Blow Moulding Process
Blow moulding Companies use a manufacturing process by which hollow plastic parts are formed.
In general, there are three main types of blow moulding:
Extrusion Blow Moulding, Injection Blow Moulding, and Stretch Blow Moulding.
In extrusion blow moulding , plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a cooled metal mould. Air is then blown into the parison, inflating it into the shape of the hollow plastic bottle, container, or part. After the plastic has cooled sufficiently, the mould is opened and the part is ejected. Continuous and Intermittent are two variations of Extrusion Blow Moulding. In Continuous Extrusion Blow Moulding the parison is extruded continuously and the individual parts are cut off by a suitable knife. In Intermittent blow molding there are two processes: straight intermittent is similar to injection moulding whereby the screw turns, then stops and pushes the melt out. With the accumulator method, an accumulator gathers melted plastic and when the previous mould 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.
The process of Injection Blow Moulding is used for the production of
hollow plastic objects in large quantities. In the Injection Blow Moulding process,
the polymer is injection moulded onto a core pin; then the core pin is rotated
to a blow molding station to be inflated and cooled. This is the least used
of the three blow moulding processes, and is typically used to make small
medical and single serve bottles. The process is divided into three steps:
injection, blowing and ejection.
The injection blow moulding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mould. The preform mold forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body.
The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.
After a cooling period the blow mould opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mould can have many cavities, typically three to sixteen depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow molding and ejection.
Another application of injection blow moulding is in the production of soft elastic gelatin capsules for pharmaceutical applications. Two strips of gelatin are pressed together in a rotary die which cuts out the desired shape of capsule while the fill liquid is injected. Afterwards, they are cooled and dried to yield a firm, strong capsule.
In the Stretch Blow Molding process, the plastic is first moulded into
a "preform" using the Injection Moulding Process. These preforms
are produced with the necks of the bottles, including threads (the "finish")
on one end. These preforms are packaged, and fed later (after cooling) into
an EBM blow moulding machine. In the SBM process, the preforms are heated
(typically using infrared heaters) above their glass transition temperature,
then blown using high pressure air into bottles using metal blow molds.
Usually the preform is stretched with a core rod as part of the process.
The stretching of some polymers, such as PET (Polyethylene terephthalate)
results in strain hardening of the resin, allowing the bottles to resist
deforming under the pressures formed by carbonated beverages, which typically
approach 60 psi.
The stretch blow moulding process. The main applications are bottles, jars and other containers. The Injection blow moulding process produces bottles of superior visual and dimensional quality compared to extrusion blow moulding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. A sign of injection blow molding is the seam where the two halves of the mould meet.
Inside the mould the preform is first stretched mechanically with a stretch rod. As the rod travels down low-pressure air is introduced blowing a 'bubble'. Once the stretch rod is fully extended, high-pressure air blows the expanded bubble into the shape of the blow mould.
Blow moulding remained a relatively small part of the plastics manufacturing scene until the introduction of Low Density Polyethylene (LDPE) in the 1940's. The production of LDPE squeeze bottles caused a rapid expansion of the industry, with plastic bottles produced to replace glass bottles for shampoos and liquid soaps.
The mass production of high density polyethylene (HDPE) and polypropylene (PP) in the 1950's led to a further increase in blow moulding demand, for producing bottles for the packaging of liquid detergents, motor oil, water and milk.
The production of polyethylene terephthalate (PET) led to the viability of reheat stretch blow moulding. The strain hardening properties of PET allowed the high volume production of bottles able to resist the carbonation pressure in soft drink applications. The high clarity and economics of PET stretch blow molding have made this a popular production method for bottles for water, detergents, and other products.