Intermittent extrusion

Single-stage EBM produces a blown product in a single integrated process cycle. BM immediately follows parison extrusion and relies on the melt condition of the parison for the deformation and flow necessary to take up the shape of the mold. There is no reheating of the parison before molding. The principal variants are the continuous extrusion and the intermittent extrusion processes. Extrusion in this distinguishing sense applies to extrusion of the parison rather than the operation of the extruder. 

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 process of parison extrusion can be continuous or intermittent. For intermittent extrusion, the melt from the continuously rotating extruder may be fed into an accumulator, from which it is periodically ejected, or a reciprocating extruder like those used for injection molding may be used. Continuous extrusion is preferred for most packaging applications. It provides higher productivity and reduces thermal degradation, since the melt is not held up inside the hot extruder. Intermittent... 

Intermittent extrusion process causes undesired interfaces between the batches, which may be demonstrated in the product, depending on the body and profile ... 

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. 

There are a multitude of commercially available machines that will adequately perform these operations in a variety of ways, e.g. intermittent extrusion, continuous extrusion, horizontal or vertical extruder arrangement. 

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... 

Fig. 19. Uniloy 350R2 8-head intermittent extrusion blow molder for the manufacture of 2-L (0.5-gal) milk bottles with handles. Production rates of over 65 bottles/min have been achieved also shown is the flash trimmer. Courtesy of Hoover Universal, Inc..

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. 

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 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. 

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

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... 

For larger containers, the molten resins have to be extruded very quickly or sagging will occur due to gravity. This is called intermittent extrusion, which usually only prodnces one or two containers for each machine cycle. For smaller containers like those for dairy prodncts and fruit juices, there are machines that can prodnce np to 16 containers at a time. This allows them to process a high quantity of containers to meet the demands of industry. 

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