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What is Extrusion Blow Molding?
Extrusion blow molding is expected to show a process energy load somewhere in between injection molding and extrusion. This is the case and typical data for an extrusion blow molding site shows a base load of 25% of the average total energy use and a process load of 1.3259 kWh/kg. This is again generally representative of an extrusion blow molding site that has not taken substantial energy management action.
The extrusion blow molding site has a broadly similar base load (both in magnitude and in percentage terms) to the injection molding and profile extrusion sites and the extrusion blow molding site has a process load between that of extrusion and injection molding.
The extrusion blow molding site has a broadly similar base load (both in magnitude and in percentage terms) to the injection molding and profile extrusion sites and the extrusion blow molding site has a process load between that of extrusion and injection molding.
Extrusion blow molding is recognizable by the hollow tube that is extruded from the machine to create the parts. This tube is referred to as a parison. A hollow mold then closes around the parison and cycles to the blow station. When at the blow station, the parison within the mold is molded through utilizing air pressure while another parison is being extruded from the machine. This allows for a quicker cycle time .
Since the parison is required to hang freely during extrusion, a higher-molecular-weight polymer is required. High-density polyethylene (HDPE) is most often used within extrusion blow molding applications. Multiple heads can be used on an extrusion blow molding machine in order to create a greater output.
The two types of extrusion blowing molding machines are the two-station machine and the continuous machine. A two-station, or “shuttle,” machine transfers a common mold back and forth between the extruding parison and the blow station. A continuous, or “wheel,” machine uses multiple, identical molds to continuously produce parts.
Extrusion blow molding can produce larger parts than injection blow molding. It is also less expensive, and there are no internal stresses within the part. However, more scrap is produced when using extrusion blow molding, and there is less control over the accuracy of the critical dimensions.
Extrusion blow molding can produce larger parts than injection blow molding. It is also less expensive, and there are no internal stresses within the part. However, more scrap is produced when using extrusion blow molding, and there is less control over the accuracy of the critical dimensions.
Extrusion blowing molding can be operated as either a continuous or semicontinuous process depending on the requirement. In extrusion blow molding a plastic material feedstock is fed through a hopper onto a feeding transfer screw. This feeds material into an especially designed blow molding die that directs the material flow to create an extruded hollow tube called a parison. This parison drops with gravity until it is a process-specific length, which triggers the mold to close around it and seal the bottom edge (that gives the distinctive end seal). A cutting knife slices the open top of the parison and a blow pin moves to the top of the tool and inflates the parison against the walls of the tool cavity. The cooled tool walls solidify the molding and after a preset time it is ejected and the process repeats with the newly extruded parison.
Extrusion blow moulding is usually used for production of bigger articles (weight of 360 g and more) unlike in the case of injection blow moldings where small articles, having well-controlled wall thickness, are produced. In both methods, the first step is that of production of a parison which is a preform to be inflated in the next step against internal mold walls. Stretch blow molding is most frequently used for crystalline polymers (e.g., PET) but biaxial stretching (orientation) can also be used in the case of PVC bottles to improve their mechanical properties (see Section 4.1.1). Many polymers are used as raw materials for blow molding processes. PVC has several important features such as high clarity, excellent acid resistance, good barrier properties to oxygen, and good oil resistance. This makes it useful for many applications. The usual drawbacks of PVC are low distortion temperature, inferior impact resistance (which can be improved by impact modification), poor heat resistance (which can be improved by proper selection of stabilizers), and average cold resistance.
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