AU2002226560A1 - Metering device for a plastics moulding machine - Google Patents

Description

METERING DEVICE FOR A PLASTICS MOULDING MACHINE

The present invention relates to a metering device for use in a plastics injection moulding machine.

An injection moulding machine conventionally comprises a mould made in separable parts which between them define a mould cavity having the desired shape of the finished article. With the mould closed, molten thermoplastic material is injected into the cavity and solidifies within the mould. The mould is then opened to allow the formed article to be ejected and the mould is again closed to allow the cycle to be repeated.

The molten thermoplastic material is formed by means of a screw. The raw thermoplastic material is fed in granular form into the screw from a hopper. The action of the rotation of the screw compress the granules as they are being heated to form the melt. The melt flows into a chamber lying ahead of a piston formed by the head of the screw. When the mould cavity is to be filled, the screw is moved axially so that the melt is injected by the piston under pressure through a feed gate, controlled by a gate valve, into the mould cavity.

No special metering device is required when forming articles in this way by injection moulding because the plastics melt is required to flow until the cavity is full. Once the cavity has been filled, the flow stops automatically on account of the back pressure that builds up in the mould cavity.

There are occasions, however, when it is not possible to rely on back pressure. It is instead necessary to meter a precise dose of the plastics melt to the mould cavity. One example of a process requiring the plastics material to be ii

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plastics melt into the cavity, the device comprising a heated block to be interposed between the feeding means and the mould cavity, a cylinder formed in the block, a piston reciprocably mounted in the cylinder, a first passage formed in the block to connect the cylinder to the feeding means and a second passage formed in the block to connect the cylinder to the mould cavity, and valves in said passages for enabling the cylinder first to be filled with the desired dose of the plastics melt by the feeding means without the melt entering the mould cavity and for subsequently enabling the desired dose of plastics melt, when ejected from the cylinder by the piston, to flow from the cylinder to the mould cavity without being returned to the feeding means .

It is preferred for the piston of the cylinder to be advanced into the cylinder by the force of the movable platen of the moulding machine, but it is alternatively possible for the piston to be operated by a hydraulic, pneumatic or electromechanical actuator.

Conveniently, the valves are constituted by a spool valve having a first position that allows communication only between the cylinder and the feeding means and a second position that allows communication only between the cylinder and the mould cavity.

To allow accurate control of the quantity of plastics material injected into the mould cavity, it is possible to provide an adjustable stop to limit the stroke of the piston in the cylinder.

The piston is preferably designed to reciprocate linearly but it is alternatively possible to use a vane that can rotate within an arcuate slot. In a mould with multiple cavities, it is well known to provide a hot runner system or manifold to allow all the cavities to be filled from a common feed screw. However, a conventional runner system cannot be used with cavities that are not filled to their maximum capacity because it relies on the back pressure from full cavities to divert the flow to cavities that have yet to be filled.

An embodiment of the present invention mitigates the above problems by providing a separate cylinder for each cavity, all the cylinders being connected to a common inlet connected to the feed screw. When the cylinders are filled, the flow of plastics material to any given cylinder will stop when that cylinder is full and thereafter the flow will be diverted to the other cylinders until all the cylinders are filled to their maximum capacity. Thereafter, each cylinder will transfer its contents only to its own associated mould cavity thereby ensuring that all the mould cavities receive quantities of plastics material that are separately and accurately metered.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a schematic section through part of an injection moulding machine fitted with a metering device of the invention, and

Figures 2 and 3 are schematic representations of the gate valve used in the embodiment of Figure 1, each figure showing the valve spool in a different end position.

In Figure 1, there is shown an open mould arranged between the bulkhead 10 and the platen 12 of an injection moulding machine. The bulkhead is stationary 10 and is connected to a conventional screw (not shown) which melts and compresses thermoplastic material fed into it by a hopper and injects the molten plastics material into the mould through the passage 26 in the bulkhead 10.

The platen 12 is moved towards and away from the bulkhead 10, to open and close the mould, by a hydraulic ram that acts on the platen through a system of toggle levers . As the moulding machine is generally conventional and the invention is not restricted to any form of moulding machine, the machine will not itself be described in greater detail.

The illustrated mould is shown in its open position and is shown for simplicity to comprise only two parts, namely a stationary part 14 formed with depressions 16 and a moving part 18 formed with projecting cores 20 that fit into the depressions 16. When the mould is closed, a gap between the depressions 16 and the cores 20 constitutes the mould cavity which has the desired shape of the finished article, in this case a cup. The invention is equally applicable to more complex moulds have more that two relatively movable parts .

Everything described up to this point is conventional. In normal operation, the mould is closed, the cavities are filled to their maximum capacity and the plastics material is allowed to set. Thereafter, the mould is opened, the formed articles are rejected and new moulding cycle is started.

The problem that is addressed by the present invention occurs when the mould cavities do not need to be filled to the point where no more plastics material can be injected by the screw into the cavity. This situation arises, for example, when the article is to be formed of a foamed plastics material which increases in volume after it has been injected into the mould cavities. Another example where this arises is when the plastics material is injected into the mould while it is partly or fully opened, to be subsequently compressed by the closure of the mould. In such cases it is necessary to meter a precise dose of the plastics material into the mould cavity during each injection without relying on the pressure build up within the mould cavity to limit the quantity of the meld that is injected.

The metering device in Figure 1 comprises a block 22 that is arranged between the bulkhead 10 and the stationary part 14 of the mould. The block 22 is heated by means of heating elements that pass through it, in order the maintain the thermoplastic material in its molten state, and acts as a distribution manifold. In particular, the block 22 has a common passage 24 in communication with the passage 26 in the bulkhead 10 that leads to the screw and different runners 28 that lead from the common passage to the individual mould cavities .

Each cavity has a feed gate comprising a spool valve which is generally designated 30 and will be described in more detail below by reference to Figures 2 and 3. The spool valve acts to control the flow between the runner 28 and a conduit 32 that leads to a cylinder 34. The conduit 32 also has a branch 40 that leads to the feed gate of the cavity, which can also be opened and closed by the spool valve 30. Each spool valve 30 is actuated by a conventional slider 42 which acts on the valve spool to move it axially towards and away from the cavity.

The spool valve is shown in different positions in Figures 2 and 3. The spool 44 has an enlarged head at its lower end, as viewed, which is engaged by the actuating slider 42. Along is length, the spool 44 has an annular groove 48 which, when the feed gate is closed as shown in Figure 2, is aligned with the runner 28 and the conduit 32. In this position of the valve spool 44, the melt can flow around the spool 44 and along the groove to pass from the runner 28 into the conduit 32. In this position of the w ω κ> t H H

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As the mould is being closed, the feed gates are opened by operation of the sliders 42. As earlier described, the spools 44 will now isolate the conduits 32 from their runners 28 and instead connect the branches 40 to the feed gate of the cavity. The closure of the mould brings the mould part 18 into contact with the piston rods 38 and the pistons 37 are thus moved down from the position shown on the left hand side in Figure 1 to that shown on the right in the same figure. This expels from the cylinder 44 the precisely metered dose of the melt stored in the cylinder 34 and this dose now travels along the conduit 32 and the branch 40 and through the open feed gate into the associated mould cavity. Precise setting of the dose can be effected by suitable adjustment of the position of the stop collar 36.

The illustrated embodiment shows cylinders having pistons that are actuated by the moving mould part 18 but this is not essential to the invention. It would be alternatively possible to use other forms of actuators, such as hydraulic, pneumatic or electromechanical actuators. In such a case, it is not necessary for the axes of the cylinders 34 to be parallel to the direction of relative movement of the mould parts 14 and 18.

Though it is convenient to construct the feed gate so that it acts as a spool valve controlling the flow of the melt into the cylinder, this is not essential. One could instead use a non-return valve which at all times only allows the melt to flow along the runner 28 in the direction from the screw to the cylinder 34.

The invention has particular application in a mould having many cavities, that is to say a multi-impression mould, as in the absence of back pressure, it would conventionally be impossible to deliver the correct dose of the melt to each cavity from a common feed screw. The invention is not however restricted to use with such moulds and can be used even with single-impression moulds to achieve accurate metering of the melt into the mould cavity.

It will be appreciated by the person skilled in the art that various other modifications may be made to the illustrated and described metering device without departing from the scope of the invention as set out in the appended claims. For example, while a linearly reciprocating piston has been shown, it would be alternatively possible to have a variable volume working chamber defined by a vane movable within an arcuate slot.

Claims (6)

1. A metering device for use in a plastics moulding machine having a mould cavity that is opened and closed by means of relatively movable platens and a means for feeding a plastics melt into the cavity, the device being characterised by comprising a heated block (22) to be interposed between the feeding means (26) and the mould cavity (16) , a cylinder (34) formed in the block (22) , a piston (37) reciprocably mounted in the cylinder (34) , a first passage (28,32) formed in the block (22) to connect the cylinder (34) to the feeding means (26) and a second passage (32,40) formed in the block (22) to connect the cylinder (34) to the mould cavity (16) , and valves (30) in said passages (28,32,40) for enabling the cylinder (34) first to be filled with the desired dose of the plastics melt by the feeding means (26) without the melt entering the mould cavity (16) and for subsequently enabling the desired dose of plastics melt when ejected from the cylinder (34) by the piston (37) to flow from the cylinder (34) to the mould cavity (16) without being returned to the feeding means (26) .

2. A metering device as claimed in claim 1, wherein the piston (37) of the cylinder (34) is advanced into the cylinder (34) by the force of the movable platen (12) of the moulding machine .

3. A metering device as claimed in claim 1, wherein the piston (37) of the cylinder (34) is advanced into the cylinder (34) by means of a hydraulic, pneumatic or electromechanical actuator.

4. A metering device as claimed in any preceding claim, wherein, the valves are constituted by a spool valve (30) the spool (44) of which has a first position that allows communication only between the cylinder (34) and the feeding means (26) and a second position that allows communication only between the cylinder (34) and the mould cavity (16) .

5. A metering device as claimed in any preceding claim, wherein an adjustable stop (36) is provided to limit the stroke of the piston (37) in the cylinder (34) .

6. A metering device as claimed in any preceding claim for use with a mould have multiple cavities, wherein a separate cylinder (34) is provided for each cavity (16) , all the cylinders being connected to a common feeding means (26) .