Method and Apparatus for the Continuous Upward Production of Foamed Material

The invention relates to the production of foamed materials and in particular of polyurethane and other polymer foams, in terms of which it is largely described.

Expanded materials, particularly polyurethane foams, are made in both batch and continuous plant.

Batch production may be at any desired rate to suit subsequent conversion but is inherently labour intensive, can give variation from block to block, and is wasteful in giving blocks where all six sides are skinned and need trimming. The blocks, foamed in moulds, also show undesirable densification at the corners, which the rising and steadily more viscous material has to be forced to occupy by a weighted, floating cover or other means.

Continuous production as currently practised also has major disadvantages. The conventional, horizontal machines have inherent characteristics of large size and high minimum production rate, arising from the nature of the foaming reaction and the newly formed foam, and the curing time needed before material can be handled. This curing time, typically 5 to 15 minutes for polyurethane, sets the length of the plant once the rate of travel of the conveyor that carries the foamed material is determined. This rate of travel in turn depends on the height of the block required, only a certain steepness of profile being supportable by the material in the early stages of foaming and setting. An over-steep profile gives problems of underrun by dense, unfoamed material, or of slumping of unsupported newly foamed material, or both. The conveyor must travel fast enough to maintain the proper profile, giving a minimum production rate of for example 100-200 kg/min for 1 meter high polyurethane blocks, and thus a machine length of 40 to 50 meters. Any attempt to reduce the speed of the conveyor to give a lower production rate and hence in principle a shorter machine steepens the profile and causes underrunning or slumping, or both, making production of a uniform block impossible. Large machines thus have to be installed, at heavy capital cost, only to remain unused for much of their time.

The limiting factors are illustrated in the accompanying sketch drawings of the principle of two known kinds of machine for foaming polyurethane, the first (FIG. 1) having liquid reactants fed direct to a slightly sloped conveyor (about 6.degree. to the horizontal) and the second (FIG. 2) having the reactants fed to a trough, from which they spill over in the first stages of reaction onto a fall plate leading to a horizontal conveyor. Such machines are described in some detail in for example U.S. Pat. Nos. 3,325,823 to D. J. Boon and U.S. Pat. No. 3,786,122 to L. Berg.

The foam profile in the machine of FIG. 1 is approximately as shown at `B`, determined by the rate of feed of reactants at `A` and the rate of travel of the conveyor. The boundary between closed-cell foaming material and open-cell material after gelling and breathing is shown at `C`, the breathing zone being marked `D`. Cut-off is at `E`. The rate of travel has a certain minimum, since while to avoid slumping, on running more slowly, a more steeply inclined conveyor would in theory steepen the profile with respect to the conveyor but leave the effect of gravity unaltered, the liquid reactants fed at `A` would then underrun the foaming mass.

In the machine of FIG. 2 the risk of underrun problems has been reduced by conducting the first stages of reaction in the trough and feeding the resulting creamy and already somewhat viscous reacting mass to the fall plate `F`. There is still however a profile `C` between closed-cell fluid froth and open-cell newly gelled material, which cannot be steepened by reducing the conveyor speed and hence production rate without danger of slumping and thus loss of uniformity in the resulting block.

The difficulties with these machines are felt only when full size foam blocks at production rates below 100-200 kg/min. are required, but it is in fact only the very largest producers that can use capacity of that order. Many machines are as noted run only for an hour or two a day, the user spending the rest of the day handling the resulting foam. A typical small foamer producing say 2000 tons or less of finished foam annually might want production at 10% of the above rate for economic operation, and cannot accept the storage and handling problems of large quantities of foam and the cost and space requirements or a large machine. Even middle range producers who can accept the big machines, with their disadvantages, would be better served by machines of lower production rate.

There is therefore a potential demand, hitherto unsatisfied, for a continuous-production machine capable of running at for example 10 to 50 kg/min producing polyurethane foam, that is to say levels convenient to a typical small foamer's production.

We have now found that if the principle of existing machines is abandoned a low production rate machine can be provided. The invention stems from a realisation that horizontal production is unnecessary and that in a suitably designed machine foam can be drawn away upwardly rather than horizontally, without interference with the foaming reaction.

Proposals have in fact been made for the continuous upward production of foam, in West German Patent Specifications Nos. 1 169 648 (Munneke) and 1 504 091 (Continental Gummi-Werke), and in East German Patent Specification No. 61613 (Berbig), but we know of no practical application of these proposals. It appears that problems of control of the reaction and production of a uniform product had not been solved by the proposals as described, so that for all their disadvantages the large horizontal machines have been in extensive commercial use. Berbig is a generalised disclosure giving no process details or other indication that it was even run, and Munneke is the same. Moreover he shows a flat-bottomed reaction chamber of a type impossible in our experience to feed uniformly with the foam forming material. Even the most detailed of the three disclosures, that by Continental Gummi-Werke, does not recognise any difficulty in relating the machine construction to the stages of the foaming reaction to secure uniformity in the finished foam.

We have found that for successful continuous upwards foam production, it is necessary that foaming takes place in a diverging expansion enclosure the walls of which are provided with surfaces travelling with the foaming material, which enclosure is defined above by foam already expanded, at the sides by said walls and below by a feed zone for the foam forming materials, and leads into an upwardly directed take off path for the foam. Curing can occur, or at least begin, in the take off path.

As explained in detail later herein, the essential lies in the provision of the travelling surfaces in the expansion enclosure.

The greater part, though not necessarily the whole, of the foaming takes place in the expansion enclosure. Specifically, it will be appreciated that as the feed zone and expansion enclosure are continuous with each other, and the division between the two is regarded as where the travelling surfaces begin, foaming can start in the feed zone. Then, the foaming material contacts the travelling surfaces at an intermediate stage of reaction.

The latest satisfactory intermediate stage will vary with the kind of foam, but in all ordinary cases the stratum of the foaming material in which contact with the travelling surface is made will be less than 50% expanded in terms of the volume change from initial foam forming materials to finished foam. More usually the material will be less than 40% and often less than 30% or even 20% expanded.

The position in the machine of the stratum of a given degree of expansion is calculated from the free foam rise profile in a box test on a batch of the material, taken with the feed rate of fresh material to the machine, by summing the volumes reached by notional successive small volumes of material during their individual residence times and relating the sum to the combined volume of the feed zone and expansion enclosure of the machine up to a given height. Examples of these calculations are given in detail later herein.

The degree of expansion of the material, determining increasing viscosity and eventual gelling of materials such as polyurethane, is thus related to the latest suitable position for take up on the travelling surfaces. The increasing slowing of material that occurs adjacent to stationary walls as viscosity increases is countered, and holding back of material at the walls leading to gelling thereon does not occur. On the contrary, a smooth flow is maintained and blocking of the flow or intermittent loss of lumps of gelled material giving non-uniform properties in the product avoided.

The expansion enclosure diverges (in the sense of having increasing cross sectional area) over the greater part though not necessarily the whole of its volume. Thus it may both begin and end, and conveniently does at least end, with a non-diverging section. The former may arise if the travelling surfaces are brought in early, lower than the level at which expansion of the foam mix commences. The latter is convenient to allow for control tolerances during a run, so that completion of expansion does not have to be matched exactly to the cross-section and there is no danger of expansion being insufficient and leaving a gap of varying width between the expanded mass and the enclosure walls.

The speed of the travelling surfaces need not be the same as the translational speed of the foaming mix adjacent to them, though desirably the two speeds approximate. It will be appreciated that where for example a rectangular section expansion enclosure has two opposite parallel walls and the divergence of cross section is provided by the other two walls, webs of paper or plastics drawn over the diverging walls to provide the travelling surfaces cannot match the speed of the enclosed material both in the diverging and in subsequent parallel portions of the path of the material.

Thus in the above way the whole body of material is kept moving during expansion reducing eddy or river bank effects, preventing development of gelled or solidified material on the enclosure walls, and ensuring that material at any given stage of expansion is essentially in the same horizontal plane throughout and that a uniform body of foam is produced. In polyurethane foam production for example, the boundary between still-liquid and gelled material is horizontal or essentially so, uninfluenced by gravity. Underruns or slumping as discussed earlier cannot arise, and any convenient production rate can be adopted with a maximum depending on the length of the material path and thus the dwell time it gives. There is in the foam produced an inherent uniformity of properties, the `gravity history` of all parts of the block being the same.

The invention further provides plant for the continuous upwards production of foam, comprising a feed zone opening into a diverging expansion enclosure, means for feeding foam forming materials to the feed zone, and a take off path for foamed material opening from the enclosure, the enclosure having walls provided with surfaces that travel with the foaming material.

The invention is in principle applicable to the production of foamed materials of all kinds, but more specifically to expanded and in particular chemically expanded polymer foams. `Cure` is any chemical or physical process by which newly formed foam becomes handlable, and `reaction` any process by which gases expanding the foaming material are developed.

If achievable without serious sealing problems the whole expansion may as noted take place in contact with travelling surfaces, when the feed zone will be small, but for materials such as polyurethane foams, where the initial reactant mix is a thin searching liquid difficult to seal, it is preferable for the feed zone to be defined by an open topped vessel fed continuously from below with the foam forming materials, the travelling surfaces being introduced where this vessel adjoins the walls of the expansion enclosure. Such a vessel can readily be dimensioned to allow for first stage of for example a polyurethane foaming reaction to take place in it, a creamy and already somewhat viscous material passing to the expansion enclosure proper.

The shape of the expansion enclosure and feed zone is a matter of convenience but is generally such that the increase in volume in the direction of travel of the foaming material is matched to the expansion curve of the reacting material. There is then a substantially constant speed of upward travel in the mass of foaming material.

Conveniently the feed zone is in the form of a horizontal channel defined by walls continued at the sides by diverging walls and at the ends by parallel walls of the expansion enclosure between which the side walls lie. Such a construction, giving a generally wedge shaped expansion enclosure, lends itself to a variable expansion enclosure geometry, for example by pivoting or flexing the diverging walls, and hence to ready variation in block size while the plant is running by correspondingly moving out the walls of the take off path. It is further possible to have the diverging walls and feed channel telescopic, to allow the depth as well as the width of the block produced to be varied.

The above construction also lends itself readily to the drawing of travelling webs of material such as paper or plastics over supporting wall surfaces, the preferred method of providing the travelling surfaces. A paper web for example, folded round at the sides so as to give a lap applied to the back of such walls, readily follows their shape when drawn through from above, even if the shape is somewhat curved, without lifting off and without creeping over the wall edge. Stability is enhanced if the edges of the diverging walls have a longitudinal rib or bead over which the paper passes, and especially if a non-extensible tape is applied down the back of the paper near the edge of the wall (next to the bead if used).

An important factor in foam production for safety and consistent cell structure, certainly with polyurethane, is control of the pressure within the foaming mass. Since an inherent feature of the upwards foaming concept is that the expansion stage must be conducted in an enclosed volume, and since chemical reactions are prone to variation in rate due to such factors as temperature, degree of mixing or reactants, and impurities, provision for control of and pressure in the expansion enclosure is desirable. It is readily provided by, for example, a pressure transducer in the feed zone wall or any other position where the foaming mix is still liquid, and control of feed or take off accordingly.

In general however it is sufficient to observe the pressure as remaining within safe limits and at a small positive figure. The gelled foam in reaction such as the production of polyurethane cannot then be pulled away from the ungelled still liquid foaming materials below. A suitable pressure is for example 10 to 30 mm/Hg gauge.

Actual control is then by matching the take off rate of the volume of foam of a given density with the feed rate of volume of reactants of a given density (allowing for any reaction loss of materials that do not appear in the finished foam). As noted earlier it is important that the expansion should not have been completed before the foamed material leaves the diverging part of its path. If it shows a tendency to do so, either the total throughput can be increased, or the reaction rate at a given throughput can be slowed, for example by lowering the temperature or the catalyst concentration. Clearly for a fast reacting foam a smaller expansion chamber is needed at a given speed of removal of finished foam than for a slow-reacting one, as less time is occupied between introduction of the materials and their full foaming and thus less volume of finished foam will have been taken off in that time. If a given expansion enclosure is proving too large therefore, slowing the reaction will compensate. In practice a given expansion enclosure volume can be selected, a throughput chosen, considering in its simplest terms weight of reactants in and weight by volume of foam out, and fine control exercised by catalyst or temperature variation, the position of the full-foaming profile moving up or down according to the reaction rate.

The upwards expansion and movement of foamed material is conveniently vertical, but a path inclined from the vertical will serve provided that during expansion material at earlier stages of expansion and thus of higher density remains below material at later stages of expansion and thus of lower density. It may be advantageous to have an inclined path if, as discussed below, conveyors with pins are in use. The weight of the material will assist engagement with the pins on one conveyor as well as the weight being partly taken direct on that conveyor, while still allowing engagement of the pins on the other conveyor with the opposite face of the block. The path of expansion and movement of the foamed material will further normally be straight, but with a flexible foam a change of direction may be made if required once the foam has developed sufficient coherence. A large arc will be required, but will give the possibility for example of long lengths of foam for joining end to end and use in the manufacture of sheets by `belt` peeling.

Ordinarily, to avoid interference with the foam production by a weight of foamed material above, the foamed material is positively drawn away, conveniently by a conveyor defining a closed path for it, but this is not an essential of the invention. A conveyor may not be needed, and further the path of the foamed material may not need to be enclosed (unless to retain heat) once foaming is complete (in the case of polyurethane or other open-celled foam once `breathing` has taken place) and the material has cured sufficiently to be self supporting.

Preferably however, to reduce frictional effects, the path of the foamed materials is defined after as well as during foaming by moving surfaces. These surfaces may act throughout as conveyors, and indeed, once the foam has developed sufficient strength to sustain tensile forces, will certainly do so where there is adhesion between the foam and the surface. Adhesion of the foam may be sufficient alone where a conveying action is required, but, according to the strength of adherence of the foam to the surfaces and the nature of the materials, positive engagement as by pins or other means may be provided on one or more of the surfaces.

Pins of convenient length for polyurethane foam for example are about 1 cm, spaced laterally 5 cm apart and vertically 5 cm apart, though there is no restriction to any particular spacing or length or distribution of pins. Pins of such length (1 cm) enter only the thickness of the block that is normally skimmed off to clean up the faces, but in any case damage by the pins to the block is generally negligible.

As noted above, a conveyor may rely on adhesion of the formed body of foam, or if required may positively engage it, for example by the pins referred to, which take hold of the frame once it has developed sufficient strength. The conveyor surface, a travelling web of release paper or plastics film as well known per se in horizontal machines, may then be penetrated by the pins as it is fed.

It will be appreciated that the block size is not limited by the foam rise as in horizontal machines. Any block within the limits of size defined by the expansion enclosure and take off path may be produced. This size may in itself be variable if provision is made for varying the separation of opposed parts of the expansion enclosure or otherwise varying the path cross section.

For example as already mentioned the bottom edge of the side walls of the expansion enclosure may be pivoted and the corresponding walls of the take off path moved in and out to match, and/or telescopic side walls (and feed channel or the like) may be used and the end walls moved in and out. This is a valuable feature for machines of moderate output such as the invention gives, allowing small runs of various sizes of block.

A preferred shape for the expansion enclosure is as discussed earlier a wedge opening into a rectangular (or square) path for the formed foam. A feed zone, in the form of a channel to which foam forming materials are fed can then conveniently be placed at the base of the wedge and the main faces or sides of the expansion enclosure can readily be made adjustable as referred to above to give a variable included angle and hence block size. The length of and dwell time in the expansion chamber can, further, suit the particular material being foamed and the production rate in use.

The walls of the expansion enclosure are conveniently flat or shaped metal or like plates, but flexible members can also be used, particularly for diverging walls. The length can then for example be adjustable by altering the separation of end rollers or other guides over which the flexible members pass. Conveniently a continuous belt is used, one or more jockey rollers or like guides taking up excess. Ordinarily the belts will be moving, conveniently fed with a release paper or plastics film web as with the fixed walls referred to earlier.

The travelling surfaces at the diverging sides of a wedge-shaped expansion enclosure with either fixed or flexible walls can if so wished move at a greater speed than a conveyor carrying the formed foam away, according to their inclination to the main direction of flow, when all `river bank` effects in the movement of the material are avoided. Separate travelling webs can readily be provided to achieve this, but as already noted, such provision is not essential and it is in fact convenient to draw webs away from the top of the machine, to avoid a multiplicity of feeds and rolls.

To seal corners against leakage of materials where the sides of the expansion enclosure meet, close contact should be maintained, and this is readily possible where for example wrap-round webs of paper or the like are used as described. Alternatively it may be expedient to feed a release paper or plastics film in such a way that it laps inside the corners onto the face of an engaging member of the expansion enclosure, so that the pressure of foam forming materials or newly formed foam engages the lap and enhances the seal. Where feed zone and expansion enclosure meet, a flap of flexible plastics lapping onto the incoming webs providing the travelling surfaces gives a good seal.

The body of foam produced as described above is conveniently rectangular or square but there is no limitation to that shape. In particular, round blocks can be made with a suitably shaped feed zone, expansion enclosure and take off path, for direct use in the production of foam sheet by the `peeling` process. Any prismatic or indeed rounded form may be produced by an appropriately shaped and if required sectional expansion enclosure and take off path, fed with as many separate webs of release paper or plastics film as may be convenient. For circular section blocks, a convenient expansion enclosure is conical, formed e.g. of two or more sections fed with release paper or plastics film webs guided to enter where the feed zone meets the expansion enclosure. Where they enter, such webs are largely external of the expansion enclosure, folded back through gaps between the sections, but as they pass up the enclosure the excess is largely drawn in. Feed problems are not serious at the low rates of travel required.

The following detailed description is by way of example of the invention.

FIGS. 1 and 2 of the accompanying drawings are of known machines as referred to earlier, but the following description refers to the further accompanying schematic drawings, wherein:

FIG. 3 shows a general view of a complete machine;

FIG. 3A shows paper folder details;

FIGS. 4A and 4B show the machine in two vertical sections at right angles to each other;

FIG. 5 shows an expansion enclosure;

FIGS. 6A-6E show density profiles for foam blocks made in three conventional inclined-conveyor machines as referred to in relation to FIG. 1, using top papers to control top skin formation and improve block shape (FIGS. 6A, 6B, 6C); a conventional machine with trough and fall plate and a free top surface as referred to in relation to FIG. 2 (FIG. 6D); and the machine of the invention (FIG. 3) operated as described in Example 1 herein;

FIG. 7 is a plot of the combined volume of the feed zone and expansion enclosure (up to the parallel section) of the last referred to machine; and

FIG. 8 is a rise profile from a box pour of the materials used in Example 1 .

The machine shown may be used for example for flexible polyurethane, rigid polyurethane, polyisocyanurate, ureaformaldehyde, phenol-formaldehyde, silicone and epoxy based foams but are described in relation to the first of these.

In the machine as shown in FIGS. 3 to 5 an expansion enclosure 1 lies below a take off path defined by opposite fixed walls 2 and spiked slat conveyors 3. The machine is built on a frame (not indicated) with an access stair to a central platform and a top platform. A generally conventional inclined-path cut-off saw (giving a square cut) is provided at 6, to sever blocks 7 (FIG. 4) from the rising foam body 8, which blocks are tipped onto a conventional roller conveyor at the top right of the machine as seen in FIG. 3.

At one side of the machine is a generally conventional mixing head 10 delivering to a feed channel 11 by means of two delivery pipes 12. The feed channel lies between the lower edges of two curved expansion-enclosure side walls, the edges of which are indicated at 13, and two end walls 14. The side walls end at the take off path walls 2, and the end walls 14 at the slat conveyors 3.

In the expansion enclosure and take off path moving surfaces are provided on the end walls by means of polyethylene webs 15 fed from rolls 16 and drawn round the lower edges of the end walls 14 by the conveyors 3. The webs remain on the end faces of the foam blocks produced. The moving surfaces for the curved side walls of the expansion enclosure are provided by paper webs 17 fed from rolls 18 and drawn through the machine by powered pinch rolls 19' (FIG. 4A) to take-up rolls 19, separating them from the side faces of the block. The paper webs pass once through the machine and are scrapped after use. The slat conveyors and pinch rolls are synchronised by a common drive below the top platform, consisting of four shafts in a square formation with pairs of bevel gears at the corners, two of the shafts (one of which 19" is shown) forming the top axes of the conveyors and the other two (of which one, 19"', is also shown) carrying chain drives 19"" to the lower rolls of the pinch rolls 19'.

At the bottom of the machine, before it passes under a guide roll 22 and onto the side walls of the expansion enclosure, each paper web is folded at each side by two-part guides 20 of per se known kind to form an edge lap 21 at each edge 13 of the expansion-enclosure side walls, passing over beads of a few mm diameter (not seen), formed on the edges of the walls. FIG. 3A shows the successive stages (i) to (iv) of formation of a lap, from above (at the left of the Fig.) and in section with the guides 20 shown (at the right of the Fig.). To further assist in maintaining the position of the paper and hence cause it when within the expansion enclosure to follow the curve of the walls, an inextensible pressure-sensitive adhesive tape 21' is fed onto the under face of the paper before the folding, from reels 21" mounted on the machine frame, so as to lie adjacent to the bead after the folding. The tape passes with the paper under the rolls 22, and strengthens it for its transit through the machine.

In operation of the machine the take off speed of the foam is balanced against the rate of feed of reactants. A slight excess pressure is maintained in the expansion enclosure, for example of a few centimeters of mercury, and a transducer 23 in the base of the feeder channel monitors this pressure continuously.

FIG. 5 shows on a larger scale the curved shape of the expansion enclosure sides, matched to the expansion of the foam. The Fig. is, further, marked up with figures for height, elapsed time and % of final expansion reached at a given level for a particular HR*polyurethane foaming reaction. The dwell time in the main (diverging) part of the expansion enclosure is in this example approximately two minutes, and the foam reaches approximately 90% of full expansion (the expansion rate is substantially linear up to that point) at the exit from the diverging path provided by the enclosure. This level is marked D on the Fig. Above this is shown marked E the point at which 100% expansion is reached, that is to say at which the cells break and the material begins to breathe. The level F (about 1 further minute from D) is that at which the material becomes sufficiently strong to support tractive forces and the conveyor takes over. The `foaming height` is therefore no more than 1-11/2 meters. The breathing takes place upwards through the open-cell material above, and an incidental but very significant advantage of the machine is that an extractor hood can very readily be provided over the top of the foam path to remove the gasses breathed from the foam, where for example toluene dissocyanate (TDI) is the isocyanate used, to keep vaporised toluene dissocyanate levels surrounding the machine down to permitted levels without use of large volumes of extraction air. This is an important practical advantage, given the high cost of extraction equipment. Compared with mould processes and with horizontal foaming the exposure of material potentially giving off such materials is very small, almost the whole of the reaction mass being enclosed and part of the TDI given off reacting as it passes up the column. Further of course the actual rate of production of TDI fumes is low compared with that from high output horizontal machines.

The following are detailed examples of formulations and operating conditions.

The machine shown in FIG. 3 was used:

A. Dimensions

Paper=100 g/m.sup.2 kraft 1.78 meters wide

Film=45 micron polyethylene film 1.10 meters wide

Feed channel and expansion enclosure volume, total to parallel section=0.95 m.sup.3

Feed channel volume=0.03 m.sup.3

Conveyor speed=0.77 meters/minute

Conveyor length=4 meters

Total chemical input=37 kg/minute

Chemical input less reaction loss of 8%=34 kg/minute

Finished foam density=26 kg/m.sup.3

Area of cross-section of finished block=1.70 m.sup.2.

C. General Conditions

Temperature of reaction mixture=20.degree. C.

Rise time for 100% expansion (box pour)=110 seconds.

Chemicals mixed continuously in multi-component mix head with rotary mixer (3500 rpm). Air injected at rate of 1000 ml/minute for nucleation of foam.

Reaction mixture passed to the feed channel by two flexible plastics hoses 18 mm internal diameter.

Pressure of the reaction mixture monitored by pressure sensor mounted in the bottom-centre of the feed channel.

Observed pressure during continuous running=10-15 mm/Hg.

D. Examination of foam blocks

Block dimensions: 1.65.times.1.03.times.2.0 meters

Trim loss-all four skins removed=4% by weight

Block cross-section was rectangular

Trimmed piece density=26 kg/m.sup.3

Density variation

maximum density=26.8 kg/m.sup.3

minimum density=25.7 kg/m.sup.3

I.L.D. (indentation load deflection) hardness (203 mm diameter indentor, 460.times.460.times.75 mm sample)

50% compression

Mean hardness=22.5 kg

Maximum hardness=23.0 kg

Minimum hardness=22.0 kg

Hot compression set=4.5%

Tensile strength=130 KPa

Elongation at break=230%

The above shows production of a foam of good properties in all respects.

The machine was set up generally in the same manner as in Example 1.

B. General Conditions

Conveyor speed=0.93 meters/minute

Total chemical input=38.3 kg/minute

Chemical input less reaction loss of 8.5%=35.0 kg/min. During the run the level of amine catalyst was varied between 0.2 and 0.35 parts per 100 parts of polyol. It was noted that at the higher level of catalyst (i.e. faster expansion reaction) the block thickness reduced ("thickness" is the dimension across the top of the diverging section). Decreasing the amine catalyst level returned the block thickness to normal.

C. Product

Foam blocks of finished density 22 kg/m.sup.3 and of good properties in all respects when tested as in Example 1 were given.

A high-resilience foam formulation based on an ethylene oxide "tipped" polyol of molecular weight 6000 and a proprietary isocyanate-Desmodur*MT58 from Bayer Chemicals Ltd. was run on the same machine as Examples 1 and 2.

Foam density=36 kg/m.sup.3

Chemical input rate (net)=35 kg/minute

Conveyor speed=0.55 meters/minute

Again, good quality foam was produced.

Generally, foam produced by the method of the invention shows properties comparable to conventionally produced foam. In respect of symmetry of property variation in the block however the foam produced is superior, as FIGS. 6A to 6E show. (The diagram give % variation about means density, with mean density (A) and trim loss figures (B) alongside.) All diagrams are after skin trimming. The prior foams of FIGS. 6A to 6D all show a variation of properties broadly symmetrical about the vertical centre line, though with some variations where conditions at the two sides have not been quite the same. Properties through the block from top to bottom however show considerable variation, undesirable in principle and noticeable in use when converted to large articles, such as mattresses, especially if cutting is vertically rather than horizontally of the block. In contrast the properties of blocks produced by the method of the invention are substantially symmetrical about the block centre, as shown in FIG. 6E for the density of the foam blocks of Example 1.

In considering the detail of control of the foaming, with reference in particular to Example 1, reference may be made to FIG. 7, showing the volume contained by the feed zone, expansion enclosure and take off path of the machine of FIG. 3 at various heights above the base of the feed channel, and FIG. 8, showing the foam expansion rise profile for a box pour of the formulation of Example 1. In FIG. 7 the cumulative volume is plotted vertically and the height above the base horizontally. Perpendicular A is dropped at the height of the start of the parallel take off path, perpendicular B at the height of the start of the pinned conveyor. In FIG. 8 the percentage expansion is plotted vertically and the rise time in seconds (t) horizontally. Perpendiculars are dropped corresponding to an expansion of 10%, 20% etc. to 100%.

These plots are used to calculate the foam volume at any interval, based on theory as follows.

Let T.sub.100 be the time in seconds at which 100% expansion is reached, D be the final foam density in kg/m.sup.3, and W be the net foam input (total reactant input less reactant loss) in kg/min, and consider a time interval t.sub.1 -t.sub.2 seconds, then, ignoring volume of original unexpanded polymer:

Area under curve between t.sub.1 and t.sub.2 ##EQU1## Now weight of foam dispensed in this time ##EQU2## Volume of foam represented by this weight ##EQU3## Substituting .circle.1 and .circle.2 into .circle.3 ##EQU4##

The procedure is thus:

1. Plot rise curve--% expansion v. time in seconds.

2. Divide into vertical columns A, B . . . J corresponding to 10%, 20% . . . 100%.

3. Measure area of each column under curve.

4. Calculate volume of foam corresponding to each column A, B . . . J using formula ##EQU5## 5. From known reaction volume (volume of feed zone and expansion enclosure) relate to expansion height

The actual calculations for Example 1 are:

i.e. foam reached 100% expansion at a point 0.1 m above start of parallel section.

B--Calculation of conveyor velocity:

Net chemical input rate must match foam output rate on weight basis. ##EQU6## C--Theory of expansion control

Assume that velocity of conveyor is set for correct final density and throughput rate.

Consider each of three cases:

1. T.sub.100 just right

All expansion occurs by end of diverging section. Cell orientation maximised in horizontal direction. Density correct.

2. T.sub.100 too low

Foam will not make up to full width, the end of the diverging section not having been reached. Density will be higher. Higher pressure in expansion enclosure.

3. T.sub.100 too high

Foam makes up to full width. Some expansion occurs in parallel section. Cell orientation more isotropic. Pin grip reduced since foam now takes longer to develop strength.

In practice it is arranged for the T.sub.100 time to be marginally higher than theoretically required so as to ensure complete filling out.

The advantages of the production of foam as described and discussed above are we believe clear. In summary they are:

reduction of capital cost and space requirement for machines;

convenient production rate with smaller, continuously occupied labour force and less attendance required on machine in any case;

reduction of storage and curing space required for product prior to further handling;

lower rate and absolute amount of fume emission, giving ready and inexpensive compliance with statutory requirements;

reduced start up and shut down waste, and similarly in grade and colour changes, owing to the lower rate of operation;

no leathery top skin except at start up and thinner skins generally;

symmetrical distribution of physical properties in the block;

accurately controlled cross section.

All these matters add up to a major advance in foam production, allowing foam to be widely and economically made in moderate sized units serving individual markets where neither capital for major installations nor transport for their products over large areas are available.