Blow rate

These mass flows are a function of the oxygen blow rate and conditions as well as gas production, which are constant over the main process time. Towards the end of the process, gas formation depends on carbon concentration and, therefore, the mass transfer conditions change. 

We will assume a small blowing rate. The lowest temperature is the wet bulb temperature i l. In (1.211), the factor is... 

The value of Klw is dependent on the blowing rate, and some hypothesis is required to establish this dependency. Most authors have assumed a zero net mass transfer of air into the surface. This requires that the air carried away from the surface by the mass motion normal to the surface be balanced by the diffusion of air toward the surface Since K1 = l-K2 and j = -j2, this balance of air motion can be expressed directly in terms of the coolant properties as... 

Equation 6.71 is the required relationship between the blowing rate of the coolant and its concentration at the wall. When the diffusion flux is expressed in terms of the mass transfer coefficient, the concentration of the coolant at the wall is given by... 

P. DeRienzo and A. J. Pallone, Convective Stagnation-Point Heating for Re-entry Speeds up to 70,000 fps Including Effects of Large Blowing Rates, AIAA J. (5) 193-200,1967. 

This is for free air but there will be a pressure buildup as the parison is inflated, so the blow rate has to be adjusted. The value of cu ft/s is obtained from Table 4-5, according to the line pressure and the orifice diameter. The final mold pressure is assumed to be the line pressure for purposes of calculation. Actually, the blow air is heated by the mold, raising its pressure. Calculations ignoring this heat effect will be satisfactory when blow times are under one second (for small to medium-size parts) but if blow times are longer, the air will have time to pick up heat, resulting in a more rapid pressure buildup and shorter than calculated blow times (2). 

Normalised blowing rate = (% yield of ureas at time, t) / (limiting urea yield) Normalised gelling rate = (% yield of urethanes at time, t) / (limiting urethane yield) Blow to gel selectivity = (Normalised blowing rate) / (Normalised gelling rate)... 

Increase material temperature Increase mold temperature Improve mold venting Increase air pressure Increase air blowing rate Decrease material temperature Decrease mold temperature Decrease part wall thickness Improve mold coring Increase material density Increase air pressure Increase melt index (with decreased stock temperature)... 

Clean the die Smooth and polish the die Increase mold temperature Increase air pressure Increase air blowing rate Increase purge time when changing materials Improve die streamlining Increase due to mandrel clearance Decrease material temperature Increase extrusion speed Decrease extrusion die temperature Increase stock temperature Decrease extrusion speed Increase die temperature Improve die streamlining Adjust material temperature Increase pinch blade land width Decrease rate of mold closing Adjust and center die-parison curls toward thin area... 

Low gloss 1. Increase material temperature 2. Increase mold temperature 3. Improve mold venting 4. Increase air pressure 5. Increase air blowing rate... 

---