Three casting methods for plastic product

Roto-casting 

Roto casting is also called rotational molding , referring to a way of forming the product inside a seamless heated mold, which rotates globally in a two-axis mechanism. Raw plastic materials are pre-loaded inside the mold, then are melt under heat and formed an even layer along the inner surface of the mold with help of the centrifugal force of rotation. While the product is still soft with heat, it can be pulled out from the opening of the mold. After cooling down, the plastic product becomes solid and shaped. 

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Note roto-casting can only be used in certain designs, which means limits. First, it requires certain cavity of the piece so as for the casting to be pulled out of the mold; so only certain size of product or part with inside hollow are applicable. Second, when plastic is still soft, it's fragile, meaning breakable; so the design of the casting piece cannot be too much "hook" shape from the pulling direction. Third, the casting can not be too small or too big; small size would leave little room for the piece to be pulled out from the mold, while large size may not fit in the furnace. Last not least, roto-casting is a slow process.

Another note is that roto-casting is run with furnace and furnace consumes a lot of power, it is not cost-effective if you only cast one at a time. In production, in fact, you will always try to fill the furnace as many pieces at a time as possible; which means multiple molds are required. Depending on size of furnace and castings, 20-30 molds per part is common seen.

Injection molding

Injection molding is a very efficient manufacturing method in casting of plastic parts. Unlike roto-casting, injection mold consists of two or more separate pieces. The molding process is like this:

  • raw materials are pre-heat
  • molds are closed up
  • raw materials are injected into molds
  • product is formed with pressure remained
  • product is cool down
  • molds are opened so product is out
  • process repeats
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In fact, the whole process is automatically operated by machine, so it can run very quickly and smoothly.

The disadvantages about injection molding include, first, due to that the molding process is under high pressure and temperature, and that it is operated by machine, it requires accurate making of the mold and durability, so the molds are expensive; second, no matter how accurate the mold is, there are always seams left on the product along the separate lines of the molds. The seams can be polished off somehow, but completely removing is difficult, marks would remain.

Blown molding

Besides roto-casting and injection molding, there is another method called blown molding, that is also commonly used in plastic processing. Blown molding is basically applying high compressed air from inside of the pre-heat material and expanding it against the enclosure molds so forming the product. Blown molding is ideal for making bottle-shape and hollow products. Blowing molds are usually two-piece set, so it will leave mold seams on products. Blown molding is lest accurate compared to both injection and roto casting.

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