Coated sand composition for light-alloy casting

A coated sand composition for light-alloy casting includes refractory particles blended with phenol resin, bromide and adsorbent which adsorbs decomposition product gas from the bromide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a coated sand composition for light alloy 
casting. 
2. Description of the Related Art 
When casting a complicated structure such as an engine cylinder head in 
light alloy such as aluminum alloy, cores and the like are shaped from 
coated sand, that is, casting sand coated with resin binder, and the cores 
are used in combination with metal molds. The mold using coated sand must 
have a strength to withstand casting pressure and at the same time must be 
well disintegrated after casting. However though being apt to disintegrate 
in the case of casting in cast iron in which the casting temperature is 
high and resin burns out during casting, such a mold is not satisfactorily 
disintegrated in the case of casting in light alloy where the casting 
temperature is low, which makes troublesome sand removal, especially core 
removal after casting. 
Japanese Unexamined Patent Publication No. 2(1990)-175044 discloses a 
coated sand composition containing therein bromide of wood material such 
as lignin bromide as disintegrator. The publication says that the bromide 
promotes disintegration of the sand mold at low temperature and does not 
give rise to a problem of metal molds. Further it has been well known to 
use inorganic bromide as disintegrator as disclosed, for instance, in 
Japanese Patent Publication No. 60(1985)-15417. 
However, though the wood bromide may promote disintegration, such a special 
compound must be synthesized, which is disadvantageous in reduction of 
cost. Further whether the bromide is organic or inorganic, the bromide 
cannot avoid being decomposed by heat during casting and generating 
bromine gas. The bromine gas causes corrosion of the metal molds. The 
corroded metal mold requires repeated polishing and padding. 
SUMMARY OF THE INVENTION 
In view of the foregoing observations and description, the primary object 
of the present invention is to provide a coated sand composition for light 
alloy molding which is excellent in disintegration after casting and does 
not cause corrosion of a metal mold. 
In the coated sand composition of the present invention, disintegration of 
a sand mold is promoted by bromide and corrosion of a metal mold is 
prevented by converting corrosive gases generated from the bromide to 
harmless materials. 
That is, in accordance with the present invention, there is provided a 
coated sand composition comprising refractory particles blended with 
phenol resin, bromide and adsorbent which adsorbs decomposition product 
gas from the bromide. 
The refractory particles may be, for instance, silica sand, and the 
adsorbent may be, for instance, zinc oxide or soda lime. 
The phenol resin serves as a binder for the refractory particles, and the 
bromide promotes disintegration of the mold shaped from the coated sand. 
Though the bromide is discomposed and generates a decomposition product 
gas due to heat during casting, the adsorbent adsorbs the decomposition 
product gas and prevents corrosion of the metal mold. 
As the refractory particles, zircon sand, chromite sand, olivine sand and 
the like can also be used. The phenol resin which can be employed in the 
present invention includes novolak phenol resin, resol phenol resin and 
mixtures of novolak phenol resin and resol phenol resin. Since resol 
phenol resin is alkaline, use of resol phenol resin is advantageous in 
preventing corrosion of the metal mold. 
Either organic bromide or inorganic bromide may be used as the bromide. For 
example, tetrabromobisphenol, hexabromobisphenyl ether or the like may be 
preferably employed as the organic bromide. Bromides of zinc, calcium, 
ammonium, aluminum, potassium, sodium and the like may be preferably 
employed as the inorganic bromide. 
In one embodiment, the coated sand composition contains therein 1 to 3 
parts by weight of phenol resin, 0.01 to 1.0 parts by weight of bromide 
and 0.01 to 0.1 parts by weight of zinc oxide per 100 parts by weight of 
refractory particles. 
That is, it becomes difficult to obtain desired disintegration of the sand 
mold when the bromide is less than 0.01 parts by weight and the mold 
strength becomes too small when the bromide is more than 1.0 part by 
weight. More preferably the coated sand composition contains bromide in 
0.05 to 0.5 parts by weight. When the zinc oxide is less than 0.01 parts 
by weight, anti-corrosive effect becomes unsatisfactory and when the zinc 
oxide is more than 0.1 parts by weight, the mold strength becomes too 
small. 
In another embodiment of the present invention, the coated sand composition 
contains therein 1 to 3 parts by weight of phenol resin, 0.01 to 1.0 parts 
by weight of bromide and. 0.01 to 0.05 parts by weight of soda lime per 
100 parts by weight of refractory particles. 
The soda lime increases alkalinity of the binder (phenol resin) for the 
refractory particles as well as adsorbs the decomposition product gas, 
thereby preventing corrosion of the metal mold. 
When the soda lime is less than 0.01 parts by weight, anti-corrosive effect 
becomes unsatisfactory and when the soda lime is more than 0.05 parts by 
weight, the mold strength becomes too small.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be described in more detail with reference to 
the following example. 
EXAMPLE 
Preparation of coated sand 
Embodiment 1 
10 kg of silica sand heated to 140.degree. C. was introduced into a 
laboratory speed muller and 69 g of solid novolak phenol resin containing 
therein 3 g of zinc oxide and 6 g of disintegrator (ECP manufactured by 
TOYO COATED SAND and containing both organic and inorganic bromides) and 
80 g of solid resol phenol resin were added. The mixture was kneaded for 
40 seconds. Then stiffener comprising 9 g of hexamine and 150 mL of water 
was added and after bulk material was broken, 10 g of calcium stearate was 
further added. After 10 seconds, the mixture was sieved and cooled, 
whereby coated sand was obtained. 
Embodiment 2 
10 kg of silica sand heated to 150.degree. C. and 4 g of soda lime ground 
to not larger than 0.5 mm were introduced into a laboratory speed muller 
and 120 g of solid novolak phenol resin was added. Then the mixture was 
kneaded for 25 seconds and then liquid resol phenol resin containing 
therein 20 g of solid component was added. The mixture obtained was 
further kneaded for 15 seconds. Then stiffener comprising 18 g of hexamine 
and 200 mL of water and 16 g of 53% zinc bromide were added and after bulk 
material was broken, 10 g of calcium stearate was further added. After 10 
seconds, the mixture was sieved and cooled, whereby coated sand was 
obtained. 
Control 1 
Coated sand was prepared in the manner similar to embodiment 1 except that 
146 g of solid novolak phenol resin containing 6 g of disintegrator the 
same as that used in embodiment 1 (but without zinc oxide) was used and 
resol phenol resin was not used. 
Control 2 
Coated sand was prepared in the manner similar to embodiment 1 except that 
146 g of solid resol phenol resin containing 6 g of disintegrator the same 
as that used in embodiment 1 (but without zinc oxide) was used and novolak 
phenol resin was not used. 
Control 3 
Coated sand was prepared by using 140 g of novolak phenol resin as the 
phenol resin and 16 g of 53% zinc bromide as the disintegrator and without 
adding soda lime. 
Evaluation of coated sand 
The fusing temperature, the folding endurance, disintegration and 
generation of corrosion in metal molds were investigated for each coated 
sand. The fusing temperature was investigated according to JACT test 
method C-1, and the other performances were investigated in the following 
manner. 
folding endurance 
A test piece of 1/4".times.1".times.2" was formed of each coated sand 
(sintered at mold temperature of 250.degree. C. for 60 seconds). The test 
piece was loaded at the center thereof with opposite end supported and the 
load at break of the test piece was measured. The value obtained by 
dividing the load at break by the cross-sectional area of the test piece 
was adopted as the folding endurance. 
disintegration 
A test piece 28 mm in diameter and 50 mm in length was formed of each 
coated sand. The test piece was wrapped with aluminum foil and heated for 
8 minutes in an oven at 700.degree. C. After cooling, the aluminum foil 
was removed and the weight of the test piece was measured. Then the test 
piece was put on a 20 mesh sieve and vibrated for 30 seconds by a jet 
shifter. The amount of the disintegrated part of the test piece was 
thereafter measured and the percentage of the amount of the disintegrated 
part to the original weight was adopted as the integration. 
corrosion in metal molds 
A test piece which was 28 mm in diameter and 50 mm in length and inserted 
with a grease-free washer at an intermediate part thereof was formed of 
each coated sand. The test piece was subjected to the same heat treatment 
as in the disintegration test and then the washer was taken out. The 
amount of corrosion on the surface of the washer was visually inspected 
and evaluated on the basis of 10 points. 
Result of the test is shown in table 1. In table 1, the compositions are in 
parts by weight. 
______________________________________ 
emb. 1 
cont. 1 cont. 2 emb. 2 
cont. 3 
______________________________________ 
silica sand 100 100 100 100 100 
phenol resin 
novolak 0.6 1.4 0 1.2 1.4 
resol 0.8 0 1.4 0.2 0 
disintegrator 
ECP 0.06 0.06 0.06 0 0 
ZnBr.sub.2 0 0 0 0.16 0.16 
zinc oxide 0.03 0 0 0 0 
soda lime 0 0 0 0.04 0 
fusing temperature (.degree.C.) 
105 109 103 102 95 
folding endurance (Kg/ 
34.6 32.5 35.4 30.1 32.9 
cm.sup.2) 
disintegration (%) 
68.0 47.1 59.2 71.4 53.4 
corrosion prevention 
9 1 3 8 1 
______________________________________ 
As can be understood from table 1, the fusing temperature less depends upon 
existence of zinc oxide or soda lime. 
Though the folding endurance less depends upon existence of zinc oxide or 
soda lime in table 1, actually the folding endurance is weakened when the 
amount of zinc oxide or soda lime is excessive. 
As for the disintegration, the embodiments 1 and 2 are better than the 
corresponding controls (more apt to disintegrate). Though the reason is 
not clear, it may be said that zinc oxide and soda lime do not deteriorate 
the effect of disintegrator (ECP or zinc bromide) and the result of the 
test shows that zinc oxide and soda lime assisted the disintegrator. 
As for the corrosion in metal molds, generation of corrosion was remarkable 
in the coated sands of controls 1 and 3 in which acidic novolak phenol 
resin was used, and generation of corrosion is observed in the coated sand 
of control 2 in which only alkaline resol phenol resin was used. To the 
contrast, generation of corrosion was hardly observed in the coated sand 
of embodiment 1 in which zinc oxide was added. Accordingly it may be said 
that zinc oxide adsorbed bromine gas and prevented generation of 
corrosion. 
Further generation of corrosion was very small in the coated sand of 
embodiment 2 in which soda lime was added, which showed that soda lime is 
effective in preventing generation of corrosion. It is considered that 
soda lime prevents generation of corrosion not only by alkalifying the 
coated sand but also by adsorbing bromine gas. This is because generation 
of corrosion was observed also in the coated sand of control 2 where the 
coated sand was alkaline and generation of corrosion was much less in 
embodiment 2 than in control 2. 
As can be understood from the description above, addition of adsorbent such 
as zinc oxide or soda lime prevents generation of corrosion in metal molds 
without deteriorating mold strength. Further such adsorbent does not 
deteriorate the mold disintegrating effect of bromide; rather it promote 
the mold disintegrating effect of bromide.