Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By mechanically introducing gas into material
Reexamination Certificate
1999-10-01
2001-09-11
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By mechanically introducing gas into material
C264S051000, C264S053000
Reexamination Certificate
active
06287494
ABSTRACT:
The present invention relates to a method of introducing gas into molten plastics prior to injection of the material into a mould for forming an article.
In conventional injection moulding, the molten plastics material is injected into a mould, where it solidifies and the mould is opened for ejection of the formed article. Cinpres Limited has proposed, in various patents for example WO 93/23228, to inject gas into the mould via a nozzle simultaneously or after the plastics material to provide voids in the formed article, thus economising on the amount of plastics material used. A similar effect had previously been obtained by injection of a blowing agent with the plastics material as described for instance in GB 2 010 168 A. In this case the blowing agent is one which decomposes at a given high temperature to release nitrogen gas. The decomposition reaction is an exothermic reaction and so this type of blowing agent is known as an exothermic blowing agent.
There is a second type of blowing agent, known as an endothermic blowing agent. These blowing agents rely on a chemical reaction to liberate carbon dioxide, or an other gas, which then dissolves in the molten plastics material. As the gas comes out of solution an endothermic effect occurs in the plastics material. This has the added advantage of cooling the moulded article, which improves the mould cycle time.
In my International Patent Application No. PCT/GB96/01706, I have described use of a blowing agent to foam thick areas of an article, to a thickness greater than that given by the mould part gap, on complete or partial opening of the mould. Further in my UK patent application No. 9624162.5, I have described similar use to foam substantially all of an article to substantially circular cross-section.
The object of the present invention is to provide a method of introducing gas, to act as a blowing agent, which relies neither on a chemical blowing agent nor on injecting the gas via a nozzle directly into the mould. Rather it relies on addition of the gas to the molten plastics material in the injection moulding machine, or in a mould in the machine upstream of the mould's gate or point of injection of molten plastics material into the mould cavity. The gas is mixed with and will usually dissolve into the material and be injected into the mould cavity with the plastics material.
According to the invention there is provided a method of adding gas to molten plastics material during each cycle of an injection moulding machine with an internal space limited by a piston, the method consisting in the steps of:
introducing gas into the space in front of the piston on backwards movement of the piston,
feeding molten plastics material to the space and
applying injection pressure to the space via the piston for expulsion therefrom of the molten plastics material with the added gas and its injection into a moulding cavity.
Preferably, the gas is of a substance which is gaseous at ambient pressure and temperature and is preferably carbon dioxide. Hydrocarbon or other gases may also be suitable. Further, it is envisaged that the gas could of a substance which is liquid at ambient pressure and temperature, but gaseous at the elevated temperature of molten plastics material, such as water.
In one embodiment, the injection moulding machine has a plasticising screw which is not adapted to reciprocate and a shooting pot having an injection piston adapted to reciprocate in an injection chamber. In another embodiment, the injection moulding machine has a plasticising screw which is adapted to reciprocate in an injection chamber. In both these embodiments, the gas is introduced into the injection chamber during each cycle of the machine.
Normally, the injection pressure is reduced after injection to a holding pressure before the mould cavity is opened and the pressure in the injection chamber is reduced below the holding pressure for introduction of the gas into the injection chamber. Additionally it is likely that the gas will be introduced during or subsequent to decompression or suck-back, following injection and application of holding pressure.
Two alternative methods are envisaged for actual introduction of the gas. For both, the gas is initially at elevated pressure. In the first alternative, on introduction the gas drives the injection piston or plasticising screw back by a controlled amount due to the elevated pressure. In the second alternative, the injection piston or plasticising screw is drawn back by a controlled amount, by the machine's mechanism for reciprocating the piston or screw, whereby a controlled amount of gas is introduced into the injection chamber. In both alternatives, a controlled amount of gas is introduced into the injection chamber. Further, in both alternatives, the gas can be introduced at its storage pressure or at a lower pressure to which the gas is reduced by a regulator. Preferably, the pressure is lowered for the second alternative, whereby the amount of gas introduced can be closely controlled. It should be noted that ability to introduce the gas at storage pressure or lower is a significant advantage, both in terms of ease of metering and the avoidance of the need for a pump.
Conveniently, the stroke by which the injection piston or plasticising screw is driven or drawn back is at least partially taken up again after gas introduction by application of back pressure. This compresses the gas into a fraction of its original volume and begins to urge it into solution in the plastics material. The back pressure is then maintained during wind back and feed of the plastics material into the chamber. During this step, the plastics material is mixed with the gas by formations on the front of the screw.
The gas can be introduced directly into the injection chamber to the side of a cylinder head of the injection chamber remote from the moulding cavity. Alternatively, the gas can be introduced to the injection space via the cylinder head or a nozzle between the cylinder head and a mould tool incorporating the moulding cavity. It is also conceivable that the gas could be introduced axially of the plasticising screw. In another alternative, the gas can be introduced to the injection space via a flow channel in a mould tool incorporating the moulding cavity.
Where the gas is introduced axially of the screw, it is conceivable that it might be introduced intermittently during wind back and/or at the end of wind back. Such introduction would be to differing parts of the melt, with a view to distributing the gas in the charge of gas and melt.
After charging of the chamber, the back pressure is preferably increased to a higher pressure, with the shut-off valve still closed, to aid dissolution of the gas in the melt.
Either the cylinder head can have a shut-off valve or the mould tool can have a mechanical shut-off gate. In either case, the valve or the gate is preferably closed during gas introduction.
Preferably the gas and the melt are mixed or further mixed during injection by passage through a mixer.
Two devices are envisaged for introduction of the gas, namely a non-return poppet valve, opening into the injection chamber, or the cylinder head, or the nozzle, or a flow channel in a mould tool, and a control valve between the poppet valve and a source of gas. The other device is a controllable inlet valve, opening into the injection chamber, or the cylinder head, or the nozzle, or a flow channel in a mould tool. The controllable inlet valve can obturate the cylinder head or the nozzle or the flow channel in a mould tool when open.
According to another aspect of the invention there is provided an injection moulding machine for performing the method of the invention, the machine having a control system incorporating an auxiliary output adapted and arranged to actuate means for introducing gas into the space in front of the piston on backwards movement of the piston.
The gas introduction means can be incorporated with the machine and comprises:
a source of gas at elevated pressure, preferably having a pressure r
Coraltech Limited
Kuhns Allan R.
Ware Fressola Van der Sluys & Adolphson LLP
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