American Synthetic Sand Practice
DEFINITION OF SYNTHETIC M OULDING SAN D
W hen m etal is cast against m oulding sand, the clay contained in the sand close to the casting is dehydrated and it loses strength. It is further weakened by norm al accretions of burned core sand. This weakened sand often is discarded, although it may be as good in refractory value and grain structure as when it was new. This is costly and inefficient. Many foundries now add bonding clays to restore strength, and in this way, effect elim ination of new bonded sand or a considerable reduction in the am ount used.
Bonding clays are strong plastic clays which develop strength rapidly, even when used in small am ounts. They may be added to new or burned silica sand to form synthetic sand.
Some sand producers now m anufacture w hat is called synthetic sand by the addition o f bonding clay to unbonded or low -bonded sand at the sand pits. As generally used in American foundry practice, however, the term synthetic sand covers any sand w hich is m ade in the foundry, as required, from m aterials selected and proportioned by the foundrym an to fit best his particular needs. A rebonded sand is one whose usefulness or life is prolonged by the addition of bonding clay.
A certain am ount of burned sand is lost from the foundry each day by adherence to castings.
Common practice in m aking the synthetic sand is to add to the used sand enough clay bond to restore strength and enough silica sand to maintain volume. Since this silica sand and the core sand used are the only new materials
* V ice-P resident, E a ste rn C lay P ro d u c ts, In c ., E ifo rt, Ohio.
entering the sand they will eventually compose the entire heap or system. It is, therefore, essential to select a grain-size of this sand which will give the finish and perm eability required.
Synthetic sand has been used in steelfoundry practice for a great m any years. Its use is so com m on that steelfoundrym en do not often speak of their mixtures as synthetic sands. The term , of course, describes such sand, but probably is used m ore often by ironfoundrym en in describing their com binations of bond clay and sands. The general procedure followed in cleaning, classifying and rebonding used sands is term ed sand reclamation. The term clay-bond is a general one and m ay refer to fireclay bonds, other plastic clays, bentonites or bentonitic clays.
The reclam ation and rebonding of used iron- foundry sands reached substantial volume in the U nited States between 1925 and 1930 and has increased steadily since. It now seems probable that a larger tonnage of castings is made annually in synthetic sand than is m ade in naturally-bonded sands.
Some objection has been raised to the term
“ synthetic sand ” because the w ord “ synthetic ” might imply that the m aterial was a cheaper and inferior substitute for naturally-bonded sands. It has been suggested that the sand be called “ artificial sand ” or “ m anufactured sand.” T he term “ synthetic ” has been in use so long th a t it will probably be continued. It is w orth noting, however, that the product is used because the user believes it to be superior as well as cheaper than the natural product.
T he w riter has also read occasional references in the British technical Press to the effect that synthetic sands are widely used in the U nited
States because there is a scarcity o f good naturally-bonded sands. T here are deposits o f high-quality naturally-bonded sands in alm ost all parts o f the U nited States and they occur in naturally-bonded sands is neither usually efficient nor refractory. T he so-called bond in natural sands also frequently contains a substantial per
centage of silt w hich adds little to strength, in
creases the am ount o f tem pering w ater required and reduces perm eability and flowability. Some low -bonded sands are desirable since the small distribution m ust also receive attention.
F o r some years a perfectly round grain was grained sands give satisfactory perm eability, and m ay even give higher perm eability than
The fines in sand have m uch higher expan
sion than the coarser grains, yet their addition to the coarser grains reduces expansion diffi
culties. It has been said th at the fines provide a cushion between the larger grains, but the author believes th at their corrective action is due to the simple fact that they reduce flow- ability, thus increasing the distance between the coarser grains in the ram m ed m ould face. T his seems corroborated by the fact that a very small addition of cornflour is the best-known m ethod of stopping these cope defects, short o f changes in the sand grain distribution; cornflour reduces flowability faster than any other com mon interested principally in grain size, green strength, dry strength, hot strength, perm e
ability, durability, flowability, m oisture re
quired, resilience, expansion, contraction and sintering point. In selecting a naturally- found—grain size, perm eability and expansion—
as they are to be established in the base sands, rapidly; good durability; adequate strength at 19
high tem peratures; m oderate contraction a t high tem peratures, and other special requirem ents for specific problem s. different results given by different clays.
Recent w ork indicates th a t the m ineral com
Base sand, unbonded silica. Average grain fineness, 53. perties, are practically identical in particle size and m ineral com position, bu t the W estern de
posit carries basic adsorbed ions while the Southern carries acid adsorbed ions.
T he high quality clays in each group can be expected to give generally sim ilar results, but there is, of course, a wide range in quality between the best and w orst clays in each group. If refractory clays o f low bonding power are elim inated, the currently available A m erican bonding clays m ay be grouped into three classes determ ined by their dom inant mineral. These classes are given in T able I,
c 2
together with their relative values for various excellent plasticity and refractory value are available in the U nited States and, as a result,
Green deform ation 0.008 0.012 0.012 0.011 0 .012
D ry compression 4 7 .0 4 2 .0 5 9 .0 6 9.0 8 3 .0 possible sintering point, it is particularly well- suited to heavy castings and the alloys which
Montmorillonite Clays
The m ontm orillonite clays used for bonding in the U nited States are the W estern bentonites produced principally in W yoming an d the Southern bentonites produced principally in Mississippi. Both types o f clays are available and carried in stock in the U nited Kingdom . These clays have the finest grain size o f any bonding clays. They develop double the green strength of the best kaolinites. The Western bentonites swell when placed in w ater while the Southern bentonites are non-swelling. The ben
tonites give the highest perm eability o f any clays and the highest or lowest dry and hot strengths depending upon the type selected.
The most im portant recent A m erican research work on sands has been on hot strengths and the most im portant recent developm ent in
prac-might have washes and cuts due to the low hot strength of Southern bentonite or difficult shake out, lum py sand and cracked castings due to the high hot strength of W estern bentonite. A com bination of the two m ay be m ade to yield the exact strength required.
T able IV shows the same progression from low to high hot strength when mixing Southern bentonite and Ohio fireclay. When difficulty is experienced with either low or high hot strength from a single bond, certainly it is only com m on sense to use a com bination that will give the hot strength required to stop cutting an d washing and no m ore. This extends sand control into a hitherto untouched and frequently troublesom e field.
Table V shows th at com binations o f Ohio fireclay and Western bentonite, instead of giving
T a b l e I V .—Hot Strength o f Combinations of Bond Clays.
Base sand, unbonded silica. Average grain fineness, 53.
Bond or combination. O.F.C.
75 per cent.
O.F.C.*
25 per cent.
S .B .f
50 per cent.
O.F.C.
50 per cent.
S.B.
25 per cent.
O.F.C.
75 per cent.
S.B.
S.B.
P er cent, bond 12 9.25 6.5 5 4
P er cent, m oisture 4 .0 3.7 3 .2 2 .6 2.6
Perm eability 85 118 139 167 178
Green compression 8 .5 11.0 10.4 12.9 9.9
Green deform ation 0.019 0.015 0.015 0.012 0.008
D ry compression 71.5 8 3.0 7 1.0 55.0 4 7.0
H ot strength :—
35 40 21
260 deg. C... 55 54
535 ... 100 59 50 48 35
815 ... 153 100 62 30 30
1,040 „ ... 550 450 238 85 25
1,370 ... 5 5 5 4 3
Strength after cooling from
425 deg. C. 65 74 43 27 20
* O.F.C. = Ohio fireclay. t S.B. = Southern bentonite.
tice has been the use of com binations o f clays instead of a single clay.
Tables II, III, IV, V and V I show ho t strength behaviour and will be discussed briefly since this inform ation is im portant in choosing the single clay or the com bination of clays to be used. Table II shows a rather startling increase in hot strength with only slight increases in clay and water content. This indicates that kaolinite clays have adequate hot strength for any job when properly handled. It may also indicate the source of some cracked castings in the m al
leable industry since an overtem pered batch of sand has far higher hot strength than the same sand properly tempered.
Table III shows the gradual increase in hot strength from Southern bentonite, through com binations to W estern bentonite. A foundry
hot strengths interm ediate to the bonds used, gave a peak strength m ore than twice as high as either com ponent. This could hardly have been predicted from low tem perature, dry strength testing. Such com binations should have excellent application for heavy iron or steel castings.
Table VI shows th at additions of cereal binder (cornflour), pitch and rosin sharply increase the dry strength of Southern bentonite, although the increase in hot strength is such as still to provide a highly collapsible sand. Such a sand should be desirable for steel castings which have a tendency to crack. It is interesting to note that the addition of cereal binder to W estern bentonite did not appreciably increase dry strength, and caused a sharp decrease in h o t strength. This indicates that steelfoundry-21
m en who have used such com bination fo r in
creased strength have n o t benefited m uch in th at respect, bu t have actually provided a m ore collapsible sand.
Choice o f Clay
F rom the results in various tables, a choice o f certain clays fo r certain w ork can be made.
N on-ferrous
(1) L ig h t:— Southern bentonite for its com bination o f high perm eability and high flow- ability.
(2) H eavy:— Fireclay for econom y, if the perm eability be n o t too im p o rta n t o r to sm ooth up sand if only coarse sands be available. W estern bentonite fo r highest perm eability com bined with high ho t strength.
F ireclay:— W estern bentonite for highest h o t strength.
M alleable
(6) L ig h t: — Southern bentonite fo r its com bination o f high perm eability, high flowability and m oderate h o t strength.
(7) M edium :— F ireclay for econom y.
S outhern bentonite for highest perm eability and flowability com bined w ith m o derate h o t strength to prevent cracked castings. F ire clay : — S outhern bentonite fo r m o derate perm eability an d m oderate h o t strength.
W estern b e n to n ite :— S outhern bentonite fo r highest perm eability, com bined w ith m o derate h o t strength.
(8) H eavy:— F ireclay for high hot
Ta b l e V .— Hot Strength of Combinations of Bond Clays.
Base sand, unbonded silica. Average grain fineness, 53.
Bond or combination. W .B.
75 per cent.
W.B.*
25 per cent.
O .F.C-t
50 per cent.
W .B.
50 per cent.
O.F.C.
25 per cent.
W .B.
75 per cent.
O.F.C.
O.F.C.
P er cent, bond 5 5 .2 6 .8 8 .5 10
P er cent, m oisture 2 .5 2 .3 2 .8 3 .5 3 .3
P erm eability 167 162 154 116 103
Green compression 9 .6 9.1 8.7 8 .8 8 .7
Green deform ation 0.012 0.013 0.017 0.018 0.0 1 5
D ry compression 8 3.0 75.0 125.0 120.0 4 7 .5
H o t stren g th
:—-260 deg. C... 75 40 65 116 38
535 ... 74 65 100 160 60
815 ... 195 120 240 250 80
1,040 „ ... 490 560 830 1,000+ 295
L370 ... 8 5 6 8 5
S trength after cooling from
425 deg. C. 55 61 89 110 —
* W .B. = W estern bentonite.
F ireclay:— S outhern bentonite for high hot strength and econom y if slightly lower perm e
ability be acceptable.
Grey Iron
(3) L ig h t:—-Southern bentonite for its com bination of high perm eability and high flowability.
(4) M edium :— Fireclay for econom y if perm eability be no t too im portant o r to sm ooth up sand if only coarse sands are available. W estern bentonite for highest perm eability com bined with high ho t strength.
F ireclay:— S outhern bentonite for high hot strength and econom y if slightly lower perm eability be acceptable.
(5) H eavy:— F ireclay for high hot strength and economy, particularly on dry sand w ork w here perm eability is no t im portant. W estern bentonite fo r highest perm eability com bined with high ho t strength.
f O.F.C. = Ohio fireclay.
strength and econom y, p articularly on dry sand w ork w here perm eability is n o t im portant. W estern bentonite fo r highest perm eability, com bined w ith high h o t strength. F ireclay:— W estern b entonite for highest h o t strength.
Steel
(9) L ig h t:— Southern bentonite plus cereal binder fo r its com bination o f high p erm e
ability and high flowability.
(10) M e d iu m :— S outhern bentonite-W estern bentonite plus cereal binder fo r high p erm e
ability, high dry strength an d m o d erate h o t strength.
(11) H eavy:— F ireclay plus cereal binder for high dry strength, high h o t strength, an d high sintering point. W estern bentonite plus cereal binder for high dry strength, high hot strength, and highest perm eability. F ire c la y :— W estern bentonite plus cereal binder 22
fo r high dry strength and highest ho t since it eliminates transportation and mixing costs as com pared to m achine mixing. The
Because W estern bentonite im m ediately swells to a gel, only a relatively small am ount can be
P erm eability 178 13S 167 152 110 128
Green compression 9.9 8.2 9 .6 9.3 11.7 8.1
Green deform ation 0.008 0.034 0.012 0.027 0.013 0.012
D ry compression 47.0 133.0 83.0 87.0 158.0 145.0
H o t strength :— water. N on-swelling Southern bentonite quickly slakes into a slurry on contact with
Southern bentonite, being non-swelling, readily slakes in w ater in the same m anner as does fire