Plastic injection molding machines generally follow a standard sequence of operations. The raw plastic material is fed into a cylinder containing a rotating screw. As the screw rotates, it heats and plasticizes the material and forces it to the front end of the barrel. Plastic melt accumulates at the forward end of the barrel and forces the screw backward.
When a sufficient amount of material has accumulated to produce the desired molded article and after the mold parts have been clamped together, the motor rotating the screw is stopped. The screw is then forced forward, injecting plastic melt into the mold cavity. A holding pressure is then maintained on the screw to pack plastic into the mold, completely filling it, and then to solidify the molten plastic. When the part has sufficiently solidified to maintain its shape without external pressure, the holding pressure is reduced, and the screw is again rotated to prepare the next shot. The part is then either cooled or cured in the mold, depending on whether the plastic is thermoplastic or thermosetting. When the plastic has sufficiently solidified and the part is sufficiently formed to be ejected from the cavity, the clamp is opened, and the part is then ejected. When the mold cavity is cleared, the mold is reclamped and the mold cycle can begin again.
In the past, it has been customary in the molding industry to control the initiation and termination of portions of the cycle both in response to some physical parameters, such as the condition of plastic in the mold, and also in response to the elapse of an amount of time from the initiation or termination of some portion of the cycle. An example of the former is the so-called "dynamic transfer" control. In dynamic transfer, the pressure in either the mold cavity or the barrel is monitored, and when this pressure reaches a predetermined value, the hydraulic pressure on the screw is changed from the injection to the holding pressure, thus terminating the injection portion of the cycle and initiating the holding portion of the cycle. Other controls have used pressures, temperatures or other parameters generated during the molding cycle to initiate or terminate various portions of the cycle.
One example of a customarily used time-dependent control is the timing of the holding stage of the cycle, during which the holding pressure is applied to the plastic in the mold. Generally, when dynamic transfer is initiated to change the pressure exerted by the pneumatic cylinder on the screw from the injection to the holding pressure, a pre-set timer is also actuated. When the timer times out, the pressure exerted by the pneumatic cylinder on the screw is reduced and the holding phase is terminated. The screw then starts rotating to prepare the next shot. Similarly, it has been customary to time the cold/cure portion of the cycle, that is, the time between the termination of the holding pressure and unclamping of the mold. Similarly, depending on the sophistication or lack thereof of the molding machine controls it has been customary to time other portions of the molding cycle.
Using a time-dependent measure for starting or terminating various portions of the molding cycle by necessity reduces production rates. Since the set time intervals are empirically determined, and since the actual times required may vary from shot to shot depending on variations in the plastic, safety factors must be used to allow for the worst case, wasting a certain amount of processing time. On the other hand, if such time is not allowed, a large number of poor quality articles may be produced. Thus, the molding machine operator must make the choice between high production and a low rejection rate.