Let's cover a few contraction rules, these will really help you when knowing when to, or when not to, use a contraction.
我们先来说一些缩略形式的规则,这些规则能够帮助你理解何时该使用,何时不该使用缩略形式。
英语百科
Casting (metalworking)
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods.