FR2635482A1 - METHOD AND DIE FOR THE EXTRUSION OF PIPES IN THERMOPLASTIC MATERIAL - Google Patents

Abstract

The thermoplastic material is extruded through a round die comprising a diverging annular channel 4 connected directly to an annular straight conformation channel 5 having a constricted annular passage zone 10 upstream of its outlet orifice so that the speed of deformation of the thermoplastic material in the annular throttled passage zone 10 is at least equal to 0.1 sec ** - ** 1. For this purpose, a round die is used in which the ratio between the section of passage upstream of the annular throttled passage zone and the section of the annular throttled passage zone is between 1.5 and 10. The method and the die are particularly suitable for the production of pipes with large diameters greater than 250 mm.

Description

Process and die for extruding thermoplastic pipes

  The present invention relates to a method and a die for the extrusion of thermoplastic pipes which are particularly suitable for the production of pipes of large diameters that is to say pipes with external diameters of at least 250 mm. For the extrusion of such thermoplastic pipes, it is necessary to use an extruder equipped with an annular round die whose function is to open the annular stream of molten thermoplastic material delivered by the extruder to the

  dimensions required for the production of the desired pipe.

  The use, for this purpose, a round die having only a diverging annular channel bringing the annular flow of thermoplastic material to the dimensions of the desired pipe & connected directly to a straight annular channel of conformation

  of the pipe, does not allow, in general, to directly reach one.

  satisfying result. Indeed, during its passage through the divergent, the thermoplastic material delivered, which is in the visco-elastic state, becomes the seat of induced radial tensions with the consequence that the pipe flows & retracts radially at the outlet of the right channel annular conformation following the relaxation of these induced radial voltages. It follows therefore that the pipe finally produced has an external diameter that is difficult to control and is significantly smaller than the external diameter of the annular orifice of the annular right channel.

of conformation.

  In order to overcome this undesirable phenomenon, it is therefore necessary, up to now, to resort to annular round dies whose diverging annular channel brings the flow of thermoplastic material discharged by the extruder to an external diameter which is clearly greater than that of the desired pipe, this divergent channel 2 2 being followed by a convergent annular channel whose dual function is to allow sufficient relaxation of the radial tensions induced in the thermoplastic material by the diverging channel and gradually reduce the flow of this material to dimensions of the annular right channel conformation of the subsequent pipe, that is to say, in fact, to the dimensions of the desired pipe. Such a route is described and illustrated on page 331 of the book "Kunststoff-Extrudertechnik" by G.SCHENKEL,

Edition C. Verlag, Munich 1963.

  However, when using this type of die for the production of large diameters pipes, we are forced to make dies that are very bulky and very weighty.

  high. In addition, the presence of a diverging annular channel

  converging causes the need to provide fins to support the inner core of the round wire, these fins resulting in the formation of longitudinal marks on the pipe product that harm its appearance and constitute areas of

mechanical fragility.

  One can certainly reduce strongly or even eliminate the latter disadvantage by providing a tapered section passage area in the right channel of conformation as it is proposed in the French patent application 88.09125 of July 4, 1988. It remains nevertheless that such dies are expensive, cumbersome and difficult to handle given

their weight.

  The present invention therefore has as its first objective to

  to provide a process for the extrusion of thermally heated pipes

  plastic which no longer entails the disadvantages mentioned above and which allows the direct use of an annular round die having a diverging annular channel directly bringing the annular stream of thermoplastic material cut to dimensions

of the desired pipe.

  The invention therefore relates to a process for the extrusion of thermoplastic pipes through a round die -3-

  ring having a diverging annular channel connected directly

  to a straight annular canal of conformation of the pipe in

  the ratio of the internal diameter of the annular section to

  divergent outlet wall and the internal diameter of the annular inlet section of the diverging portion is at least 2 and wherein the annular straight pipe conformation channel has an annular passage zone constricted upstream of its exit orifice which characterized in that the geometry of the constricted annular passage zone is selected as a function of the extrusion rate of the thermoplastic material such that the deformation rate of the thermoplastic material in the annular passage zone is at least equal to

  0.1 sec -1, and preferably greater than 0.5 sec -1.

  It has been found that, in keeping with the

  above mentioned it is possible to extrude thermoplastic pipes of large diameters using an annular round die having a diverging annular channel leading

  directly the flow of molten thermoplastic material to the dimensions

  of the pipes to be produced and this, by very strongly reducing and even suppressing any tendency of radial contraction of

  extruded pipes coming out of the die.

  In fact, it is assumed that the rate of deformation imposed on

  the thermoplastic material as it passes through the annular zone.

  The strangulated air has the effect of inducing in it longitudinal internal tensions which seem to neutralize the effects of the radial internal tensions induced in it by its passage.

  through the diverging annular channel.

  The process according to the invention can be used for the production of pipes of any thermoplastic material but it is particularly advantageous, given the simplicity of the die used, for the implementation of

  thermoplastic materials deemed to be heat-sensitive.

  The process according to the invention can not only be used for the production of pipes made of a single thermoplastic material but also for the production of pipes.

  multilayer composites extruded from thermo

  different plastics some of which may be expandable and this, using a conventional distribution block disposed upstream of the die; The process according to the invention is particularly advantageous for the extrusion of large diameter pipes, given the fact that it allows the use of lighter dies than convergent divergent-convergent dies, It is obvious that his interest is by no means limited to the production of such pipes. It is also obvious that the process according to the invention is in no way limited to the production of circular section pipes but that it can be exploited for the production of hollow profiles having any sections determined by the geometry of the right channel.

of conformation.

  The invention also aims to provide a die which is particularly suitable for carrying out the method described above.

  The invention thus also relates to a circular round die

  having a diverging annular channel connected directly

  to an annular right channel of conformation in which.

  the ratio between the internal diameter of the annular section of

  outlet of the divergent and the internal diameter of the annular section

  The inlet flue of the diverging element is at least equal to 2 and in which the annular conformal annular channel has an annular zone of constricted passage upstream of its outlet orifice which is characterized in that the ratio between the upstream flow section of the annular zone of constricted passage and the section of the annular zone of constricted passage is between 1.5 and 10 and preferably between 2 and 8. It has in fact been

  found that by using a sector corresponding to this

  it is possible to produce very satisfactory pipes

  with a range of industrially acceptable productivities

tables.

  According to a particular embodiment of the die according to - 5--

  the invention, the annular zone of constricted passage is constituted

  killed by a linearly progressive increase in the internal diameter of the annular passage section of the annular conformation channel. According to another embodiment, the annular zone of constricted passage is constituted by a linear decrease.

  progressive outer diameter of the annular crossing section.

  the annular canal of conformation. However, it is quite obvious that the annular zone of constricted passage may be constituted both by an increase and by a decrease in the inner and outer diameters of the annular passage section of the shaping channel. In order to avoid any risk of

  stagnation of thermoplastic material, the

  and / or decrease in internal and / or external diameters

  above mentioned are made on slopes of 5 to 35 degrees.

  The length of the annular zone of constricted passage is

  not critical, it can vary between 1 and 100 mm.

  The annular zone of constricted passage may be located directly at the entrance of the annular right channel of conformation or be separated from this entry, but it is appropriate that the distance between the end of this zone and the exit orifice of the annular right channel of conformation of at least 50 mm so as to ensure the conformation, in the end part of this channel, of a pipe having the required wall thickness, notwithstanding the

  presence of the annular zone of constricted passage.

  The rate of deformation experienced by the thermo-

  plastic in the annular zone of constricted passage of the die according to the invention can be expressed by the relation: (to 1) in which: e is the strain rate Qv is the volume flow rate of thermoplastic material L is the length of the convergence zone S1 is the passage section upstream of the annular zone of constricted passage -6- S2 is the passage section in the annular zone of

Strangled passage.

  This rate of deformation can also be expressed with a good approximation by the relation: () V2_e2) \ e in which: V2 is the average speed of the thermoplastic material in the annular zone of constricted passage and is the thickness of the flow of thermoplastic material before the annular zone of constricted passage e2 is the thickness of the flow of thermoplastic material in the annular zone of constricted passage

  These relationships therefore make it possible to

  Geometric meters of the annular passage zone constricted according to the desired extrusion rate and geometric characteristics (diameter, thickness or section of the pipe to be produced).

  In FIG. 1 of the attached drawings, it has also been shown

  schematically sketch a round die according to the invention in

  which the different geometrical parameters have been explained.

cities.

  The die according to the invention is furthermore

  cited in more detail in the following description of a

  example of practical embodiment and in which reference will be made to FIG. 2 of the accompanying drawings, Fig.2 being a sectional view of a die according to the invention operated with a block of

  distribution for the production of a multilayer pipe.

  As it appears in this figure, the round die 1 according to the invention consists of an inner core 2 and an outer sleeve 3 which delimit successively between them a diverging annular channel 4 and a right channel of conformation of which the section corresponds substantially to that of the pipe to

  produce. The inner core 2 is attached directly to a prolon-

  6 of the inner core 7 of a conventional distribution block 8 -7partially represented and designed to output an annular flow and multilayer plastic constituent of the pipe to produce. Because the inner core 2 is lightened and reduced in length it is not necessary to provide support by kernel trellis. The outer sheath 3 of

  the die 1 is fixed directly to the sheath 9 of the riprap block.

8.

  According to the invention, the straight conformation channel 5 has a constricted passage zone 10 which is obtained by a local reduction of the outer diameter of the annular section of

passage of this canal.

  The results obtained by the process and thanks to the die according to the invention are furthermore demonstrated by the practical tests described below and in which the test 1R given as a reference for comparison is excluded from the scope of the present invention.

present invention.

  Test iR To carry out this test, a die was used as illustrated in FIG. 2 of the accompanying drawings but practically without a constricted passage zone in the right conformation channel, said die being connected directly to an extruder discharging an annular flow. polyvinyl chloride monolayer. The characteristics of this sector as well as the

  Flow characteristics are shown in Table 1 below.

  It can be seen that in this test the rate of deformation of the

  plastic material at the level of the conformation channel is

  less than 0.1 sec-1. The last column of the table De / Df gives the ratio between the outer diameter of the pipe obtained and the outer diameter of the right channel of conformation. It thus appears that the product pipe retracts radially so

  considerable I its exit from the die.

  Test 2 and 3 The procedure was as in Test 1R but using dies as illustrated in FIG. 2 of the accompanying drawings and this time equipped with a constricted passage zone conforming to FIG.

  the invention in the right channel of conformation.

  The geometric characteristics of the dies and the flow characteristics are also shown in Table 1 below. It is found this time that the rate of deformation of the plastic material in the constricted passage zone is greater than 0.1 sec-1 and that the pipe product-does not substantially shrink in the radial direction after its exit from the die. The w -o Test Di D2 D2 / D1 el e2 L siS2 QM QV V2 ú From / Df mm mm: mm mm mm2 mm2 kg / h mm3 / sec mm / sec 1 / sec

  1,103 308.5 2,995 9 8.5 10 8,72,103 8,24,103 315 6,25,104 7.59 0.04 0.79

  2 104.5 314 3. 10 2 10 9.86.103 1.97.103 197 3.91.104 19.81 1.6 0.92 l

  3 104.5 313 2.995 10 3 10 9.83.103 2.95.103 385 7.64.104 25.89 1.83 0.95

  I.- oC (in Table Q (A is the masaic flow and Qv is the volumetric flow, these flows being connected by the

  relationship: Qv - QM / p, where p is the density of the extruded thermoplastic material).

lIx ul t ty

- 10 -

Claims (4)

R E V E N D I C A T I 0 N S   1 - Process for the extrusion of thermoplastic pipes through an annular round die having a diverging annular channel directly connected to an annular straight duct forming channel in which the ratio between the internal diameter of the annular outlet section divergent and the internal diameter of the annular inlet section of the diverging portion is at least 2 and wherein the annular conformal annular channel has an annular passage zone constricted upstream of its outlet orifice, characterized in that chooses the geometry of the annular passage zone constricted according to the extrusion rate of the thermoplastic material such that the deformation rate of the thermoplastic material in the annular zone   choked passage is at least equal to 0.1 sec-1.   2 - die for carrying out the method according to claim 1 comprising a divergent annular channel connected directly to a straight annular channel of conformation in   the ratio of the internal diameter of the annular section to   divergent outlet wall and the internal diameter of the annular inlet section of the diverging portion is at least equal to 2 and in which the annular conformal annular channel has an annular passage zone constricted upstream of its exit orifice, characterized in that the ratio of the passage section upstream of the annular passage zone and the section of the annular passage zone is included between 1.5 and 10.   3 - Die according to claim 2 characterized in that the annular zone of throat passage section is constituted by a linearly progressive increase in the internal diameter of the annular section of the channel passage annular right of conformation.   4 - die according to claim 2 characterized in that the annular zone of constricted passage section is constituted by a linearly progressive decrease of the - he -   outer diameter of the annular passage section of the canal annular right of conformation.   - Die according to claim 2 characterized in that the distance between the end of the constricted passage zone and the outlet of the right annular channel conformation is at least 1 50 mm.