Blow molding is a manufacturing process by which hollow plastic parts are formed: it is also used to form glass bottles. This is the most common method for manufacturing hollow articles from thermoplastics. The simplest products and articles made by blow molding are bottles, ventilation ducts, suitcases and automobile fuel tanks.
In general, there are three main types of blow molding:
- extrusion blow molding,
- Injection and blow molding, and
- Injection blow molding and stretch.
The blow molding process begins with melting the plastic and forming it into a preliminary shape or, in the case of injection molding and injection stretch blow (ISB), a preform. The parison is a tube-shaped piece of plastic with a hole at one end through which compressed air can pass.
a.Extrusion and blow molding:
An extruded tube or parison is stretched to achieve the required length. The parison is then closed by the mold, which is then cut by a blade. It is then transported to the blowing station. At the blowing station, a blowing 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 opened mold is then transferred to position below the extrusion die to receive a new parison. The process then continues.
blow molding process explained step by stepB. 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 into plastics approaching 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.
w. Injection Blow Molding:
In injection and blow molding, a short tubular part (parison) is injection molded into cold dies. (Preliminary shapes can be made and stored for later use.) The dies then open and the preliminary shape is transferred to a blow molding die by an indexing mechanism (Fig.). Hot air is injected into the parison, expanding it towards the walls of the mold cavity. Typical products manufactured are plastic beverage bottles (typically made from polyethylene or polyetheretherketone, PEEK) and small, hollow containers.
Injection blow molding diagram:
Injection and blow molding processPVC, polypropylene and polyethyl terephthalate are some of the materials used in this process.
Advantages and disadvantages of blow molding:
Advantages of blow molding:
The advantages of blow molding include:
- Low cost of tools and dies;
- Fast production rates;
- Ability to mold complex parts;
- Handles can be incorporated into the design.
Disadvantages of blow molding:
Disadvantages of blow molding include:
- Limited to hollow parts,
- Low resistance,
- To increase the barrier properties, preliminary multilayer forms of different materials are used, which are therefore non-recyclable.
- To make wide-necked jars, rotary trimming is necessary.
