FR2550990A1 - METHOD FOR MANUFACTURING A HOLLOW BODY BY EXTRUSION AND BLOWING OF PLASTIC MATERIAL AND INSTALLATION FOR CARRYING OUT SAID METHOD - Google Patents

Abstract

PROCESS FOR MANUFACTURING A HOLLOW BODY BY EXTRUSION AND BLOWING OF PLASTIC MATERIAL, IN WHICH PLASTIC MATERIAL IS SUPPLIED IN AN EXTRUDER AND AN EXTRUSION DIE 3, AT THE OUTPUT OF WHICH A PARAISON 28 SHAPES THE SHAPE OF THE HOLLOW BODY AND IN WHICH AIR IS BLOWN, THIS PARAISON BEING THEN INTRODUCED IN A MOLD 6 PRESENTING AN IMPRESSION 7 OF THE BODY TO BE MOLDED; THE MOLD 6 BEING OPEN, AIR BLOWS INTO PARAISON 28 UNTIL ITS LENGTH IS EQUAL TO THE LENGTH OF THE DESIRED HOLLOW BODY, THEN WE CUT THE AIR BLOWING TO OBTAIN A DEFLATED PARAISON 31, WE CLOSE THE MOLD 6 AND AIR IS BLOWN AT HIGH PRESSURE INTO THE PARAISON 31 TO OBTAIN THE MOLD BODY 9A. THIS PROCESS ALLOWS A UNIFORM THICKNESS OF THE MATERIAL TO BE OBTAINED IN THE VARIOUS TRANSVERSAL SLICES OF THE CONTAINER.

Description

The present invention relates to a method of manufacturing a body

  hollow by extrusion and blowing of plastic material, in particular of containers of complex shape, as well as an installation for the implementation of this process. As we know, the manufacture of bodies

  any form of hollow plastic can be produced by extrusion blow molding machines.

  One of the most delicate phases of this manufacturing process is the suitable distribution of the material, for which the known blow-molding installations are equipped with automatic programming devices of thickness 15 and length of the parisons (it that is to say bubbles of blown plastic, not yet shaped in a mold), and of punches or ovalized dies, this ovalization being possibly

controlled by a programmer.

  Thus, the known installations comprise one or more plastic extrusion dies connected to plasticized material supply means, such as an Archimedes screw ensuring the plasticization of the plastic material in a sheath and its routing to an extrusion head, means for blowing air into the parison obtained at the outlet of the die, means for cutting the parison and welding the base of the parison, a mold intended to receive the parison and having an internal cavity corresponding to the desired shape for the hollow body to be molded, at least one air blowing rod inside the mold and a device for moving the mold from the extrusion die to the rod blowing and vice versa for the next parison, as well as

  means of evacuating finished hollow bodies and waste.

  The thickness of the parison at the outlet of the extrusion die 5 is programmed in the vertical direction so as to obtain a good distribution of the material in the direction of flow of the parison, that is to say say between the bottom and the top of the hollow body Current techniques also consist in blowing air at relatively low pressure, for example 0.3 bar, into the parison previously welded to the base, so as to obtain a kind of bubble of plastic material of length much less than that of the hollow body targeted, then closing the two parts of the mold on this parison, and blowing air at high pressure (of the order of 6 or 7 bars), inside the parison trapped in the mold, in order to inflate it and apply it

  evenly across the mold footprint.

  However, it can be seen that the distribution of the material in the horizontal direction, by considering successive horizontal sections of the bottle finally obtained, is not uniform. In fact, the measurements carried out have made it possible to highlight areas of weakness and extra thickness zones: for example for a 5 liter bottle intended to receive

  cleaning products or detergent, the weak areas are those where the thickness is less than 0.70 mm, and are located at the bottom of the bottle, on its curved front part, and on the shoulder of the neck 30 as well than on the handle.

  The areas of excess thickness, that is to say of thickness greater than 1.20 mm are located at the welds at the bottom of the bottle and at the base of the blanks. These variations in thickness in successive horizontal sections of the bottle have the following disadvantages: during the operations of filling the containers, then of storage and handling, the containers sink more or less

  each other by their zones of weakness.

  In addition, the existence of these zones entails risks of local rupture, therefore of subsequent leaks, during handling operations. Finally, there are difficulties in properly affixing the label of the bottle on the face provided for this purpose, due of

lack of flatness thereof.

  This harmful irregularity in the distribution of the material inside the same horizontal section of the container can be explained by the fact that at the start of the blowing of the parison inside the mold, the contact between the plastic hot 20 and the cooled mold walls (at 20 OC-approximately), do not

  is not done simultaneously at all points of the mold.

  However, the contact between the hot plastic material and the wall of the mold cavity immediately causes the plastic material to solidify, especially on the external wall of the parison, which therefore becomes less malleable. parison blowing process makes it possible to establish that the plastic material does not come into contact with the bottom, the front convex, the handle and the neck 30 after the start of blowing The material already cooled, must therefore be very stretched to reach the distant points, which explains their thinness On the other hand, when the mold is closed, it is the blanks which come into contact with the mold most quickly The plastic material is therefore frozen there and then moves more difficult,

  which leads to the observed excess thicknesses.

  To solve this problem in order to ensure a regular distribution of the material in the same horizontal section of the bottle during blowing, various methods were first tried, without making it possible to solve the problem properly. Finally,

  this has been resolved as follows.

  The method according to the invention is characterized in that, during the formation of the parison whose base has been welded by two jaws, air is blown into the parison through the extrusion die to strongly inflate the parison outside the mold until its length is

  substantially equal to the desired final length of the hollow body, then the mold is closed on the parison, and air is blown into the latter at a pressure sufficient to press its wall on the imprint.

  It can be seen that in each horizontal section considered of the bottles obtained by this process, the excess thicknesses and zones of weakness have disappeared, thanks to the elimination of the premature contact of the plastic with the large faces.

mold imprint.

  According to a particular feature of the process targeted by the invention, air is blown into the parison outside the mold until the cross section of the parison is equal to that of the mold cavity, in order to distribute the material uniformly in each horizontal or transverse section of the parison, then deflate it by cutting off the air blowing and putting it in the atmosphere or in depression, in order to give it a flaccid appearance, and we close the mold on the deflated parison, the walls of which are thus kept out of any contact with the cold walls of the cavity, until the resumption of air blowing in the parison trapped at

inside the mold.

  Thus, the parison very stretched by the prior inflation, regains a flaccid appearance after deflation, which allows it to be housed inside the mold without premature contact of said

  parison with the cooled walls of the imprint.

  It is therefore understood that the subsequent inflation at high

  Parison pressure ensures simultaneous and uniform application of this on the impression.

  Furthermore, the prior inflation of the parison out of the mold constitutes an essential characteristic of the process according to the invention, since it is this inflation outside the mold, at suitable pressure, which makes it possible to obtain a uniform distribution of the material. in the transverse direction inside a determined horizontal slice Of course, the thickness of the successive horizontal slices can

be properly scheduled.

  Other features and advantages of

  the invention will appear in the course of the description which follows, given with reference to the appended drawings, which illustrate an embodiment thereof by way of

  d: nonlimiting example: FIG. 1 is a simplified front elevation view of an installation for blowing plastic material for the manufacture of hollow bodies by implementing the method according to the invention; Figure 2 is a simplified side elevational view of the installation of Figure 1; Figure 3 is a set of diagrams illustrating the successive steps of the method of manufacturing a hollow body according to the invention, by means of the installation of Figures 1 and 2;

  FIG. 4 is a diagram of an embodiment of the device for controlling the air blowing inside the parison at the outlet of the extrusion head, the parison repeating outside the mold.

  The installation shown in Figures 1 and 10 2 is intended for the manufacture of a hollow body by

  extrusion and blowing of plastic material, in particular of containers of complex shape such as polyethylene bottles of various capacity, intended to receive, for example, cleaning or detergent products.

  In a manner known per se, this installation comprises an extruder 1 provided with a horizontal screw for plasticizing the material which is introduced into the screw 1 by a hopper 2 two extrusion dies 3 arranged vertically and

  supplied with plastic material by the extruder 1.

  means 4 for blowing air into the parisons 5 obtained at the outlet of the dies 3, means 15 for cutting successive parisons, a main mold 6 made up of two half-molds 6 a in which two half-cavities are formed 7 corresponding to the desired shape for the hollow body to be molded, two air blowing rods 8 inside the mold 6, a device (not shown) for moving the main mold 6 from the dies 3 to the rods blowing 8 below these and vice versa as well as means of evacuation (not shown) of the finished hollow bodies 9 and of the waste The latter are received by a conveyor 11 to be recycled in the extruder 1,

in a known way.

  The assembly of the above members rests on a fixed frame 12 The installation also includes a mold 13 for taking up the containers 9 a formed in 5 the mold 6, a jack 14 associated with each extrusion head to move the punch arranged in a manner known per se inside thereof, in order to adjust the thickness of the wall of the parison 5, and two jacks 16, 17 associated with the blowing rods 8, the 10 jack 16 being pneumatic and the jack 17 being hydraulic The installation is provided with a motor 18 associated with a reduction gear 19 for driving in rotation the screw of the extruder 1, as well as electronic programming means and jacks (not shown) capable of moving both

  half-molds 6 a, on the one hand in order to move them apart or to bring them closer to one another to close the mold, and on the other hand to make the mold 6 closed on the parisons move between the heads 20 of extrusion 3 and the blowing rods 8.

  According to the invention, the installation comprises (FIG. 4) two solenoid valves 21, 22 each associated with the air blowing means in the heads or dies 3, and controlled automatically by a solenoid valve 23 which is itself electrically controlled by programmable timed relays The solenoid valves 21, 22 each control, according to their position, the passage of inflation air from the corresponding parison 5 through the punch 24, 25 of the associated extrusion head 3, and this by pipes axial 26, 27 schematically represented in the

Figure 4.

  Given that the process according to the invention, the steps of which are illustrated in the

  Figure 3, is implemented by means of the air blowing control device in the parisons by the extrusion heads 3 by means of the system of solenoid valves 21, 22, 23, we will first describe the operation of this control device.

  In the position of the solenoid valves 2 t to 23 shown in solid lines, the electric control Y of the solenoid valve 23 (composed of two time relays not shown) making it possible to program the triggering and stopping of the venting) , is at rest, the return member X therefore maintains the solenoid valve 23 in the position of non-passage of the pressure pl (indicated by the arrow in solid lines) The pressure p 2 on the push-buttons 15 (not shown) of the solenoid valves 21 and 22 maintains these in the position of direct passage of the inflation air pressure p in the parisons by the axial pipes 26, 27 and the punches

24, 25.

  In the second position of the solenoid valves 21 to 23, illustrated by the arrows in phantom, the parisons are vented as follows: the electric control Y is excited, so that the air at the pressure pl passes through l solenoid valve 23 and actuates the solenoid valves 21, 22 which

  then pass to the deflation position of the gobs.

  The arrows p indicate that the supply of lines 26, 27 with inflation air is interrupted. While new gobs must

  be inflated, the automatic programming of the device by the aforementioned relays triggers the rest of the Y control, and the above cycle begins again.

  We will now describe with reference to Figure 3 the process more particularly targeted by the invention. The two half-molds 6 a are spaced from each other, and plastic material leaves the extrusion head 3 with air blowing into the parison 28, automatically controlled by the solenoid valve 21 (or 22) associated with the head 3 and which is itself triggered by the solenoid valve 23 as described above. 10 The air is blown into the parison 28 which inflates gradually, at a pressure of around 0.6 bar for example, the start of this step being

illustrated by diagram a.

  The blowing is continued at this pressure 15 until the parison 29 is practically the length of the finished container 9 and a cross section equal to that of the cavities 7 when the mold 6 is closed This step is indicated by diagram b, and the air supply is interrupted by the solenoid valve 23 when the parison has taken the form

referenced 29 in step b.

  During this step, the plastic is therefore distributed uniformly in each horizontal section of the parison 29, the thickness of which is thus constant in a given section. Then cut the air supply (step

  c) to deflate the parison which assumes the flaccid appearance referenced 31, consequently putting it in the atmosphere or in depression As indicated previously, the interruption of the air blowing is obtained by excitation of the electric control Y of the solenoid valve 23.

  We close the mold 6 on the disgusting parison.

  folded 31 (step d) whose walls are thereby kept out of contact with the cold walls of the cavity 7 The mold 6 is then moved (which can mold two containers 9a, in Example 5 illustrated in Figure 1) to bring it below the air blowing rods 8 Air is blown through it at high pressure, for example of the order of 7 bars, as indicated by the arrows on the diagram d, so that the walls of the parison Sa 10 trapped inside the mold 6 are applied

  simultaneously at all their points on the imprint 6.

  The air trapped between the parison 31 and the imprint 7 is distributed and forms a screen which is eliminated at the time of blowing, by passages (not 15 shown) provided for this purpose as known per se.

between the two half-molds 6 a.

  During blowing, the material is therefore quickly and uniformly applied over the entire surface of the impression 7, including the points furthest from the axis of the parison Sa, because the expansion

  necessary has already been roughed out of the mold 6.

  After interrupting the air supply,

  open the mold 6 in order to extract the molded containers Sa therefrom, deburr them in a manner known per se, and the finished containers 9 are removed.

  The mold 6 is replaced on standby under the dies 3 and the cycle begins again for the following parisons,

  It can be seen that, as already indicated, the thickness of the wall of the containers 9 is strictly uniform in a given transverse section, and this from one end to the other of the container.

  This advantageous result is made possible by the prior inflation of the parisons outside of the t 1 mold (steps a, b) and their deflation (closure of the mold) before closing the mold thereon This prevents any premature contact between the 'imprint 7 and the parison and therefore premature local solidifications 5 of the material opposing its uniform stretching. There is also surprisingly

  that the process according to the invention makes it possible to slightly reduce the weight of the bottles, and therefore the amount of plastic material necessary for their manufacture.

  In addition, the resistance of the various bottles capable of being obtained by the invention is significantly improved compared to that of the bottles obtained by the prior method, which is all the more advantageous when the bottles have a weight.

slightly decreased.

  The invention is not limited to the embodiment described and may include variant embodiments It is notably possible to replace the solenoid valves 21, 22 for programming the air blowing in the extrusion heads 3 by any other

equivalent device.

Claims (4)

  1 Method for manufacturing a hollow body by extrusion and blowing of plastic material, in particular of containers (9) of complex shape, in which a plastic extruder (1) is supplied with plastic material comprising a screw for plasticizing the material and a extrusion die (3) equipped with a movable punch (24, 25) for adjusting the thickness of the wall of the hollow body (91, and a parison (28) is obtained at the outlet of the die (3) roughing out the shape of the hollow body (9), the parison is separated from the plastic material extruded upstream, this parison is then inflated by blowing air inside a mold (6) in which an imprint is formed ( 7) of the desired shape for the body (9) then the mold is opened   (6) to release the molded body (9 a) on the impression (7), characterized in that the mold (6) being open.   air is blown into the parison (28) through the extrusion die (3) to inflate the parison outside the mold (6) until its length   is substantially equal to the desired final length of the hollow body (9), then the mold (6) is closed on the parison (31), and air is blown into the latter at a pressure sufficient to press its wall 25 on the imprint (7).   2 Method according to claim 1, characterized in that air is blown into the parison (28) outside the mold (6) until the cross section of the parison (29) is equal 30 to that of the mold cavity (7), in order to distribute the material uniformly in each horizontal section of the parison (29), then the parison (29) is deflated by cutting off the air blowing and setting it to the atmosphere or depression   in order to give it a flaccid appearance, and the mold (6) is closed on the deflated parison (31) whose walls are thus kept out of any contact with the cold walls of the imprint (7) until the 5 recovery blowing air into the parison (31) trapped inside the mold (6).   3 Method according to claim 2, characterized in that the air is blown into the parison (26) out of the mold (6) at a pressure of the order of 0.6 to 0.7 bar, and into the parison (31) to   inside the mold at around 7 bars.   4 Installation for the implementation of   method according to one of claims 1 to 3, comprising at least one plastic extrusion die (3), connected to supply means   (1, 2) in plastic material, means for cutting and welding the base of the parison (15), means for blowing air into the parison obtained at the outlet of the die (3), a mold ( 6) intended to receive the parison and having an internal cavity (7) corresponding to the desired shape for the hollow body (9) to be molded, at least one cane (8) for blowing air inside the mold, a device for moving the mold (6) from the extrusion die 25 (3) to the blowing rod (8) and vice versa, as well as means for discharging the finished hollow bodies (9) and waste, characterized in that it comprises a first solenoid valve (21, 22) associated with the air blowing means in the extrusion head 30 (3) associated and controlled by a solenoid valve (23) for setting the parison (29 ) corresponding to the atmosphere after inflation, then to cut off the venting after a predetermined time and so   allow the next parison to be inflated.   Installation according to claim 4, characterized in that the deflation of the parison is programmable by means of time relays constituting the electric control (Y) of the second solenoid valve (23), which pneumatically controls the first solenoid valve ((21, 22).