Melt flow concentration, wall thickness

Melt flow is influenced by fiber content and concentration, wall thickness, melt temperature, and pressure (see Figs. 4-12 through 4-20). 

Pipe and Tubing. A typical die for extmding tubular products is shown in Figure 4. It is an in-line design, ie, the center of the extmded pipe is concentric with the extmder barrel. The extmdate is formed into a tube by the male and female die parts. The male die part is supported in the center by a spider mandrel. Melt flows around legs of the mandrel and meets on the downstream side. The position of the female die part can be adjusted with bolts adjustment is requited to obtain a tube with a uniform wall thickness. 

The smooth flow of the melt in a mould is interrupted by sharp comers and abrapt changes in wall thickness. Both types of feature have other undesirable effects, and should be avoided where possible. Sharp internal comers act as stress concentrators, and are one of the most frequent sites for crack initiation in moulded parts. Stress concentration factors can be calculated using standard formulae given in books on stress anafysis. They should preferably be limited to a value of 1.S, and when a sharp comer cannot be avoided, the radius should be not less than 0.5 mm. 

Fig. 8.13 illustrates the manner in which stress concentrates in inside corners by plotting the ratio of the inside radius (R) to the wall thickness (WT against the stress concentration factor. At ratios below 0.5, the stress concentration factor rises dramatically. At the ratio of 0.25, it has reached 2.25. That should be regarded as an absolute minimum. Below 0.25, the stress concentration factor reaches toward astronomical levels. Even a little radius is better than none at all. In addition to the reduction in stress concentration factor, radiused corners also improve the flow of plastic in the mold resulting in a more uniform melt and a shorter molding cycle. 

Abrapt transitions in wall thicknesses cause high stress concentrations, but also disturbance in flow of the melt... 

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