The state of the art includes various arrangements for hot runner manifold systems to transfer molten material, typically plastic resin, from an injection molding machine to a mold. Hot runner manifold systems are well known and typically include a manifold plate, a manifold housed in the manifold plate, and a backing plate that supports the manifold and manifold plate. The manifold system routes molten material from a central sprue, which connects to an injection unit on an injection molding machine, to a plurality of nozzles which inject the molten material into cavities in the mold. The manifold system divides the flow of the molten material into several branches as it flows from the central sprue to the nozzles. It is desirable that flow of molten material through the manifold system be balanced so that material arriving at each nozzle has approximately the same temperature and pressure to produce uniform parts in each mold cavity. Toward that end, manifold systems are preferably designed so that each branch provides substantially the same size and length of flow path for the molten material. With uniform flow paths at each branch, temperature and pressure differences between branches should be minimized. However, for molds with a high number of cavities, such uniform flow paths are not always possible due to location limitations on the manifold.
Referring to FIGS. 1 and 2, a prior art manifold system using two plates is shown with portions of the plates and main manifold cut away to reveal internal detail. For injection molding systems with many cavities in the mold, a manifold assembly 10 has a plurality of sub-manifolds 12 arranged in manifold plate 14 and fed by a main manifold 16 mounted in backing plate 18. Sprue 20 connects to the main manifold 16 at a central location. Main manifold 16 has a melt channel 22 with branches to each arm 24 of main manifold 16 and connecting to an inlet of each sub-manifold 12. Each sub-manifold 12 has its own melt channel network that communicates the molten material from main manifold 16 to nozzles (not shown) connected to each sub-manifold 12. In the example illustrated, each sub-manifold 12 accommodates twenty-four nozzles. Typically, valve-gate type nozzles are used with such a system, and have pneumatic valve actuators at the upper end of the nozzle that actuate valve stems in the nozzle. The valve stems extend through apertures 26 in the sub-manifolds 12 and the actuators are housed in actuator cavities 28 formed in backing plate 18.
Such prior art manifold systems have significant limitations and shortcomings. Specifically, since the main manifold 16 and actuator cavities 28 are both in backing plate 18, and the main manifold 16 cannot pass through actuator cavities 28, the transverse spacing of actuator cavities 28, and hence the nozzles, can be greater than desired. That leads to the mold being larger than optimum, and flow length of the molten material being increased.
Air lines 30 are routed to each actuator through the backing plate 18. The location of the air lines is constrained by the location of the manifold 16. Also since the location of the arms 24 of main manifold 16 is constrained by the location of actuator cavities 28, flow of molten material to portions of sub-manifolds 12 is not optimum. In the example illustrated, arm 24a conducts molten material through melt channel 22 to branches 32a and 32b to two sub-manifolds 12a and 12b at portions 34a and 34b located at the periphery of sub-manifolds 12a and 12b. Material then flows to a central location in the sub-manifolds and subsequently through multiple channels to the nozzles. Such a flow path increases the likelihood of the molten material having less uniform temperature and pressure throughout the sub-manifolds 12, which can lead to unbalance in the system.
Physical coupling, typically through the use of bolts, between the backing plate 18 and the manifold plate 14 stabilizes the layered structure by restricting bowing during the injection cycle. Plate bowing arises as a consequence of the injection pressure and pressure from spring-loaded seals at interfaces between the sub-manifolds 12 and nozzles and also between the sub-manifolds 12 and the arms 24 of the main manifold 16. If the plates bow, leakage can occur at those interfaces. Pillars 36 are provided in manifold plate 14 where possible, and numerous bolt holes 38 are provided through backing plate 18 to facilitate such bolting. However, bolts cannot be put through the melt channel 22 of manifold 16, so to make the bolt spacing adjacent the manifold 16 as tight as possible, the arms 24 of manifold 16 are made as narrow as possible. To maintain structural integrity of such narrow portions, the manifold 16 may have to be hardened or be made from a stronger material than is desirable.