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BACKGROUND OF THE INVENTION
The present invention relates to methods for making negatives which are
intended for the production of bodies which contain cavities of
predetermined shapes. The term "negative" as used herein, therefore
signifies a disposable core member for example as used in some molding
techniques.
When manufacturing such a body it is in fact a wide-spread practice to make
a negative from an easily disposable substance which negative wholly or
partly matches the cavities which it is desired to form; to then wholly or
partly fill the empty spaces left in the negative with the material of
which the body will finally be composed; and then to dispose of the
negative.
One of the best known methods of producing such bodies is the lost-wax
casting method.
The shape of the negatives will obviously depend on the shape of the
cavities which it is desired to form.
A primary object of the invention is to produce negatives the structure of
which makes it possible to obtain bodies that include cavities which form
two independant systems of regularly distributed intersecting channels
with the channels in any one system intercommunicating via ducts which are
themselves regularly distributed. It is for example possible to have
intersecting channels which are flat and horizontal connecting ducts which
are cylindrical or polygonal and vertical. In what follows such bodies
will be referred to as "modules" and may be used in the field of
heat-exchanging and also in the field of filtration.
Another object of the invention is to allow the techniques of producing the
negatives to be adapted to the various methods of producing the module
proper, namely by applying the material of which the module is made to the
negative, by injecting the material into the empty spaces left in the
negative, and so on.
SUMMARY OF THE INVENTION
A first technique according to the invention for making such negatives
consists in stacking up plates made of an easily disposable substance
which plates are perforated with regularly spaced holes which leave
between them continuous bands of the substance. The intersections of the
bands from nodes and the width of the bands is less than the maximum
dimension of the holes. The plates are so stacked that holes match with
holes and nodes with nodes and they are spaced apart by means of spacers
the maximum cross-sectional area of which is the same as that of the nodes
and which are positioned over each of the nodes and the height of which is
greater than the thickness of the plates.
This first technique is particularly suitable where the material forming
the module is then applied to the negative in layers the thickness of
which is such that the height of the spacers is equal to the thickness of
one plate plus twice the thickness of the applied layer. In effect, in
this case, after the said material has been applied (and after any
treatment which may be needed to make it cohere) and the negative has been
disposed of, a module is obtained in which one of the two independent
systems of channels and their corresponding ducts is formed by a
proportion of the empty space in the negative, and second system being
formed by the voids left by the negative (namely the plates and spacers)
when it is removed.
The number of plates and their dimensions, the size, number and
distribution of the holes, and the size and number of the spacers depend
on the dimensions of the module which it is desired to obtain and on the
dimensions, number and geometry of the channels and ducts which it is to
contain.
The holes may be of any cross-sectional shape whatever: round, square, etc.
Similarly in the case of the spacers, which may be parallelepiped,
cylindrical etc.
The continuous bands of substance separating the holes may intersect at any
angle whatever.
The nature of the substances of which the negative (plates and spacers) is
formed depend principally on the nature of the material from which the
module will be formed, these substances and this material needing to be
compatible in the circumstances under which the module proper is produced,
and the substances of which the negative is made needing to be capable of
being easily removed without damage to the material of which the module is
made.
A second technique according to the invention for producing negatives may
be considered as a modification of the previous technique. It consists of
stacking plates which have the same characteristics as those in the
previous case, but doing so in such a way that the centres of the holes in
one of the plates, if projected perpendicularly, coincide with the centres
of the nodes in the previous plate, the said plates being spaced apart by
spacers the cross-sectional area of which is smaller than that of the
holes and which are positioned over each and every node.
This second technique is particularly suitable where the material of which
the module is made is introduced into the negative in such a way as
entirely to fill empty spaces in it. In fact, in this case, after the said
material has been introduced and possibly treated to make is cohere, and
after the negative has been removed, a module is obtained in which the two
independent systems of channels and their corresponding ducts are formed
by the voids which are left by the negative when it is removed, each
system being formed by the succession of voids which are left by pairs of
non-consecutive plates and their associated spacers.
The remarks made concerning the dimensions and number of the plates, holes
and spacers in the case of the first technique are equally applicable in
this case, allowing of course for the changes which have to be made in
view of the differences between them.
The same applies to the shape of the holes and spacers.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be more clearly understood, reference will
now be made to the accompanying drawings which show some embodiments
thereof by way of example and in which:
FIG. 1 shows a perforated plate with holes separated by nodes,
FIG. 2 shows a plan view of the stack of plates which is made,
FIG. 3 shows two cross-sections, taken along the lines A1 and A2 of FIG. 2
arranged contiguously side by side, through the stack, which are taken
through the same plate at the levels of the holes in the case of A2 and
the nodes in the case of A1 respectively,
FIG. 4 shows a modification, and
FIG. 5 shows how a third technique is operated.
In the drawings, for greater clarity, the holes are of square cross-section
and the spacers of circular cross-section.
SPECIFIC DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to the drawings, FIGS. 1, 2 and 3 are illustrative of the
second technique hereinabove referred to, FIG. 1 representing a plate
showing square-shaped holes 2 separated by nodes 3.
In FIGS. 2 and 3, the centres of holes 2a in a plate 1a coincide, when
projected, with the centres of the nodes on a plate 1b. Similarly, the
centres of the holes 2b, in a plate 1b coincide, when projected, with the
centres of the nodes on a plate 1a.
Over the nodes on plates 1a are positioned spacers 4a which hold two
consecutive plates 1a apart. Similarly, over the nodes on plates 1b are
positioned spacers 4b which hold two consecutive plates 1b apart.
The remarks made regarding the substances forming the negative in the case
of the first technique are equally valid in this case. It may simply be
pointed out that where the material for the module is dense, the spacers
may be made of the same substance as the plates and that where the module
material is compressible and needs to be compressed before the negative is
removed, the spacers need to be of a substance which is more compressible
than the material of the module.
Both these techniques give very good results but they have the drawback
that they involve the lengthy, painstaking and costly operation of
positioning the spacers at the nodes on each plate.
A first economising modification to these techniques consists in replacing
the plates and spacers with plates which are perforated with holes as
before but which are recessed at each node in a cylindrical,
parallelepiped or other shape. Each plate as shown in FIGS. 1-3 is
replaced by two recessed plates which are combined in such a way that the
recessed portions project from the combination, thus forming halves of
spacers as used in the previous techniques.
FIG. 4 shows this modification as a non-limiting illustration; it is a
cross-sectional view of a stack of such combinations made up of plates 5
which are perforated with holes 6 and contain recessed portions 7. The
stacking in this case takes place as for the second technique according to
the invention.
It may be pointed out that in cases where the material of the module is
dense, the stack of plates needs to be held together under light pressure
so that no material can penetrate along the plane of contact between the
recessed areas.
In cases where the material of which the module is made is compressible and
needs to be compressed, the recessed portions are compressed at the same
time.
A second modification to these techniques consists in replacing the plates
and spacers by plates which are perforated with holes as before but which
have at each node two reliefs which are symmetrical with respect to the
plate and which each act as half a spacer. The reliefs may be of
cylindrical, paralellepiped or any other shape. Having such relief, each
perforated plate performs the same function as the combination of two
perforated plates with recessed portions which were described in the case
of the previous modification.
It may be mentioned that in cases where the material of which the module is
made is not to be compressed, the plates may be of a non-compressible
substance. On the other hand, when the said material is to be compressed,
obviously the plates will need to be made of a compressible substance.
With all the plate systems described above the recessed plates or the
perforated plates and their spacers may be stacked manually or with
suitable equipment. In this case it is essential for them to be placed in
a mould the cross-section of which is of identical dimensions to the
plates, and when they have to alternate, for them to be trimmed.
A third technique according to the invention for producing negatives
consists in winding two series of strips of an easily disposable substance
alternately around a solid plate, which strips may be identical and are
regularly spaced apart from one another and each series of which is wound
in parallel directions, the said series of strips being wound onto the
said solid plate in two alternating directions which intersect.
This technique is comparable with the first one. It is in fact particularly
suitable in cases where the material forming the module is applied to the
negative in layers which are thin in comparison with the thickness of the
strips. As in the case of the first technique, after the said material has
been applied and possibly treated to make it cohere, and after the
negative has been removed, a module is obtained in which one of the two
systems of channels and associated ducts is formed by a proportion of the
empty space in the negative, and the second system by the voids which are
left by the negative when it is removed.
Each turn of the strips forming one series as it were represents a spacer
for the next turn of the strips forming the other series.
The dimensions of the solid plate, the dimensions and number of the strips
and their spacing, the numbers of turns of the strips, and the angles at
which the windings intersect, depend on the dimensions of the module which
it is desired to obtain and on the dimensions, number and geometry of the
channels and ducts which it is to contain.
FIG. 5 shows this third technique by way of non-limiting illustration. Onto
a solid plate 8 two series of regularly spaced strips, 9a and 9b, are
wound alternately. To make it easier to position the strips the edges of
the plate may be cut out as shown in the Figure. In this instance the two
series of strips are shown wound alternately in two mutually orthogonal
directions.
It should be noted that with this technique two negatives, corresponding to
two modules, are built up simultaneously, one on top of the plate, the
other below. It is even possible to build up 2.times. negatives
corresponding to 2.times. modules with this winding technique.
As with the previous techniques, the nature of the substance of which the
strips consist depends chiefly on the nature of the material of which the
module is made, this substance and this material needing to be compatible
in the conditions under which the module proper is produced, and the
substance of which the strips is made needing to be capable of easy
eradication without damage to the material forming the module.
A fourth technique according to the invention for producing negatives may
be considered as a modification of the third one. It consists in winding
two series of strips of an easily disposable substance alternately round a
solid plate under exactly the same conditions but with the various strips
held apart from one another by suitable inserts.
This technique is particularly suitable in cases where the material forming
the module is so introduced into the negative as completely to fill all
its voids. In this case, after the said material has been introduced and
possibly treated to make it cohere and the strips have been removed, a
module is obtained in which the two independent systems of channels are
formed by the voids left by the strips in the following manner: in any one
plane of strips, one strip of each two goes to form the first system and
the other goes to form the second system.
It should be noted that in this case, depending on the inserts used, it may
sometimes be necessary to make holes for the ducts which connect the
channels forming any one system in the finished module, as will be seen
below.
In effect, the inserts may be of different forms, viz:
a. they may be formed by spacers of an easily disposable substance which
are positioned on each strip at the projected points of intersection with
the other strips making up the same system. When the module is produced,
these spacers will be eradicated along with the strips, and the voids
which they leave will form the ducts between the various channels. The
height of the spacers depends on the thickness required for the wall which
separates the channels in one system from those in the other.
In this case a negative is obtained which is just like that obtained with
the second technique and the remarks made there are equally applicable
here.
The inserts just described, and shown in FIG. 3 have the drawback that they
require a lengthy positioning operation.
b. they may be formed by a third series of strips of a disposable substance
which are wound in another direction between each each of the two layers
of strips shown in FIG. 5 forming the negative proper. When the module
proper is produced, after the material has been fed into the voids in the
negative, this third series of strips is selectively removed while the
first two series of strips remain in place and is replaced in a second
operation by the material forming the module. The first two series of
strips are then removed.
c. they may be formed by perforated plates of a disposable substance which
perform the same function as the third series of strips in (b) and which
consequently involve the same operations when the module is being made.
The perforated plates concerned may advantageously be of the same material
as the module. When this is the case they remain in the module and are
embedded in the material forming the module when the module is made.
d. they may be formed by thicker strips than those forming the negative
proper, these thicker strips being arranged at the sides of the windings
between each layer of strips forming the negative proper, which latter are
stretched over these thicker strips. Such inserts may be used only in
cases where the method by which the module is produced does not cause the
stretched strips to distort.
As in the previous case, the dimensions of the solid plate, the dimensions
and number of the strips and their spacing, the numbers of turns of the
strips, and the angles which the windings make, depend on the dimensions,
number and geometry of the channels and ducts which the module is to
contain. The only difference results from the inserts, the thickness of
which depends on the thickness of the wall which is to separate the
various channels in the module.
It should also be noted that since the angle formed between the windings
may be of any size whatever, it is possible to produce negatives which
give modules in which the angle at which the channels intersect may
likewise be of any size whatever. This is particularly useful in cases
where the module is a heat exchanger. If the channels intersect at a large
angle, the streams of fluid flowing in the channels will be relatively
close to parallel streams (flowing in the same or the opposite direction).
Furthermore it is possible in certain cases that the pressure of the fluids
which flow in the finished module will be high and will call for channels
of relatively small cross-sectional area separated by a fairly thick wall.
To obtain a negative which meets these requirements it is then merely
necessary to reduce the width of the strips, or if need be to replace them
with wires, and to increase the thickness of the inserts.
Where the inserts are those referred to above under (b), (c) or (d), when
the module has been produced it is necessary to pierce holes in it at the
points where the nodes of the systems are situated, i.e. at the points
where the spacers are placed in (a), so as to connect the various channels
in each system together.
With this technique, as with the previous one, two negatives for two
modules are built up at the same time, one on top of the plate and one
below, and it is even possible to build up 2.times. negatives for 2.times.
modules using the winding technique.
As with the previous techniques, the nature of the substances of which the
strips and the inserts consist (when these latter have to be removed)
depends chiefly on the material of which the module is to consist, these
substances and this material needing to be compatible in the conditions
under which the module proper is made.
It should be noted that with this technique the strips may be removed
mechanically.
To seal the module at the sides, it is possible:
either, after introducing the material to form it and possibly treating it
to make it cohere, to trim the four sides and bond or weld on a layer of
the said material to seal the channels which open at these sides:
or, before introducing the material to form the module, to place the
negative in a mould in such a way that its sides do not touch the wall of
the mould.
Also, to use negatives produced with the last two techniques, the bundles
of strips are clamped between two plates and the strips are trimmed at the
sides.
In cases where 2.times. negatives are prepared for 2.times. modules, the
2.times. modules may be formed simultaneously and then cut up.
Finally, a few remarks may be made which apply to all the techniques:
the plates and strips employed may contain small holes which, when the
module proper is made, allow bridges of material to be created between the
walls which separate the various channels in the module, these bridges
endowing the module with greater strength and being capable of creating
turbulence in the fluids which flow through the channels, which they will
do to a greater extent the more carefully the holes in the plates and
strips are distributed and their cross-sections calculated.
from negatives produced with these techniques it is possible to obtain
modules:
the separating walls in which may be very thin, of the order of 1mm to
1/100mm or less,
the cross-section of the channels in which may be 1mm to 1/100mm or less,
in which the flow length for the fluids may be 1/10mm to 1cm and above.
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