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with nowhere near the defect severity of the first sample run. Te simulations showed locations of shrinkage in the defect region, but they were minor in relation to the defect size seen in the sample run. Te simulation inputs were double-checked and all the
process parameters were modified to the possible extreme, yet the simulated defects varied only slightly. Eventually, the engineers decided the casting defect was not shrinkage porosity at all, but a condition in the process that was not being captured—core gas. Magmasoft currently does not directly calculate the evolution of gas from core and mold binders. However, the conditions that cause off-gassing of cores and their migration into the casting are accounted for by the software’s results. In order for a core to emit a gas, uncured binder must be
present in the core or mold, and these core/mold materials must reach high temperatures. Images from the simulations at Custom Castings showed that the core possessed all the required conditions to produce gas. Te second condition of core gas-related porosity is an
entry point for the gas to flow into the casting. Once a skin has formed on the outside of the casting, the likelihood of gas migrating into the casting is small, as the gas will take the path of least resistance. Simulation software results were used to identify the locations of the casting that formed a skin last and identify the entry point of the gas into the casting and the amount of time in which it was able to do so (Fig. 2). Tese two results, in combination with a sectioned core,
revealed all the conditions necessary for the core to produce gas and the location within the casting where the gas would reside. With the true defect identified, Custom Castings’ engi-
neering department worked with its core materials supplier to troubleshoot the variability in the coremaking process. Te company reduced the binder content of the sand, which yielded a slight improvement in the defect severity. During the investigation of the cores, Custom Castings discovered the core molds were not heating the core evenly, causing the uncured binder issue. To verify the sand cores were the root cause of the problem, the cores were replaced with an alter- nate material to completely remove the potential for core gas porosity issues. A sample run of the castings was conducted using the new material, and it resulted in 100% sound castings
and verified the cores were the source of the porosity issue. ■ Visit
www.magmasoft.com for more information.
显示铸件不应该存在缺陷。虽然模拟结果显示在空洞出 现的区域有可能产生缩孔，但模拟尺寸和实际尺寸相差 甚远。
我们仔细检查了模拟的输入数据，把所有工艺参数 都调到了极端情况，可是模拟出来的缺陷变化很小。 最后，工程师认定缺陷是由型芯气体而不是缩孔引起 的。Magmasoft目前还不能直接计算型芯和粘接面产 生的气体。但是通过软件模拟的结果，依然可以解释为 什么会出现型芯气体到铸件的迁移。
为了让型芯释放气体，必须在型芯和铸型上添加未凝 固的粘结剂并使其达到一定的高温。Custom Castings 公司模拟结果显示只有条件合适，型芯才会产生气体。 第二种产生型芯气体的条件是：一些气孔是气体进入 铸件的通道。一旦铸件的表皮形成，由于气体会沿阻力 最小的通道流动，因此气体就很难进入铸件。通过模拟 结果可以看出铸件最后形成表皮的部分，以此来确定气 体进入铸件的位置和时间。（图2）
上面的两种条件和对型芯的切片观测共同揭示了铸件 中气体产生的条件和位置。 Custom Castings公司的研发部门和其型芯供应商结 合生产缺陷，寻找砂芯生产中存在的问题。公司降低了 砂型的粘结剂含量，在一定程度上减少了缺陷。在对型 芯的研究过程中，Custom Castings公司发现型芯没有 被均匀加热，导致出现未固化的粘结剂界面。为了验证 砂芯就是产生缺陷的原因，采用一种不产生气体的替代 材料来进行试验。采用新材料生产的型芯后，铸件的质
量100%合格，这也证实了砂芯是产生孔隙的原因。■ 获取更多信息，请登录
www.magmasoft.com。
Fig. 2. The defect prone region of the casting took 140 seconds after filling to completely form a skin. Pictured at left is the inital skin formation. At right is the final casting skin.
图2.有可能产生缺陷的区 域在浇注140秒后产生了 完整的表皮。左边的图是 表皮开始产生，右边的图 是表皮完全形成。
Fall 2011
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