Patent Publication Number: US-2007104021-A1

Title: Screw for plastication of resin material and a plasticizing mechanism

Description:
TECHNICAL FIELD  
      The present invention relates to a screw for plastication of resin material and a plasticizing mechanism for resin material which are employed in an injection molding machine, an extrusion molding machine or the like for obtaining moldings by discharging a plasticized resin material, more specifically relates to a screw for plastication of resin material and a plasticizing mechanism for resin material which are applied to a plasticizing cylinder (heating cylinder), an injection cylinder or the like for an injection molding machine, an extrusion molding machine or the like of small size and are used for making a plastication state of a resin material uniform and discharging the resin material.  
     BACKGROUND ART  
      In a screw for plastication (hereinafter, referred to simply as a “screw”) used in injection molding and extrusion molding of resin material which is installed in a plasticizing cylinder in order to plasticize and discharge a resin material, when an L/D ratio which is obtained by dividing a length L of a section of the screw on an outer surface of which a spiral screw flight is formed (hereinafter, this length is referred to as an effective length L of the screw) by a diameter D of the top end of the screw flight (hereinafter, this diameter is referred to as an external diameter D of the screw) is 10 or less, a plastication state of the resin material is not stabilized generally. Due to this, when using a screw designed to have such an L/D ratio, unmolten resin or half-molten resin is discharged from a plasticizing cylinder or an injection cylinder, causing incompleteness or infeasibility in molding.  
      In order to prevent incompleteness or infeasibility in molding, for example, a screw designed to have the L/D ratio of  24  is used for wire covering and a screw designed to have the L/D ratio of 18 to 20 is used for injection molding. With such L/D-ratio design, even though the external diameter D of the screw is set to be about 20 mm for example, the effective length L thereof becomes about 480 mm in the case of the screw for wire covering and becomes 360 mm or more in the case of the screw for injection molding. In this way, the effective length L and the entire length of the screw cannot be decreased even by making the external diameter D of the screw smaller, and accordingly, it is difficult to achieve downsizing of an injection molding machine or an extrusion molding machine.  
      As a configuration which achieves compatibility between decrease in length of the screw and stable feeding of the resin material in a uniform plastication state, proposed are configurations, for example, such that the effective length L of the screw is decreased while the external diameter D is enlarged and large-scale shear between an internal surface (heating surface) of the plasticizing cylinder and an outer surface (plasticizing surface) of the screw is generated to plasticize the resin material (see Japanese Patent Application Unexamined Publication No. Hei 6-312443), and such that the temperature is controlled by a temperature controller installed on an outer surface of the plasticizing cylinder while the L/D ratio of the screw is set to be  1  to  3  (see Japanese Patent Application Unexamined Publication No. 2000-71252). In addition to the above-described configurations, proposed is a configuration such that a screw in a cone shape is employed (see Japanese Patent Application Unexamined Publication No. 2002-67110).  
      The configurations described in the above-mentioned references are for ensuring stabilization of a plastication state of the resin material by employing a screw of large diameter or in a cone shape in order to increase an area providing the resin material with shear to promote plastication by heat generated by shear. Employing the screw of large diameter can decrease the effective length of the screw; however, it does not always decrease an occupied volume thereof. In addition, in order to drive the screw of large diameter, it is required to upsize a driving system such as a motor. Due to this, it is difficult to ensure downsizing of the injection molding machine, the extrusion molding machine or the like. In addition, it is relatively hard to fabricate a screw in a cone shape and a cylinder in a cone shape so as to be in combination.  
      In addition, a widely employed configuration for promoting plastication of the resin material is such that a barrier flight or a sub flight is formed, a Dulmage structure is provided, a shear element is provided, or the number of threads is increased. However, a screw having a well-known kneading structure such as a shear element and a screw on which a plurality of threads are formed generally have a disadvantage that a discharge rate or measurement is difficult to stabilize at the time of continuous discharge compared with a full flight screw. Additionally, in such a configuration, it is necessary to employ an appropriate screw in accordance with the variety of resin materials so that discharge can be made under optimum conditions. Employing such a screw takes a lot of troubles with maintenance and replacement; therefore it is not considered that the configuration is favorable for the use at actual manufacturing premises.  
      Further, also employed is a configuration such that a spindle-shaped member called a torpedo (also called a spreader) is installed in the vicinity of an end of the plasticizing cylinder. This configuration is for ensuring stabilization of a plastication state of the resin material by decreasing a cross sectional area of a flow path of the plasticized resin material in order to develop a shear rate of the resin material to promote heat generation by shear. For example, a configuration is proposed such that a material consisting of a powder of a cellulosic material and a resin are fed into a resin reservoir formed between the screw and the torpedo using the screw and the screw is made to run forward to inject a molten resin through the flow path formed between the torpedo and a barrel (see Japanese Patent Application Unexamined Publication No. Hei 11-198164).  
      However, though the Publication JP Hei 11-198164 describes well-known arts in which a resin is plasticized by heat generated by shear and the shape of flutes of the torpedo is changed to adjust a surface appearance and a touch of the plasticized resin, structures how to support and mount the torpedo are not clearly described. In addition, the screw employed therein is described as a short and specific one; however, the extent of a difference between its L/D ratio and the L/D ratio of 18 to 20 which is for general injection molding or a concrete structure of the screw is not disclosed.  
      Consequently, the present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a screw for plastication of resin material and a plasticizing mechanism for resin material which ensure downsizing of an injection molding machine, an extrusion molding machine or the like by decreasing the L/D ratio without extremely enlarging the screw in external diameter and ensure compatibility between maintenance of a uniform plastication state of a resin material and stability of discharge of a plasticized resin material even in the case of a short screw.  
     DISCLOSURE OF THE INVENTION  
      To achieve the objects and in accordance with the purpose of the present invention, the invention described in claim  1  is intended to provide a screw for plastication of resin material installed in a plasticizing cylinder for plasticizing a resin material for molding, wherein an external diameter D of a metering section formed at a top part of the screw is 100 mm or less, an L/D ratio obtained by dividing a length L of a section of the screw on an outer surface of which a spiral screw flight is formed by the external diameter D of the metering section is 10 or less, and a pitch of the screw flight is designed so that a thread length thereof falls within a range of 30 to 300% of a thread length of a screw in which an L/D ratio is 20 to 24 and a pitch of a screw flight is designed to be the same as an external diameter D of a metering section.  
      Further in the screw for plastication of resin material, as described in claim  2 , it is desirable that an external diameter of a feed section for feeding the resin material into the plasticizing cylinder is designed to be larger than the external diameter of the metering section for keeping an extrusion amount of the resin material uniform and an external diameter of a compression section for plasticizing the resin material, and a channel depth of the feed section formed by the screw flight is designed to be larger than a channel depth of the compression section.  
      Further in the screw for plastication of resin material, as described in claim  3 , it is desirable that a pitch of the screw flight in the feed section for feeding the resin material into the plasticizing cylinder is designed to be larger than a pitch of the screw flight in the metering section for keeping the extrusion amount of the resin material uniform and smaller than the external diameter of the metering section and a pitch of the screw flight in the compression section for plasticizing the resin material is designed to become smaller gradually from the feed section toward the metering section, and furthermore, as described in claim  4 , it is desirable that the pitch of the screw flight in the feed section for feeding the resin material into the plasticizing cylinder is designed to be more than 1.5 times as large as the pitch of the screw flight in the metering section for keeping the extrusion amount of the resin material uniform.  
      In addition, the invention described in claim  5  is intended to provide a plasticizing mechanism for resin material, wherein the screw for plastication of resin material according to any of claims  1  to  4  is installed in the plasticizing cylinder for plasticizing the resin material, and a torpedo plate in which a torpedo is supported so as to be positioned in a central part of a path of the resin material is arranged to be mountable and demountable at a downstream part of a flow of the resin material in the plasticizing screw and the resin material inside the plasticizing cylinder is conveyed to flow around the torpedo in the torpedo plate.  
      When the flight pitch is designed so that the thread length of the screw is 30 to 300% of the thread length of the screw of the same diameter in which the L/D ratio is designed to be 20 to 24 and the flight pitch is designed to be the same as the external diameter D of the metering section (hereinafter, the screw designed as such is referred to as a screw with a square pitch) as in the invention described in claim  1 , the thread length can be secured to be long even in the case of the L/D ratio of 10 or less.  
      The long thread length increases a distance in the plasticizing cylinder where the resin material is sheared and also increases a residence time of the resin material in the plasticizing cylinder if the number of revolutions of the screw is the same as before, increasing heating time, and thereby the plastication of the resin material is promoted. In addition, a discharge rate decreases in the case of the same revolution number as before; accordingly, when the revolution number is increased to keep the discharge rate, a greater sharing force is applied to the resin material, and thereby the plastication is promoted. Thus, a plastication state of the resin material can be stabilized while the external diameter D of the screw is not extremely enlarged; therefore compatibility is ensured between the stabilization of the plastication state of the resin material and downsizing of an injection molding machine or an extrusion molding machine.  
      When the external diameter of the feed section for feeding the resin material is enlarged compared with that of the metering section for metering the resin material as in the invention described in claim  2 , the channel depth of the feed section can be enlarged. Owing to this, a sufficient amount of resin material can be fed even though the flight pitch of the feed section cannot be enlarged enough compared with a pellet size of the resin material. Then, in the compression section, the external diameter of the screw is gradually decreased, so that compression brought by decrease in spatial volume between the screw threads is also applied to the resin material. Therefore, plastication of the resin material is rapidly achieved even in the case of a short screw, allowing stabilization of a plastication state of the resin material.  
      When the flight pitch of the feed section is formed to be larger than that of the metering section as in the invention described in claim  3 , a feed rate of the resin material fed into the plasticizing cylinder can be sufficiently secured in the feed section, and the resin material can be fed stably. Then, the smaller channel depth of the compression section as well as the gradual decrease in the flight pitch thereof apply compression to the resin material. Therefore, plastication of the resin material is rapidly achieved even in the case of a short screw, allowing stabilization of a plastication state of the resin material.  
      When the flight pitch of the feed section is more than 1.5 times as large as that of the metering section as described in claim  4  and is not larger than the external diameter of the screw in the metering section, the thread length of the screw can be secured for stabilization of a plastication state while the resin material in the feed section can be fed stably.  
      Here, owing to the plasticizing mechanism employing the above-described screw in combination with a configuration that the torpedo plate including the torpedo is installed in the vicinity of the top end of the screw in the plasticizing cylinder as described in claim  5 , further stabilization of a plastication state of the resin material to be discharged can be ensured without incurring complexity of a structure or a driving mechanism of the screw nor upsizing of an injection molding machine. In addition, as the torpedo plate is made exchangeable, injection of the resin material can be achieved under optimum conditions. Therefore, there arises no need to prepare a plurality of screws in accordance with the variety of resin materials, allowing costs for equipment to be curbed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A  to  1 C are external plan views showing structures of screws consistent with the first embodiment of the present invention, and the screw shown in  FIG. 1A  is designed to have the external diameter D of 22 mm and a flight pitch of 11 mm, the screw shown in  FIG. 1B  is designed to have the external diameter D of 22 mm and a flight pitch of 8 mm, and the screw shown in  FIG. 1C  is designed to have the external diameter D of 22 mm and a flight pitch of 22 mm;  
       FIGS. 2A and 2B  are plan views diagrammatically showing structures of screws consistent with the second embodiment of the present invention and states in which the screws are installed in plasticizing cylinders, and the screw in  FIG. 2A  has a large root diameter in a feed section and the screw in  FIG. 2B  has a small root diameter in a feed section;  
       FIG. 3A  is an external plan view showing a structure of a screw consistent with the third embodiment of the present invention, and  FIG. 3B  is a view showing a conventional example for the sake of comparison;  
       FIG. 4A  is an external perspective view showing an exploded state of a plasticizing cylinder in which a plasticizing mechanism for resin material consistent with the present invention is installed, and  FIG. 4B  is a front view of a torpedo plate which is installed in the plasticizing cylinder; and  
       FIG. 5  is a sectional view showing a structure of the plasticizing cylinder in which the above-described plasticizing mechanism for resin material is installed. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      A detailed description of one preferred embodiment of the present invention will now be given with reference to the accompanying drawings.  
      While the type of a resin material to which a screw consistent with the present invention is applicable is not limited, the screw is favorably applicable especially to a polybutylene telephthalate (PBT) resin, a polypropylene (PP) resin, a commonly-used thermoplastic elastomer material and the like. In addition, a pellet of commercially available size and shape, for example, one which is 3 mm in diameter and 2 mm in length is applicable.  
      For the use on a common office desk, it is preferable for the screw to have the external diameter of about 90 mm or less, and in order to apply to a small-size plasticizing device, it is more preferable for the screw to have the external diameter of 60 mm or less. Hence, inorder that the screw may also be used for such purposes, to the screw consistent with the present invention, one having the external diameter of about 100 mm or less, specifically about 90 mm or less is favorably applied. Here, an L/D ratio is obtained by dividing an effective length L of a screw by an external diameter D thereof. When the screw is designed to have the L/D ratio of  10  or less (e.g., the L/D ratio of 5 or 10), a flight pitch is designed so that a thread length of the screw falls within a range of about 30 to 300% (hereinafter, this range is sometimes referred to as a “preferable range”), more preferably a range of about 60 to 150% (hereinafter, this range is sometimes referred to as a “more preferable range”) of a thread length of a screw with a square pitch having the same diameter and the L/D ratio of 20 to 24. With such design, compatibility between maintenance of a favorable plastication state of the resin material and downsizing of an injection molding machine brought by downsizing of the screw is ensured.  
      In other words, according to the screw in which the flight pitch is adjusted so that the thread length falls within 30 to 300% of the thread length of the screw with a square pitch having the same diameter and the L/D ratio of 20 to 24, when the number of revolutions of the screw is made the same as before, a discharge rate per revolution decreases; therefore, a residence time of the resin material becomes about 70 to 700% in comparison as before to show that the resin material is allowed to stay for the same period of time as or longer than before. Generally, it is said that a main heat source for rapid plastication of the resin material in a plasticizing cylinder is heat generated by shear in the resin material. However, heat applied to the plasticizing cylinder also brings about plastication of the resin material, not as rapid as the plastication by heat generated by shear. Accordingly, when the resin material is allowed to stay more time in the plasticizing cylinder, the resin material is plasticized by heat generated by shear and heat applied to the plasticizing cylinder without leaving a half-molten resin or an unmolten resin, allowing stabilization in the plastication state of the resin material.  
      Meanwhile, when the flight pitch is made smaller, the discharge rate of the plasticized resin material per revolution of the screw decreases, so that it is necessary to raise the revolution number of the screw to secure the discharge rate of the resin material. When the revolution number of the screw is raised in order to maintain the discharge rate as same as before, a large-scale shearing force is applied to the resin material to bring about easier plastication, allowing stabilization in the plastication state of the resin material.  
      Therefore, according to the screw having the L/D ratio designed as described above, an entire length of the screw can be decreased without a necessity to extremely enlarge the screw in external diameter while maintaining the favorable plastication state of the resin material. Owing to the downsizing or the decrease in length of the screw, an injection molding machine or an extrusion molding machine can be downsized. Therefore, the compatibility is achieved between the stabilization of the plastication state of the resin material to be discharged and the downsizing of an injection molding machine or an extrusion molding machine.  
      Table 1 in the following shows: results of calculation of thread lengths and flight pitches of screws having the L/D ratio of 5 or 10 and the external diameter D of 22 mm, the screws being consistent with the first embodiment of the present invention; comparisons of the thread lengths of the screws with a thread length of a screw with a square pitch having the L/D ratio of 20; and assessments of plastication states of resin materials in cases where the respective screws are used.  
                                       TABLE 1                                           Ratio                               of   Assessment           External       Number   Thread   Thread   of           Diameter   Pitch   of   Length   Length   Plastication       L/D   mm   mm   Pitch   mm   %   State                                                            L/D = 20   22   22   20.0   1551.39   100.0   ⊚       L/D = 10   22   22   10.0   775.70   50.0   ◯       L/D = 10   22   11   20.0   1424.62   91.8   ⊚       L/D = 10   22   8   27.5   1929.98   124.4   ⊚       L/D = 10   22   5   44.0   3413.06   220.0   ◯       L/D = 5   22   22   5.0   387.85   25.0   X       L/D = 5   22   11   10.0   712.31   45.9   ◯       L/D = 5   22   8   13.8   964.99   62.2   ⊚       L/D = 5   22   5   22.0   1528.91   98.6   ⊚                  
 
      The assessments were made respectively as to a polybutylene telephthalate resin and a polypropylene resin. For a pellet, one which is 3 mm in diameter and 2 mm in length was used. Besides, no filler was used. The revolution number of the screws was set in a range of 150 to 360 rpm. In addition, the plastication cylinders were heated using a heater. Heating temperatures by the heater were set to be 300 to 360° C. in the case of the polybutylene telephthalate resin and 200 to 280° C. in the case of the polypropylene resin.  
      In the item “Assessment of Plastication State” in Table 1, “⊚” indicates a plastication state in which the resin material is completely plasticized and there arises no problem in injection molding, “∘” indicates a plastication state in which the resin material is plasticized while the temperature becomes slightly unstable, and “X” indicates a unfavorable plastication state in which the resin material is sometimes mixed with the not-completely plasticized one.  
      Besides, the screw in the top row in Table 1 having the L/D ratio of 20, the external diameter of 22 mm and the pitch of 22 mm is a conventionally-designed screw, which is provided for comparison purposes.  
      Firstly, a description is given to the screw designed to have the L/D ratio of 10. As shown in Table 1, when the flight pitch is designed to be 22 mm , the thread length becomes 50% of that of the screw having the L/D ratio of 20. When the flight pitch is designed to be 11 mm , the thread length becomes 92% of that of the screw having the L/D ratio of 20. When the flight pitch is designed to be 8 mm , the thread length becomes 125% of that of the screw having the L/D ratio of 20. When the flight pitch is designed to be 5 mm , the thread length becomes 220 mm of that of the screw having the L/D ratio of 20. By designing the flight pitches in this manner, each of the thread lengths can be adjusted to fall within the range of 30 to 300% being the “preferable range”.  
      As for the plastication states of the resin materials, when the screw in which the flight pitch is designed to be 11 mm or 8 mm was used, the respective resin materials were completely plasticized to bring about a state where there is no problem with injection molding. These screws have the thread lengths which fall within the “more preferable range”, i.e., the range of 60 to 150 mm of the thread length of the screw having the L/D ratio of 20. In addition, when the screw in which the flight pitch is designed to be 22 mm or 5 mm was used, the resin materials were plasticized. These screws have the thread lengths which fall within the range of 30 to 300% of the thread length of the screw having the L/D ratio of 20 (i.e., the “preferable range”) but fall outside the “more preferable range”.  
      Next, a description is given to the screw designed to have the L/D ratio of 5. When the flight pitch is designed to be 11 mm , the thread length becomes 45.9% of that of the screw having the L/D ratio of 20. When the flight pitch is designed to be 8 mm , the thread length becomes 62% of that of the screw having the L/D ratio of 20. When the flight pitch is designed to be 5 mm , the thread length becomes 98% of that of the screw having the L/D ratio of 20. By designing the flight pitches in this manner, the thread lengths can be adjusted to fall within the “preferable range”. However, if the flight pitch is designed to be 22 mm , the thread length becomes 25% of that of the screw having the L/D ratio of 20, falling outside the “preferable range”.  
      As for the plastication states of the resin materials, when the screw in which the flight pitch is designed to be 8 mm or 5 mm was used, the respective resin materials were completely plasticized to bring about a state where there is no problem with injection molding. These screws have the thread lengths which fall within the “more preferable range”. In addition, when the screw in which the flight pitch is designed to be 11 mm was used, the resin material was plasticized. This screw has the thread length which falls within the “preferable range” but falls outside the “more preferable range”. On the other hand, when the screw in which the flight pitch is designed to be 22 mm was used, the plastication state of the resin material was unfavorable where the plasticized resin material was mixed with a not-completely plasticized resin material. This screw has the thread length which falls outside the “preferable range”.  
      As described above, for the screws having the L/D ratio of 10, in order to have the thread length fall within the “preferable range”, the flight pitch may be designed to be any of 5 mm, 8 mm, 11 mm and 22 mm. However, in order to have the thread length fall within the “more preferable range”, it is preferable to design the flight pitch to be 8 mm or 11 mm. Meanwhile, for the screws having the L/D ratio of 5, in order to have the thread length fall within the “preferable range”, it is preferable to design the flight pitch to be 5 mm, 8 mm or 11 mm, and in order to have the thread length fall within the “more preferable range”, it is preferable to design the flight pitch to be 5 mm or 8 mm.  
       FIGS. 1A, 1B  and  1 C are external plan views showing structures of the screws designed to have the L/D ratio of 10 among the examples shown in Table 1. A screw la in  FIG. 1A  has a flight pitch of 11 mm and a thread length of 92% of that of the screw with a square pitch having the L/D ratio of 20. A screw  1   b  in  FIG. 1B  has a flight pitch of 8 mm and a thread length of 125 mm of that of the screw with a square pitch having the L/D ratio of 20. In this way, these screws la and  1   b  respectively have the thread lengths which fall within the “more preferable range”. A screw  1   c  in  FIG. 1C  has a flight pitch of 22 mm and a thread length of 50% of that of the screw with a square pitch having the L/D ratio of 20. That is, the screw  1   c  is an example of the screw which has the thread length falling within the “preferable range” but falling outside the “more preferable range”.  
      Incidentally, the “preferable range” of the thread length of the screw consistent with the present embodiment is 30 to 300% of that of the screw with a square pitch having the same diameter and the L/D ratio of 20 or 24; however, it changes depending on an amount of a filler of the resin material. Additionally, in order to ensure further stabilization of the plastication state of the resin material, another configuration may be employed where various well-known structures which promote plastication of the resin material are additionally provided or used in combination: for example, a barrier flight or a sub flight is formed on the screw, a Dulmage structure or a shear element is provided in the screw, or the thread number is increased.  
      In addition, the applicable resin material is not limited to the polybutylene telephthalate resin and the polypropylene resin mentioned above, and a resin material which is plasticized at temperatures in the above-mentioned ranges is applicable. For example, polyamide, polyphthalamide and a syndiotactic polystyrene (SPS) resin are cited.  
      Next, a description is given to screws consistent with the second embodiment of the present invention. The screws consistent with the present embodiment are formed so that channel depths of feed sections are made larger than those of metering sections, so that feed rates of the resin materials in the feed sections can be secured sufficiently. Besides, thread lengths and flight pitches of the screws consistent with the present embodiment are designed the same as those consistent with the first embodiment.  
       FIGS. 2A and 2B  are sectional views diagrammatically showing specific shapes of the screws consistent with the present embodiment and states in which the screws are installed in plasticizing cylinders. Besides, the plasticizing cylinders are shown in section while outward appearances are shown regarding the screws. In addition, the screws are shown while enlarged radially in order to easily discern differences between external diameters and root diameters of the screws (in the present specification, a root diameter is referred to as a diameter which is obtained by subtracting the channel depth from the external diameter of the screw), so that the screws shown are different in shape from the actual ones.  
      Screws  50   a  and  50   b  in  FIGS. 2A and 2B  are formed so that feed sections  51   a  and  51   b  are larger than metering sections  53   a  and  53   b  in external diameter. The screw  50   a  in  FIG. 2A  is formed so that the feed section  51   a  is larger than the metering section  53   a  and a compression section  52   a  in root diameter. The screw  50   b  in  FIG. 2B  is formed so that the feed section  51   b  is smaller than the metering section  53   b  and a compression section  52   b  in root diameter. Besides, a plasticizing cylinder  55  is formed so that an internal surface thereof fits the external diameters of the respective sections of the screw.  
      A detailed description is given to these screws  50   a  and  50   b . As for the screw  50   a  shown in  FIG. 2A , the feed section  51   a  is formed to have a uniform external diameter and a uniform root diameter to provide cylindrical shapes. A section  59   a  of the compression section  52   a , which is closer to the feed section  51   a , has an external diameter and a root diameter both of which are narrowed gradually from the feed section  51   a  toward the metering section  53   a . The narrowed volume of the external diameter is greater than that of the root diameter, so that a channel depth becomes smaller gradually toward the metering section  53   a . In addition, a section  58   a  of the compression section  52   a , which is closer to the metering section  53   a , has a uniform external diameter to provide a cylindrical shape while having a root diameter which becomes larger gradually toward the metering section  53   a . Accordingly, a channel depth of the compression section  52   a  becomes smaller gradually toward the metering section  53   a . The metering section  53   a  is formed to have a uniform external diameter and a uniform root diameter to provide cylindrical shapes.  
      As for the screw  50   b  shown in  FIG. 2B , the feed section  51   b  is formed to have a root diameter smaller than that of the metering section  53   b . The feed section  51   b  and the metering section  53   b  are formed to have the uniform root diameters to provide cylindrical shapes. A section  59   b  of the compression section  52   b , which is closer to the feed section  51   b , is formed to have a root diameter the same as that of the feed section  51   b  to provide a cylindrical shape. A section  58   b  of the compression section  52   b , which is closer to the metering section  53   b , is formed to have a root diameter which becomes larger gradually from the section  59   b  closer to the feed section  51   b  toward the metering section  53   b  forming a tapered shape, smoothly connecting the section  59   b  closer to the feed section  51   b  and the metering section  53   b .  
      The feed section  51   b  and the metering section  53   b  are formed to have uniform external diameters to provide cylindrical shapes. In addition, the feed section  51   b  is formed to have the external diameter larger than that of the metering section  53   b . The section  59   b  of the compression section  52   b , which is closer to the feed section  51   b , has an external diameter which is narrowed gradually from the feed section  51   b  toward the metering section  53   b . Meanwhile, the section  58   b  of the compression section  52   b , which is closer to the metering section  53   b , is formed to have a uniform external diameter, which is the same as that of the metering section  53   b .  
      When the channel depths of the feed sections  51   a  and  51   b  are larger than those of the compression sections  52   a  and  52   b  and those of the metering sections  53   a  and  53   b  as described above, the feed rates of the resin materials in the feed sections  51   a  and  51   b  can be secured sufficiently. Additionally, when the channel depths of the compression sections  52   a  and  52   b  are formed to become smaller gradually from the feed sections  51   a  and  51   b  toward the metering sections  53   a  and  53   b , the resin materials are compressed to promote plastication. Therefore, compatibility between decrease in length of the screws and stabilization of a discharge state of the resin material can be achieved.  
      Next, a description is given to the third embodiment of the present invention. Plasticizing screws consistent with the present embodiment are formed so that a flight pitch of a feed section is larger in order to stabilize the feed rate of the resin material in the feed section. Then, a flight pitch of the compression section is formed to become smaller gradually toward the metering section to compress the resin material. Accordingly, the screws, even short, can achieve rapid plastication of the resin material. Besides, thread lengths of the screws are designed the same as those consistent with the first embodiment.  
       FIG. 3A  is an external plan view showing a structure of the screw consistent with the third embodiment. A screw  30  shown in  FIG. 3A  is designed to have the L/D ratio of 5. Besides, a screw  502  shown in  FIG. 3B  is a comparative example. On the screw  502  in  FIG. 3B , a flight pitch is formed uniformly over its entire effective length.  
      The screw  30  of the present embodiment is formed so that an external diameter is uniform over its entire effective length to provide a cylindrical shape. A feed section  31  and a metering section  33  are formed to have uniform root diameters to provide approximately cylindrical shapes. Besides, the metering section  33  is formed to have the root diameter larger than that of the feed section  31 . In addition, in a compression section  32 , a root diameter at an end close to the feed section  31  is the same as that of the feed section  31  and a root diameter at an end close to the metering section  33  is the same as that of the metering section  33 , and the root diameter of the compression section  32  is arranged to become larger gradually from the end close to the feed section  31  toward the end close to the metering section  33 .  
      Then, with the thread length of the screw within the “preferable range”, a flight pitch P f  of the feed section  31  is designed to be larger than a flight pitch P m  of the metering section  33 . Especially, it is desirable that the flight pitch P f  of the feed section  31  is designed to be more than 1.5 times as large as the flight pitch P m  of the metering section  33  and smaller than the diameter D of the metering section  33 . In addition, a flight pitch of the compression section  32  becomes smaller gradually from the end close to the feed section  31  toward the end close to the metering section  33 , smoothly connecting the screw flights of the feed section  31  and the metering section  33 .  
      Incidentally, for smooth connection of the screw flights at the boundary between the metering section  33  and the compression section  32 , there emerges a need to form such a section also in the metering section  33  that the flight pitch becomes smaller toward an end of the screw. Due to this, the flight pitch of the metering section  33  becomes nonuniform; it is therefore desirable to have the flight pitch uniform by four pitches from the end of the metering section  33 , and it is more desirable to have it uniform by six pitches. With such design, stable discharge of the plasticized resin material can be achieved.  
      As described above, when the flight pitch P f  of the feed section  31  is made larger, a stable feed rate of the resin material in the feed section  31  is allowed. To the compression section  32 , compression brought by the smaller flight pitch is applied in addition to compression brought by a smaller channel depth, allowing rapid plastication of the resin material even the screw  30  is short. As the flight pitch P m  of the metering section  33  is formed uniform, the stable discharge of the plasticized resin material can be achieved. Therefore, compatibility between the decrease in length of the screw and the stabilization of a plastication state and a discharge state of the resin material to be discharged can be achieved.  
      Next, a description is given to a plasticizing mechanism for resin material used favorably in combination with the respective screws above described. According to the respective screws having the above-described configurations, the screws can be shortened while stabilizing the plastication state of the resin material. However, the resin material sometimes becomes difficult to plasticize because of excessive addition of a filler to the resin material, and the like. In addition, when the screw is designed to have the thread length fall within the “preferable range”, the flight pitch of the feed section may sometimes become excessively small depending on the pellet size of the employed resin material or the number of the threads provided to the screw, possibly causing difficulties in securing the feed rate of the resin material as well as difficulties in setting the thread length to fall within the “preferable range”.  
      In such cases, the various well-known configurations to promote plastication of the resin material can be added. Specifically cited is the configuration, as mentioned above, such that the barrier flight or the sub flight is formed on the screw, such that the Dulmage structure or the shear element is provided, or such that the number of threads is increased. The plasticizing mechanism for resin material consistent with the present invention is applied in addition to, or instead of such a configuration. Specifically, the plasticizing mechanism for resin material is configured such that a torpedo plate for promoting plastication of the resin material is installed ahead of the screw in the plasticizing cylinder, i.e., at a downstream part of a flow of the plasticized resin material.  
       FIG. 4A  is an external perspective view showing an exploded state of one end of the plasticizing cylinder in which the plasticizing mechanism of the present invention is incorporated. As shown in  FIG. 4A , to one end of a plasticizing cylinder  10 , a cylinder head  11 , a torpedo plate  13  in which a torpedo  12  is arranged, and a spacer  14  are piled to be fixed.  
      The torpedo plate  13  has a configuration such that a through-hole is formed at the approximate center of a plate-shaped member and the torpedo  12  approximately spindle-shaped is arranged inside of the through-hole. Here,  FIG. 4B  is an external plan view of the torpedo plate  13 . As shown in  FIG. 4B , the torpedo  12  is supported by one or more fins (supporting pieces)  16 . Besides, in  FIG. 4B , a configuration in which four fins support is shown as an example. Between an outer surface of the torpedo  12  and an internal surface of the through-hole, a path  17  of molten resin is formed like a clearance.  
      The spacer  14  is a plate-shaped member similar to the torpedo plate  13 , at the approximate center of which formed is a path  19  of resin material being a through-hole. Besides, for the purpose of smooth flow of the plasticized resin material, it is desirable for the path  19  of resin material to have the caliber approximately the same as the diameter of the through-hole formed in the torpedo plate  13 .  
      In the cylinder head  11 , in order not to interfere with the torpedo  12 , a concave part  20  is formed at the approximate center on a surface mated to the torpedo plate  13 . At the center of the concave part  20 , formed is a path  21  of resin material being a through-hole of which the caliber is smaller than the paths  17  and  19  of resin material of the torpedo plate  13  and the spacer  14 . Accordingly, at the center of the cylinder head  11 , formed is a through-hole which is cross-sectionally in a funnel shape as a whole.  
      At the one end of the plasticizing screw  10 , the spacer  14 , the torpedo plate  13  and the cylinder head  11  are piled to be installed in this order from a part closer to the plasticizing screw  10 , and they are fixed to the one end of the plasticizing screw  10  with bolts  15  via bolt holes  25  formed in the respective members.  
       FIG. 5  is a sectional view showing a state in which the plasticizing mechanism for resin material consistent with the present invention is incorporated. Besides, in  FIG. 5 , the screw  1   a ,  1   b  and the torpedo  12  are shown in an external view, not in a sectional view. In the state of incorporation, a part of the torpedo  12  which projects from a circular-plate-shaped surface of the torpedo plate  13  is accommodated in the concave part  20  of the cylinder head  11  with a predetermined clearance in order to be kept from contact therewith. Meanwhile, a part of the torpedo  12  which projects toward the screw  1 a,  1   b  is fixed using the spacer  14  at a position such that the part does not interfere with the screw  1   a ,  1   b . Additionally, heaters  101  for heating the resin material are sometimes installed on an outer surface of the plasticizing cylinder  10 .  
      According to such a configuration, the plasticized resin material conveyed from the plasticizing cylinder  10  flows through the path  17  of resin material of the torpedo plate  13  and the clearance between the outer surface of the torpedo  12  and an internal surface of the concave part  20  of the cylinder head  11 . Then, the resin material is conveyed from the path  21  of resin material of the cylinder head  11  to a nozzle and the like (not illustrated) mounted to the cylinder head  11 , and discharged.  
      Incidentally, the torpedo  12  arranged in the torpedo plate  13  does not always have to be formed in such a manner that one end or both ends thereof project from an end surface/end surfaces of the torpedo plate  13 . For example, the configuration may be arranged such that the torpedo plate  13  is made thicker in order that the torpedo  12  does not interfere with the screw  1   a ,  1   b  without the use of the spacer  14 .  
      In addition, when a screen member for removing foreign particles (e.g., a mesh plate material such as a stainless-steel wire gauze) needs to be installed to remove foreign particles mixed in the resin material, the configuration may be arranged in such a manner that a breaker plate is inserted either in front of or behind the torpedo plate  13  in order to hold the screen member.  
      As described above, at the time of continuous discharge using the above-described short screw having the L/D ratio of 10 or less, especially using the screw having the L/D ratio of 5 or less, employing such a configuration that the torpedo plate in which the torpedo is arranged is installable in the plasticizing cylinder and improving the plastication state of the resin material using the torpedo plate do not result in complexity or upsizing of a driving mechanism of the screw. Therefore, further stabilization of the plastication state of the resin material can be ensured while keeping an injection molding machine or an extrusion molding machine small.  
      In addition, the shape of the torpedo  12 , an area of the flow path such as the clearance between the torpedo  12  and the through-hole, surface finishing of the torpedo plate  13 , and the number and shape of the fins (supporting pieces)  16  for supporting the torpedo  12  vary depending on the resin to be discharged. Therefore, it is desirable to prepare in advance various kinds of torpedo plates in accordance with the variety of resin materials so that they can be exchanged.  
      The torpedo plates  13  can be exchanged only by mounting and demounting the bolts  15  from the outside of the plasticizing cylinder  10 , which is easier compared with exchange of the screws. Therefore, operation of adjustment to obtain the optimum plastication states for the respective resin materials (i.e., the exchange of the torpedo plates) can be conducted in a short period of time, ensuring efficiency in adjustment operation of an injection molding machine or an extrusion molding machine.  
      Further, since such a member as the torpedo plate is generally low in cost compared with the screw, costs for equipment can be curbed even though a plurality of torpedo plates are prepared in accordance with the variety of resin materials to be plasticized when compared with a case where a plurality of screws are prepared in accordance with the variety of resin materials.  
      In addition, shown in  FIG. 5  is the configuration in which one piece of torpedo plate is installed; however, the configuration may be arranged such that a plurality of torpedo plates are installed. For example, a plurality of torpedo plates which are different in the number of, or in the positions of fins for supporting the torpedoes are employed in combination to provide the fins with a function of a static mixer. Accordingly, combining a plurality of torpedo plates having different structures also provides another advantage than the uniform plastication of the molten resin.  
      Incidentally, in the configuration described above, the torpedo plate is installed between the nozzle and the plasticizing cylinder; however, if performing extrusion molding using a cross head, the configuration may be arranged such that the torpedo plate is installed between the plasticizing cylinder and the cross head. In addition, the resin material applicable is not limited to, for example, the polybutylene telephthalate, the polypropylene and the commonly-used thermoplastic elastomer which are described in the embodiments.  
      The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description; however, it is not intended to limit the invention to the preferred embodiments, and modifications and variations are possible as long as they do not deviate from the principles of the invention.