Control of part weight in injection molding of amorphous thermoplastics

Two methods are proposed for the control of part weight in injection molding. In the PT method, part weight control is achieved by controlling the temperature and pressure of the melt in the cavity at gate freeze time. This time is considered to occur when the cavity melt pressure starts to decrease from its peak value. For the purposes of control, the bulk melt temperature is estimated from measurements by surface thermocouples at strategic locations in the cavity, A cascade scheme is implemented for the control of bulk temperature from cycle to cycle. A self-tuning algorithm, with an observer, is employed for controlling the cavity pressure-time profile, to follow a set point trajectory during a cycle, In PWT control, the coolant temperature is controlled, while the peak cavity pressure is adjusted in a given cycle to compensate for bulk melt temperature deviations measured in the previous cycle. Both PT and PWT control reduce variance in part weight. PWT control appears to yield the best results.