Friability Friability is a measure of how easily the molding sand is
abraded from the compacted sand surface. Sands with high friability are not able to withstand the erosive flow of molten metal and are prone to producing erosion and sand inclusion defects. Loss of detail and sharp edges also are common. Tis corresponds with the fin/flash defects that Landers Foundry experienced. Te metalcasting facility’s sand lab discovered that its
sand’s initial friability was more than 12%. Te reason for the high friability was the low active clay content, low strength and the high amount of non-silica components that were competing with the clay for moisture.
Clay Content While Landers Foundry’s live MB clay content was low
(4.9%), the dead clay content measured higher than the live clay content at 7.46%. High amounts of dead clay content raise the moisture requirement, which increases the tendency for gas defects. Dead clay also competes with live clay for moisture and hinders the strength of bentonite bonding by diluting the live clay with the inert dead material. Tis increases friability and makes the sand prone to erosion and inclusions. It also lowers the silica content of the mold- ing sand as a whole, which lowers its heat conductivity and makes it prone to metal penetration.
LOI and Volatiles
Te LOI test is a measure of the organics and combustibles in the sand used to determine the amount of carbons, such as seacoal. Molding sand for cope and drag iron casting normally has an LOI of 3.5-4%. Te initial LOI of the sand at Landers Foundry was 14.6%. Te excess material likely contributed to gas defects and increased moisture requirements, as well as friability, low strength and low heat conductivity. Te original excessive carbon content may have been
the result of the casting facility adding extra seacoal in an attempt to improve surface finish. If carbons are too low, surface finish will suffer and expan- sion defects can occur. However, this happens when the correspond- ing silica level is high. In Landers Foundry’s case, the rough castings and penetration were
The facility’s working and available bond levels have been brought into greater consistency over time.
脆性
脆性用来衡量型砂从砂型表面被擦落的难易程度。 过脆的型砂无法抵挡金属液流，容易产生冲刷、夹砂 等缺陷。铸件尖锐边角和表面细节丢失也是很常见的 缺陷。这印证了Landers 铸造厂经历的毛刺、飞边缺 陷。
铸造厂的型砂实验室发现砂子的初始脆性超过12% 。高脆性原因是活性粘土量低、低强度和高含量的非 石英砂成分，该成分和粘土争水分。
膨润土含量
Landers 铸造厂的活性膨润土含量偏低，仅为4.9% ，而死土含量却高达7.46%，甚至高于活性土含量。 死土含量过高增加了对水分的需求，从而增加了气孔 的概率。死土会和活性土争水分，而且死土中的惰性 成分会稀释活性土，使膨润土的粘结强度降低。这些 都使型砂变脆，增加冲刷和夹砂的发生概率。死土还 会降低硅砂在整体型砂中的含量，进而降低导热性， 引发粘砂缺陷。
灼减测试和挥发性
灼减测试可以测定型砂中有机物与可燃物如煤粉的 含量，从而确定碳含量。在铸铁中，用于上下箱的型 砂灼减量通常为3.5-4%。Landers 铸造厂的型砂最初 灼减量为14.6%。多出的材料增加了对水分的需求， 可能是产生气孔的原因之一，同时也使型砂变脆，强 度和导热性降低。
铸造厂向型砂中添加了多余的煤粉以改善表面光洁 度，这也许就是最初碳含量偏高的原因。如果碳含量 偏低或者硅砂含 量过高，表面光 洁度会变差导致铸 件不平整。对于 Landers 铸造厂， 硅砂含量偏低和砂 粒粗糙并且粒度分 布集中才是表面粗 糙与粘砂的原因， 增加煤粉含量于事 无补。
有效粘结成分与总粘 结成分测试中，均静 置一段时间，以达到 更好的一致性。
Summer 2011
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