6. Increase Freezing Rate Rather Than Risering
Flow and solidification simulation software is useful to predict where shrinkage is likely to occur in castings. At that point, metalcasters have the choice to add risering to feed more liquid iron to the spot where shrinkage is predicted or somehow make that part of the casting freeze more quickly with chilling techniques. In large castings, producing structures with high nodule counts and high nodularity can be a struggle. In these situations, using chilling to avoid shrinkage is useful because it also provides an improve- ment in the structure. Risers may make the structure worse by prolonging the freezing time and reduce the iron yield. Heavy shrink-prone areas can be chilled in a num-
ber of ways. Figure 5 shows holes drilled into the cores and the subsequent cooling pins cast during mold filling. These iron pins serve as radiators to transfer heat from the casting into the sand more rapidly, to avoid shrinkage in those areas. In some cases, pins or fins can be added to a pattern. If they can’t be included automatically with patterns or coreboxes, they can be drilled into the pattern and then removed from the casting during or after cleaning. In Figure 6, a coiled spring is used to rapidly freeze
a section of a casting prone to shrinkage. The greater surface area of a spring can chill the iron more rapidly than a straight wire or bolt. In some cases, a hole is drilled through the center of the area where the spring is located, and in other cases, the spring is machined entirely out. In both cases, the drilled surface must not reveal shrinkage voids of any size. Chills also may be embedded into a core as it is
produced in order to provide chilling of the iron. The metal chills do not melt into the casting but extract heat more quickly from the shrinkage-prone sections of the casting. Metal chills must have clean, dry sur- faces and often are coated with a ceramic wash. In Figures 7 and 8, bolts were formed as “ram-up” type
inserts set into the pattern before creating the mold section to accelerate freezing in a green sand mold.
7. Produce Uniformly Strong, Rigid Molds Ductile iron can be produced without risers if
molds are suitably strong. This normally means using a nobake rather than a green sand mold. Green sand molds must be as strong as possible to avoid shrinkage induced by wall movement. This means attention must be paid to sand properties and molding machine main- tenance. For example, valve wear on impact machines can result in weaker molds. Methods to maintain mold strength when the pattern has deep pockets should be considered, including adequate venting of air expelled during impact compaction.
Tis article is based on “Avoiding Shrinkage Defects and Maximiz- ing Yield in Ductile Iron” published in the 2012 AFS Proceedings.
6.设置冒口不如提高凝固速度
可以用充型过程的模拟和凝固模拟软件来预测铸件上 可能发生收缩缺陷 的部位。在这些部位，铸造者可以选 择加冒口向可能出现收缩缺陷的位置补充更多的铁液， 或加冷铁使铸件该部位更快速冷却。对于大型铸件，石 墨球数多，石墨形态好的组织是不容易的。在这种情况 下，采用加冷铁以避免收缩是有用的，因为用冷铁得到 较好的组织。加冒口可能因延长凝固时间使组织变差， 而且降低铸件的工艺出品率。
厚达、易发生收缩缺陷易的部位可以多种方式激冷。 图5表示在砂芯上钻孔，然后在充型时铸成有冷却作用 的销。这些铁销起到散热器的作用，使热量从铸件更快 地传递到砂子，以避免这些部位产生收缩缺陷。在某些 情况下，销子或散热片可以加在模样上。如果模样或芯 盒本身不含销子或散热片，可以在模样上钻孔，销子清 理时或之后将其除去。
在图6中，在铸件容易产生收缩缺陷的部位用一螺旋 弹簧，使之快速凝固。弹簧有较大的表面积，与直金属 丝或螺栓相比，能更快地使铸件冷却。在某些情况下， 弹簧被置于中心区域的钻孔内，而在其他情况下，弹簧 被完全放在外面。在这两种情况下，钻孔的表面必须没 有任何收缩缺陷。
为了让铁液快速冷却，也可在制芯时，将冷铁也嵌 入到砂芯中。金属冷铁不熔入铸件，但可以更快地从铸 件易发生收缩缺陷的部位吸取热量。金属冷铁必须有清 洁、干燥的表面，通常都涂有陶瓷涂料。 在图7和图8中，螺栓是预埋式插件，造型前装在到 模样上，加速粘土湿砂型中铸件的凝固。
7.制造均强度一致、刚性好的铸型
如果铸型的强度足够，球墨铸铁件可以无冒口生产。 这通常意味着采用自硬砂工艺比用粘土湿砂型好。粘土 湿砂型铸型必须尽可能提高强度，以避免因型壁运动引 起的收缩缺陷。这意味着必须注意型砂性能和造型机的 维护。例如，冲击造型上的阀的磨损可导致砂型强度较 差。当模样上有深的凹坑时, 应充分考虑维持砂型强度 的方法，包括冲击坚实和时有足够的排气通道。
本文根据2012年美国铸造协会（ AFS）的诉讼文件“避免球 墨铸铁的收缩缺陷、保证最大收益”完成。
May 2013
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