Intermittent extrusion blow molding

Decide which of the two processes (continuous extrusion blow molding or intermittent extrusion blow molding) is more suitable for the production of bottles from ... 

The intermittent extrusion blow molding process is generally used in the fabrication of large volume articles (e.g., > 20 liter bottles). An accumulator is employed to increase the parison extrusion speed (see diagram below). After the mold closes there is no additional mold movement. Refer to Injection Stretch Blow Molding. 

In intermittent extrusion blow molding, the parison is formed immediately after the blow-molded product is removed from the blow mold in most machines. In some large machines, the parison is cut and closed within the blow mold and then... 

The extrusion blow-molding process can be operated either continuously or intermittently. In the continuous process, the parison, in essence, is never-ending. As such, it is necessary to move the mold to and away from the extruded parison (see Fig. 9-6). The intermittent operation, sometimes called short extrusion, uses a reciprocating screw similar to the type used with injection-molding processes (see Fig. 9-7). Intermittent extrusion blow molding is faster than the continuous extrusion operation. 

Fig. 13.7 depicts an intermittent extrusion blow molding machine with a ram (piston) accumulator remote from the die head. 

Intermittent extrusion blow molding is best suited to long or heavy parisons and to thermoplastics with a low melt viscosity or low melt strength. 

Reciprocating screw intermittent extrusion blow molding machines use an extruder unit in which the screw is capable of axial as well as rotational movement. The function is virtually identical to that of the injection unit on an injection molding machine, except that the blow molding unit functions at a lower melt... 

The extrusion blow molding process can be continuous or intermittent, as shown in Fig. 14.16. The former, employed commonly for parts less than 1 gal, has a continuously rotating screw extruder, extmding parisons through one or more dies. The latter may use either an accumulator head with a piston-driven extrusion forming of the parison, or a reciprocating screw, such as the one used in injection molding. 

Blow molding can be divided into three major processing categories (Figure 6.1) extrusion blow molding (EBM) with continuous or intermittent melt producing a parison from an extruder (Chapter 5), injection blow molding (IBM) with noncontinuous melt from an... 

Figure 1.2(b) Extrusion Blow Molding of a Bottle (Miller, 1983. Hopper, reciprocating extruder screw, die head, extruder parison, and molds shown in open position. The extruder rotates and reciprocates continuously, providing continuous mixing of resin with intermittent extrusion of parisons. 

In 1950, Continental Can Company received the first patent for a continuous extrusion blow molding machine. The term continuous applies to the constant extrusion of the parison as compared to an intermittent parison forming process. Plax Company and Continental Can Company owned a strong patent position on the blow molding process imtil about 1960. After the patents expired, Zam Company pioneered the first HOPE milk bottle for Borden Dairy in North Carolina using Uniloy machines [29]. 

In extrusion blow molding, a tubular preform (parison) is placed into the blow mold, where, with the use of internal pressure, it is shaped into the final product. There are two basic methods for producing the tubular preform, either continuous extrusion or intermittent extrusion. 

With this method, the extruder also works continuously, but the melt is stored in a tubular piston accumulator (Figure 1.139) from which the plastic is intermittently extruded to produce the tubular preform. Figure 1.140 illustrates an extrusion blow molding machine equipped with an accumulator head as it is used, for example, in the manufacture of plastic fuel tanks, barrels, liners, and other similar large structures. The removal of the finished part is done with grippers and a transfer mechanism. The accumulator heads work best with the first in, first out (FiFo) principle. This is necessary in order to accommodate for the limited thermal stability of the plastic. 

Extrusion blow molding can be classified into two major categories continuous extrusion and intermittent extrusion. Intermittent extrusion is further... 

The intermittent parison extrusion blow molding machine using a reciprocating screw is often referred to as shot extrusion. The screw rotates and retracts, plasticizing or melting toe resin as it moves back, then charging toe shot in front of toe screw. The screw then rams forward via hydraulic or electric means. 

Figure 13.5 Intermittent parison extrusion blow molding machine using a reciprocating screw.

Intermittent accumulator EBM machines use a normal conventional axially fixed continuously operating extruder to prepare the melt. The accumulator is a heated reservoir where the melt is temporarily stored in the intervals between parison extrusion (similar action of a two-stage injection molding machine (IMM) as reviewed in Chapter 4. Also used is a conventional reciprocating IMM that delivers melt through a die into a blow mold. 

To form hollow parts such as thin-walled bottles, the blow molding process (Figure 10.11) is used. Blow molding starts with a cylindrical hollow preform of smaller diameter called a parison which is first produced by extrusion in a continuous molding process, or by injection in an intermittent molding process. 

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