Abstract:
A novel barrier screw scientifically designed with new structural features that eliminate the shortcomings of the previous barrier screws and further improve their performance. The important new structural features include a structure of originating the barrier flight without creating any dead-spot while maintaining a constant solid channel width, a structure of terminating the barrier flight without creating any dead-spot while assuring good melt quality, a melt channel structure of avoiding excessively deep channel depth while assuring sufficient melt conveying capacity, and a solid channel structure of avoiding excessively large channel area with shallow channel depth while assuring good melt quality.

Description:
FIELD 
     This invention relates to an improved screw design for use in a single-screw extruder. 
     BACKGROUND 
     Single-screw extruders are most widely used in processing plastic materials for melting solid plastic into molten state, or melt, suitable for forming into desired shapes. The performance of an extruder basically depends on the geometrical features of the screw. Among various types of special screws developed to improve the extruder performance, barrier screws utilizing a barrier flight have been most successful. The barrier flight with a tight clearance divides the screw channel into a solid channel and a melt channel. Only molten plastic material can flow over the barrier flight from the solid channel into the melt channel. 
     Although the barrier screws significantly improve the extruder performance, all of them have some undesirable structural features and their structures still can be improved. Problems encountered with previous barrier screws include, but are not limited to, blockage to solid bed movement, degradation of plastic material at dead-spot or in deep melt channel, small solid bed pieces entering melt channel from solid channel with a large depth at the end, overheating of melt at the end of solid channel with a very shallow depth at the end, poor metering capability of very deep melt channel at the end, and undesirable melt distribution from deep melt channel into shallow metering channel in the direction opposite to drag flow. 
     SUMMARY 
     A screw for a single-screw extruder comprising a feeding section with a deep depth at the feed end of an extruder, a metering section with a shallow depth at the discharge end of the extruder, and a compression section between the feeding section and the metering section. 
     The compression section has at least one pair of a helical main flight with a minimum clearance to the barrel and a helical barrier flight with a tight clearance to the barrel. The main flight originates at the feed end of the screw and maintains a substantially constant lead in the feeding section, forming a helical feed channel with a substantially constant channel width and a substantially constant channel depth in the feeding section. The main flight increases its lead at or near the end of the feeding section. 
     The barrier flight originates proximate to the pushing side of the main flight, but is sufficiently separated from the main flight without creating a dead-spot, at a point downstream of the main flight shortly after the main flight increased its lead, forming a helical solid channel and a helical melt channel in the compression section; 
     The minimum clearance of the main flight, or the main flight clearance, being the possible minimum in machining and assembly of the screw and the barrel. 
     The tight clearance of the barrier flight is substantially more than the main flight clearance, allowing the melt to flow through the barrier flight clearance from the solid channel into the melt channel, and preventing solid plastic materials from entering the melt channel. The feed channel of the feeding section continues to become the solid channel with substantially the same channel depth and width into the compression section without substantially reducing its channel area, and without blocking the movement of tightly packed solid plastic materials from the feed channel into the solid channel. 
     The melt channel quickly increases its width to about 30-50% of the solid channel width over about the initial 10-30% of its length by quickly increasing both leads of the main flight and the barrier flight, and then keeping its width substantially constant until about 70-90% of its length. The melt channel starts with a depth substantially the same or deeper than the metering channel depth, that is significantly greater than the tight clearance of the barrier flight, with an opening to the solid channel or the feed channel without creating a dead-spot. The melt channel gradually increases its depth to about 150-200% of the metering channel depth over about 70-90% of its length; 
     The melt channel, after about 70-90% of its length, quickly increases its width to about 50-80% of the combined channel width of the solid channel and the melt channel over about the last 10-30% of its length by increasing the lead of the barrier flight and reducing the solid channel width by substantially the same amount, while quickly increasing its depth to about 170-220% of the metering channel depth over the same length. 
     The solid channel starts with substantially the same width as the feed channel width and keeps its width substantially constant over about 70-90% of its length. The solid channel starts with substantially the same depth as the feed channel depth and gradually decreases its depth to substantially the same as the metering channel depth over about 70-90% of its length; 
     The solid channel, after about 70-90% of its length, quickly decreases its width to about 20-50% of the combined channel width of the solid channel and the melt channel over about the last 10-30% of its length. The solid channel quickly decreases its depth to about 20-50% of the metering channel depth over substantially the same length and also the clearance of the barrier flight over the same length is significantly increased to about 20-50% of the metering channel depth. At the end, the solid channel has a relatively shallow depth of about 20-50% of the metering channel depth to prevent a significant amount of solid materials from entering the metering channel, while providing a sufficiently large opening to the metering channel without creating a dead-spot and minimizing the channel area with the shallow depth to avoid excessive heat generation. 
     The main flight and the barrier flight interchange their roles at the end of the compression section by converting the main flight to become a second barrier flight and converting the barrier flight to become a second main flight. The flight interchange results in switching the positions of the melt channel and the solid channel relative to screw rotation, making the melt distributed from the deep melt channel into the shallow solid channel occur by the drag force of screw rotation without the need of high pressure in the melt channel. 
     The melt channel located on a trailing side of the second main flight after the flight interchange quickly decreases its depth from about 170-220% of the metering channel depth to the metering channel depth over an axial length of the screw of about 1-3 times the diameter of the screw. The solid channel located on the pushing side of the second main flight after the flight interchange quickly increases its depth from about 20-50% of the metering channel depth to the metering channel depth over the same axial length of the screw. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic representation of a single-screw extruder with a conventional screw. 
         FIG. 2  is a schematic representation of a typical conventional screw. 
         FIG. 3  shows an idealized cross-section of a conventional screw channel during melting. 
         FIG. 4  shows an unwrapped screw channel of barrier screws with a diagonal barrier flight. 
         FIG. 5  shows an unwrapped screw channel of a barrier screw with a parallel barrier flight and a constant main flight lead. 
         FIG. 6  shows an unwrapped screw channel of a barrier screw with a constant solid channel width throughout the feeding and compression sections with increasing main and barrier flight leads. 
         FIG. 7  shows an unwrapped screw channel of a barrier screw with an open parallel barrier flight, constant flight leads, and flight interchange. 
         FIG. 8  shows an exemplary embodiment of an unwrapped screw channel of the scientifically designed barrier screw of this invention. 
     
    
    
     DETAILED DESCRIPTION 
     General understanding of the geometries and functions of a screw in a single-screw extruder as presented in BACKGROUND will be helpful in understanding and appreciating the novel structural features and advantages of the exemplary embodiments of this invention. For more background information, please see Chan I. Chung,  Extrusion of Polymers, Theory and Practice  (1 st  ed. 2000); which is hereby incorporated by reference in its entirety. 
     Aspects of the invention are disclosed in the following description and related drawings directed to specific embodiments of the invention. Alternate embodiments may be devised without departing from the spirit or the scope of the invention. Additionally, well-known elements of exemplary embodiments of the invention will not be described in detail or will be omitted so as not to obscure the relevant details of the invention. Further, to facilitate an understanding of the description, discussion of several terms used herein follows. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the terms “embodiments of the invention”, “embodiment” or “invention” do not require that all embodiments of the invention include the discussed feature, advantage or mode of operation. 
     An exemplary single-screw extruder  100  with a conventional screw  102  is shown schematically in  FIG. 1 . The screw  102  is rotated by a motor  104 , and a barrel  106  is heated by heaters  108 . The design of the conventional screw  102  is shown schematically in  FIG. 2 . The screw  102  has one helical thread or flight  202 , forming a continuous screw channel  204  with a channel depth  206  along the screw  102 . The flight  202  has a minimum clearance to the barrel. The axial distance that the flight  202  travels in one turn is the pitch or lead  208  of the screw  102 . Screw  102  consists of three distinct sections: a feeding section  210  with a constant depth at the feeding end of the extruder, a metering section  212  with a constant depth at the discharge end of the extruder, and a transition or compression section  214  between the feeding section  210  and the metering section  212 . The depth of the feeding section  210  is much greater than that of the metering section  212 , typically by 2-5 times, and the depth of the transition section  214  decreases from the depth of the feeding section  210  to the depth of the metering section  212 . The length L of the flighted portion of the screw  102 , or the screw length, is commonly expressed by the multiples of the screw diameter D. For example, the screw length with 30 length to diameter ratio or 30 L/D is equal to 30 times the screw diameter D. The L/D of most screws ranges about 20-45, usually around 30. The lengths and depths of the three distinct sections as well as the lead  208  of the screw  102  are designed for the specific properties of the plastic material and the specific requirements of the process. In a simple design, the lengths of the three distinct sections of the screw  102  are made about the same, each section with about one third of the screw length L, and the lead  208  of the screw  102  is made substantially equal to the screw diameter D. 
     Referring to  FIGS. 1 and 2 , a solid plastic material, in the form of pellet, chip, powder, or flake, etc., is fed from a hopper  110  into the screw  102 . The solid plastic material fed into the screw  102  is moved downstream from the feed end to the discharge end of the extruder and compacted by the rotation of the screw  102 , forming a tightly packed solid bed in the screw channel  204 . Substantially complete compaction of the solid bed occurs usually at about 4-7 L/D from the hopper  110 . The solid bed rotates with the screw  102 , rubbing on the barrel  106  at almost the same peripheral velocity of the screw  102  and melts gradually as it slowly slides down along the screw channel  204 . 
     The melting mechanism of the solid bed  302  is depicted in  FIG. 3 . A thin film of the molten plastic material or melt film  306  is formed between the solid bed  302  and the hot barrel  304 , and the melt film  306  is highly sheared by the rubbing force of the solid bed  302  on the barrel  304 . The motor supplies the rubbing force. Melting occurs mainly on the top surface of the solid bed  302  in contact with the barrel  304  by the heat generated in the melt film  306 . The thickness  308 , not the width  310 , of the solid bed  302  decreases along the screw channel by melting. Plastic materials have a very low thermal conductivity, and melting by heat conduction from the hot barrel  304  is usually minimal. In fact, too much heat is generated in the melt film  306  at high screw speeds, and the barrel  304  must be cooled to remove the excess heat in order to avoid overheating the melt. The melt formed on the barrel  304  is collected by the wiping action of the rotating flight of the screw into a melt pool  312 . The amount of the solid bed  302  decreases gradually along the screw channel, while the amount of the melt pool  312  increases gradually. Complete melting of the solid bed  302  before the end of the screw  102 , forming a homogeneous melt pool with uniform temperature and mixing in the entire screw channel, is required for good product qualities. 
     Referring again to  FIG. 1 , the screw  102  pumps or meters out the melt through a die  112 . The die  112  is attached to the barrel  106  by an adaptor. The melt is filtered by a screen pack  114  placed at the end of the screw  102  to remove large foreign solid particles, and the screen pack  114  causes a large pressure drop. The pressure at the end of the screw  102  or the head pressure P is the sum of the pressure drops through the screen pack  114 , the adaptor, and the die  112 . 
     The screw of an extruder performs three main functions; solid conveying, melting, and metering. The solid conveying capacity and the metering capacity of the screw increase almost proportional to screw speed, but the melting capacity of the screw increases far less than proportional to screw speed. Thus, complete melting of the sold bed occurs at a point farther down the screw channel as screw speed increases. Eventually, incomplete melting of the sold bed occurs inside the screw as the screw speed is increased, limiting the production rate. Furthermore, it is observed that the solid bed becomes unstable and breaks up into small solid pieces, to be called “small solid bed pieces”, when it becomes small towards the end of the screw. The small solid bed pieces are mixed into the melt pool and make the melt pool inhomogeneous, also limiting the production rate. 
     Numerous special screw designs have been developed in order to increase the melting capacity of a screw and/or to prevent the small solid bed pieces from mixing into the melt pool. The most successful special screw designs are the barrier screws, such as those disclosed in background art U.S. Pat. Nos. 3,358,327 by Maillefer, 3,375,549 by Geyer, 3,271,819 by Lacher, 3,698,541 by Barr, 3,858,856 by Hsu, 3,867,079 by Kim, 3,650,652 by Dray and Lawrence, and 4,000,884 by Chung. These barrier screws generally have a barrier flight in the compression section in addition to the main flight, dividing the screw channel into two channels, a solid channel and a melt channel. The barrier flight generally has a tight clearance to the barrel, only allowing the melt to flow through the tight clearance but preventing incompletely molten plastic material and the small solid bed pieces from passing over the barrier flight from the solid channel into the melt channel. Although these barrier screws perform better than conventional screws, all of these barrier screws have some undesirable structural features that cause blockage to the solid bed movement, stagnation of the melt, or poor metering capability. Blockage to the solid bed movement results in the output fluctuations, and stagnation of the melt results in degradation of the melt. Poor metering capability reduces the output rate per screw revolution and increases the temperature of the melt, or the melt temperature, exiting from the extruder. 
     The background art barrier screws of U.S. Pat. Nos. 3,358,327 by Maillefer, 3,375,549 by Geyer, and 3,271,819 by Lacher essentially have a diagonal barrier flight in the compression section  416  as shown in  FIG. 4 . The metering channel  402  is completely separated from the feed channel  404  by a diagonal barrier flight  406 . Thus, no small solid bed piece is expected in the metering channel  402  of these barrier screws. However, the diagonal barrier flight  406  makes the width of the solid channel  408  decrease and the width of the melt channel  410  increase along the screw channel. The decreasing width of the solid channel  408  forces the solid bed to reduce its width along the solid channel  408 . The solid bed is tightly packed and resists any deformation in its width because the solid bed melts mainly on its surface in contact with the barrel and only the thickness of the solid bed decreases upon melting. Thus, the decreasing width of the solid channel  408  causes blockage to the solid bed movement along the solid channel  408 . Furthermore, the diagonal barrier flight  406  creates dead-spots  412  at its originating point and its terminating point because it is connected to the main flight  414 . These barriers screws with a diagonal barrier flight suffers from output fluctuations, especially for rigid polymer materials, caused by blockage to the solid bed movement as well as degradation of the melt at the dead-spots. 
     The background art barrier screws of U.S. Pat. No. 3,698,541 by Barr and U.S. Pat. No. 3,858,856 by Hsu have a barrier flight parallel to the main flight, while maintaining the lead of the main flight constant throughout the entire screw. Both the width of the solid channel and the width of the melt channel stay constant once they are formed. The constant solid channel width does not cause any blockage to the solid bed movement along the solid channel. However, the barrier flight  502  in the compression section  504  originates rapidly either from the pushing edge of the main flight  506  in the case of U.S. Pat. No. 3,698,541 by Barr shown in  FIG. 5 , or from the trailing edge of the main flight in the case of U.S. Pat. No. 3,858,856 by Hsu, drastically reducing the area of the feed channel  518  from the feeding section  508  into the solid channel  520  in the compression section  504 , usually by about 35%, at the point of its origination as well as creating a dead-spot  510  as shown in  FIG. 5 . 
     The drastic reduction of the channel area causes severe blockage to the solid bed movement, causing output fluctuations. The barrier flight  502  terminates merging with the main flight  506 , creating another dead-spot  512 . Furthermore, the narrow melt channel  514  becomes very deep at the end in order to accommodate increasing amount of the melt. The metering capability of a screw channel decreases as the depth to width ratio of the screw channel increases because the melt adhered on the large surfaces of the flight does not move. Thus, the narrow and deep melt channel  514  at the end has a poor metering capability. These barrier screws suffer from degradation of the melt at the dead-spot and also in the deep melt channel. The melt accumulated in the deep melt channel  514  on the pushing side of the main flight must be distributed into the metering channel  516  at the end of the compression section  504 . However, the required direction of the melt distribution is in the opposite direction to the natural melt flow driven by screw rotation as explained below, and a high pressure in the melt channel  514  at the end is required for the melt distribution. The high pressure at the end of the melt channel  514  further reduces the metering rate along the melt channel, adversely affecting the performance of the screw. The melt inside a screw channel adheres on all screw surfaces and the barrel surface. The melt adhered on the screw surfaces rotates with the screw, but the melt adhered on the barrel surface does not move and stay with the stationary barrel. Thus, the stationary melt adhered on the barrel surface moves in the screw channel from the trailing side to the pushing side of the flight in the direction opposite to screw rotation as shown in  FIG. 5 , which is called “the drag flow”. The drag flow gives the metering capability of the screw. 
     The background art barrier screw of U.S. Pat. No. 3,867,079 by Kim is shown in  FIG. 6 . This screw originates a barrier flight  602  from the main flight  604  at the end of the feeding section, forming a melt channel  606 . The melt channel width is gradually increased to become about 50-100% of the width of the feed channel  608  by increasing both leads of the main flight  604  and the barrier flight  602 . The feed channel  608  from the feeding section is not divided by the barrier flight  602  and its width is kept constant in the compression section  610 . Thus, the width of the solid channel  612  is kept constant throughout the feeding and compression sections. This barrier screw design eliminates the blockage problem to the solid bed movement. However, both leads of the main flight  604  and the barrier flight  602  become very large towards the end of the compression section  610 , and the conveying capacities of both channels decrease. The problem of a dead-spot  614  at the origination of the barrier flight  602  still remains. The solid channel  612  is open to the metering channel  616  in this design. If the end of the solid channel  612  is deeper than the clearance of the barrier flight  602  and closer to the size of the solid plastic material, incompletely molten plastic material can go into the metering channel  616 . If the end of the solid channel  612  is shallow close to the clearance of the barrier flight  602 , there is a large solid channel area with a very shallow channel depth and over-heating of the melt will occur in that area. The barrier screw of U.S. Pat. No. 3,650,652 by Dray and Lawrence is similar to the barrier screw of U.S. Pat. No. 3,867,079 by Kim except the major difference that the melt channel width is kept constant once it is increased to a desired width. 
     The background art barrier screw of U.S. Pat. No. 4,000,884 by Chung is shown in  FIG. 7 . It has a barrier flight  702  parallel to the main flight  704  and the main flight lead stays constant similar to the barrier screw of U.S. Pat. No. 3,698,541 by Barr, but the barrier flight  702  is not connected to the main flight  704  at both the originating point and the terminating point. Thus, the melt channel  706  is open to the feed channel  708  without a dead-spot, and the solid channel  710  is open to the metering channel  712  without a dead-spot. At the end of the compression section  714 , the main flight  704  terminates and the barrier flight  702  converts to become a new main flight  716 . This flight interchange switches the positions of the melt channel  706  and the solid channel  710  relative to screw rotation, making the melt distribution from the deep melt channel  706  at the end into the shallow metering channel  712  occur by the drag flow of screw rotation without the need of high pressure in the deep melt channel  706 . However, the width of the feed channel  708  is drastically reduced by the melt channel  706  at the start of the compression section similar to the barrier screw of U.S. Pat. No. 3,698,541 by Barr. Thus, blockage to the solid bed movement occurs at the start of the compression section  714 . Because the solid channel  710  is open to the metering channel  712 , this barrier screw also suffers from the same problems discussed for the barrier screw of U.S. Pat. No. 3,867,079 by Kim. 
     An objective of this invention is to eliminate all of the problems encountered with all of the previous barrier screws, such as blockage to solid bed movement, degradation of plastic material at dead-spot or in deep melt channel, small solid bed pieces entering melt channel from solid channel with a large depth at the end, overheating of melt at the end of solid channel with a very shallow depth at the end, poor metering capability of very deep melt channel at the end, and undesirable melt distribution from deep melt channel into shallow metering channel in the direction opposite to drag flow. 
     Another objective is to achieve homogeneous melt quality with uniform temperature and mixing delivered from an extruder in order to obtain good product quality and increased production rate. 
     Objectives ancillary to the foregoing objectives are to teach and provide a novel barrier screw to accomplish said objectives. 
     Aspects of the invention are disclosed in the following description and related drawings directed to specific embodiments of the invention. Some exemplary embodiments include exemplary ranges or dimensions for descriptive purposes. Additional embodiments, ranges, dimensions or similar descriptive characteristics may be utilized by a person having ordinary skill in the art without deviating from the scope of the present invention. 
       FIG. 8  is an exemplary embodiment of the present invention. Six specific locations along the screw axis from the feed end of the screw are denoted by Locations  1  to  6 . A main flight  802  with a minimum clearance to the barrel originates at the feed end of the screw, forming a feed channel  804 . The main flight  802  maintains its lead substantially constant in the feeding section and increases its lead at or near the end of the feeding section at Location  1 . A first barrier flight  806  with a tight clearance to the barrel between Location  2  and Location  4  originates at Location  2  proximate to the pushing side of the main flight  802  shortly after the main flight  802  increased its lead. Location  2  is downstream of Location  1  by about 5-15% of the feed channel width. The first barrier flight  806  originates separately from the main flight  802  without creating a dead-spot, and it forms a solid channel  808  and a melt channel  810  in the compression section  812 . The clearance of the first barrier flight  806  is more than the clearance of the main flight  802 , allowing the melt to flow through the clearance from the solid channel  808  into the melt channel  810  and preventing the solid plastic material from entering the melt channel  810 . 
     The feed channel  804  at Location  1  continues to become the solid channel  808  with substantially the same channel depth and width at Location  1  without substantially reducing its channel area, thus without blocking the solid bed movement. The melt channel  810  starts with a depth substantially the same or deeper than the metering channel depth, which gives an opening to the solid channel  808  and/or the feed channel  804  between Location  1  and Location  2  without creating a dead-spot. The exemplary structural feature of originating the barrier flight  806  downstream of the main flight  802  and separately from the main flight  802  without creating a dead-spot at the start of the melt channel  810 , while converting the feed channel  804  to become the solid channel  808  without substantially reducing the width and depth to avoid blockage to the solid bed movement, is a new feature of an exemplary embodiment of this invention. 
     The melt channel  810  quickly increases its width to about 30-50% of the solid channel  808  width over about initial 10-30% of its length from Location  1  to Location  3  by quickly increasing both leads of the main flight  802  and the first barrier flight  806 , and, afterward, keeps its width substantially constant until about initial 70-90% of its length from Location  3  to Location  4 . The melt channel depth gradually increases to about 150-200% of the metering channel depth over about initial 70-90% of its length from Location  1  to Location  4 . Then, the melt channel quickly increases its width to about 50-80% of the combined channel width of the solid channel  808  and the melt channel  810  over about final 10-30% of its length from Location  4  to Location  5  by increasing the lead of the first barrier flight  806  and reducing the solid channel width by substantially the same amount. The melt channel  810 , after about initial 70-90% of its length, quickly increases its depth to about 170-220% of the metering channel depth over about the last 10-30% of its length from Location  4  to Location  5 . The solid channel  808 , starting with substantially the same width and depth as those of the feed channel  804 , keeps substantially the same width over about 70-90% of its length from Location  1  to Location  4 , and gradually decreases its depth to about the same as the metering channel depth over the same length. Then, the solid channel  808  quickly decreases its width to about 20-50% of the combined channel width of the solid channel  808  and the melt channel  810  over about the last 10-30% of its length from Location  4  to Location  5 , and also quickly decreases its depth to about 20-50% of the metering channel depth over the same length. A second barrier flight  816  between Location  4  and Location  5 , has a substantially larger flight clearance than first barrier flight  806  to accommodate a higher flow rate of the melt from the solid channel  808  into the melt channel  810  resulting from quickly decreasing solid channel area. The solid channel  808  at the end, although it has a relatively shallow depth to prevent solid bed pieces from entering the metering channel  814 , is sufficiently open to the metering channel  814  without creating a dead-spot, and the solid channel area with the relatively shallow depth near Location  5  is made relatively small to substantially avoid excessive heat generation. The combined width of the solid channel  808  and the melt channel  810  is substantially constant between Location  3  and location  5 . Although the width and depth of the solid channel  808  are quickly reduced towards the end of the compression section  812  between Location  4  and Location  5 , blockage to the solid bed movement is substantially prevented because the solid bed becomes small and weak, and substantially breaks up after reaching Location  4 . The structural feature of quickly changing the channel areas of the solid channel  808  and the melt channel  810  and utilizing the second barrier flight  816  towards the end of the compression section  812  is another new feature of this exemplary embodiment of the invention. 
     At the end of the compression section  812  at Location  5 , the main flight  802  converts to become a third barrier flight  818  between Location  5  and Location  6 , and the second barrier flight  816  converts to become a second main flight  820 . This flight interchange switches the positions of the melt channel  810  and the solid channel  808  relative to screw rotation. The melt channel  810  was located on the pushing side of the original main flight  802  before the flight interchange until Location  5 , but it is located on the trailing side of the second main flight  820  after the flight interchange from Location  5 . The melt channel  810  now located on the trailing side of the second main flight  820  quickly decreases its depth from about 170-220% of the metering channel depth to the metering channel depth between Location  5  and Location  6 , while the solid channel  808  now located on the pushing side of the second main flight  820  quickly increases its depth from about 20-50% of the metering channel depth to the metering channel depth between the same locations. The depth of the entire screw channel becomes substantially the same as the metering channel depth at Location  6 . The melt distribution from the deep melt channel  810  into the shallow solid channel  808  substantially occurs between Location  5  and Location  6  effectively by the drag flow of screw rotation without the need of high pressure in the deep melt channel  810 . The axial distance between Location  5  and location  6  is usually about 1-3 times the screw diameter. The flight clearance of the third barrier flight  818  is designed to be sufficiently large enough to achieve the melt distribution mostly by the drag flow but small enough to further improve the melt quality during the melt distribution. It is possible to eliminate the third barrier flight  818  between Location  5  and Location  6  to simplify the design or shorten the screw length between Location  5  and Location  6 . 
     At least two screws of one exemplary embodiment of the invention were utilized in practice with improved performances. One screw with 120 mm diameter and about 30 L/D length was used for extruding polypropylene at the output rate of about 450 kg/hr at the screw speed of about 95 rpm and the head pressure of about 160 kg/cm 2  very stably without any noticeable fluctuation of the head pressure. The other screw also with 120 mm diameter and about 30 L/D length was used for extruding amorphous polyethylene terephthalate at the output rate of about 750 kg/hr at the screw speed of about 85 rpm and the head pressure of about 63 kg/cm 2  very stably without any noticeable fluctuation of the head pressure. 
     The foregoing description and accompanying drawings illustrate the principles, preferred embodiments and modes of operation of the invention. However, the invention should not be construed as being limited to the particular embodiments discussed above. Additional variations of the embodiments discussed above will be appreciated by those skilled in the art. 
     Therefore, the above-described embodiments should be regarded as illustrative rather than restrictive. Accordingly, it should be appreciated that variations to those embodiments can be made by those skilled in the art without departing from the scope of the invention as defined by the following claims.