Manufacture of polymeric foam

Details Manufacture of polymeric foam Manufacture of polymeric foam

1992-02-11

1995-02-28

Ball, Michael W.

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

156344, 264 458, 264 462, B29C 6722, B32B 3114

Patent

active

053933613

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to an improved method of manufacturing slabstock polymeric foam, especially but not exclusively polyurethane foam, and the invention also relates to an apparatus for use in the production of foam blocks by the improved method.
It is well known and common current practice to make polyurethane foam blocks by a continuous process involving the essential steps of laying down a mixture of foam reactants onto the bottom of a channel-shaped conveyor, which serves as a moulding trough of open-topped rectangular cross-section, allowing the mixture to foam and expand as it progresses along the channel-shaped conveyor, and when the expanded foam has cured, cutting the slabstock foam into blocks. It is usual for the bottom and sides of the conveyor to be lined with foils of paper and/or plastics which advance with the foam and prevent it from sticking to the conveyor walls. When the foam expands, the resulting foam bun tends to take on a crowned or domed shape at the top, as seen in cross-section. At least in part, this crowning effect is caused by the expansion of the foam being resisted by the foam being in frictional contact with the lining foils or walls at the sides of the conveyor. The convex shape at the top of the foam bun is highly undesirable since it reduces the useful volume of foam obtained and hence detracts from the economy of the manufacturing process.
Various proposals have been made for preventing the formation of the crowned top. For example, it has been suggested to raise the foils lining the side walls during the expansion in attempt to eliminate the frictional resistance. Another idea is to apply a top cover sheet, e.g. of paper, to the upper surface of the foam mixture soon after it is laid down on the conveyor, and as the foam rises, pressure is applied to the top surface through the top cover sheet to constrain the expanding foam and achieve a flat top transverse to the direction of conveyor movement. In GB 1487848 (Planiblock) for instance there is described an apparatus in which a series of rollers or runners (plates) carried on pivoted arms are arranged to bear down on the expanding foam. According to another proposal described in GB 1392859 (Hennecke), a levelling device in the form of a slatted grating or grid is arranged to float on the expanding foam mixture. The Hennecke system has been used commercially and has enjoyed a certain degree of commercial success. Applying the top cover sheet has an additional benefit in that it reduces the tendency for the foam to form a skin on the top surface due to the foam collapsing at this surface. However, this benefit is counteracted to some extent by the application of substantial pressure to the top surface by the weight of the levelling device, and as a consequence the foam has to rise against this pressure, and a skin of significant thickness is still produced in practice. A further drawback of applying substantial pressure to the top surface during foam expansion is that it can increase the variation in density over the height of the foam block, and especially in the upper layers, which means that some of the foam can fall outside specification requirements.
Another technique which has proved very successful in producing flat-topped foam slabs is described in GB 1354341 (Unifoam). According to this technique the foam reactants are delivered into a vessel or trough which extends transversely to the direction of conveyor advancement and which has an overflow outlet over which the mixture spills onto a downwardly inclined ramp or fall plate extending to the conveyor itself. The fall plate angle is adjusted so that, in essence, the foam expands downwardly and the top surface is maintained substantially level throughout the foam expansion process. This process is exploited commercially under the name "Maxfoam" and has been in successful operation for several years. The Maxfoam process is very effective in producing flat-topped blocks, whereby the conversion of the blocks is very economic. Nonethel

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Webster's II New Riverside Dictionary, 1984.

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