For cold runner systems, the runner is the first place where air and gases can be removed during injection. To assess whether runner venting is necessary, consider the following factors. Hot runner systems feeding the part directly do not have a large volume of air and gases ahead of the melt that need to be removed. In contrast, cold sprue-gated systems may have a large amount of air ahead of the melt, but these systems do not allow for the removal of air before the melt reaches the cavity, unless the mold is equipped with a vacuum system.

While nearly all molds with cold runners will benefit from runner venting, the effect will be very noticeable in some molds, and less noticeable in others. A mold with a large cavity volume and a much smaller runner/sprue volume will see little improvement in the process window. In contrast, a multi-cavity mold with a large runner/sprue volume and small total cavity volume may see a dramatic increase in the process window from correct runner venting.

Large runner and small parts. Multi-cavity molds can often produce small parts using branched runner systems that have a total volume much larger than that of all the cavities combined. In many cases, only the ends of each runner branch are vented, and this venting typically extends only to the depth of the cavity vents.

When injecting plastic into a cold runner system, the melt front will pressurize the air ahead of it before it even starts to fill the cavities. Some air will escape through vents at the end of each runner branch, but since air is a fluid, it will follow the path of least resistance. Any air not vented from the runner system will be forced through the gates, pressurizing the air already in the cavities before the plastic even reaches the cavities. This extra air must then escape through the cavity vents.

As air is pressurized, it becomes hotter. High-pressure hot air can significantly heat the steel around the vents and at the plastic flow front, which could result in burning. Making the cavity vents deeper to stop burning at the end of fill may result in excessive flash at the vents. Then the vents are usually welded up and re-cut again. This is a vicious cycle. Often in these cases, the technician will slow the injection speed, allowing air to bleed off to prevent burning. This reduces the process window, adds cycle time and can add stress to the parts.

A better solution is to add more and deeper vents to the runner. During the fill phase, the plastic in the runner is under low pressure until it reaches the gate, where it begins to increase. Plastic under low pressure does not flash easily. Once the runner’s skin freezes, it is very difficult to flash. Many years ago, in a molding seminar, I remember the presenter stressing, “Let the runners flash! Nobody says they can’t regrind the runners because they have flash!” As long as the flash doesn’t reach the secondary vent relief channel, the runner flash will not be detrimental to the process.

Short-sprue mold feeds PEEK medical washer. A single-cavity mold had a short, small-diameter sprue and a short, thin runner feeding a PEEK washer-shaped medical part. The washer was about 0.75-inch (19mm) in diameter and 0.2 inch (5mm) thick with a hole through the center.

The customer would not allow any primary vent in the cavity, but there was a secondary relief channel at end of fill. The part, predictably, had a heavy weld line, which was unacceptable and could not be eliminated through the process. When I tried to correct it, I couldn’t convince the customer to add a vent at end of fill, but they did agree to vent the runner, although they were skeptical that it would have any effect.

Runner venting should be evaluated for every cold runner mold. If it is modeled into CAD and cut with a CNC mill, then the machining cost is minimal. The benefits, though, could be enormous.

The toolmaker first cut secondary vents on both sides of the runner to a point near the gate, leaving a land of about 0.12 inch (3mm), then ground in vents 0.0015 inch (0.04mm) deep. This small amount of additional venting removed enough gas to increase the process window to eliminate the heavy weld line.

Unusual runner vent. An unusual approach to runner venting from an article published several years ago. I tried the approach on several molds with great success. The author suggested creating a wide, stepped land from the runner to the secondary vent channel. The land in contact with the runner was quite deep. Perhaps, 0.005 inch (0.13mm) and wide, 0.25 inch (6mm) or more, then stepping up to a shallower depth of about 0.002 inch (0.05mm).

Venting at least one side, as well as the end, of each branch up to the gate will remove as much excess air as possible before plastic reaches the cavities. Runner vents must create a path of least resistance for air and gases, while preventing excessive plastic flow into the vents before freezing. Runners with vent depths greater than the cavity vent depths will more easily remove these gases.

If a mold has a large cavity volume and a small runner volume, molten plastic may reach the gate before much of the gases can be evacuated from the runner. Still, it is a good idea to at least vent near the gate. On the other hand, if the runner volume is a significant percentage of the shot size, it makes sense to add as much venting to the runner as is reasonably possible.

Runner venting should be evaluated for every cold runner mold. If it is modeled into CAD and cut with a CNC mill, then the machining cost is minimal. The benefits, though, could be enormous.