GB2028216A - Backflow seal for screw injection moulding machines - Google Patents
Backflow seal for screw injection moulding machines Download PDFInfo
- Publication number
- GB2028216A GB2028216A GB7927588A GB7927588A GB2028216A GB 2028216 A GB2028216 A GB 2028216A GB 7927588 A GB7927588 A GB 7927588A GB 7927588 A GB7927588 A GB 7927588A GB 2028216 A GB2028216 A GB 2028216A
- Authority
- GB
- United Kingdom
- Prior art keywords
- screw
- sealing element
- space
- axial bore
- pressure
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- 238000001746 injection moulding Methods 0.000 title claims abstract description 12
- 229920003023 plastic Polymers 0.000 claims abstract description 28
- 239000004033 plastic Substances 0.000 claims abstract description 28
- 239000000463 material Substances 0.000 claims abstract description 11
- 238000007789 sealing Methods 0.000 claims description 78
- 230000010006 flight Effects 0.000 claims description 7
- 238000011144 upstream manufacturing Methods 0.000 claims description 5
- 230000029058 respiratory gaseous exchange Effects 0.000 abstract 1
- 238000002347 injection Methods 0.000 description 16
- 239000007924 injection Substances 0.000 description 16
- 238000000034 method Methods 0.000 description 4
- 238000010276 construction Methods 0.000 description 3
- 239000008187 granular material Substances 0.000 description 2
- 230000033001 locomotion Effects 0.000 description 2
- 235000001674 Agaricus brunnescens Nutrition 0.000 description 1
- 229920000426 Microplastic Polymers 0.000 description 1
- 230000000295 complement effect Effects 0.000 description 1
- 230000006835 compression Effects 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000001105 regulatory effect Effects 0.000 description 1
- 229920001169 thermoplastic Polymers 0.000 description 1
- 239000004416 thermosoftening plastic Substances 0.000 description 1
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
- B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
- B29C45/00—Injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould; Apparatus therefor
- B29C45/17—Component parts, details or accessories; Auxiliary operations
- B29C45/46—Means for plasticising or homogenising the moulding material or forcing it into the mould
- B29C45/47—Means for plasticising or homogenising the moulding material or forcing it into the mould using screws
- B29C45/50—Axially movable screw
- B29C45/52—Non-return devices
Landscapes
- Engineering & Computer Science (AREA)
- Manufacturing & Machinery (AREA)
- Mechanical Engineering (AREA)
- Injection Moulding Of Plastics Or The Like (AREA)
Abstract
Backflow seal of injection moulding machine comprises a mushroom- shaped body 15 which is sealingly guided with its shaft in an axial bore 14 provided on the end face of the screw head 10 and is movable between two stop positions. In the front stop position, the head 17 of the seal releases passages 11 for the plastics material in the screw head 10; in the rear stop position these passages are closed by the head 17 of the seal. Since the mushroom-shaped seal is freely slidable in the axial bore, it is automatically kept in the open position, without a throttling action which would restrict flow, by means of the plastic material which is conveyed forwards by the screw 2. On the other hand, the "breathing" of the plastic mass in front of the screw 2, which occurs at the end of the plasticising stage, suffices to press the seal into the closed position. <IMAGE>
Description
SPECIFICATION Bacidlow seal for screw injection moulding machines
The invention relates to injection moulding machines and more particularly to a backflowseal at the screw head of screw injection moulding machines.
Injection moulding machines are known which possess a sealing element which is guided in the axial bore of the screw head and is axially slidable between a sealing position and an open position in front of a sealing surface of the screw head. The sealing element, in its open position in which it is lifted off the sealing surface, releases passages formed in the sealing element and/or in the screw head which connect the accumulator space in front of the screw with the screw fights, and possesses a space formed at the rear end of the axial bore in the screw head to the rear end face of the sealing element, this space being sealed from the accumulator space and the screw flights.
Backflowseals of the type described above are
known (compare West German Austegeschnft 1,201,039 and West German Offenlegungsschrift 2,260,345). In these known backflow seals the sealing element is formed by an approximately mushroom-shaped body, of which the rearwardextending shaft is slidably guided in the axial bore of the screw head. The sealing element can be guided with its "mushroom head" forming a seal against the inner wall of the injection cylinder, and can contain the passages mentioned, and optionally comprises two parts (German Auslegeschrift 1,201,039) or one part only, the passages advantageously being present in the screw head which forms the seal against the inner wall of the injection cylinder (West German Offenlegungsschrift 2,260,345).In both cases, the shaft which extends through the axial bore carries, on its rear end, a spring seat on which seats a pressure spring, the other end of which rests against the screw head. The pressure spring is pre-stressed so that it keeps the sealing element constantly pressed into the sealing position on the sealing seat. The sealing element is pressed into the open position only during the plasticising stage, namely when as a result of the rotation of the screw, plastics material is conveyed forwards through the screw flights, and as a result a pressure is build up and lifts the sealing element off the sealing face against the action of the pressure spring.
It is true that the above construction ensures that the sealing element already produces a seal, under the influence of the pressure spring, at the end of the plasticising stage, when the conveying pressure of the plastic material drops, instead of only coming into operation at the beginning of the subsequent injection stroke, due to flow processes as is the case with backflow seals which employ pressure rings. As a result, substantially more precise setting and reproducibility of the shot weight can be achieved.
However, it is a disadvantage that the opening stroke of the sealing element is determined by the foot of the pressure spring, by the viscosity of the plastic
material and by the rate of flow of the plastic
material through the passages, and therefore cannot
be externally infuenced, that the pressure in the
plastic material present in the accumulator space is necessary substantialy lower than at the front end fo the screw flights, because this pressure has to overcome the force of the pressure spring in order open the sealing element, and that employing a spring to keep the element closed entails additional design effort and manufacturing costs.
According to the present invention, there is provided injection moulding apparatus for plastics materials, the apparatus comprising: a screw; an accumulator space; a screw head between the screw and the accumulator space and having an axial bore; and a sealing element which is guided in the axial bore of the screw head and is axially slidable between a sealing position and an open position in front of a sealing surface of the screw head, the sealing element, in its open position in which it is lifted off the sealing surface, releasing passages formed in the sealing element and/or in the screw head, which passages connect the accumulator space in front of the screw with the screw flights, there being a space formed at the rear end of the axial bore in the screw head and associated with the upstream end face of the sealing element, the space being sealed from the accumulator space and the screw; in which the sealing element is arranged to be freely slidable in the axial bore.
It has been found that merely by omitting the spring mechanism for holding the seal closed, so that the sealing element is freely slidable in the axial bore between its sealing position and its open position, all the disadvantages mentioned can be overcome without in any way impairing the desired functioning of the sealing element. Thus, in particular, the construction ensures, just as before, that at the end of the metering stroke the sealing element is already forced into its sealing position, so that during the subsequent injection stroke no backflow losses whatsoever occur.
As has been found, at the end of the plasticising stage the sealing element is pushed into the sealing position merely by the plastic mass present in the accumulator space. This is due to the fact that at the end of the metering stroke, when the "backpressure" exerted by the injection plunger of the screw, against which the screw moves back during the plasticising stage, is reduced to zero, the plastics melt present in the accumulator space is correspondingly relieved of pressure and, by virtue of its compressibility, abruptly expands. As a result of this expanion process, the screw is usually pushed back a short distance further.In the invention, this expansion process is utilised to bring the sealing element into the sealing position, which is possible because of the fact that in the open position of the sealing element a pressure force acts rearwardly on this element. the force being calculable from the shaft cross-section of the shaft guided in the axial bore and the pressure diference between the accumulator space and the space located behind the axial bore. Since the presssre in the space located behind the axial bore can be kept at virtually as low a value as desired and since, as a result, the pressure difference is considerable, the sealing element which is of substantially lower weight than the screw, is pushed into the sealing position before the screw itself can react.
In the plasticising stage, the sealing element is pushed into the open position, a a result of the forward-conveyed plastic mass, until it strikes a stop, and remains there during the plasticisation, with the plastics mass flowing round the sealing element experiencing a flow resistance as a result of which a certain pressure gradient is set up over the flow gap released by the sealing element. Because of this pressure gradient, the return force which acts on the sealing element, and which can be calculated from the pressure prevailing in the accumulator space multiplied by the shaft cross-section of the sealing element shaft, is overcome. This can be achieved for all pressure conditions encountered, by choosing an appropriate dimension of the shaft cross-section.At the end of the screw rotation, the flow of melt into the accumulator space is also interrupted, as a result of which the presssure gradient built up over the flow gap disappears and hence pressure equalisation around the sealing element occurs. As this point in time, the pressure built up in the accumulator space as a result of the back-pressure acting on the screw has not yet bee lost via the screw as a result of compression relief, so that the return force comes into action on the sealing element adn slides the latter into the sealing position.
The space located behind the axial bore can, for example, be vented to the atmosphere through an axial bore passing completely through the screw, so that atmospheric pressure constantly prevails in this space.However, it is possible to dispense with such an axial bore and have a space of closed construction, since the pressure fluctuations occurring in this space during the movements of the sealing element into the open position or sealing position can be kept negligibly small by choosing an appropriate size of space. On the contrary, it is even conceivable to utilise a slight vacuum, occurring in the open positon of the sealing element, to return the sealing element into the sealing position.
The invention will be further described with reference to the accompanying drawings, in which:
Figure 1 shows a schematic axial section through the injection cylinder of an injection moulding machine embodying the present invention; and
Figure 2 shows an axial section, on a larger scale, through the injection cylinder and the screw in the region of the screw head, from which figure the backflow seal can be seen more clearly.
An injection moulding machine, for, for example, thermoplastics, in which the present backflow seal is employed possesses an injection cylinder 1 on which a screw 2 is rotatably and axially slidably arranged. Plastic granules are introduced via a hopper 3. A rotational drive 4 and an injection plunger 5, to which a pressure medium can be applied, are only indicated at the rear end of the screw. Pressure medium can be applied by means of pump P, via a line 7, to the pressure space 6, in which the injection piston 5 is movable.A connection to the tanks can be produced via a line 8 in which a pressure-regulated valve 9 is located, and by means of the latter the outflow of pressure medium from the pressure space 6 can be throttled so that a predeterminable pressure builds up in this space.
These details are known to an expert in injection moulding and therefore do not require more detailed explanation here.
As can be seen from Figure 2, the screw head of the screw 2 is formed by an insert 10 which is sealingly guided on the inner wall of the injection culinder 1 and possesses passages 11. The insert 10 is screwed into an axial threaded bore 13 of the screw 2 and additionally has a straight-through axial bore 14 which serves to guide a sealing element 15 in a ieaktight manner. The axial bore 14 in the screw 2 is continued rearwards beyond the end of the insert 10 and forms a space 16 which is completely sealed from the exterior so that plastic mass cannot enter it.
The sealing element 15, which is approximately mushroom-shaped, possesses, at its head 17, a rearward-pointing conical surface 18, which cooperates with a sealing surface 19 of complementary shape to the conical surface, the sealing surface being formed by widening of the axail bore 14. The passages 11 terminate upstream of the sealing surface 19, in a small space 20.
The shaft of the sealing element 15, guided in the axial bore 14, is provided with a series of leakage grooves 21 and possesses, at its rear end, an external thread onto which a stop nut 22 is screwed.
The stop nut 22 is secured by a pin 23. The stroke of the sealing element between the open position and the sealing position is determined by the sealing surface 19, on the one hand, and the stop nut 22, on the other hand. The stroke h of the sealing element can be altered by means of the stop nut 22, which can also consist of two lock nuts. It is therefore possible to suit the stroke h to the particular plastics to be processed and even, by appropriately choosing the stroke h to introduce, in a controlled manner, additional friction into the plastics mass so that as a result a particular viscosity is achievable immediately before the plastic mass enters the accumulator space.
In the forward direction, the head 17 of the sealing element 15 is in the shape of a pointed cone, so as to offer as little resistance as possible to the plastic flow.
The mode of action of the backflow seal is as follows: during the injection stroke, in which plastics material is expelled through the nozzle of the injection cylinder 1, the sealing element 15 is in the sealing position shown in Figure 2. Following the injection stroke, the rotational drive 4 for the screw 2 is switched on, so that the screw 2 conveys plastics granules, supplied from the hopper 3, forwards in the screw flights and in doing so plasticisesthe granules. The plasticisation process can, if desired, be assisted by heating, not shown in the drawing, of the injection cylinder 1. The plasticised plastics if forced through the passages 11 into the space 20 and as a result slides the sealing element 15 into the open position, not shown in the drawing, in which the stop nut 22 rests against the rear end face of the insert 10.This sliding motion is resisted bythe sealing element 15 through the frictional forces arising in the axial bore, through the return force which can be calculated from the pressure in the accumulator space and the shaft cross-section, and through virtually negligibly small suction forces resulting from an increase in volume in the space 16.
Accordingly, in the open position, the plastics material can pass into the accumulator space 12 through the passages 11 and the conical annular gap which has formed between the surfaces 18 and 19. During the plasticising stage, the pressure space 6 is connected to the tank T via the line 8, and at the pressure-regulating valve 9, called into play at this stage, a "back-pressure" is set, against which the screw 2 conveys the plastics into the accumulator space 12, and by virtue of which back-pressure a correspondngly high pressure builds up in the plastics present in the accumulator space.
At the end of the plasticising stage, when the screw 2 has completed its metering stroke, the rotational drive 4 is switched off and at the same time the pressure space 6 is directly switched to the tank T, with by-passing of the pressure-regulating valve 9, so that the pressure in the pressure space 6 drops abruptly. The switching-off of the rotational drive 4 first of all interrupts the flow of melt, the pressure gradient at the conical annular gap 18, 19 disappears and as a result the opening force acting on the sealing element disappears. Due to the pressure drop in the pressure space 6, the pressure exerted by the screw on the plastics mass present in the accumulator space 12 disappears, so that the plastics mass can expand abruptly in accordance with its compressibility.
As a result of the pressure drop and the expansion, the sealing element 15, which is substantially lighter than the screw 2, is pushed back in the axial bore 14, into the sealing position. Hence, during the subsequent renewed injection stroke, no backflow whatsoever of plastics mass present in the accumulator space 12 into the screw flights is to be expected.
Claims (4)
1. Injection moulding apparatus for plastics materials, the apparatus comprising: a screw; an accumulator space; a screw head between the screw and the accumulator space and having an axial bore; and a sealing element which is guided in the axial bore of the screw head and is axially slidable between a sealing position and an open position in front of a sealing surface of the screw head, the sealing element, it its open position in which it is
lifted off the sealing surface, releasing passages formed in the sealing element and/or in the screw
head, which passages connect the accumulator space in front of the screw with the screw flights, there being a space formed at the rear end of the
axial bore in the screw head and associated with the
upstream end face of the sealing element, the space
being sealed from the accumulator space and the screw; in which the sealing element is arranged to be freely slidable in the axial bore.
2. Apparatus according to Claim 1, wherein the sealing element possesses, at upstream end, a stop for determining its open position.
3. Apparatus according to Claim 1 or 2, wherein the space associated with the upstream of the axial bore is closed.
4. Injection moulding apparatus constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19782836165 DE2836165A1 (en) | 1978-08-18 | 1978-08-18 | BACKFLOW BLOCK FOR SNAIL INJECTION MOLDING MACHINES |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2028216A true GB2028216A (en) | 1980-03-05 |
Family
ID=6047349
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB7927588A Withdrawn GB2028216A (en) | 1978-08-18 | 1979-08-08 | Backflow seal for screw injection moulding machines |
Country Status (4)
| Country | Link |
|---|---|
| DE (1) | DE2836165A1 (en) |
| FR (1) | FR2433409A1 (en) |
| GB (1) | GB2028216A (en) |
| IT (1) | IT7950007A0 (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0323556A2 (en) * | 1988-01-07 | 1989-07-12 | Cincinnati Milacron Inc. | Injection moulding apparatus |
| US5044926A (en) * | 1990-06-25 | 1991-09-03 | Cincinnati Milacron Inc. | Anti-backflow valve for injection molding machines |
| EP0541048A1 (en) * | 1991-11-08 | 1993-05-12 | Spirex Corporation | Plastic injection moulding machine with automatic shut-off valve |
| EP0590037A4 (en) * | 1991-05-13 | 1994-02-16 | Robert F Dray | Positive-type non-return valve. |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3240872C2 (en) * | 1982-11-05 | 1986-10-09 | Erich 7141 Steinheim Blattert | Non-return valve for a screw injection molding machine |
| DE3814673C2 (en) * | 1988-04-30 | 1994-05-05 | Krauss Maffei Ag | Non-return valve for injection molding machines |
| CN110509493B (en) * | 2019-09-10 | 2021-10-29 | 张弛 | Oil leakage prevention device of injection molding machine |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1201039B (en) * | 1964-01-21 | 1965-09-16 | Reifenhaeuser Kg | Backflow stop provided with an axially displaceable ring body on the screw head of screw injection molding machines for thermoplastic plastics |
| DE2515530A1 (en) * | 1975-04-09 | 1976-10-21 | Froendenberger Masch App Gmbh | Plastics injection moulding plasticizing cylinder - with centering ring screwed to screw point and rotatable or sliding non-return element |
| FR2363429A1 (en) * | 1976-09-07 | 1978-03-31 | Manceau Marcel | Screw head valve for screw injection moulding machine - to prevent melt reverse flow during injection |
-
1978
- 1978-08-18 DE DE19782836165 patent/DE2836165A1/en not_active Withdrawn
-
1979
- 1979-08-08 GB GB7927588A patent/GB2028216A/en not_active Withdrawn
- 1979-08-08 IT IT7950007A patent/IT7950007A0/en unknown
- 1979-08-17 FR FR7920904A patent/FR2433409A1/en not_active Withdrawn
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0323556A2 (en) * | 1988-01-07 | 1989-07-12 | Cincinnati Milacron Inc. | Injection moulding apparatus |
| EP0323556A3 (en) * | 1988-01-07 | 1990-05-09 | Cincinnati Milacron Inc. | Injection moulding apparatus |
| US5044926A (en) * | 1990-06-25 | 1991-09-03 | Cincinnati Milacron Inc. | Anti-backflow valve for injection molding machines |
| EP0590037A4 (en) * | 1991-05-13 | 1994-02-16 | Robert F Dray | Positive-type non-return valve. |
| EP0590037A1 (en) * | 1991-05-13 | 1994-04-06 | DRAY, Robert F. | Positive-type non-return valve |
| EP0541048A1 (en) * | 1991-11-08 | 1993-05-12 | Spirex Corporation | Plastic injection moulding machine with automatic shut-off valve |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2433409A1 (en) | 1980-03-14 |
| DE2836165A1 (en) | 1980-03-06 |
| IT7950007A0 (en) | 1979-08-08 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |