WO2008056971A1

WO2008056971A1 - Injection moulding machines

Info

Publication number: WO2008056971A1
Application number: PCT/MY2007/000075
Authority: WO
Inventors: Gregory L. Foster
Original assignee: FOSTERASIA Sdn Bhd
Current assignee: FOSTERASIA Sdn Bhd
Priority date: 2006-11-08
Filing date: 2007-11-07
Publication date: 2008-05-15
Anticipated expiration: 2009-05-08
Also published as: MY177126A

Abstract

An injection moulding machine comprising a two part mould (40,50), a first part of which has a plurality of injection points (70) through which flowable material can be introduced into the mould (40,50), a plurality of reservoirs (80,100) arranged for receiving a charge of molten material at a low pressure and subsequently injecting its charge of molten material through its associated injection point (70) into the mould at a higher pressure, one reservoir (80,100) being associated with each injection point (70), the reservoirs (80,100) being heatable to maintain the temperature of the molten material above the temperature at which it will flow readily through the mould, means for closing the mould by bringing the two or more mould parts (40,50) together and means (130) for causing the molten material to be ejected from the reservoirs into the mould cavity, when the mould (40,50) has been closed.

Description

INJECTION MOULDING MACHINES

FIELD OF INVENTION

This invention relates to injection moulding machines, and more particularly to multipoint injection moulding machines that can be used for the formation or complicated - shaped mouldings from high viscosity moulding compositions.

BACKGROUND OF THE INVENTION

Injection moulding of shaped products, that cannot be produced, for example, by extrusion or pressure moulding, from thermoplastics materials is a technique that has been used for many years. Molten material is injected into a mould formed of at least two parts and allowed to harden. The mould parts are then separated and the moulding is removed. In cases where the shape is complicated and the molten material has to follow a tortuous flow path it is common practice to use a hot runner plate having various outlet points through which material passes into the mould under high pressure in order that the molten material can flow to all points of the mould before it hardens in the unheated mould. In such a multi-point injection the molten material in the hot runner plate is at the same pressure as the pressure in the extruder, or other device that is feeding the material to the injection moulding machine and is fed into the mould at that pressure.

It has hitherto been considered that there is a limitation on the materials that are suitable for forming by injection moulding. If the material has too high a softening point such that it reaches its deterioration point before it becomes fluid enough to be satisfactorily injected into the mould that material is considered to be unsuitable for injection moulding. As examples of such materials there may be mentioned thermoplastic material with a very high filler content such as, the materials filled with vegetable fibres, especially wood fibres, described in WO 00/40384, which have very highly desirable properties and if they could be formed easily into such articles as door and window frames and other similar products that are currently made from timber it would be very beneficial.

This invention provides an injection moulding machine that operates very satisfactorily using such materials that are highly viscous when molten although it can also be used with other thermoplastic materials having a lower melt viscosity and which are susceptible to injection moulding using conventional machines.

SUMMARY OF THE INVENTION

The invention is based on the surprising observation that materials such as wood fibre filled thermoplastic composites when heated to a temperature at which the thermoplastics matrix that binds the wood fibres becomes molten have flow properties, presumably because the molten matrix material acts as a lubricant, and hence the materials can be injected into an injection mould provided that they are injected into the mould at high pressure through multiple injection points arranged such that the material injected through any given injection point only has to travel a short distance through the mould before it meets and coalesces with material injected through one or more other injection points.

According to the invention an injection moulding machine comprises a two part mould, a first part of which has a plurality of injection points through which flowable material can be introduced into the mould, a plurality of reservoirs arranged for receiving a charge of material at a low pressure and subsequently injecting its charge of material through its associated injection point into the mould at a higher pressure, one reservoir being associated with each injection point, the reservoirs being heatable to maintain the temperature of the material above the temperature at which it will flow readily through the mould, means for closing the mould by bringing the two or more mould parts together and means for causing the material to be ejected from the reservoirs into the mould cavity, when the mould has been closed. The reservoirs may be fed from a hot runner plate which receives the moulding material from an extruder or other feed device.

A hot runner plate, when used, will normally be formed in two parts with the runner channels connecting the individual reservoirs to the incoming feed channel being formed on the surface of one part only but this is only for the sake of economy and ^•there is no reason why mirror image channels should not be formed in both parts.

In some cases the material being used in the moulding process will not travel through a runner plate sufficiently readily under the injection pressure to ensure that the requisite amount of material is fed to each of the reservoirs at the correct time and in such cases it may be necessary to modify the runner plate to provide additional means for forcing the material towards the reservoirs. The runner plate may be provided, for example, with flexible screw drives in its channels to force the material towards the reservoirs. The drives may be, for example flexible wire springs that can deflect to allow them follow the curves in the channels in the runner plate.

With some especially viscous materials, however, even this adaptation may not solve the problem and it will be necessary to use an alternative method for feeding material to the reservoirs. One alternative is to manually feed the reservoirs but this is, of course, very labour intensive and some form of robotic feeder is preferably used. "

Using individual feeders for the reservoirs has the advantage that when changing the- form of the mouldings to be produced it is not necessary to replace a complicated runner plate but instead it is only necessary to change the positioning of the reservoirs in relation to the injection points of the new mould and to reprogram the robotic feeders. It also allows the machine to be used with multiple moulds, which may be identical or different, and this greatly increases the flexibility of the machine.

The reservoirs preferably each comprise a cylinder and a piston, and the means for causing ejection of the material from the reservoirs comprises a moveable member, such as a plate that impacts on the pistons of all the cylinders simultaneously to force them into their cylinders thereby ejecting at least a part of the molten material therein into the mould over substantially the same period of time. Simultaneous ejection is necessary to ensure that the material in one part of the moulding does not harden before it has bonded with other moulding material flowing in the mould.

When the reservoir is a piston and cylinder arrangement and the machine has a hot runner plate, the cylinder is preferably formed as a bore in the hot runner plate with a restriction at the injection point into the mould ensure that the material from the reservoir is injected under sufficient pressure to flow through the narrowest of the channels in the mould. In such a case the heat for heating the reservoirs is provided by the runner plate.

With other reservoir/runner plate arrangements the reservoir may need supplementary heating arrangements.

Other possible reservoir structures are, for example, a bellows type arrangement or independent screw activators. The main requirement is that at exit from the reservoirs the pressure of the material being ejected is high enough for the material at the injection temperature to flow through the narrowest of channels in the mould and to bond perfectly with a flow of material from another injection point flowing in the opposite direction.

In order that air trapped between two converging flows of material does not interrupt bonding of the materials vents are, of course, located at the points at which two flows coincide.

In a preferred embodiment, of machine with a runner plate, the. runner plate is fixed and the second mould component is arranged for vertical movement to open and close the mould. In a conventional injection moulding machine the mould parts are brought together using, for example, hydraulic rams or other force applying mechanisms, and the same method may be used in the injection moulding machine of the invention. However the forces generated during the injection process in the machine of the invention, especially when injecting high viscosity molten material, are very much greater than in a conventional machine and have a tendency to force the two mould parts apart against the hydraulic forces. Because of this two mould parts are preferably positively locked together on closure of the mould, for example, by an automatically operating lock, such as a cam lock, instead of merely being held together using the hydraulics of the machine operating system.

The moulding material is preferably fed to the injection moulding machine by an extruder, which may be a single- or multi-screw extruder and which, in addition to melting the material and bringing it to injection temperature, may also be used to mix other components, such as fillers or other compounding ingredients into a basic thermoplastic material.

hi a further preferred embodiment, two or more injection moulding machines according to the invention are arranged to be fed by a common feed means so that when one injection moulding machine has been charged and is carrying out its moulding and cooling cycles the feed is switched to the another machine. By operating in this manner the feed can be continuous and the non-productive elements of the cycle for one or more machines, for example cooling and discharging are carried out while another machine is carrying out its productive elements.

hi another embodiment it is possible to produce composite articles comprising, for example a material that can only be injection moulded using a machine according to the invention and a rubber, which produces a flexible joint between individual such articles. In this way, for example a flexible strip of a plurality of such articles with adjacent articles being joined by rubber inserts that are firmly bonded to the articles. This may be achieved by first injection moulding a plurality of spaced apart articles using a first die and then substituting the original die plate by a different die plate and injecting a rubber into the spaces between the individual articles. Such articles as flexible bed slats, which are normally made of plywood can easily be made.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail, by way of example, witib. reference to the accompanying drawings, in which;

Fig. 1 is a vertical section through one form of injection moulding machine according to the invention with the mould, which is for forming a pallet, closes;

Fig. 2 is a plan view of the hot runner plate of the injection moulding machine shown in Fig. 1; and

Fig. 3 is a plan view showing one way of feeding two injection moulding machines from a single feed extruder.

DETAILED DESCRIPTION OF THE INVENTION

As shown in Figs. 1 and 2, the injection moulding machine comprises a frame (10) on which is fixed a hot runner plate (20) for receiving molten moulding material from a feed extruder (not shown). Hot runner plate (20) on its lower surface (30) has one part

(40) of a two part mould. The other part (50) of the mould is formed on a platen (60) mounted on frame (10) for slideable movement in a vertical direction, movement being achieved by means of hydraulic or pneumatic rams (not shown). Mould part (40) has a number of injection points (70) through which molten thermoplastic material can be injected into the mould.

Hot runner plate (20), as shown in Fig. 2, has a number of through bores (80) coaxial with injection points (70), the positions of which are determined to ensure that when molten thermoplastic material injected into the mould formed by bringing parts (40) and (50) together through one injection point (70) flows to a predetermined part of the mould (40, 50) where it meets a second flow from an adjacent injection point (70) both flows are still molten and the material can blend intimately and ultimately bond. Channels (90) passing internally through runner plate (20) interconnect bores (80) and also connect to the feed extruder. Bores (80) form cylinders, which together with pistons (100), form a plurality of reservoirs arranged for receiving a charge of molten material through channels (90) under lower pressure than the pressure in the feed extruder. The piston rods (110) of pistons (10) extend above the upper surface of hot runner plate (20). Pistons (100) can be forced into bores (80) by means of a second slideable platen (120), movement being achieved by means of further hydraulic or pneumatic rams (not shown), which can impact on the protruding ends of piston rods (110) to inject at least a part of the charges of molten material in the reservoirs (80,100) into the mould (40,50). The lower ends of bores (80) are formed as nozzles (130), aligned with, and of the same diameter as, injection points (70), this diameter being reduced compared with the diameter of bores 8, to ensure that material is injected into the mould at a higher pressure than the pressure in reservoirs (80, 100). Locking means, which automatically engage when parts (40) and (50) come together, hold the mould closed.

As shown in Fig. 3, two injection moulding machines (140, 150) of the type shown in Figs. 1 to 3, are arranged side-by-side with a feed extruder (160) between them. Extruder (160) is arranged such that it feeds a predetermined charge of material to one of injection moulding machines (140) or (150). When a full charge has been delivered a sensor tells the extruder (160) to stop delivery to the first injection moulding machine and to start charging the other machine. Thus the extruder (160) can operate continuously and does not have long down periods while the non-productive parts of the cycle of a single machine are going on. A valve arrangement (not shown) between each machine (140, 150) and extruder (160) is used to switch delivery between the two machines (140, 150). In an injection moulding operation using the apparatus of Figs 1 and 3, molten material from extruder (160) is forced into hot runner plate (20) where it passes through channels (90) to fill cylinders (8) of the reservoirs, the pistons (100) of which are retracted. At the same time the empty mould part (50) is bought together with mould part (40) and the mould (40, 50) is locked closed. When the mould has closed platen (130) is lowered to impact on piston rods (110) and force pistons (100) into their cylinders (80) to force the molten material through nozzles (130) into the mould under high pressure. When the mould (40, 50) is full, movement of platen (120) ceases and platen (130) is withdrawn. Lower platen (60) is also withdrawn thereby opening mould (40, 50) to allow removal of the moulded part.

If the arrangement is as shown in Fig. 3, at this stage the feed from extruder (160) switches to the other injection moulding machine and the operation is repeated.

Claims

1. An injection moulding machine comprising a two part mould (40, 50), a first part of which has a plurality, of injection points (70) through which flowable material can be introduced into the mould (40, 50), a plurality of reservoirs (80,100) arranged for receiving a charge of material at a low pressure and subsequently injecting its charge of material through its associated injection point (70) into the mould at a higher pressure, one reservoir (80,100) being associated with each injection point (70), the reservoirs (80,100) being heatable to maintain the temperature of the material above the temperature at which it will flow readily through the mould, means for closing the mould by bringing the two or more mould parts (40, 50) together and means (130) for causing the material to be ejected from the reservoirs into the mould cavity, when the mould (40, 50) has been closed.

2. An injection moulding machine according to claim 1, wherein the reservoirs (8, 10) are fed via a hot runner plate (20).

3. An injection moulding machine according to claim 2, wherein the hot runner plate (20) is formed in two parts with the runner channels (90) connecting the individual reservoirs (80, 100) to the incoming feed channel being formed on the surface of one part with the second part having a flat surface.

4. An injection moulding machine according to any one of claims 1 to 3, wherein the machine has a fixed hot runner plate (20) and the second mould (50) component is arranged for vertical movement to open and close the mould.

5. An injection moulding machine according to claim I₅ which has individual feeding means for each reservoir (80, 100).

6. An injection moulding machine according to claim 4, is a robotic device- that feeds the reservoirs (80, 100) sequentially.

7. An injection moulding machine according to any one of claims 1 to 5, wherein the reservoirs (80, 100) each comprise a cylinder (80) and a piston (100) and the means (130) for causing ejection of the material from the reservoirs (80, 100) comprises a moveable member (120) that impacts on the pistons (100) of all the cylinders simultaneously to force them into their cylinders (80) thereby ejecting at least a part of the molten material therein into the mould (40, 50) over substantially the same period of time.

8. An injection moulding machine according to claim 7, wherein the moveable member is a moving platen (120).

9. An injection moulding machine according to any one of claims 1 to 8, wherein the two mould parts (40, 50) are positively locked together on closure of the mould.

10. An injection moulding machine according to claim 9, wherein the two mould parts (40, 50) are positively locked together on closure of the mould by an automatically operating lock.

11. An injection moulding machine according to claim 10, wherein the automatic lock is a cam lock.

12. An injection moulding machine according to any one of claims 1 to 8, wherein the feed means for feeding molten material to the hot runner plate (20) is an extruder.

13. Injection moulding apparatus comprising two or more injection moulding machines (140, 150) according to any one of claims 1 to 12 arranged to be fed by a common feed means (160) so that when one injection moulding machine has been charged and is carrying out its cooling and discharge cycles the feed is switched to another machine.

14. Injection moulded articles made using the injection moulding machine according to any one of claims 1 to 12 or the injection moulding apparatus according to claim 13. - li -

is. Composite Injection moulded articles comprising two or more different materials made using the injection moulding machine according to any one of claims 1 to 12 or the injection moulding apparatus according to claim 13.