Sodium silicate of various molecular ratio was added to molding sand to improve the compressive strength and the collapsibility of CO₂ mold.
  Some properties of CO₂ mold were investigated such as the compressive strength just after gassing and after holding 24 hrs the retained strength after heating for an hour at elevated temperature, and the collapsidility in a practical test. The tests were done in both of cases with and without 2% pitch powder in CO₂ molding sand.
  The results obtained were as follows :
  1) Sodium silicate of higher molecular ratio hardens the mold rapidly by gassing.
  2) Sodium silicate of lower molecular ratio gives higher compressive strength in the specimens just and after holding 24 hrs after gassing compared to that of the higher molecular ratio.
  3) The strength has a maximum value at about 200°C, then begins to decrease and falls to minimum at about 600°C, and it increases remarkably beyond the temperature. Sodium silicate of higher molecular ratio has always lower strength.
  4) The collapsibility is good in the following conditions :
  (1) In small amount of sodium silicate addition.
  (2) Using higher molecular ratio.
  (3) Thin castings.
  5) The addition of spheroidal pitch powder decreases the retained strength, and improve the collapsibility.

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  With a view to examine the acceptability of conventional practice of collapsibility testing of the carbon dioxide sand mold, where the standard specimen of the sand mold first heated up to a given high temperature and then cooled down to a room temperature, experiments were conducted by checking the compressive strength. In these experiments, the cores made of the carbon dioxide process were used to obtain several ring casting of different metals with different thicknesses in order to check the process of temperature rise and the casting conditions which result in achievement of four different levels of nearly constant temperature of 250°C, 500°C, 830°C and 1130°C. Then, pouring was carried out to the ring mold, and castings obtained were subjected to cooling down to the room temperature, followed by wedging test, by which the collapsibility of the core was examined, according to the final number of wedge driving for perfect penetration of the wedge. Other measurements were made on the standard specimens of the carbon dioxide mold which have been heated up in an electric furnace in nearly a same process of the temperature rise by heating as in the core during the preceding cast testing, to determine the residual strength. Finally, comparison was conducted with respect to the obtained results between residual strength testing of the standard specimen, and wedging test of the core often test castings and got the following conclusions;
  1) Residual strength of the tested standard specimen offers a good guide to estimate the collapsibility of casting core, provided that heating has been made within a temperature range below 830°C, where no substantial sintering is seemed not taking place.
  2) The core, checked by the collapsibility test where heating occurer up to high temperature exceeding 1000°C cannot determind with respect to its collapsibility from the residual strength, obtained by testing the standard specimen, because penetration of cast metal sintering of sand grain are likely to take place.

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  For the purpose of improving the collapsibility of CO₂ process sand for cast iron as well as preventing from penetration, we mixed coke sand made by crushing coke cake into CO₂ process sand and tried a test. The good result were obtained as follows and now it has been applied practically.
  1) If about 15% coke sand is mixed in silica sand No. 6 for CO₂ process, collapsibility is remarkably improved and penetration is much prevented with effective results.
  2) It is, however, unpractical to mix coke sand more than 30% to silica sand.
  3) The sodium silicate of high SiO₂/Na₂O ratio is proved effective to be used for the sand mixed with the coke sand, and comparing with the sand using the sodium silicate of low SiO₂/Na₂O ratio, gassing time of CO₂ can be much shortened to 1/2-1/3.
  4) Out of various materials which improve better breakdown, it has been proved that coke sand is superior on the whole and moreover it can decrease the breakdown time of core to 1/3-1/2 in comparison with conventional sand.

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  It is important for CO₂ mould to solve the mechanism of sand adherence on steel castings. The authors made an apparatus by which the conditions at large steel castings can be given on the laboratory mould specimens and it was investigated that the effects of SiO₂/Na₂O ratio of sodium silicate on the critical penetrating pressure and on the depth of metal penetration in CO₂ mould.
  The following results were obtained:
  1) The optimum region of SiO₂/Na₂O ratio of sodium silicate is found to be 2.3∼2.7 at 1070mm ferrostatic pressure.
  2) The increase of sodium silicate content to CO₂ mould tends to result in the high penetrating pressure and decreased the depth of metal penetration throughout the mole ratio.
  3) In the case of CO₂ mould using sodium silicate of high mole ratio, remarkable penetrations were observed inspite of relatively high penetrating pressure.

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  It is well-known that CO₂ process is the hardening method of sodium silicate bonded sand by carbon-dioxide and the process improves the efficiency of the work, but it is more expensive owing to the more consumption of carbon-dioxide than the theoretical volume.
  The present investigation dealt with the dry-air hardening of the sand bonded with sodium silicate of high molecular ratio.
  The following results were obtained.
  1) The sand bonded by sodium silicate of molecular ratios 2.1 and 3.0 (mixed 1 : 1) was hardened by passing dry-air for 2-3 minutes with pressure 0.5-1.0 kg/cm².
  2) The strength after hardening increased slowly for 48 hours.
  3) The dry strength increased very much, and the maximum strength was obtained heating at 120-160°C for 45-60 minutes.
  4) In molding and casting tests, the results were similar to the CO₂ process.

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