Introduction to Extrusion Blow Molding
How Plastic is Formed into Bottles and Other Hollow Components
Extrusion blow molding is a manufacturing process used to create hollow parts from a plastic material. The extrusion blow molding process is very similar to glass blowing in many ways, and the process is directly based on the process of creating molded glass bottles.
The Extrusion Blow Molding Process
A cylinder of plastic material, also called a parison, is extruded, and one end of the cylinder is pinched closed. Most extrusion blow molded parts will have an obvious point on its base where the pinch occurred. The parison is placed in a mold, and air is blown into the cylinder. The air pressure expands the parison against the walls of the mold and forces the plastic to conform to the shape of the mold. After the plastic part has cooled, it is removed from the mold.
Depending on the part and its use, post-processing may be required, such as trimming away excess material from the neck or mold parting lines, or adding paint, decals, or labels to the part. Unlike injection molding, extrusion blow molding results in very little waste product, and any waste product can generally be recycled.
Controlling Wall Thickness in Extrusion Blow Molded Parts
The primary concern in producing extrusion blow molded parts is controlling the wall thickness. Depending on the shape of the mold, the parison may stretch or compress in areas, causing the wall thickness of the parison to change. There are two ways to control the wall thickness:
- Designing a mold that restricts the amount of stretch. As with die casting or injection molding, certain shapes and contours are better for extrusion blow molded parts, and sharp angles and corners are generally avoided.
- Programming the extrusion process to vary the wall thickness of the parison as it is extruded. If a certain portion of the mold causes the parison to stretch, the thickness of the parison in that location can be increased so that after it has stretched, it is the same thickness as the rest of the part.
So, what can extrusion blow molded parts, be used for?
Uses for Extrusion Blow Molded Parts
The primary market for extrusion blow molded parts is bottles. This includes plastic soda bottles, as well as bottles for liquid soap, detergent, motor oil, and other liquids. Plastics are non-reactive to these types of liquids, allowing them to be stored for long periods of time. The biaxial oriented polyethylene terephthalate (PET), developed in the 1970s and used in soda bottles, is designed to be lightweight, yet able to withstand the pressures of a carbonated beverage.
Extruded blow molded parts can be found nearly anywhere, from kitchen cabinets and cleaning closets to stadium seating and toys. This cost-effective process allows for fast, efficient mass-production of consumer and industrial products.
PET beverage bottle
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Plastic bottles before processing
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