GB2260101A - Accumulator device for injection moulding machine - Google Patents

Abstract

An accumulator means 1, for increasing the shot size capacity of an injection moulding machine adapted, in use, to be interposed between the plastics material outlet of a conventional barrel 2 of an injection moulding machine and a die 3, the accumulator means 1 comprising a heatable cylinder 6, a plunger 9 reciprocable in the cylinder 6, a bore 14 for conveying plastics material to and from the interior 11 of the cylinder 6, a pressure sensitive check valve 12 to allow or prevent flow of material from the barrel 2, and an actuable valve 18 to allow or prevent flow of material to the die 3, and means 15, (16, 17 Fig 2 not shown) to displace the plunger 9 to eject plastics material from the interior 11 of the cylinder. <IMAGE>

Description

INJECTION MOULDING MACHINES This invention relates to injection moulding machines and to an accumulator means for such machines.

In conventional injection moulding machines, it is ordinarily the case that plastics material in an appropriate granular or bead form is fed from a hopper to one end of a heated plasticising chamber or barrel in which is provided a feed screw, to feed the plastics material to a holding chamber at the opposite end of the screw, the combined effect of heat and the shear action from the screw providing a homogenous mass of molten plastics material in the holding chamber, of a volume to fill a die cavity of a die head attached to the outlet from the holding chamber. At that point, a valve at the outlet from the holding chamber is opened, and the feed screw is powered axially towards the holding chamber to serve as a piston to eject the molten plastics material out of the holding chamber and into the die cavity.

To enable the removal of a moulding, it is ordinarily the case that a split die is used and held between clamping plates. At the injection stage, a considerable force is applied to ensure the complete filling of the die cavity requiring an equally considerable force between the clamping plates and die halves to prevent the die halves from inadvertent opening during the injection stage.

Such machines operate with a considerable efficiency when it is a case of injection moulding thin-walled plastics components and when the shot size capacity of the holding chamber is matched, as is to be expected, to the overall volume of the die cavity. However, when it is a case of the injection moulding of thick walled components, there is a considerable difficulty in gathering the required greater volume of molten plastics material in the holding chamber, and delivering it in a single injection stage to the die cavity as the die cavity volume exceeds the shot size capacity of the holding chamber, and the alternative of using or acquiring a larger size machine, is frequently uneconomical.

This has led to the development of the so-called flow moulding technique, where a machine of the type discussed above has its holding chamber open to the die cavity, and such that plastics material is urged, in a first stage by rotation of the feed screw into the holding chamber and on into the die cavity to provide a partial filling, and in a second stage with axial movement of the feed screw after the manner of a piston, to drive the final proportion of the required volume of plastics material from within the holding chamber and into the die cavity.Such a technique serves the purpose of avoiding the need for a larger capacity machine by avoiding the need for a massive clamping force on the die halves, but has the disadvantage that the initial, relatively slow partial filling of the die cavity and the inevitable cooling of plastics material in the die cavity prior to the final injection stage, invariably results in the moulding having a poor surface finish.

It is the object of the invention to provide a means of injection moulding particularly suited to thick walled plastics moulding, that avoids those disadvantages mentioned above.

According to a first aspect of the present invention, there is provided an accumulator means, for increasing the shot size capacity of an injection moulding machine adapted, in use, to be interposed between the plastics material outlet of a conventional barrel of an injection moulding machine and a die, the accumulator means comprising a heatable cylinder, a plunger reciprocable in the cylinder, a bore for conveying plastics material to and from the interior of the cylinder, a pressure sensitive check valve to allow or prevent flow Qf material from the barrel, and an actuable valve to allow or prevent flow of material to the die, and means to displace the plunger to eject plastics material from the interior of the cylinder.

According to a second aspect of the invention, there is provided an injection moulding machine incorporating an accumulator means as defined above.

Thus, in principle, the accumulator means provides an attachment for a conventional injection moulding machine which, when put into use, increases the shot size capacity of the machine to increase the flexibility of the machine by enabling the self-same machine to mould relatively thickwalled articles and, when taken out of use, enables the machine to revert to the moulding of relatively thin-walled articles.

Thus, with the accumulator means and machine of the invention, the heated cylinder and reciprocable feed screw operate in relatively conventional manner to provide a homogenous volume of molten plastics material, and the initial volume of molten plastics material so formed flows under the action of the screw feed, via the check valve, into the accumulator means with progressive displacement of the plunger in the accumulator cylinder. At the point that the accumulator means has been filled, the remaining volume of molten plastics material gathers in the holding chamber until the volume of plastics material required to fill a die cavity has been prepared.At that point, displacement means of the plunger is activated, to commence ejection of the plastics material from within it and the actuable valve is opened to permit flow of material into the die cavity with the higher pressure generated with the plastics material upon plunger activation displacing the check valve to close off the barrel and hence prevent material supply from the holding chamber of the barrel until such time as the plunger has completed its stroke whereupon the ensuing fall in pressure results in the check valve becoming unseated by the higher material pressure within the holding chamber to permit ejection of material from the holding chamber upon the feed screw being activated to serve as a piston and to eject the molten plastics material from within the holding chamber and into the die cavity.As a result, the larger volume of molten material required for thick walled castings can be injected into a mould cavity in a time scale considerably shorter than is inevitably so with flow mould techniques, the two-stage pressure injection of plastics material ensuring adequate filling of the die cavity in the absence of a deleterious cooling of the plastics material and with then the production of thick walled mouldings with an acceptable high quality surface finish.

Preferably, the displacement means for the plunger comprises hydraulic ram means. In detail, two spaced apart hydraulic rams may be provided, operable about axes parallel to that of the plunger and connected to the plunger by a common crosshead.

Preferably, an hydraulic cylinder operated shut-off linkage is provided at the outlet side from the accumulator means to control movement of the actuable valve, which linkage is held closed during the loading of the accumulator means and the filling of the barrel holding chamber, and which at the point of activating the plunger is displaced to a valve open position.

Desirably, and to ensure the complete ejection of plastics material from within the cylinder, the plunger is provided with a profiled tip and the outlet end of the accumulator means is correspondingly profiled. Preferably, the accumulator means has an outlet communicating with a bore extending through a body communicating the outlet from the holding chamber to the inlet to the die cavity, the shut-off linkage and the check valve being provided in that bore and to either side of the communication of the bore with the accumulator means.Thus, with the plunger in its position where plastics material in the accumulator means has been ejected completely, only a small surface area of the plunger is exposed to the bore, and as a consequence and during ejection of plastics material from the holding chamber, an insufficient force is applied to the plunger to cause it to move and hence ensure that plastics material ejected from the holding chamber is passed directly to the die cavity.

The accumulator means of the injection moulding machine of the invention may be taken out of service simply by holding the plunger in its position to close the opening to the accumulator means so that a dedicated machine with an inbuilt accumulator means has the advantage that it then leaves the machine free to produce conventional thin-walled mouldings. In this condition a valved outlet from the holding chamber at the end of the screw feed cylinder can be held closed until the required volume of plastics material has been gathered in the holding chamber, followed by the opening of the valve and the ejection of the plastics material into the die cavity.

A further advantage of the invention is that the accumulator means and its associated hydraulically activated plunger may itself be a discrete unit and able to be located between an existing and conventional injection moulding machine and its die head and whereby said existing conventional machine is provided with the facility for the effective production of thick walled mouldings.

The invention will now be described in greater detail, by way of examples, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional view through a first embodiment of accumulator means in accordance with the first aspect; Figure 2 is a section on the line II-II of Figure 1; and Figure 3 is a diagrammatic plan view of a second embodiment.

In both embodiments, like components are accorded like reference numerals.

Basically, an accumulator means 1 in accordance with the first aspect of the invention, is mounted on the structure of a conventional injection moulding machine, and specifically is interposed between the conventional machine barrel ~2 housing the conventional screw and holding chamber, and a die 3 split at 4 and having a cavity 5 in which a relatively thick-walled article is to be moulded.

It will be seen that in the embodiment of Figures 1 and 2, the accumulator means 1 is operative in a vertical plane, and in the embodiment of Figure 3 in a horizontal plane.

In detail, the accumulator means 1 comprises a cylinder 6 heatable by electric heaters 7 and having, in the embodiment of Figures 1 and 2, a frusto-conical base 8 or, in the embodiment of Figure 3, a planar base 8A. A plunger 9 having a tip 10 also of frusto-conical profile in Figures 1 and 2 and planar in Figure 3, is reciprocable within the cylinder 6 in a filling direction, wherein cylinder interior 11 increases in size, as the plunger 9 is displaced by plastics material supplied by the screw of the machine barrel 2 via a check valve 12 housed in a base plate 13, which is also heatable, and a bore 14 in the base plate 13 in plastics flow communication with the cylinder interior 11.At its other end the plunger is secured to a crosshead 15 connected to a pair of hydraulic rams 16 having an oil supply 17 and serving, when activated, to urge the plunger 9 in the opposite direction to discharge plastics material from the cylinder interior 11 upon an actuable valve 18 in the bore 14 and beyond the cylinder interior 11 being opened by a hydraulic control cylinder 19 via a linkage 20, and at the same time closing the check valve 12 to hold back plastics material in the machine barrel 2 and its holding chamber;.

Beyond the valve 18 the bore 14 continues to a sliding nozzle tip 21 to give easy alignment with the die 3, with the pressure of plastics material in the nozzle area holding a nozzle sleeve 22 against the conventionally provided mould sprue bush. When the plunger 9 reaches the end of its stroke and the tip 10 engages the base 8, 8A, a pressure drop in the plastics material in the bore 14 etc. results in the check valve 12 being unseated by the higher material pressure within the barrel 2, whereby the screw may advance axially in the normal manner to complete the accurate fill, pack and hold sequences until the conventionally provided timer signals the end of the injection cycle. At this point, the screw rotates and, being held in the forward position by oil pressure, and with the oil pressure in rams 16 released, generates sufficient pressure to push back the plunger 9 to re-fill the interior 11 with the next charge of material, with further rotation of the screw pushing the screw back until a trip is activated to complete the total cycle.

Claims (13)

1. An accumulator means, for increasing the shot size capacity of an injection moulding machine, adapted, in use, to be interposed between the plastics material outlet of a conventional barrel of an injection moulding machine and a die, the accumulator means comprising a heatable cylinder, a plunger reciprocable in the cylinder, a bore for conveying plastics material to and from the interior of the cylinder, a pressure sensitive check valve to allow or prevent flow of material from the barrel, and an actuable valve to allow or prevent flow of material to the die, and means to displace the plunger to eject plastics material from the interior of the cylinder.

2. An accumulator means as claimed in Claim 1, wherein the displacement means for the plunger comprises hydraulic ram means.

3. An accumulator means as claimed in Claim 2, wherein the hydraulic ram means comprises two spaced apart hydraulic rams operable about axes parallel to the plunger, and connected to the plunger by a common crosshead.

4. An accumulator means as claimed in any preceding Claim, wherein the actuable valve is movable by an hydraulic cylinder-operable shut-off linkage.

5. An accumulator means as claimed in any preceding Claim, wherein the plunger is provided with a frusto-conical tip to engage a frusto-conical seal.

6. An accumulator means as claimed in any preceding Claim, comprising a sliding nozzle tip adapted, in use, to align with the die.

7. An accumulator means substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

8. An accumulator means substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

9. An injection moulding machine comprising an accumulator means as defined in any preceding Claim.

10. An injection moulding machine as claimed in Claim 9, wherein the accumulator means is in-built as an integrated unit

11. An injection moulding machine as claimed in Claim 9, wherein the accumulator means is a discrete unit, capable of ready attachment and removal.

12. An injection moulding machine substantially as hereinbefore described with reference to Figures 1 and 2.

13. An injection moulding machine substantially as hereinbefore described with reference to Figure 3.