US3786857A - Metal casting apparatus with mechanism for immersing jig and mould - Google Patents

Abstract

This invention relates to a process for casting articles from molten metal by the immersion of a mould or stack of moulds carried in a jig into a melt of the metal to be cast such that the cavity or cavities in the or each mould become filled with molten metal as the jig is immersed in the melt, whereafter the jig carrying the mould or stack of moulds is removed from the melt. Each mould cavity has a permanently open ingate and at least some of the heated air or gas within the cavity escapes through the ingate as the ingate passes through the surface of the melt, thereby preventing the entry of impurities from the surface of the melt into the cavity.

Description

ilnited States Patent 1191 Sutherland 1 Jan. 22, 19M

[5 METAL CASTING APPARATUS WITH 2,450,755 /1948 Higgins 164/136 x MECHANISM FOR IMMERSING JIG AND 3,295,171 1/1967 Strange et a1 MOULD 3,367,189 2/1968 Curry, Jr v 2,716,790 9/1955 Brennan [75] Inventor: Alan Sutherland, Bradford, England 2,959,8 9 9/1 57 Brennan 1,291,390 1 1919 B dbent ASSigYIeeI g pcn g g l g Llmlted, 3,463,221 8/1969 Ti iiZr 164/350 m an FOREIGN PATENTS OR APPLICATIONS [22] May 1971 553,551 6/1932 Germany 164/136 [21] Appl. No.: 141,114

' Primary Examiner-Robert D. Baldwin Related Apphcatmn Dam Assistant ExaminerV. K. Rising [63] Contmuauon-in-part of Ser. No. 762,201, Sept. 16, Attorney, Agent, or Fi H b w h ill &

1968, abandoned. Brisebois Foreign Application Priority Data ABSTRACT May 14, 1970 Great Britain 23508/ Th1s 1nvent1on relates to a process for castlng art1cles [52 us. 01 164/350 164/136 164/361 fmm metal by the immersiO 0f muld 164/129 249/119 249/126 stack of moulds carried in a jig into a melt of the [51] Int. Cl .1 B22d 23/00 metal to be cast Such that the cavity or cavities in the of Search or each mould become molten metal 35 the 164/335 130 g jig is immersed in the melt, whereafter the jig carrying the mould or stack of moulds is removed from the melt. Each mould cavity has a permanently open in- [56] References Cited gate and at least some of the heated air or gas within the cavity escapes through the ingate as the ingate UNITED STATES PATENTS passes through the surface of the melt, thereby preventing the entry of impurities from the surface of the ran 2,807,845 10/1953 Sawyer 164/37 melt mto the 2,968,848 1/1961 8 Claims, 9 Drawing Figures Carter 164/136 UX PATENTEBJANZPISH SHEET 2 [1F 3 SHEEI 3 [1F 3 PATENTED JAN 2 2 I974 ///A i I Fig. 8

This case is a continuation-in-part of US. Pat. Ser. No. 762,201 filed Sept. 16, 1968, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to an improved apparatus for the casting of articles from molten metal.

Conventional metal casting processes wherein the molten metal is introduced into a mould by pouring have a number of disadvantages.

Firstly, pouring under gravity introduces turbulence in the metal stream which, by mould erosion or other disturbance of the mould and metal interface, leads to the inclusion of non-metallicsin the casting.

Secondly, decanting metal from a furnace to a pouring ladle can lead to a loss of metallurgical control and metal temperature, which is aggravated in cases where relatively small volumes of molten metal are poured some distance from the melting furnace.

Furthermore the running systems required to distribute metal from the pouring cup to the mould cavities proper necessitate considerable wastage of mould material and cast metal. This wastage can assume major proportions in small thin-section castings of high surface-area-to-volume ratio, even when these are unfed. Also extensive running systems involve progressive heat loss in the advancing metal stream which may seriously restrict the range of conditions within which satisfactory castings can be made.

Moreover, metallostatic pressures exerted on nonrigid moulds can lead to excessive mould distortion/db lation with attendant shrinkage problems and loss of accuracy.

In addition, the pouring operation represents a major technical obstacle to the development of more highly mechanised foundry processes.

Attempts have been made to cast metal by immersing a mould or moulds in a melt of the metal to be cast. However such methods have required the provision of valves or mechanisms to work within the melt, which have not proved to be satisfactory and have been extremely costly to produce. Other immersion casting processes require the use of costly two-part moulds or rotating moulds and again do not give satisfactory and economic results.

It is an object of this invention to provide a simple, economic and effective immersion casting apparatus for metals.

It is a further object of the invention to provide a metal casting apparatuswhich prevents impurities on the surface of the melt from entering the mould cavity as the mould is immersed, without requiring the provision of complex and expensive valves.

It is another object of the invention to provide a metal casting apparatus which may employ either expendable gas-permeable moulds or permanent moulds. By a permanent mould is meant a mould that can be used more than once.

SUMMARY OF THE INVENTION From one aspect, the invention provides a apparatus for the casting of articles from molten metal wherein a mould or a stack of moulds, the or each mould including at least one cavity having a permanently open ingate, is held in a jig and said jig carryingsaid mould or stack of moulds is immersed into a melt of the ferrous metal to be cast such that the or each cavity in the or each mould is filled with molten metal through its ingate as the jig is immersed in said melt and at least some of the heated air or other gas within the cavity escapes through the ingate as the ingate passes through the surface of the melt, thereby preventing the entry into the cavity of impurities from the surface of the melt, and wherein the jig and mould or stack of moulds are removed from the melt after the mould cavity or cavities have been filled.

From another aspect, the invention provides apparatus for the casting of articles from molten metal comprising a mould or a stack of moulds, the or each mould including at least one cavity having a permanently open ingate arranged to communicate directly with the melt when the mould is immersed therein, a jig for supporting said mould or stack of moulds, and a mechanism for immersing said jig carrying said mould or stack of moulds into the melt of ferrous metal to be cast, such that the cavity or cavities in the or each mould is/are filled with molten metal through the ingate, and for withdrawing said jig and mould or stack of moulds from the melt after the cavity or cavities have been filled.

The mould or moulds may be made of conventional gas permeable mould materials such as green sand, dry sand, oil sand, CO sand or resin bonded sand, and are expendable after casting although the mould material may be reclaimed in some cases. Permanent moulds may be made from metal, e.g. an alloy of molybdenum. The support for the mould or moulds during casting comes from the surrounding metal mass so equalising the pressure difference between the outside of a mould and the casting cavity. In the case of a stack of moulds the moulds may be made of a material of lower density than the metal to be cast and become clamped together by the difference in density between the moulds and the casting metal. Thus, the moulds of a stack may be loosely held in the jig and tend to float upwards in the jig on entry into the molten metal so producing a clamping force between the moulds which is maintained during immersion. The ingates and venting systems are designed to ensure that the metal from the mould cavity does not drain away on withdrawal from the metal and that air and generated gases are safely and adequately carried away during casting.

The apparatus is particularly suitable for small components made in quantities which justify large scale melting furnaces and which hitherto have demanded relatively complex multipiece mould assemblies; i.e., stack moulds. In the present process, the mould has no pouring cup or running system, and each cavity is gated direct from the metal bath with no necessary interconnecting passages between the separate moulds of the stack. If the castings are unfed, it is only necessary to ensure that the ingates freeze sufficiently rapidly to prevent metal running out of the mould as it is withdrawn from the melt. This can be achieved by suitably dimensioning the ingate of each mould in relation to the cross-section of the mould cavity. if some feeding is required, the mould cavity geometry must be altered to ensure retention on withdrawal of an adequate reservoir of molten metal, or the retention of the mould in the metal until the casting has solidified.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of a gas permeable mould for making a small casting such as a piston ring.

FIG. 2 shows, partly in section, a stack of such moulds assembled in a jig, ready to carry out the process of the invention.

FIG. 3 is a diagrammatic view, in section, showing the immersion of a stack of moulds in a melt of metal to be cast.

FIG. 4 is a perspective view of a metal casting plant for carrying out the process of the invention;

FIG. 5 is a plan view of a further embodiment of mould.

FIG. 6 is a perspective view of a permanent mould for making a small casting such as a piston ring;

FIG. 7 is a cross-section of a stack of such moulds assembled in a jig;

FIG. 8 is a diagrammatic view, in section, showing the immersion of a stack of permanent moulds in the melt of ferrous metal to be cast; and

FIG. 9 is a plan view of a further embodiment of mould.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION FIG. 1 shows a mould for castinga piston ring, the mould being in the form of a biscuit made for example as an air-blown compact in resin-bonded shell sand. The biscuit 1 contains an annular cavity 2 with an ingate 3 and vent gates 4, and forms a complete mould when a similar biscuit is stacked on top of it. The vent gates open into a central vent 4a. More than one ingate can be provided if necessary. There is virtually no surplus sand in the construction of the biscuit since the mould is reduced to the minimum dimensions needed to support the components and can be any external shape desired. In fact the mould walls can be of such a thin section that they would distort or fail if used for casting by pouring molten metal, although they are of adequate strength for the process of this invention.

FIG. 2 shows a stack of such moulds mounted in ajig J ready for immersion in the melt with the outside of the moulds exposed to the melt. The stack may comprise moulds 1, with a cover mould la provided to cover the cavity in the top mould. The jig may be made of mild-steel, coated with carbon blackening or other suitable materials. Such mild steel jigs appear to have an indefinite life, provided they are maintained coated between casting operations. The jig J comprises a steel base 5 on which the moulds are stacked, the base itself being mounted on a supporting yoke 7 whose upper end carries a tubular guide 8 which is free to move up and down relatively to a spindle 9. A refractory block 6 e.g. of carbon, provided with vent holes 60, is suspended from the centre spindle 9, whose upper end is provided with a part 9a so that it can be attached to a suitable mechanism for lowering the assembly into the molten metal and withdrawing it therefrom.

The jig loaded with moulds is immersed in the bath of molten metal to a level about halfway up the block 6. At the start of immersion the bottom moulds are clamped tight by the weight of the moulds above them and commence to cast. The whole stack then floats up until the top mould is in contact with the block 6 at which point the clamping pressure is sustained by this floating effect. Apart from this floating action, the

moulds remain static in the jig. After a period of time, sufficient to fill the moulds, they are withdrawn and allowed to cool. The expansion of the air entrained in the moulds prevents premature entry of partly chilled metal into the mould cavity and thus promotes clean castings. It also prevents impurities on the surface of the melt from passing through the ingates 3 into the mould cavities. The equalisation of metallostatic pressure inside and outside the mould minimises mould distortion, whilst the provision of direct entry from the casting metal via the ingate 3 of each mould cavity improves consistency of the castings obtained. There is little turbulence, and the whole operation is amenable to automatic control. For denser mould materials, e.g. zircon sand, mechanical clamping can replace the displacement clamping of the moulds described above. This can be achieved by threading the spindle 9 in the tubular guide 8 so that the stack of moulds can be mechanically clamped between the base 5 and the block 6 and remain static in the jig.

FIG. 3 shows diagrammatically the start of the immersion of the stack of moulds into the melt of metal 10. The bottom mould has filled with molten metal through the ingate 3 and the next mould is just at the surface 10a of the melt. During the immersion of the stack of moulds, those moulds which have been immersed in the casting metal at any instant are generating gases which have to be evacuated. The evacuation of these gases relies upon the gas permeability of the moulding material assisted by the vent gates 4 to the central vent 4a. The gases pass to atmosphere through the apertures 6a in the hollow block. Another important feature associated with this natural generation of gases and expansion of entrapped air in the moulds is that, because of the mould permeability and the connection of the ingates 3 to atmosphere, a proportion of these gases under pressure is forced out of the ingates 3, mould walls 1, and joint lines of the mould not yet immersed, and this phenomena ensures that the impurities 10b on the surface 10a of the metal bath 10 are blown away, thereby ensuring that only clear metal can enter an ingate 3 when the pressure of the molten metal overcomes the gas pressure and running of the casting commences. The directions of the gas flow is shown by the arrows in FIGS. 2 and 3.

Tests on piston rings have shown that problems associated with shrinkage which are prevalent on castings made of low carbon equivalent cast irons produced by conventional metal pouring methods of casting have either been substantially eliminated or drastically reduced by means of the process of the present invention. Such shrinkage problems have usually been dealt with in metal pouring processes of casting by the provision of feeders either incorporated in the runner system and/or separate feeders at strategic points and by tending to thicken up the ingate at its point of connection with the casting so that it does not freeze off before the feed metal has had time to compensate for the liquid shrinkage taking place during casting solidification. However, the immersion casting process of the present invention has the advantage of only requiring a short ingate and therefore the heat losses between the molten metal in which the mould is immersed and the point of entry into the mould cavity are negligible and almost immeasurable. Whereas normal moulds poured from a ladle may have heat losses as high at to 200 C between the furnace temperature and the temperature of the metal at point of entry into the mould cavity, with immersion casting according to the present invention using a furnace temperature of say l,400 C, the temperature of entry into the mould cavity will be about l,400 C. This is an important advantage because it means that the cross-sectional area of the ingate 3 at the point of connection to the casting can be reduced to an area which would be impossible to run with poured casting processes. It has been found that ingate areas of less than two square millimetres (6 mm X 0.28 mm) have fed the castings very successfully and this is generally the reverse of normal and accepted practice.

FIG. 4 shows a perspective view of a complete metal casting plant. The immersion jigs 23 complete with monorail carriers 23a are mounted on an endless monorail 25. The jigs 23 are fed past the moulding machines 24 where they are stacked with moulds made on these machines. The jigs stacked with moulds are then transported on the monorail 25 to the reciprocating im-- mersion mechanism 26 situated over the furnace 27,

such as a mains frequency induction furnace, where the parts are cast by immersion of the moulds in the molten metal. The jigs are then withdrawn and are transported to the station 28 where the cast moulds are pushed off the jigs and the moulds and castings are separated. The empty jigs 23 are then moved to the refractory coating station 29, where the jigs are dipped into a tank of refractory wash and then allowed to dry during the rest of the cycle back to the moulding machines. If the supporting yoke 7 of the jig is in the form of a single rod extending through the central vent 4a (FIGS. 2 and 3) it is masked by the mould or moulds and it is then unnecessary to provide the refractory coating 19.

FIG. 5 is a plan,view of a multi-cavity mould which may be used in the process of this invention. References corresponding to those in FIG. l have been employed. Thus the mould 1, made for example from green sand, is provided with six annular mould cavities 2 each having a separate ingate 3'. Vent gates 4 opposite each mould cavity communicate with a central venting aperture 4a.

In the embodiment of FIG. 6, there is shown a permanent metal mould ll for casting a ferrous metal. The mould may be made from an alloy comprising molybdenum with 0.4 percent titanium and 0.1 percent zirconium. The mould ll has an annular cavity 12, an ingate l3, and one or more vent gates 14 (two diametrically opposed gates 14 aligned with the ingate 13 being shown) communicating with a central vent aperture 14a. A mould is completed, to close the cavity 12, when a similar mould is stacked on top of it. More than one ingate 13 may be provided.

Referring to FIG. 7, a stack of metalmoulds l1, having aligned vent holes Ma, is mounted in a jig J, which may hold, for example, 20 moulds. The outer surface of the moulds is exposed to the melt; A cover mould Ila may be provided to cover the cavity 12 in the top mould 11. The cover mould also has a central vent aperture 14in and the apertures 14a of the moulds are aligned forming a vent passage through the stack. The jig J is made of mild steel coated with a refractory material, e.g. graphite, and includes a base 15, on which the moulds ll are stacked, attached to a yoke 117 whose upper end carries a tubular guide 118 which is free to move up and down relative to a vertical spindle 119. A refractory block 116, e.g. of cast iron, provided with vent holes 16a, is suspended from the spindle 19, whose upper end is provided with a part 19a so that it can be attached to a suitable reciprocating mechanism for lowering the assembly into molten metal and withdrawing it therefrom.

The external surfaces of the moulds 111 may also be provided with a refractory coating to prevent them from the effects of repeated immersions in the molten metal.

The jig J loaded with moulds 1 l is immersed in a bath 20 of molten iron to a level above the top of the moulds, say about half-way up the refractory block 16, just below the vent holes 160. At the start of immersion the bottom moulds are clamped together by the weight of the moulds above them, and molten iron flows into the cavities 12 through the ingates 13.

The metal moulds illustrated have a lower density, i.e., a lower specific gravity, than the ferrous metal being cast and accordingly beging to float upward, until the cover mould 1 la is in contact with the block 16, the moulds being clamped together by the effect of their buoyancy. After the lapse of sufficient time to fill the moulds, the jig is withdrawn from the bath and allowed to cool. However, if the moulds have a higher density than the metal being cast, they can be mechanically clamped together prior to immersion. This may be achieved by threading the spindle 19 in the tubular guide 118 so that they stack of moulds can be mechanically clamped between the base 15 and block 16.

The start of immersion of a stack of moulds into the melt 20 is shown in FIG. 8. As shown, the bottom mould has filled with molten metal and the next mould is just at the surface 21 of the melt. The moulds are generally impermeable, but heating of the moulds resulting from immersion causes expansion of the air or other gas in the cavity 12, the air being displaced partly through the ingate 13, which is designed to have a narrow throat cross-section. This cross-section is so designed in relation to the speed of immersion that, while the ingate is passing through the impurities 22 on the surface 2i of the melt, the air is expanding out through the ingate, and flow of metal into the cavity only begins when the ingate is below the surface of the melt. Air remaining in the cavity at this time passes out through the vents 14, up the central vent apertures Ma, into the interior of the block 16, whence it escapes to atmosphere through the vent holes 160.

As previously mentioned, if the castings are unfed with melt during cooling, it is only necessary to ensure that the ingates freeze sufficiently rapidly to prevent metal running out of the moulds as they are withdrawn from the melt. This can be achieved by suitably dimensioning the ingate of each mould in relation to the cross-section of the mould cavity. However, if some feeding is required, the mould cavity geometry must be shaped to ensure retention on withdrawal of an adequate reservoir of molten metal, or the retention of the mould in the melt until the casting has partially or wholly solidified.

Referring now to FIG. 9, there is shown a multicavity permanent mould in accordance with the invention, the mould 31 being made of metal and having six annular mould cavities 32 each having a separate ingate 33. Vent gates 34 opposite each mould cavity communicate with a central venting aperture 34a.

The metal casting process of the present invention enables a much higher yield of castings/molten metal used to be obtained and a stack of castings can be rapidly produced e.g. in a few seconds. Satisfactory castings have been produced using malleable cast iron to BS. 1452 grade 14 and Air Ministry specification D.T.D. 485. Moreover the process may be used with any metal melting furnace without modification.

In addition, where gas permeable moulds are used, the moulds can be of relatively thin section and need only be of handleable, i.e., green strength, even though no supporting mould box is employed. This is because during the entry of metal into a mould cavity, the mould is supported externally by the pressure of the molten metal in which the mould is immersed.

Although the process and apparatus of the invention has been specifically described in relation to a stack of moulds it will be understood that in the apparatus as shown the stack of moulds may be replaced by a single mould for casting a single component, such as cylinder liner.

The metal casting process of this invention possesses several advantages over prior art processes. The most important are:

l. lmmersion of the moulds in the melt of metal to be cast pressure balances them, thereby minimising mould distortion.

2. Direct communication between the melt and the mould cavity via a short ingate minimises metal wastage so that in fact less metal per casting need be melted. This reduces operating costs and also there need be no allowance for temperature loss due to pouring, thus reducing the temperature of which the melt must be maintained and reducing operating costs still further.

3. Unlike a pouring operation, very little skill is required on the part of an operator. This again reduces operating costs and/or facilitates automation.

4. Shrinkage is almost completely absent and excellent quality castings are obtained.

5. Surface impurities and slag are automatically excluded from the mould cavities by the arrangement of the ingates, thus producing good quality castings without the complication of valves for the ingate, which valves would have to operate under extremely ardous conditions.

I claim:

1. Apparatus for the casting of articles, said apparatus including a mould body having upper and lower ends, said mould body comprising an inner wall extending from said upper to said lower end and defining a central aperture in said body extending through both of said ends and an outer wall extending from said upper to said lower end and surrounding said inner wall, said body defining in the region between said central aperture and said outer wall at least one mould cavity having upper and lower regions, a permanently open ingate in said outer wall leading to the upper end of each cavity and arranged to communicate directly with a body of molten metal when said at least one mould is immersed therein, and a vent gate being provided in said inner wall leading from each cavity to said central apertures, a jig supporting said mould with its upper end uppermost, and a mechanism for immersing said jig and mould into said body of molten metal to permit each cavity in each mould to be filled with molten metal through its ingate, and for withdrawing said jig and at least one mould from said molten metal after each cavity has been filled.

2. Apparatus for the casting of articles from molten metal including at least one mould body which is made of sand, has upper and lower ends and has a wall of such thin section that it would distort if filled without providing external pressure to support it, said mould body comprising an inner wall extending from said upper to said lower end and defining a central aperture in said body extending through both of said ends, and an outer wall extending from said upper to said lower end and surrounding said inner wall, said body defining in the region between said central aperture and outer wall, at least one mould cavity having upper and lower regions, a permanently open ingate in said outer wall leading to the upper end of each cavity a arranged to communicate directly with a body of molten metal when said at least one mould is immersed therein, a jig supporting said mould with its upper end uppermost, and a mechanism for immersing said jig and mould into said body of molten metal to permit each cavity in each mould to be filled with molten metal through its ingate, and for withdrawing said jig and at least one mould from said molten metal after each cavity has been filled.

3. Apparatus as claimed in claim 1, wherein each mould has a substantially unsupported wall of such thin section that it would distort or fail if used unsupported for casting by a metal pouring process, but is sufficiently supported by the mass of metal surrounding it during immersion in the melt.

4. Apparatus as claimed in claim 1, wherein each mould is made of metal.

5. Apparatus as claimed in claim 1, wherein each mould is made of a gas permeable material.

6. Apparatus as claimed in claim 1, wherein a stack of moulds is arranged in the jig between a supporting base and an upper member.

7. Apparatus as claimed in claim 4, wherein the stack of moulds is mechanically clamped between the supporting base and the upper member.

8. Apparatus as claimed in claim 4, wherein there is an aperture through each mould and at least one vent gate between each cavity and said aperture, said apertures through the moulds of the stack being aligned to form a passage through the stack and said upper member having holes which communicate with said passage.

Claims (8)

1. Apparatus for the casting of articles, said apparatus including a mould body having upper and lower ends, said mould body comprising an inner wall extending from said upper to said lower end and defining a central aperture in said body extending through both of said ends and an outer wall extending from said upper to said lower end and surrounding said inner wall, said body defining in the region between said central aperture and said outer wall at least one mould cavity having upper and lower regions, a permanently open ingate in said outer wall leading to the upper end of each cavity and arranged to communicate directly with a body of molten metal when said at least one mould is immersed therein, and a vent gate being provided in said inner wall leading from each cavity to said central apertures, a jig supporting said mould with its upper end uppermost, and a mechanism for immersing said jig and mould into said body of molten metal to permit each cavity in each mould to be filled with molten metal through its ingate, and for withdrawing said jig and at least one mould from said molten metal after each cavity has been filled.

2. Apparatus for the casting of articles from molten metal including at least one mould body which is made of sand, has upper and lower ends and has a wall of such thin section that it would distort if filled without providing external pressure to support it, said mould body comprising an inner wall extending from said upper to said lower end and defining a central aperture in said body extending through both of said ends, and an outer wall extending from said upper to said lower end and surrounding said inner wall, said body defining in the region between said central aperture and outer wall, at least one mould cavity having upper and lower regions, a permanently open ingate in said outer wall leading to the upper end of each cavity a arranged to communicate directly with a body of molten metal when said at least one mould is immersed therein, a jig supporting said mould with its upper end uppermost, and a mechanism for immersing said jig and mould into said body of molten metal to permit each cavity in each mould to be filled with molten metal through its ingate, and for withdrawing said jig and at least one mould from said molten metal after each cavity has been filled.

3. Apparatus as claimed in claim 1, wherein each mould has a substantially unsupported wall of such thin section that it would distort or fail if used unsupported for casting by a metal pouring process, but is sufficiently supported by the mass of metal surrounding it during immersion in the melt.

4. Apparatus as claimed in claim 1, wherein each mould is made of metal.

5. Apparatus as claimed in claim 1, wherein each mould is made of a gas permeable material.

6. Apparatus as claimed in claim 1, wherein a stack of moulds is arranged in the jig between a supporting base and an upper member.

7. Apparatus as claimed in claim 4, wherein the stack of moulds is mechanically clamped between the supporting base and the upper member.

8. Apparatus as claimed in claim 4, wherein there is an aperture through each mould and at least one vent gate between each cavity and said aperture, said apertures through the moulds of the stack being aligned to form a passage through the stack and said upper member having holes which communicate with said passage.

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