Comparison of Olivine, Silica, Chromite and Zircon as Moulding Materials
Under what condition should a change to Olivine Sand be made ?
|Specific Gravity gm/c3>
|Bulk Density (g/cm 3)
|Melting Point oC
|Sintering Point o C
|High Temperature Reaction Acid
|Specific heat 20-1000oC Cal/go C
||Sub angular to to angular
||Rounded to sub angular
|Loss of Ignitions
Better quality of product, simplified manufacture and lower costs are the usual motives for change of method or material in an industrial process. Where a change has been made to moulding materials based on Olivine sand, those expectations have been fully borne in mind in few cases.
Olivine sand has several advantages also in the production on non-ferrous castings. A stable mould is obtained which makes it possible to avoid expansion defects. Thus heavy aluminum castings, poured in green moulds facilitates other moulding sands. The high cooling ability of Olivine moulds facilitates the production of mould castings in alloys with vide solidication ranges.Choice of Olivine Sand Grading
When selecting the proper grading for a given purpose, one should not only select the average grain size (AFS Number), but also the grain size distribution and the shape of the grains. Unwashed Olivine Sands are characterized by a relatively disperse grain size distribution, which generally makes it possible to lower the content of clay-binders, especially Bentonite. Washed Sand is more concentrated and have a minor part of fines. This means that these sands are suitable also for fluid binders used in core production and in resin-coated sand for shell moulding. the washed sands are characterized by a higher permissibility an a minor ignition loss. The grain surfaces of Olivine Sand being a crushed product are somewhat more angular in shape than many silica sands. However, they are considerably less angular than in chrome sand.
The manufacture of Olivine sand involves crushing, grinding and screening with a strict control on these processes. The chemical composition and sieve analysis have been found to be very consistent from time to time.
Inorder to benefit from the advantages of Olivine Sand further, reclaiming of used sand is preferable. The Olivine Sand may to a very high extent be re-used because of its long durability and the fact that its grainsize changes very little. The quality of the sand when recycled, is improved through a gradual reduction of the inherent gas content. As Olivine has a higher density, the source to metal ratio can be as low as 1 : 1.What changes are needed in mould-making and pouring?
Applications of Olivine :
- Patterns : Experience has shown that no change is needed in the construction and appearance of the pattern. This applies to relief angle, contractor allowance matching allowance, core print clearance the finish and durability of the pattern.
- Ramming : Olivine sand packs better and has greater fluidity than quartz sands. New sand is slightly less advantages in this respect than reclaimed sand.
- Strike Off : Strike off of the mould has been found to be simpler. Olivine sand requires greater compression for the same degree of ramming which may be one of the reasons why it is sometimes not preferred.
- Rein for Cement : No change in the extent of reinforcement has been necessary despite the greater weight of the Olivine Sand.
The extent of sprigging and nailing work has been greatly reduced in some foundries, while others have noticed no change when compared to moulds made by Silica Sand.
- Pattern Removal : With reclaimed sands of Olivine the pattern is some what more difficult to remove than with quartz, owing to the greater liability of the pattern, or core box to stick to the sand. The impairment is not troublesome, however the removal is assisted in the conventional manner by means of liquid partings. In foundries which use other silica sands the problem of sticking of patterns and core boxes is thought to be less when compared with Olivine Sand.
- Gas Vents : Owing to the greater gas evolution in Olivine sand, more attention should be paid to venting than with silica sands. When using uncalcined new sand, troublesome cores and moulds should be provided with a larger number of suitably shaped air and gas vents with the conventional moulding/core making processor.
- Surface Drying : Experience has shown that, at the same moisture content the reclaimed sand dries more quickly than new sand, which must be compensated by increasing the moisture content from four percent for new sand and six percent for reclaimed sand.
- Tooling : The plasticity of Olivine sand makes it easier to cut gates and runners by hand than in silica sands. Olivine mould surface are easy to tool and have a smooth feel,possibly due to the mica constituents in the Olivine.
- Painting: Experience shows that for dry sand moulds generally one coating usually suffices except in mould parts and cores which are subject to penetration it has also been possible to reduce painting in some cases even as unpainted moulds for manganese steel and for small castings of plain carbon steel.
- Gating and Risering : Inorder to avoid sand erosion in sprue cups and ducts may be used to a greater extent than usual. No change is necessary in the size, shape, location or number of risers.
- Pouring Temperature & Pouring Speed : No change has been made in the size of the usual nozzles in the pouring temperature for unalloyed, large and medium sized steel castings. For high alloy chromium and manganese steel and thin sections, it has often been necessary to raise the pouring temperature and shorten the pouring time to avoid the risk of cold and incomplete filling of the mould keeping in view of the general practices of gating/risering for the patterns castings.
- Sand Removal : The residual strength of Olivine sands after casting is higher than that of silica sands, especially in the case of new sand.With mechanical shakeout the higher residual strength is no particular draw-back in practice.
Defects occasioned by properties of the moulding sand cause less work than with silica. For castings of high alloy manganese steel the total cleaning and repair times are generally 25 - 50 percent shorter for Olivine sand. Equal savings, compared with quartzite, have been made by one foundry for unalloyed low alloy and stainless steel casting.
- Surface Finish : Several foundries have found that the castings had a improved finish, with a smoother and cleaner appearance, due to reduced occurrence metal penetration in the sand and as a result fewer welds. High grade manganese steel, in particular has a considerably better finish.
The slight linear thermal expansion of Olivine sand results in a greatly reduced scabbing tendency.
Castings of carbon steel poured in green sand moulds some times get crater like sand spots, if there are loose sand grains in the mould.
There is less tendency to enclosure of sand below the skin, so less need for deep chiseling. This is probably due to the more uniform thermal expansion of Olivine sand.
An increased tendency for cold shuts to appear has been observed in high alloy chromium and manganese steel, the fluidity of which diminished through oxidation of alloying elements in the steel surface. Some risks of cold shuts may be expected in thin walled castings, which may be effected by the greater cooling capacity of Olivine Sand at low temperature. Olivine sand does not influence the occurrence of pin - holes shrinkage of tears.
- Dimensional Accuracy : The contraction allowance has been found to be the same when using Olivine or Silica Sands, so that no change of pattern is needed when converting to Olivine. No difference of dimension has been noticed between castings produced in Olivine and Silica Sand moulds. Some foundries consider that Olivine Sand moulds give more accurate dimensions.
Olivine has proved successful as a moulding material for the shell range of castings especially steel foundry producing manganese steel. In countries like Europe, U.S.A,Japan the use of Olivine is so far mainly limited to production of manganese and other high alloyed steels.
In U.S.A non-ferrous foundries producing aluminum brass and bronze employ Olivine sand . It is claimed to give accurate and smooth surfaces due to its good cooling stability.
Several properties of Olivine Sand and the smaller size proved to be splendid base sand for shell or process.
- Olivine Flour for Mould Washes.
- Rammed ladle linings, especially when pouring manganese Steel.
- Heat insulating material in hot tops for ingot moulds and feeders.
- Olivine in lumps and gradings thereof for night shortage heaters and refractory bricks. These bricks produced from magnesium brick Olivine is used in different kinds of heating furnaces and heat treatment furnaces as well as refractory bricks in glass and open earth furnaces.
- Cupola spout lining can also be made with Olivine aggregate.
- Abrasives used as sand blasting environment friendly.
- Minimizes oil spillage on water.
- Blast furnace adaptive replaces the combination of Dolomite/Quartz.
- Tap Hole Filler for steel industries.