For many plastic parts, basic parting line vents are adequate. However, with complex geometries, knowing where and how to vent air traps may not be intuitive.

Locating Potential Air Traps

It is not always obvious how molten plastic will flow in complex mold cavities. If there is any question as to how the cavity will fill and where potential air traps will occur, I highly recommend using mold filling simulation. In addition to locating air traps, mold filling simulations can help determine warp, stress, fill pressures, etc. If the mold builder or molder doesn’t have this in-house capability, there are many companies that offer mold filling simulation services and will provide a detailed analysis of the results.

One common cause of air trapping is what is known as “race tracking.” Very simply, it is gating into a thick wall that extends around a thinner wall. The molten plastic more easily flows through the larger gap of the thick wall and slowly fills the thin wall. Once the plastic completely surrounds the thin wall area, no gases can escape through the parting line. The interior of the thin wall needs another vent or vents to completely evacuate the gases.

There is also danger that the flow front will stall in parts of the thin wall, leading to voids or weak weld lines, even with adequate venting. Faster injection speeds can sometimes overcome hesitation in the thin walls, but will require more or deeper vents, or vacuum venting.

Core Pin Venting

Core pins can sometimes be used to vent trapped air from interior surfaces. One downside to using core pins is that, since they are stationary, the buildup from escaping gases can block the vents over time and may require disassembling the mold to clean.

Making the vents as deep as possible (enabling acceptable flash) and polishing vent surfaces, along with deep secondary channels, will reduce buildup and help increase mold cleaning intervals.

Ejector Pin Vents

A ring vent, which runs 360 degrees around the end of the pin, along with a secondary vent channel positioned a short distance from the end and connected to a deep flat secondary channel that links to additional channels in the cavity block or mold base, is significantly more efficient than using flat vents on the end of the pin. For example, a vent pin with a diameter of 0.500 inches (12.7 mm) and a ring vent depth of 0.001 inches (0.025 mm) has an effective venting surface area of 0.0016 square inches (1.03 mm²). While this may not seem substantial, the same vent pin with four flats ground to the same depth would only provide an effective venting surface area of 00012 square inches (0.08 mm²). This represents about 93% less venting efficiency compared to a ring vent of the same depth.

Ejector pin vents are preferred over core pin vents for one main reason: their vents can usually be cleaned simply by stopping the cycle, moving the ejectors forward and wiping off the vents. Ring vents are preferred to vent flats because there is much more vent surface for the same vent depth.

Which ejector pins should be vented? Definitely vent ejector pins in any gas trap locations and near the last place to fill.

Testing the Venting

After the mold is built, two tests will show if there is adequate venting. One is to do a series of short shots to see the last areas of fill. This should be done by reducing shot size, not reducing speed or pressure. Check this area to see if more venting should be added before developing a process.

The second is to push the injection speed, increasing it to the maximum fill speed that could reasonably be used for that part. If any short shots or burning shows up, that means the air could not escape fast enough before either burning or freezing occurred. More venting is needed to eliminate those conditions.

In some cases, the vent depth cannot be increased without danger of flashing. It may be that too much air is trapped in a corner and needs to be evacuated before it reaches that dead end. Full perimeter vents can help, reducing the amount of gas trapped in the corners. For some parts that cannot be vented, a mold vacuum may be the only reasonable approach.

Plastic flowing around obstacles will often trap air on the opposite side of the flow. This can lead to unacceptable weld lines. Cores that form holes through the part should always be vented to help reduce weld lines.

Venting in Practice

I once had a mold built for a medical housing unit. The dimensions of the housing were approximately 8 inches (200 mm) wide, 11 inches (280 mm) high and 6 inches (150 mm) deep. It featured an open front and had a recessed battery compartment on the bottom, enclosed by four walls. It had a sprue gate on the back, slides formed the two sides and another slide formed the bottom battery compartment.

I wasn’t sure exactly how the plastic would fill the mold, so I had mold filling analysis done. It identified the best location for the gate, but also showed air would be trapped near the middle of the battery compartment, somewhere near the center of the slide. I instructed the mold designer to place cooling lines in the slide around the perimeter of the battery compartment, but to leave the center open to add vent pins later.

After the mold was built, the first sample showed air trapped in a rough “c” shape, near where the mold filling analysis showed. I had the toolroom add vent pins along the centerline of the unfilled “c”, then sampled the mold again. There was still some air trapping, but it was smaller and slightly moved from the original location. I had the toolroom add a few more small pins in the center of the new void. Another sampling proved that the air trap was completely eliminated.

The housing mold continued to run well, if the mold cleaners remembered to pull out and clean the slide vent pins. If they skipped this step a few times, the void and burning would occur in the center of the battery compartment again.

What changed? If a mold running consistently begins to experience voids or burning near vent pins, ejector pins or vented inserts, the vents may have been blocked by gas deposits.

Air traps can occur in unexpected locations, especially in complex parts where the plastic flow may not be intuitive. Computerized mold filling analysis and mold sampling may be necessary to determine the best areas to locate venting to eliminate these issues.