Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29B—PREPARATION OR PRETREATMENT OF THE MATERIAL TO BE SHAPED; MAKING GRANULES OR PREFORMS; RECOVERY OF PLASTICS OR OTHER CONSTITUENTS OF WASTE MATERIAL CONTAINING PLASTICS
  • B29B7/00—Mixing; Kneading
  • B29B7/30—Mixing; Kneading continuous, with mechanical mixing or kneading devices
  • B29B7/34—Mixing; Kneading continuous, with mechanical mixing or kneading devices with movable mixing or kneading devices
  • B29B7/38—Mixing; Kneading continuous, with mechanical mixing or kneading devices with movable mixing or kneading devices rotary
  • B29B7/40—Mixing; Kneading continuous, with mechanical mixing or kneading devices with movable mixing or kneading devices rotary with single shaft
  • B29B7/42—Mixing; Kneading continuous, with mechanical mixing or kneading devices with movable mixing or kneading devices rotary with single shaft with screw or helix
  • B29B7/428—Parts or accessories, e.g. casings, feeding or discharging means
  • B29B7/429—Screws
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/36—Means for plasticising or homogenising the moulding material or forcing it through the nozzle or die
  • B29C48/50—Details of extruders
  • B29C48/505—Screws
  • B29C48/64—Screws with two or more threads
  • B29C48/65—Screws with two or more threads neighbouring threads or channels having different configurations, e.g. one thread being lower than its neighbouring thread
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/03—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion

Definitions

• This invention relates to a method and apparatus for melting and conveying plasticated material.
• this invention relates to a ⁇ r method and apparatus for melting and conveying plasticated
• the apparatus includes a barrel having a cylindrical bore therethrough, a shaft rotatable in said bore which has a substantially circular cross-section, the shaft and bore
• the shaft is provided with a helical first channel means integral with the shaft for conveying said plasticated material in a downstream direction through the feed section, the tran-
• a helical second channel means for producing an initial melt film is provided integral with the shaft for rapidly subjecting the material in the second channel means to high initial shear rates to form a melted film of plasti-
• the second channel means terminates at a point of beginning of the transition section whereupon the - first channel means has a step through the transition sec ⁇ tion which step disappears before a point of beginning of
• a well established melt film at the surface of the 35 bore provides the necessary difference between the coef- ficient of friction between the bore and shaft or root of the screw so that effective conveying and pressure genera ⁇ tion can be acheived. Until the melt film is properly established, efficient melting cannot be obtained in a con- ventional extruder having a smooth bore.
• melt film is normally not formed until several turns of the flight from the beginning of the feed section, since the cold pellets of material must be heated at their surfaces to their respective melting point by either an external heat source on the barrel or by shearing of the pellets against the bore by the rotating flight. This process is at best unstable and can be disturbed by changes in the bulk density or the viscosity of the material or downstream pressure changes.
• screw designs include an auxilliary channel located in the vicinity of the transition section.
• This auxilliary channel is separated from a main channel by an auxilliary flight or "barrier" which is fre ⁇ quently undercut from the main flight diameter and acts as a restrictive orifice.
• the pellets of solid material are exposed to high shear rates as they cross the barrier and should be substantially melted as they enter the auxilliary channel. Examples of this type of design may be seen in the patents to Geyer, U.S. Patent No. 3,375,549; and Dray et al, U.S. Patent No. 3,650,652.
• the melt film in these examples is continuously drained off into the auxilliary channel located in the transition section to permit the exposure of more of the unmelted material in the main chan ⁇ nel to shearing stresses presented by the auxilliary flight and existing within the main channel.
• these designs depend upon a traditional solids conveying or feed section which must be carefully designed to establish a stable melt film.
• Barrier type designs may exhibit a reduced capacity to convey in a transition section since the volume of the primary channel is continuously reduced. Difficulties are sometimes encountered because the auxilliary flight is in essence a restrictive orifice, and the melting resulting from the high shear rates across the barrier is highly sen ⁇ sitive to changes in the bulk density and viscosity of the material being processed. The end result may be unstable melting and surging.
• the shear rates to which the material is subjected may be too low with resulting reduction in melting and efficiency. Conversly, if the primary channel were to be too shallow, the shear rate may reach unacceptably high values causing the melt to be overheated.
• Another object of the invention is to provide an independent formation of a stable melt film in the vicinity of the beginning of the feed section of the screw.
• Yet another object of the invention is to improve melting in a single screw extruder by increasing the shear rate in the transition section of the screw without reducing the capacity of the transition section or producing excessi ⁇ vely high temperatures in the material.
• Still another object of the invention is to provide a method and apparatus for melting and conveying plasticated material which will not subject the material to unusually high temperatures within a channel which might cause degra ⁇ dation of the material.
• Another object of the invention is to provide in a screw design the ability to increase the conveying capacity of a feed section by increasing its pitch or lead and its depth.
• Another object of the invention is to provide a capa- bility for controlling the thickness of sheet material being processed in sheet extrusion by increasing or decreasing the initial temperature of the external heaters in the vicinity of the feed section, rather than by increasing or decreasing the rotational speed of the screw.
• the pre ⁇ sent invention provides a method and apparatus as disclosed for melting and conveying plasticated materials, which includes a barrel having a cylindrical bore therethrough.
• a shaft which is rotatable within the bore is provided with a first helical flight which defines a feed section, a metering section, and a transition section.
• a second heli ⁇ cal flight commences at a point of beginning of the feed section and extends to the beginning of the transition sec ⁇ tion.
• the first and second flights define a pair of channels in the feed section, which are disclosed as parallel in the preferred embodiment.
• One of the channels is shallower than the other channel which is identified as a first channel, in order to form a melt film in the second channel at a location very near to the inlet end of the apparatus.
• the early formation of the melt film "wets" the bore of the barrel and provides for the efficient conveying of the material in the deeper channel.
• the second flight and the second channel terminate at the end of the feed section, whereafter the channel is defined by the first flight and the shaft of the screw.
• the channel in the transition sec ⁇ tion is provided with two unequal adjacent diameters within the channel which define a helical step, the smaller diameter being oriented downstream from the larger diameter.
• a helical land formed by the larger diameter provides high shear rates and a large amount of melted material downstream toward the end of the transition section.
• the smaller diameter remains substantially equal to its diameter in the feed section, and continues well into the transition sec ⁇ tion. Toward the end of the transition section the smaller diameter, gradually increases to the diameter of the shaft at the beginning of the metering section.
• the diameter of the land gradually decreases through the transition section so as to likewise become equal to the shaft diameter at the point of beginning of the metering section.
• the step disap ⁇ pears at such point due to the equivalence of the shaft and land diameters.
• melted material in the vicinity of the land and solid material in the vicinity of the shaft diameter are commingled which reduces the overall melt temperature as the heat of the melted material is transferred by conduction to the remaining unmelted material which allows " higher screw rota ⁇ tional speeds and greater outputs.
• Figure 1 is a view of an extruder embodying the invention.
• Figure 2A is a detail view of a portion of the apparatus involved in a feed section of the preferred embo- di ent of the invention.
• Figure 2B is a detail view of a portion of the apparatus involved in a transition section of the preferred embodiment.
• Figure 3A is a section taken along line 3A-3A of Figure 2A.
• Figure 3B is a section taken along line 3B-3B of Figure 2B.
• Figure 4A is a section taken through a channel of an extruder screw representing the prior art not otherwise illustrated.
• Figure 4B is a section similar to Figure 3A showing the formation of a melt film in a second channel of the pre ⁇ ferred embodiment.
• Figure 4C is a section similar to Figure 3B showing melting taking place in a transition section of the pre ⁇ ferred embodiment.
• Figure 5A is a view of an "unwrapped" or developed channel of a screw having a configuration similar to that disclosed herein showing a second channel becoming narrower.
• Figure 5B is another view of an unwrapped or deve ⁇ loped channel showing a first channel becoming narrower.
• Figure 1 depicts a portion of a plasticizing extruder indicated generally by reference numeral 7.
• Barrel 11 is generally cylindrical and is provided with cylindrical bore 13 therethrough, which has an axis for rotation 8.
• Screw 31 includes shaft 39 and is rotatable about axis 8 within bore 13, shaft 39 has a diameter 41 which varies along axis 8 in a downstream direc ⁇ tion indicated generally by reference numeral 2B.
• Extruder 7 also has inlet end 16 in proximity to hopper 6 and outlet end 21 in proximity to outlet 23 where the melted material is ultimately discharged.
• pellets of plasticated material indicated generally by reference numeral 44, which are con- tained in hopper 6, are conveyed in a downstream direction as indicated by reference numeral 28, due to first helical flight 76 and second helical flight 91, both of which com ⁇ mence at beginning point 103 of feed section 106 and have flight diameter 92.
• flight diameter 92 is slightly less than the diameter of bore 13.
• the diameter of second flight 91 may be made smaller or "undercut" from the diameter of first flight 86.
• first flight 76 has a pitch (sometimes referred to as a lead) which is greater than the flight diameter 92.
• first flight 76 has pushing side 81, and trailing side 84, pushing side 81 being oriented downstream from trailing side 84.
• Second flight 91 is downstream from first flight 76 and has pushing side 94 and trailing side 97, pushing side 94 being downstream from trailing side 97. It is important to note that first flight 76 and second flight 91 both commence at beginning point 103 of feed section 106, as do first channel 56 and second channel 68.
• First flight 76 and second flight 91 are shown on Figure 1 as being parallel, however, as will be discussed hereinafter, first flight 76 and second flight 91 need not be, depending upon the type of material being processed and if additional compaction is required for very low density material, for example.
• Screw 31 may be seen in Figure 1 to be divided into feed section 106 for solids conveying, which section is located in proxi ⁇ mity to inlet end 16, metering section 112 for pumping the melted material, which section is located downstream from feed section 106 in proximity to outlet end 21 and outlet 23, and transition section 109, which is located between feed section 106 and metering section 112.
• Trailing side 84 of first flight 76, shaft 39 and pushing side 94 of second flight 91 define first channel 56, which is approximately rectangular in cross section.
• plasticated material 44 is conveyed in downstream direction 28 sequentially through feed section 106, transition section 109 and metering section 112 and outlet 23.
• screw 31 may have more than one stage and may include multiple feed metering sections and further be provided with a venting section, mixing devices or multiple flights depending on the characteristics of the material to be pro ⁇ Listed.
• first channel 56 has a first channel depth 116 as measured radially bet ⁇ ween flight diameter 92 and and shaft diameter 41, which first channel depth 116 is greater than second channel depth 122, which is measured radially between flight diameter 92 and land diameter 71.
• First flight 76 and second flight 91 are continuous throughout feed section 106.
• Second flight 91 and second channel 68 terminate at a point where feed section 106 ends and transition section 109 commences.
• first channel depth 116 remains constant while second channel depth 122 becomes shallower in downstream direction 28 throughout feed section 106, as will be further illustrated in an example of an actual con ⁇ figuration for screw 31.
• second flight 91 and second channel 68 have terminated at a point of beginning 108 of transition section 109.
• First flight 76 continues throughout transition section 109 and first chan ⁇ nel 56 is therein defined by shaft 39, pushing side 81 and trailing side 84 of first flight 76.
• shaft diameter 41 and land diameter 71 continues in transition section 109.
• Shaft diameter 41 is downstream from and adjacent to land diameter 71, and form a step 126 between shaft diameter 41 and land diameter 71, throughout transition section 109.
• shaft diameter 41 may be seen first remaining at its former diameter as in feed section 106 for a predetermined distance 113 along transition section 109 in a downstream direction 28 and then increasing gradually as it aproaches point of termination 111 of transition section 109 while land diameter 71 gradually decreases until at a point of ter- mination 111 of transition section 109 which is also the commencement of metering section 112, shaft diameter 41 and land diameter 71 become substantially equal, whereupon step 126 disappears.
• Predetermined distance 113 depends upon the particular melting characteristics of material 44, but as will be shown in a typical application, it may extend approximately 78% of the length of transition section 109.
• step 126 is also helical and has a pitch which may or may not be equal to the pitch of first flight 76.
• the pitch of first flight 76 in transition sec- tion 109 is reduced so as to be approximately equal to the nominal flight diameter 92 of screw 31.
• first flight 76 could have a pitch approximately equal to flight diameter 92 in a transition section 109, a reduc ⁇ tion from the pitch in feed section 106 as discussed above.
• the pitch of the first flight 76 may be reduced because the increased volume is no longer needed.
• Step 126 may be pro- vided with a pitch which is identical to the pitch of first flight 76, in which case step 126 and first flight 76 will be parallel and land width 73 will remain constant throughout transition section 109.
• step 126 may have a pitch greater than the pitch of first flight 76, in which case land width 73 will increase throughout tran ⁇ sition section 109 and step 126 will approach trailing side 84 of first flight 76 or conversely step 126 may have a pitch which becomes less than the pitch of first flight 76 in transition section 109, in which case land width 73 will decrease throughout transition section 109.
• step 126 having a pitch larger than the pitch of first flight 76 so that land width 73 increases throughout transition section 109, while simultaneously step 126 has disappeared in a smooth fairing at the point of termination 111 of transition section 109, due to the increasing of shaft diameter 41 and the decreasing of land diameter 71.
• First channel 56 has a depth 116 which is uniform at point of termination 111 of transition section 109.
• Metering section 112 is disclosed as having a uniform shaft diameter 41.
• First flight 76 continues into metering section 112 and is shown together with a third flight designated by reference numeral 133.
• Third flight 133 begins approximately at a point where step 126 terminates and is parallel to first flight 76.
• third flight 133 is not critical to the operation of the invention but provides two parallel sets at flights in metering section 112 for improved mixing and pumping of types of materials.
• Length of continuation of feed section shaft diameter 41 into transition section 109 in a downstream direction 28 is approximately 78% of the length of transition section
• first channel depth 116 in feed section 106 is approximately 13% of flight diameter 92; second channel depth 122 in feed section 106 decreases from 11.1% to 1.1% of the flight diameter 92 in downstream direction 28; shaft diameter 41 increases through transition section 109 in a down ⁇ stream direction 28 by approximately 21%; land diameter 71 decreases through transition section 109 in downstream direction 28 by approxi ⁇ mately 9.1%; pitch in feed section 106 (first flight 76 and second flight 91) is 1.222 times flight diameter 92; pitch of first flight 76 in transition section 109 is equal to the first diameter 92; pitch of step 126 in transition section 109 is 1.034 times flight diameter 92; and pitch of first flight 76 and third flight 133 in metering section 112 is equal to flight diameter 92.
• first flight 76 and second flight 91 in feed section 106 are larger than would normally be expected.
• first channel depth 116 is greater than that which would normally be encountered.
• Figure 5A shows feed section 106, in schematic form for purposes of clarity, having first channel 56 and second channel 68
• first channel 56 may be seen as having a constant width 117 whereas second channel 68 has a width 123 which decreases in downstream direction 28.
• width 117 of first channel 56 narrows while width 123 of second channel remains constant in downstream direction 28.
• width 123 of second channel 68 might be narrowed by increasing width 128 of first flight 76 such that pushing side 81 of first flight 76 approaches trailing side 97 of second flight 91, thus narrowing second channel 69.
• width 117 of first channel 56 might be narrowed by increasing width 128 of first flight 76 such that trailing side 84 of first flight 76 approaches pushing side 94 of second flight 91. Conversely, width 131 of second flight 91 might be increased to accomplish a similar * effect.
• first channel width 117 or second channel width 123 is narrowed, material 44 in the respective channel is compacted without any appreciable increase in the shear rate, as would normally occur if com ⁇ paction were attempted by way of decreasing first channel depth 116 in feed section 106. Due to the larger than nor ⁇ mal initial pitch of first flight 76 and second flight 91, there is an ample margin for the narrowing of either first channel width 117 or second channel width 123 and yet still maintain an adequate volume in either.
• first channel 56 and second channel 68 may be specifically configured to enhance the melting and con- veying of a particular material.
• second channel 68 Since, as may be seen in Figure 3A, second channel 68 has a very shallow depth 122, it immediately subjects material 44 to high shear rates very early in feed section 106, with the result that a melt film is established in close proximity to inlet end 16 of screw 31. Meanwhile, the material 44 in first channel 56 is subjected to lower shear rates since first channel depth 116 is greater than second channel depth 122 and less melting of material 44 takes place. A depiction of this may be seen in Figure 4B, which shows the material 44 in second channel 68 nearly melted while the material in first channel 56 is conveyed down ⁇ stream in unmelted form. By way of contrast.
• Figure 4A shows a section of channel at an equivalent location along a screw similar to screw 31 which contains mostly solid material with some melt film beginning to form, which would normally be 4 to 5 turns of the flight downstream from the inlet end 16.
• the melt film immediately forming in second channel 68 can "wet" bore 13 and is much more stable than the melt film formed in a conventional feed section.
• the difference in the coefficients of friction between bore 13 and shaft 39 is well established and the conveying of material 44 in first channel 56 is much more efficient due to the earlier forming of the melted film of material 44.
• This early formation of a melt film in second channel 68 may be seen to be indepen ⁇ dent of solids conveying in first channel 56.
• second flight 31 and second channel 68 terminate at the end of feed section 106 and the beginning of tran ⁇ sition section 109.
• the melt film has been well established and is quite stable, thus permitting effi ⁇ cient solids conveying.
• second channel 68 has "emptied" into first channel 56 which is now defined by first flight 76, shaft 39 and land 67 and first channel 56 has in addition step 126 between shaft diameter 41 and land diameter 71.
• Land diameter 71 provides a narrower and shallower area for the maximum production of melt film.
• shaft diameter 41 remains substan- tially the same as in feed section 106 or possibly shaft diameter 41 may be initially reduced in transition section 109.
• Step 126 is able to permit the gradual controlled intermixing of material at two different temperatures within first channel 56, since as step 126 disappears, the unmelted and melted material is commingled throughout transition section 109 in first channel 56.
• Metering section 112 which may be provided with third flight 133 in conjunction with first flight 76 pumps the melted material through outlet 23 and a die which, although not illustrated, may be provided at outlet end 21.
• a transition section which increases melting rates, thus enhancing the melting taking place in the main channel. Due to the commingling of the melted and solid material well into the transiton section, melt temperatures remain within acceptable limits for the prevention of the degradation of the material. The capacity of the transition section is not reduced due to the increase of pitch of the feed section which is able to convey solid material at a greater rate without the danger of "surging", since melting is already well developed in an auxilliary channel in the feed section.
• the initial melt film is formed independently in the second channel at the beginning of the feed section, the differential between the frictional coefficients of the bore and the shaft or "root" of the screw are well established, and the feed section is much less sensitive to a shortened residence time of the material in the feed sec ⁇ tion if the pitch of the flight were to be increased.
• the pitch of the flight may be increased without loss of mechanical advantage; and further, the first channel depth in a feed section can be increased without detriment to melting occurring in the first channel.
• the characteristics and design of the feed section of the screw is less dependent upon the physi- cal and thermal characteristics of the material.
• the melt film can be established in a second channel for a wide variety of materials since it is independent of what is occurring in the first or main channel. If there should appear an increase in downstream pressure, the melt film in the second channel is not as readily disturbed as in the prior art. In fact it has been observed that such pressure increases serve to assist the entire melting process by further compacting the yet unmelted material in the vicinity of the step in the transition section, thus permitting improved heat transfer from the melted material as the two commingle toward the end of the transition section.
• melt film is formed in the second channel as an independent event, it can likewise be controlled by independent means.
• barrel heaters are used in the vicinity of the feed section to assist the formation of a melt film. It has been observed that by increasing the initial temperatures in the feed zone by increasing the heat imput from the heaters, melt formation in the second channel is increased. This, in turn, provides an improved "wetting" of the bore together with more melted material to commingle with the yet unmelted material in the transition section, resulting in an increase in output, sometimes measured in the art as the variable "pounds per hour per revolutions per minute" or "lbs./hr./rpm".
• heater temperature may be used to vary output which permits a greater degree of control without producing adverse changes in bulk density, pressure or viscosity or without allowing slight fluctuations of these variables, if they do occur, to adversely effect the quality of the extru ⁇ ders output.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Extrusion Moulding Of Plastics Or The Like (AREA)
• Processing And Handling Of Plastics And Other Materials For Molding In General (AREA)

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• 1986
  • 1986-04-24 DE DE19863690220 patent/DE3690220T1/de not_active Withdrawn
  • 1986-04-24 WO PCT/US1986/000861 patent/WO1986006325A1/en active Application Filing
  • 1986-04-24 GB GB8630021A patent/GB2184975B/en not_active Expired
  • 1986-04-24 JP JP61502675A patent/JPS62502677A/ja active Pending
  • 1986-04-24 EP EP86903023A patent/EP0221141A1/de not_active Withdrawn
  • 1986-04-25 CA CA000507662A patent/CA1284413C/en not_active Expired - Lifetime

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