Internal Gate Cutting for Thermoset Molding

How does one determine whether or not to incorporate automatic gate cutting into the thermoset molding process? A paper from Norman Akens of GE at the 41st Annual Thermoset Molding Conference in October gave some good guidelines. The paper was presented by Leon Meyer of Durez.

The place to start is the amount of labor involved in gate removal. If the labor required is one or more full-time people, then the cost to modify a mold with a gate cutter can be recouped in less than one year. The appearance of the part is also improved markedly because there's no gate scar.

Part properties also increase. For example, a big increase in tensile strength was achieved as compared to thin tab gates, but warpage was shown to be only .040 inch over 5 inches, vs. .225 inch over 5 inches, in a Durez 118-14SI test. In other words, the properties approach those of compression molded parts.

But changes must be made in the mold and machine. There must be a high-quality mold, on which you'll need guided ejection and thicker ejector plates. Occasionally, additional ejection push rods will be needed to equalize the cut-off force. For those presses without programmable ejection, hydraulic cylinders can be added to the outside of the mold to provide the cut-off function. These are run off of the corepull sequence; it's also used on die-head ejection.

The best sequence for cut-off is for the blade to move forward before the end of the injection hold portion of the cycle. A multiple stroke is recommended. The first time a new gate cutter blade is used, it will often gall. After the run, remove all gall marks and it won't happen again. It is important not to overpack the mold with material, as it will bind up the cut-off system and break the blades.

There are several problems that could occur. If the gate doesn't cut, the cause might be that the hydraulics are not sufficient, or the injection hold pressure is too high. Perhaps the ejector sequence is not working, or there isn't enough time on the gate hold sequence. If the part doesn't fill completely, the cutter may come in too early, or the material may set up before the cutter is activated.

For the right application, a gate cutter eliminates labor needed to remove the gate and the scrap associated with gate removal and broken gates. A GE mold running in Puerto Rico, for example, required a full-time operator; adding in-mold cutting paid off in one year.