GB2092509A - Moulding apparatus and method of manufacturing foamed plastics articles. - Google Patents

Abstract

The apparatus (11) is a combination of a vented sectional mould (15) and an exhaustable carrier box (13) for encasing the mould during a moulding operation. The mould is supported in the box on a carrier frame (25) and secured in a closed condition by an inflated bag (43). The invention also includes a method of moulding using the apparatus. The apparatus and method are designed for the moulding of fast-reacting, highly resilient foams, especially MDI-based flexible foams. <IMAGE>

Description

SPECIFICATION Moulding apparatus and method of manufacturing foamed plastics articles This invention relates to the moulding of foamed plastics articles and, more particularly, articles made of fast-reacting, high resilience foams.

In moulding foamed plastics articles using fast foaming piastics-forming mixtures, such as the MDI-based flexible foam-forming mixtures particularly described in UK Patent Specification No. 2053778 (our application No. 8016427), we have found that it is very important to assist the removal of air or to reduce the pressure in the mould. If the mould is inadequately vented, the pressure above the expanding foam increases and inhibits further foam expansion with the result that the mould may be incompletely filled.If extra foam is used to counteract underfilling the foamed article may be of an unsatisfactorily or uneconomically high density. increasing the size of the vent in the mould allows the air to escape faster and reduces the pressure built up over the rising foam but a large vent will allow foam to escape and can cause local collapse of newly risen foam adjacent to it owing to the accentuated pressure gradient across the foam.

One solution to this problem has been proposed in the the peviously mentioned UK Patent Specification No. 2053778 in which the mould is vented by vacuum assistance through a permeable insert, such as a reinforcing segment of reclaimed, rebonded flexible foam of the type commonly used in motor car seating.

The insert prevents foam exudation through the vent. This solution has not proved altogether satisfactory, however, partly because of difficulties in sufficiently exhausting air through the insert.

We have now devised a moulding apparatus which enables air in moulds to be exhausted quickly and efficiently without the deleterious effects encountered when using large vents.

According to the present invention we provide moulding apparatus for the manufacture of a foamed-plastics article which comprises the combination of a vented closed mould and, for encasing said mould during a moulding operation a carrier box contructed to sustain an internal pressure less than ambient pressure and connectable to a pressure reducing device.

The mould may be vented by a plurality of small holes i.e. holes of diameter not greater than approximately 3 mm. The number of holes and their location will depend interalia on the shape, size and complexity of the mould. There will be as many holes as is necessary to sufficiently exhaust air in the mould during a moulding operation and the holes will normally be spaced apart. Alternatively, the mould may be vented through a long narrow gap at a flange.

Closed moulds and methods of using them for moulding foamed plastics articles are well known. Usually they are sectional moulds, that is moulds constituted by two or more sections or parts which can be dismantled to remove a moulded article and assembled for a moulding operation. Any such mould suitably vented may be used in the apparatus of the invention. The mould can be vented by, for example, drilling in it as many holes up to 3 mm diameter as required.

The carrier box for encasing the mould during a moulding operation is an airtight box conveniently formed by enclosing a commonly used mould carrier frame. It is connectable to a pressure reducing source by, for example, an integral valved exhaust pipe and is usefully provided with a separate valved air inlet port for venting the box. Means for regulating the pressure in the box is thereby provided.

It is an advantage of the present invention that lines to the pressure reducing device may be permanently connected to the carrier box and may thus be larger and more robust than if they were otherwise used to serve directly individual moulds.

Further, the use of a carrier box ensures a balanced pressure drop across all mould vents and avoids the possibility of air leakage into the mould caused by the pressure within the mould being reduced below that of the ambient atmosphere. Foaming at pressures below atmospheric pressure to give controlled foam density reduction is thus more easily and reproducibly done with the present invention.

In a preferred embodiment of our invention we provide moulding apparatus for the manufacture of a foamed plastics article which comprises the combination of (a) a closed mould which can be dismantled into upper and lower component parts to release an article moulded therein and assembled for a moulding operation, the upper part having a plurality of vent holes of diameter not greater than approximately 3 mm and an aperture through which a plastics foam-forming liquid mixture can be introduced into the mould; (b) for encasing the mould a carrier box constructed from closable lid and base sections to sustain when in a closed condition an internal pressure less than ambient pressure and provided with means for (i) connecting the box to a pressure reducing device (ii) venting the box and (iii) regulating the pressure therein; and (c) clamping means for securing the mould parts in an assembled condition while encased in the carrier box during a moulding operation; the upper mould part being held in the lid section of the carrier box with its aperture coincident with an aperture in the lid section so that together the two apertures form an inlet to the mould only, said inlet being closable by a closure member, and the lower mould part being held in the base section of the box, the combination being such that when the carrier box is opened the mould is dismantled and when closed the mould is assembled and securable in an assembled condition by the clamping means.

A suitable clamping means comprises an inflatable bag located between the base section of the box and the lower mould part. Conveniently the inflatable bag is provided with means extending through the base section of the box for connecting the bag to a pressure intensifying source.

The lid and base sections of the carrier box are conveniently hinged and securable in a closed condition by a toggle clamp. Attapered plug hinged externally to the lid section may be used as an inlet closure member and similarly secured in a closed condition by a toggle clamp.

The invention also separately includes a carrier box constructed to sustain an internal pressure less than ambient pressure, connectable to a pressure reducing source and adapted to detachably hold a vented closed mould for the purpose herein described and a vented closed mould adapted to be detachably held by such a carrier box for the same purpose.

Further the invention includes a method of manufacturing a foamed plastics article which comprises introducing into a vented closed mould a plastics foam-forming liquid mixture, allowing the mixture to foam and set and, while the mixture is allowed to foam, venting the mould to a region maintained at a pressure lower than the pressure in the space above the foaming mixture in the mould.

Any plastics foam-forming mixture may be used in the process of the invention but of most interest to us are isocyanate-based foam-forming mixtures. These include polyurethane and polyisocyanuratefoams whose methods of manufacture are fully documented in the relevant literature and well known to those skilled in isocyanate-based foam technology.

The invention finds special application to the diphenylmethane diisocyanate (MDI)-based flexible foams which are particularly described in UK Patent Specification No. 2053778. These MDI-based foam-forming mixtures may be formulated to give foam rise times of less than 25 seconds when foamed in closed metal moulds having a controlled mould surface temperature within the range of from 40 to 600C.

The present invention will now be further described, by way of example only, with reference to the accompanying drawings in which Figure 1 is a cross section through moulding apparatus in an open condition and Figure 2 is a cross section through the same apparatus in a closed condition ready to receive a plastics foam-forming liquid mixture.

In the drawings, a moulding apparatus 11 comprises a carrier box 13 and a sectional mould 15 of the closed mould type. The carrier box 13 is formed from a base section 17 and hinged lid section 19 and when closed, as shown in Figure 2, is secured by a catch 22. Integral exhaust pipe 21 connectable to an evacuated rese oir, and vent pipe 23 are controlled by valves X and Y, respectively. Mould 15, supported on a carrier frame 25, comprises upper and lower halves 27 and 29. The upper mould half 27 is drilled with 3 mm vent holes 31 and has an aperture 33 which coincides with an aperture 35 in the lid section 19 of the carrier box 13.

The areas of the upper mould half 27 of the mould and the lid section 19 immediately surrounding the apertures are in sealed contact. Together the apertures form an inlet 37 to the mould 15. The inlet may be closed by a plug 39 which can be secured in a closed condition by a toggle clamp 41. An inflatable bag 43, located between base section 16 of the box 13 and carrier frame 25, is connectable to a pressure intensifying source by pipe 45 which extends through the base section 16 and is controlled by a valve W.

In operation, with the exhaust pipe 21 connected to an evacuated resevoir at a pre-set pressure and pipe 45 connected to an elevated pressure supply, the mould carrier box 13 and hence the mould 15 are closed. The carrier box is latched and the airbag 43 pressurised when valve W is opened to clamp the mould halves together inside the box. Afoam reaction mixture is injected into the mould through the inlet 37 which is then closed with the plug 39. The plug is not clamped at this stage but allowed to rest in the inlet to act as a one way valve during foam rise. With valve Y closed, valve X is opened to exhaust the carrier box which is then tilted, if necessary, to bring the mould vents to the most effective position. This will be determined by the need to provide maximum venting capacity for as long as possible during foam rise. When foaming is nearly complete, the inlet closure plug is secured in a closed condition with the toggle clamp 41. When the reaction mixture has foamed and set, valve Y is opened to admit atmospheric air to the mould carried box. The vapours are flushed out of the box and the exhaust valve X is closed. Pressure in the air bag is released, the mould box and hence the mould are opened and the moulded article, which in this example is a cushion, is demoulded.

A typical operating sequence for a high speed, high resilience MDI-based foam moulding, which preserves the environmental advantages of controlled exhaust of mould gases is given below. The total time for the moulding operation is 120 seconds and the time is given from the beginning of the operation at which each step is taken. The particular foam used is one which takes 27-30 seconds to fully rise.

OPERATING SEQUENCE Step Operation Time (seconds) 1 Carrier box and mould closed 0 2 Air bag pressurised to clamp mould 2 3 Foam reaction mixture injected 3-4 4 Plug dropped into inlet 5 5 Exhaust valve X opened 6 6 Mould tilted to optimum position 6-10 7 Plug secured by toggle clamp 25 8 Valve Y opened 116 9 Valve X closed 117 10 Pressure in air bag released 118 11 Mould box and mould opened and cushion 120 demoulded An alternative mode of operation, especially suitable for moulds with permanently attached self-cleaning mixers for the foam-forming ingredients is to reduce the pressure in the mould carrier box and hence the mould before or during foam injection.

Claims (9)

1. Moulding apparatus for the manufacture of a foamed-plastics article which comprises the combination of a vented closed mould and, for encasing said mould during a moulding operation a carrier box constructed to sustain an internal pressure less than ambient pressure and connectable to a pressure reducing device.

2. Apparatus according to claim 1 in which the mould is vented by a plurality of holes of diameter not greaterthan approximately 3 mm.

3. Moulding apparatus for the manufacture of a foamed plastics article which comprises the combination of (a) a closed mould which can be dismantled into upper and lower component parts to release an article moulded therein and assembled for a moulding operation, the upper part having a plurality of vent holes of diameter not greater than approximately 3 mm and an aperture through which a plastics foam-forming liquid mixture can be introduced into the mould, (b) for encasing the mould, a carrier box constructed from closable lid and base sections to sustain when in a closed condition an internal pressure less than ambient pressure and provided with means for (i) connecting the box to a pressure reducing device, (ii) venting the box and (iii) regulating the pressure therein, and (c) clamping means for securing the mould parts in an assembled condition while encased in the carrier box during a moulding operation; the upper mould part being held in the lid section of the carrier box with its aperture coincident with an aperture in the lid section so that together the two apertures form an inlet to the mould only, said inlet being closable by a closure member, and the lower mould part being held in the base section of the box, the combination being such that when the carrier box is opened the mould is dismantled and when closed the mould is assembled and securable in an assembled condition by the clamping means.

4. Apparatus according to claim 3 in which the clamping means comprises an inflatable bag located between the base section of the box and the lower mould part.

5. Apparatus according to claim 4 in which the inflatable bag is provided with means extending through the base section of the box for connecting the bag to a pressure intensifying source.

6. Moulding apparatus according to claim 1 substantially as herein described with reference to the accompanying drawings.

7. A carrier box constructed to sustain an internal pressure less than ambient pressure, connectable to a pressure reducing source and adapted to detachably hold a vented closed mould for the purpose herein described.

8. A vented closed mould adapted to be detachably held by a carrier box according to claim 7 for the purpose herein described.

9. A method of manufacturing a foamed plastics article which comprises introducing into a vented closed mould a plastics foam-forming liquid mixture, allowing the mixture to foam and set and, while the mixture is allowed to foam, venting the mould to a region maintained at a pressure lower than the pressure in the space above the foaming mixture in the mould.