Injection-molded and blow-molded plastic parts are so pervasive in everyday life that the two processes are sometimes thought of as interchangeable. While the two may be used in an individual application — perhaps a blow-molded fluid reservoir is attached to a custom injection-molded medical device — injection molding and blow molding serve different purposes and markets.
The process of blow molding follows the basic steps found in glass blowing. The blow molding process is designed to manufacture high volume, one-piece hollow objects. If you need to make lots of bottles, this is the process for you. Blow molding creates very uniformly, thin-walled containers. And, it can do so very economically.
Injection molding requires a great deal of upfront engineering to develop detailed tooling or molds. Crafted from stainless steel or aluminum, molds are injected with liquid polymers at high temperatures under extreme pressure. The molds are then cooled to release complete plastic parts.
Perhaps the most noticeable difference between parts made by injection molding and parts made by blow molding is that injection molding creates solid parts, while blow molding creates hollow parts. If you’re producing something that needs one rigid wall, injection molding may be what you’re looking for. Some common examples of parts created with injection molding are bottle caps, hair combs, and housings for computers and televisions.
If you need a piece that can be either flexible, structural, or can hold a fluid, blow molding is more likely what you need. The most common example of a blow molded product is a bottle. Billions of bottles are created at very low prices using blow molding. But blow molding can also be used to make a variety of more industrial part shapes like coolers, fuel tanks, and stadium seats. See an extensive list of blow molded products here.
There is a substantial difference between blown and injection as well. For injection, the plastic is sealed in the injection chamber and mold throughout the entire process. On the other hand, the blow mold begins when the mold is removed from the plastic, which gives some more freedom for the blowing to expand the product in order to reach the necessary, final size.
Air is the worst enemy of injection molding. If air is present during the manufacturing process, it can create air pockets or bubbles. These abnormalities create weak spots in the product, ultimately leaving it defective and, likely tossed out. With the blowing method, air is very important. Blow molding revolves around air forced into the mold in order to push out and expand the product. There is a limited amount of air the mold can hold before separating and leaving holes in the molten product. Ultimately though, without air, blow molding is not possible to perform.
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