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Fig. 5. Each dot on this standard deviation chart represents the average of 30 methylene blue clay tests.


Watching Size Distribution A third key independent variable to measure is the size


distribution of washed green sand. Size distribution is not as easy to control because new sand is a product supplied by a vendor, not something you can change in-house. But metalcasting facilities should try to control size distribution because it has a big effect on the sand system. Size distribution can be expressed through:


• AFS Grain Fineness Number (GFN). • Surface area. • Percent fines. • Individual screens. • Permeability. When performing the permeability test, it is important to keep in mind that while sand size is the biggest influencer, other variables can affect permeability, such as moisture and clay levels. Grede-St. Cloud runs a permeability test every time it runs lab


green sand tests—every two or three hours. Te casting facility conducts the other four size distribution tests, which are more labor-intensive, daily and plots the results on a run chart. Sand casting operations that are performing these tests less frequently than weekly are opening themselves up to missing a problem. It’s also important to build a history of normal ranges to be able to determine when something is not operating regularly. Vendors deliver new sand to a set specification, but it can


range from the high or low end of that specification. Because not much can be done with the new sand, improving permeability or size distribution in the plant is done by adjusting the dust collec- tion system through key collection points in the facility—such as by the conveyor discharge points, rotary screen or coolers. If the sand in the system is too fine, open up the draw more at the key collection points. If the sand is too coarse, close the draw a little. At Grede-St. Cloud, some jobs are highly cored while others


have little core input. When the facility is running highly cored jobs with fresh new sand, the sand in the green sand system can become too coarse. If not enough fines are available, it may be necessary to adjust the grade or amount of sand added directly to the green sand system. It’s not recommended to adjust the core sand grade because that could affect core strength. Variation in the sand mix inhibits the ability to produce


consistent, quality castings. In order to eliminate the variation, a good sand testing program will include daily tests and then use the resulting data to monitor and make adjustments to the system as needed. 


图5. 这张标准偏差图上每个点代表30次亚甲基蓝法测试 的平均值。


布,因为它对型砂系统的质量有很大的影响。 粒度分布可以用下列参数表示: • AFS细度,相当于我国标准规定的“平均细度” (GFN)


• 表面面积 • 细粒百分含量 • 各筛上的存留量 • 透气性


进行透气性试验时,需要记住是虽然型砂粒度是最大 的影响因素,但其他变量也可以影响透气性,如水分和 粘土的含量。 Grede-St. Cloud公司每隔两三个小时要在实验室中 做的型砂测试都包括透气性测试。在铸造厂进行其他 4种粒度分布测试,工作量非常大,需每天在运行图上 绘制结果。砂型铸造厂进行这些测试的频率低于每周一 次,都可能面临错失发现问题的机会。同样重要的是建 立规定范围的数据档案,以便能够确定什么时候有些作 业运行不正常。


供应商按设定的规范供应新砂,但新砂的粒度范围可 能处于该规范的高限到低限之间。因为对于新砂没有多 少工作可做,要提升车间内型砂的透气性或粒度分布水 平可以通过调节工厂灰尘收集系统的关键集合点实现, 例如调节输送带卸载点,旋转筛或冷却器。如果系统中 的砂粒太细,开大一点关键收集点的吸尘气流以便收集 更多细小砂粒。如果大砂粒太多,应关闭一点。 在Grede-St. Cloud公司,有些产品有大量的芯子而 有些产品芯子非常少。当工厂生产用新砂制成的芯子多 的产品时,湿型砂系统中粒度大的砂子含量会增多。如 果没有足够的细粒砂可用,可能有必要调整直接加入到 湿型砂系统中的新砂粒度等级或数量。不建议调整芯砂 的粒度,因为这可能会影响砂芯强度。


型砂的变量会抑制产生一致的、高质量铸件的能力。 为了消除变量的影响,良好的型砂的测试方案包括每日 检测,然后使用所得到的数据来监测,并根据需要调整 型砂系统。 


June 2015 FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 65


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