It is often asked what variables of an injection molding process should be monitored or controlled to achieve and maintain part quality. From the plastics perspective, only 4 variables need to be considered. Any deviation of these 4 variables will indicate a variation in part dimension, weight and even cosmetics such as gloss. These 4 variables include plastic temperature, plastic pressure, plastic cooling rate and plastic injection rate. These variables must be observed inside the mold since this is where the part resides. Variables outside the mold such as machine settings are secondary to the 4 primary plastic variables. The machine parameters are numerous. Consistent machine variables only indicate a healthy machine and can be deceptive when they are used to measure the symptoms of the parts being molded. Variations in the plastic polymer, mold mechanics and weather will affect the 4 primary plastic variables and will go unnoticed if only the health of the machine is being monitored.
Plastic pressure, cooling rate, and temperature require some investment. Internal mold pressure and temperature sensors are required. The number of sensors and placement is critical and varies with each mold. Large production volumes and difficult scrap issues usually warrant this more sophisticated approach. Proper training and upper management dedication is a requirement for a good return on investment. Remember, Plastic temperature, pressure, fill rate and cooling rate inside a mold will never be equivalent to the machines set points and actual values.
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