Metal casting patterns

A metal casing pattern of expanded thermoplastics material such as polystyrene or polymethyl methacrylate has a filter comprising a porous ceramic body, such as a body having pores extending from one face to another face or a foam structure, incorporated therein.

This invention relates to metal casting patterns of expanded thermoplastic 
material having a filter therein. 
Patterns of expanded thermoplastic material, such as expanded polystyrene, 
are used in the so-called Full Mould or Lost Foam process of casting 
molten metal. One or more such patterns, corresponding to the metal 
casting to be produced and to the sprue, and mould runner system are 
coated with a refractory coating and embedded in unbonded sand in a 
moulding box to form a mould. When molten metal is poured into the mould 
the pattern is decomposed and replaced by the metal which solidifies to 
produce a casting having the shape and configuration of the pattern. 
The expanded polystyrene pattern which is used in the process is commonly 
produced by partially expanding polystyrene beads containing a volatile 
blowing agent such as pentane under the action of heat, injecting the 
partially expanded beads into a mould, usually made of metal, such as 
aluminum, and then further expanding the beads under the action of heat in 
the mould so causing the beads to fuse together to form the pattern. 
Usually heating in the mould is done by injecting steam into the mass of 
partially expanded beads. 
In the Full Mould or Lost Foam process it is common practice to produce a 
number of castings in a single mould, using a simple running system 
consisting of a sprue or sprue and runner bar and a series of ingates, 
each ingate linking either the sprue or the runner bar to a casting 
pattern, and the casting pattern, sprue, runner bar and ingates are all 
made of expanded polystyrene. Each ingate must be relatively large as it 
must support the mass of the casting pattern when the assembly is coated 
with refractory coating and invested in the unbonded moulding sand. 
In such a process it is diffucilt to incorporate conventional slag traps 
and it is also difficult to ensure that the runner bar remains full of 
molten metal during casting so that any slag floates to the top and is 
trapped in the runner bar when the metal solidifies. For these reasons 
slag defects are common in iron castings and oxide film defects are common 
in aluminum castings. 
Similar defects can be prevented in other types of process using bonded 
sand moulds because a print can be incorporated in the runner system and a 
filter can be inserted into the print. This remedy is not possible in the 
Full Mould or Lost Foam process because all the parts of the mould are 
formed of expanded polystyrene and there is nowhere for the filter to be 
located. 
It has now beed found that a filter can be used in the Full Mould or Lost 
Foam process if the filter is incorporated in a pattern of expanded 
thermoplastics material during production of the pattern. 
According to the invention there is provided a metal casting pattern of 
expanded thermoplastics material having incorporated therein a filter 
comprising a porous ceramic body. 
According to a further feature of the invention there is provided a method 
of making a metal casting pattern of expanded thermoplastics material 
having a filter comprising a porous ceramic body therein comprising 
locating the filter in a die or mould having an internal shape conforming 
to the shape of the pattern so that all porous surfaces of the filter are 
accessible to thermoplastics material when the thermoplastics material is 
introduced into the die or mould, introducing beads of the thermoplastics 
material into the die or mould until the die or mould is filled and 
heating the beads so as to expand and cure the beads and form the pattern. 
The metal casting pattern according to the invention may be a pattern for 
forming the sprue, runner or a part of the sprue or of the runner of a 
Full Mould or Lost Foam process mould, or the pattern may form the metal 
casting and the runner or a part of the runner and have the filter 
incorporated in the runner or runner part. 
The expanded thermoplastics material is preferably expanded polystyrene or 
polymethyl methacrylate and it is in relation to the production of 
expanded polystyrene patterns that the invention will be described in 
detail. 
The porous body may be for example a honeycomb type of structure having 
pores which extend from one face of the body to another face or a 
structure having interconnecting pores such as a ceramic foam. 
Foam structures are preferred and such structures may be made using a known 
method of making a ceramic foam in which an organic foam, usually a 
polyurethane foam, is impregnated with an aqueous slurry of ceramic 
material containing a binder, the impregnated foam is dried to remove 
water and the dried impregnated foam is fired to burn off the organic foam 
to produce a ceramic foam. The production of ceramic foams by this method 
is described in U.S. Pat. No. 3,090,094 and in British Pats. Nos. 923862, 
916784, 1004352, 1054421, 1377691, 1388911, 1388912 and 1388913. 
When the pattern of the invention is produced it is essential that at least 
the exterior surfaces of the filter which are to be in contact with the 
molten metal during casting are substantially covered with polystyrene, 
otherwise coating material could penetrate inside the filter when the 
pattern is coated with the refractory coating prior to use, or sand could 
enter the filter when the pattern is invested with unbonded sand. 
When the filter is of the honeycomb type, i.e. having pores which extend 
from one face of the filter to the opposite face, it is only necessary to 
ensure that the filter is located in a die or mould for producing the 
pattern in such a manner that partially expanded polystyrene beads are 
blown around the faces which are to contact molten metal because the 
remaining surfaces of the filter are not porous and no problems could 
arise if these surfaces are exposed to the refractory coating or to the 
unbonded sand. 
Similarly if the filter is a ceramic foam in which those surfaces which are 
not intended to pass molten metal are rendered impermeable, for example by 
the application of a glaze as described in British Pat. No. 1419762, by 
the application of a plastics coating or by means of adhesive tape, only 
the porous faces need be covered by polystyrene during production of the 
pattern. 
Ceramic foam filters are usually in the foam of square or rectangular 
cross-section boards whose major surfaces are intended to pass molten 
metal. During the production of such filters organic foam which has been 
impregnated with a slurry of ceramic material is usually passed through 
rolls to distribute the slurry and remove excess slurry. As a result two 
opposite minor faces of the filters are closed by the squeezing operation 
while the other two minor faces remain open. 
In order to produce patterns according to the invention containing such 
filters it is necessary to ensure that not only the major faces of the 
filter but also the minor faces are substantially covered by polystyrene. 
The filter must therefore be located in a die or mould so that there is a 
gap between the minor faces and the inner surface of the die and partially 
pre-expanded beads of polystyrene can be blown around all faces of the 
filter. The gap is preferably at least 2 mm so that a minimum of 2 mm of 
polystyrene covers each face. 
Ceramic foam filters have a dimensional tolerance of about 1 mm, so in 
order to allow for size variations and to ensure that all filters of a 
given nominal size can be located accurately in the die or mould and 
substantially covered with polystyrene, movable means such as spring 
loaded wedges or knife edges may be used to locate and hold the filters in 
the desired position. In order to prevent the filter from being abraded by 
the wedges or knife edges it may be desirable to protect those surfaces 
which are contacted by the wedges or knife edges by means of a coating or 
an adhesive tape. If desired the spring loaded wedges or knife edges may 
be used in combination with fixed wedges or knife edges. 
During production of the metal casting pattern according to the invention 
it is desirable that polystyrene beads enter and at least partially fill 
the surface pores or surface cells of the filter. When the filter is a 
ceramic foam in cells are usually larger than the size range of partially 
pre-expanded polystyrene beads which are used for pattern production and 
penetration of the beads into the cells will occur automatically. The size 
of the pores or channels in a ceramic honeycomb type of filter is usually 
smaller than the size of the cells in a ceramic foam and it may be 
necessary to choose a polystyrene bead size which is sufficiently small to 
ensure that penetration takes place. As a result of the penetration of the 
polystyrene into the filter a strong pattern is produced and there is no 
danger of the polystyrene surrounding the filter breaking away from the 
filter during handling or use of the pattern. 
In use the pattern of the invention is coated with a refractory coating, 
surrounded by dry sand in a suitable moulding box and vibrated to compact 
the sand and metal is then poured, according to conventional practice, 
with or without vacuum applied to the sand. When the metal is cast the 
expanded polystyrene pattern is destroyed and replaced by the metal.

Referring to FIG. 1 an aluminum die for producing an expanded polystyrene 
mould runner pattern having a ceramic foam filter therein consists of an 
upper half 1 and a lower half 2 shaped so as to form together cavities 3 
and 4 for the runner pattern and cavity 5 for a ceramic foam filter 6 in 
the shape of a square cross-section board. The upper half 1 of the die has 
filler nozzles 7 for admitting partially pre-expanded polystyrene beads 
and both the upper half 1 and the lower half 2 have vents 8 for admitting 
and venting steam. The inner surface of both halves 1, 2 around the cavity 
5 has a combination of fixed and spring loaded wedges for locating and 
holding the filter 6. The horizontal surface of the lower half 2 has a 
fixed wedge 9 and the horizontal surface of the upper half 1 and the 
vertical surfaces of both halves 1, 2 (apart from the vertical surfaces 
adjacent face 11 of the filter 6) have spring loaded wedges 10. The filter 
6 is located in the desired position so as to leave a gap of at least 2 mm 
between the inner surface of the die and all faces of the filter 6. The 
pattern is produced by injecting partially pre-expanded polystyrene beads 
through the nozzles 7 into cavity 4 and then cavity 3 until the die is 
filled. Steam is then injected through the vents 8 so as to fully expand 
and cure the beads to form the mould runner pattern. Prior to use in a 
mould the pattern is coated with a refractory coating. 
Referring to FIGS. 2 and 3 an expanded polystyrene runner pattern which 
consists of a first runner section 12 and a second runner section 13 and 
between the two runner sections a section 14 containing the ceramic foam 
filter 6 is surrounded by dry sand 15 in a moulding box (not shown) and 
the box is vibrated to compact the dry sand 15 around the pattern. At 
points corresponding to the positions of the wedges in the die used to 
produce the pattern there are wedge shaped recesses 16 and 17 in the 
surface of the pattern. The cross-sectional area of the runner section 12 
is larger than that of the runner section 13. When molten metal is poured 
into the mould so that it flows in the direction indicated by the arrows 
the expanded polystyrene is destroyed by the advancing metal front. Under 
the pressure of the metal the filter 6 is pushed against the refractory 
coating and sand at the outlet side 18 of the mould thus preventing molten 
metal leaking around the sides and outlet face 11 of the filter 6. If the 
runner sections 12 and 13 were of the same cross-sectional area pressure 
would not be exerted on the filter 6 and it would be possible for metal to 
leak around the filter 6 once the polystyrene surrounding the sides of the 
filter 6 was destroyed.