GB2108423A

GB2108423A - A back flow seal in an injection cylinder

Info

Publication number: GB2108423A
Application number: GB08230534A
Authority: GB
Inventors: Siegfried Hirmer
Original assignee: Bucher Guyer AG
Current assignee: Bucher Guyer AG
Priority date: 1981-10-28
Filing date: 1982-10-26
Publication date: 1983-05-18
Also published as: ATA354982A; FR2515102A1; AT384989B; FR2515102B1; DE3142756A1; DE3142756C2; GB2108423B

Abstract

An injection moulding apparatus has a screw (2), rotatably and axially displaceably mounted in a cylinder (1), by the rotation of which is plasticised the material in the cylinder, and by the axial forward displacement of which the plasticised material is injected into a mould. A closure ring (17) is guided on the screw head (8) and is axially displaceably and is in sealing contact with the inner surface (19) of the cylinder. The closure ring bears, during plasticising, by axial spaced apart projections (22), provided on its end face facing in the direction of said forward displacement, on to a shoulder (26) of the screw head, and during the injection phase is brought to bear by its sealing surface, provided on its end face (20) on to a complementary sealing surface (21) of the screw. The projections of the closure ring are provided with bearing surfaces (24), which in axial section make an acute angle with the direction of forward displacement of the screw, and bears on to the shoulder 26 which is formed as a bearing surface (27) complementary to the surfaces (24). <IMAGE>

Description

SPECIFICATION A back flow seal for a screw extruder The invention relates to a back flow seal for a screw extruder having a screw, rotatably and axially dis placeablysituated in a cylinder, by the rotation of which is plasticised the material fed in, and by the axial forward displacement of which the plasticised material is injection moulded, and further having a closure ring guided on the head of the screw axially displaceably and in sealing contact with the inner surface of the cylinder, which ring may bear, during plasticising, by axial projections, provided on its end face facing in the direction of said forward displacement, on to a shoulder of the screw head and during injection moulding may be brought to bear by its sealing surface, provided on its end face facing in the direction opposite to that of said forward displacement, on to a complementary sealing surface of the screw.

The purpose of such back flow seals is to prevent during injection moulding flow of the plasticised material back to the screw extruder. This is needed because in a screw extruder of the described kind the material which is plasticised during rotation of the screw flows past the screw head into a storage space situated in front of the screw head, whereupon follows the injection moulding by axial displacement of the screw during which the plasticised material positioned in front of the screw head is pushed out of the extruder. Because the pressure caused thereby in the plasticised material acts in all directions, there is during injection moulding also a back forcing pressure component which would cause undesirable back flow to the screw extruder.The back flow seal must therefore on the one hand during plasticising enable forward flow of the plasticised material towards and past the screw head, and on the other hand must prevent during injection moulding back flow of the plasticised material.

In a known back flow seal of the mentioned kind a shoulder, on to which bear the axial projections of the closure ring, is formed by a bearing surface of the screw head, the surface extending in the radial plane of the screw extruder and the end faces of the projections, which also lie in a radial plane, bear on it. Because the bearing takes place under a forward pressure of the material, which is plasticised and conveyed by the screw, the closure ring is pressed by a considerable force on to the screw head, this force acting on the said surfaces at right angles due to the radial position of the bearing surface of the screw head and of the end faces of the closure ring.

Unless the closure ring is carried by the screw during its rotation due to its pressing-on without slippage this causes considerable frictional forces between the screw head and the closure ring, with which are associated high stressing and wear of the parts.

However, as long as the closure ring follows the rotation of the screw, undesirable frictional forces are generated between the inner curved surface of the cylinder and the outer curved surface of the closure ring, whereby the sealing in this zone is worn off and deteriorates. In the known back flow seal are therefore provided on the outer curved surface of the closure ring two axially spaced apart sealing rings in a manner similar to piston rings, which should improve the sealing between the outer curved surface of the closure ring and the iner curved surface of the cylinder. However, the increased stresses caused particularly during the processing of thermosetting materials, due to their higher viscosity compared with thermoplastic materials, cannot be thereby avoided.

Also back flow seals are known, in which the conical screw head is provided on its periphery with axially extending passages due to which the screw head acquires a shape similar to a spear, on to which may bear a closure ring during plasticising, so that the plasticised material may pass through said passages. It is also known to provide on the conical screw head star shaped projecting arms which form the front stop of a closure ring. These known back flow seals are suitable for the processing of thermoplastic materials, while during the processing of thermosetting materials, in case of unexpected hardening, the material provides a positive connection with the screw head provided with passages for the star-shaped arms, with respect to the inner wall of the cylinder, which may cause breaking of the screw head.Such damages are caused when the operation is interrupted, as is the case with breakdowns of all kinds, breakdowns caused by tools, breakage of material or in unmanned operation, and particularly where such interruptions of operation are long lasting. In addition, due to higher viscosities of thermosetting materials, higher frictional forces are generated in the region of the back flow seal, so that its stressing is increased. This results in a high wear on the bearing surfaces of the screw head and of the closure ring, which, after a time, requires the replacement of these parts.

The aim of the invention is to avoid the described disadvantages and to devise a back flow seal of the mentioned kind which, particularly even if it is used in the processing of thermosetting materials, is subjected to a very small wear.

This aim is achieved according to the invention in that the projections of the closure ring are provided with bearing surfaces which in axial section make an acute angle with the direction of forward displacement of the screw, and bear on to the shoulder of the screw head, which shoulder is formed as a bearing surface complementary thereto.

In a seal according to the invention the bearing surfaces, formed on the projections, lie on the curved surface of an acute-angled cone, which opens in the direction of displacement, and are supported on the bearing surface of the shoulder of the screw head, which surface is complementary thereto and is also in the shape of a conical envelope. During plasticising the plasticised materials, such as thermoplastic materials or thermosetting materials, can therefore be conveyed through the closure ring, and also through the passages between the projections, past the screw head into the storage space situated downstream thereof.

During the rotation of the screw, which is connected therewith, the bearing surfaces of the closure ring and of the screw head slide on each other with a considerably smaller friction, because these surfaces are stressed due to their inclination not at right angles, but with a much smaller partial component of the pressure. Because the bearing surfaces have due to their inclination an increased axial dimension, also the specific surface pressure is reduced, In addition the shape of the closure ring enables cleaning of the back flow seal by spraying it with a so-called cleaning granulate of a thermoplastic material, which is needed in case of intentional or unexpected interruption of operation when hardening thermosetting materials are used.Although even the non-hardened thermoplastic cleaning granulate solidifies, it can be transferred by reheating to a liquid state and injected from the screw extruder.

In order to achieve a very low surface pressure between the bearing surfaces of the projections and of the complementary bearing surface of the screw head, which surface is in the shape of a conical envelope, it is further advantageous if the whole cross-sectional area of the orifice, formed between the projections provided with the bearing surfaces, is at least as large as the cross-sectional area of the flow formed in the front portion of the screw, which portion faces the screw head between the outer diameter and the shaft diameter of the screw. This dimension, which has been established after many experiments, is particularly advantageous for a low wear undisturbed operation. In a preferred embodiment according to the invention the projections, provided with the bearing surfaces, are uniformly distributed in the peripheral direction of the screw and are at least two in number.This brings about advantageously a symmetrical distribution of the generated pressure on the periphery of the closure ring. In this connection, one embodiment which is particularly advantageous from the aspect of its manufacture, has three projections provided with the bearing surfaces.

According to the invention the projections may be at their foot radially thickened relative to the adjacent inner surface of the closure ring, and the bearing surface may extend in axial section across the portion of the projection between the free end of the projection and the thickening at the foot. This brings about not only better strength of the projections but also, due to the larger axial dimension, increased bearing surface, and consequently smaller surface pressure. It is further proposed by the invention that the bearing surface of the projection be formed as a lubrication pocket, the opening of which, which is formed by a small angle between the azimuthal tangential directions of the bearing surface of the projection and the bearing surface, complementary thereto, of the screw head, is situated in a direction opposite to the direction of rotation of the screw.Due to this provision the plasticised material can during rotation of the screw enter between the respective bearing surface of the projection and the bearing surface, complementary thereto, of the screw head, which brings about reduced friction and hydrodynamic relief on the bearing surfaces.

Finally in a preferred embodiment according to the invention the end face portions of the closure ring, which are situated between the projections provided with the bearing surfaces, are formed as flat elements parallel to the bearing surfaces in the shape of conical envelopes of the screw head. The advantage of this provision resides in that the plasticised material, conveyed between these front face regions and the bearing surface of the screw head, is, due to this arrangement, exposed to only a small flow resistance, so that even due to this the pressure to which is exposed the closure ring, and also the surface pressure on the bearing surfaces, are reduced.

Further features, details and advantages of the invention will be apparent from the following description and drawing to which reference should be made as regards details not specifically mentioned in the text. In the drawing: Figure 1 is an axial section along line l-l in Figure 2 of a screw extruder with a back flow seal, and Figure2 is a radial section along line Il-Il in Figure 1.

As is apparent from Figure 1 the screw extruder for an injection moulding machine for plastics has a screw 2, positioned inside a cylinder 1, the screw 2 being both rotatably and axially displaceably mounted in the cylinder 1. The screw 2 is provided in its end part, which ends in a radial end face 3, with a central axial bore 4 which is at a distance from the end face 3 provided with a screw thread 5. The part of the axial bore 4, which extends from the screw thread 5 up to the end face 3, has a diameter which is slightly larger than that of the screw thread 5.

A screw head 8, which is on its free end, removed from the screw 2, in the form of a cone 7, opening towards the screw 2, has at its end, adjacent the screw 2, a threaded end piece 9, screwed into the thread 5, and a shaft 10, which forms a continuation thereof in the direction to the free end 6 and made with a fitting for accurate centering of the screw head 8 in the part of the axial bore 4 adjacent to the end face 3. A collar 11, which forms continuation of the shaft 10 at its end face facing the free end 6, forms a shoulder 12 projecting towards the shaft 10.

On this shoulder 12 is supported a thrust ring 13, provided with a shoulder complementary thereto and positioned on the shaft 10, the ring 13 bearing by its radial end face 14, situated opposite to the shoulder 12, on to the end face 3 of the screw 2. The cylindrical outer surface 15 of the thrust ring 13 forms a smooth continuation of the outer surface 16 of the screw shaft.

Between the free end 6 of the screw head 8 and the thrust ring 13 is axially displaceably guided a closure ring 17 with its outer curved surface 18 in sealing contact with the inner surface 19 of the cylinder 1. Its axially symmetrical end face 20, facing the thrust ring 13, is inclined at an obtuse angle to the direction of displacement of the screw 2 to the free end 6 of the screw head 8, and is arranged opposite to the axially symmetrical end face 21 of the thrust ring 13, which is inclined at the same angle. When the thrust ring 13 is axially displaced in a direction opposite to the direction of forward displacement, the two end faces 20 and 21 impinge on to each other so that due to the accurate fitting the inner space of the cylinder 1, facing the screw 2, is sealed.

The closure ring 17 is further on its end face, facing in the direction of the foward displacement i.e. in the direction to the free end 6 of the screw head 8, provided with axial projections 22 which are according to Figure 2 uniformly distributed on the periphery of the closure ring 17 and which are three in number in the embodiment illustrated in Figure 2.

The radial outer surface 23 of the projections 22 forms a smooth continuation of the cylinderical outer surface 18 of the closure ring 17. The opposite radially inner sides of the projections 22 are made as bearing surfaces 24 which are inclined, in axial section, at an acute angle to the direction of forward displacement of the screw 2. The bearing surfaces 24 of the projections 22, which are three in number in the embodiment according to Figure 2, lie therefore substantially on the curved surface of a cone opening in the direction of foward displacement.The screw head 8 has also on its side, facing the screw 2, of a cylindrical portion 25, extending from the free end 6 to the screw 2, a tapering shoulder 26, which is inclined, in axial section, in a direction opposite to the direction of the forward displacement at the same angle as the bearing surfaces 24 of the projections 22 so that a frustoconical bearing surface 27 is formed which is complementary thereto.

Finally the portion 29 of the screw head 8, which extends from the radially inner end portion 28 of the shoulder 26 up to the end face 21 on the thrust ring 13, is substantially cylindrical, while its outer curved surface 30 smoothly merges, in the vicinity of the collar 11, into the end face 21.

The operation of the screw extruder includes plastification and injection moulding. During plastification the screw 2 only rotates, whereby the material supplied to the screw extruder, such as a thermosetting or thermoplastic material e.g. in the form of granules, is conveyed, and simultaneously plasticised, by the screw thread 31 of the screw 2 in the direction to the free end 6. The closure ring 17 is during this time, due to the flow pressure exterted thereon by the plasticised material, supported on the screw head 8 in that the bearing surfaces 24 of the projections 22 bear on to the bearing surface 27 of the screw head 8.The plasticised material flows between the end face 21 of the thrust ring 13 and the end face 20 fo the closure ring 17, and also between the outer curved surface 30 of the portion 29 and the inner curved surface 32 of the closure ring 17, which surface 32 has a larger diameter than the portion 29, and through the passages 33, defined between the axial projections 22, and past the cylindrical portion 25 of the screw head 8, which portion 25 has a smaller diameter than the cylinder 1, and arrives finally into the storage space situated downstream of the free end 6 of the screw head 8.

The screw 2 is during injection moulding axially displaced in the direction of forward displacement, whereby the plasticised material is injected from the screw extruder. Due to the reverse pressure component, caused by the injection pressure, the closure ring 17 is displaced against the direction of forward displacement, so that its end face 20 bears sealingly on the end face 21 of the thrust ring 13 and back flow of the material towards the screw 2 is not possible.

The conical arrangement of the end faces 20 and 21 promotes sealing between these end faces.

This inclined arrangement of the end faces 20 and 21 forms at the same time a channel for easy flow of the plasticised material during plasticising. In addition, in order to create suitable flow conditions, the end face portions 34 of the closure ring 17, situated between the projections 22, are formed as surface portions parallel to the shoulder 26 or to the conical bearing surface 27.

The passages 33 between the projections 22 are dimensioned for the purpose of favourable conditions for the flow of the plasticised material and also for achievable minimum surface loading between the bearing surfaces 24 and 27, such that their whole cross-section available for flow is at least as large as the flow cross-section between the inner surface 19 of the cylinder 1 and the outer surface 16 of the screw shaft in the part of the screw 2 facing the screw head 8.

As is apparent from Figures 1 and 2 the axial projections 22 have at their foot end axially opposite to their free end 35, a thickening 36 extending radially inwards. The bearing surface 24 extends, in axial section, over the whole area of the respective axial projection 22, namely the area between the free end 35 and the thickening 36 at the foot. This results on the one hand in an increased strength of the projections 22 and on the other hand in a particularly large area of the bearing surfaces 24, so that surface pressure is further reduced.

As is apparent particularly from Figure 2, the bearing surfaces 24 are provided with lubrication pockets. In order to form these lubrication pockets the bearing surface 24 is so undercut that at each ppint of its inner periphery the azimuthal tangential direction of the bearing surface 24 makes with the azimuthal tangential direction of the complementary bearing surface 27 of the screw head 8 a small angle a. The opening, formed by this small angle a, is situated in a direction opposite to the direction of rotation of the screw 2 indicated by an arrow 37. A gap 38, indicated in Figure 1 by a dashed line, is formed in this way between the bearing surface 27 and the respective bearing surface 24 on the axial edge 39 of the projection 22, the edge 39 being positioned against the direction of rotation of the screw 2. The plasticised material is during rotation of the screw 2 conveyed into the lubrication pocket so that hydrodynamic relief is obtained on the bearing surfaces due to the introduced plasticised material.

Claims

1. A back flow seal for a screw extruder having a screw, rotatably and axially displaceably mounted in a cylinder, by the rotation of which is plasticised the material fed in, and by the axial foward displacement of which the plasticised material is injectionmoulded, and further having a closure ring guided on the head of the screw axially displaceably and in sealing contact with the inner surface of the cylinder, which ring may bear, during plasticising, by axial spaced apart projections, provided on its end face facing in the direction of said forward displacement, on to a shoulder of the screw head, and during injection moulding may be brought to bear by its sealing surface, provided on its end face facing in the direction opposite to that of said forward displacement, on to a complementary sealing surface of the screw, wherein the projections of the closure ring are provided with bearing surfaces, which in axial section make an acute angle with the direction of forward displacement of the screw, and bear on to the shoulder of the screw head, which shoulder is formed as a bearing surface complementary thereto.

2. A back flow seal according to Claim 1, wherein the whole cross-sectional area of orifices defined between the projections is at least as large as the cross-sectional area of the flow formed in the front portion of the screw between the outer diameter and the shaft diameter of the screw.

3. A back flow seal according to Claim 1 or 2, wherein the projections are uniformly distributed in the peripheral direction of the screw and are at least two in number.

4. A back flow seal according to any one of Claims 1 to 3, wherein the closure ring has three said projections.

5. A back flow seal according to any of Claims 1 to 4, wherein the projections are at their foot end radially thickened relative to the adjacent inner surface of the closure ring, and the bearing surface extends in axial section across the portion of the projection between the free end of the projection and the thickening at the foot thereof.

6. A back flow seal according to any one of Claims 1 to 5, wherein the bearing surface of at least one projection is formed as a lubrication pocket made by undercutting at a small acute angle subtended between the aximuthal tangential directions of the bearing surface of the projection and the bearing surface, complementary thereto, of the screw head, the inlet of which faces in a direction opposite to the direction of rotation of the screw.

7. A back flow seal according to any one of Claims 1 to 6, wherein end face portions of the closure ring, which are situated between the projections, are formed as flat elements parallel to the bearing surfaces of the screw head.

8. A back flow seal constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompany ing drawing.

9. A device including a back flow seal according to any one of Claims 1 to 8.

10. A product made with the use of a back flow seal according to any one of Claims 1 to 8, or with the use of a device according to Claim 9.