Mehmet Özer, 030070050, Bonus Words - Part 2

4. Extrusion Blow Molding (Manufacturing Method)

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This form of blow molding consists of the cycle illustrated in Figure 13.30. In most cases, the process is organized as a very high production operation for making plastic bottles. The sequence is automated and often integrated with downstream operations such as bottle filling and labeling.

It is usually a requirement that the blown container be rigid, and rigidity depends on wall thickness among other factors. We can relate wall thickness of the blown container to the starting extruded parison [12], assuming a cylindrical shape for the final product. The effect of die swell on the parison is shown in Figure 13.31.The mean diameter of the tube as it exits the die is determined by the mean die diameter Dd. Die swell causes expansion to a mean parison diameter Dp. At the same time, wall thickness swells from td to tp. The swell ratio of the parison diameter and wall thickness is given by

When the parison is inflated to the blow mold diameter Dm, there is a corresponding reduction in wall thickness to tm. Assuming constant volume of cross section, we have

Solving for tm, we obtain 

Substituting Eq. into this equation, we get

The amount of die swell in the initial extrusion process can be measured by direct observation; and the dimensions of the die are known. Thus, we can determine the wall thickness on the blow-molded container.

(Groover M.P., Fundamentals of Modern Manufacturing: Materials, Processes, and Systems 4^th Edition, pp. 298-300)