GB2054446A - Moulding foamed plastics articles - Google Patents

Abstract

Flexible polyurethane foam blocks are made by introducing a foamable liquid through a plurality of inlet ports 27 in the base 15 of a mould 11. Continued gradual introduction of foamable liquid can take place during foam rise without disturbing the foam being formed. An upper mould half 21 may be lifted by the rising foam to counter doming of the block. The inlet ports 27 are preferably controlled by mushroom valves 31. <IMAGE>

Description

SPECIFICATION Method and apparatus for moulding foamed plastics articles This invention relates to the moulding of foamed platics articles and especially to the batch production of blocks of flexible foam.

According to our invention we provide a method for making a foamed plastics article which comprises introducing, against gravity, a foam-forming liquid mixture through a plurality of inlet ports in the base of a mould and allowing the mixture to foam to fill the mould and set. The invention also includes the aticle so obtained.

An advantage of this method over moulding methods used hitherto is that it permits the continued, gradual introduction of a foam-forming mixture into a mould during foam rise without disturbing the foam being formed. Thus in one embodiment of our invention at least part of the foam-forming mixture is allowed to commence foaming while the remainder of the mixture is introduced into the mould.

The method is particularly applicable to the manufacture of flexible foam blocks in which a large quantity of a foam-forming mixture is used and in which difficulty may arise in introducing all the foam mix before foaming begins. The method of the present invention not only enables this difficulty to be circumvented but by allowing a deliberate, unhurried introduction of foam mix the quality and consistency of the foam product can be better controlled.

One problem encountered in foam block manufacture is the doming of the top of foam blocks formed in open box moulds. This doming effect is caused by drag of the peripheral regions of the rising foam against the side walls, the central portion of the block being able to expand unimpeded.

It is desirable, therefore, either to allow the walls of the mould to rise or the base of the mould to fall in synchronism with the foam rise so that the zone of contact between the upper surface of the foam and the mould walls does not substantially move. This may be done by using, for example, a mould cover attached to the mould walls, the mould walls being raised as the expanding foam raises the cover. In similar manner the expanding foam may be allowed to depress the base of the mould with mould walls attached.

In particular the method of the present invention may be used in conjunction with the moulding process described in British Patent Specification No.

1425612 in which a foam-forming reaction mixture is deposited on a support, which, together with one or more side shaping members, defines a mould cavity, and in which the support is lowered relatively to the shaping members when foaming begins so as to increase the volume of the cavity at such a speed that the zone of contact between the upper surface of the foam and the shaping members does not move.

In this process the expansion of the foam is not used to move the support thus avoiding surface densification and density variations throughout the body of the product obtained.

In adapting this process for use with the method of the present invention, the support is provided with a plurality of inlets and the foam mix introduced therethrough. The rates at which the mixture is introduced and at which the support is lowered can be varied to suit requirements and to take account of the reactivity of the particular foam-mix used.

Any plastics foam-forming mixture may be used in the method of the invention but of particular interest to us are mixtures for forming polyurethane foams and especially flexible polyurethane foams of high resilience suitable for use in mattresses.

The mixtures are prepared by methods well known to those skilled in the technology of the particular foamed plastics material to be used.

In a further aspect of the present invention we provide apparatus for making foamed plastics articles which comprises a mould having a mould base and side walls defining a mould cavity, the mould base being provided with a plurality of valved inlet ports through which a foam-forming mixture can be introduced into the mould cavity.

In one suitable embodiment the inlet ports are provided with mushroom valves whose upper surfaces lie flush with the mold base when the valves are closed. The valves sit in bevelled seats formed where the outlets meet the mould base.

In a preferred embodiment the mould base is moveable relative to the side walls so that-during a moulding operation the zone of contact between the upper surface of the foam and the side walls does not substantially move. Such an embodiment is obtained by providing the base support of the moulding apparatus described in British Patent Specification No. 1425612 with a plurality of valved inlet ports for the passage of a foam-forming mixture. The modification of this previously known moulding apparatus in accordance with the present invention will be readily put into practice by a skilled technician in this field.

Alternatively the mould of the present invention may be provided with a cover having attached side walls which are overlapped by side walls attached to the base, the cover side walls rising in synchronism with the expanding foam as the foam raises the cover.

By way of illustration one embodiment of the invention will be further described with reference to the accompanying drawings in which Figure 1 is an interrupted diagrammatic cross-section through moulding apparatus according to the present invention; and Figure 2 is a detail from Figure 1 showing more clearly an inlet port valve.

In the drawings, a mould 11 consists of a lower mould half 13 having a base 15 and side walls 17 defining a mould cavity 19, and an upper mould half 21 having a cover 23 and side walls 25 which are a running fit within side walls 17. A plurality of inlet ports 27 are located in formation in the base 15 and are connected to a manifold 29 which is connectable to a supply of a plastics foam forming mixture (not shown). Mushroom valves 31 provide closure means for inlet ports 27, the valve stems 33 passing through fluid-tight seals in manifold 29, and being connected to operating means (not shown).

In operation, with valves 31 open, a foamable plastics mixture is introduced into the mould cavity 19 through manifold 29 and inlets 27. As the foam rises lifting the upper mould half, freshly mixed liquid is spread at the bottom of the mould until sufficient has been introduced to form the desired size block of foam. The valves 31 are simultaneously closed and the foamable mixture allowed to complete its foaming and set. The foam block is removed from the mould and the mould interior sprayed, as necessary, with a mould release agent ready for the next moulding process.

Claims (11)

1. A method for making a foamed plastics article which comprises introducing, against gravity, a foam-forming liquid mixture through a plurality of inlet ports in the base of a mould and allowing the mixture to foam to fill the mould and set.

2. A method as claimed in claim 1 in which at least at least part of the foam-forming mixture is allowed to commence foaming while the remainder of the mixture is introduced into the mould.

3. A method as claimed in claims 1 or 2 in which the walls of the mould are allowed to rise or the base of the mould to fall in synchronism with the foam rise so that the zone of contact between the upper surface of the foam and the mould walls does not substantially move.

4. A method as claimed in any one of the preceding claims in which the foam-forming mixture forms a polyurethane foam.

5. A method as claimed in claim 4 in which the polyurethane foam is flexible and of high resilience.

6. A method as claimed in claim 1 substantially as herein described with reference to the drawings.

7. Apparatus for making foamed plastics articles which comprises a mould having a mould base and side walls defining a mould cavity, the mould base being provided with a plurality of valved inlet ports through which a foam-forming mixture can be introduced into the mould cavity.

8. Apparatus as claimed in claim 7 in which the inlet ports are provided with mushroom valves whose upper surfaces lie flush with the mould base when the valves are closed.

9. Apparatus as claimed in claims 7 or 8 in which the mould base is moveable relative to the side walls so that during a moulding operation the zone of contact between the upper surface of the foam and the side walls does not substantially move.

10. Apparatus as claimed in claim 1 substantially as herein described with reference to the drawings.

11. A foamed plastics article whenever made by a method as claimed in any one of claims 1 to 6 or in an apparatus as claimed in any one of claims 7-10.