Balancing cavities in family molds

Anyone who molds low-volume parts knows the problems with justifying the cost of a good tool over the run of a single part. Many molders turn to the family mold to make molding more economical. But family molding comes with its own problems: The parts are not identical, thus it's tough to properly balance the cavities. If the cavities in a multicavity tool aren't perfectly balanced, the end result is often inconsistent quality. Parts may be overpacked or underpacked, they may crack from the stress, and they may not perform as they were designed.

Traditional Balance

There are ways that molders try to get those cavities in balance, but they aren't always satisfactory. If you adjust the gates, too much shear may be imparted to the parts with the small gates. Balancing by adjusting runner size may restrict the runners that feed the smaller cavities too much, or the runner in a cold runner tool may get too small for effective packing. Then what if the processing conditions change? You may find yourself back in the tool room. Maybe you want to use a hot runner tool and control the fill through temperature differentials in the manifold and at the gates. This may work, but it is costly for a low-volume part and increases tooling lead time.

Well, there may be a better way; someone has finally developed a system to help balance the filling of a family mold. Murray Feick, a plastics industry veteran, has established Cavity Balancing Systems, Kitchener, ON, to promote and market his development of Adjustable Thickness Overflow Molding (ATOM), a patented fill balancing technique. The objective is to bring the flow fronts to the end of each part at precisely the same time. His method of doing that is an adjustable extra cavity, which he calls the EquaFill overflow cavity. In a family mold, one or more of these hang off the end-of-fill position of the first-filling cavity or cavities as an appendage to the molded part (Figure 1).

An exit gate is located at the edge of the part that fills first, and a channel connects the gate and the overflow cavity. Material flows through all the cavities and into the overflow cavity or cavities after each part cavity fills. The thickness of the cavity determines how much material flows into it and thus when end-of-fill actually occurs. What's different about this overflow cavity is that it is adjustable while it is in the mold, so that all cavities in the mold can be "tuned" by short-shot trials. Thus they fill uniformly during setup, without having to remove the tool for rework. If process conditions or the molding machine or material change, you can rebalance the mold with more short-shot trials.

Constructing the Tool

With a complicated part, you may want to do a mold filling analysis to quickly locate the last area to fill and determine how much compensation the overflow cavity needs to make. You can model the overflow cavity and its channel in the analysis, and adjust the thickness until the simulation shows equal fill. Alternatively, an open channel fill may be done during an actual mold trial. Once the location is determined, select an overflow cavity with a diameter at least one-third the total length of the connecting channel and cavity diameter together. Here are the steps involved (see Figure 2).

Balancing the Cavities

The overflow cavity adjusts from the mold face with a tamper-proof spanner driver. Rotating the overflow's piston increases or decreases the thickness of the overflow cavity During the short-shot trials, the wall thickness of each cavity is adjusted until the flow fronts all reach the end of their paths at the same time. When the mold is balanced, you can remove the EquaFill cavity and tighten its locking screw. Flats on the overflow help the grip. Once the mold is balanced, a locking mechanism in the EquaFill cavity stops any rotation from occurring during molding.

What does this cost? The overflow itself is around $750, and it takes about 15 hours of a moldmaker's time to integrate it into the tool. Because of its small footprint (Figure 3), and because it is joined to the smaller cavity, there is usually plenty of room in a tool, even in an existing family mold, where this technique can be retrofitted. Since the cavity is shallow and fills with low injection pressure, a minimum amount of steel between the overflow and the edge of the mold is needed. Complete dimensional details for a specific application will be provided by CBS.

Feick says the economics of the total run, however, substantially favor this method. The table below illustrates costs that might be incurred with the various methods for a two-part medium-impact ABS part, molded on a 40-second molding cycle, 50,000 sets per year. Has it worked on a real part? Yes, Feick says, in research conducted at the Industrial Research & Development Institute in Canada. Brahim Brahimi of IRDI says that his analysis indicates this concept can provide more flexible cavity balancing, and thus more quality (Figure 4) than conventional runner-based techniques.

*Machine rates inclue operator and profit. All costs in U.S. dollars.