Molding Cycle Time on Injection Moulding
– By Zafar Kamal
The important thing to remember, while considering the cycle time is that, once the conditions are set into the controlling instruments of the machine, the identical cycle will be repeated accurately for the duration of the production run or until some effective change has been made.
In a typical cycle action is started by closing the safety gate at the die area. As it is closed the gate trips two safety limit switches, one for the machine hydraulic system, the other for the overall electrical system. From this point on, machine operation is automatic. Assuming that the machine is equipped with a hydraulic clamp, oil enters a jackram housed within the main ram, causing the movable platen to advance at high speed but under low pressure. Just before the mold halves make contact, a limit switch is tripped, causing oil to be diverted from the jackram to the main cylinder. Platen speed immediately drops off, but pressure increases. As the mold halves make contact, pressure builds up, sensed by the pressure switch that signals the injection unit to begin its cycle.
At this point, the reciprocating screw is in its retracted position behind a previously plasticized shot. Upon receipt of the signal from the pressure switch, the shutoff valve opens and the injector’s hydraulic system is actuated. This causes the screw to advance as a plunger and to inject the shot at pressure as high as 20,000 psi. This initial pressure-high to counter the chilling effect of the mold is controlled by an adjustable relief valve or other flow control device.Length of screw stroke, and therefore shot size, is also variable and is controlled by a timer. When the mold cavities are filled, a signal from this timer to an injection-speed controller reduces pressure on the screw, permitting injection to continue at a reduce rate. The purpose of maintaining melt pressure is to minimize the possibility of heat sinks resulting from contraction during part cooling. This phase of the cycle is called the dwell time.
At the end of the dwell time the timer signals the shutoff valve to close and the screw to resume its rotation. Plasticized melt again advances to the front of the screw, forcing it to retract against the back pressure until the shot-control limit switch is contacted. At this point, the clamp is still closed and the molded parts are cooling. On the signal from the timer, set according to the cooling-time requirements, the clamp opens slowly under pressure. Slow movement, mandatory to protect the molded parts, continues until a fast-return limit switch is contacted, at which point the platen speed picks up. Fast retraction continues until another limit switch is contacted by a control rod actuated by the platen, to reduce speed for slow, smooth ejection of the molded parts, which drop into a box, or water tank, or onto a conveyer. Thus the machine cycle and its subordinate, injection unit cycle are completed. In some cases automatic operation of an injection machine is interrupted for manual removal of parts that might be damaged by automatic ejection.
The overall molding cycle may be described as the total time required to produce one complete shot of one or more parts, depending upon the number of cavities in a given mold.
The molding cycle is not merely the time that the materials remains in the mold but includes the time necessary for the mold to close and clamp, any safety or delay time required at the start of the cycle, the injection time (time required to
fill the cavity), the dwell or time required to cool the molten material, and the ejection time. Most molders refer to the sum of these elements as the gate-togate time or the total overall cycle.
