Reciprocating screw blow molding

Fig. 14.16 Schematic view of (a) continuous extrusion blow molding (b) reciprocating screw blow molding and (c) ram-accumulator blow molding. [Reprinted by permission from Modem Plastics Encyclopedia, McGraw-Hill, New York, 1976-1977.)...

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. 

Use is made of screw tip valves, popularly called non-return valve, ball check valve, or sliding ring valve. They are used in reciprocating injection and injection blow molding machines (IMM and IBMM) to control the melt flow in one direction (Chapter 4). There is also the smearhead for IMM, IBMM, and extruder. Back flow will not occur... 

SBC can be processed on most conventional blow molding and injection blow molding equipment designed for many other types of polymers with little or no equipment modifications. This includes continuous, accumulator head, or reciprocating screw extrusion blow molding equipment. Bottles made from SBC have been used widely for applications such as honey bears. The SBC will withstand the moderate hot fill temperatures required for this application. 

Figure 1.2(b) E.xtrusion Blow Molding of a Bottle (Miller, 1983J. 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 extmsion of parisons. 

Fig. 2. Three-station injection blow-molding machine utilizing a horizontal reciprocating screw produced by Bekum, Berlin, Germany.

A reciprocating screw extrusion blow-molding machine sketch is shown in Figure 18. This is the process where the parison is extruded, then the blow molds cut the parisons and close, and the containers are then blown. Only after the blow molds open and the blown containers exit the machine is the parison once again extruded (see Fig. 18). 

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. 

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.

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... 

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