Stack molding is well known in the injection molding art and provides various advantages. In particular, stack molding enables the output of an injection molding machine to be at least doubled without significantly increasing its size or clamping tonnage. Stack molds are typically double or quadruple-level, although there could be any number of stacks in a molding machine. For example, some rubber molds use up to ten levels.
A double level stack mold generally comprises a stationary first platen, a movable center platen and a movable second platen, with two single face mold plates mounted back to back. A first mold (single cavity or multi-cavity) is defined by one of a mold cavity or core plate which is located on the face of the movable center platen adjacent the stationary first platen and the other of the mold cavity or core plate which is located on the stationary first platen. A second mold is defined by one of a mold cavity or core plate which is located on the other face of the moveable center platen adjacent to the movable second platen and the other of the mold cavity or core plate located on the moveable second platen. The molds are opened and closed by a single machine force actuator (generally a hydraulic ram) applied to the moveable second platen and transferred from the second platen to the center platen by a suitable linkage. In a quadruple stack mold, an additional two moveable platens are provided and mold cavity plates and/or mold core plates are located thereon to define additional molds.
To supply molten resin to the cavities of the closed molds, conventional stack molds employ a sprue bar which runs from the machine through the stationary platen to the center platen and which serves as a direct channel between the extruder nozzle of the injection molding machine and the mold's hot runner distributor, which is mounted in the center platen of the stack mold. Alternatively, a movable sprue bar located outside of the mold stack can convey the resin to the center section, as described by Bertschi in U.S. Pat. No. 5,011,646. Sprue bars generally include heaters along their length to maintain the molten state of the resin traveling therethrough and must cope with the relatively high pressure at which the molten resin passes through them.
For injection molding applications where there are more than two levels in the stack mold, multiple sprue bars can be used for delivering a split stream of molten resin to the hot runner distributors in the multi-level injection mold. In this case, after the resin stream is split, the sprue bars carry the resin to the hot runner distributors in their respective mold sections comprising the injection mold. With multi-sprue bar applications, a single source injection unit channel is typically used with a machine nozzle that divides the single source channel into a plurality of channels aligned with the individual sprue bars, as described in U.S. Pat. No. 5,522,720 to one of the present inventors, and assigned to the assignee of the present invention.
In such cases, the sprue bars are normally attached to the respective mold section to which the molding resin is being delivered. Because injection mold sections in a multi-level stack mold generally move in the longitudinal or vertical direction when the mold is open and closed, the sprue bars must be displaced with the mold sections. Accordingly, the sprue bars are not rigidly attached to their source of resin, i.e.--the machine nozzle or the channel splitting device. Consequently, the sprue bar arrangement must be designed so that the sprue bars will return to their sources of resin and reform a seal therewith at the beginning of each molding cycle.
In particular, several design problems are typical for stack molds with more than two levels where resin must flow from a single source injection unit to multiple levels spaced progressively farther from the stationery platen. For example, in a four level stack mold, a sprue bar will feed the first and second level via channels in the mold plate between the two levels and a second sprue bar will feed the third and fourth levels via channels in the mold plate between these two levels. It is desired that sprue bars be as short as possible to reduce pressure losses and to minimize the manufacturing expense of the sprue bars. A further difficulty occurs as, due to the progressive arrangement, the two sprue bars will necessarily be of different lengths and thus the pressure drop that occurs between the inlet end of the sprue bar adjacent to the injection nozzle and the outlet end of the bar is much larger in the longer sprue bar than in the shorter.
When molding shallow parts, and thus opening the mold to a relatively small degree, the length of the sprue bars is generally not large, and the difference in the sprue bar length is relatively small. Consequently, the pressure drop is of minor consequence, generally on the order of 3 to 5 MPa. However, a relatively large pressure drop, on the order of 25 MPa, can occur when molding tall parts because the sprue bars are necessarily longer. This large pressure drop must be compensated for at the injection molding machine and, more importantly, the differential in the pressure drop between the sprue bars can cause insufficient mold packing in the molds furthermost from the injection nozzle.
Another difficulty with sprue bars is that variations in their length occur due to thermal expansion effects, as the sprue bars are heated to allow resin to flow through them. Accordingly, when the mold is closed, the position of the end of each sprue bar relative to the stationery platen and the channel splitting nozzle on the injection molding machine will vary, due to these thermal variations and due to variations in the position in which the mold plates close at the various levels. The combination of these variables makes it very difficult to predict the location of the two sprue bar ends each time the mold is closed and the sprue bar is returned to the channel splitting nozzle. Therefore, some resin leakage from the joint between the nozzle and the sprue bars is inevitable. Resin also tends to leak or "drool" from the nozzle gates or the open channel of the sprue bars when the mold is opened. This drool cannot be tolerated at any of the parting lines of the mold cavity and core sections. At best, such drool prevents complete mold closing and allows flashing to occur and, at worst, can cause permanent damage that requires expensive repairs.
U.S. Pat. No. 5,522,720 to one of the present inventors, and assigned to the assignee of the present invention, discloses a nozzle that tolerates misalignment with the two sprue bars while still forming a tight and repeatable seal between the sprue and the nozzle. Although this design overcomes the problem of drool between the nozzle and sprue bar during injection (mold closed), it does not solve the problem of drool from the gates when the mold opens, or the substantial pressure drops to the lengths of the two sprue bars.
U.S. Pat. No. 4,207,051 to one of the present inventors and assigned to the assignee of the present invention shows a stack mold wherein molten resin is supplied to a center platen through a telescoping tube assembly which is mounted externally to the mold. Essentially, the two tubes form an expandable single sprue bar to deliver molten resin to the mold hot runner and the sprue bar thus need not be detached from the injection nozzle. However, it has proven difficult to construct and operate such a telescoping tube system to accommodate the very high injection pressures (exceeding 20,000 psi) experienced at the nozzle in a multi-cavity, multi-level stack arrangement.
U.S. Pat. No. 5,458,843 discloses a four level stack mold that utilizes a single sprue bar with feed connectors extending through mold components. Drool is reduced via a valveless anti-drool arrangement whereby a spring-activated extension of an outwardly tapered piston into the manifold flow passage reduces its internal pressure and thereby minimizes backflow and resin drool from the feed connector. However, no provision is made for the possibility of drool in the central distributor side of the feed connector. Therefore, the risk of leakage at the mold parting line still exists.
U.S. Pat. No. 4,212,626 to Gellert dispenses with sprue bars entirely and instead uses a combination of control valve units abutted together to transfer the pressurized melt through mechanically operated valve gates from the stationary platen, where the machine nozzle resides, to the moving platen, where the hot runner manifold resides. Several problems are inherent in this approach. First, drool at the parting lines is likely to occur over time, as the valve gates must remain aligned at the parting mold faces with extreme precision over millions of injection cycles. Second, melt channel capacity is limited by the size of the valve gates through which the melt must pass through. Therefore, large parts cannot be successfully molded using this arrangement. Third, the mold shut height is much greater to accommodate the arrangement of the valve gate construction, leading to slower cycle times and greater material expense (the platens are thicker and therefore have greater mass).
U.S. Pat. No. 4,611,983 to Bielfeldt discloses a transfer molding system for fiber-reinforced thermoset resins whereby the molten resin is transferred to an injection cylinder via a feed bore. The injection piston is connected to a telescoping sleeve, so that as the piston moves up inside the injection cylinder to fill the mold cavity with resin, the sleeve also rises and seals off the feed bore. Also, the inner diameter of the sleeve is larger than the root diameter of the injection piston, so that any resin drool flows out of the annular clearance. However, this technique suffers from various disadvantages and does not work if applied to a high injection pressure, multi-cavity, multi-level stack mold arrangement with at least two hot runner systems.
U.S. Pat. No. 4,586,887 to Gellert shows two opposed hot tip bushings meeting at a stack mold parting line. Each hot tip bushing has a first, inner heating element and a second, outer heating element, each running along the length of the hot tip. A temperature difference is created by heating the inner heating element of the first nozzle and the outer heating element of the second nozzle, which, when combined with the taper of the gates, is intended to reduce tip drool, as the tips freeze off each time the mold faces separate. Excess melt is intended to be absorbed into the system when the mold is opened. However, this arrangement has been less than satisfactory in eliminating tip drool.
U.S. Pat. No. 4,891,001 to Gellert was directed to overcome some of the drooling problems of U.S. Pat. No. 4,586,887 discussed above. This reference teaches an arrangement wherein, rather than heating the tip with two coaxial heaters running the length of the tips, localized heat control is provided on the tip directly at the orifice in the mold face. The first tip has a heating element proximate the mold face, while the second tip had a heating element that is distal the mold face. A similar principal of creating unequal temperatures of the tips at mold separation is intended to result in freezing. However, this arrangement has also been found to experience unacceptable drool.
Accordingly, it is desired to have a stack mold and a sprue bar assembly therefor which does not suffer from the above-mentioned or other disadvantages.