The 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.
The parison is then clamped into a mould and air is blown into it. The air pressure then pushes the plastic out to match the mould. Once the plastic has cooled and hardened, the mould opens up and the part is ejected.
Blow moulding is most commonly used to make containers, packaging (such as soda bottles), or industrial applications (like fuel tanks). There are multiple variants, detailed below.
Extrusion blow moulding can be used to process many different polymers including polyethylene, polyvinyl chloride, polypropylene and more. The process begins with the conventional downward extrusion of a tube. When the tube reaches the desired length, the mould is closed, catching and holding the neck end open and pinching the bottom end closed. Then a blow-pin is inserted into the neck end of the hot tube to form the threaded opening and inflate the tube inside the mould cavity. When the mould is completely cooled, it is opened to eject the bottle and the excess plastic is trimmed from the neck and bottom areas.
Furthermore, there are (at least) two variants of extrusion blow moulding.
Injection blow moulding is the least-used of the three blow moulding processes. IBM is used for producing hollow plastic (or glass) objects in large quantities, typically small medical and single-serve bottles. The process is divided into three steps: injection, blowing and ejection.
The process starts with the injection moulding of a polymer onto a core pin which is then rotated to a blow moulding station to be inflated and cooled.
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 hot runner manifold where it is injected through nozzles into a heated cavity and core pin. The cavity mould forms the external shape and is clamped around a 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, similar in appearance to a test tube with a threaded neck.
The preform mould opens and the core rod rotates and clamps into the hollow, chilled blow mould. The end of 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 rotates to the ejection position. The finished article is stripped off the core rod and as an option can be leak-tested prior to packing. The preform and blow mould can have many cavities, typically three to 16, depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow moulding and ejection.
The process first requires the plastic to be injection moulded into a “preform” with the finished necks (threads) of the bottles on one end.
The preform is then heated above its glass transition temperature and blown, using high pressure air, into bottles using metal blow moulds. At the same time, a core rod stretches the preform to fill inside of the mould.
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. Strain hardening occurs as part of the stretching process of some polymers (such as Polyethylene Terephthalate) which allows the bottles to resist deforming under the pressures resulting from carbonated beverages (typically around 60 psi).
Injection stretch blow moulding is the common method for producing soda bottles. The process begins with an injection moulded perform. The perform is typically pre-heated then stretched in the axial direction and blown into its final shape by a stretch blow moulding machine.
Single-stage process is again broken down into three-station and four-station machines. In the two-stage injection stretch blow moulding 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 a reheat stretch blow moulding machine. In the ISB 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 moulds. The preform is always stretched with a core rod as part of the process.
In the single-stage process, both preform manufacture and bottle blowing are performed in the same machine. The older four-station method of injection, reheat, stretch blow and ejection is more costly than the three-station machine which eliminates the reheat stage and uses latent heat in the preform, thus saving costs of energy to reheat and 25 percent reduction in tooling.
Coextrusion allows blow moulders to improve the barrier properties of their products by keeping either oxygen or moisture out, which is critical in food packaging applications. In industrial applications such as gas tanks, the barriers layers are designed to regulate hydrocarbon emissions.
Products often have an excess of material due to the moulding process, which must be trimmed off by spinning a knife around the container which cuts the material away. This excess plastic is then recycled to create new mouldings.
Spin trimmers are used on a number of materials, such as PVC, HDPE and PE+LDPE. The physical characteristics of different types of the materials affect trimming. Mouldings produced from amorphous materials are much more difficult to trim than crystalline materials. Using titanium-coated blades rather than standard steel can increase life.
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