Simulation of Blow Molding

During the hanging of the parison in the case of extrusion blow molding, there is some cooling of the parison as the hang times may be on the order of 10 to 20 s for large parts. There is probably little heat transfer at the inner surface, and hence, this can be considered as an insulated surface. At the exterior 

FIGURE 10.20 Blow molding process showing an extruded parison of cylindrical shape leaving an annular die and the walls of the mold. 

In two-step processes, such as used for blow molding PETP, the preform must be heated up to a temperature suitable for inflation. This is usually accomplished by means of radiation heating as discussed in Section 5.4. As this topic was discussed in some detail in Section 5.4, we do not discuss it further here. 

Sagging of the parison as discussed above leads to a nonuniform distribution of the wall thickness. If the wall becomes too thin, it will fail during the inflation process. In the next example we consider how to approach the modeling of sagging. 

Equation 10.79 can be solved numerically using the following boundary conditions  

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S. Tanue, T. Kajiwara, K. Funatsu, K. Terada, andM. Yamabe, Numerical Simulation of Blow Molding - Prediction of Parison Diameter and Thickness Distribution in the Parison Formation Process, Polym. Eng. Sci., 36, 2008-2017 (1996). 

The material properties needed for simulation of blow molding, blown film extrusion, and thermoforming are listed in Table 11.8. The... 

TABLE 11.8 Material Properties Needed for Simulation of Blow Molding, Blown Rim ... 

Busby, W.J. and Kouba, K. (1998). The use of computer simulation of blow molding. Proceedings of the Advances in Blow Molding Conference, Loughborough, UK. 

Numerical simulation of blow molding -prediction of parison diameter and... 

H. G. deLorenzi, H. Nied, and C. A. Taylor, Simulation and Analysis of Blow Molding Using the Finite Element Method, SPE ANTEC Tech. Papers, 34, 797-799 (1988). 

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. 

The use of simulation software to predict the process of blow molding can save considerable time and money in the product development and is becoming more widespread. However, for parison formation simulation, the current finite element (FE) software is suitable only for the situation where the die gap is fixed. In this work, a new method was proposed to apply the FE simulation to the varying die gap parison formation. In order to evaluate the availability of the new method, the predicted parison thickness distributions were compared with the experimental results. It is demonstrated that the new method has certain accuracy and reliability in predicting the parison thickness from a varying die gap. 

Figure 13.14 Simulated axial pressure and temperature profile for the original blow-molding screw at a rate of 156 kg/h and a sorew speed of 12 rpm, for a specific rate of 13.0 kg/(h-rpm)...

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).]...

H. G. deLorenzi and H. F. Nied, Finite Element Simulation of Thermoforming and Blow Molding, SPEANTEC Tech. Papers, 33, 418 120 (1987). 

A. Rodriguez-Villa, J. F. Agassant, and M. Bellet, Finite Element Simulation of the Extrusion Blow-Molding Process, Numiform, 95, 1053 (1995). 

S. Wang, A. Makinouchi, and T. Nakagawa, Three-dimensional Viscoplastic FEM Simulation of a Stretch Blow Molding Process, Adv. Polym. Technol., 17, 189-202 (1998). 

B. Debbaut, B. Hocq, and J. M. Marchal, Numerical Simulation of the Blow Molding Process, SPEANTEC Tech. Papers, 39, 1870-1872 (1993). 

Contour plots of the wall thickness distribution for the injection blow molding simulations. 

One development for coinjection equipment was the inclusion of separate hot-runner temperature control systems for the different resins, for example, in order to allow an EVOH core layer to be processed at a temperature 70 C lower than the PET skin layers. Another development was the use of computer simulation to design tooling that manipulates the barrier resin distribution to fortify critical areas such as wall sections. Both wide mouth and narrow neck coinjection stretch blow molded bottles are available. 

Blow molding is complicated by the complex stress field set up in the materials when the parison is inflated. This amounts to a biaxial stretching of the molten polymer and it is difficult to obtain material data under these conditions so that simulation may be performed. Despite this, much work on the inflation stage has been done, mostly with the aim of determining the final thickness distribution. Recently parison inflation has been simulated using three-dimensional finite elements and with remeshing of the parison as it inflates to minimize error from element distortion. ... 

D. Laroche and F. Erchiqui, 3D Modehng of the Blow Molding Process, in J. Hu6tink and F. P. T. Baaijens, eds.. Simulation of Materials Processing Theory, Methods and Applications, A. A. Balkema, Rotterdam, 1998. 

M. Belief, A. Rodriguez-Villa, and J. F. Agassant, Finite Element and Automatic Remeshing Methods for the Simulation of Complex Blow Molded Polymer... 

Color labs are outfitted with laboratory size equipment that simulates the larger machines used for production internally and by their customers. Typical processing equipment found in the lab are small extruders, two-roll mills, ban-burry mills, and media mills. Small rotational, injection and blow molding machines are used to duplicate the customers process. Instruments and computers are required for testing physical properties and color. Most labs have a computer-controlled color measuring system and a light booth to evaluate color. The spectrophotometer with computer is initially used to assist in colorant formulation and later as a quality control (QC) tool to provide certification of the quahty of match to standard. The light booth provides a standardized set of conditions to visually observe color and appearance. 

Also useful are simulation programs for the blow molding process [10]. These continuously optimized programs can be used as a form of expert system (e.g., the process engineering process is mathematically written and given to the program user). 3D molds and article data are necessary to use simulation programs. 

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