Extrusion blow moulding
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 hollowbottle, container, or part. After the plastic has cooled sufficiently, the mold 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 moulding 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 mold 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.
EBM processes may be either continuous (constant extrusion of the parison) or intermittent. Types of EBM equipment may be categorized as follows:
Examples of parts made by the EBM process include dairy containers, shampoo bottles, hoses/pipes, and hollow industrial parts such as drums.
Injection blow moulding
The process of injection blow moulding (IBM) is used for the production of hollow glass and plastic objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow moulding 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 mold. 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 mold 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 mold 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 moulding and ejection.
Stretch blow moulding process
In the stretch blow moulding (SBM) process, the plastic is first molded 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 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. 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 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 main applications are bottles, jars and other containers.
Advantages of blow moulding include: low tool and die cost; fast production rates; ability to mold complex part; produces recyclable parts
Disadvantages of blow moulding include: limited to hollow parts, wall thickness is hard to control.