Method of manufacturing a hollow casting mold

A method of manufacturing a casting mold includes the steps of forming a casting mold body of a flowable binder-free backfilling material, stabilizing the casting mold body by negative pressure, and providing in the body a lost form of synthetic plastic material, particularly foamed synthetic plastic material.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of manufacturing a casting mold 
of a flowable, binder-free backfilling material, stabilized by negative 
pressure. 
It is known to utilize lost forms of foamed synthetic plastic material for 
manufacturing of cast parts. These forms are surrounded by sand provided 
with binding medium. Into the foamed synthetic plastic form molten metal 
is introduced so as to vaporize the form and to occupy its place. This 
technique, known as the full mold process, is widely utilized. However, it 
has the disadvantage that the outer face of the final cast part is rough 
and many cores are necessary. 
It has been proposed to pour the foamed synthetic plastic form in a 
binder-free sand. This method has the disadvantages that during the 
vaporization of the foamed synthetic plastic material a strong shrinkage 
of the form takes place and thereby the dimensions of the cast parts do 
not correspond to the initial dimensions of the form. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
of manufacturing of a hollow casting mold, particularly stabilized by 
negative pressure, which avoids the disadvantages of the prior art. 
More particularly, it is an object of the present invention to provide a 
method in accordance with which burnable or vaporizable forms of synthetic 
plastic material are utilized, so as to produce with relatively low 
technical expenditures cast parts having correct dimensions and smooth 
outer surfaces and so that, due to the omission of cores, cast parts may 
be produced which are of complicated spacial construction. 
In keeping with these objects and with others which will become apparent 
hereinafter, one feature of the present invention resides, briefly stated, 
in a method of manufacturing a negative pressure-stabilized cast mold of 
flowable binder-free backfilling material, in which a lost vaporizable or 
burnable form of synthetic plastic material is utilized as a form. 
Particularly, the lost form may be constituted of foamed synthetic plastic 
material. 
In accordance with the present invention, in the negative pressure 
stabilized casting mold of flowable binder-free material, particularly 
quartz sand, the negative pressure takes place in the backfilling mass. 
However, in the interior of the foamed synthetic plastic form a normal 
pressure or even a certain negative pressure depending upon the foaming 
process in the closed individual cells of the foamed synthetic plastic 
body takes place. Thereby, the foamed synthetic plastic form withstands 
the static loading through the backfilling mass and is not compressed by 
the same. 
The vaporizable or burnable synthetic plastic form may be designed as a 
full or integral form of foamed synthetic plastic material. It is also 
possible that it is composed of plates constituted by foamed synthetic 
plastic material which plates together form a hollow space wherein a 
suitable air pressure is provided. The latter also ensures that the 
negative pressure-stabilized backfilling sand will not be compressed. 
In order to improve separation of normal pressure in the foamed synthetic 
plastic form from the negative pressure in the binder-free backfilling 
mass, the foamed synthetic plastic form, especially formed by plates and 
bars as a hollow form, is outwardly coated by a synthetic plastic foil. 
This purpose can also be attained in a different manner. In accordance with 
another inventive feature, the form is coated by grained material having 
small particles and mixed with warm-hardenable or cold-hardenable binding 
material. After vaporizing or burning-out of the synthetic plastic form, 
this mixture forms the outer surface or wall of a hollow of the mold. Such 
a shell is more or less thick, in dependence upon the dimension of the 
form and the characteristics of the molten metal, especially its 
temperature. In such a method, the utilization of negative pressure in the 
backfilling material is not necessary in many cases. 
When the lost synthetic plastic form is coated with a refractory coating 
resistant to the molten metal, it is further proposed to heat the 
binder-free sand before pouring of the molten metal. The heating acts for 
vaporizing of the synthetic plastic form so that the vaporization is not 
performed only by introducing the molten metal. When the temperature of 
the binder-free mold material is not sufficient for full or partial 
vaporization of the synthetic plastic form, then the heated casting mold 
causes good flowing out of the molten metal, since it is only 
insignificantly cooled by the mold. 
The heating of the binder-free mold material can be performed so that the 
heated binder-free mold material is brought onto the coated lost form. It 
is also possible to bring the binder-free mold material to conventionally 
utilized temperatures in the casting box or the like casting containers 
and to heat it in the latter. This may be performed by flame heating or by 
electric heating. 
When the binder-free backfilling mass in heated state is applied to the 
lost form provided with the shell, it is recommended to perform this in a 
casting box or container which is under vacuum, whereby the pressure 
acting upon the form is not so high. 
In accordance with a further feature of the invention, after pouring of 
molten metal when hardening of a cast part starts, a portion of the 
binder-free mold material is withdrawn from the casting box. This 
withdrawal has the purpose of avoiding stresses caused by shrinkage of the 
cast parts. The withdrawal of the binder-free mold material is performed 
advantageously from inner regions of a hollow box-shaped cast part. The 
thus-withdrawn binder-free mold material which is heated by the molten 
metal is suitable for bringing into a next casting mold before casting. 
Thereby the synthetic plastic form of the next casting mold is either 
burned out or vaporized, or heated as described so that the molten metal 
is not significantly cooled and flows well. In accordance with this method 
it is possible to cast relatively thin-walled parts of metals which do not 
have high flow characteristics. 
The withdrawal of the binder-free mold material after sufficient hardening 
of the cast part so as to maintain the cast contours, can be attained by 
various means. For example, this can be attained by providing lateral or 
bottom openings in the casting box. Especially advantageous is the 
withdrawal by aspiration. 
The inventive method is especially utilized in connection with the foamed 
synthetic plastic form provided with fibers which increase the rigidity of 
the cast part. These fibers, particularly formed as a fabric with 
sufficient distance between weft fibers and warp fibers, can be 
constituted of carbon fibers which are utilized as reinforcement in 
manufacture of synthetic plastic materials. The fibers can be mineral 
fibers. Finally, metallic fibers may be provided. For example, when an 
alloy of aluminum is utilized, fibers or wires of a fine steel may be 
provided. Regardless of the dimensions and thickness of the cast part to 
be manufactured, this fabric and especially the wire, has the thickness of 
the reinforcement mats. 
When a foamed synthetic plastic form or foamed synthetic plastic plates 
with interposed wires, fabric or mats of metal are provided, the wires, 
fabric or mats are electrically heated, in accordance with a further 
inventive feature, before casting of metal, so that the foamed synthetic 
plastic material is vaporized or burnt-out. When the electric heating is 
not so strong as to fully burn-out the lost synthetic plastic form, the 
heating acts to ensure that the molten metal will not be so quickly cooled 
and thereby has good flowing characteristics. 
In accordance with the invention simple means are utilized for 
manufacturing of very complicated and especially thin-walled cast parts. 
For example, a ship hull having a length over 10 m of an aluminum-silicon 
alloy may be manufactured, and advantageously reinforcement from wires or 
a fabric of rust-proof steel may be utilized. For manufacturing of forms, 
the plates or foils of synthetic plastic material or foamed synthetic 
plastic material are utilized between which the reinforcement is arranged. 
After manufacturing of the lost form the latter is coated by a refractory 
mass. Finally, the backfilling material with binder-free mold material 
follows the coating. Advantageously after or during the filling, a 
negative pressure takes place in the binder-free mold material. 
Finally, the binder-free mold material is heated. The heating can be 
performed with the aid of tubular conduits located inside the binder-free 
mold material, and fillable by highly heated air, for example to 
1000.degree.C. In addition to or independently of this, the reinforcement 
wires or mats of steel are subjected to current, so that they are heated 
and thereby the synthetic plastic material is burnt out, or vaporized, or 
heated to prevent cooling of the molten metal poured into the mold. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS. 1 and 1A show a form of foamed synthetic plastic material. In 
accordance with FIG. 2 the form of FIG.1, arranged in a casting box 11 
which is closed from above by a cover plate 12. The cover plate may be 
constituted of a film of synthetic plastic material, and also of a sheet 
material. An inlet portion 13 with a pouring-in funnel 14, both 
constituted of a foamed synthetic plastic material, are arranged on the 
form 10. The casting box 11 has at its side a suction tube 15 and a filter 
15a. A vacuum pump is connected with the suction tube 15 so that a 
binder-free backfilling material 16, particularly quartz sand, is under a 
negative pressure. 
Since the closed cells of the foamed synthetic plastic material of the form 
10 are under a normal pressure, the backfilling material which is under 
the negative pressure exerts only a small static pressure upon the form, 
so that the contour of the latter remains unchanged. The molten material 
is supplied in the direction of the arrow through the pouring-in funnel 
14. It evaporizes the foamed synthetic plastic material and replaces the 
same. During this casting process the backfilling material 16 is under 
negative pressure. The negative pressure remains stable so that when the 
vacuum pumps operates during the casting process the products of 
combustion of the foamed synthetic plastic material are carried away 
through the funnel 14 and thereby can be so treated as to cause no 
problems. 
FIG. 3 shows that the negative pressure in the binder-free material leads 
to such consistency that the utilization of special cores is not 
necessary, since the negative pressure imparts sufficient rigidity to the 
binder-free filling material. 
FIG. 4 shows that the form 10 of the foamed synthetic plastic material is 
coated by a refractory mass. In other words, it is a coating of a known 
composition, which withstands the action of casting molten material. In 
the case of a low meltable alloy, such as aluminum, this coating mass may 
be constituted of gypsum, especially foamed gypsum. It is also possible to 
utilize mixtures of materials known in fine casting processes, which can 
be applied in especially effective and technically simple manner as 
coating on the lost form before enveloping of the same by the binder free 
material 16. The coating may have a low but also a higher gas 
permeability, whereas the gas impermeability of the lost form 10 due to 
the closed pores provides pressure a differential between the form and the 
binder-free backfilling material. The refractory coating can be produced 
by spraying, brushing or dipping of the form in a liquid or paste-like 
coating material. The coating, additionally to the binding material 
containing the same, is under negative pressure. The gas permeability of 
the coating leads to problemless casting. The form of the foamed synthetic 
plastic material of FIG. 4 further has an insulated or exothermic feeding 
insert. The latter may be formed as a hollow ball-shaped feeder 18 which 
is not provided with a coating 17. 
The form of foamed synthetic plastic material may also be coated by a foil 
of a synthetic plastic material which during the contact with the molten 
metal within binder-free loose sand such as quartz sand, forms a crust, 
whereas the foil acts as a binder medium. 
The casting inlet 13 with the pouring-in funnel 14 is formed by a hollow 
thin-walled tube of synthetic plastic material. It is also coated. Since 
the plate 12 is constituted of synthetic plastic foil, it overlaps the 
casting funnel from above. It is especially advantageous to provide a 
baffle of a thin metal lamella 19 which melts first when the casting 
funnel is filled with the liquid metal. Thereby the negative pressure is 
maintained in the mold. Similarly, the cylindrical funnel insert 18 is 
closed from above by the foil. 
FIG. 5 shows the heating elements 19a are arranged in the binder-free 
backfilling material 16. They may be formed by tubes through which hot air 
flows. They also may be formed as electrically resistant rods. 
FIG. 6 shows that the tubes 19b may be formed as components of the casting 
box. 
FIG. 7 shows in proportion that the form 10 is assembled from plates 10a 
and 10b of foamed synthetic plastic material between which fibers, wires, 
or fabric of carbon filaments, mineral fibers, or metallic wires are 
arranged. When for example an aluminum casting alloy is utilized, it is 
recommended to utilize fibers fabric of carbon fibers or of fine steel. 
Thereby the foamed synthetic plastic model is reinforced by fibers, wires 
and the like. Since later the foamed synthetic plastic material is 
replaced by the molten metal, indirectly or directly, the fibers, wires 
and the like reinforce the cast part. Such a reinforcement is advantageous 
when the casting material does not have high mechanical fluidity as, for 
example, in the case of well meltable aluminum-silicium alloy. In such 
cases it is recommended to utilize reinforcement from carbon fibers or 
advantageously a fabric or fine steel. The alloy is, however, advantageous 
for other metallic materials. Thus, for example, the alloy of cast iron 
has a reinforcement of steel. The thus obtained reinforcement of the cast 
part is independent of the coating of the lost form or the utilization of 
negative pressure in the backfilling material 16. 
The lost form 10a, 10b in FIG. 7 is also provided with a coating. In 
accordance with the embodiment shown in this figure the wires constituted 
of metal are so heated electrically that the foamed synthetic plastic 
material is vaporized or burnt out. The thus-produced hollows are 
maintained by the coating 17. When the synthetic plastic material is not 
vaporized, the casting material is poured into the form. 
In order to obtain unimpeded shrinkage of the cast member after the 
casting, the binder-free material 16 is aspirated through a suction 
conduit 21 and applied to a following casting box. This contributes to 
heating of the binder-free backfilling material in the next casting box so 
as to vaporize or burn out the embedded lost form of synthetic plastic 
material. In this case the special heating means shown in FIGS. 5 and 6, 
for heating already filled backfilling material, is no longer necessary. 
FIG. 8 shows that the lost form of synthetic plastic material is assembled 
from the plates 10a and 10b, and a reinforcing mat of steel wires is 
located therebetween. The plates 10a and 10b are glued to one another. 
FIG. 9 shows that the plates 10a and 10b are arranged at a distance from 
one another, and rods 21 and 21a of foamed synthetic plastic material are 
located therebetween. Mats of fibers or wires are located on sides of the 
plates, the sides facing toward one another, whereby two such mats are 
arranged at a distance from one another. Several such layers may be 
provided in a respectively designed lost form. The plates of synthetic 
plastic material located one above the other advantageously overlap the 
reinforcing mats. 
FIG. 10 shows that the foamed synthetic plastic plate 10a having a contour 
corresponding to that of the lost mold, is provided with a synthetic 
plastic foil 22. The latter leads to a very smooth outer surface of the 
cast part regardless of whether no coating is provided as shown in FIG. 2, 
or an additional coating 17 is provided as shown in FIG. 4. This foil of 
synthetic plastic material is simultaneously a "separating wall" between 
the backfilling material which is under negative pressure, and the form of 
foamed synthetic plastic material, when a negative pressure is utilized, 
which is not necessary in many cases. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
constructions, differing from the types described above. 
While the invention has been illustrated and described as embodied in a 
method of manufacturing a hollow casting mold, it is not intended to be 
limited to the details shown, since various modifications and structural 
changes may be made without departing in any way from the spirit of the 
present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.