Standardized stack molds are now possible.

Few molders would consider stack molds an off-the-shelf item, but a new modular design takes them a step closer by pairing a new runner-less mold base with interchangeable core-and-cavity sets. "This is the first and only standardized stack mold in the world," claims John Von Holdt, president of Plas-Tool Co., Niles, Ill., the new mold's originator. A key factor in the stack molds' versatility is a new hot-runner valve-gate bushing that permits adjustment of the gate size while running.

Intended for thin-wall-container or over-cap applications, the Modular Unit Mold System (MUMS) represents a joint development by two well-known tool makers: Plas-Tool can supply the system's core-and-cavity sets and adjustable-gate size hot runner system. And National Tool Co. of Kenilworth, N.J., will supply the mold bases as well as hot runners.

STAYS ON THE MACHINE

MUMS, which appeared for the first time at the Plastics Fair in Chicago last month, has several features that set it apart from conventional stack molds. Most notably, the bulk of the mold remains on the machine all the time. According to Von Holdt, the support and clamp plates, leader pins, bushings, ejector system, water and air systems, screws, rack and pinion, and gear mechanisms all remain in place during product changeovers. Even the mold's hot-runner plate stays on the machine. It's supported by an adjustable outrigger system resting on the tiebars. This outrigger telescopes on roller bearings to reach the tiebars of presses between 200 and 700 tons.

During changeovers, the only parts that are exchanged are the modular core-and-cavity sets and stripper plates and rings. The standard mold bases supplied by National Tool can accommodate a variety of core and cavity sizes: In a four-cavity design, the maximum part volume is 128 oz, while a 48-cavity tool would accommodate part volumes up to 10 oz. Two styles of core-and-cavity sets are available. One mounts the cores and cavities on standard retainer plates, allowing all of them to be pulled at one time using a sling chain. In the other style - designed for larger parts - the cores and cavities are removed individually while leaving the mold base in place. In both styles, the cores and cavities employ a patented interlock device to maintain uniform wall thickness, says Von Holdt.

He also notes that MUMS will support hot-runner systems from manufacturers other than Plas-Tool or National Tool. The same goes for core-and-cavity sets. "The mold base will accept all our competitors' stuff," he says.

ADJUSTABLE VALVE GATE

Another aspect of the system is a new valve-gate sprue bushing, called the Shutter Gate. It lets users fine tune gate size opening with a few turns of an exterior hand crank during production runs. Inside the tool, a drive shaft turns the bushing in a threaded mount which moves the bushing tip into or out of the gate area, changing its size.

Von Holdt explains that this adjustable-gate feature not only helps balance flow into multiple cavities, but it also reduces the occurrence of molded-in stresses by allowing the gate-freeze time to be matched to the part wall's freeze time.

Von Holdt adds that the Shutter Gate is especially appropriate for stack molds and is standard on MUMS tools; but it is also available for conventional molds as well.

A COST SAVER

While the price of the first mold using the MUMS approach is comparable to that of a conventional stack mold, according to Von Holdt, the cost (and delivery time) of subsequent molds is only about 25% of an entire stack mold since only a core-and-cavity set need be purchased.