Hold time for injection molding is currently controlled open loop, using a fixed time delay set by the machine operator. Insufficient hold time leads to inconsistent moldings, while excessive hold time increases the cost of the part. We have developed new sensors that monitor the solidification of polymer in the mold. A closed loop strategy is demonstrated where the hold time is automatically controlled based on feedback from a solidification sensor. Using this strategy, the hold time is set once in reference to the sensor signal, providing the minimum proper hold time for each part under changing processing conditions.