The Relative Viscosity (RV) test is often used to help identify an ideal fill speed/time for the mold. Fill speed and the resulting fill time are an indication of how fast the plastic is injected into the mold. The speed affects how much shear heating and shear thinning the plastic experiences, which in turn affects the material’s viscosity, the pressure, the temperature of the plastic inside the cavities, and the overall part quality (dimensional, aesthetics, strength, etc.). Though there are many different approaches to selecting the idea point, those who teach this method generally agree that for most parts the fill time should be located somewhere on the flatter portion of the curve (Figure 1).
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