Basic Extrusion Blow Molding Process

The marks left from the removal of the flash serve as an easy means for identification of extrusion blow-molded containers. Usually this is easiest to see on the bottom of the container. It typically appears as a rough area along the mold parting line, centered in the middle of the bottom and running half or so of the distance to the heel of the bottle. It is also possible, on careful examination, to identify the roughness at the top of the finish, or on other areas where flash was trimmed. 

The flash, after being trimmed, usually is immediately ground up and fed back into the extruder at a controlled rate, mixed with the virgin resin. The use of regrind can be problematic for heat-sensitive resins like PVC, especially if the proportion of flash is high. 

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 

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Fig. 4-1. Basic extrusion blow molding process. A = Parison cutter, B = Parison, C = blow mold cavity, D = blow pin.

Figure 7-57. The basic extrusion blow-molding process. A = a parison (plastic tubular melt leaving the extruder die) cutter B = parison C = the two blow-mold cavities, D = the blow pin.

Two basic processes of blow molding are extrusion blow molding and injection blow molding. These processes differ in the way in which the parison is made. The extrusion process utilizes an unsupported parison, whereas the injection process utilizes a parison supported on a metal core. The extrusion blow-molding process by far accounts for the largest percentage of blow-molded objects produced today The injection process is, however, gaining acceptance. 

Designers and processors to produce products at the lowest cost have unconsciously used the basic concept of the FALLO approach. This approach makes one aware that many steps are involved to be successful, all of which must be coordinated and interrelated. It starts with the design that involves specifying the plastic, and specifying the manufacturing process. The specific process (injection, extrusion, blow molding, thermoforming, and so forth) is an important part of the overall scheme and should not be problematic. 

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. 

Extrusion blow molding consists basically of the extrusion of a predetermined length of parison (hollow tube of molten plastic) into a split die, which is then closed, sealing both ends of the parison. Compressed air is introduced (through a blowing tube) into the parison, which blows up to fit the internal contours of the mold. As the polymer surface meets the cold metal wall of the mold, it is cooled rapidly below Tg or Tm- When the product is dimensionally stable, the mold is opened, the product is ejected, a new parison is introduced, and the cycle is repeated. The process affords high production rates. 

Fig 19.39. Basic blow molding process (1) extrusion of parison (2) compressed air inflates parison (3) ejection of blow molded container. Reprinted from Blow Molding Handbook, Rosato end Roseto, Fds., by courtesy of Cad Hanser Verhg.)... 

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

Like the extrusion of metals, the extrusion of plastics involves the continuous forming of a shape by forcing softened plastic material through a die orifice that has approximately the geometric profile of the cross-section of the work. The extruded form is subsequently hardened by cooling. With the continuous extrusion process, such products as rods, tubes, and shapes of uniform cross-section can be economically produced. Extrusion to obtain a sleeve of the correct proportion almost always precedes the basic process of blow molding. 

Blow molding requires an understanding of every element of the process, starting with the basic extruder used in conventional extrusion and IM machines. (For information on the machines used to plasticate/melt materials for BM, see the previous sections on injection molding and extrusion.)... 

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