The requirements and topics of interest regarding hot runner systems and their suppliers vary to some degree from a mold manufacturer’s perspective versus those of the injection molder. Mold makers are often concerned about price, delivery, and quality…all of which are important considerations that benefit the mold and the mold maker’s reputation.
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).
Short shot mold diagnostics is a seemingly simple yet very effective method of evaluating a molds performance. The use of short shots can provide a window into how a mold fills. Uses can include diagnostics of filling imbalances in multi-cavity molds, evaluation of filling patterns within a given cavity and evaluation of the molding pressure development within the mold. Despite the simplicity of this method, it is common that the test is done incorrectly and results in false information. Below are instructions on how to properly collect short shot data.
We previously talked about Flow Group ID’s and how they relate to the pressure drop equation (Figure 1) to help identify root cause of mold filling and part quality variations. Using Flow Groups help separate out the root causes into either steel or viscosity variations.