United States Patent (19)
`Jakobsen et al.
`
`11)
`(45)
`
`4,405,546
`Sep. 20, 1983
`
`(54)
`
`METHOD FOR PRODUCING ATUBULAR
`OBJECT OF POLYETHYLENE
`TEREPHTHALATE OR SIMLAR
`THERMOPLASTIC MATERIAL FROMA
`TUBULAR BLANK OF AMORPHOUS
`MATERIAL
`Kjell M. Jakobsen, Hökvågen; Claes
`75) Inventors:
`T. Nilsson, Pramvagen, both of
`Sweden
`Assignee:
`73
`PLM AB, Malmo, Sweden
`Appl. No.:
`21)
`264,665
`(22)
`May 18, 1981
`Filed:
`Foreign Application Priority Data
`(30)
`May 29, 1980 SE Sweden ........................'ss so so 8004003
`51) Int. Cl. ....................... B29C 17/02; B29C 17/14
`52 U.S. C. .................................... 264/159; 264/527;
`264/532; 264/291; 264/296; 425/393
`58 Field of Search ............... 264/159, 527, 532, 291,
`264/296; 425/392,393
`References Cited
`U.S. PATENT DOCUMENTS
`2,338,210 1/1944 Snyder ................................ 264/159
`3,205,289 9/1965 Carpenter .....
`... 264/291 X
`3,757,718 9/1973 Johnson ...
`... 264/296 X
`4,198,370 4/1980 Suzuki ................................. 264/291
`4,212,204 7/1980 St. Amand ...................... 264/291 X
`
`(56)
`
`Primary Examiner-Jan H. Silbaugh
`Attorney, Agent, or Firm-Hane, Roberts, Spiecens &
`Cohen
`ABSTRACT
`57
`An object is produced from a tubular blank (10,15,17) of
`thermoplastic material of polyester or polyamide, pref
`erably of polyethylene terephthalate by reducing the
`thickness of amorphous material in one or several areas
`of the blank by means of one or several consecutive
`re-shaping operations. A mechanical shaping device
`(23,29,71,81) moves a transitional zone (13,14,113) situ
`ated between thicker and thinner material along the
`blank and simultaneously elongates the blank in the
`moving direction of the transitional zone while reduc
`ing the thickness. After the last re-shaping operation the
`thinner material preferably has an oriented state. Dur
`ing the re-shaping operation the temperature of the
`material in the transitional zone (13,14,113) is controlled
`at a level which, immediately before the re-shaping
`operation, is within or close to the range of the glass
`transition temperature (TG). In one embodiment the
`reduction of the wall thickness of the material is begun
`in a track (12) in the wall of the blank and continues
`symmetrically around the track. Clamping devices
`(41,51,61) are used to maintain the axial position of the
`blank. The blank is suitable for re-shaping, for instance,
`into a container in which the mouth also consists of
`oriented material.
`
`20 Claims, 15 Drawing Figures
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`U.S. Patent—Sep. 20, 1983 Sheet 1 of 11 4,405,546
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`U.S. Patent
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`Sheet 2 of 11
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`4,405,546
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`U.S. Patent
`U.S. Patent
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`Sep. 20, 1983
`Sep. 20, 1983
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`Sheet 3 of 11
`Sheet 3 of 11
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`4,405,546
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`U.S. Patent
`
`Sep. 20, 1983
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`Sheet 4 of 11
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`4,405,546
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`U.S. Patent
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`Sep. 20, 1983
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`Sheet 5 of 11
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`U.S. Patent Sep. 20, 1983
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`U.S. Patent
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`Sep. 20, 1983
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`Sheet 8 of 11
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`4,405,546
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`4,405,546
`U.S. Patent Sep. 20, 1983
`Sheet 9 of 11
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`U.S. Patent Sep. 20, 1983
`4,405,546
`U.S. Patent—sep. 20, 1983
`4,405,546
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`U.S. Patent
`U.S. Patent
`
`Sep. 20, 1983
`Sep. 20, 1983
`
`Sheet 11 of 11.
`Sheet 11 of 11
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`4,405,546
`4,405,546
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`1
`
`METHOD FOR PRODUCING ATUBULAR
`OBJECT OF POLYETHYLENE TEREPHTHALATE
`OR SIMLARTHERMOPLASTIC MATERAL,
`FROM ATUBULAR BLANK OF AMORPHOUS 5
`MATERAL
`
`O
`
`15
`
`4,405,546
`2
`non-stretched material. This condition will arise, for
`instance, if a central section of a tubular blank is
`stretched while the-material in the blank is at an initial
`temperature below the glass transition temperature
`(TG) and if the blank is then separated into two parts so
`as to form two separate objects. The difference is due to
`the fact that one of the objects contains the start zone
`for the yielding of material produced by the stretching
`operation, while the other object contains the stop zone
`for the yielding of material.
`When practising the invention in accordance with
`our U.S. application Ser. No. 154,888 an annular transi
`tional zone between the thinner material which has been
`stretched and the thicker material which has not been
`stretched will normally be produced. In the zone the
`surface of the material will form an angle of approxi
`mately 45 with the surfaces of the stretched and non
`stretched material respectively. Axially displaced taper
`ing areas will occasionally be produced in the essen
`tially annular transitional zone during the drawing oper
`ation, the presence of said areas usually requiring the
`object to be scrapped.
`SUMMARY OF THE INVENTION
`The present invention contemplates a method for
`producing tubular objects with areas of the wall in
`which, in the case of polyethylene terephthalate, the
`original thickness has been reduced to approximately
`of its initial value, in which areas the material is oriented
`mainly along the axis of the object only, in which areas
`crystallinity is less than approximately 30% and is pref.
`erably within the range of 10-25%, and in which areas
`the crystallinity produced in the material by the afore
`mentioned orientation will achieve a maximum value of
`17%. The material in these areas will also have been
`oriented mainly along the axis of the object.
`The speed at which the thickness of the material is
`reduced is greater in accordance with the present inven
`tion than by previously disclosed methods. Further
`more, the transitional zone between material with the
`original wall thickness and material with reduced wall
`thickness will always have a predetermined shape, at
`the same time as the length of the sections with reduced
`wall thickness will always be precisely defined due to
`the fact that the re-shaping of the blank in the transi
`tional zone itself is mechanically controlled. For exam
`ple, when starting from a tubular blank in order to make
`an object to be used for two preforms which face each
`other, the rate of production will additionally be dou
`bled by starting the drawing operation at a central point
`and by then continuing it simultaneously towards the
`two ends of the tube. The object which has thus been
`produced may be cut and the cut sections sealed to form
`two preforms for future processing, in which the transi
`tional zone between material with the original wall
`thickness and material with reduced wall thickness will
`always have a predetermined shape and will always
`contain material with characteristics which are idential
`from one preform to another.
`;
`: In accordance with the present invention, it is also
`possible to produce an object in which the thickness of
`the material is reduced along the entire length of the
`object or in one or more cylindrical sections of the
`object. The object is in the form of a tube which is open
`at both ends or, in certain applications, in the form of a
`tube which has been sealed at one end. The object is
`mainly intended to be moulded into containers,
`
`FIELD OF THE INVENTION
`The present invention relates to a method for produc
`ing a tubular object of polyethylene terephthalate or a
`similar thermoplastic material from tubular blank of
`amorphous material. The object has one or more cylin
`drical areas of material in which the thickness of the
`material in the blank has, in the case of polyethylene
`terephthalate, been reduced by one or more shaping
`operations until it is approximately. of the original
`thickness of the material. In certain applications, the
`thickness of the material is reduced along the entire
`length of the object. In the areas with reduced wall
`thickness the material is axially oriented and has, in the
`case of polyethylene terephthalate, a crystallinity of less
`than approximately 30%, usually of the order of
`10-25%. The initial temperature of the material during
`shaping is preferably below the glass transition tempera
`ture (TG) and the shaping operation takes place with 25
`the temperature of the material controlled in the area of
`shaping, where the material is in surface contact with an
`external draw ring and/or an internal shaping device.
`PRIOR ART
`Previously disclosed in our U.S. application Ser. No.
`154,888, is an object in which the wall thickness prefera
`bly at a central point of said object has been reduced to
`approximately of the original thickness of the mate
`rial. The reduction in thickness is produced by clamping
`35
`each end of a tubular blank in which the material is at an
`initial temperature.below the glass transition tempera
`ture (TG) in two jaws which are then moved apart. By
`causing an annular area of the material to have a higher
`temperature than the surrounding material, the obtained
`40
`material properties of the area will cause the reduction
`in thickness to commence in the area during the stretch
`ing operation. In certain applications, the internal diam
`eter of the blank is stabilized by the use of an internal
`mandrel. The process as disclosed will produce an ob
`45
`ject in which the material is axially oriented and has a
`crystallinity of less than approximately 30%, usually
`being of the order of 10-25%.
`Already disclosed in our U.S. application Ser. No.
`154,887 is a method for causing the thickness of the 50
`material in a tubular blank to be reduced mechanically
`by the use of an external device. The device consists of
`one or more rollers which make contact with the exter
`nal and/or internal surface of the blank with such force
`that the desired reduction in thickness will occur as the 55
`initial temperature of the material approaches the glass
`transition temperature. The external device is moved
`around the periphery of the blank and along the axis of
`the blank simultaneously. This method, will also pro
`duce an object in which the crystallinity in the areas of 60
`reduced material thickness will be less than approxi
`mately 30% and will be of the order of 10-25%. How
`ever, the material will not have been oriented axially to
`such a high degree as when the process described in the
`previous paragraph is used.
`In accordance with the inventions described above,
`certain applications will produce objects which differ in
`respect of the transitional zone between stretched and
`
`20.
`
`30
`
`65
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`4,405,546
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`3
`whereby each single object is formed into either a single
`operation reduces the wall thickness in the grooves to
`about of its original thickness at the same time that the
`container or a number of containers. In the latter case,
`the object is divided into a number of parts which are
`blank is elongated axially. Further reduction of the
`thickness of the wall of the blank is produced by posi
`then moulded into containers.
`The production of an object of polyethylene tere- 5
`tioning the draw ring in the aforementioned groove or
`grooves and moving it axially along the blank. Certain
`phthalate in accordance with the present invention
`starts with a tubular blank or a similar thermoplastic
`applications use two rings, whereby the reduction in
`wall thickness takes place simultaneously from a spe
`material. The material in this blank will be in an amor
`phous state. In one single operation or a number of
`cific groove outwards towards the two ends of the
`consecutive operations the thickness of the material in 10
`blank.
`the blank is reduced to approximately of its original
`In the case of a blank which is sealed at one end and
`thickness. This reduction in thickness takes place either
`in which the wall thickness is to be reduced adjacent to
`the seal, the draw ring should preferably be moved from
`along the entire length of the blank or in one or more
`sections of the blank. Use is made of a draw ring in
`the seal towards the other end of the blank. In certain
`certain applications, in which the relationship between 15
`applications the reduction in thickness is allowed to
`continue along the entire length of the blank.
`the internal circumference of the draw ring and the
`In those applications in which the blank is in the form
`external circumference of the blank is such that the
`of a tube open at both ends, this may be sealed in certain
`thickness of the material will be reduced as the draw
`ring is moved axially along the blank. The temperature
`cases after the reduction in thickness by heating the
`material at one end of the object and then by press
`of the material immediately before the reduction in 20
`moulding the material in a mould which may be cup
`thickness takes place must be in the range of or lower
`than the glass transition temperature which will be
`shaped, for instance. German Patent DE PS No. 1 704
`119 contains examples of methods suitable for achieving
`abbreviated as TG hereafter, and should deviate from
`TG preferably by no more than 15 C. Although the
`sealing of this kind.
`technical effect of the present invention may be 25
`The reduction of the thickness of the material in sev
`achieved at a much lower temperature, it is advanta
`eral stages is used mainly in the event that the material
`is so thick that problems arise in transferring sufficient
`geous to use an initial temmperature close to TG, for
`heat away from the transitional zone. By reducing the
`instance a temperature which is between 1 to 3° C.
`below TG, since material at this initial temperature will
`thickness of the material a number of times before the
`final reduction in thickness is made, a thinner material
`permit the draw ring to be moved at high speed. In 30
`certain typical applications, the draw ring operates in
`will be produced which will facilitate the transfer of
`conjunction with an internal shaping device located
`heat away from the transitional zone to the adjacent
`draw ring and/or internal shaping device.
`inside the blank, whereby the external dimension of the
`Two draw rings are included in one device for pro
`shaping device fits the internal surface of the blank. In
`other applications only the internal shaping device is 35
`ducing an object in accordance with the invention. A
`mandrel around which a tubular blank is placed during
`used. The thickness of the material in the blank is re
`the drawing operation is positioned axially inside the
`duced by contact with the draw ring and/or the internal
`shaping device as said draw ring and/or shaping device
`two draw rings. Separate clamping devices are also
`is moved axially along the blank. During the re-shaping
`positioned near the end sections of the blank. Each
`operation a transitional zone is formed between material 40
`draw ring consists of two draw ring halves which are
`moved by the driving device between an operating
`with the original thickness and material with reduced
`thickness, said transitional zone gradually moving axi
`position in which the halves of the draw ring are in
`contact with each other and an open position in which
`ally along the blank. The material in the transitional
`the halves of the draw ring are separated from each
`zone is kept at a temperature close to TG during the
`other. The open position is used when inserting the
`re-shaping operation by the transfer of heat to the draw 45
`tubular blank or removing the made object from the
`ring and/or the shaping device located inside the tubu
`lar blank. In certain applications the material in the
`device.
`Driving devices are also positioned in such a way as
`transitional Zone is, however, allowed to assume a tem
`to move the draw rings and clamping devices axially
`perature which exceeds TG by no more than 30°C., and
`along the blank. At least while the draw rings are mov
`50
`preferably by no more than 15 C.
`In certain applications the material in the area adja
`ing towards the ends of the tubular blank, the move
`cent to the transitional zone is cooled to a temperature
`ment of the draw rings is connected to the movement of
`the clamping devices in such a way that the draw ring
`below TG immediately after it has been reduced in
`and clamping device which are adjacent to each other
`thickness.
`In accordance with the present invention, the possi- 55
`are moved in the same direction and in such a way that
`bility is offered of producing an object with areas with
`the correlation between the speed of the respective
`draw ring and clamping device is determined by the
`mainly monoaxial orientation and in which the material
`reduction in thickness which it is wished to produce in
`has been reduced in thickness and whose external cir
`the material in the tubular blank. For example, when
`cumference has been reduced and/or the internal cir
`reducing the wall thickness of the blank to of its origi
`cumference has been increased compared to the circum- 60
`nal thickness, the ratio between the speeds of the clamp
`ference of the corresponding sections of material in the
`ing device and the draw ring shall be 3, with a ratio of
`blank.
`; when reducing the thickness of the material to of its
`In the case of sections with reduced wall thickness to
`original thickness, etc.
`be produced in areas situated between the ends of the
`blank, the reduction in thickness is begun by forming 65
`The clamping devices are fitted with axially movable
`spring components against which the edges of the tubu
`one or more peripheral grooves in the wall of the blank
`lar blank make contact. Any longitudinal tolerances
`at the same time as external devices are used to subject
`the blank to tensile stress along its axis. This stretching
`which are present in the blank both before and during
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`the drawing operation are absorbed by the spring com
`only. This embodiment preferably makes use of an inter
`ponents.
`nal mandrel as a dolly which will take up the axial
`In one embodiment of the invention, each draw ring
`forces which arise as the wall thickness is reduced. The
`usually consists of three ring sections with a certain
`movement of the draw ring is interrupted in certain
`degree of thermal insulation between the sections. Each
`applications before it passes over the edge of the tubular
`ring section is provided with channels for a fluid, said
`blank. The edge will thus be bordered by a rim in which
`fluid either heating or cooling the ring section. The
`the wall thickness has not been reduced after the draw
`aforementioned mandrel is also provided with liquid
`ing operation is complete. The object which is pro
`channels. The central ring section includes that part of
`duced in this way is suitable for use as a preform, possi
`the draw ring against which the transitional zone be
`bly after a certain amount of re-shaping of the rim, i.e.
`10
`tween material with its original thickness and material
`which will become the edge of the mouth, for a con
`with reduced wall thickness is formed during the draw
`tainer intended to be closed by means of a "crown
`ing of the tubular blank. Of the neighboring ring sec
`cork', for instance. Before the object is moulded into a
`tions, the one with the largest internal diameter is in
`container, the area which will become the mouth is
`contact with the material with its original thickness
`stabilized by heating and by thermal crystallization,
`during the drawing operation, while the other ring
`with crystallization usually being allowed to continue
`section with the smallest internal diameter is in contact
`until the material in the aforementioned rim becomes
`with the material with reduced thickness.
`opaque.
`When drawing a tubular blank, said blank is posi
`In a preferred embodiment of the present invention
`tioned on the mandrel in such a way that it is held se
`the blank is provided with a starting track or groove of
`20
`curely by the clamping devices. A central peripheral
`such axial length that space is provided in the track for
`groove in the base of which the wall thickness has been
`a clamping device together with the draw ring or draw
`reduced to approximately of its original thickness is
`rings. At least during the initial stage of the movement
`formed in the tubular blank in the manner described
`of the draw ring or draw rings, the clamping device
`above. The draw ring halves are then moved to their
`holds sections of the wall of the material in the base of
`25
`operating position. The shape and the axial length of
`the track pressed against the mandrel. In this way the
`those parts of the draw rings which are introduced into
`position of the blank relative to the mandrel is fixed. In
`the groove match the shape and the axial length of the
`the embodiment of the present invention in which two
`groove. The material of the blank in the area which is to
`draw rings are moved away from each other, the clamp
`be drawn should preferably be heated to a temperature
`ing device is designed to hold the wall of the blank in an
`30
`close to but below TG before the blank is positioned on
`area situated between the draw rings. By holding the
`the mandrel. The material will be raised to the correct
`blank securely against the mandrel, this will prevent
`drawing temperature through the contact surfaces
`any axial movement of the blank in relation to the man
`formed between the material of the blank and the draw
`drel which might arise depending on the axial forces
`rings and mandrel.
`which occur at the contact surfaces between the respec
`35
`During the actual drawing operation, the draw rings
`tive draw ring and the material in the transitional zone
`and the clamping devices are moved away from the
`between the wall of the blank with reduced thickness
`groove by the driving devices, while the aforemen
`and the wall of the blank with non-reduced thickness.
`tioned relative speed between the draw rings and the
`Any movement could, for instance, lead to the disad
`clamping devices is maintained. Thus the thickness of
`vantage that in the variant of the invention in which an
`40
`the material in the blank will continue to be reduced by
`area surrounding the track has the thickness of its wall
`the draw rings for as long as movement continues. At
`reduced, the area with reduced wall thickness may be
`the same time the blank is lengthened along its axis.
`positioned unsymmetrically in relation to the starting
`An essential factor relating to the device is the con
`track.
`trol of the temperature of the material in the transitional
`The use of a clamping device in accordance with the
`45
`zone between amorphous material with the original
`previous paragraph offers the possibility of producing
`wall thickness and material which has been reduced in
`any desired axial length in the area in which the wall
`thickness. The draw ring has an internal profile which
`thickness of the material has been reduced. In certain
`matches the change in the thickness of the material
`applications this involves producing such a reduction in
`which occurs in the transitional zone. The profile has
`the thickness of the wall of the material along the entire
`been selected in such a way that it will form contact
`length of the blank, whereas in other applications the
`surfaces with the internal surface of the draw ring not
`reduction in thickness is interrupted before the draw
`only in the transitional zone, but also ahead of and after
`ring or draw rings reach and pass beyond the end or
`it during the drawing operation. The draw ring is able,
`ends of the blank. By leaving an area of amorphous
`in this way, to control the shape of the transitional
`material at the very end or ends of the blank, a section
`surface in the transitional zone. The temperature of the
`of material will be produced which is highly suitable,
`material in the blank is regulated during the entire
`for example, for sealing to produce the base of a pre
`drawing operation by the transfer of heat between the
`form by the use of a method which is described in Ger
`material in the blank and the draw rings and/or the
`man Patent DE PS No. 1 704 119 or, after having been
`mandrel. It is particularly important that the section of
`subjected to thermal crystallization, for forming a rim
`60
`the draw ring which is in contact with the material in
`for use with the "crown cork” type of closure.
`the transitional zone should maintain the material in the
`In an alternative embodiment, the draw ring is used
`blank at a temperature close to TG.
`to reduce the wall thickness of the material in a tubular
`A simplified embodiment of the present invention
`blank with a sealed bottom, the mouth of which is al
`makes use of a single draw ring which is moved from a
`ready provided with a means of closure, for example
`65
`starting groove preferably all the way to the edge at one
`screw threads. The blank in which the material is in
`of the ends of the blank. In this way an object is pro
`tended to be subjected to a reduction in thickness is
`duced in which the wall thickness is reduced at one end
`produced by a conventional method, for example by
`
`50
`
`15
`
`55
`
`EX-1009 PGR2024-00035
`
`

`

`4,405,546
`7
`injection moulding or by extrusion followed by sealing
`and moulding the bottom and mouth. In certain applica
`tions, the starting track is formed in the manner de
`scribed above, while in other applications a starting
`point or the starting track in whole or partly is pro
`duced during the injection moulding of the preform.
`When using an external draw ring to reduce the
`thickness of the material in the wall of the blank, a
`certain reduction in the internal diameter of the blank
`will also be produced, as has already been mentioned.
`The mandrel inside the blank thus acts as a shaping
`device which will determine the extent of the reduction
`in diameter. Surprisingly, it has been found that, for
`material within the range of temperatures indicated
`above during the re-shaping operation, the contact be
`15
`tween the wall of the material and the mandrel and
`produced by the contraction, generates relatively little
`contact pressure between the internal surface of the
`wall of the blank and the external surface of the man
`drel, for which reason no problems are encountered in
`20
`removing the finished blank from the mandrel after the
`shaping (drawing) of the blank is complete.
`No internal mandrel is called for in certain types of
`application for the device, in which the object is al
`lowed to assume an internal circumference which is
`25
`smaller than its original circumference. By selecting an
`internal mandrel with an external circumference which
`is smaller than the internal circumference of the blank,
`it is possible in other typical applications to control the
`reduction in the internal circumference of the blank
`during the drawing operation to a value which is suit
`able for the envisaged specific application.
`BRIEF DESCRIPTION OF THE FIGURES OF
`THE DRAWING
`The present invention is described in greater detail
`with reference to a number of FIGURES, in which
`FIG. 1 shows a perspective view of a drawing device;
`FIG. 2 shows a longitudinal section through the
`drawing device in FIG. 1 in which the draw rings of the
`drawing device are in the initial position before draw
`1ng;
`FIG. 3 shows part of a longitudinal section corre
`sponding to FIG. 2, in which the draw rings have been
`separated from each other;
`45
`FIG. 4 shows a support plate fitted with a draw ring
`in its operating position;
`FIG. 5 shows a longitudinal section through a tubular
`blank with a draw ring in its initial position;
`FIG. 6 shows a longitudinal section through a tubular
`blank during drawing in the direction away from its
`sealed part;
`FIG. 7 shows a section through a tubular blank with
`a draw ring in the initial position prior to drawing the
`blank in a short section at one end;
`FIG. 8 shows a section of the tubular blank in FIG. 7
`with the draw ring in its final position;
`FIG. 9 shows a longitudinal section through a draw
`ing device with a central clamping device and with the
`draw rings or the clamping device in the initial position
`before drawing:
`FIG. 10 shows part of a longitudinal section corre
`sponding to FIG. 9 with the draw rings moved apart;
`FIG. 11 shows the central area in accordance with
`FIG. 9 in detail;
`FIG. 12 shows the central area in accordance with
`FIG. 10 in detail; and
`FIGS. 13, 14, and 15 show alternative embodiments.
`
`8
`DETAILED DESCRIPTION
`FIG. 1, which shows an overall perspective view of
`a drawing device in accordance with the present inven
`tion, includes a base 30 from which a number of guide
`pins 31a–31d project vertically. The rest of the descrip
`tion which follows will relate to the device in this posi
`tion, although the concept of the invention is in no way
`restricted to a special position of this kind. The designa
`tions “upper' or "lower' or other similar designations
`are occasionally used below in connection with a vari
`ety of components, said designations being used only for
`the purpose of clarification.
`The base is fitted with driving devices with gearing
`(not shown in the Figure) for a number of drive screws
`32a-32d which lie parallel with the guide pins. Four
`support plates 50,60,70,80 are positioned at right angles
`to the guide pins and the drive screws. Each support
`plate is fitted with bearings which fit the guide pins
`31a-31d and threaded holes 52a-52d, 62a-62d, 72a-72d,
`82a-82d operate in conjunction with a number of the
`drive screws 32a-32d. The plates are also provided with
`openings 58a-d, 68a–d, 78a–d, 88a-d for those drive
`screws which are not in threaded contact with the ac
`tual support plate. The top and bottom support plates 50
`and 60 are arranged to operate in conjunction with the
`two drive screws 32a and c, whereas the two interjacent
`support plates 70 and 80 are arranged to operate in
`conjunction with the two remaining drive screws 32b
`and d. Furthermore, the upper parts of the drive screws
`which operate in conjunction with the support plates 50
`and 70 are threaded in opposite directions to the lower
`parts of the drive screws which operate in conjunction
`with the support plates 60 and 80. This means that as the
`drive screws are rotated the two upper support plates
`will move in the same direction, although this will be
`opposite the direction in which the two lower support
`plates move.
`Since the top and bottom support plates are driven by
`the sc