Process for producing foam slabs of rectangular cross-section

In the continuous production of rectangular foam slabs wherein a foamable reaction mixture is foamed on a forwardly moving floor web and lateral confining webs, and in which a cover web is applied during the foaming process to the surface of the rising foam, the improvement which comprises supplying said cover web from a supply to the surface under controllable tension, thereby exercising a controllable pressure on the surface of the rising foam, removing the cover web from the foam surface beyond the point at which the rising mixture has substantially reached its maximum height and passing the web rearwardly and then forwardly to serve as the floor web for receiving further foamable mixture. Advantageously the cover web is heated before application to the foaming mixture, it is pressed against the foam surface with a pressure of about 2 to 4 kilograms per square meter, it is removed from the foam surface about 2 to 4 meters downstream of where the foam mixture has reached its maximum height, and it is dried before it becomes the floor web. A corresponding apparatus is provided, with vertically adjustable transversely extending horizontal control members governing the locations where the cover web contacts and leaves the foam surface. The control members may be as long as the cover web is wide or they may be from about 15 to 25 cm shorter.

BACKGROUND 
It is known in the continuous manufacture of rectangular foam slabs, in 
which a foamable polyurethane reaction mixture is freely foamed, to apply 
a gas-permeable overlay to the free surface of the rising foam during the 
foaming process. Such an overlay is intended to prevent the formation of a 
convexly curved surface on the foam slab during the blowing. The curvature 
is produced on the one hand by the sticking of the flanks of the slab or 
strand to the lateral boundary surfaces of the apparatus, and on the other 
hand by the formation of a solid skin on the top, which stretches 
arcuately. This undesirable cupping is supposed to be prevented by the 
known methods by having the overlay keep the surface of the slab flat. 
From German "Offenlegungsschrift" No. 2,123,216, it is known to lay 
gas-permeable webs on the rising foam underneath a lattice-like or 
jalousie-like decking for the production of rectangular soft foam slabs. 
The gas-permeable web is intended to eliminate the danger that the decking 
might sink into the foam mixture. 
The apparatus of "Offenlegungsschrift" No. 2,123,216, has, however, the 
disadvantage that the decking for the equalization of the foam surface is 
contaminated by the rising mixture. Therefore, German 
"Offenlegungsschrift" No. 2,165,902 provides that the gas-permeable web 
itself is in turn covered by an impermeable shielding web which is 
intended to prevent the foam mixture from wetting the superimposed 
equalization means. 
The above-described, known methods have the disadvantage that the 
gas-permeable web is clogged at least in part by the rising foam mixture 
and thus the uniform removal of the evolving reaction gas, which emerges 
from the foam surface especially after the end of the foaming process, is 
not assured. Furthermore, the use of a gas-permeable web which is removed 
from the foam slab after it has set and then discarded adds undesirably to 
the cost of the process. The use of an additional, fluid-tight shielding 
web, which consists for example of paper, makes the process still more 
expensive and complicated. 
THE INVENTION 
The object of the present invention therefore consists in devising an 
improved method of making rectangular foam slabs, whereby it will be 
possible to minimize the rejection of material through an optimum control 
of the cross-sectional dimensions of the foam slab during the foaming 
process, and at the same time improve production by a simpler and better 
arrangement of the apparatus. 
The subject matter of the invention is a method for the continuous 
production of rectangular foam slabs, in which a foamable reaction 
mixture, especially a polyurethane reaction mixture, is foamed on a 
forwardly moving floor web carried by a lowerable conveyor belt, and in 
which a covering web is applied to the surface of the rising foam during 
the foaming process, the method being characterized in that the web 
applied to the surface is under a controllable tension, and thus exercises 
a controllable pressure on the surface of the rising foam, the web is 
removed from the foam surface approximately at the point at which the 
foaming process is completed, the removed web is carried back against the 
direction of transport and dried, and is then returned to the process as a 
floor web. 
The pressure which the covering web controlledly exercises on the foam 
surface through a drag roll and, in some cases, an additional roll is 
approximately from 1 to 6, and especially 2 to 4, kilograms per square 
meter. The pressure to be applied will depend on the formula of the foam 
mixture. Fluid foam mixtures require lower covering web pressure than 
viscous foam mixtures. The pressure of the covering web on the surface of 
the foam mixture is produced by the drag roll. The drag roll is driven by 
a motor, an oil motor for example. By the reduction of the rotatory speed 
of the drag roll, the web which is advancing at the same speed in the 
foaming channel as the foam slab, is correspondingly tightened and thus 
the pressure on the foam slab surface is increased. 
Also subject matter of the invention is drawn to the use of an apparatus 
for the practice of this method, which has a lowerable conveyor belt 
systems for mixing and feeding the reaction mixture, vertical webs serving 
as walls for the lateral limiting of the rectangular foam slab, a floor 
web on the lowerable conveyor belt, and an upper covering web, the 
apparatus being characterized by a uniform, continuous web forming the 
covering web and the floor web, a supply roll, a drag roll, vertically and 
horizontally adjustable control rolls, a heating element and idle rolls 
for the guidance of the web. 
The control rolls and additional roll, if used, have a maximum length which 
corresponds to the width of the foam slabs, i.e., up to about 2.25 m. 
However, rolls can be used having a length approximately 5 to 20%, and 
especially about 10 to 15%, smaller than the corresponding slab width. In 
this manner the slab width can be increased or reduced during the foaming 
process by from 5 to 20%, and especially by 10 to 15%, without the need to 
replace the rolls. 
The continuous covering web and floor web preferably covers the entire 
width of the foam slab surface. The upper section is lifted away from the 
foam surface by passing around the control roll at a moment shortly 
following the transition of the foam from the plastic to the solid state, 
at which time most of the gas formed in the reaction begins to escape. 
This point is located about 1 to 6 meters, especially 2 to 4 meters, after 
the point at which the rising mixture has reached its greatest height. 
In the present invention, the foam mixture is, for example, a polyether or 
polyester-polyurethane foam. 
The conveyor belt can be lowered vertically also during the foaming. The 
angle at which it is lowered from the horizontal amounts to as much as 
10.degree. , and especially 3.degree. to 4.degree.. 
The method of the invention are especially suited for the production of 
rectangular soft foam slabs.

In FIG. 1 there is shown a supply roll 1 from which a web 4, preferably a 
paper web, is delivered over a drag roll 3 to the surface of the rising 
foam. If desired, a device 2 can be disposed between the supply roll 1 and 
the drag roll 3 for folding over or trimming the edges of the web 4. The 
point at which the web is laid on the surface of the foam is determined by 
the first horizontally and vertically adjustable control member 5. This 
point is preferably located where the initially concave surface of the 
rising foam undergoes a transition to a convex surface. The control member 
5 provides so that the web 4 will contact the foam surface without the 
roll 5 exercising pressure on the foam mixture. At a second horizontally 
and vertically adjustable control member 7, the covering web is lifted 
from the surface of the foam slab which has now reached its full height, 
and after the web has been carried over idle rolls 8 and 8' it passes 
below a heating element 9 for drying. 
The control members 5 and 7, mounted on horizontally shiftable pivoting 
arms, can be in the form of rolls and/or slides. The control members 5 and 
7 can be displaced in the vertical and/or horizontal direction by means of 
the adjustable pivoting arms. In this manner the point of first contact 
between the covering web and the reaction liquid and the point of removal 
from the foamed slab can be precisely adjusted. The control member 5 will 
be applied preferably at the point at which the initially concave surface 
of the reaction liquid or foam, as the case may be, undergoes a transition 
to a convex surface. The position of control member 7 will be preferably 
such that the slab will already have attained its full height or 
thickness. The position can be shifted, however, to any desired point 
within a relatively great length. The covering web can be lifted away 
shortly before or after the end of the liquid phase range of the foam. It 
is also possible, however, to refrain from removing the web until after 
the pores of the foam have burst open. 
The supply roll 1 can be associated, if desired, with an edge cutting means 
2 which laterally seams the covering web 4. The control roll 5 can be 
preceded, if desired, by a heating element which heats the web 4 before it 
is applied to the surface of the foam mixture. Between the control members 
5 and 7 there can be provided in some cases, if the reaction mixture 
requires it, an additional horizontally and vertically adjustable roll or 
slide 6. 
In addition to conventional heating elements which are capable of drying 
the covering web while it is moving past them, infrared heating systems 
are especially suitable. 
By means of idle rolls 10 and 11 assurance is provided that the web will be 
kept in alignment with the conveyor belt until it meets it and will remain 
centered thereon. 
Two lateral webs 12 and 12', which consist for example of paper or flexible 
plastic material, are joined to the floor web 4 by means of inserted 
strips of paper, for example, which have been folded at an angle. The 
joint can be produced, however, also by causing the side webs 12 and 12' 
to be folded around slightly laterally and then to be laid on top of or 
underneath the margins of the floor web 4. 
The apparatus for use in the invention can be adjusted with no special 
difficulty to different slab or block widths. All that is needed is to 
shift the side webs laterally. If the width of the foam slab is increased 
by more than about 20 to 25 cm, it will be necessary to replace the 
control members 5 and 7 on the shiftable pivoting arms with 
correspondingly longer ones. The width set in the apparatus can be 
increased during the foaming process from, for example, 2 m to about 2.25 
m, without the need to replace the control members 5 and 7 with longer 
control members. The apparatus will produce perfect slabs even if the 
members 5 and 7 are as much as about 15, and even about 25, cm narrower 
than the slab being formed. 
The foam film adhering to the removed covering web is uniformly spread out 
over the web and is very thin. The weight of this film amounts to about 50 
to 150 g/m.sup.2 depending on whether the covering web is not pulled off 
until after the pores burst or whether it is pulled off beforehand from 
the still liquid to semisolid foam surface. 
FIG. 2 shows a cross section taken along line A--A through the apparatus of 
the invention. The reaction mixture is here restricted by the floor web 4, 
the side webs 12 and 12' and the covering web 4. The control member 5 
extends over virtually the entire surface of the foaming reaction mixture. 
The laterally shiftable supporting surfaces of the foam channel have been 
omitted, as they have in FIG. 1. 
FIG. 3 shows a cross section taken through an embodiment of the invention 
in which an additional pressure roll 6 is used for the equalization of the 
surface. FIG. 3 shows a cross section taken through the apparatus along 
line B--B. 
FIG. 4 shows a cross sectional view taken through the apparatus of the 
invention along line C--C of FIG. 1. The covering web has been lifted from 
the surface. 
The principal advantages of the method and apparatus of the invention lie 
in the fact that the web 4 running over the control members 5 and 7 
represents a new means of applying the web which is much simpler than the 
known web applying means in the form of bars coupled together or 
individually laid-on bars or slide members. The pressure acting on the 
surface can be regulated substantially more accurately and with great 
precision. This regulation is accomplished especially by the drag roll 3 
and additionally, if desired, by roll 6. The rising foam mass can thus 
foam up freely without the danger of collapsing the foam by locally 
excessive downward pressure. 
The upper web is used as a floor web, which makes the process particuarly 
economical. 
The synchronization between the upper web and the foam slab and between the 
speed of the upper web and the floor web is provided by the simple fact 
that the floor web and the covering web are not separate but consist of a 
single web. This prevents the exercise of shear forces on the foam in the 
process of the invention, which are caused when the floor web and the 
cover web move at different speeds. These shear forces can result in an 
uneven surface on the foam slab or in cracks in the foam slab. 
The foam blocks or slabs produced with the apparatus of the invention have 
no differences in density either horizontally or vertically. The surface 
and side walls of the slabs are flat and display no unevenness. 
With the apparatus for use in the invention, foam slabs can be produced 
having a thickness of 1.10 to 1.25 m and a width up to 2.25 m. The 
thickness of the foam slabs, however, is not limited by the method or by 
the apparatus. The rate of advancement of the foam slab, which depends in 
part on the chemical composition of the foam mixture, amounts to about 3 
to 10, and especially 5.5 to 8, meters per minute. 
It will be appreciated that the instant specification and examples are set 
forth by way of illustration and not limitation, and that various 
modifications and changes may be made without departing from the spirit 
and scope of the present invention.