To eliminate the secondary operation of degating, injection molds in which the parts are automatically degated from the runner system by the opening motion of the mold were already developed.
Principle of self-degating
Self-degating requires two parting lines in the mold. In the first parting line，melt is distributed from a central sprue through runners and flows through risers in an intermediate plate at the end of which the gate to the cavity is located. The parts to be molded are located in the second parting line of the mold. During the opening motion of the mold，the parting line containing the runner system opens first and the runner system is held on the stationary side in order that the gate in the intermediate plate be severed from the molded part.
Suitable measures must be taken to ensure that the runner system and sprue are ejected once this parting line has opened a suitable amount. The second parting line containing the molded parts now opens. After this second parting line has opened the necessary amount，the parts are ejected in the usual manner.
The sequential opening of two parting lines requires a sufficiently large opening stroke for the mold, a situation that unfortunately has not been taken into consideration in the design of many injection molding machines. A relatively large mounting height for the mold is also necessary，especially for deep parts，in order to provide enough space for the actuating mechanisms for the two-step opening.
Surprisingly，it has been found that with the so-called pinpoint gate (and this is always the type in a three or four-plate mold) very small gate openings suffice if certain prerequisites are fulfilled. It has also been found that thin walls can be filled better with pinpoint gating than with a gate located in the parting line. It is also easier to prevent sink marks in thick-walled parts with pinpoint gating than with a larger gate placed in the parting line.
The explanation is probably that with proper design of the pinpoint gate，even though there is a severe pressure drop at the restriction, the sudden heating of the melt as a result of this pressure drop results in better flow into the cavity.
Local heating of the steel around the gate is a further consequence, as the result of which melt can continue to flow into the filled cavity through the pinpoint gate under holding pressure to compensate for shrinkage because the local increase in temperature of the steel surfaces prevents premature freezing of the gate.
However, an essential prerequisite for easy filling of the cavity and maintenance of holding pressure is that the runners be large enough to prevent premature solidification of the melt on the one hand and too great a pressure drop during flow through the runners on the other. The pinpoint gate always requires high injection pressure from the very start of injection.
This prerequisite can always be fulfilled in hydraulically actuated machines. If，on the other hand，the injection rate is limited by the type of drive, as is the case with spindle and toggle drives in plunger machines， the pressure may be too low to force the melt through the pinpoint gate at the plunger speed attainable because of the high flow resistance at the restricted cross section. In this case，the pinpoint gate must be opened up to reduce the flow resistance.