Blow Molding 102
Blow molding appeared sometime in the 1800s but blow molding companies didn’t produce polyethylene bottles until the early 1940s. Most of the products produced with blow molding are hollow plastic containers. However, production capabilities are not limited to that just that. Blow molding produces strong plastic containers because chains of polymers are held together by a strong and fluid bond. This is why pop bottles are strong enough to withstand carbonated pressure and still be relatively thin.
Most blow molding processes are completed in one step. Basically a mold has to be created first. Then the mold is put through the blow molding machinery. Warm plastic is then pushed into the mold. After enough plastic enters the mold compressed air at low-pressures stretch the plastic to form the sides of the mold. This is supposedly an expensive process but is extremely efficient for mass production. That explains plastic bottles and other hollow containers make up the majority of the goods manufactured this way.
I have two different images in my head when I think of blow molding. Both of these imagines are most likely false incantations. Firstly, I think of glass blowing on a massive scale. My imaginary blow molder takes the plastic then blows it into particular shapes. Second I imagine a machine pushing play-doh into silly molds. I Google image searched blow molding and got some intense looking machines and some weird diagrams. I have to admit, I figured the whole thing looking more like an assembly line than an oven.