Principles of Blow Molding

Figure 7.80 Schematic illustration of blow molding process. From Z. Tadmor and C. G. Gogos, Principles of Polymer Processing, Copyright 1979 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

In principle, thermoforming is quite similar to the parison inflation stage of blow molding. A complication is the use of plugs to assist forming. The physics of the interaction between the molten material and the plug is not well understood and is difficult to simulate. As a result, there are some limitations on what can be simulated today. 

Figure 14.1 illustrates the general principles of extrusion blow molding. A molten tube of polymer, known as a parison, is extruded vertically downwards from an extrusion die, as shown in Fig. 14.1 a). The two halves of the blow mold surround and then close on the... 

Parison cooling is an integral part of the process that has been treated by a number of researchers (103-105). The principles are based on contact solidification without deformation, as discussed in Chapter 5. A special complication is the frictional heat generation in injection blow molding between the rod and the parison. 

Parison - A parison is a tube used for blow molding, which in principle at least has a constant wall thickness at all points around its circumference and along its length. Thickness along the parison length may vary due to tensile thinning caused by the weight of the dependent parison. 

Principles of process control Instrumentation Data acquisition/monitoring Servo control for injection molding Control of extrusion processes Blow molding/parison control SPC/SQC... 

Blow molding is a collective term for the manufacture of hollow thermoplastic products, characterized by the principle that a preform is blown to its desired shape in a blow mold. Blow molds have their origin in the glass industry. 

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. 

FIGURE 1.161 Principle of parison forming for dip blow molding... 

These general principles apply to a number of distinct extrusion blow molding processes which are described in the following sections of this chapter. 

---