The Blow Molding Process

Blow molding is a process that is used to mass-produce many modern products. Blow molded tables, chairs, and containers are light weight and durable. Moreover, the manufacturing process is relatively inexpensive once one has the proper machinery. Therefore, blow molded plastic products are often ideal for outfitting banquet halls, schools, or office meeting areas where a large amount of movable furniture is needed to create uniform work spaces.

The technology for blow molding was developed in the late 19th century. The first application was for the production of baby rattles. Since then, blow molding has been used to produce a plethora of products. It is especially useful for containers and plastic furniture where a uniform shape and thickness is required to meet product specifications. In 1942, the first blow molded plastic bottle went into mass production. Today, blow molding is used to produce nearly 40 billion polyethylene bottles every year in the United States. And this is only one of the many products manufactured by blow molding.

The blow molding process is fairly simple, allowing for fast production on an assembly line. The first step is to heat thermoplastic until it reaches a molten state. This molten plastic is then pushed out through a die head in the form of a hollow tube. This hollow tube (known as a “parison”) is placed in two halves of a mold. The shape of these two mold halves when closed together comprises the form of the intended final product. After the two mold halves are closed together the parison tube is inflated to fill out the shape of the mold. Then, the molten plastic is cooled so it hardens into the shape of the mold. Once the plastic is solidified the mold is opened and the component is removed from the molding. The plastic now retains the shape of the mold and the lightweight component is complete.

There are several variations of this process. The basic aforementioned process is known as “extrusion blow molding.” The mold is water cooled, which allows for the solidification of the plastic. Another type of blow molding is known as “injection blow molding.” Injection blow molding adds a step prior to the inflation process within the mold. This involves injecting the molten thermoplastic into a cavity before it is placed in the mold halves. This cavity contains a central stem which creates a hollow void in the center allowing for space within the final component. This initial injection cavity also allows for the weight specifications to be measured. Injection blow molding is useful for the manufacture of hollow vessels such as bottles or other containers.

Yet another blow molding process is known as “stretch blow molding.” Again, this involves adding a station prior to the inflation process. Because this process involves both injection molding and extrusion molding it is usually applied only for mass production, as the expense for the equipment is high and only justified by the production of a large amount of components. In stretch blow molding an injection mold is used to create a “preform,” which is then extruded into the mold halves and inflated. Some companies specialize in manufacturing just the preformed components. With the preformed material already created by an outside vendor another company can purchase these preforms and complete the manufacturing process by blow molding this material at their own plants. This two-part process is known as “RHB” or “reheat and blow molding.” Stretching and reheat blow molding is accomplished by stretching the injection molded preforms with a “stretch rod.” The mechanical stretching of the thermoplastic increases the durability of the final product.

Blow molded plastic manufacturing is great for the production of lightweight furniture. Because it allows for hollow components, it is possible to produce chairs and tables that are extremely light and easy to move to different configurations. Moreover, the stretching process increases impact resistance so plastic blow molded furniture and other products are usually very durable. Therefore, they generally make for a solid investment, especially if you wish to purchase a large order of uniform tables and chairs.