In order to eliminate the secondary operation of demo […]
In order to eliminate the secondary operation of demolding, injection molds have been developed that automatically demold the parts from the runner system through the opening movement of the mold.
Self demolding requires two parting lines in the mold. On the first parting line, the melt is distributed from the central runner through the runner and flows through the riser in the intermediate plate, and the end of the intermediate plate is provided with a gate leading to the cavity. The part to be formed is located on the second parting line of the mold. During the mold opening movement, the parting line containing the runner system is opened first, and the runner system is kept on the stationary side in order to separate the gate in the middle plate from the molded part.
Proper measures must be taken to ensure that the runner system and runner are ejected after the parting line is opened for an appropriate amount. The second parting line containing the molded part is now open. After the second parting line is opened by the required amount, the part is ejected in the usual way.
The sequential opening of the two parting lines requires the mold to have a sufficiently large opening stroke. Unfortunately, many injection molding machine designs do not take this into consideration. The installation height of the mold also needs to be larger, especially the deep part, in order to provide enough space for the drive mechanism of the two-step opening.
Surprisingly, it has been found that if certain prerequisites are met, the use of so-called precision gates (which are usually the type in three- or four-plate molds) very small gate openings is sufficient. It was also found that the use of precision gates can better fill thin walls compared to the use of gates located in the parting line. Compared with placing larger gates on the parting line, it is easier to prevent sink marks in thick-walled parts with precision gates.
The possible explanation is that with the precisely designed gate, even if there is a severe pressure drop at the limit, this pressure drop causes the melt to suddenly heat up and better flow into the cavity.
The local heating of the steel around the gate is a further result, because the local increase in the surface temperature of the steel can prevent premature, so under the holding pressure, the melt can continue to flow into the filled cavity through the precise gate to compensate for shrinkage and freezing door.
However, the necessary prerequisite for easy filling of the cavity and maintaining pressure is that the flow channel is large enough to prevent premature solidification of the melt on the one hand, and to prevent excessive pressure drop in the flow channel on the other hand. Precise gate requires high injection pressure from the beginning of injection.
This prerequisite can always be met in hydraulically driven machines. On the other hand, if the injection speed is limited by the type of drive, such as the spindle and toggle drive in a plunger machine, the pressure may be too low to force the melt through the precise gate at the achievable plunger speed because it is restricted. The cross section has high flow resistance. In this case, the precise gate must be opened to reduce flow resistance.