GB2060473A

GB2060473A - Valved Head for Extruding Plastics Material Tubes

Info

Publication number: GB2060473A
Application number: GB8031597A
Authority: GB
Original assignee: BELL AG MASCHF; Bell Maschinenfabrik AG
Current assignee: BELL AG MASCHF; Bell Maschinenfabrik AG
Priority date: 1979-10-03
Filing date: 1980-10-01
Publication date: 1981-05-07
Also published as: FR2466334A1; JPS5658832A

Abstract

The head 7, 8 for extruding tubes for blow moulding, includes a mandrel 4 supported by one part (8), the other part (7) comprising at least one conical wall portion (10) which, with a corresponding mandrel wall portion (9), forms a conical flow space section (11) whose throughflow cross-section can be varied by the relative displacement of the two telescopically engaging parts (7, 8). The variable conical flow space section may be provided downstream of the mandrel support when a conical outlet orifice may be simultaneously altered in width, or a pair of such variable sections 28, Fig. 4, may be provided which are simultaneously varied in opposite sense. <IMAGE>

Description

SPECIFICATION Extruder Head The invention relates to an extruder head which is intended for the production of plastics material tubes, particularly in a machine for the production of blow moulded hollow plastics parts or articles. In such heads, a body forms the external boundary of a duct for the plastics material, the duct having substantially cylindrical and conical wall portions alternately following one another with smooth transitions. A torpedo is arranged centrally in the duct, for forming a symmetrical annular flow space, the torpedo having alternately substantially cylindrical and conical wall portions following one another with smboth transitions so as to correspond with the outer wall portions, to form respectively a cylindrical or conical flow space portion.

For construction of hitherto known extruder heads it is considered advisable so to arrange the duct or the flow space geometrically that during throughflow through the extruder head the plastics material is not thermally or mechanically damaged as a result of excessive pressure or too high a throughflow speed. In these cases, specific properties of the particular plastics material being dealt with are taken into account. But even if the form of the duct or flow space is arranged to allow for the particular properties of the material in question, disturbances occur during operation.

Particularly in the case of the plastics which are highly sensitive to pressure and temperature, for example the PVC-based recipes prescribed for the production of foodstuffs packaging for hygienic reasons, so-called hard PVC, scorching or burning of the material very often occurs in the extruder head. Often the flow of plastics material does not unite, or only to an inadequate extent, downstream of the webs which extend through the flow space and which are provided for holding the torpedo. These factual reasons lead to the extrusion of a defective plastics tube with a nonuniform structure. The wall thickness is variable or streaking or clouding is found to have occurred in the tube.If this happens, attempts have to be made by experiment to find the conditions under which the plastics material can be treated in the duct or the flow space, as regards temperature, pressure, throughflow speed and friction, to avoid forming a defective tube. For example it is attempted to heat or cool the extruder head in certain regions, or various annular elements are arranged experimentally in the flow space to modify the cross-section thereof, so as to try and find a suitable shape for the various sections.

Experiments of such kinds are very laborious, and correspondingly expensive, since they always interrupt operation of a possibly high-capacity machine. It will be apparent that hitherto known extruder heads constructed in the manner described are very restrictively designed for a single specific material which is to be extruded, and cannot be used for other materials having other specific properties. In the case of the aforesaid high-sensitivity PVC materials, blocking of the duct or flow space with burned or baked-on material is practically unavoidable. In such an event the entire machine has to be stopped, to disassemble the extruder head to clean the blocked regions.

The invention seeks to provide an extruder head having a relatively wide field of possible use; in other words, a head with which tubes of various plastics materials can be produced to the desired high quality standard, and with which it is possible to vary to the necessary extents, even during operation, without disassembly, the pressure conditions or the speed of the plastics material in the duct or flow space to adapt to the specific properties of the material being extruded.

It is also desirable to be able to make unavoidable accumulations of damaged material flow out of the duct or flow space. Further, the invention aims to provide for the wall thickness of the tube being extruded to be regulated or modified operatively during working operations.

According to the invention the body of the extruder head is divided at least into two parts which are constructed to be longitudinally displaceable relatively to one another and overlap each other telescopically in the region of the flow space, the torpedo being secured to one part and the other part comprising at least one wall portion extending substantially parallel to a conical wall portion of the torpedo and bounding externally a conical flow space section, so that by relative displacement of the two parts of the body the throughflow cross-section of the flow space section formed between the said two conical wall portions situated parallel to one another is variable.

In an extruder head according to the invention it is possible at least at one region of the flow space to provide a throughflow profile suitable for the particular properties of the material being extruded. This can be done without disassembly of the extruder head, and even during operation.

By reducing the throughflow cross-section a pressure accumulation is built up before the relevant flow space section, and a greater throughflow speed achieved in the section.

Because of this, the material in the section is heated by increased friction, so that it flows more readily. If the throughflow cross-section is increased, the inverse effect is obtained. It is also possible as a result to make any damaged material particles which may have accumulated upstream of this flow space section, flow out from such regions.

The invention will now be described by way of example and with reference to the accompanying drawings in which each figure shows in crosssection one embodiment thereof.

An extruder head, the body of which is given the general reference numeral 1, is normally vertically situated, and comprises an elbow conduit 2 with which the extruder head is connected to a horizontally situated extruder or a horizontally situated distribution conduit. The elbow conduit is indicated with broken lines in Figure 1. A duct for the plastics material, given the general reference numeral 3, leads through the body 1. The duct 3 is bounded externally by substantially cylindrical and conical wall portions which follow one another alternately and with stepless transitions. Centrally in the duct 3 a torpedo 4 is arranged, to form a symmetrical annular flow space 5, and it is held on the body 1 securely by means of fins or webs 6 extending through the flow space.The torpedo 4 comprises alternating conical and cylindrical wall portions which connect with one another at smooth transitions and in each case form a conical or cylindrical flow space section respectively with the opposite wall portions of the body 1 which bound the duct 3 externally. The body 1 is divided in the region of the flow space 5 at least into two parts 7 and 8 which overlap one another telescopically and are constructed to be longitudinally displaceable relatively to one another. The torpedo 4 is secured to the one part 8 by means of the webs 6.The other part 7, the upstream part considered in the direction in which the plastics material flows through the extruder head, comprises a wall portion 10 which is disposed substantially parallel to conical wall portion 9 of the tip of the torpedo 4 and which bounds externally a conical flow space section 1 1 between these two walls 9 and 10. By a relative displacement of the two parts of the body 1, the one part 7 with the wall portion 10, and the other part 8 with the torpedo 4 and its wall portion 9, for example by means of an adjusting screw 12, it is possible to vary the throughflow cross-section of the flow space section 1 1 formed between the said two conical wall portions 9 and 10 which are situated parallel to one another.

The first part 7 extends to the base line 13 of the wall portion 11 which widens conically in the material flow direction, extends substantially parallel to the wall 9 forming the tip of the torpedo 4, and forms the external boundary of the conical flow space section 11.

The first part 7 overlaps telescopically with the second part 8 of the body 1, in that it is given a cylindrical neck 14 the diameter of which is equal to the diameter of the base line 13 of the wall 11.

This neck 14 fits displaceably in the cylindrical wall portion 1 5 in the part 8, the latter wall portion bounding externally a cylindrical flow space section 16 immediately following the conical flow space section 1 The second part 8 with the cylindrical wall portion 15 is constructed to form an adequately long guide portion for the neck 14 of the part 7, so that the relative telescopic displacement of the two parts 7 and 8 is easily possible. The fit of the neck 14 relatively to the cylindrical wall portion 15 forming the neck guide is also close enough to prevent plastics material flowing out along this guide. Relative displacement of these parts 7 and 8 modifies the throughflow cross-section of the conical section 1 1, in other words the flow space in the region of the torpedo tip.

The use and arrangement of the adjusting screw 12 and of a suitable screwthread in the part 8 is shown only as one possibility for a device for effecting the relative displacement of the parts of the body of an extruder head according to the invention. Other devices suitable for this purpose are also usable. Various remotecontrolled servo devices could conceivably be used with which the displacement of the parts i.e. the adjusting of the throughflow cross-section in a flow space section is further simplified and made more operatively effective.With such a servo device it would then be possible e.g. to increase briefly the size of a relevant flow space cross-section, to free the duct 3 or the flow space 5 from a blockage with damaged plastics material particles or, with a short-duration varying of the cross-section, to modify briefly the wall thickness of the produced tube if required, as will be explained later.

In another constructional example, which is shown more particularly in Fig. 2, a flow space 5 comprises downstream of the webs 6 used for fixing the torpedo 4, considered in the direction of flow of the material, a flow space section 1 7 which is conical and narrows in the flow direction and is formed between conical wall portions 18 and 19 situated substantially parallel to one another. The body 1 of the extruder head comprises a first part 7 and a second part 8. The first part 7 is provided for securing the torpedo 4.

For manufacturing reasons it is divided, so that the webs 6 of the torpedo are held between the parts thereof. For that purpose a tie bolt 20 is provided whereby the part 7 is assembled with the torpedo. A second part 8 of the body 1, following the first part 7, starts at the base line 21 of the conical wall portion 19 which externally bounds the said conical flow space section 17.

The two parts 7 and 8 overlap one another. The second part 8 comprises a cylindrical neck 22 the diameter of which is equal to the diameter of the base line 21 of the wall portion 19, and fits displaceably on the cylindrical wall portion 23 in the part 7, which forms the external boundary of a cylindrical flow space section immediately preceding the conical flow space section 17. By relative displacement of the two mutually overlapping parts 7 and 8, e.g. by means of an adjusting screw 12, it is possible to vary the throughflow cross-section of the said conical flow space section 1 7, downstream of the webs 6 in the direction of material flow. This flow space section 17 of the space 5 is important for the reuniting of the flow of material which was divided at the webs 6, so that varying, or correctly adjusting, its throughflow cross-section allows satisfactory re-uniting of the flow of material to be achieved.

In this constructional example, as shown in Fig.

2, a last section 24 of the annular flow space 5, as considered in the material flow direction, is conically shaped to form an annular slot nozzle through which the material issues from the extruder head, between wall portions 25 and 26 disposed substantially parallel to one another.

Since the torpedo 4, whose wall portion 25 helps to form the conical section 24 with the annular slot nozzle, is secured in the preceding part 7 in the direction of material flow, and since the second part 8 comprises the wall portion 26 bounding externally the conical section 24 with the annular slot nozzle, the throughflow crosssection of the conical section 24 and the annular slot nozzle can be modified by displacement of the parts 7 and 8 relatively to one another. The wall thickness of the plastics material tube being produced is thereby variable.

Fig. 3 shows a constructional example wherein the features of the constructional examples described hereinbefore with reference to Figs. 1 and 2 are combined. In the case of this extruder head it is possible to modify the throughflow cross-section of the flow space both in the region of the tip of the torpedo 4, in the conical flow space section 11 flaring in the flow direction, and also in the narrowing conical flow space section 17 downsteam of the webs 6, or in the conical section 24 with the annular slot nozzle.

The body 1 is divided into three parts: there is a first part 7 which extends to the base line 13 of a wall portion externally bounding the section 11, and comprises a cylindrical neck 14. There is a second part 8 which starts at the base line 21 of a conical wall portion 19 forming the external boundary of the narrowing conical section 17 and ends with the wall portion 26 externally bounding the section 24 with the annular slot nozzle, and comprises a cylindrical neck 22 with the diameter of the base line 21. Between these two parts 7 and 8 there is arranged a third part 27 which is provided for securing the torpedo 4 by means of the webs 6, and is to guide the cylindrical neck 14 of the part 7 and the cylindrical neck 22 of the part 8.For this, the part 27 comprises a cylindrical wall portion 15 whose diameter matches the diameter of the neck 14, or the base line 1 3, such as to allow sliding. On the other hand the part 27 comprises a cylindrical wall 23 the diameter of which matches the diameter of the neck 22 of the part 8 or the base line 21, such as to allow sliding.

Adjusting screws 12 are used for relative displacement of the parts 7 and 8 with respect to the part 27. For manufacturing reasons the individual parts 7, 8 and 9 are of divided construction, and are held together with tie bolts 20.

An extruder head body 1 which is shown in Fig.

4 comprises a flow space with a section 28 which has the form of two cones with their apexes pointing to one another. These cone-shaped sections of flow space section 28 are designated as 28' and 28R respectively in Fig. 4. Wall portions 29 and 30 which bound the narrowing conical section 28', the first in the flow direction, extend parallel to one another, likewise wall portions 31 and 32 which bound the second conical section 28", which widens in the flow direction. To vary the throughflow cross-section in flow space section 28 a part 33 of the body 1 is provided which starts at the base line 34 of the outer wall portion 30 of the first narrowing section 28' and extends to the base line 35 of the outer wall portion 32 which helps to form the second, widening section 28".The part 33 overlaps telescopically and is made longitudinally displaceable relatively to another part 7 of the body 1, the torpedo 4 being secured in this part.

An adjusting screw 12 is used for the relative displacement of the two parts 33 and 7, and thus of the part 33 with respect to the torpedo 4. Thus, displacement causes a reduction in the size of the throughflow cross-section of the first, narrowing, conical section 28' whilst at the same time increasing the size of the throughflow crosssection of the second, broadening, conical section 288 and vice versa.

The displaceable part 33 has a cylindrical neck 36, which has the diameter of the base line 34 and matches a cylindrical wall portion 37 in the preceding body part 7, such as to allow sliding. In the region of the conical wall portion 32 of the second conical section 28" the part 33 has a second cylindrical neck 38, which has the diameter of the base line 35 of the relevant conical section 28" and matches a cylindrical wall portion 39 in a third, downstream part 8 of the body 1, such to allow sliding. The displaceable part 33 overlaps with the two parts and is guided telescopically. The relative position of the part 7 relatively to the part 7 in the axial direction is to be adjusted by means of a tie bolt 40 with nuts 41.It will be apparent that displacement of the third, lower part 8 relatively to the part 7 in which the torpedo 4 is fixed, also allows the conical section 24, forming an annular slot nozzle at the discharge end of the flow space, to be modified.

Thus in this constructional example it is possible to vary the throughflow cross-sections of three sections 28', 28" and 24.

It is possible for this extruder head which has just been described to be further developed, or used in combination, for example by the use of the measure described in Fig. 1 or Fig. 3, so that as a result the flow space would be adjustable additionally in the region of the tip of the torpedo 4. Fig. 5 shows such a combination.

Finally, it would be conceivable for the various parts of the body which have been described, and also the individual conical and cylindrical elements of the torpedo, to be designed on the unit construction principle. From such modular system elements extruder heads of varying duct or flow space forms could be assembled. For example a torpedo assembled from individual cylindrical and conical elements could be held together by means of a tie bolt. A tie bolt of this kind is indicated for example in Fig. 4 in broken lines, and is designated as 42.

Claims

1. An extruder head for the production of plastics material tubes comprising a body defining the outer wall of a duct for the plastics material, the duct having substantially cylindrical and conical wall portions alternately following one another with smooth transitions; and a torpedo arranged centrally in the duct, for forming a symmetrical annular flow space, the torpedo having alternately substantially cylindrical and conical wall portions following one another with stepless transitions corresponding to the outer wall portions, to form respectively cylindrical and conical flow space sections, wherein the body is divided into at least two telescopically engaged parts which are constructed to be longitudinally displaceable relatively to one another, the torpedo being secured to one part and the other part comprising at least one wall portion extending substantially parallel to a conical wall portion of the torpedo and externally bounding a conical flow space section, the throughflow cross-section of said flow space section formed between the two conical wall portions extending parallel to one another can be varied by relative displacement of the two parts.

2. An extruder head according to Claim 1, wherein an upsteam part of the body is provided which extends to the base line of a wall portion which forms the external boundary of a conical flow space widening in the direction of material flow, said wall portion extending substantially parallel to the wall portion forming the tip of the torpedo, the torpedo being fixed in a downsteam part of the body.

3. An extruder head according to Claim 1 including webs securing the torpedo to one part of the body and wherein a conical flow space section is formed downstream of the webs, which section narrows in the direction of flow and is formed between substantially parallel walls, the outer wall being formed on a said other part of the body downstream of the one part.

4. An extruder head according to Claim 1 wherein the flow space has a portion comprising two conical flow space sections with their apexes directed towards one another, the walls thereof being defined by the torpedo and a said other part of the body, whereby said relative displacement simultaneously reduces the throughflow crosssection of one said flow space section and enlarges the throughflow cross-section of the other said flow space section.

5. An extruder head according to any preceding Claim wherein the last flow space section in the direction of material flow forms an annular slot nozzle between the torpedo and a part of the body, and wherein the torpedo is fixed in an upstream part of the body, the said parts being longitudinally displaceable whereby such displacement varies the throughflow crosssection of the annular slot nozzle.

6. An extruder head substantially as described herein with reference to any one of the Figures in the accompanying erawings.