an injection mold (not shown) with the screw 3a stopped, and a foamed molded product is obtained. The high-pressure resin having a high pressure in the high-pressure resin forming section is kept higher than the resin pressures of the stirring section 33 and the measuring section 12 until the injection is completed.

According to the molding method using the injection molding machine 1a configured as described above, since the high-pressure resin forming portion 32 is provided between the plasticizing portion 31 and the stirring portion 33 of the screw, the high-pressure resin is formed. The forming portion 32 forms a resin portion between the plasticizing portion 31 and the agitating portion 33, which has a higher pressure than the resin pressure of the agitating portion 33. It is prevented by this high-pressure resin part. Moreover, the high-pressure resin forming portion 32 is provided with the second stirring blade 36.
Since only the reverse flight structure is used, the resin is kneaded also in the high-pressure resin forming section 32. Therefore, the plasticizing part 3
As compared with the case where a check ring is provided between the No. 1 and the stirring unit 33, the resin can be sufficiently kneaded even if the screw length of the injection molding machine 1a is shortened.

Since no check ring is provided between the plasticizing section 31 and the stirring section 33, the screw rod 30
It can be connected to one screw, and there is no problem in screw strength even in the case of small diameter injection molding machines. Therefore, it is possible to form a small molded product. Further, since the molten resin is impregnated with the carbon dioxide gas in the expanded gas impregnation zone 11 where the pressure is lower than that of the metering unit 12, the gas can be uniformly dispersed in the molten resin in a short time without increasing the supply pressure of the carbon dioxide gas so much. Can be impregnated. That is, it is not necessary to make the entire device a pressure resistant structure, and the manufacturing cost of the device can be reduced.

FIG. 5 shows a second embodiment of the injection molding machine according to the present invention. As shown in FIG. 5, the injection molding machine 1b includes a high-pressure resin forming section 3 of a screw 3b.
The second embodiment is the same as the above embodiment except that the second stirring blade 36 'of 2'has a dullage type stirring blade structure. 6 and 7 show the third embodiment of the injection molding machine according to the present invention.
FIG. As shown in FIGS. 6 and 7, the injection molding machine 1c includes a high-pressure resin forming section 39.
Is similar to the above embodiment except that it is formed by providing a weir 39a in a ladder shape on the screw rod 30 between the flights between the plasticizing part 31 and the stirring part 33 of the screw 3c. There is.

The present invention is not limited to the above embodiment. For example, in the above-described embodiment, the number of the gas supply path 5 is one, but a plurality of gas supply paths 5 are provided, and a seal box is individually provided at a portion facing the gas introduction port of each gas supply path, and the gas to the gas supply port is The supply paths may be individually provided. In the above-described embodiment, the screw rod 30 is provided with the gas supply passage 5 passing therethrough, and the carbon dioxide gas is supplied to the molten resin in the cylinder 2 through the gas supply passage 5, but the stirring portion is provided. A gas supply port may be provided on the wall surface of the facing cylinder, and gas may be supplied from this gas supply port. Further, the gas may be supplied from both sides of the screw rod side and the cylinder side.

[0043]

Since the injection foam molding method according to the present invention is configured as described above, the backflow of the molten resin to the plasticizing portion side is caused by the resin pressure at the time of injection of the gas-impregnated molten resin in the measuring portion. Can be reliably prevented and a small molded product can be manufactured. Moreover, the resin can be sufficiently kneaded even if the screw length is short, and the device can be downsized.

According to the molding method of the third aspect, it is not necessary to make the cylinder a pressure resistant structure, and the size of the apparatus can be reduced. According to the molding method of the fourth aspect, a good molded product can be obtained without degrading the resin.

Since the injection molding machine according to the present invention is constructed as described above, it is possible to manufacture a small molded product. Moreover, the resin can be sufficiently kneaded even if the screw length is short, and the device can be downsized. In particular, if the molding machine according to the eighth aspect is used, it is not necessary to make the cylinder a pressure resistant structure, and the device can be downsized.

Since the screw of the present invention is as described above, the molding method of the present invention can be carried out only by setting the cylinder of the existing injection molding machine.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a first embodiment of an injection molding machine according to the present invention.

FIG. 2 is a cross-sectional view of main parts of the injection molding machine of FIG.

3 is a cross-sectional view of a seal box portion of the injection molding machine of FIG.

FIG. 4 shows the seal material of the seal box of FIG.
The figure (a) is the front view, and the figure (b) is the side sectional view.

FIG. 5 is a cross-sectional view of essential parts showing a second embodiment of the injection molding machine according to the present invention.

FIG. 6 is a cross-sectional view of essential parts showing a third embodiment of an injection molding machine according to the present invention.

FIG. 7 is an enlarged cross-sectional view of the main parts of FIG.

[Explanation of symbols]

1a, 1b, 1c injection molding machine 12 Weighing department 2 cylinders 3a, 3b screw 30 screw rod 31 Plasticizing part 32, 32 ', 39 High-pressure resin forming part 33 Stirrer 35 First stirring blade 36,36 'Second stirring blade 37 Third stirring blade

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Claims (10)

[Claims] 1. A molten resin, which has been plasticized and melted in a plasticizing section in the cylinder by the rotation of the screw in the cylinder of an injection molding machine, flows easily into the cylinder at a position facing the stirring section of the screw. A gas impregnating step of supplying a gas acting as an agent and / or a foaming agent to mix and impregnate the molten resin with the molten resin in a cylinder, and weigh the obtained gas-impregnated molten resin, and then mold the measuring resin into a mold. An injection molding method including an injection molding step of injecting into a cavity to obtain a molded product, wherein the molten resin pressure is higher than the molten resin pressure of the stirring section between the plasticizing section of the screw and the stirring section. An injection molding method, characterized in that a resin forming portion is provided to prevent the molten resin from flowing back to the plasticizing portion side. 2. Between the plasticizing section and the stirring section of the screw,
The injection molding method according to claim 1, wherein a high-pressure resin forming section having a stirring blade structure is provided in which the absolute value of the resin feed rate is smaller than the absolute value of the resin feed rate of the plasticizing section. 3. The injection molding method according to claim 1 or 2, wherein the gas is supplied into the cylinder through a screw rod and a gas supply passage opened to a stirring portion of the screw. 4. The injection molding method according to claim 1, wherein the gas is a non-reactive gas with respect to the resin. 5. A plasticizing section having a stirring blade for the screw to melt the resin supplied into the cylinder by the rotation of the screw and convey the resin toward the tip of the cylinder, and a molten resin melted in the plasticizing section. An injection molding machine having a stirring section for stirring and kneading, and a gas supply means for supplying a gas acting as a freezing agent and / or a foaming agent to the molten resin in the cylinder at a position facing the stirring section of the screw. The screw is equipped with a high-pressure resin forming section having a stirring blade structure between the plasticizing section and the stirring section, in which the absolute value of the resin feeding rate is smaller than the absolute value of the resin feeding rate in the plasticizing section. An injection molding machine characterized in that 6. The injection molding machine according to claim 5, wherein the stirring blade of the high-pressure resin forming portion is formed in a reverse flight structure with respect to the stirring blade of the plasticizing portion. 7. The injection molding machine according to claim 5, wherein the high-pressure resin forming portion is formed in a dullage-type stirring blade structure. 8. The gas supply means is provided with a gas supply passage that is opened from the gas inlet provided at the rear end of the screw through the inside of the screw to the wall surface of the stirring portion of the screw.
The injection molding machine according to claim 7. 9. A plasticizing section having a stirring blade that melts the resin supplied into the cylinder by the rotation of the screw by the stirring blade and conveys the resin toward the tip of the cylinder.
A stirring unit that stirs and kneads the molten resin that has become molten in the plasticizing unit with stirring blades, and stirring that is provided between the plasticizing unit and the stirring unit and that is smaller than the absolute value of the resin feeding speed of the plasticizing unit. A screw provided with a high-pressure resin forming part having a blade structure. 10. The screw according to claim 9, further comprising a gas supply passage which is provided at a rear end of the screw, passes through the inside of the screw rod, and opens to a wall surface of a stirring portion of the screw.