Blow moulding – Process , advantages and Disadvantages
Blow moulding is a manufacturing process by which hollow plastic parts are formed: It is also used for forming glass bottles. This is the most common method for manufacturing hollow articles from thermoplastics . The most simple products and articles that are made by blow molding are bottles, ventilation ducts, suit cases and fuel tanks for automobiles.
In general, there are three main types of blow moulding:
- Extrusion blow moulding,
- Injection blow moulding, and
- Injection stretch blow moulding.
The blow molding process begins with melting down the plastic and forming it into a parison or in the case of injection and injection stretch blow moulding (ISB) a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass.
a .Extrusion Blow Moulding :
An extruded tube or parison is elongated to reach the required length . The parison is then enclosed by the mold , which is then cut by a blade. It is then conveyed to the blowing station. In the blowing station, a blow mandrel is inserted into the mold and the actual process is subsequently carried out. It is then cooled and the finished product is removed by opening the mold. The open mold is then transferred to the position below the extrusion die inorder to receive a new parison. The process then continues.
b .Stretch Blow Molding :
This is a special form of blow molding in which the mechanical properties are considerably improved. A high degree of orientation is introduced in the plastics by drawing near the glass transition temperature or crystalline melting points. The molding material is drawn not only in the circumferential direction, but also in the machine direction. A mechanical ram is used for drawing in the longitudinal direction. The process is carried out under low temperature with the application of high deformation forces.
c. Injection Blow Moulding :
In injection blow molding, a short tubular piece (parison) is injection molded into cool dies. (Parisons may be made and stored for later use.) The dies then open, and the parison is transferred to a blow-molding die by an indexing mechanism (Fig.). Hot air is injected into the parison, expanding it to the walls of the mold cavity. Typical products made are plastic beverage bottles (typically made of polyethylene or polyetheretherketone, PEEK) and small, hollow containers.
Injection Blow Moulding Diagram :
PVC, polypropylene and poly ethyl terephthalate are some of the materials for use with this process.
Blow Moulding advantages and Disadvantages :
Advantages Of Blow Moulding :
Advantages of blow molding include:
- Low tool and die cost;
- Fast production rates;
- Ability to mold complex part;
- Handles can be incorporated in the design.
Disadvantages Of Blow Moulding :
Disadvantages of blow molding include:
- Limited to hollow parts,
- Low strength,
- To increase barrier properties multilayer parisons of different materials are used thus not recyclable.
- To make wide neck jars spin trimming is necessary.
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