HDPE stands for high density polyethylene. It is a commonly-used thermoplastic resin made from natural gas or obtained through refining petroleum. Polyethylene can undergo a low- or high-pressure process of forming polymer chains (polymerization). High-density polyethylene has longer chains with fewer side branches that provides higher crystallinity, and thus greater stiffness, hardness, strength and heat resistance. HDPE is designated with the recycling code No. 2. All No. 2 plastics are some type of HDPE.
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Blow molding is a manufacturing process that involves melting plastic and shaping it into a preform. Next, pressurized gas expands the hot preform which is then pressed against a mold cavity. Blow molding is commonly used for milk and detergent containers.
An injection-mold process consists of delivering material into a heated barrel and then mixing and forcing it into a mold cavity. The plastic cools and hardens to the configuration of the mold cavity. The cavity is generally comprised of steel or aluminum and designed specifically to form the features of the desired shape. Injection molding is commonly used for butter and yogurt containers.
Colored and natural HDPE
HDPE can also involve additive color pigments mixed in when the plastic is heated. Natural HDPE refers to a clear or semi-translucent color. These bottles have good barrier properties and stiffness and are good for packaging products that do not last long, such as milk. Colored HDPE containers have better stress crack resistance as the pigments help enforce chemical resistance. They are commonly used for products such as laundry detergent and shampoo that have long shelf lives.