The blow molding process
The blow moulding process, in comparison with injection moulding, is a low pressure process with typical blow air pressures around 25 to 150 psi.
The process of blow molding involves melting plastic pellets and extruding it through a die, forming a round hollow tube called a parison.
Once the parison reaches the correct length, the mold is closed around the parison and air is injected into the tube forcing the plastic to the interior walls of the mold while cooling and forming the plastic part.
There are several different types of blow moulding. An overview of each process is outlined below:
Injection Blow Moulding:
Injection molding makes solid parts, like a Frisbee, while blow molding makes hollow parts, like a soap dispenser.
With injection molding, making the mold is 90% of the job.
In blow molding, making the mold is only 50% of the job — a host of other variables must be managed closely, such as the type of plastic, the process temperature, the velocity or pressure of the blown air, and the mold close speed.
Extrusion Blow Moulding:
Blow moulding produces hollow three-dimensional articles from many of the thermoplastics materials which are available as granules or powders. The simplest tool consists of two female parts which contain a cavity when closed.
Granules or powder are softened in a plasticising cylinder and extruded into a vertical tube or “parison”.
The soft, warm parison is surrounded by the open mould which is then closed, thereby sealing the lower end of the parison.
This is then inflated pneumatically (from the other end) to conform with the surface of the mould.
Stretch Blow Moulding:
The main applications of stretch blow moulding includes jars, bottles, and similar containers because it produces items of excellent visual and dimensional quality compared to extrusion blow moulding.
The process first requires the plastic to be injection moulded into a ‘preform’.
The preform is then heated above its glass transition temperature and blown, using high pressure air, into bottles using metal blow molds.
At the same time the preform is stretched with a core rod to fill inside of the mould.
Strain hardening occurs as part of the stretching process of some polymers (such as Polyethylene Terepthalate) which allows the bottles to resist deforming under the pressures resulting from carbonated beverages (typically around 60 psi).