Extrusion Blow Molding Variations

Two basic methods are used in this process to deliver material to the processing units. These are extrusion and injection. In the next step, the preformed material is expanded to form parison. There are many commercial variations on this basic technique some of which include continuous-extrusion-blow-molding, coextrusion-and-sequential-blow-molding, and injection-stretch-blow-molding. Both extrusion and injection molding are the subjects of later discussions below, we will concentrate here on the parison formation, its processing, and the related effects. 

A variation on extrusion blow molding is injection blow molding, a two-stage process. In the first stage, parisons are injection molded into a tubular shape complete with the threaded and formed top. These parisons may then be stored until time for the second step. 

Blow molding parison programming In extrusion blow molding bottles and containers of complex geometry are produced. To compensate for the variation in bottle geometry and... 

Commercial blowing processes come in two basic variations extrusion blow molding (EBM) and injection blow molding (IBM). 

In continuous processes the parison or preform must have sufficient melt strength so that it does not sag under its own weight. Sagging leads to unacceptable variations in the wall thickness of the part. For polymers such as PETP it is usually not possible to use extrusion blow molding because of severe sagging problems. To overcome sagging problems preforms are injection molded in a separate step where they can rapidly be quenched to inhibit crystallization and hence remain clear. The preforms are then heated by means of radiation to a temperature about 30 °C above Tg, where crystallization kinetics are slow, but the material is deformable. The heated preforms are then transferred to the mold, where they are inflated by means of air pressure. This type of two-step process can be used for resins such as PPS, which also has slow crystallization kinetics and low melt strength. 

Blow molding is a versatile conversion process used to produce a wide variety of objects by inflating a molten tube of polymer inside a hollow mold. Two major variations of the method exist extrusion blow molding and injection blow molding. The former is predominant in polyethylene molding. Common items fabricated by blow molding include bottles, drums, chemical storage tanks, and toys. 

Extrusion. Sheet, film, and profiled articles are made by extrusion (20). The resin is melted and forced through a die plate or head. Variations include multilayer and blown film applications. In multilayer coextrusions, different combinations of plastics are separately but concurrently extruded to form layered sheet or film. In the packaging industry, specialty resins such as high barrier ethylene—vinyl alcohol copolymers are combined with heat- and impact-resistant thermoplastics for food packages. The properties of each resin layer are additive, as opposed to the "averaging of property" in blends. Multilayers are also used for blow-molded containers, films, and sheet products (see also Film AND sheeting materials). 

Draw resonance occurs in processes where the extrudate is exposed to a free surface stretching flow, such as blown film extrusion, fiber spinning, and blow molding. It manifests itself in a regular cyclic variation of the dimensions of the extrudate. An extensive review [169] and an analysis [170] of draw resonance were done by Petrie and Denn. Draw resonance occurs above a certain critical draw ratio while the polymer is still in the molten state when it is taken up and rapidly quenched after take-up. 

In injection blow molding, the parison is formed by injection molding rather than extrusion. A variation known as stretch (or orientation) blow molding is responsible for the now ubiquitous plastic soda-pop bottle. In this process, parisons (often called preforms in this case) are injection molded with the bottom end closed and the threads and neck molded on the open top. They are allowed to cool to room temperature. Prior to blowing, they are reheated in a radiant-heat oven in which close control is exercised over the temperature profile of the parison. When introduced to the blow mold, the parison is normally a fraction of the length of the mold (Figure 17.7), but before blowing, a rod... 

Although there are a number of variations in the way in which blow molding is carried out, there are a number of common steps. First, conventional melt processes are used to make a cylindrical tube. (Note The preformed sample may be of other shapes.) When extrusion is used, this tube is referred to as a parison, and when injection molding is used, it is referred to as a preform. The softened preformed tube is transferred... 

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