In extrusion blow molding (EBM), plastic is melted and extruded into a hollow tube (a parison). Blow molding is the forming of a hollow object by “blowing” a thermoplastic molten tube called a parison in the shape of a mold cavity.
Extrusion blow molding is the most widely used of many blow molding methods. This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected. There are two extrusion blow processes: continuous and intermittent.
Continuous Extrusion Blow Molding
Continuous and Intermittent are two variations of Extrusion Blow Molding. In Continuous Extrusion Blow Molding the parison is extruded continuously and the individual parts are cut off by a suitable knife.
EBM processes may be either continuous (constant extrusion of the parison) or intermittent. Types of EBM equipment may be categorized as follows:
Continuous extrusion equipment
rotary wheel blow molding systems
shuttle machinery
Examples of parts made by the EBM process include dairy containers, shampoo bottles, hoses/pipes, and hollow industrial parts such as drums.
Intermittent blow molding
Intermittent extrusion may be also called shot extrusion. Parison shot extrusion is accomplished by means of a reciprocating screw almost identical to those used in injection molding machines.
In Intermittent blow molding there are two processes: straight intermittent is similar to injection molding whereby the screw turns, then stops and pushes the melt out. With the accumulator method, an accumulator gathers melted plastic and when the previous mold has cooled and enough plastic has accumulated, a rod pushes the melted plastic and forms the parison. In this case the screw may turn continuously or intermittently.
Intermittent extrusion machinery
reciprocating screw machinery
accumulator head machinery
Controlling Wall Distribution
Control of wall distribution is the heart of blow molding. There are two primary techniques in extrusion blow molding for controlling wall distribution: Programming and die shaping. Programming is the control of the wall thickness, from top to bottom, of the parsion as it emerges from the die head tooling during extrusion.
In die shaping, sectors of the die bushing or mandrel are machined to thicken the parison longitudinally in those areas where the part being formed requires greater thickness. The diameter of the die tooling is very important, for it determines the parison diameter. Too small a parison will rupture or “blow out” because of too much stretch. Too large a parison will result in too much flash, and cause trimming problems.
Plastic pellets are supplied via a hopper into a rotating screw whose spiral shape moves the pellet forward through a heated chamber.The pellets are heated gradually and turns into a homogeneous plastic melt as it approaches the end of the helical screw.
The molten mass is extruded through a die into a tubular profile, in which a cavity is created by the center core comprising a tube.
A: The profile then passes between two separated mold halves.
B: When the profile reaches the mold halves the bottom of the molds are closed which pinch the lower end of the profile and the mold clamps the center of the die core. Then the extruded profile is pressurized from the inside through the center core allowing it to expand and finally conform to the mold walls.
After the mass solidifies the mold halves are separated and the part is removed and the process is repeated.