Plastic Blow Moulders in South Africa
Blow moulding process
Extrusion blow molding
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> In Extrusion Blow Moulding (EBM), plastic is melted and extruded into
a hollow tube (a parison). This parison is then captured by closing it
into a cooled metal mold. Air is then blown into the parison, inflating
it into the shape of the hollow bottle, container or part. After the plastic
has cooled sufficiently, the mold is opened and the part is ejected. Extrusion
Blow Moulding processes may be either continuous (constant extrusion of
the parison, or intermittent. Examples of parts made by the Extrusion
Blow Moulding process include dairy containers, shampoo bottles, and hollow
industrial parts such as drums.
Basic polymers, such as PP, HDPE, PVC and PET are increasingly being coextruded with high barrier resins, such as EVOH or Nylon, to provide permeation resistance to water, oxygen, CO2 or other substances. In dairy applications, it is possible to extrude a black light-blocking layer in the center layer of containers, with opaque white resin used in the inner and outer layers.
Compared to injection molding, blow molding is a low pressure process, with typical blow air pressures of 25 to 150 psi. This low pressure process allows the production of economical low-force clamping stations, while parts can still be produced with surface finishes ranging from high gloss to textured. The resulting low stresses in the moulded parts also help make the containers resistant to strain and environmental stress cracking.
Stretch blow moulding
In the Stretch Blow Molding process, the plastic is first molded into
a "preform" using the Injection Molded 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 Extrusion Blow Moulding 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 Terepthalate) 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 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 molding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. A sign of injection blow moulding is the seam where the two halves of the mold meet.
This picture shows what happens inside
the blow mould. The preform is first stretched mechanically with a stretch
rod. As the rod travels down low-pressure air of 5 to 25 bar (70 to 350
psi) is introduced blowing a 'bubble'. Once the stretch rod is fully extended,
high-pressure air of up to 40 bar (580 psi) blows the expanded bubble
into the shape of the blow mould.
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