GB2042410A - Process and apparatus for moulding embossed foamed plastics articles and the articles so moulded - Google Patents

Abstract

In one embodiment, the apparatus is a temperature controlled sectional mould 37 having an inlet 53 and a detachable reticulated grid 57. The grid is mounted on hydraulic jacks 59 and 61 which extend through fluid-tight seals 63 and 65. The grid is lowered onto the mould base after a liquid foam- forming mixture has been introduced into the mould and distributed over the base. The mixture then foams to fill the mould and sets to produce a foamed plastics article having an embossed or patterned surface in high relief. The invention is useful for the batch manufacture of plastics foam mattresses. In a second embodiment, the apparatus comprises a mould 13 or the bottom wall 27 which can be placed on grid 25 and which is closed by a lid 15 held in place by clamps 19. <IMAGE>

Description

SPECIFICATION Process and apparatus for moulding foamed plastics articles and the articles so moulded This invention relates to a process for moulding foamed plastics articles having embossed or patterned faces, to apparatus for carrying out the process and to the articles so obtained.

In US Patent Specification No. 4073020 a process is described for making foam mattresses having a crown area divided into decorated segments by a geometric pattern of grooves. In this process a flexible polyurethane foam-forming mixture is dispensed from a plurality of spray heads in a fan-shaped pattern into a mattress mould of rectangular shape. The mould is closed and the mixture is allowed to foam and cure before the mattress so formed is stripped from the mould. A hexagonal pattern, in high relief, on the bottom of the mould gives a complementary hexagonal grooved effect to the crown area of the mattress.

To obtain a satisfactory product it is important to ensure that the right amount of foamforming mixture is dispensed into each of the depressions in the patterned mould bottom before any significant rise of the foam has taken place. In the process described in US Patent Specification No. 4073020 this is achieved by controlled movement of the spray heads relative to the mould. Unfortunately the equipment used is expensive. The process also presents an environmental hazard because the mould is necessarily open during spraying of the toxic polyurethane ingredients.

The present invention offers a way of avoiding these disadvantages.

According to our invention, we provide a process for making a moulded foamed plastics article having an embossed or patterned face, which comprises introducing a liquid foamforming mixture into a mould, allowing the mixture to flow over a mixture-receiving surface in the mould until distributed as required, bringing to bear a reticulated grid against the mixture-receiving surface and allowing the mixture to foam to fill the mould and set. The invention also includes the moulded foam articles so obtained.

Any plastics foam-forming mixture may be used in the process of the invention but we are mainly interested in using mixtures for forming polurethane foams, and especially flexible polyurethane foams of high resilience suitable for the moulding of deeply patterned mattresses.

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

Polyurethane foam-forming mixtures are prepared by mixing together a polyisocyanate, a polyol, a blowing agent and usually a catalyst. The type of polyurethane foam obtained will vary considerably depending inter alia on the nature and quantity of the polyisocyanate and polyol used and on other ingredients present which may include "chain extenders" and "cross-linking agents''. The blowing agent is usually a volatile orgnic liquid or water. Water reacts with the polyisocyanate to form a polyurea and carbon dioxide which "blows" the plastics material into a foam.

Thus the term polyurethane foam is understood to include materials which contain a substantial proportion of polyurea groups as well as urethane groups and also isocyanurate groups formed by trimerisation of the polyisocyanate with a suitable catalyst.

The foam-forming mixture may be introduced, for instance by pouring it, into an open mould which is closed with a suitable lid before foaming has proceeded too far, or, preferably, to avoid toxic fumes, into a vented closed mould through one or more injection points conveniently in the mould lid.

The mixture-receiving surface will normally be the bottom surface of the mould and the mixture may flow over this surface of its own accord until it is evenly distributed. Distribution may be assisted by tilting the mould.

Whether this is necessary will depent inter alia on the speed of reaction of the mixture, the number of injection points and size and shape of the mould.

The reticulated grid may be of any desired shape or size; it being important only that the mixture can foam freely through the interstices. Suitable grids include those with a mesh- and lattice-work structure and those in the foam of a honeycomb with hexagonal or circular interstices. The grid chosen will be complementary to the embossed or patterned effect which it is desired to produce on the moulded article.

If an open mould is used the grid may be brought to bear against the lower mould surface by hand or by mechanical means. When a closed mould is used, the grid is conveniently lowered from a raised position into the distributed foam-forming mixture by hydraulically or pnemuatically operated jacks extending through the bottom of the mould in fluidtight manner. In either case the grid is preferably pressed firmly against the lower mould surface to remove any residual mixture from the interface leaving the requirement amount of mixture in the cells formed within the interstices of the grid and the contiguous lower mould surface.

The amount of foam-forming mixture introduced into the mould will be such that it will foam to fill the mould. When a flexible polyurethane foam-forming mixture of high resilience is used, normally sufficient will be introduced into the mould to give a 15% 'over-fill' or thereabouts. 'Over-filling', sometimes called "overpacking' is a term used to express how much additional space the foam would occupy in relation to the mould cavity were it free to rise unrestricted by the mould.

With the mould closed, the foam-forming mixture is allowed to react and expand to fill the mould cavity and to set. The temperature of the moulding process is conveniently controlled by circulating conditioning water through ducts in the mould sections or through coils. The temperature chosen will depend inter alia on the nature of the reactants and desired residence time for each moulding.

After the foam has set and, normally, after an initial curing period, the moulded article is removed from the mould.

The inner surface of the mould and the grid are usefully coated with a release agent before the foam-forming mixture is introduced. A wax release agent is suitable for polyurethane foam-mouldings and can be sprayed over the mould surfaces before each moulding operation. De-moulding may be further assisted by tilting the mould base and blowing air between the moulded article and mould surface.

Raising the grid may also help to ease removal of the moulded article.

After de-moulding, high resilience polyurethane foam articles may be processed through a cell crushing mangle and a post cure oven.

Crushing the closed cells of these articles reduces shrinkage after curing. Raising the grid to eject the moulding may give some useful crushing effect.

In a further aspect of our invention we provide apparatus for making a foamed plastics article having an embossed or patterned face which comprises a sectional mould having an inner mould surface defining a mould cavity, part of the mould surface being a mixture-receiving surface, and a reticulated grid displaceable from a position remote from the mixture-receiving surface to a position in which it bears against the mixture receiving surface.

By sectional mould we mean a mould constituted by a plurality of sections or parts which can be clamped, interlocked or otherwise held firmly together during the mould process, the mould being dismembered into its component sections or parts to release the moulded article.

For moulding mattresses, the mould is conveniently a two-part mould, the two parts being a lid and base which can be clamped together by, for example, quick-acting toggle clamps.

In use, the foam-forming mixture may be poured into the base of the mould (the bottom surface of which forms a mixture-receiving surface), the grid placed in the bottom of the mould and the lid clamped in position before appreciable foaming takes place.

In our preferred apparatus the mould is provided with an inlet for introducing the foam-forming mixture into the mould cavity and a mechanism for displacing the grid within the mould cavity when the mould is closed.

One suitable mechanism for displacing the grid comprises one or more jacks actuated by means external to the mould and extending through fluid-tight seals into the mould cavity for attachment to the grid. Conveniently the jacks are operated hydraulically or pneumatically.

Preferably the grid is readily attached and detached from the grid displacement mechanism so that it can be removed for cleaning or replacement with a grid of different design.

Thus our invention includes, for use in the preferred apparatus, a grid adapted to be attached to the grid displacement mechanism.

It also includes the preferred apparatus devoid of the grid, the grid displacement mechanism being adapted to attachably receive a grid.

By way of example only, two embodiments of the invention will now be further described with reference to the accompanying drawings in which Figure 1 is a cross section of a first mould containing an embossing grid; Figure 2 is a plan view of a part of the grid shown in Fig. 1; Figure 3 is a perspective view of a different grid shown in relation to a lower mould surface; and Figure 4 is a cross section of a second mould containing a grid of similar design to that shown in Fig. 3.

In Fig. 1, a two part aluminium mould 11 comprises a box section 13 and lid 15 clamped together by G-clamps 17 and 19.

The mould 11 has an inner mould surface 21 which defines a mould cavity 23 having the approximate dimensions 600 x 450 X 150 mm3. A reticulated grid 25 rests on a mixturereceiving surface 27 which forms part of the inner mould surface 21.

The reticulated grid 25, shown more clearly in Fig. 2, comprises a regular array of mild steel rings 29 embedded in a polyester resin plate 31 about 50 mm thick.

In one example of the use of the mould, a polyurethane foam-forming mixture, prepared as described below, is poured into the box section 13, with the lid 15 and grid 25 removed. When the mixture has spread itself evenly over the mixture receiving surface 27 the grid, is inserted and pressed down into contact with the surface 27. The lid is clamped in position and the mixture allowed to foam to fill the mould. After the foam has set and undergone an initial curing period, it is removed from the mould together with the grid. The grid is then removed separately from the foam.

The inner mould surface and grid are spra yed with a wax release agent before the foam mixture is introduced.

The foam mixture is prepared by mixing together 36.7 parts by weight of an isocyanate blend comprising one part by weight of polymeric diphenyl methane diisocyanate and 4 parts by weight of 80/20 (2,4-/2,6-) tolylene diisocyanate, and 100 parts by weight of a polyol blend comprising 95% by weight of mainly a polyether triol, which is a 15% ethylene oxide tipped oxypropylated glycerol (MW 5300; OH value 32 mg/KOH; 75% primary OH groups), and 5% by weight of other ingredients including water, a catalyst, a silicone and surfactant.

The foam mixture has a viscosity of 20 to 25 c.p.; an expansion ratio of about 30; a cream time of approximately 7 seconds and a rise time of approximately 80 seconds.

In a second embodiment of the invention, shown generally in Fig. 4, a mould 37 comprises top and bottom mould plates 39 and 41, respectively, and side walls 43 held together by quick-acting toggle clamps (not shown). Lagged conditioning water coils 49 and 51 are welded to mould plates 39 and 41, respectively. The mould has an inner mould surface 45 defining a mould cavity 47 and an inlet 53 centrally located above a mixture-receiving surface 55. A grid 57 is detachably mounted on hydraulic jacks 59 and 61 which extend through fluid-tight seals 63 and 65 into the mould cavity 47 and which are raised and lowered by means (not shown) external to the mould.

Grid 57 is similar in design and movement to grid 33 shown in Fig. 3 which is displaceable from a position remote from a mixturereceiving surface 35 in a direction 'A', perpendicular to surface 35, to a position in which it rests against surface 35.

The mould is mounted on trunnion bearings (not shown) so that it can be tilted at an angle of up to 30 to assist the de-moulding process.

In operation, the inner mould surface and grid are sprayed with a release agent before the grid 57 is attached to the jacks 59 and 61. With the mould closed and the grid in a raised position (as shown in Fig. 4) remote from mixture-receiving surface 35, a foamforming mixture, prepared as described above, is injected through inlet 53. The inlet is closed and the foam mixture allowed to spread itself over surface 35 before the grid is lowered and held firmly against the mixturereceiving surface. During this time heated water is circulated through the coils 49 and 51 to maintain the mould at a temperature of about 46 C.

The foam mixture is allowed to expand, set and partially cure before being removed from the mould.

Ancillary equipment is commercially available for mixing and dispensing the foamforming ingredients, for temperature conditioning and for post-treatment of the demoulded article. This equipment can readily be adapted for use with the present invention by those skilled in this area of technology.

Claims (8)

1. A process for making a moulded foamed plastics article having an embossed or patterned face, which comprises introducing a liquid foam-forming mixture into a mould, allowing the mixture to flow over a mixture .receiving surface in the mould until distributed as required, bringing to bear a reticulated grid against the mixture-receiving surface and allowing the mixture to foam to fill the mould and set.

2. A process as claimed in claim 1 substantially as herein described with reference to the accompanying drawings.

3. Apparatus for making a foamed plastics article having an embossed or patterned face which comprises a sectional mould having an inner mould surface defining a mould cavity, part of the mould surface being a mixturereceiving surface, and a reticulated grid displaceable from a position remote from the mixture-receiving surface to a position in which it bears against the mixture-receiving surface.

4. Apparatus as claimed in claim 3 in which the mould is provided with an inlet for introducing the foam-forming mixture into the mould cavity and a mechanism for displacing the grid within the mould cavity when the mould is closed.

5. Apparatus as claimed in claim 4 devoid of the grid, the grid displacement mechanism being adapted to attachably receive a grid.

6. A grid adapted to be attached to the grid displacement mechanism in the apparatus claimed in claim 4.

7. Apparatus as claimed in claim 3 substantially as herein described with reference to the accompanying drawings.

8. Moulded foamed plastics articles whenever obtained by a process as claimed in claim 1 or 2 or from an apparatus as claimed in claim 3 or 4.