WO2004082921A1 - Procede et appareil de moulage par injection avec assistance gazeuse et expulsion de matiere plastique - Google Patents

Procede et appareil de moulage par injection avec assistance gazeuse et expulsion de matiere plastique Download PDF

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Publication number
WO2004082921A1
WO2004082921A1 PCT/GB2004/001255 GB2004001255W WO2004082921A1 WO 2004082921 A1 WO2004082921 A1 WO 2004082921A1 GB 2004001255 W GB2004001255 W GB 2004001255W WO 2004082921 A1 WO2004082921 A1 WO 2004082921A1
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WO
WIPO (PCT)
Prior art keywords
mould cavity
plastics
plastics material
pressure
cavity
Prior art date
Application number
PCT/GB2004/001255
Other languages
English (en)
Inventor
Terence Colwyn Pearson
Original Assignee
Cinpres Gas Injection Limited
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Filing date
Publication date
Application filed by Cinpres Gas Injection Limited filed Critical Cinpres Gas Injection Limited
Publication of WO2004082921A1 publication Critical patent/WO2004082921A1/fr

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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
    • B29C45/1703—Introducing an auxiliary fluid into the mould
    • B29C45/1704—Introducing an auxiliary fluid into the mould the fluid being introduced into the interior of the injected material which is still in a molten state, e.g. for producing hollow articles
    • B29C45/1711—Introducing an auxiliary fluid into the mould the fluid being introduced into the interior of the injected material which is still in a molten state, e.g. for producing hollow articles and removing excess material from the mould cavity by the introduced fluid, e.g. to an overflow cavity
    • 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
    • B29C45/1703—Introducing an auxiliary fluid into the mould
    • B29C45/1732—Control circuits therefor
    • 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
    • B29C45/76—Measuring, controlling or regulating
    • B29C45/762—Measuring, controlling or regulating the sequence of operations of an injection cycle
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76003—Measured parameter
    • B29C2945/76006—Pressure
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76003—Measured parameter
    • B29C2945/76083—Position
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76003—Measured parameter
    • B29C2945/7611—Velocity
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76177—Location of measurement
    • B29C2945/7618—Injection unit
    • B29C2945/76187—Injection unit screw
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76177—Location of measurement
    • B29C2945/7618—Injection unit
    • B29C2945/762—Injection unit injection piston
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76344—Phase or stage of measurement
    • B29C2945/76384—Holding, dwelling
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76494—Controlled parameter
    • B29C2945/76498—Pressure
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76494—Controlled parameter
    • B29C2945/76545—Flow rate
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76494—Controlled parameter
    • B29C2945/76551—Time
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76494—Controlled parameter
    • B29C2945/76585—Dimensions, e.g. thickness
    • B29C2945/76591—Dimensions, e.g. thickness volume
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76655—Location of control
    • B29C2945/76732—Mould
    • B29C2945/76752—Mould runners, nozzles
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76655—Location of control
    • B29C2945/76765—Moulding material
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76655—Location of control
    • B29C2945/76775—Fluids
    • 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
    • B29C2945/00—Indexing scheme relating to injection moulding, i.e. forcing the required volume of moulding material through a nozzle into a closed mould
    • B29C2945/76—Measuring, controlling or regulating
    • B29C2945/76929—Controlling method
    • B29C2945/76939—Using stored or historical data sets
    • B29C2945/76943—Using stored or historical data sets compare with thresholds

Definitions

  • the present invention relates to a process and apparatus for injection moulding hollow plastics articles.
  • Injection moulding of plastics materials is well known and widely practised as a means of manufacturing an ever- increasing diversity of plastics components for industrial and consumer use.
  • versions of the process globally referred to as "gas assisted injection moulding" have been developed and used to overcome some of the problems inherent in conventional injection moulding, and to reduce costs and improve the quality of the final products .
  • a gas such as nitrogen
  • the low viscosity gas flows into the paths of least resistance within the more viscous plastics, thereby forming hollow channels within the plastics.
  • the process is particularly beneficial for thick section mouldings, such as handles, and weight savings of up to 45% or more can result. Also, the moulding time cycles can be substantially reduced.
  • the injected gas tends to flow into the thicker sections, again forming hollow continuous channels through which pressure may be transmitted through the medium of the gas .
  • the gas provides a medium for transmitting pressure to the plastics as it cools and solidifies, instead of the moulding machine which applies the pressure in conventional injection moulding.
  • a gas pressure is applied at the exit from the gas controller, a similar pressure is applied virtually instantaneously throughout the length of the continuous gas stream or channel . Therefore the gas enables a similar internal pressure to be exerted throughout the moulding.
  • the pressure is transmitted more uniformly and efficiently and therefore lower pressures can be used than in conventional injection moulding. This adds to the scope of the designer and removes some of the design restrictions of conventional injection moulding.
  • One known gas assisted plastics moulding process employs filling the mould cavity with plastics material
  • shut-off valve is provided in a runner connecting the mould cavity and a secondary cavity.
  • the valve may be opened by a pressure of injection of the gas, the gas being injected whilst resin is expelled from the mould cavity into the secondary cavity.
  • a further known process allows a portion of the molten plastics material to be expelled from the mould cavity into at least one secondary cavity through a valve controlled runner.
  • the mould cavity With the valve closed, the mould cavity is filled with molten plastics material.
  • Pressurised gas is injected into the plastics in the mould cavity with the valve maintained closed, whereby the pressure of the gas on the plastics in the mould cavity can be held for a period of time in order to apply a packing pressure to the plastics material to be expelled into the or each secondary cavity.
  • the valve is opened to allow a portion of the molten plastics in the mould cavity to be expelled into the secondary cavity.
  • the remaining plastics material in the mould cavity is permitted to solidify before exhausting the gas from the mould cavity.
  • the mould is then opened and the article removed.
  • a principal advantage of the above process is that the provision of the valve in the runner between the mould cavity and the secondary cavity allows the plastics to be pressurised, immediately after filling the mould cavity, with pressure applied by the moulding machine. Increasing pressure within the plastics serves to obtain a good replication of the mould cavity surface onto the surface of the article.
  • a process for injection moulding a hollow plastics article comprising the steps of: injecting a quantity of molten plastics material into a mould cavity to fill fully the mould cavity; injecting pressurised fluid into the plastics material in the mould cavity; opening at least one valve to allow a portion of the molten plastics material in the mould cavity to be expelled into at least one secondary cavity coupled to the mould cavity, sufficient plastics material being expelled to fill the at least one secondary cavity; permitting the remaining plastics material in the mould cavity to solidify; exhausting the fluid from the mould cavity; and removing the plastics article from the mould; the process including determining when the step of filling full the mould cavity is at or near to completion, and initiating the step of injecting pressurised fluid in response to that determination.
  • completion or near completion of the filling of the mould cavity is determined when a selected plastics pressure is reached during or after the injection of the plastics material, and the selected pressure is employed to initiate the step of injection of pressurised fluid.
  • completion or near completion of the filling of the mould cavity is determined when a piston or screw for injecting the plastics material into the mould cavity reaches a selected position, and the selected position is employed to initiate the step of injection of pressurised fluid.
  • the process preferably includes the step of holding the pressure of the fluid on the plastics material in the mould cavity to apply a packing pressure to the plastics material, before opening the at least one valve to allow a portion of the molten plastics material in the mould cavity to be expelled into the at least one secondary cavity.
  • the process includes determining a selected pressure of the pressurised fluid, and opening the valve which allows a portion of the molten plastics material in the mould to be expelled into the at least one secondary cavity in accordance with that determination.
  • the invention also provides an apparatus for injection moulding a hollow plastics article comprising: a mould with a mould cavity therein defining at least a portion of the article; means for injecting a quantity of molten plastics material into the mould cavity to fill fully the mould cavity; means for injecting pressurised fluid into the plastics material in the mould cavity; at least one secondary cavity coupled to the mould cavity by at least one valve controlled connection whereby when the valve means in the at least one connection is closed, no plastics or fluid can flow into the at least one secondary cavity, but with the valve means open a portion of the molten plastics in the mould is allowed to be expelled into the at least one secondary cavity, and control means for opening the valve means to allow a portion of the molten plastics material in the mould cavity to be expelled into the at least one secondary cavity, prior to the remaining plastics material in the mould cavity solidifying and the fluid being exhausted from the mould cavity to allow the removal of the plastics from the article from the mould; the apparatus including means for determining when the step of filling full the mould cavity
  • monitoring means for determining when a selected plastics pressure is reached during or after the filling step is complete or near to completion, and for initiating the means for injecting the pressurised fluid into the plastics material in the mould cavity.
  • a piston or screw is provided for injecting plastics material into the mould cavity. and positional sensing means are provided which monitor the movement of the piston or screw, the sensing means determining the position of the piston or screw to indicate that the step of filling the mould cavity is complete or near to completion, and activating the means for injecting the pressurised fluid.
  • the apparatus preferably includes means for opening the valve means which are activated by a selected pressure of the pressurised fluid.
  • control means are adapted to hold the pressure of the fluid on the plastics material in the mould cavity to apply a packing pressure to the plastics material, before opening the valve to allow a portion of the molten plastics material in the mould to be expelled into the secondary cavity.
  • Figures 1A - IE illustrate the sequence of steps of a first embodiment
  • Figure 2 is a pressure/time graph of the first embodiment, in which a selected plastics pressure initiates gas injection and a selected gas pressure activates opening of the valve between the mould cavity and the secondary cavity;
  • Figures 3A - 3E illustrate the sequence of steps of a second embodiment
  • Figure 4 is a pressure/time graph of the second embodiment in which a selected position of the plastics injection piston or screw initiates gas injection and a selected gas pressure activates opening of the valve between the mould cavity and the secondary cavity;
  • Figures 5A and 5B illustrate an alternative arrangement of the secondary cavity, showing the valve member of the valve between the mould cavity and the secondary cavity in its valve closed and valve open positions.
  • Figures 1A - IE illustrate the sequence steps of a first embodiment of the process referred to generally by the reference numeral 10.
  • a quantity of molten plastics material 20 is positioned in the barrel or cylinder 21 of an injection moulding machine.
  • the injection moulding machine can be of any conventional type and can expel the plastics material from the nozzle 22 of the cylinder 21 in any conventional manner, such as by a helical screw or a piston or ram 23, as illustrated.
  • the nozzle 22 of the injection moulding machine is coupled via a sprue or runner 24 to a mould cavity 25.
  • the mould cavity 25 is positioned in a conventional- mould which in turn is mounted in a conventional moulding machine.
  • the mould cavity 25 represents a thicker section of the article to be moulded.
  • a secondary cavity 26 is also provided within the mould in communication with the mould cavity 25, and coupled to the mould cavity by a flow runner or conduit 27.
  • a valve member 28 is positioned in the runner 24 between the nozzle 22 and the mould cavity 25, while a second valve member 29 is positioned in the flow runner or conduit 27 leading to the secondary cavity 26.
  • the valve members 28, 29 can be of any conventional type, but in this embodiment are shut-off type valve members, and can be operated in any conventional manner, such as pneumatically, hydraulically or electrically.
  • the quantity of plastics material in the cylinder 21 of the injection moulding machine is sufficient to fill fully the volume of the mould cavity 25.
  • the plastics material 20 may be of any conventional type such as a thermoplastics or thermosetting resin and may be a filled material, that is mixed with a glass or mineral material.
  • the second step in the process is shown in Figure IB.
  • the plastics material 20 is injected by the piston 23 into the mould cavity 25 to fill fully the mould cavity.
  • valve member 28 of the valve in the sprue or runner 24 is opened, but valve member 29 of the valve in the runner 27 is maintained closed in order to prevent any plastics from flowing out of the mould cavity 25 into the secondary cavity 26.
  • the plastics pressure is maintained for a short period of time, for example, 1 - 5 seconds. This assists to ensure good reproducibility of the mould surface on the moulded article in the mould cavity 25.
  • a signal representing the selected plastics pressure initiates the commencement of the gas cycle in which pressurised gas, e.g. nitrogen, is injected through inlet conduit 30.
  • pressurised gas e.g. nitrogen
  • the signal representing the selected plastics pressure can be applied to initiate the step of injecting the pressurised gas either before the completion of the step of filling the mould cavity with plastics material, or on completion of filling the mould cavity, or after the completion of filling the mould cavity, as in this embodiment.
  • the gas pressure needs to overcome the plastics pressure before it can penetrate the plastics and begin creating a hollow in the plastics material.
  • the mould cavity will be filled fully with plastics material before the commencement of gas injection into the plastics material .
  • the signal of plastics pressure in the mould may be picked up at a variety of different positions.
  • the plastics pressure signal may be taken from sensing means which act as monitoring means within the moulding machine.
  • the sensing means may include a pressure transducer or other sensor in the plastics extruder of the moulding machine.
  • the plastics pressure signal is taken from a pressure transducer or other sensor positioned in the runner 24 to the mould cavity or in the mould cavity 25, for example, at a position near to the valve member 29 controlling the outflow through runner 27 to the secondary cavity 26.
  • an in-mould pressure transducer or other sensor is located inside the valve member 29 of the valve controlling the outflow of plastics through the runner 27 to the secondary cavity 26.
  • the valve member 29 is shown in its valve closed position and in Figure 5B the valve member 29 is in its valve open position.
  • the valve member 29 acts in line with the runner 27, and the pressure transducer or other sensor (not shown) is flush with the end of the valve member 29 adjacent the mould cavity 25.
  • a signal corresponding to the completion or near completion of the filling of the mould may be taken from a predetermined pressure of a further pressurised medium employed to generate the supply of pressurised plastics material.
  • the further pressurised medium may be in an hydraulic or pneumatic circuit for operating the plastics extruder.
  • the signal taken from the hydraulic or other fluid is employed to initiate the commencement of gas injection.
  • the plastics extruder may have an electric drive.
  • the monitoring means are in the electric drive, and a predetermined power demand of the electric drive, corresponding to the completion or near completion of the filling of the mould cavity, may be employed to initiate the commencement of gas injection.
  • microprocessor having a software programme for controlling the monitoring means of the particular embodiment.
  • the valve member 29 Initially, on gas beginning to enter the plastics in the mould cavity 25, the valve member 29 remains closed until receiving a signal activated by a selected pressure of the gas.
  • the pressure of the gas creates a hollow cavity in the molten material and applies an internal pressure to the plastics which forces the plastics against the inner wall surfaces of the mould cavity 25 until an outer skin of the plastics has solidified, whilst a core portion of the plastics remains molten, thereby assisting to provide a good surface finish on the article.
  • the nozzle valve member 28 is closed and the secondary cavity valve member 29 is opened.
  • the selected pressure of the gas 31 in the molten plastics material in the mould cavity 25 activates the opening of the valve member 29, which allows the gas to expel a portion of the plastics into the secondary cavity 26 through the runner 27.
  • the portion expelled is from the more fluent core of plastics in the thicker section or sections in the article.
  • valve member 29 is activated by a signal from monitoring means in the circuit supplying pressurised gas to the mould cavity 25.
  • the monitoring means may be a pressure sensor, for example, a pressure transducer.
  • the amount of plastics expelled is sufficient to fill the secondary cavity 26.
  • the volume of the secondary cavity 26 is predetermined so that it is of an appropriate size to accept and be filled by the amount of plastics 32 which is required to be expelled to ensure that the gas path within the plastics extends throughout the thicker section or sections of the article. Adjustment of the size of the secondary cavity 26 is performed during mould test trials, and may be achieved by metal machining or removal from the mould to increase the volume, or by the addition of metal to reduce the volume.
  • the gas 31 is exhausted, for example, back through conduit 30. This is shown in Figure IE.
  • the expelled gas can be collected and reclaimed for further use, but is generally expelled to atmosphere.
  • Mechanisms and systems for venting the gas are well known in gas assisted injection moulding, and a conventional mechanism or system may be used.
  • the piston 23 of the moulding machine is typically retracted to its rest position, as illustrated, in preparation for the next moulding cycle .
  • the mould is then opened and the moulded article is ejected or otherwise removed from the mould cavity 25.
  • the expelled plastics 32 is similarly ejected or removed from the secondary cavity 26.
  • the plastics 32 in the secondary cavity 26 can be re-ground for further use, or the secondary cavity can be used to form another plastics article.
  • FIG. 2 A graph illustrating the sequence of steps described above is shown in Figure 2.
  • the graph charts the moulding cycle time against the plastics pressure and the gas pressure in the mould.
  • the plastics pressure 34 initially increases as the plastics 20 is injected into the mould cavity 25.
  • the plastics pressure increases further until it reaches a selected pressure 35.
  • the gas injection cycle is activated to commence.
  • the plastics pressure continues along the path as indicated in the graph.
  • the process involves determining when the step of filling the mould cavity with plastics material has been completed, and initiating injection of the pressurised gas in response to that determination.
  • the process may involve determining when the step of filling the mould cavity with plastics material is near to completion, whether that is before or after completion of the filling step, and then initiating the step of injecting the gas in response to that determination.
  • the gas pressure 36 increases at a selected rate or ramp.
  • the gas pressure exceeds the plastics pressure, the gas begins to create a hollow in the plastics in the mould cavity 25 and, in this embodiment, applies an internal packing or hold pressure indicated by the line 37.
  • the gas pressure is again increased to a predetermined pressure 38 which activates the opening of the valve member 29 in the runner 27 connecting the mould cavity 25 and the secondary cavity 26.
  • the gas pressure is raised further, as illustrated, whilst the gas expels a portion of the plastics 32 from the centre of the thicker section of the article through the runner 27 to fill the secondary cavity 26.
  • the secondary cavity 26 has a volume predetermined to receive the required amount of plastics which allows the gas to extend throughout the thicker section.
  • Gas "hold-on" or “packing” pressure 39 is maintained in the mould cavity 25 either at a constant pressure or first constant and then at a reduced pressure, as illustrated, whilst the remaining plastics in the mould cavity cools and solidifies. This may take a period of 10 to 25 seconds.
  • the gas pressure then reduces substantially as shown by line 40 to atmospheric pressure as the gas 31 is vented from the mould cavity through conduit 30, prior to the mould being opened to allow the article to be ejected or otherwise removed from the mould.
  • the moulding machine is first programmed to inject sufficient plastics to fill the mould cavity 25, and then to further increase the pressure within the plastics to obtain good replication of the mould cavity surface on the surface of the article.
  • Plastics pressure continues to rise to a programmed maximum. On reaching the maximum, or preferably at a preselected intermediate pressure 35, a signal is relayed to the gas controller to commence the introduction of gas into the mould, in the manner explained above. The gas pressure continues to rise until it exceeds the plastics pressure, thereby initiating penetration of the gas into the still molten plastics to create a hollow. This is possible due to the volumetric shrinkage of the plastics as it begins to cool. The gas pressure is programmed to continue to rise and thereby applies an internal pressure to the plastics.
  • Signalling the start of introducing the gas by a specified plastics pressure, whether that is before, at, or after the mould cavity is filled with plastics material, is more precise and repeatable than signalling by timing.
  • the above process is very effective in moulding articles with an excellent surface finish and a consistent expulsion of plastics into the secondary cavity 26, the secondary cavity having a pre-selected volume which determines the amount of plastics expelled.
  • the process is repeatable and consistent in operation.
  • a selected position of the piston or ram 23 or moulding machine injection screw during the forward movement of the piston or screw which is its direction of movement during the injection of plastics into the mould.
  • the selected position is at or close to its forward position. It can be achieved by employing a software programme which accepts one or more signals from a positional sensor which, in this embodiment, is a linear displacement transducer (LDT)40.
  • LDT linear displacement transducer
  • the transducer conveniently comprises an electric coil 42, which is mounted longitudinally on the cylinder 21 of the injection moulding machine, and a moving core 43, which is mechanically linked (shown diagra matically as link 44) to the piston rod 45 of the piston 23 or to the screw, if provided, for injecting plastics material 20 into the mould cavity 25.
  • the resistance of the core 43 moving through the coil 42 monitors the forward movement of the piston 23 or screw, and gives a signal which determines the selected position of the piston 23.or screw within the cylinder 23, and activates the gas controller equipment according to the selected position of the piston or screw to commence the gas cycle.
  • the transducer 40 may be used to signal directly the start of gas injection when the plastics has filled the mould cavity 25, or just before the completion of filling occurs (as illustrated by line 41 in the graph of Figure 4) , or after filling and as the plastics starts to cool and shrink when the piston or moulding machine screw continues to move slowly forward.
  • selecting a position of the transducer movement eliminates the need for a timer, and can be more precise in interpreting the operation of the process compared with the use of a delay timer.
  • the graph of Figure 4 is similar to the graph of Figure 2 except that the gas injection cycle is signalled to commence by the linear displacement transducer 40 just before the end of the forward stroke of the piston 23 or extruder screw completes the filling of the mould cavity 25 with plastics material 20, as indicated by line 41. Thereafter, the plastics pressure continues along the path indicated in the graph. At the same time, the gas pressure increases at a selected rate until it exceeds the plastics pressure. The gas can then penetrate the plastics and begin to create a hollow in the plastics. The gas applies an internal packing or hold pressure indicated by the line 37. A further rise in gas pressure activates the opening of the valve member 29, which allows a portion of the plastics 32 to be expelled to fill the secondary cavity 26, and the gas path to extend throughout the thicker section. Gas pressure is maintained during the cooling stage, which is then followed by venting of the gas 31, before opening the mould to remove the article.
  • the gas or fluid injection cycle is commenced when the mould cavity 25 is full or nearly full of plastics material.
  • the skilled person will appreciate that the pressure of the fluid to be injected into the plastics material may be built up before commencement of the injection cycle to minimise or avoid any delay between commencement of the fluid injection cycle and the pressure of the fluid exceeding that of the plastics material in the mould cavity 25.
  • the pressurisation of the fluid may commence when a predetermined quantity of plastics material has been introduced into the mould cavity 25. For example, when the mould cavity 25 is 80% full of plastics material the pressurisation of the fluid may commence.
  • the mould cavity 25 may be less than 80% full of plastics material when the pressurisation commences.
  • the pressure of the fluid to be injected may therefore build up while more plastics material is injected into the mould cavity 25.
  • the pressurised fluid only enters the plastics material when its pressure exceeds that of the plastics material.
  • the pressure of the fluid to be injected preferably exceeds the pressure of the plastics material only after the moulding cavity 25 is full or nearly full of plastics material; the quantity of plastics material injected into the moulding cavity before the fluid pressurisation cycle is initiated is preferably selected to ensure that this is the case.
  • a valve (not shown) may be provided in the inlet conduit 30 so that the pressure of the fluid to be injected may exceed that of the plastics material in the mould cavity 25.
  • the valve may be opened to facilitate injection of the fluid only when the mould cavity is full or nearly full of plastics material.
  • the means for determining when the mould cavity 25 has been filled by a predetermined quantity of plastics material to determine when to commence pressurisation of the fluid to be injected into the plastics material may operate in the same way as the means for determining when the mould cavity 25 is full or nearly full of plastics material.
  • a pressure sensing means, a timer, or a positional sensing means may determine when the predetermined quantity of plastics material has been injected into the moulding cavity 25 and, thus, when pressurisation of the fluid commences.
  • the same sensing or timing means may determine when a predetermined quantity of plastics material has been introduced into the mould cavity 25 to commence pressurisation of the fluid to be injected into the plastics material , and subsequently determine when the mould cavity is full or nearly full of plastics material to initiate injection of the pressurised fluid.
  • a pressurised liquid e.g. water
  • a pressurised fluid instead of a pressurised gas.
  • each connecting runner 27 may also be more than one secondary cavity 26 connected to the mould cavity 25, each connecting runner 27 containing a valve member 29, which is actuated to open by a specified pressure 38 of the pressurised fluid within the plastics in the mould cavity 25. It will also be appreciated that a plurality of connecting runners 27 may connect the mould cavity 25 to each secondary cavity 26.

Landscapes

  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Mechanical Engineering (AREA)
  • Moulds For Moulding Plastics Or The Like (AREA)
  • Injection Moulding Of Plastics Or The Like (AREA)

Abstract

La présente invention concerne un procédé de moulage par injection d'articles creux en plastique. Ce procédé comporte une opération par laquelle on injecte dans une cavité de moule (25) une quantité donnée de matière plastique en fusion (20) de façon à combler la cavité du moule. Cette opération est suivie de l'injection d'un fluide pressurisé (31) dans la matière plastique à l'intérieur de la cavité du moule. L'ouverture d'au moins un clapet (29) permet à une partie de la matière plastique en fusion d'être chassée de la cavité du moule pour arriver dans au moins une cavité secondaire (26) couplée à la cavité du moule, une quantité suffisante de matière plastique étant chassée jusqu'à combler l'une au moins des cavités secondaires. On laisse prendre le reste de la matière plastique dans la cavité du moule, après quoi le fluide est évacué de cette même cavité. On retire ensuite du moule l'article en plastique. Ce procédé implique, d'abord de déterminer l'instant où l'opération de comblement de la cavité du moule est presque ou totalement achevée, puis de lancer l'opération d'injection du fluide pressurisé en réaction à cette détermination.
PCT/GB2004/001255 2003-03-20 2004-03-22 Procede et appareil de moulage par injection avec assistance gazeuse et expulsion de matiere plastique WO2004082921A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0306426.8 2003-03-20
GB0306426A GB2399533A (en) 2003-03-20 2003-03-20 Process and apparatus for injection moulding

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WO2004082921A1 true WO2004082921A1 (fr) 2004-09-30

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US10357910B2 (en) 2013-10-31 2019-07-23 Subsea 7 Limited Techniques for coating pipes
DE102020206712A1 (de) 2020-05-28 2021-12-02 Volkswagen Aktiengesellschaft Verfahren zur Herstellung von Kunststoffbauteilen

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Cited By (2)

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Publication number Priority date Publication date Assignee Title
US10357910B2 (en) 2013-10-31 2019-07-23 Subsea 7 Limited Techniques for coating pipes
DE102020206712A1 (de) 2020-05-28 2021-12-02 Volkswagen Aktiengesellschaft Verfahren zur Herstellung von Kunststoffbauteilen

Also Published As

Publication number Publication date
GB2399533A (en) 2004-09-22
GB0306426D0 (en) 2003-04-23

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