GB2285628A

GB2285628A - Casting core composition

Info

Publication number: GB2285628A
Application number: GB9425605A
Authority: GB
Inventors: Takuya Arakawa; Hiroshi Tako; Toru Tohata
Original assignee: Aichi Machine Industry Co Ltd; Nissan Motor Co Ltd
Current assignee: Aichi Machine Industry Co Ltd; Nissan Motor Co Ltd
Priority date: 1993-12-24
Filing date: 1994-12-16
Publication date: 1995-07-19
Anticipated expiration: 2014-12-16
Also published as: DE4446352A1; GB9425605D0; JPH07178506A; JP3186005B2; DE4446352C2; GB2285628B

Description

2285628 CASTING CORE COMPOSITION The present invention relates to a

casting core composition, and more particularly to a casting core composition which is to be molded by a so-called shell mold process.

A casting core is used for forming internal cavities in a cast product. In fact, a casting core is inserted between two halves of a mold (cope and drag). Then, molten metal is poured into the mold. After solidification of the metal, the mold is disassembled and then the cast product is removed. After that, the casting core is broken away and removed from the cast product. With this, the cast product will have internal cavities having certain specific shapes.

Nowadays, many casting cores for automobile cast products are produced by the shell mold process. In this process, the casting core is molded out of a mixture of silica sand grains and a thermosetting resin as a binder for binding the silica sand grains. Silica sand contains Si02 as a main component thereof. However, if the casting core of this type (silica sand grains bound with a thermosetting resin) is used for casting, for example, an aluminum-alloy automobile cylinder block under a high casting pressure (at least 800 kgf/cm2), it is necessary to provide the casting core with a certain sufficient strength to withstand the high casting pressure. Silica sand grains themselves have variable polygonal shapes. Thus, a casting core prepared from silica sand grains tend to have spaces between silica sand grains, upon molding of the casting core. With this, the casting core may be broken under the high casting pressure. To prevent this, it is considered to increase the amount of the thermosetting resin to, for example, a range of from 3.5 wt% to 4.2% based on the total weight of the silica sand grains and the thermosetting resin. However, with this, percentage of contraction of the casting core's longitudinally center portion in the direction of the thickness thereof becomes large (for example, 15-17%) after casting, and the amount of so-called deformation of the casting core's center portion also becomes large (for example, 1.2-1.5 mm) after casting (see the aftermentioned Comparative Example 1). With this, the cast product becomes inferior in dimensional precision. The definition of the amount of this deformation will be explained in detail in the following DESCRIPTION OF THE PREFERRED EMBODIMENTS of this application, with reference to Fig. 6.

It would therefore be desirable to be able to provide an improved casting core composition for providing a casting with a low percentage of contraction and a small amount of deformation even when the casting core is used in casting under a high pressure of at least 800 kgf/cm2. According to a first aspect of the present invention, there is provided a casting core composition comprising: refractory grains including, as a main component, mullite grains, said refractory grains being substantially spherical in shape and in a range of from 78 to 110 in terms of a sand grain size according to American Foundrymen's Society; and 20 a binder for binding said refractory grains, said binder amounting to a range of from 1.0 to 2.2 wt% based on the total weight of said refractory grains and said binder. According to a second aspect of the present invention, there is provided a casting core composition comprising: 25 refractory grains consisting essentially of mullite grains, said refractory grains being substantially spherical in shape and in a range of from 78 to 110 in terms of a sand grain size according to American Foundrymen's Society; and a binder for binding said refractory grains, said binder amounting to a range of from 1.0 to 2.2 wt% based on the total weight of said refractory grains and said binder. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph illustrating the binder amounts and percentages of contraction of casting cores according to the present invention and conventional casting cores using silica sand grains No. 7, after casting under a pressure of 800 kgf/cm2; 1 Fig. 2 is a graph similar to Fig. 1, but illustrating the binder amounts and the amounts of deformation of casting cores according to the present invention and conventional casting cores using silica sand grains No. 7, after casting under a pressure of 800 kgf/CM2; Fig. 3 is a graph illustrating the casting pressures and percentages of contraction of casting cores according to the present invention and conventional casting cores using silica sand grains No. 7, after casting; Fig. 4 is a graph similar to Fig. 3, but illustrating the casting pressures and the amounts of deformation of casting cores according to the present invention and conventional casting cores using silica sand grains No. 7, after casting; Fig. 5 is a graph illustrating the binder amounts and the amounts of the mullite grains remained in each internal cavity of a cast product, after casting under a pressure of 800 kgf/CM2 and then shot blasting; and Fig. 6 is a schematic plan view showing by solid line a casting core before casting, and by dotted line a deformed casting core after casting under a high pressure. DESCRIPTION OF THE PREFERRED EMBODIMENTS

An improved casting core composition according to the present invention will be described in the following. As will be clarified hereinafter, this composition provides a casting core with a low percentage of contraction and a small amount of deformation thereof even when the casting core is used in casting under a high pressure of at least 800 kgf/cm2. This high pressure casting is very suitable for producing castings which are superior in dimensional precision and surface finish, with a high productivity.

A casting core composition according to the present invention comprises refractory grains and a binder for binding the refractory grains. The refractory grains contain, as a main component, mullite grains. The preferable mullite content of the refractory grains is usually at least about 97 wt%. In other words, the refractory grains consist essentially Of MUllite grains.

Therefore, the terms of "the refractory grains" and "the mullite grains" will be used interchangeably hereinafter. Mullite is a mineral having a chemical composition of 3A1203.2SiO2 2A1203-SiO2. A casting core prepared from the refractory grains according to the present invention is more improved in strength than that prepared from silica sand grains. Therefore, the former casting core does not tend to be broken even upon casting under the high pressure. Furthermore, the former casting core becomes substantially small in contraction after casting under the high pressure.

is According to the present invention, the refractory grains are substantially spherical in shape. This means in the present application that the refractory grains may be somewhat oval in shape, too. With this, the refractory grains become closely packed when a casting core is molded. Therefore, contraction of this casting core is substantially decreased even after casting under a relatively high pressure.

In the invention, the refractory grains are in a range of from 78 to 110 in terms of sand grain size according to American Foundrymen's Society (AFS). With this, upon casting, the degree of penetration of molten metal into the molded casting core becomes small. In other words, the degree of penetration of molten metal into pores of the molded casting core which are defined between the reftactory grains becomes small. Furthermore, it becomes easy to mold a casting core. Still furthermore, upon molding of a casting core, packing density of the refractory grains becomes adequate. With this, the molded casting core does not have void spaces therein. Thus, upon casting under the high pressure, the degree of contraction and the degree of deformation of the casting core become substantially small.

If the sand grain size of the refractory grains is less than 78 in terms of sand grain size according to AFS, their grain size becomes too large. With this, penetration of a molten metal into the molded casting core increases too much upon casting. If the sand grain size of the refractory grains is more than 110 in terms of a grain size according to AFS, their grain size becomes too small. With this, it becomes difficult to mold a casting core.

Furthermore, upon molding of a casting core, packing density of the refractory grains decreases too much. With this, the molded casting core will have void spaces therein. Thus, upon casting under the high pressure, contraction and deformation of the casting core become too much.

The amount of the binder is in a range of from 1.0 to 2.2 wt% based on the total weight of the refractory grains and the binder. With this, the casting core becomes adequate in strength.

Thus, it becomes easy to completely remove the casting core from the cast product, after casting. If it is less than 1.0 wt%, it becomes difficult to uniformly mix the refractory grains with the binder. With this, strength of the casting core becomes insufficient. If it is greater than 2.2 wt%, strength of the casting core becomes excessive. With this, it becomes difficult to completely break away and remove the casting core from the cast product, after casting. It is usual to use a thermosetting resin such as a phenolic resin as the binder, A casting core is molded out of the casting core composition by a shell mold process such as a so-called dumping shell-mold process or a so-called blowing shell-mold process.

The present invention will be illustrated with the following nonlimitative examples.

EXAMPLE 1

Three batches of CERABEADS #750 (a trade name) of NAIGAI CERAMICS CO., LTD. as the spherical mullite grains which are 78 in terms of a sand grain size according to AFS (hereinafter, CERABEADS #750 will be referred to as AFS 78) were respectively mixed with 1.4 wt%, 1.8 wt%, and 2.2 wt%, of a phenolic resin (binder), based on the total weight of AFS 78 and the binder. Separately, four batches of CERABEADS #1000 (a trade name) of NAIGAI CERAMICS CO., LTD. as the spherical mullite grains which are 90 in terms of a sand grain size according to AFS (hereinafter, CERABEADS #1,000 will be referred to as AFS 90) were respectively mixed with 1.0 wt%, 1.4 wt%, 1.8 wt%, and 2.2 wt%, of a phenolic resin (binder), based on the total weight of AFS 90 and the binder. Still separately, only one batch of CERABEADS #1450 (a trade name) of NAIGAI CERAMICS CO., LTD. as the spherical mullite sand grains which are 110 in terms of a grain size according to AFS (hereinafter, CERABEADS #1,450 will be referred to as AFS 110) was mixed with 1.2 wt% of a phenolic resin (binder), based on the total weight of AFS 110 and the binder. Then, a casting core for forming a coolant passage (water jacket) of an aluminum alloy cylinder block of an automotive engine was molded out of each of all the above mixtures by a blowing shell mold process. In this process, the blowing pressure for blowing each mixture was controlled within a range from 2.5 to 4.0 kgf/cm2, the curing temperature for curing the phenolic resin was controlled within a range from 180 to 250 OC, and the curing time was controlled within a range from 30 to 50 sec.

The thus molded each casting core was inserted into a die casting mold for forming the coolant passage. Then, the cylinder block was cast under a high pressure (800 kgf/cm2). After casting, each casting core of the cast cylinder block was subjected to shot blasting, two times each for 40 seconds, so as to break away and remove the casting core from the cast cylinder block.

Percentage of contraction of each casting core's longitudinally center portion in the direction of the thickness thereof was measured. The results are shown in Fig. 1.

The amount of deformation of each casting core's longitudinally center portion was measured. The results are shown in Fig. 2. The definition of the amount of deformation of each casting core will be described in the followin, with C) 9 reference to Fig. 6. Fig. 6 is a schematic plan view showing by solid line X1 a casting core before casting under a high pressure, and by dotted line X2 a deformed casting core after casting under a high pressure. That is, a curved casting core before casting tends to deform and become straight after casting under C) a high pressure. The amount of deformation is defined as the distance between al which is a center point of a casting core 1 before casting and a2 which is a center point of a deformed casting core after casting. A line bi is a center line of a casting core before casting, which is defined in the longitudinal direction thereof. A line b2 is a center line of a casting core after casting, which is defined in the longitudinal direction thereof. As is shown in Fig. 6, the points al and a2 are on a center line of a casting, which is defined in the direction of the casting core's width.

The amount of the mullite grains remained in each internal cavity of the cylinder block was determined. The results are shown in Fig. 5. In Fig. 5, the upper and lower ends of a line segment at each binder amount (1.0, 1.4, 1.8, or 2.2 wt%) respectively represent the maximum and minimum mullite grains' amounts remained in each internal cavity of the cylinder block at each binder amount. It should be noted that the amount of the remained mullite grains is not influenced by the pressure variation of casting. Therefore, the amount of the mullite grains remained in each internal cavity of the cylinder block was determined only for the casting cores after the casting under a pressure of 800 kgf/cm2. In other words, the determination of the amount of the remained mullite grains was not conducted in the following Examples 2-6 and Comparative Examples 1-3.

EXAMPLES 2-6

In each of Examples 2-6, Example. I was repeated except that a plurality of batches of only AFS 90 were respectively mixed with 1.0 wt%, 1.4 wt%, 1.8 wt%, and 2.2 wt%, of a phenolic resin (binder), based on the total weight of AFS 90 and the binder, and that each cylinder block was cast under a pressure of 500, 600, 700, 900, or 1,000 kgf/m2.

The results regarding the above-defined percentage of contraction and the amount of deformation of each casting core are respectively shown in Figs. 3 and 4. Thus, it should be noted that all the data shown in Figs. 3 and 4 are concerned with each casting core prepared from a mixture of AFS 90 and the binder.

Furthermore, it should be noted that all the data regarding AFS after a casting under a pressure of 800 kgf/m2 shown in Figs.

is 1 and 2 are respectively copied as the data after a casting under a pressure of 800 k-gf/m2 shown in Figs. 3 and 4. COMPARATIVE EXAMPLE I In this Comparative Example 1, Example 1 was repeated except that only two batches of silica sand grains No. 7 were respectively mixed with 3.5 wt% and 4.2 wt% of a phenolic resin (binder), based on the total weight of the silica sand grains No. 7 and the binder, and that the shot blasting of Example 1 was omitted.

The results regarding the above-defined percentage of contraction and the amount of deformation are respectively shown in Figs. 1 and 2. Furthermore, the results regarding the above-defined percentage of contraction and the amount of deformation of the casting core prepared by using the mixture of silica sand grains No. 7 and 3.5 wt% of the phenolic resin are also shown in Figs. 3 and 4 (see the data at a casting pressure of 800 kgf/cm2 in Figs. 3 and 4).

The reason why the binder amounts of Comparative Examples 1-3 (3.5 and/or 4.2 wt%) are different from those of Examples 1-6 (1.0-2.2 wt%) will be discussed in the following. If 1.0-2.2 wt% of the binder were used in Comparative Examples 1 3, it is expected that the percentage of contraction and the amount of deformation would become much much higher than those shown in Figs. 1-4. With this, it becomes difficult to neatly show the data of Comparative Example 1-3 and the data of Examples 1-6 in one graph. Therefore, in Comparative Examples 1-3, 3.5 and 4.2 wt% were chosen, instead of 1.0-2.2 wt%.

COMPARATIVE EXAMPLES 2-3 In each of Comparative Examples 2-3, Example I was repeated except that only two batches of silica sand grains No. 7 were respectively mixed with 3.5 wt% of a phenolic resin, that the cylinder blocks were respectively cast under pressures of 500 and 700 kgf/m2, and that the shot blasting of Example 1 was omitted. The results regarding the above-defined percentage of contraction and the amount of deformation of each casting core are respectively shown in Figs. 3 and 4.

It is understood from Figs. 1 and 2 that the percentage of contraction and the amount of deformation of each casting core according to Comparative Example 1 were respectively much greater than those according to Example 1. Furthermore, it is understood from Figs. 1 and 2 that there are tendencies that the percentage of contraction and the amount of deformation respectively increase as the grain size of the mullite grains becomes finer. In comparison between AFS 78, AFS 90 and AFS 110, AFS 78, AFS 90 and AFS 110 are respectively coarse, medium, and fine in terms of grain size.

With reference to Figs. 3 and 4, it is understood that the percentage of contraction and the amount of deformation of each casting core according to Comparative Examples 1-3 were respectively much greater than those according to Examples 1-6, throughout the range of casting pressure (500-1,000 kgf/cm2).

Furthermore, with reference to Figs. 3 and 4, it is understood that the percentage of contraction and the amount of deformation of each casting core according to Comparative Examples 1-3 respectively increased more steeply by increasing the casting pressure, as compared with those according to Examples 1-6. It is understood from Fig. 4 that there is a tendency that the amount of deformation of the casting core according to Examples 1-6 increases as the amount of binder becomes smaller.

With reference to Fig. 5, it is understood that the amount of the mullite grains remained in each internal cavity of the cylinder block, after casting under a pressure of 800 kgf/cM2, increases by increasing the amount of binder. The reason of this is considered that strength of the casting core increases by increasing the amount of binder.

Claims

CLAIMS:

A casting core composition comprising: refractory grains including, as a main component, mullite grains, said refractory grains being substantially spherical in shape and in a range of from 78 to 110 in terms of sand grain size according to American Foundrymen's Society; and a binder for binding said refractory grains, said binder amounting to a range of from 1.0 to 2.2 wt% based on the total weight of said refractory grains and said binder.
2. A casting core composition according to Claim 1, wherein said binder is a thermosetting resin.
3. A casting core composition according to Claim 2, wherein said thermosetting resin is a phenolic resin.
4. A casting core composition comprising: refractory grains consisting essentially of mullite grains, said refractory grains being substantially spherical in shape and in a range of from 78 to 110 in terms of sand grain size according to American Foundrymen's Society; and a binder for binding said refractory grains, said binder amounting to a range of from 1.0 to 2.2 wt% based on the total weight of said refractory grains and said binder.
5. A casting core composition substantially as described with reference to any of the embodiments specified in Examples 1 to 6 herein.