Aside from the center block motion system, the key element of a stack mold is the hot runner system. The transfer of plastic melt from the machine barrel into the mold center block has traditionally been achieved by a heated sprue bar,

which leads the melt from the machine nozzle through the fixed mold half and through the first mold parting surface into the hot runner manifold, which is located in the center block of the stack mold.

The relatively simple sprue bar has the disadvantage of getting disengaged from the machine nozzle at each cycle when the mold opens for part ejection. The sprue bar may also become an obstacle at part removal, especially with robot handling equipment because it always remains inside the first mold parting surface. Consequently, better melt transfer methods into the mold center block, like the sprue-bar-less gate-to-gate transfer, have evolved.

Mold cavities, which by design, cover the center surface of the molding area do not allow a center sprue bar, so the hot runner has to bypass the melt by leading it with an off-set manifold to the outside of the mold, and from the side position through the first mold parting line into the center block for further melt distribution through the nozzles to the cavities.

Gate-to-Gate Melt Transfer
Increasingly more popular is the melt transfer through the first mold parting surface without a sprue bar because it eliminates the sprue bar as an obstacle. The basic principle of a sprue-bar-less melt transfer is a nozzle-to-nozzle melt transfer

As the mold parting surfaces close, two nozzle front ends establish a melt passage. These nozzles separate when the mold opens. A leak-free connection or seal at 2,000 bar or 30,000 psi is accomplished with the specially designed manifold system for stack molds. The front surfaces of the melt transfer nozzles contact each other at clamp force in the mold parting surface and establish a connection between the melt channels. This passage can be opened and closed through valve pin actuation during injection. During mold opening, the transfer nozzles provide a dry surface with absolutely no stringing or drooling of the melt.

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