CA1102995A - Mould for casting at low pressure - Google Patents

Abstract

ABSTRACT
Mould for casting at low pressure, of the type defining a moulding cavity whose walls consist of agglomerated sand and which is connected by an inlet of small section to a vertical supply pipe closed at its upper end, characterised in that the walls of the moulding cavity are constituted by a layer of sand adhering to an outer metal shell. Costs are reduced and the production rate is increased in that less said and binding agent are required and the cooling is much more rapid.

Description

~a s ~2~5 ~ ne ~)resent iaventio~ relatec to low ~ressure fo7lnd~~ o,l~ c;
irl which a moulding cavity i~ used ,hose Jalls c~nsist of a~g~o~e~at~d sand and this cavity ;~ supplied throu~rh a vertica~' pipe closed at its up?er end and connected to -the mouldi~, cavit,y by an inlet of s;~all section.

~ `he inve~;ion relates in particular,'but not exclusivel~" to the casting of lamellar or spheroidal gra~hite cast iron.

It is known that moulding or castin,g at low pressure are freque~tly carried out in sand moulds, in particular in order to obtain certain metallic structures in the castings. ~his is the c~se for exan~le when one wishes to ~roduce rough castiQgs of sp~eroidal graphite cast iron, which contain no hydrocar'~ons, in t~e rough cast state, without requiring subsequent heat tre~t.~ent. -. -.
~ his result is due es~entially to the refractory nature of the sa~d for~ing a thernal barrier which prevents a phenomenon of hardening of the cast metal.
., One also benefits from the permeability of the sand to gas.

q'hus, moulds comprising walls of sand prevent foundry defects a~d reduce wastage.

~ he method of castin~ at low pressure is also kQo~rn, a~cording to whic~n the mould is filled under ~ressure, the ~ressure i~ maintained until the inlet has solidified, then the prescure is released. ~he inlet is calibrated to solidify after a period of time ~lhich is slightly greater tha~ the time necessary for sU~?
t~e lloulding cavity with an adequate qua~tity of molten metal in order to obtain a sound casti~g cand the section of -this inlet lS

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much less than that of the pipe, such that in practlce all the metal contained in this pipe re-descends into theca~tina ladel when the pressure is released.
However, the fact that -this method uses conventional sand moulds, which are solid, introduces drawbac]cs: owing to the fact that the Eilling pressure is mairltained for a prolonged period of time, the moulds must have relatively high mechanical strength. ~-t is therefore necessary to use sand having a higher proportion of binding agent and/or moulds which are thicker than in the case of casting by simple gravity. But this increases the cost price of the castings, since on the one hand, the moulds, which cannot be re-used, are more expensive to produce and on the other hand, cooling of the castings, is too slow, which considerably reduces the production rate, which is also due to the time required for the solidification of the inlet.
The invention intends to remedy these drawbacks.
To this end, the invention relate to a method for the low pressure casting of liquid metal in which the metal is supplied to a moulding cavity having walls of agglomerated sand by means of a vertical supply pipe closed at the top and a casting inlet of small section connecting the pipe to the moulding cavity, this method being characterised in that the thickness of agglomerated sand ~orming the moulding cavity is reduced and this moulding cavity is reinforced.
Cooling of the casting is speeded up by this method, whilst increasing the mechanical strength of the mould and en-suring a sound structure of the casting.
According to one variation, the method of the invention is characterised in that during casting, the molten metal is cooled more quickly in the region of the casting inlet than in the region of the moulding cavity, thus eliminating at least partly the thermal barrier constituted by the thickness of sand ''"'"
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in the reaion of the cas-tin~ inlet.
The invention also relates to a mould for carrying out the above-described method of casting at low pressure, character-ized in that the walls of the moulding cavity are constituted by a layer of agglomerated sand adherina to an outer metal shell.
Thus, with a small volume of material which can he re-used, the metal shell gives the mould the necessary mechanical streng-th and rigidit~ and the sliqht thickness of the sand and resin mixture may easily be chosen to give a relativelv rapid, predetermined cooling speed of the cast metal.
It is also possible to produce the supply pipe and casting inlet from a layer of agglomerated sand reinforced by the same shell.
As a variation, the casting inlet may have metal walls in order to increase the cooling of the metal cast at this point, thus eliminating the thermal barrier constituted by the sand and making it possible to release the pressure more quickly at the end of casting, which increases theproduction rate.
Further features and advantages of the invention will become apparent from the ensuing description, given as a non-; limiting example and with reference to the accompanying drawings in which:
Figure 1 is a diagrammatic sectional view of an in-stallation for casting at low pressure provided with a mould according to the invention;
Figure 2 is a partial detailed view in section of a variation of the mould according to the invention;
Figure 3 is a diagrammatic sectional view of another variation of the mould according to the invention, comprising cores for moulding two parts at the same time;

Fiaure 4 is a sectional view on line 4-4 of figure 3.
The casting installation of figure 1 comprises a ladle , 1 for castina under pressure, of known type, containing li~uid spherioidal or lamellar graphite cast iron 2. This ladle is providec~ with a removable cover 3, fixed in a sealed manner. A
vertical refractory casting tube ~ passes -through the cover 3 and its lower part terminates the vicinity of -the bo-t-tom of the ladle 1. A conduit 5 for supplying a pressurized aaseous fluid, for example compressed air, controlled by a valve 6, opens into -the ladle 1 throuyh the cover 3, above the maximum level of liquid cast iron 2.
The ladle 1 is placed in a frame 7 having an upper horizontal table 8 able to serve as a support for a series of successive moulds 9 according to the invention. As known, the top 10 of the refractory casting tube 4 projects slightly above the table 8 oE the frame 7.
The mould 9 is in two parts, a lower part 11 and upper part 12, separated by a horizontal median interface 13. Accord-ing to the invention, it is constituted by a metal shell 14 of outer parallelepipedal shape and a layer 15 of agglomerated sand adhering to the inner wall of this shell.
The inner wall of the shell 14 firstly defines a vertical blind cvlinder 16 open at the bottom secondly a cavity 17 similar to the moulding cavity. An approximately horizontal channel 18, whose section is much less than that of the cylinder 16, connects this cylinder to the cavity 17, on either side of the interface 13.
As a variation, the channel 18 could be inclined and/or located above or below the interface 13.
The wall 16 17-18 is not machined, but on the contrary rough. A series of channels 19 connects this wall to the res-pectively lower and upper outer sides of each half-shell.
The layer 15 is constituted by sand agglomerated for example by a polyisocyanate resin hardened by a chemical reaction ~r, - 5 ' ' :

s carried out cold with a gas containing an amine, bv the so-called "cold-box" method. This layer 15 of virtually uniform depth, covers the entire wall 16-17-18, with the exception of the lo-~Jer inlet of the cylinder 16. It thus defines a supply pipe 20, connected by an inlet 21 to the moulding impression or cavity 22, these three parts havinq walls consisting of agglomerated sand.
To produce the mould 9, the two half-shells which are themselves rough cast, are placed in a machine for blowing or "drawing" cores, of any suitable known type. The sand/resin mixture is injected through the channels 19, then a cold hardening gas, then compressed air are passed into the mass moulded in this way, through the same channels 19. The use of a shell which has not been machined, facilitates adhesion of the layer of sand and is very economical. Using this method, it is possible to obtain a layer of slight thickness, of the order of 10 mm, very quickly.
It has been found that this method also makes it possible to speed up stripping of the casting and thus to increase the pro-duction rate and the rate of re-cycling the shells.
In addition, since the operation is carried out cold, any thermal deformation of the shell is prevented.
~he operation of the installation is as follows:
An empty mould 9 is put in position on the table 8 of the frame 7 in order to connect the inlet of the pipe 20 to the upper end 10 of the casting tube 4. The junction may be sealed by any known means, for example by means of a refractory pasty coating as described in French Patent 2,295,808.
; The weight 23 is placed on the mould to prevent the - upper part 12 of this mould from rising when the metal is cast.
For example, the weight 23 may be a plate connected to the piston of a pressing ram.
Air, at a given pressure greater than atmospheric ,,/"'.'~

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p~-eSSI~I-C iS introduced through the pipe 5 a},ove the ]evel Or the li~uid cast iron 2. Owing to the fact ~hat the molJ]diny im--pression is empty, the liquid cast iron rises in the tube4, the pipe 20, passes through the inlet 21 and fills the cavity formed by the moulding impression 22. The gases liberated as the cast iron rises escape easily through -the pores of the layer of sand 15 and throuyh the blowing holes 19, which are thus used as air holes. This contributes to obtaining a sound casting.
The air pressure necessary for causing the cast iron to rise into the moulding cavity is maintained in the pipe 5 for the period of time necessary for filling the impression 22 of the mould and for solidification of the cast iron located in the casting inlet 21.
T'le a-r pressure in the ~i~e ~ is then released, such thac virtually all the liquid cast iron contained in the pipe 20 and ~he casting tube 4 drops quickly into the ladle, at the most leaving solidified residues on the walls of the pipe 16 and in the inlet 21.
The weight 23 is then removed and the mould 9 is removed from the installation, since cooling and strippina of the casting ill be continued outside the installation.
Owing to this installation and in particular owing to the mould 9 having a layer of sand 15 reinforced by a shell 14, the mould 9 is very rigid. In fact, the shell 14 gives the mould 9 increased mechanical strength, thus preventing any danger of cracking of the impression 22 consisting of sand. It is thus possible to cast parts of more complicated shape and to produce in the foundry casting details sometimes of small dimensions.
This great rigidity of the composite mould 9 is also advantageous for dimensional aceuraey of the casting, for pro-dueina a sound casting and for reducing wastage. In faet, expansior.
of the sand by pushing of the cast iron and thermal expansion is ~ - 7 -: ' ;
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composite mould of this type also benefits from the porosity of the sand and its refractory nature making it possible to elimate occlusions of gas and -the e~fect of surface hardening, which e~plai.ns the soundness of the castina. Furthermore, the technique of casting under pressure for which this rigidity of the composite mould is particularly well-suited makes it possible either to cast parts from metals having a high melting point at a temperature lower than when casting by gra~ity (for example at approximately 1320C instead of approximately 1420C for the cast iron), which speeds up cooling and consequently increases the .. :

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producti.on rate, or to cast thinner parts than when casting by gravity, at the same temperature (for example approximately 1420~C).
Furthermore, the consumption of agglomerated sand is much less in the case of the invention than in the case of moulds made comple-tely of agglomerated sand of the prior art and this results ln an appreciable saving in sand and time, as shown by the following examp].e:
Example:
In order to cast a manhole Eor a road, according to the invention one uses a layer of sand 15 having a depth of the order of lOmm, which for the weight of the rough cast part of 70 kg, limits the consumption of sand to 60 kg instead of lOOkg for a prior mould consisting entirely of agglomerated sand. For a rate of binding agent of 0.6% by weight of ASHLAND resin sand number 606 +0.6% by weight ASHLAND resin sand No. 306, the numerical data are:
- casting pressure of the cast iron: 0.8 bars;
- time for the increase in pressure: 1.8 secs;
- time fox filling the composite mould: 2.5 secs;
- stripping of the casting: 20 to 3Q minutes after theheainning of blowing of the mould, instead of 11/2 hours in the case of conventional mouldiny.
Thus, with respect to the use of solid moulds, the invention makes it possible to reduce all the times occurring in a cycle for low pressure moulding:
- the filling time, since the presence of the metal reinforcement 14 makes the sand mould sufficiently rigid and strong to withstand a rapid intake of cast iron;
- the time dur~ng which the pressure is maintained, since, owing to the slight depth ofthe layer of sand, the metal cools relatively quickly and consequently a supply sufficient for compensating for shrinkage as well as solidification of the inlet, `

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which is sliahtly later by constructio~, are obtained quickly.
~ he two above-mentioned periods of time relate to the presence of the mould on the table 8 and shortening of the latter makes lt possible to speed u the casting rate. But in addition, the other times relating to the overall installation are also reduced:
- -the cooliny time before stripping, owing to the slight depth of the sand;
- the time Eor producing a mould from two half-shells ; 10 owing to the small volume of sand used.
Consequently, for mass production, it is possible to use only a small number of metal shel]s which are re-cycled quickly, the arrangement consti-tuting a "rnoclular" installation of relatively low bulk. The advantage consists of great flexi-bility of the equipment, the number of "modules" being able to be modified easily as required.
According to the variation of figure 2, with the aim of increasing the production rate still further, a metal casting inlet 21a is used instead of a sand inlet. Metal "inserts" 24, for example of cast iron, copper or steel, are fixed to the shell 14, at the location of the casting inlet, for example eliminating part of the layer of sand at this point. Cooling of the casting inlet 21a is thus accelerated with respect to cooling of the cast iron contained in the impression 22 and the cast iron is trapped more quickly inside themoulding impression 15, which for a given cross section of the inlet 21a, makes i-t possible to release the pressure in the pipe 5 sooner, so that the cast iron drops back in the pipe 21 and tube 4, without any danger of suction of the cast iron contained in the impression 22. It is thus possible for the solidification time of the inlet to approach the period of time during which the impression is supplied with metal, leading reliably to a sound casting. It is thus possible for example .~
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3~i to reduce the period of time during which the moulaing pressure is maintained, for the same casting, from approximately 40 seconds in the case of the sand inlet 21 to approximately 15 seconds in -the case of the metal inlet 21a.
As a variation, instead of using inserts 24, the channel 18 of the shell may :itself consti-tute the metal inlet 21a. It is also possible to provide graphite inser-ts 24.
Naturally, with a metal inlet 21a, the structure of the moulded cast iron in the inlet is hardened, but this does not impair the quality of the casting in the impression 22.
The variation of the mould of figures 3 and 4 advantag-eously makes it possible to mould two parts simultaneously, with-out producing a burr, this without these parts being attached to each other at the time of stripping. In this example, a frame 25 of a manhole for a road and its cover 26 are cast simultaneously.
One uses a mould 9a in two parts lla and 12a, which is essentially similar to that of figure 1.
The frame 25 is cast in the lower part lla and the cover 26 in the upper part 12a. An inner mass of sand 27, forming a core, produces the inner shape of the frame 25 and the shape of the lower side of the cover 26~ The outer shape of the frame 25 as well as that of the other sides of the cover 26 are produced by the sand impression 22.
The supply pipe 20 is provided at its upper end with a radial protuberance 28 forming a supply basin at which point two horizontal casting inlets 29 and 30 originate, one for the frame 25, the other for the cover 26. The inlets 29 and 30 are separated by a core 31 of agqlomerated sand in the shape of a plate.
The edae of this plate opposite the pipe 20 rests on the mass of sand 27, whereas its side edges are received in shoulders 32 of the lower half mould lla and sandwiched between these shoulders and khe upper half mould 12a.

\ ,i' It should be noted that it is not absolutely necessary for the intakes 29 and 30 to be horizontal, nor parallel: they may be slightly inclined, convergent, divergent or in the form of a siphon.
Figure 3 shows the level N of the ]iquid cast iron 2 at -the -time when the air pressure in the pipe 5 has been released.
The cast iron falls back in the pipe 20 at the most leaving only traces in the casting basin 28 and in the pipe 20 itself. Small quantities of solidified cast iron remain in the inlets 29 and 30 which are separated from each other. The castings 25(frame) and 26 (cover) may thus be stripped independently, without any connection produced by casting.
This advantageously results in an improvement in the ratio of the weight of casting/weight of metal solidified in the mould. Time is also saved during casting, since two parts are cast at the same time and after casting, since it is no longer necessary to detach the two parts one from the other and energy is also saved due to the presence of a common supply pipe 20.
On condition of retaining the core 31, it is also possible to replace all or part of the sand walls of the inlets 29 and 30 by metal inserts or by the wall of the shell 14 itself.
It should also be noted that in the case of static casting by simple gravity, it would be possible to cast -the frame and cover of a manhole for a road in the same mould, but it would not be possible to obtain castings which are not attached by casting connections. In fact, the latter would remain full of cast iron, since, contrary to the technique of moulding at low pressure, cast iron is not removed from the supply pipes after casting.
Naturally, the invention may also be applied to the casting of aluminium or other metals.

Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method for casting liquid metal at low pressure, in which a moulding cavity comprising walls of agglomerated sand is supplied by way of a vertical supply pipe closed at its upper end and a casting inlet of small section connecting the pipe to the moulding cavity, wherein the depth of agglomerated sand forming the moulding cavity is reduced and the moulding cavity is reinforced.

2. A method according to Claim 1, wherein the liquid metal is cooled more quickly in the region of the casting inlet than in the region of the moulding cavity, thus eliminating at least partly the thermal barrier constituted by the depth of sand in the region of the casting inlet.

3. A method according to Claim 1 or 2, wherein the sand of the moulding cavity is agglomerated by means of a polyisocyanate resin which can be hardened by a chemical reaction when cold with a gas containing as amine, by the so-called "cold box" method.

4. A mould for casting liquid metal at low pressure, of the type defining a moulding cavity whose walls consist of agglomerated sand and which is connected by an inlet of small section to a vertical supply pipe closed at its upper end, wherein walls of the moulding cavity are constituted by a layer of agglomerated said adhering to an outer metal shell.

5. A mould according to claim 4, in which the supply pipe has walls of agglomerated sand, wherein the outer metal shell surrounds the walls of the pipe.

6. A mould according to claim 4 or 5, wherein the inlet is also constituted by agglomerated sand surrounded by the shell.

7. A mould according to claim 4, wherein the walls of the inlet are metallic.

8. A mould according to claim 7, wherein the walls of the inlet are constituted by the shell.

9. A mould according to claim 7, wherein the walls of the inlet are constituted by at least one metal part fixed in a passage in the shell.

10. A mould according to claim 4, wherein the shell is provided with channels connecting said walls of the moulding cavity to the outside of the shell.

11. A mould according to claim 4, wherein the depth of agglomerated sand is approximately constant.

12. A mould according to claim 4, wherein the sand is agglomerated by a polyisocyanate resin.

13. A mould according to claim 4 or 5, wherein the inner walls of the shell are rough.

14. A mould for casting liquid metal at low pressure, of the type defining a moulding cavity which is connected by an inlet of small section to a vertical supply pipe closed at its upper end, wherein the moulding cavity and the inlet are divided by cores into two impressions and into two inlets each extending from the supply pipe to one of the two impressions.

15. A mould according to claim 14, characterised in that the pipe comprises a radial protuberance where the two inlets originate.