Nipple height

The maximum nipple height is established by the designer, so we can establish an upper limit on a. in terms of the physical constants of the material, the applied pressure, and the thickness of the sheet. The value of v is between 0.35 and 0.5 for most plastics, so we will take the upper limit and substitute into 10.3. Rearranging the equation, we get... 

Therefore, the engineer needs to determine the maximum nipple height, the temperature-dependent modulus, and the molding vacuum to solve for the permissible diameter of the vent holes. After determining the correct vent hole size, the value is rounded down to the next smallest drill size. 

The diameter of the vent holes is limited by the acceptable height of the nipples that will form on the mold side of the part. The drawing of the plastic into the vent holes is dependent on the diameter of the hole, the modulus and Poisson s ratio of the polymer, and the sheet thickness. The problem is similar to a beam suspended across an opening, as shown in Fig. 10.13. 

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