GB2145365A - Extrusion and blowing of a hollow body of plastics material - Google Patents

Abstract

Method for producing a hollow body by the extrusion and blowing of a plastics material, comprising feeding plastics material to an extruder and an extruding die 3 at the outlet of which there is obtained a parison 28 roughly producing the shape of the hollow body, and blowing air into the parison, thereafter introducing this parison into a mould 6 having a cavity 7 whose shape corresponds to the shape of the body to be moulded, wherein, with the mould open 6, air is blown into the parison 28 until its length is equal to the length of the desired hollow body, then the blowing air is cut off so as to obtain a deflated parison 31, the mould 6 is closed and air at high pressure is blown into the parison 31 so as to obtain the moulded body 9a. This method results in a uniform thickness of the plastics material in the various cross-sections of the container. <IMAGE>

Description

SPECIFICATION Method and apparatus for extrusion and blowing of a hollow body of plastics material The present invention relates to a method for producing a hollow body by extrusion and blowing of a plastics material, and in particular containers of complex shape, and apparatus for carrying out the said method.

As is known, hollow bodies of a wide variety of shapes may be produced from plastics material by means of extruding-blowing machines.

One of the most delicate stages of this production method is the appropriate distribution of the material for which known extruding-blowing units are provided with devices for automatically programming the thickness and the length of the parisons (i.e. bubbles of plastics material which have been blown but not yet put into shape in a mould), and ovalised plungers or dies, this ovalisation being possibly controlled by a programmer.

Thus, known units comprise one or more dies for extruding plastics material and connected to means for feeding plasticised material, such as an Archime dean screw, which ensures the plasticisation of the plastics material in a sleeve and its feed to an extruding head, means for blowing air in the parison obtained at the outlet end of the die, means for cutting the parison and welding the base of the parison, a mould for receiving the parison and having an interior cavity corresponding to the desired shape of the hollow body to be moulded, at least one blowing tube for blowing air in the mould, a device for shifting the mould from the extruding die to the blowing tube and viceversa for the following parison, and means for discharging finished hollow bodies and scrap.

The thickness of the parison at the outlet of the extruding die is programmed in the vertical direction so as to achieve a good distribution of the material in the direction of flow of the parison, i.e.

between the base and the upper end of the hollow body. Present techniques also involve blowing air at relatively low pressure, for example at 0.3 bar, into the parison which has been previously welded at the base, so as to obtain a kind of bubble of plastics material whose length is much less than that of the desired hollow body, then closing the two parts of the mould onto the said parison, and blowing air at high pressure (of the order of 6 or 7 bars) inside the parison trapped in the mould so as to inflate the parison and apply it uniformly against the cavity of the mould.

Now, it has been found that the distribution of the material in the horizontal direction, with respect to successive horizontal layers, of the bottle finally obtained is not uniform. Indeed, measurements effected have revealed thin regions and excessively thick regions. For example, in respect of a bottle of 5 litres capacity adapted to receive household, maintenance or washing products, the weak regions are those where the thickness is less than 0.70 mm and are located at the bottom of the bottle on its front crowned part and on the shoulder of the neck and on the handle.

The excessively thick regions, i.e. the regions having a thickness exceeding 1.20 mm, are located in the region of the welds at the base of the bottle and at the base of the sides.

These variations in thickness in successive horizontal sections of the bottle have the following drawbacks: during the container-filling operations and then the storage and handling thereof, the containers more or less press into each other in their weak regions; moreover, the existence of these regions creates the risk of localised rupture and consequently subsequent leakages in the course of the handling operations; lastly, difficulties are encountered for suitably adhering the label of the bottle on the side provided for this purpose owing to the fact that this side is not planar.

This harmful irregularity in the distribution of the material within a given horizontal layer or section of the container can be explained by the fact that, at the beginning of the blowing of the parison inside the mould, the contact between the hot plastics material and the cooled walls of the mould (at about 20 C) does not occur simultaneously at every point of the mould. Now, the contact between the hot plastics material and the wall of the mould cavity immediately produces a commencement of solidification of the plastics material, above all on the outer wall of the parison which consequently becomes less malleable. In practice, the analysis of the parison blowing procedure shows that the plastics material comes first into contact with the bottom, then the bulging front part before the handle and with the neck only after the beginning of the blowing.The material already cooled must therefore be considerably drawn or stretched in order to reach to remote points, which explains their reduced thickness. Further, at the moment of the closure of the mould, it is the sides which are the quickest to come into contact with the mould. The plastics material is then consequently set and thereafter moves with more difficulty and this produces the observed excessive thicknesses.

In order to overcome this problem and ensure an even distribution of the plastics material in a given horizontal section or level of the bottle in the course of the blowing, various methods were first of all tried without suitably solving the problem. Finally, the latter was solved in the following manner.

The method according to the invention is characterised in that, during the forming of the parison whose base was welded by two jaws, air is blown into the parison through the extruding die so as to inflate highly the parison outside the mould until its length is sustantially equal to the desired final length of the hollow body, then the mould is closed onto the parison and air is blown into the latter at sufficient pressure to apply its wall against the cavity of the mould.

It has been found that in each horizontal section of the bottles obtained by this method, the excessive thicknesses and the weak regions have disappeared owing to the avoidance of the premature contact between the plastics material and the large sides of the cavity of the mould.

According to a feature of the method according to the present invention, air is blown into the parison outside the mould until the cross-section of the parison is equal to that of the cavity of the mould so as to distribute uniformly the material in each horizontal or transverse section of the parison, then it is deflated by cutting off the supply of blowing air and it is put into contact with the atmosphere or subjected to a depression or partial vacuum so as to impart thereto a flaccid form, and the mould is closed onto the deflated parison whose walls are thus maintained out of any contact with the cold walls of the cavity of the mould until the blowing of the air into the parison trapped inside the mould is resumed.

Thus, the parison which was considerably stretched or drawn by the prior inflation, resumes a flaccid form after deflation which enables it to be placed inside the mould without premature contact of the said parison with the cooled walls of the cavity of the mould. It will therefore be understood that the subsequent inflation of the parison at high pressure ensures that the parison is applied simultaneously and uniformly against the cavity of the mould.

Further, the prior inflation of the parison outside the mould constitutes an essential feature of the method according to the present invention, since it is this inflation outside the mould at a suitable pressure which permits the attainment of a uniform distribution of the material in the transverse direction within a given horizontal section. It will be understood that the thickness of successive horizontal sections may be suitably programmed.

The invention may be put into practice in various ways and one specific embodiment will be described by way of example to illustrate the invention with reference to the accompanying drawings, in which: Figure 1 is a simplified front elevational view of an extruding-blowing unit for plastics material for producing hollow bodies by the method according to the present invention; Figure 2 is a simplified side elevational view of the unit shown in Figure 1; Figure 3a to 3d are diagrams illustrating the successive steps of the method for producing a hollow body according to the present invention by means of the unit shown in Figures 1 and 2; and Figure 4 is a diagram of an embodiment of the device for controlling the blowing of air inside the parison at the outlet end of the extruding head, the parison being outside the mould.

The unit shown in Figures 1 and 2 is adapted to produce a hollow body by the extrusion and blowing of plastics material and in particular containers of complex shape such as bottles of polyethylene of various volumes adapted to receive for example maintenance, cleaning or washing products.

In known manner, this unit comprises an extruder 1 (Figure 2) provided with a horizontal screw for plasticising the material which is introduced into the extruder through a hopper 2, two extruding dies 3 (Figure 1) disposed vertically and fed with plasticised material by the extruder 1, means 4 for blowing air into the parisons 5 obtained at the outlet end of the dies 3, means 15 for cutting successive parisons, main mould 6 formed by two semi-moulds 6a in which are defined two semicavities 7 corresponding to the shape desired for the hollow body to be moulded, two tubes 8 for blowing air inside the mould 6, a device (not shown) for shifting the main mould from the dies 3 to blowing tubes 8 below the latter and vice-versa, and means (not shown) for discharging the finished hollow bodies 9 and scrap.The scrap is received by a conveyor 11 and recycled to the extruder 1 in known manner.

All of the foregoing component parts rest on a fixed frame 12. The unit also includes a mould 13 for receiving the containers 9a formed in the mould 6, a ram 14 associated with each extruding head for shifting the plunger disposed in the known manner within this head so as to adjust the thickness of the wall of the parison 5, and two rams 16, 17 associated with the blowing tubes 8, the ram 16 being pneumatic and the ram 17 hydraulic. The unit is provided with a motor 18 associated with a speed-reducer 19 for rotating the screw of the extruder 1 and electronic programming means and rams (not shown) for shifting the two semi-moulds 6a for the purpose of moving them away from each other or toward each other for closing the mould, and for shifting the mould 6, closed onto the parisons, between the extruding heads 3 and the blowing tubes 8.

According to the present invention, the unit includes (see Figure 4) two electro-valves 21, 22 each associated with air blowing means for blowing air into the heads or dies 3 and automatically controlled by an electro-valve 23 which is itself controlled electrically by programmable timed relays.

The electro-valves 21, 22 each control, in accordance with their position, the passage of the air for inflating the corresponding parison 5 through the plunger 24, 25 of the associated extruding head 3 by way of axial passageways 26, 27 diagrammatically shown in Figure 4.

Bearing in mind the method according to the present invention, the steps of which are illustrated in Figure 3 and which is carried out by means of the device controlling the flowing of air into the parisons through the extruding heads 3 by means of the electro-valve system 21, 22, 23, the operation of this control device will first of all be described.

In the position of the electro-valves 21-23 shown in full lines, the electric control Y of the electrovalve 23 (consisting of two timed relays not shown) permitting the programming of the start and stopping of the communication with the atmosphere, is at rest and the return means X therefore maintains the electro-valve 23 in the position preventing the passage of the pressure P, (indicated by the arrow in full lines). The pressure P2 exerted on the push-buttons (not shown) of the electro-valves 21 and 22 maintains the latter in the position for the direct passage of the inflating air pressure P into the parisons through the axial pas sageways 26, 27 and the plungers 24, 25.

In the second position of the electro-valves 2123, illustrated by the arrows in dashed lines, the parisons are put into communication with the atmosphere in the following manner: the electric control Y is excited so that the air at pressure P, passes through the electro-valve 23 and actuates the electro-valves 21, 22 which then move to the position for deflating the parisons. The arrows P indicate that the supply of inflating air to the passageways 26, 27 is interrupted.

When new parisons must be inflated, the automatic programming of the device by the aforementioned relays puts the control Y at rest and the foregoing cycle recommences.

There will now be described with reference to Figure 3 the method to which the invention is more particularly directed.

The two semi-moulds 6a are spread apart and the plastics material issues from the extruding head 3 with a blowing of air into the parison 28, automatically controlled by the electro-valve 21 (or 22) associated with the head 3 and itself actuated by the electro-valve 22, as described previously.

The air is blown into the parison 28 and the latter gradually inflates, the pressure of the air being about 0.6 bar for example and the beginning of this step being illustrated by the diagram of Figure 3a.

The blowing at this pressure is continued until the parison 29 has substantially the length of the finished container 9 and a cross-section equal to that of the cavities 7 when the mould 6 is closed.

This step is indicated in the diagram of Figure 3b and the blowing of air is interrupted by the action of the electro-valve 23 when the parison has assumed the shape indicated by the reference numeral 29 in Figure 3b.

In the course of this step, the plastics material is therefore uniformly distributed in each horizontal section of the parison 29, the thickness of which is consequently constant within a given section.

Thereafter, the blowing air is cut off (step c) so as to deflate the parison which assumes the flaccid state shown at 31 by putting it in communication with the atmosphere or a depression (partial vacuum). As mentioned before, the interruption of the blowing air is achieved by the excitation of the electric control Y of the electro-valve 23.

The mould 6 is closed onto the deflated parison 31 (see Figure 3d) whose walls consequently do not contact the cold walls of the cavity 7 and indeed, in effect, are held out of contact with the walls of the mould. The mould 6 (which may mould two containers 9a simultaneously in the embodiment illustrated in Figure 1) is then shifted so as to bring it below the air blowing tubes 8. Air under higher pressure, for example of the order of 7 bars, is blown through the tubes 8 as indicated by the arrows in the diagram of Figure 3d so that the walls of the parison 9a trapped within the mould 6 are simultaneously blown outwardly and thus applied at every point against the cavity 6.

The air tranoed between the parison 31 and the cavity 7 is evenly distributed and constitutes a screen which is eliminated upon the blowing by way of passages (not shown) provided for this purpose in known manner between the two semimoulds 6a.

During the blowing, the material is consequently rapidly and uniformly applied against the entire surface of the cavity 7, including the points the most remote from the axis of the parison 9a, since the necessary expansion or stretching of the polymer layer had already been roughly achieved outside the mould 6 in the stage shown in Figure 3b.

After the interruption of the blowing of the air, the mould 6 is opened for the purpose of extracting the moulded containers 9a, removing the burrs and fins in the known manner and discharging the finished containers 9.

The mould 6 is placed in its standby position under the dies 3 and the cycle is recommended for the following parisons.

It is found that, as already mentioned, the thickness of the wall of the containers 9 is of greatly increased uniformity and preferably approaching exact uniformity in a given transverse section from one end to the other of the container. This advantageous result is made possible by the prior inflation of the parisons outside the mould (steps a and b) and the deflation thereof (step c) before the closure of the mould onto these parisons. This avoids any premature contact between the cavity 7 and the parison and consequently premature localised solidification of the material which would otherwise oppose its uniform stretching or drawing.

It is also found surprisingly that the method according to the invention permit a slight reduction in the weight of the bottles and consequently in the amount of plastics material required for their production.

Further, the strength of the various bottles capable of being produced by the method of the present invention is distinctly improved over that of the bottles obtained by the method of the prior art, which is especially advantageous since the bottles can have a slightly reduced weight.

It must be understood that the scope of the present invention is not intended to be limited to the manner of carrying out the invention described since modifications may be made therein. It is in particular possible to replace the electro-valves 21, 22 for programming the blowing of the air into the extruding heads 3 by any other equivalent device.

Claims (10)

1. A method for producing a hollow body by an extrusion and blowing of plastics material and in particular containers of complex shape, comprising supplying plastics material to an extruder having a screw for plasticizing the material and an extruding die equipped with a movable plunger for adjusting the thickness of the wall of the hollow body, obtaining at the outlet of the die a parison giving the rough shape of the hollow body, separating the parison from the plastics material extruded at the upstream end, thereafter inflating said parison by blowing air inside a mould in which is defined a cavity of the desired shape of the body, then opening the mould for releasing the body moulded against the mould cavity, wherein, with the mould open, air is blown into the parison through the extruding die so as to inflate the parison outside the mould until the length of the parison is substantially equal to the desired final length of the hollow body, and the mould is closed onto the parison and air is blown into the parison at sufficient pressure to apply the wall of the parison against the mould cavity.

2. A method of blow moulding plastics material in which the parison is inflated outside the mould to a length and maximum transverse dimensions substantially those of the mould in which it will be moulded, is then caused or allowed to deflate so that at least its transverse dimensions are substantially less than those of the mould and is then placed in the mould and inflated into contact therewith.

3. A method as claimed in Claim 1 or Claim 2, comprising blowing air into the parison outside the mould until the cross-section of the parison is equal to the cross-section of the cavity of the mould so as to distribute uniformly the material in each horizontal section of the parison, then deflating the parison by cutting off the blowing air and putting it in communication with the atmosphere or a depression or partial vacuum so as to impart to the parison a flaccid state, and closing the mould onto the deflated parison whose walls are thus maintained out of any contact with the cold walls of the mould cavity until the blowing of air into the parison trapped inside the mould is resumed.

4. A method as claimed in Claim 1, 2 or 3 in which the air is blown into the parison outside the mould at a pressure of the order of 0.6 to 0.7 bar and into the parison inside the mould at about 7 bars.

5. A method as claimed in any one of Claims 1 to 4 substantially as specifically described herein with reference to the examples.

6. Apparatus for carrying out a method as claimed in any one of Claims 1 to 5, comprising at least one extruding die for extruding plastics material connected to means for feeding plasticized material, means for cutting the parison and welding the base of the parison, means for blowing air into the parison obtained at the outlet of the extruding die, a mould for receiving the parison and having an inner cavity corresponding to the desired shape of the hollow body to be moulded, at least one tube for blowing air inside the mould, a device for shifting the mould from the extruding die to the blowing tube and vice-versa, and means for discharging the finished hollow bodies and scrap, wherein there are provided a first control means associated with means for blowing air into the extruding head associated therewith and controlled by a second control means for putting the corresponding parison in communication with the atmosphere after inflation and then for interrupting the communication with the atmosphere after a predetermined time and thus permit the inflation of the following parison.

7. Apparatus as claimed in Claim 6 in which the deflation of the parison is programmable by means of timed relays constituting the electric control of the second control means which controls the first control means.

8. Apparatus as claimed in Claim 6 or Claim 7 in which the first and second control means are electro-valves.

9. Apparatus as claimed in Claim 8 in which the second electro-valve pneumatically controls the first electro-valve.

10. Apparatus as claimed in Claim 6 substantially as specifically described herein with reference to the accompanying drawings.