GB2128541A

GB2128541A - Hydraulic press

Info

Publication number: GB2128541A
Application number: GB08228514A
Authority: GB
Inventors: Basil Frederick Dockree; Paul Mungo Dockree
Original assignee: TAVISHELM HYDRAULICS INTERNATI
Current assignee: TAVISHELM HYDRAULICS INTERNATI
Priority date: 1982-10-06
Filing date: 1982-10-06
Publication date: 1984-05-02
Also published as: GB2128541B

Abstract

A hydraulic press has fixed and movable platens 10, 11 and at least one hydraulic ram 14 for moving the movable platen towards the fixed platen in a two stage cycle of operation, at least the second stage being a hydraulically actuated pressure stage. The hydraulic ram comprises a cylinder 17 and piston 16 of which the cylinder is attached to the movable platen for movement therewith relative to the piston during the second stage of operation of the ram, the pressure fluid being fed into the cylinder between the movable platen and the adjacent face of the piston. The platen 11 is held in a raised position by chocks 21 engaging a groove 20 in the piston rod 16. On withdrawal of the chocks the platen 11 falls under gravity in a first stage. The chocks 21 are then re-engaged with a higher groove 20 in the piston rod during the second stage. <IMAGE>

Description

SPECIFICATION Hydraulic presses This invention relates to hydraulic presses.

More particularly but not exclusively, the invention relates to large hydraulic presses suitable for applying the high pressures necessary for dough moulding of articles from plastics materials, e.g.

the roof of the cab of a lorry.

Such hydraulic pressures generally employ one or more pressure applying rams each connected to both a prefill tank for supplying an initial high flow, low pressure oil feed to the ram and also a pressure source for supplying a subsequent low flow, high pressure feed of oil. However, the bulk of oil necessary is found to be undesirable since the second source still has to provide a considerable quantity of oil both to effect further expansion of the ram and to compensate for the compression of the oil under load. It is also necessary to apply the full pressure as quickly as possible and thus very high powered pumps have to be employed.

According to the invention there is provided a hydraulic press comprising fixed and movable platens, and at least one hydraulic ram for moving the movable platen towards the fixed platen in a two stage cycle of operation, at least the second stage being a hydraulically actuated pressure stage, said hydraulic ram comprising a cylinder and piston of which the cylinder is attached to the movable platen for movement therewith relative to the piston during the second stage of operation of the ram, the pressure fluid being fed into the cylinder between the movable platen and the adjacent face of the piston, and the piston being adapted to resist the pressure of the fluid applied thereto.

It is preferred that during the first stage of operation of the ram the piston is adapted to move with the movable platen, a mechanical lock being provided for locking the piston during the second stage pf operation of the ram.

In a preferred embodiment of the invention the press operates vertically and the first stage of the operation of the ram is actuated by gravity.

Alternatively the press may operate horizontally in which case the first stage of the operation of the ram is actuated hydraulically.

The mechanical lock may be at least one retractable latch which is engageable in a peripheral groove in the piston rod. In one embodiment the latch is one of a pair of retractable chocks operated by hydraulic rams in a transverse plane relative to the direction of movement of the pressure applying ram.

Separate hydraulic rams may be provided for returning the movable platen.

By way of example, a specific embodiment in accordance with the invention will be described with reference to the accompanying diagrammatic drawings in which: Figure 1 is a perspective view of a hydraulic press; Figures 2 to 4 are elevations indicating three operational positions of the press of Figure 1: and Figure 5 is a vertical section through part of the main ram of the press of Figure 1.

This example is concerned with large hydraulic presses of the kind used for dough moulding of articles from plastics materials. Such presses usually operate vertically and need to provide a fast downward stroke and then a holding stage under high pressure. If the operating stroke and the application of that pressure is not sufficiently quick the plastics material, e.g. glass reinforced polyester resin, may be cured by the heated mould to an unacceptable level. An optimum time in respect to polyester resin may be about 5 seconds. It may then be necessary to maintain that pressure for about 1 5 minutes before the formed article is released from the mould.

Referring to the drawings a vertically operating hydraulic press has a lower fixed platen 10 and an upper movable platen 11 between which the two parts of a moulding tool (not shown) are fixed in known manner. The movable platen 11 is guided by four pillars 12 extending upwardly from the fixed platen 10, through the movable platen to a second fixed platen 13. The downward operating stroke of the movable platen 11 is effected by a single acting hydraulic ram 14 and a separate pair of hydraulic rams 1 5 are provided for raising the movable platen.

The main ram 14 has a two stage cycle of operation, the first stage effecting movement of the platen 11 to interengage the two parts of the moulding tool and the second stage applying the pressure stroke to close the moulding tool and to maintain the required pressure for a predetermined time to form the article being moulded. In this embodiment the first stage is effected by gravity and the second stage is effected hydraulically. There is also provided a mechanical lock to determine the extreme positions of the first stage of operation of the ram 14.

To this end, the ram 14 has a piston 1 6 and cylinder 17 arrangement at the lower end of a piston rod 18, the base 19 of the cylinder being bolted to the movable platen 11 for movement relative to the piston 16. The piston rod 18 extends upwardly from the cylinder 17 through a slide bearing 30 in the upper fixed platen 1 3.The piston rod 1 8 also has a plurality of peripheral grooves 20 which are engageable by a pair of retractable chocks 21 mounted directly beneath the upper fixed platen 1 3. Hydraulic rams 22 are provided to effect a reciprocal movement of the chocks 21 into and out of engagement with one of the grooves 20 of the piston rod 18 thereby providing the above-mentioned mechanical lock precluding axial movement of the piston rod 18 at both end positions of the first stage of operation of the ram 14.

Within the cylinder 1 7 the piston 1 6 comprises an end cap which is attached to the piston rod 1 8 by bolts 23 and carries a sealing ring 24 held captive in a peripheral recess by a retaining ring 26 bolted to the piston. The sealing ring 24 has a circumferential groove 27 into which the pressure fluid, which is oil in the present embodiment, may enter to assist the sealing effect of the ring. The oil for the ram 14 is a passageway 28 in the base 19 of the cylinder 1 7 which is open to the bore of the cylinder beneath the piston, the oil entering an annular recess 25 defined by the underside of the piston when the piston and cylinder arrangement is fully contracted (Figure 3).

In operation the movable platen 1 1 is initially raised and the chocks 21 are engaged with the lowermost peripheral groove 20 of the piston rod 18 (Figure 2). A block of moulding material is inserted into the female part of the heated moulding tool. The chocks 21 are withdrawn by their respective rams 22. With the returns rams 1 5 open, the movable platen 11 then falls under gravity and the chocks 21 are re-engaged with the piston rod 1 8 in a higher groove 20. The piston rod 1 8 is thereby locked against further axial movement relative to the upper fixed platen 13 (Figure 3).

Expansion of the piston and cylinder arrangement applies the pressure stroke to the movable platen 11, the pressure being held for the required time (Figure 4). The return rams 1 5 are then operated to contract the piston and cylinder arrangement and subsequently to iift the piston rod 18 after the chocks 21 have been withdrawn.

The chocks are again re-engaged with the piston rod and the formed article is removed from the moulding tool. The cycle of operation may then be repeated.

The main advantage of the embodiment described above is that the provision of the mechanical lock at the end of the first stage of the ram 14 allows the second stage to become immediately effective whilst minimising the amount of oil and power required to apply the necessary pressure to form the article being moulded within a given time.

The invention is not restricted to the specific details of the embodiment described above. For example, the press may have more than one main ram 14.

Also the press may operate horizontally. In this case the or each main ram would have a hydraulically actuated first stage prior to the application of the mechanical lock and the subsequent hydraulically actuated pressure stroke.

Claims

1. A hydraulic press comprising fixed and movable platens, and at least one hydraulic ram for moving the movable platen towards the fixed plate in a two stage cycle of operation, at least the second stage being a hydraulically actuated pressure stage, said hydraulic ram comprising a cylinder and piston of which the cylinder is attached to the movable plate for movement therewith relative to the piston during the second stage of operation of the ram, the pressure fluid being fed into the cylinder between the movable platen and the adjacent face of the piston, and the piston being adapted to resist the pressure of the fluid applied thereto.

2. A hydraulic press as claimed in Claim 1, wherein during the first stage of operation of the ram the piston is adapted to move with the movable platen, a mechanical lock being provided for locking the piston during the second stage of operation of the ram.

3. A hydraulic press as claimed in Claim 1 or Claim 2, wherein the piston of the ram has a piston rod mounted for sliding movement relative to a fixed part of the ram or the press during the first stage of the operation of the ram.

4. A hydraulic press as claimed in Claim 2 or Claim 3, wherein the press operates verticaily and the first stage of the operation of the ram is actuated by gravity.

5. A hydraulic press as claimed in Claim 3 or Claim 4 when dependent on Claim 2, wherein the mechanical lock is at least one retractable latch which is engageable in a peripheral groove in the piston rod.

6. A hydraulic press as claimed in Claim 5, wherein the latch is one of a pair of retractable chocks operated by hydraulic rams in a transverse plane relative to the direction of movement of the pressure applying ram.

7. A hydraulic press as claimed in any one of the preceding claims, including separate hydraulic rams for returning the movable platen.

8. A hydraulic press substantially as hereinbefore described with reference to and as shown in the accompanying drawings.