Processing blow molding

Describe both the extrusion blow molding and injection blow molding processes. Why would one method be chosen over the other ... 

Polyesters exhibit excellent high temperature strength and electrical properties making them a good choice for many demanding applications. They also are physiologically inert allowing them to be used in food contact applications. The two common polyesters, polyethylene terephthalate and polybutylene terephthalate, are both used in injection molded products. Polyethylene terephthalate is often used in both extrusion and blow molded processes also. 

Regulatory systems, environmental impact assessment, 10 232-233 Rehalogenating bleaches, 19 261 Reheat blow-molding process, 20 47 Rehm-Weller equation, 19 112 Rehydration bonded alumina, 2 395, 396-397... 

Table 11.8 Screw Geometry for a 152.4 mm Diameter Screw for a High-Speed Blow-Molding Process Running an HOPE resin (Original Design). The Screw had an L/D of 33 and a Barrier Melting Section...

Increasing the Rate of a Large Part Blow-Molding Process... 

Blow-molding processes consists of five main operations plastication of the resin, formation of the parison, inflation of the parison, solidification of the part, and removal of the part from the tooling. The best process economics will occur with a part optimized for weight and a minimum cycle time. In order to have a minimum cycle time, the cooling operation must be the rate-limiting step. For the case study... 

Table 13.11 Screw Channel Dimensions for a 152.4 mm Diameter Extruder for a Large-Part Blow-Molding Process Running a PC/ABS Blended Resin...

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.

Blow molding is rather difficult and limited to the direct blow molding process, because of the rheologic characteristics. For the same reason the injection blow molding process does not work. Tradenames appearing in the references are shown in Table 4.8. 

Blow Molding Processing Guide, Zytel Nylon Resins, H06024, E. I. du Pont de Nemours Co., Wilmington, Del., Oct. 1988. 

Figure 3.56 Schematic of the extrusion blow molding process.

Fig. 14.15 Schematic representation of the blow molding process, (a) The extruder head with the blowing pin and open mold (b) the extrusion of the parison (c) the mold closed with the parison pinched in the bottom and sealed at the top (d) the inflated parison forming a bottle.

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. 

It is possible to blow mold several layers of material by coextrusion blow molding processes. By appropriate material selection, the various parts of the structure can be optimized for the best balance between properties and cost. 

In the injection blow molding process, the parison is formed by injection molding of the preshaped parison onto a steel rod, as shown in Fig. 14.18. The rod with the molded thread already completed is moved to the blowing station, where the article is inflated free of scrap. The parison thickness distribution is determined in the injection mold without the need of further control. Some axial orientation is introduced during injection, resulting in an article with partial biaxial orientation. 

Fig. 14.19 Thermomechanical histories of the two variants of the injection stretch blow molding process (a) the two-step, and (b) the less common one-step.

Fig. 14.20 Schematic representation of the injection stretch blow-molding process. Step (1) producing preforms may be carried out in different location from the stretch blow molding process. [Reprinted by permission from Schmidt et al., Experimental Study and Numerical Simulation of the Injection Stretch/Blow Molding Process, Polym. Eng. Sci., 38, 1399 (1998).]...

Fig. 14.21 Schematic representation of the crystallization behavior of PET relative to the stretch blow molding process. [J. S. Schaul, Drying and Injection Molding PET for Beverage Bottle Preform, SPE ANTEC Tech. Papers, 26, 534 (1980).]...

Fig. 14.23 Typical blow molding die A, choke adjusting nut B, mandrel adjustment C, feed throat D, choke screw E, die head F, plastic melt G, die barrel H, heater band /, choke ring J, centering screw K, clamp ring L, die heater M, die N, mandrel. [Reprinted by permission from J. D. Frankland, A High Speed Blow Molding Process, Trans. Soc. Rheol., 19, 371 (1975).]...

Fig. 14.24 Detailed cross-sectional representation of the die exit and the parison formed in the blow molding process, with the key design variables and the thickness and outer diameter swell.

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