Patent Application
20070090571
THIS INVENTION relates to a moulding method and apparatus that has particular application in injection moulding of plastics materials.
In a conventional injection moulder, synthetic plastic is heated to a molten state and the molten plastics material is then forced at high pressure, through gates, sprues and runners to a mould cavity.
The rates at which the mould cavity can be filled is limited by the rate at which the molten plastics material, can flow through the narrow sprues, runners and gates. The molten plastics material has to be fed to the mould cavity at high pressures, which are costly to apply and which require substantial structural strength of the mould, feed equipment, etc. These disadvantage can be overcome by increasing the cross sections of the passages via which the molten plastics material is fed to the mould, which also allows molten plastics material with inclusions such as long fibres, particulate matter from recycled plastics, or the like, to be used. With increased cross sections of the feed passages, lower feed pressures are required.
However, large cross sectional feeds to mould cavities have the disadvantage that the port where the feed enters the mould, needs to be closed off when the molten plastics material is allowed to freeze in the mould. This can be achieved by accumulating the molten plastics material needed to fill the mould cavity, in a cylindrical holding chamber adjacent the mould and closing the port with a piston, the leading face of which becomes part of the peripheral wall of the mould cavity, when closed. However, in apparatus of this type, the holding chamber is open to the mould cavity and molten plastics material flowing into the holding chamber contacts the part of the mould cavity immediately adjacent the feed port, where it starts to freeze before the piston forces the molten plastics material into the chamber.
Further, the piston face in this type of mould apparatus is typically internally cooled to cool with the rest of the mould wall, when the molten plastics material in the mould cavity is frozen. When the piston is withdrawn to refill the holding chamber for a following mould cycle, the molten plastics material that flows into the holding chamber contacts the cold piston face and begins to freeze. The partial freezing of the molten plastics material in some areas, prior to the piston filling the mould, increases the viscosity of the molten plastics material and thus requires higher feed pressure to fill the mould cavity and is prone to leaving marks on the moulded products.
The disadvantages of filling a large cross sectioned holding chamber adjacent the mould cavity, before urging the molten plastics material into the holding cavity, are ameliorated to some extent in the invention disclosed in U.S. Pat. No. 6,464,910, to Smorgon et al. This patent discloses a moulding cycle in which molten plastics material is fed to an accumulator, from where it is fed to the mould cavity at low pressure, via a large cross sectioned passage and wherein the feed port of the passage leading into the mould cavity is closed by a piston of a valve. The molten plastics material is thus not accumulated in a holding cavity immediately adjacent the mould, where some of the molten plastics material may have frozen.
However, in the Smorgon et al process, molten plastics material is fed continually from an extruder to the accumulator and from the accumulator to the mould cavity, so that there is no volumetric control over the quantity of molten plastics material fed to the mould cavity. Instead, the pressure within the mould cavity is measured. The consequence is that the mould cavity is filled completely before the piston of the valve starts to close the feed to the mould. The valve has large cross sectional dimensions and the advancement of the valve piston feeds a considerable volume of additional molten plastics material into the mould, thus causing over filling of the mould. Smorgon does not describe what happens to the overflow of molten plastics material fed to the mould, but it is presumably received in an overflow reservoir and goes to waste.
Further, in the Smorgon et al process the molten plastics material is fed continuously under low pressure from the accumulator to the mould cavity until the mould cavity is full and the pressure in the mould increases. Only when this increase in pressure is detected, does the valve piston start its movement to close the mould. It can thus safely be assumed that the flow of the molten plastics material is momentarily interrupted before the valve piston movement starts. The interruption of the flow momentarily increases the residence time of the molten plastics material in the valve adjacent the mould and causes changes in the rheology of the plastics material. The valve piston then forces this material into the mould cavity, resulting in visible marks and/or local weakness within the product. Apart from the stagnation that occurs in the valve, the melt front velocity of molten. plastics material that flows into the mould is also momentarily disrupted, which affects the physical properties and appearance of the moulded product.
One object of the present invention is to provide an improved moulding method and apparatus which allow molten plastics material to be fed uninterruptedly to a mould cavity through a large cross sectioned passage.
Another object of the present invention is to provide an improved moulding method and apparatus which limit wastage by limiting overfilling of a mould cavity with molten plastics material which is fed uninterruptedly to the cavity.
According to one aspect of the present invention there is provided a method of moulding an article, said method comprising: feeding molten plastics material into a metering cavity, feeding a predetermined quantity of molten plastics material from the metering cavity into a mould cavity via a transition passage, the transition passage being defined adjacent the mould cavity, and urging the molten plastics material from the transition passage into the mould cavity with a working stroke of a packing piston, until the packing piston closes a port defined between the transition passage and the mould cavity and a leading face of the packing piston forms part of the peripheral wall of the mould.
Less than ninety percent of the mould cavity may be filled with molten plastics material when the packing piston starts its working stroke.
The packing piston may starts its working stroke while molten plastics material is still being fed from the metering cavity and the packing piston may close an inlet into the transition passage from the metering cavity, during said working stroke.
A predetermined volume of molten plastics material may be fed from the metering cavity to the mould, via the transition passage. The molten plastics material may be fed from the metering cavity by a working stroke of a metering piston that is displaceable within the metering cavity, and the displacement of the metering piston during its stroke may be monitored, to determine the volume of molten plastics material that is fed from the metering cavity.
Instead, the molten plastics material may be fed from the metering cavity by a working stroke of a reciprocating injection moulding screw that is displaceable within the metering cavity.
According to another aspect of the present invention there is provided apparatus for moulding an article, said apparatus comprising: a metering chamber and a displaceable member, together defining a metering cavity of predetermined variable volume, said volume of said metering cavity being variable by reciprocal displacement of the displaceable member in the metering cavity and said metering cavity being connectable to a supply of molten plastics material, a packing chamber and a packing piston, together defining a transition passage that is in flow communication with an inner cavity of a mould via a port and that is in flow communication with the metering cavity via a passage, the packing piston being reciprocally displaceable within the transition passage between a retracted position and a forward position, the packing piston closing said port between the transition passage and the mould cavity and a leading face of the packing piston forming part of the peripheral wall of the mould cavity, when the packing piston is in said forward position.
Said packing piston may be configured to start a working stroke from its retracted position to its forward position when less than ninety percent of the mould cavity is filled with molten plastics material.
The displaceable member may be a metering piston or an injection moulding screw.
It would be appreciated by a person skilled in the art that the method of plasticizing and/or compounding the polymer and/or additives or reinforcing means alluded to in the specification can be achieved by different types of screws, i.e. single or twin screws, continuous, intermittently or of a stop/start type. The molten polymer and/or compound can also be fed from these screws to multiple moulds and clamping devices or to multiple cavities in the same mould.
For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of non-limiting example, to the accompanying drawings in which:
Referring to the drawings, moulding apparatus in accordance with the invention is indicated generally by reference numeral 10. Some of the features of the apparatus 10 is only visible in some of the drawings.
The apparatus 10 comprises a metering chamber 12 within which there is a reciprocally displaceable member in the form of a metering piston 14, so that a metering cavity 16 of variable volume is defined inside the metering chamber. The metering cavity 16 has an inlet opening 18 that can be connected to a supply of molten plastics material, e.g. a heated pipe leading from an extruder or compounder, and has an outlet leading into a passage in the form of a melt flow path 20. The metering piston 14 is upwardly and downwardly displaceable between a fully down position shown in
The metering cavity 16 can be substituted by a conventional reciprocating injection moulding screw which would provide a metered quantity of molten plastics material to the mould cavity.
The apparatus further comprises a packing chamber 22 in the form of a barrel within which there is a reciprocally displacable packing piston 24, so that a cavity in the form of a transition passage 26 (as can best be seen in
The transition passage 26 is defined immediately adjacent the mould cavity (not shown) and is open to the mould cavity, so that a port 30 is defined where the transition passage opens into the mould cavity. The mould defining the mould cavity has been omitted form the drawings, but it will be clear to those skilled in the art that the mould cavity will, in use, be immediately to the left of the transition passage 26, as illustrated.
The packing piston 24 is driven externally, e.g. by a double acting hydraulic piston, to be displacable between a retracted position shown in
In use, when the mould cavity is filled with molten plastics material that can freeze to form a moulded article, the apparatus is in the condition shown in
While the molten plastics material in the mould cavity freezes, the metering cavity 16 is filled with molten plastics material that enters the metering cavity via the inlet opening 18 and the metering piston 14 gradually lifts to increase the volume of the metering cavity to accommodate the inflow, until the apparatus 10 is in the condition shown in
Once the plastics material in the mould cavity has frozen, the mould is opened and the article ejected. The mould then re-closes and the mould cavity is ready to be filled with molten plastics material to make another article in a next cycle of operation, the packing piston 24 is withdrawn to its position shown in
The metering piston 14 is lowered a predetermined distance from its raised position to its fully down position, so that the predetermined metered volume of molten plastics material is displaced by the metering piston, from the metering cavity 16, via the melt flow path 20 and the inlet 28 into the transition passage 26, from where it flows uninterrupted through the port 30 into the mould cavity. The volume of molten plastics material fed from the metering cavity 16 during this stroke of the metering piston 14, generally equals the volume required for an optimised quality part coming out of the mould cavity.
While the metering piston 14 moves downwardly, the molten plastics material flows continuously and uninterrupted from the metering cavity 16 to the mould cavity, so that the residence time of the plastics material in the transition passage 26 is kept to a minimum.
While the metering piston 14 is still travelling downwardly, but is nearing its fully down position, the packing piston 24 starts a working stroke from its retracted position, towards the port 30. When the packing piston 24 reaches the inlet 28, it closes it. In the illustrated embodiment of the invention, the packing piston 24 first travels a short distance from its retracted position, before it reaches the edge of the inlet 28 and starts closing the inlet. This is not essential, but it allows the packing piston 24 to accelerate and start displacing molten plastics material from the transition passage 26 and allows the metering piston 14 to reach its fully down position so that flow in the melt flow path 20 stops, before the packing piston reaches the inlet 28. The apparatus 10 is configured so that the metering piston 14 reaches its bottom position just as the packing piston 24 is about to start closing the inlet 28.
The metered volume displaced by the metering piston 14 generally equals the volume of the mould cavity, while the volume of the transition passage 26 is made as small as possible. However, the cross section of the transition passage 26 should preferably be large for reasons provided above and the length of the packing piston's stroke cannot be smaller than what is allowed by physical constraints such as the thickness of the mould wall, the diameter of the melt flow path 20, etc. The volume of the transition passage 26 is preferably about ten percent to forty nine percent of the metered volume and it follows that the mould cavity is filled about fifty one percent to ninety percent with molten plastics material that is fed by movement of the metering piston, before the last ten to forty nine percent of the mould cavity is filled by material fed by movement of the packing piston.
The overlapping reciprocal movements of the metering piston 14 and the packing piston 24 causes the flow of molten plastics material from the transition passage 26 into the mould, to be continuous and thus maintains the melt front velocity as the material flows into the mould. The flow may be accelerated momentarily when both the pistons 14, 24 are moving, but this is generally inconsequential. What is important, is that the flow of molten plastics material does not stagnate in the transition passage 26 and that the residence time of molten material in the transition passage is kept to a minimum. The uninterrupted flow of material during the process benefits moulding materials affected by long residence times during the injection cycle.
Keeping the residence time of plastics material in the transition passage 26 to a minimum, is advantageous since the face 32 of the packing piston 24 and the walls of the transition passage 26 adjacent the mould cavity may be cooler than the internal walls of the metering cavity 16 and the melt flow path 20, owing to thermal losses that occur when the walls of the mould cavity (including the piston face 32) are cooled to form an article in the mould. Those skilled in the art would appreciate that all the apparatus 10 will preferably be kept at temperatures at which the plastics material remains molten, but at which thermal degradation is kept to a minimum. However, the outer walls of the mould have to be cooled to freeze molten plastics material in the mould cavity and thermal losses from parts of the apparatus immediately adjacent the mould, are practically inevitable.
When the packing piston 24 reaches its forward position, its face 32 forms part of the mould wall and the molten plastics material in the mould can be allowed to freeze. The apparatus 10 is now again as shown in
The position of the packing piston 24 relative to the port 30 is monitored at this stage, to monitor the extent of filling of the mould cavity with the volume of molten plastics material fed from the metering cavity. The stroke of the metering piston 14, i.e. the height to which it is raised in the following operational cycle can be automatically adjusted to correct over or under filling of the mould in relation to the stopping position of the packing piston in its forward stroke.
The inlet opening 18, melt flow path 20, inlet 18, transition passage 26 and port 30 are all of cross sectional diameters that far exceed the diameters of the narrow sprues, runners and gates of conventional injection moulding apparatus and as a result, the melt velocity of the molten plastics material at the port 30 is at least 10 times less than what is the case in an optimised conventional injection moulding process. Further, the time required to fill the mould is less than half that required to fill a mould in a conventional injection moulding process and because filling happens so much faster, the front face of the packing piston 24 and the mould cavity can be kept cooler, leading to decrease in total cycle time as well as a decrease in the dimensional instability of moulded articles.
An additional benefit of this low shear injection process through a large gate is that thinner in-mould decorative skins and other layers can be used that would not be pierced as easily as what can be observed in conventional injection moulding processes.
The moulding of long glass fibres in excess of average 4 mm in a polyolefin matrix can be achieved due to the uninterrupted, minimal restrictive passages or gates that the composite has to move through before entering the mould.
