Method for the treatment of the steel belts of a double belt press

In a double belt press, in which the steel belts are supported on the back by rollers, inherent compressive stress occurs on the side of the steel belts of a double belt press that are supported on the back by rollers, resulting in an undesirable bowl-shaped deformation of the steel belts. In order to avoid this deformation, at least the side of the steel belts which faces away from the rollers is subjected to treatment by shot blasting before being installed in the double belt press.

DETAILED DESCRIPTION 
The present invention relates generally to a method for treating steel 
belts and more particularly to a method for treating steel belts of a 
double belt press. 
Double belt presses of the above-mentioned type are disclosed in DE Patent 
Nos. 22 43 465, 28 19 943 and 37 04 940. These three references relate to 
thermal stresses caused by temperature differentials between the center 
and the edges of the steel belts, which produce a bowl-shaped deformation, 
particularly on the top side of the lower molding belt on which the 
dispersed material forming particle board is applied. The deformation of 
the steel belt significantly impairs the uniformity of the dispersion and 
the deviation from its horizontal alignment which accompanies this 
deformation. In addition, due to the different temperatures, additional 
tension occurs in the molding belts which is superimposed on the applied 
tension stresses and the bending stresses occurring at the deflection 
drums, reducing the useful lifetime of the steel belts. To reduce the 
thermal stresses, DE Patent No. 22 43 465 discloses the step of providing 
additional heat to the edges of the steel belts which project laterally 
beyond the roller area and the dumping area. DE Patent No. 28 19 943 
provides for corrugation of the steel belt edges, which results in a 
certain resilience. In DE patent 37 04 940, an edge material free of 
binder is allowed to run along the edge of the belt, thus maintaining the 
steel belts in contact with the rollers at the edge area and also ensuring 
good heat transfer at that location, thus producing a uniform temperature 
perpendicular to the steel belts. 
During the operation of the double belt presses of the type discussed 
above, there is another phenomenon, in addition to the tendency to produce 
thermal stresses, which is independent of the temperature but which also 
gives rise to undesirable tensions and deformations of the steel belts. It 
turns out that due to the constant movement of the plurality of rollers 
which transfer their force to one side of the steel belts in limited, 
almost linear regions while under high surface pressure, a certain plastic 
deformation coupled with compression of the surface occurs over a long 
period of time, which in turn leads to compressive stress in the steel 
belts in a direction perpendicular to their longitudinal axes. Since this 
compressive stress occurs on only one side of the steel belt, resulting in 
a state of tension that is therefore asymmetrical over the thickness of 
the steel belt, the compressive stress also produces a bowl-shaped 
deformation in the steel belts. This deformation is undesirable because of 
the increase in stresses produced at the deflection drums and the 
resulting disturbance of the bulk material as already described, with the 
latter, in particular, occurring in connection with the lower steel belt, 
on which are applied the glue-coated chips that form the layer which yield 
the particle board, in front of the actual pressing segment. Of course, 
the layer must be as uniform as possible over the width of the strip. 
It is known to attach an additional unit to a double belt press of the 
above-mentioned type which counteracts any undesirable deformations that 
have occurred during operation, by exposing to shot blasting the lower 
steel belt, thus compressing the surface in a manner similar to the effect 
caused by the rollers, but on the opposite side of the belt. 
However, a disadvantage of this arrangement is that the steel belts must 
first undergo the undesirable deformation before the treatment begins. In 
addition, the treatment requires an extended interruption in the operation 
of the press, which produces significant economic losses for production 
systems of this size. Finally, in practice, it is extremely difficult to 
prevent one of the shot particles from the shot blasting device installed 
in the double belt press from getting caught between the rollers. Even 
just one shot particle may cause damage in the area of movement of the 
rollers, resulting in the press being shut-down and significant repair 
expenditures. 
Therefore, the problem in the prior art is that there is no way to treat 
the steel belts in such a manner that deformations of the press do not 
occur without interruptions in the operation of the press or other 
complications. 
SUMMARY OF THE INVENTION 
The present invention provides a method for treating steel belts of a 
double belt press that exert pressure on a continuous strip for producing 
materials such as particle board. The materials are produced by passing 
the strip between the steel belts that form continuous loops such that the 
steel belts extend over the width of the strip and convey the strip in a 
forward direction. The double belt press also includes rollers rotatable 
in a longitudinal plane that extends perpendicular to the continuous 
strip, and a support construction forming pressure transfer elements that 
transfer the working pressure from the support construction to the steel 
belts. The rollers are disposed between the steel belts and the support 
construction. The method includes the step of surface treating at least 
the lower steel belt on at least a side thereof facing away from the 
rollers, whereby inherent compressive stress is provided in a region close 
to the surface of the lower steel belt facing away from the rollers. The 
surface treating step is performed before the lower steel belt is 
installed in the double belt press. 
The essential idea of the invention is that the compression treatment of 
the steel belt surface is performed before the belt is installed in the 
press. In other words, an undesirable deformation of the press does not 
first have to become evident, and the press also does not have to be shut 
down, in order to counteract the deformation. 
The inherent compressive stress generated in advance on the side facing 
away from the rollers equalizes the inherent compressive stress generated 
on the side facing the rollers during operation. A prerequisite for this, 
however, is that the inherent compressive stress does not constantly 
increase due to local deformation of the steel belt surface, but rather 
that their formation comes to a stop. This requirement is met for the 
steels which may be used to form the belts because of their inherent 
capacity to compress under plastic deformation. 
According to the invention, the treatment which takes place before startup 
may be performed on only the side of the steel belt facing away from the 
rollers or it may take place on both sides of the belt at the same time. 
If the pretreatment is carried out on only one side, the steel belt is 
first bent in a direction opposite to the undesirable deformation, but 
after a short time has passed during which the press is in operation, the 
inherent compressive stress which results on the side of the belt facing 
the rollers, a uniform stress state begins to form, which comes to a stop 
after some time, where the stress distribution is approximately 
symmetrical and the belt is essentially flat when in a state free from 
external forces. When both sides of the belt are pretreated, the inherent 
stress state produced in this manner is symmetrical right from the start, 
and thus the steel belt is installed in the double belt press in a flat 
state. Because the inherent stress formation has been anticipated ahead of 
time, and now comes to a stop, no changes which result in deformation 
occur when the operation of the press begins. 
For production of the inherent stress profile according to the invention, 
any suitable methods may be used in principle, such as thermal methods in 
which surface transformations are produced by means of a plasma or by 
means of laser or electron radiation, accompanied by an increase in the 
specific volume. 
In one particular embodiment, however, the inherent stress profile is 
produced by shot blasting. 
In this method, small steel balls are shot at high speed in an air jet 
perpendicular to the steel belt surface and essentially hammer against 
this surface, causing the elasticity limit to be locally exceeded, due to 
the very high surface pressure at the impact points, thus resulting in 
deformations which lead to the compression of the surface and thus forming 
inherent compressive stresses. 
The hammering and compressing effect is of foremost significance here, as 
opposed to wear, such as occurs in blasting with sharp-edge, very hard 
particles, such as during sand-blasting. This is also the reason for the 
use of balls, which have an overall convex shape that is not suitable for 
cutting and wear.

DETAILED DESCRIPTION 
FIG. 1 shows a double belt press for the production of particle boards, 
wood fiber boards and other board-shaped materials which consist of 
particles bonded by means of a binder that hardens under the effect of 
pressure and heat. The press includes an upper molding belt 1 made of a 
steel sheet with a thickness of approximately 1 to 1.5 mm, and a similar 
lower molding belt 2. Between the steel belts 1 and 2, a strip 4 of a bulk 
material 4', which consists of a material which can be dumped, is 
compressed in a pressing segment 3, which after the pressing produces one 
of the aforementioned materials. 
The upper steel belt 1 runs around rollers or drums 5 and 6 arranged 
perpendicular to the strip 4. The drum 6 is mounted in a fixed stand 7, 
while the drum 5 is mounted in a stand 9 which can pivot around an axis 
that is perpendicular to the strip 4. The stand 9 pivots about a bearing 8 
positioned on the floor. The stand 9 is moved via hydraulic cylinders, and 
the steel belt 1 is tightened in this manner. 
Similar to the belt the steel belt 2 extends around drums 11 and 12 
arranged perpendicular to the strip 4. The drum 11 is mounted in a fixed 
stand 13, while the drum 12 is mounted in a mobile stand 14 that is 
movable on rails. The stand 14 can be moved in the longitudinal direction 
(defined by the longitudinal axis of the strip) via hydraulic cylinders 
15, and the steel belt 2 can be tightened in this manner. The steel belts 
1 and 2 are driven via the drums 5, 6, 11 and 12. 
The steel belts 1 and 2 run through the device in the direction indicated 
by the arrows 16 and thus the bulk material 4' applied to the right side 
of the apparatus as seen in FIG. 1 is drawn into the pressing segment 3. 
The outgoing compressed strip 4 is removed from the left region of the 
steel belt 2 as seen in FIG. 1 by means of any suitable device, which is 
not shown. In the pressing segment 3, an upper support construction 17 is 
provided in the inner region of the steel belt 1, which cooperates with a 
lower support construction 18. The support constructions 17 and 18 support 
the regions of the steel belts 1, 2 which face the strip 4 and press them 
together flat with a great force. 
The support constructions 17 and 18 are each formed from individual beams 
19 and 20, which are each arranged opposite one another above and below, 
respectively, the steel belts 1 and 2 and the strip 4 (see FIG. 2). Each 
pair of beams 19 and 20 is clamped in place with side spindles 21 (see 
FIG. 3), so that individual pressure elements, with the force contained in 
the unit, are formed. 
Disposed between the beams 19, 20 and the steel belts 1, 2, are thick 
plates 26, 27, which transfer the force exerted by the individual beams 
19, 20 evenly over the surface of the steel belts 1, 2. The plates 26 and 
27 contain channels 40 (see FIG. 4) in which heating elements are arranged 
or through which a heating medium is passed. 
Roller chains 30 are arranged between the sides of the plates 26, 27 facing 
each other and the steel belts 1, 2. The steel belts 1, 2 roll on the 
roller chains 30 relative to the plates 26, 27. The steel belts 1, 2 run 
around the plates 26, 27 in a continuous manner, forming a loop in a 
vertical longitudinal plane. The rollers of the roller chains 30 transfer 
both the pressure and the heat of the plates 26, 27 to the steel belts 1, 
2, and thus to the strip 4 that is being formed. 
The roller chains 30 may return to the actual pressing region, i.e. between 
the beams 19, 20 and the plates 26, 27, after the chains pass a certain 
point at the end of the segment 3, which as indicated in FIGS. 2 and 4 is 
at the plate 26. An advantage of this embodiment is that the roller chains 
30 essentially maintain the same temperature during their course of 
travel. However, it is also possible to pass the roller chains 30 around 
the outside of the support construction, as can be seen in the bottom of 
FIG. 2, where the roller chains pass outside the support construction 18. 
According to FIG. 4, the plates 26, 27 are composed of a heating and 
support plate 43 and a separate return plate 44 with return grooves 42 for 
the roller chains 30. FIG. 4 is a partial cross-section through an edge 
region located above the strip 4 seen in to FIG. 2. 
The plates 43 contain the channels 40, which are connected together via 
pipe bends 45 to form a continuous conduction path. The plates 43 also 
contain smooth contact surfaces 41, which form the common rolling surfaces 
for the roller chains 30 arranged next to one another, as is evident in 
FIG. 5. 
When the steel belts I, 2 move forward, the roller chains 30 roll between 
them and the contact surfaces 41 of the plates 43 which face one another. 
Adjacent roller chains 30 are directly opposite one another with their 
outer frontal surfaces. 
It is essential to note that in the chain arrangement each of two adjacent 
roller chains 30 can be moved forward independently of one another. The 
totality of the support elements of the steel belts 1, 2 form a field 
which is divided into individual parts in the longitudinal direction, 
which can shift relative to one another with corresponding stress in the 
longitudinal direction. Therefore, no constraining forces can develop 
within the roller chain arrangement as the result of different transport 
by the molding belts. 
In the example of FIG. 2, the lower steel belt 2 is longer than the upper 
steel belt 1, so that it projects longitudinally beyond the upper belt 1 
on the right side, as viewed in FIG. 2. Therefore, a dispersion device 
(not shown) can be made accessible to this projecting region by arranging 
it above the top side of the steel belt 2. A layer 33 of wood chips or any 
other particles that may be used is applied to the steel belt 2 in a 
dispersion region 39 by the dispersion device, and the layer enters the 
pressing segment 3 in the direction of the arrow 16. The outer edge 31 of 
the bulk material 33 compressed to form the strip 4 lies within the edges 
of the steel belts 1, 2, as is evident from FIG. 4. In the pressing 
segment 3, the strip 4 exerts significant pressure against the steel belts 
1, 2, which is caught by the roller chains 30 and passed on to the contact 
surfaces 41. 
After the press has been in operation for a certain period of time, it 
turns out that the top side of the lower steel belt 2 in particular, has a 
bowl-shaped deformation in its cross-section in the force-free state, i.e. 
in the zone of the steel belt 2 projecting to the right according to FIG. 
2, in which the dispersion region 39 is located, even when cold or at 
uniform temperature. It must be understood that if a bulk material which 
must be transported further in an unsolidified state is applied to such a 
curved surface, uneven areas cannot be avoided. 
The occurrence of the bowl-shaped deformation of the steel belt 2 is 
explained with the aid of FIGS. 6 and 7. FIG. 6 shows a single roller 30' 
of a roller chain 30, on which the steel belt 2 rests and against which 
the steel belt 2 is pressed from above, under great pressure from the 
strip 4. In the contact area 46, which is exaggerated in the Figure, the 
steel belt 2 is elastically compressed, with material being displaced in 
the direction of the arrows 47. Locally, the elasticity limit may even be 
exceeded in the essentially linear zone 46, particularly in the center in 
an area around hard joint sites, giving rise to local plastic deformation. 
Such deformation processes are repeated when new rollers 30' constantly 
roll over one and the same site, so that over a certain period of 
operating time, an inherent stress state E develops, which is plotted in 
FIG. 7 over the thickness of the steel belt 2. On the side 2' of the steel 
belt 2 which faces the rollers 30', constant compression occurs, with the 
formation of compressive stress, countered by tension stresses in a region 
adjacent to the side 2' facing the bulk material 4, due to equilibrium. 
The stress distribution results in the steel belt 2 being more or less 
pressed apart at the "bottom" 2', in the manner evident from FIG. 5, so 
that a bowl-shaped deformation occurs as soon as the steel belt is left in 
its own inherent stressed state, without being influenced by outside 
forces. 
In order to avoid this phenomenon, the steel belt 2 is subjected to surface 
treatment by shot blasting before it is installed in the double belt 
press. In FIG. 8, a first embodiment is shown, in which the treatment is 
undertaken only on the "top" 2" of the steel belt 2 (i.e., the side of the 
steel belt 2 nearest the bulk material 4), which faces away from the 
rollers 30' in the installed state. The steel balls 48 are shot against 
the surface on the side 2" in an air stream, at high speed, and exert a 
similar local compression effect on the steel belt 2 upon impact as the 
roller 30' exerts on the opposite side 2' in the situation illustrated in 
FIG. 6. After treatment for a certain period of time by means of the shot 
blasting indicated in FIG. 8, an inherent stress state VE occurs, which is 
illustrated by solid lines in FIG. 8, and which approximately corresponds 
to a mirror image of the inherent stress state E shown in FIG. 7. Of 
course, this artificially induced inherent stress state which is brought 
about before installation of the steel belt 2 in the double belt press 
results in the steel belt 2 having a tendency to bend in the opposite 
direction relative to the bending shown in FIG. 5. 
If the steel belt 2 has then been installed and operated with continuous 
roller movement over the side 2' by the rollers 30', there are also 
inherent compressive stresses according to FIG. 7 in the vicinity of the 
sides 2'. An inherent stress state E according to FIG. 7 is therefore 
superimposed on the artificially induced inherent stress progression VE 
according to FIG. 8, produced previously, so that finally, the inherent 
stress state E' shown with a broken line in FIG. 8 is produced. The stress 
state E' is essentially symmetrical about the center plane of the steel 
belt 2, and thus does not lead to any bending of the steel belt 2. Since, 
due to the properties of the steel, the formation of inherent compressive 
stress does not constantly continue as the rollers 30' roll over the 
surface, but rather comes to a stop after a certain deformation has been 
produced, the symmetrical inherent stress state E' is maintained even 
during further operation of the steel belt 2 in the double belt press. 
In the embodiment of the invention shown in FIG. 9, treatment with shot 
blasting is undertaken on both sides 2' and 2" of the steel belt 2 before 
its installed in the double belt press, where the impacting steel balls 48 
produce surface compacting with inherent pressure stress close to the 
surface, so that after a certain period of treatment, an inherent stress 
state VE' occurs. The steel belt 2 is installed in the double belt press 
with this inherent stress state produced by the preliminary treatment, and 
this inherent stress state does not significantly change during subsequent 
startup of the double belt press because the inherent stresses no longer 
form after the steel has been subjected to continued stress for a period 
of time, due to the properties of the steel, particularly because the 
steel surface solidifies and thus the elasticity limit is no longer 
locally exceeded by the balls 48 or the rollers 30. In this second 
embodiment of the invention, the steel belt 2 also remains flat during its 
operation. 
A typical steel which may be used to form the steel belts 1, 2 has, for 
example, the following alloy components: 
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C &lt;0.09 
Cr 15.0 
Ni 7.0 
Cu 0.7 
Ti 0.5 
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(in % by weight)