Landers Foundry saw a spike in permeability that it was later able to bring down to standard levels.
of the other lighter components. Low density, low specimen weight and low silica content contribute to metal penetration. Tis can be mechanical in nature, when com- pacted density is low due to poor compac- tion, or thermody- namic, contributing to penetration even with good compaction. Moisture in molding sand has a strong cooling effect on
the metal and sand. Moisture, however, is driven away quick- ly from the casting and sand at the mold-metal interface. Of the remaining dry components, silica has the highest heat conductivity, so it is the most effective for extracting heat from the metal. If the silica level is low, the sand at the mold-metal interface retains more heat, increasing the tendency of the metal to penetrate into the sand. Silica also is the most refractory of the components in molding sand, and if its level is too low, the molding sand can begin to vitrify or turn glassy. Tis can cause a mold metal interface reaction, leading to burn-in.
Permeability, AFS Grain Fineness and Distribution Te permeability of Landers Foundry’s sand measured
low, at 89 g/sq. cm/minute. Tis was attributed to the fine- ness of the sand (67.5 AFS GFN) and the quantity of the fines, which was 7.1%. Low permeability hinders the venting of gases from the mold. A large amount of gas is generated upon pouring as moisture in the mold is converted to steam and carbons burn. Cores also generate gases. If these gases cannot vent quickly from the mold, they create back pres- sure, increasing pouring time and causing turbulent metal flow, which leads to misruns, gas defects and spalling ( the cracking or flaking of particles from the casting’s surface).
Green Compression Landers Foundry’s sand lab determined its green compres-
sion strength was low due to low active MB clay content, which was 5-6%. Te high amount of moisture-absorbing ad- ditives also contributed because they were competing with the clay for moisture. Consequently, the calculation for available bond, which is a rough estimate of live clay based on moisture, was being thrown off. While the actual MB clay may have been 4.9%, the available bond was calculated as 9.2% since ad- ditives other than clay also required moisture. In fact, initially available bond decreased while working bond increased.
Landers 铸造厂的透 气性曾出现一个高峰， 后来又将其降至正常 水平。
致粘砂的问题。这 可能是因为机械原 因导致的，如紧实 度不够就会导致密 度偏低。然而即使 紧实良好，铸件也 有可能因为其它热 力学问题而导致机 械粘砂。
型砂中的水分对 于金属和砂型有很
强的冷却作用。然而在浇铸过程中，金属与砂型界面 附近型砂水分会受热而迅速散失，剩下的干组分中， 硅砂的导热性最好，因此硅砂对于金属散热最有效。 如果硅砂含量偏低，金属与砂型界面处的型砂会过度 受热，从而增加了金属渗入型砂导致机械粘砂的概 率。
同时，硅砂还是型砂组分中耐火度最好的，硅砂含 量过低时，型砂受热容易玻璃化，引起界面处的化学 反应，最终导致化学粘砂。
透气性，AFS粒度及其分布
Landers 铸造厂的型砂透气性测试结果偏低，仅 为89 g/sq. cm/min（89克/平方厘米/分）。这主要 是由于型砂粒度（67.5AFS粒度）和7.1%的微粒数 量导致的。透气性低使得气体难以从砂型中排出。浇 注时，砂型中的水分的蒸发和碳的燃烧均产生大量的 气体，砂芯受热也会产生气体。如果这些气体不能迅 速从砂型中排出，就会产生向外的压力，延长浇注时 间，并且导致金属液紊流，产生浇不足、气孔和表面 脱皮剥落等缺陷。
湿压强度
Landers 铸造厂型砂实验室确定湿压强度偏低的原因 为活性膨润土含量偏低，仅为5-6%。其它吸水性的添 加物含量过高也是原因之一，因为这些组分与膨润土争 水分。因此，由含水量估算活性膨润土含量计算出的有 效粘结成分含量完全不准确，因为除膨润土外其它组分 也会吸取水分。如实际活性膨润土仅为4.9%时，由含水 量估算出的粘结成分含量却高达9.2%。实际上，当有效 粘结成分增加后，最初的粘结组分含量反而降低了。
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION Summer 2011
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