WO1991005648A1 - Continuous moulding of expanded plastics foam - Google Patents

Abstract

A mould tunnel (5) for the continuous, step by step moulding of a foamed plastics block is divided into two functionally distinct zones, namely a front zone (9) and a rear zone (10). A metering hopper (32) and sliding gate (13) provide for charging the front zone with a predetermined quantity of expandable plastics beads. An ejector wall (7) defining the front end of the front zone is connected to a cross-head (28) on a jack-screw (29) and may be driven to and fro within the front zone in consequence of motorised reversible rotation of the screw. The walls, floor and roof of the tunnel, the sliding gate and the ejector wall are slotted or perforated and jacketed, with the jackets of the rear zone being distinct from those of the front zone. The jacketing is such that, under independent control, steam under pressure or vacuum may be applied to the front zone through its side walls, the front zone may be connected to atmosphere through its floor, the ejector wall and gate, and vacuum may be applied to the rear zone through its floor, walls and roof. In use a still soft moulded section is held in the rear zone by the application of vacuum thereto. Beads are charged into the front zone; the ejector wall is moved slightly to compress the charge of beads; air is then flushed from the front zone by flowing steam through it, which also warms and softens the beads; vacuum is then applied to the front zone to extract condensate and residual air; steam under pressure is then applied to the front zone to expand the beads and bond the expanded mass to the section in the rear zone; steam is discontinued to the front zone and vacuum applied to quickly reduce pressure therein; vacuum is broken to both zones and the newly moulded section is ejected into the rear zone to displace the prior moulded section from the tunnel into a curing chamber; and the process is repeated.

Description

CONTINUOUS MOULDING OF EXPANDED PLASTICS FOAM

TECHNICAL FIELD

This invention relates to the production of large blocks of expanded plastics foam, for example, polystyrene foam, by continuous moulding processes.

BACKGROUND ART

Typically such blocks are made by filling a mould with expandable polystyrene beads (EPS beads), blowing steam through the mould space until the beads start to react and expand, then closing the steam exhaust passage from the mould space and allowing the steam pressure to build up to complete the expansion and fusion process, then exhausting steam and condensate from the mould and finally opening the mould or ejecting the finished block therefrom.

In conventional, non-continuous machines the newly formed block must remain in the mould until the internal pressure has dropped sufficiently to allow the mould to be opened, or the block to be ejected, without causing further swelling, and thus cracking, of the block. This has limited the production rate, and of course the size of the block has been limited by the size of the mould.

In order to overcome or alleviate those limitations step-by-step continuous moul^! ng has been proposed wherein the mould is in the form of a tunnel, one end of which is closeable by a longitudinally moveable ejector wall and the other end of which opens into an open ended curing "chamber".

In using the last mentioned apparatus the EPS beads are charged into the mould tunnel and processed much as before, with a previously formed foam block section in the curing chamber being relied upon to close the end of the tunnel adjacent to it, to enable the tunnel to contain steam under pressure. The block section in the tunnel fuses with the block section in the curing chamber and in due course the ejector wall pushes the new section into the curing chamber, an operation which is facilitated by conveyor devices acting on the block section already in the curing chamber.

DISCLOSURE OF INVENTION

Difficulties have been experienced in the operation of prior known step-by-step moulding apparatus. The block section in the curing chamber is necessarily still soft and resilient, although it has of course commenced to shrink as the internal pressure drops. Thus it tends to be compressed by the expanding block section in the steam tunnel, which thereby exhibits deficiencies normally associated with under-filling the mould space at the outset. Also there is a tendency for steam to leak into the curing chamber. This not only results in a waste of steam but also damages the edge of the block section in the chamber, leading to poor quality joints between sections in the finished block.

An object of the present invention is to overcome or at least alleviate the above-indicated deficiencies in prior known step-by-step continuous moulding apparatus by very simple means.

The invention achieves that object by providing a block moulding apparatus in which the mould tunnel has two functionally distinct ^'zones, a front zone to which steam may be supplied, and a rear zone to which a degree of vacuum may be applied. In preferred embodiments both zones of the mould tunnel are considerably shorter than prior known tunnels. Each operation of the ejector transfers a freshly expanded block section from the front zone to the rear zone of the tunnel, and thus each step of the step-by-step operation produces a shorter length of the overall block than has been the case hitherto. Any- perceived objection to that is more than made up for by the quality of the joints between the sections, the possibility of utilising higher foam pressures, and the simplicity of the operation and control of the short stroke apparatus of preferred embodiments of the invention, by comparison with other machines intended to overcome the above-indicated deficiencies in the basic step-by-step apparatus referred to above.

Therefore, the invention, in one aspect, consists in a step-by-step, continuous, expanded plastics foam moulding apparatus of the kind comprising a mould tunnel, means to charge expandable plastics beads into said tunnel, means to admit steam into said tunnel, a curing chamber and ejector means able to eject freshly moulded block sections from said tunnel into said curing chamber, characterised in that said tunnel is divided into two functional zones, a front zone to which steam may be admitted and a rear zone to which no steam is admitted but to which a vacuum may be applied, and in that said ejector means operate to transfer freshly expanded block sections from the front zone to the rear zone of the tunnel.

The operation of the above-described moulding apparatus of the invention may now be briefly described.

Assuming operation is in progress following an earlier start up and a freshly expanded block section has been ejected into the rear zone of the tunnel, then vacuum is applied to the rear zone to cause the freshly expanded block section therein to swell into pressure contact with the rear zone walls to hold that section against bodily movement and create a steam seal between it and those walls. The front zone may then or simultaneously be charged with expandable plastics beads, steam is then supplied to the front zone to cause the beads to expand into a fresh block section, thereby to produce a pressurised interface between it and the section held in the rear zone and to effect a bond therebetween, steaming is then discontinued and vacuum may be applied to the front zone to speed up pressure, drop in the said fresh block section. As soon as pressure conditions permit the vacuum is relieved in both zones and the newly expanded section in the front zone is ejected into the rear zone causing the prior made section therein to be displaced into a curing chamber, the process is then repeated.

Therefore, the invention, in a second aspect, consists in a method of moulding expanded plastics foam blocks comprising the steps of displacing a prior made block section from a rear zone of a mould tunnel by moving a freshly expanded block section into said rear zone, applying a degree of vacuum to said rear zone to secure said freshly expanded block section therein, charging a front zone of the tunnel with expandable plastics beads, admitting steam to said front zone to expand the beads into a second freshly expanded block section bonded to said section in the rear zone, relieving said degree of vacuum and repeating the aforesaid steps. BRIEF DESCRIPTION OF DRAWINGS

By way of example, an embodiment of the above described invention is described in more detail hereinafter with reference to the accompanying drawings.

Figure 1 is a diagrammatic partly sectioned side elevation of a moulding apparatus according to the invention, shown immediately prior to the ejection of a freshly expanded foam block section.

Figures 2 and 3 are sectional elevations taken on lines 2-2 and 3-3 respectively of figure 1, drawn to a larger scale.

BEST MODE OF CARRYING OUT THE INVENTION

The illustrated apparatus comprises a square or rectangular sectioned tunnel 5 closed at its front end by an end wall 6, incorporating a moveable ejector wall 7 and opening at its rear end into a similarly dimensioned curing chamber 8. The curing chamber may be furnished with conventional clamping devices to prevent any movement of the block section therein. The tunnel 5 comprises two zones, namely a front zone 9 and a rear zone 10 distinguishable from each other by their respective jacketing and functions, as described below. The walls 11, floor 12 and a slideable gate 13 of the front zone are slotted or perforated, as is the ejector wall 7. The walls 11 are covered by jackets 1 defining a jacket space 15 into which a steam supply duct 16 and a gas extraction or vacuum duct 17 both open. The gate 13 is double skinned, and the front wall 6 serves as a jacket for the ejector wall 7, defining a jacket space 18 for the ejector wall 7. The interior of the gate 7 is also open to the space 18. Likewise the interior of the floor 12 is open to the space 18 by a series of holes in the walls of the structural member therebetween. An exhaust duct 19 opens into the space 18. A seal 20 preserves the integrity of the jacket space 18 when the gate 13 is in the closed position, as shown in full line in figure 1.

The walls 21 , floor 22 and roof 23 of the rear tunnel zone 10 are also slotted or perforated and jacketed by jackets 2 defining a jacket space 25 into which a second gas evacuation or vacuum duct 26 opens.

Flow in the respective ducts 16, 17, 19 and 26 is individually controlled by valves (not shown) providing control over the flows into and from the respective jacket spaces. It is emphasised that jacket space 25 is distinct from jacket space 15. Thus, by appropriate connection of steam duct 16 to a boiler, vacuum ducts 17 and 26 to a vacuum pump and exhaust duct 19 to atmosphere, the interior of the tunnel may be steam heated and pressurised, evacuated and exhausted as required.

A critical difference between the illustrated tunnel of the invention and the corresponding component of conventional apparatus is the limitation of the steam jacketing to the front zone 9 of the tunnel. Furthermore, the control system for the opening and closing of the various valves permits synchronised differing conditions to be established in the front and rear zones of the tunnel respectively, as will be described more fully below.

The ejector wall 7 is connected to four push rods 27 extending from the ejector wall through clearance openings in the front wall 6 to a cross- head 28 including a central nut or threaded opening pierced by a jack-screw 29. The clearance openings are fitted with 0-ring type seals within which the push rods 27 may slide. The jack-screw 29 is supported for rotation in bearings 30 and is drive connected to a reversible motor 31. Thus operation of the motor 31 causes the cross-head to advance or retract along the jack-screw 29 and thereby effect movement of the ejector wall 7 to and fro between the positions shown in full and broken line respectively in figure 1.

In other embodiments the front end wall itself may be modified and movable enabling it to function as the ejector wall. In that event the end wall may be moved by a jack-screw extending from it through electrically or hydraulically rotated nuts fixedly located on the machine frame. Preferably those nuts are of known kind incorporating balls to reduce the friction between the nuts and the screws. Furthermore, in all embodiments hydraulic thrustors operating directly on the ejector wall or other conventional drive means for the reciprocation of the wall through an ejection and return stroke may be used in lieu of the preferred illustrated arrangements.

In the present instance the stroke of the ejector wall 7 is substantially 1 metre, which, of course, corresponds with the length of each zone 9 and 10 and the length of the individual block sections. This contrasts with typical values for prior art apparatus producing blocks of similar cross-section in 2.5 metre long sections or thereabouts. As a rule of thumb, for blocks having conventionally shaped and sized cross- sections, that is to say square or rectangular with an aspect ratio no greater than say 2 to 1 , apparatus according to the invention preferably has a stroke of no more than say 5/8 of the perimeter of the block's cross- section. Due to the short length of the block section being formed in the front zone 9 of the tunnel, the ejector wall 7 may move f ster than in conventional apparatus, the filling of the front zone of the tunnel may be effected substantially wholly during the return of the ejector wall, and ejection may proceed at comparatively high foam pressures. These factors enable a fast cycle time of from 70 to 90 seconds for conventional foam blocks to be achieved by the embodiment of the invention now being described.

In conventional apparatus the ejector wall or the cross-head 28 would be coupled to clamping side plates within the curing chamber 8 operable to grip cured block sections therein as the ejector wall moves through its ejection stroke to thereby facilitate the movement of the various bonded block sections through the apparatus. Alternatively the curing chamber walls would be lined with slatted chain conveyors, operable in concert with the ejector wall to facilitate such movement. Such arrangements could be provided in apparatus according to the invention, however the short stroke of preferred embodiments enables them to be dispensed with in favour of simpler clamping devices to grip the block section in the chamber during the steaming stage as the ejector means may be relied upon to expel the cured section from the curing chamber unaided.

A metering hopper 32 is provided above the front zone 9 for the infeed of EPS beads. The beads are delivered to the hopper by entrainment in an air stream created by a blower 33- The air escapes through a fabric cover 34 leaving the beads behind. A sensor 35 responsive to the depth of beads in the hopper 32 controls the operation, of the blower 33. The beads in the hopper 32 are discharged into the front zone 9 when the gate 13 is slid from under the hopper 32 by contraction of a hydraulic thrustor 36. When closed the gate 13 is sealed to the hopper to prevent beads from escaping by brushlike seals 37. The hopper, gate and their mode of operation are quite conventional and need no further description herein.

The apparatus would operate in conjunction with other conventional ancillary components (not shown), such as a run out table to take the emergent block from the curing chamber, hot wire or other cutter means for cutting the emergent block into required lengths and a programmable computer or like control means whereby the desired operating cycle for each product may be set.

To carry out the method of the particular embodiment of the invention now being described, and assuming a prior expanded block section is in the rear zone 10 of the tunnel, that zone has vacuum applied to it to secure that section therein and that the ejector wall has been returned to its full line position as seen in figure 1 , the first step is to fill the front zone 9 of the tunnel with EPS beads. This is effected by retracting the gate 13 briefly to allow the charge of beads accumulated in the hopper 32 to fall into the tunnel. The gate 13 is then returned to the closed position to enclose the charge of beads in the front zone 9 in contact with the previously moulded block section in the rear zone 10.

Preferably, the ejector wall 7 may be moved inwardly slightly after the front zone is charged with beads to compensate for any under filling and to enable the density of the finished block to be adjusted to meet specification. This precompression is particularly desirable if the in-feed material contains re-cycled regranulate particles as well as fresh beads, in that such particles otherwise affect the density of the finished block. Due to the short length of the front zone the degree of precompression is substantially uniform throughout. This density adjustment facility is a marked advantage of apparatus according to the invention over the longer stroke prior art apparatus, wherein any similar movement of an end of the mould space only substantially affects some of the material therein.

Steam is then blown from duct 16 for a brief period through the side walls 14 of the front zone to exhaust through the gate 13_> floor 12 and ejector wall 7 thereof into duct 19- Thus, the steam flows through the mass of loose beads, replacing most of the air in the front zone and starting the expansion and fusion of the beads. Steam flow is stopped in this preconditioning stage before the beads start to react strongly.

Once the preconditioning steam flow is stopped, the exhaust duct 19 is closed and vacuum is applied to the front zone 9 by way of duct 17 from a vacuum tank of sufficient size to lower the internal pressure to about half an atmosphere absolute in a few seconds. As the zone 9 is filled with hot steam with little residual air, this evacuation of about 50% ensures that there is now very little air in the front zone 9 of the tunnel.

The main steaming stage follows. The vacuum is cut off from the front zone 9 and steam is introduced as before. That steam may initially flow to exhaust through the gate 13, floor 12 and the ejector wall 7 as soon as the internal pressure reaches atmospheric, by way, for example, of a non-return valve.

The steam exhaust is then fully closed and pressure builds up in the front zone of the tunnel. Next all duct valves are closed and the expansion, bead adhesion and section to section bonding allowed to go to completion.

The front zone 9 is then opened to exhaust and vacuum is applied to it. This causes the foam pressure in the front zone 9 to drop to a value at which ejection may commence.

The vacuum is then broken in both the front zone 9 and rear zone 10 and the newly expanded section in the front zone 9 is pushed by the ejector wall 7 into the rear zone 10, thereby displacing the section then in that zone into the curing chamber 8.

Vacuum is then re-applied to the rear zone 10 and the cycle repeated.

Claims

1. A step-by-step, continuous, expanded plastics foam moulding apparatus of the kind comprising a mould tunnel (5), means to charge expandable plastics beads into said tunnel (36,13,32), means to admit steam into said tunnel (16,14), a curing chamber (8) and ejector means (7,27,28,29,30,31) able to eject freshly moulded block sections from said tunnel into said curing chamber, characterised in that said tunnel is divided into two functional zones, a front zone (9) to which steam may be admitted and a rear zone (10) to which no steam is admitted but to which a vacuum may be applied, and in that said ejector means operate to transfer freshly expanded block sections from the front zone to the rear zone of the tunnel.

2. Moulding apparatus according to claim 1 further characterised by the provision of means to apply vacuum (17) to said front zone.

3• Moulding apparatus according to claim 2 further characterised by means to exhaust (19) said front zone to atmosphere.

4. Moulding apparatus according to claim 1 further characterised in that said ejector means comprise an ejector wall (7), a jack-screw (29), a non-rotatable cross-head (28) engaged by said jack-screw, reversible motor means (3 ) for rotating said jack-screw and a plurality of push rods (27) extending from said cross- head to said ejector wall.

5. Moulding apparatus according to claim 1 wherein said steam is admitted to said front zone through slotted or perforated side walls (11) thereof.

6. Moulding apparatus according to claim 1 further characterised in that said vacuum is applied to said rear zone through slotted or perforated side walls (24), floor (22) and roof (23) thereof.

7. Moulding apparatus according to claim 1 wherein the length of each said zone is no more than five eighths of the length of the perimeter of its cross-section.

8. A method of continuously moulding an expanded plastics foam block of the kind comprising successively moulding sections of the block in a mould tunnel and ejecting each section as it is moulded into a curing chamber extension of the tunnel, characterised by the steps of applying a vacuum to a rear zone of the mould tunnel to expand and retain a previously moulded section therein, charging a front zone of the tunnel with expandable plastics beads, expanding said beads by- admitting steam into said front zone thereto to cause the beads to expand into a fresh block section, thereby to produce a pressurised interface between it and the section held in the rear zone and to effect a bond therebetween, relieving the vacuum in the rear zone, ejecting the newly expanded section in the front zone into the rear zone to cause the prior made section therein to be displaced into the curing chamber, and then repeating the foregoing steps.

9. A method according to claim 8 further characterised by the step of preconditioning the beads in the front zone before the step of expanding them by blowing steam through the front zone to expel air therefrom and warm the beads to an extent insufficient to produce full expansion thereof.

10. A method according to claim 9 further characterised by the step of applying vacuum to the front zone at the end of said preconditioning step to extract air and condensate therefrom.

11. A method according to claim 8 further characterised by the step of mechanically compressing the charge of beads prior to expanding same, thereby to adjust the density of the foam in the finished section.

12. A method according to claim 1 further characterised by applying vacuum to the front zone after the step of expanding the beads to reduce the foam pressure and reduce the time needed to allow ejection to proceed.