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Steps in Making a Milk Jug From High-Density Polyethylene

You've seen plastic milk jugs so often their shape probably seems familiar and simple to you. If you tried to make one by hand, however, you might find it more difficult than you'd think. Fortunately, there's no need to make these items by hand. With the aid of a technique called blow molding, manufacturers can rapidly mass-produce milk jugs and similar items from HDPE plastic.

Melting
- High-density polyethylene, or HDPE, belongs to a class of materials called thermoplastic resins. Like the other resins in this group, it can be liquefied and remolded multiple times. These properties are especially useful in making milk jugs, because melting the plastic by heating it to 400 degrees Fahrenheit is the first step in the process. Once it's melted, the hot, soft resin flows through a metal part called a die head or an extrusion die.
Extrusion
- As it drops from the die head of the extruder, the hot molten plastic forms a roughly cylindrical tube called a parison. The two halves of a water-cooled mold clamp down around the parison as soon as it reaches a chosen length. Molds are typically made from aluminum and designed so they force the plastic to adopt the correct shape as it cools. One mold, for example, might be shaped to make plastic jugs, while another might be designed for making shampoo bottles.
Blowing
- Once the mold is in place, a pin punctures the top of the parison, and a jet of high-pressure air flows through the hole, inflating the parison like a rubber balloon. The soft, semisolid plastic expands until it reaches the walls of the mold, where it freezes to the sides as it cools. When the plastic has cooled to the point where it is completely solid, its shape will be the same as the shape of the mold that held it.
Final Steps
- The stream of water cooling the mold ensures that the temperature of the plastic inside falls rapidly, because aluminum is an excellent conductor of heat. Once the newly formed plastic jug has cooled, the mold reopens and another blast of plastic air ejects the jug. A fresh tube of molten plastic flows through the extruder, and the cycle begins anew. Manufacturers can use this simple cycle to manufacture a large quantity of plastic jugs in a very short time.

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