The primary function of any molding material is to maintain the shape of the mold cavity until the molten metal solidifies. Silica sand is the most widely used molding material, particularly for metals that melt at high temperatures. It serves well because it is readily available, low in cost, easily formed into complicated shapes, and able to withstand the molten metal.
The three major parts of a molding sand are:
1) the sand grains, which have the necessary refractory properties to withstand the intense heat of the molten metal; 2) a bonding material, which may be natural or added clay, cereal, etc., to hold the grains together; and 3) water to coalesce the grains and binder into a plastic molding material.
Molding Sand
Natural sands contain only the binder mined with them and are used as received with water added. They have the advantages of maintaining moisture content for a long time, possess a wide working range for moisture, and permit easy
Figure 9-18. Color coding.
patching and finishing of molds. Sometimes it is desirable to change the properties of a natural sand by adding bentonite (a clay binder). Such a sand is known as a semi-synthetic sand.
Synthetic sands are formulated from various ingredients. The base may be a natural sand with some clay content or a washed sand with all clay removed. A binder, such as bentonite, and water are added. The sands have these ad- vantages over natural sands: 1) more uniform grain size, 2) higher refractory, 3) moldability with less moisture, 4) require less binder, 5) easier control of properties, and 6) require less storage space since one type of sand may be used for different kinds of castings.
Loam sand is high in clay, as much as 50%
or so, and dries hard. Loam molding is done by making the mold, usually for a large casting, of brick cemented and lined with loam sand and then dried.
Properties
The way a sand performs in a mold to pro- duce good castings can be tested by and largely depends upon its green permeability, green strength, and dry strength. These properties are determined chiefly by the sand’s grain fineness, grain shape, and clay and moisture content. Other properties of less influence are heat strength, sintering point, deformation, and collapsibility.
Green permeability. The porosity from the openings between grains allows permeability of the sand. This gives passage for air, gases, and steam to escape when the molten metal is poured in the mold. Permeability is measured in a common test by passing a definite amount of air through a standard test specimen under specified conditions. The result is expressed by:
P = 501.2/pt (Eq. 9-1)
where:
P = permeability
p = pressure, grams/cm2 t = time, min
Thus the permeability number is larger as the sand becomes more porous.
Grain fineness is measured by passing sand through standard sieves, each with a certain number of openings per linear mm (in.). Com-
mercial sands are made up of grains of various sizes. Grain size of a sand is designated by a number that indicates the average size as well as the proportions of smaller or larger grains in the mixture. Size is determined by a procedure described in handbooks.
Finer grains impart a smoother finish to a casting. However, permeability decreases as the grains and thus the voids between grains become smaller. The same condition results from a large proportion of fine grains in a mixture. The best compromise must be reached. For large castings that require coarse sand for high permeability, the surface of the mold cavity may have a thin layer of fine-grained facing material.
There are two distinct sand grain shapes:
angular and rounded, with many degrees of roundness and angularity between the two ex- tremes. Sharp angular grains cannot pack to- gether as closely and consequently have higher permeability than rounded grains. This is dem- onstrated by the graph in Figure 9-19.
Both the type and the amount of binder have a decided effect on the permeability of sand. An illustration of the permeabilities imparted by two common types of clay is given in Figure 9-20.
Over a wide range of moisture content, benton- ite was found to give more permeability than fireclay. Permeability may decrease with an increase in clay content, as depicted by the 2%
moisture curve for bentonite in Figure 9-21. The upper curve of 4% moisture content indicates fairly constant permeability over a wide range of bentonite content. In general, clay content is optimum when present to the extent of coating
Figure 9-19. Relationship of molding sand permeability to moisture content and grain shape.
the sand particles completely without filling the spaces between the grains.
With a low moisture content, fine clay par- ticles clog the spaces between the grains, and permeability is low. More moisture softens and agglutinates the clay around the grains for optimum conditions. An excess of moisture fills the voids and decreases permeability.
Peaks of maximum permeability are seen in Figure 9-19. The optimum moisture content is not the same for all molding sands, although it generally lies between 2 and 8%.
Green strength. Green strength is a charac- teristic of a sand ready for molding and, if the metal is poured immediately, represents the abil- ity of the sand to hold to the shape of the mold. It may be expressed in kPa (psi) required to rupture a standard specimen.
Figure 9-20. Permeability of two kinds of clay for molding sand with various amounts of moisture.
Figure 9-21. How the permeability of a molding sand varies with its bentonite content.
The finer the sand grains, the larger the surface area of a given bulk, and the larger is the amount of binder needed to cover the area. The con- tacts and bonds between grains are more numerous, and thus green strength is higher with finer grains. Figure 9-22 shows that as the grain size becomes larger, green strength decreases under normal conditions.
Round grains pack together more closely than angular grains and as a result are bonded to- gether with a higher green strength than angu- lar grains. A comparison is given for two types of grains in Figure 9-23.
Some binders provide higher green strength than others. Comparison of bentonite and fireclay
Figure 9-22. Green strength of a molding sand in relation to the sizes of its grains for one set of conditions.
Figure 9-23. Relationship of green strength of a molding sand to moisture content and grain shape.
at different moisture levels is given in Figure 9-24. Green strength increases in proportion to the amount of binder in a molding sand; how- ever, as pointed out earlier, too much binder is detrimental to permeability and a compromise must be accepted.
The effect of moisture on green strength is similar to the effect on permeability. Green strength increases with the first additions of moisture, reaches a maximum strength, and then starts to decrease as depicted in Figure 9-25. Also shown is that excessive moisture has a weakening effect, even nullifying the influence of grain size.
Figure 9-24. Comparison of the green strengths of two samples of molding sand with different clay binders at different moisture levels.
Figure 9-25. How the green strength of molding sand varies for several sizes of grains.
In addition to other factors, mulling or mixing practice affects green strength as will be ex- plained in the section on the preparation of sand.
Dry strength. Dry strength is a character- istic of sand that has been dried or baked. In general, dry strength varies with grain fineness, grain shape, and moisture content in the same way as green strength. However, different bind- ers can affect dry strength and green strength differently. For example, in contrast to western bentonite, southern bentonite produces high green strength and low dry strength, which is conducive to easy shake out of castings.
Control
Many substances are added to molding sands to impart or change certain properties. Many new ones become available each year, particu- larly substances to minimize heating and save energy. Space is not available here to list many additives, so only the basic ones are cited. Cereal (such as wheat and corn flour), dextrine, rosin, and similar substances are often added to aug- ment or modify clay binders. It is important to realize that the action of each additive is somewhat different. Corn flour improves green strength slightly but dry strength markedly.
Wheat flour improves collapsibility. Dextrine binders are a form of sugar and produce a much higher dry strength than cereal binders but decrease green strength. A rosin-bonded core has a hard surface when baked but absorbs moisture on standing and should be used as soon as possible.
The binder is burned out and the proportion of fine particles is increased when foundry sand is exposed to the heat of molten metal. As a re- sult, green strength and permeability decrease as the sand is reused. By measuring properties periodically with sand-testing equipment, a sand technician can make appropriate addi- tions to restore the sand before it deteriorates to the point where it must be discarded. Uniform properties may be maintained from day-to-day by making continuous checks and adding small amounts of binder.
Preparation
Under a microscope, sand grains are seen to exist in clusters of small particles adhering
to large particles. This is thought to be the result of previous use. Clusters are less refractory and harder to ram than individual particles. So they need to be broken up, particularly to prepare sand for cores and for facing patterns to make mold linings. Breaking down the clustered particles is desirable for all sand and is usually done by stir- ring and kneading it mechanically. A popular machine for that purpose is a muller, which kneads, shears, slices through, and stirs the sand in a heavy iron pot by means of several revolving rollers and knives. The time needed for mulling depends upon the type of muller and the type and amount of binder. After the optimum length of mixing time in each case, there is no further in- crease in green strength as shown by the example in Figure 9-26. Mulling sand distributes the binder over the grains. Thus less binder is re- quired for satisfactory green strength and perme- ability is higher than with hand-mixed sand.
Sand is aerated to make it fluffy so it flows readily around and takes up the details of the pattern. This is done to some extent in mull- ing. One device for aeration is a sand cutter that hurls the sand from a rapidly moving belt against bars or springs to break the grains apart.
Sands are mixed into many formulas for vari- ous metals and purposes. For instance, for steel castings, three sands may be prepared: one for the green facing, another for the green backing, and still another for the facing to be surface dried in the mold by a torch. Formulas for foundry sands are given in reference texts and handbooks.
Figure 9-26. Effect of mulling time on the green strength of a molding sand.
Core Sand
Core sand must have much the same proper- ties of permeability, cohesiveness, and refractory as molding sand, but must also possess collaps- ibility and friability. Collapsibility means that the core gives way easily as the casting cools and shrinks to avoid inducing hot tears and cracks in the metal. Friability means that the core crumbles and falls apart when it must be removed from the casting. These properties are imparted by the type and amount of binder, ad- ditives to the binder, and by the curing of the cores. For example, if cracks occur in the cored area of a casting, a weaker binder or less binder may be used, or a small amount of wood flour may be added to the binder.
Core sand mixes are started with clean, dry silica sand. Among various binders used for core sand are corn flour, dextrine, fish oil, raw linseed oil, and commercial core oils. Linseed oil is an ingredient in most proprietary core oils. A long list could be made of binders, each imparting a particular property to the core. As the ingredi- ents are added, the sand is thoroughly mixed in a muller but not excessively, which would cause stickiness.
Oils and other core binders that must be baked have been largely replaced by other sub- stances requiring less energy. Popular among these is sodium silicate (water glass) that is mixed with the sand and hardened in the core in a few seconds when exposed to carbon dioxide (CO2) gas. The core can then be used in the mold immediately. A major disadvantage of sodium silicate and some other no-bake binders is that they must be mixed with the sand a short time before use because they harden rather soon on exposure to air. Other no-bake binders with longer shelf lives are available and widely used.
Among them are plastics materials that can be catalyzed in the core for quick hardening. One in particular is hardened by exposure to sulfur dioxide (SO2) for a few seconds. New cold-cure substances are being brought out each year and are too numerous to be mentioned here.