It seems intuitive that when forcing molten plastic into a cavity filled with air, the air has to be displaced in order to completely fill the cavity.

How Air Can Be Removed
Air within the cavity can be removed by one of two methods: traditionally, the flow front of molten plastic forces the air out ahead of it into gaps in the mold (vents); or, alternatively, a vacuum is used to pull air from the cavity prior to injection. The molds need to be built somewhat differently for each method.

In the traditional method, primary and secondary vents are cut into the mold around the perimeter of the cavity and at any points where air would be trapped, enabling air and gases to escape into the atmosphere. For a mold vacuum, the cavities and runners must be sealed, typically with O-rings or other elastomeric seals, and have vent channels which lead to a vacuum pump to draw air out prior to the injection of molten plastic. This can be particularly challenging in molds with stepped parting lines, side actions, lifters and other features.

Effects of Inadequate Venting
Inadequate venting can cause several issues. The most common are burning at the end of fill, weak weld lines (which can sometimes exhibit burning), mold erosion, blisters, bubbles, difficulty filling, short shots, high melt and mold temperatures, splay, slow fill rates, long cycle times and reduced process windows.

Many mold shops and individual moldmakers are reluctant to cut vents to the specified depth because they believe that more venting can be added later, if needed, and they don’t want to weld up a new mold to reduce the vents if they flash. It is usually easier to cut vents to the prescribed depths rather than to increase them later, especially with contoured parting lines.

As a rule of thumb, the thicker the wall section, the deeper the vent can be without flashing.

Too often, though, inadequate venting is never corrected, resulting in a mold that runs high scrap and long cycle times for the life of the mold. Process technicians often process around molding defects, which, during process validation, can lock a reduced process window around inadequate vents.

The most common reaction to flashing parting lines is to blame the vents for being too deep. While I have seen this a few times (two instances that come to mind, one due to the vent being cut too deeply and the other because of unusual part geometry), in most cases, the flash was caused by other factors. I will cover the causes of flash in a later article. Where venting is cut too deep, in most cases, it would be easier to grind 0.0002-0.0005 inch (0.005-0.013mm) off the parting line than to weld and recut the vents.

“Natural” Venting
Early in my career, I worked for captive molders. When I had molds made, I didn’t specify any venting, and the tool shops typically did an adequate job of venting the molds. The first time I received a mold with “natural” venting from a shop I had not used before, I did not understand what the tool shop had done until we ran the mold.

Natural venting is simply using gaps between mold components and irregular surfaces, such as those from EDM or grind marks, to enable air to escape without any deliberate attempt to evacuate or channel it outside the mold. We ended up cutting vents and creating secondary channels to successfully run production. That was the last time I allowed natural vents, and I began specifying vent depths and locations during the mold concept stage.

A few years later, while working for another captive molder, they had a series of approximately 12 medical molds built to create irregularly shaped cylindrical parts. These parts had walls that were about 0.03 inch (0.75 mm) thick and were 0.75 inch (19 mm) high, with diameters ranging from about 0.5 inch (13 mm) to 1.25 inches (32 mm). Each cavity had four slides that shut off on a central core pin with sleeve ejection. Three subgates were equally spaced around the top. The material was a medical-grade acetal (POM). The molds were successfully sampled and validated before production commenced. Sometime after running production, they began having issues with weak weld lines breaking when the parts were flexed. They checked the process and tried different lots of material, but the parts still kept breaking.

I heard about the issue and asked if I could look at the tooling. I found that there were no primary vents at the end of fill where the weak weld lines were located! They had secondary channels but did not cut any vents. The parts could not have any loose flash, and everyone knows that acetal flashes too easily.

As these molds with natural venting underwent several thousand cycles, it appeared that the microscopic grinding and polishing marks became flattened when the slides opened and closed. This effectively blocked the natural venting, leading to weak weld lines.

Having molded millions of acetal gears in the past, many of which were produced with the same 0.03-inch (0.75 mm) wall thickness and 0.0007-inch (0.018 mm) deep vents, I suggested that they add 0.0007-inch (0.018 mm) deep primary vents at the ends of the fill. They suggested this approach to the customer, who agreed but requested that only vents of 0.0005 inches (0.013 mm) deep be added to one mold — the largest size in the series, which was rarely used. This mold produced parts without flash and featured strong weld lines which did not break, even after repeated flexing. They then incorporated this venting method into the remaining molds in the series and conducted a revalidation, successfully addressing the weak weld line issue.

Proper venting must be designed into every mold, verified before production and monitored periodically as the mold ages.

How Deep Can You Go?
Plastic suppliers once published guidelines for vent depths specific to their materials, but many have since discontinued this practice. However, generic guidelines for common materials are still available and are suitable for most applications. Typically, these guidelines provide vent depth recommendations within a range, such as 0.001 to 0.002 inches (0.025 to 0.05 mm).

As a rule of thumb, the thicker the wall section, the deeper the vent can be without flashing. Also, glass- and mineral-filled materials require deeper vents than the same materials unfilled.

When designing a new mold, if you have a tool and a process optimized for a similar part, a good starting point is to use the vent depths from that mold for the new design. However, don’t rely solely on the numbers from the previous tool design. Instead, measure the vent depths on the existing mold to ensure they match the specifications on the tool drawing.

During my time at a custom molder, I designed a small test mold for a box. During a slow time, I had the toolroom build it using a surplus MUD insert. The mold featured vents of varying depths along the parting line. On one side of the box, the vents were shallow near the gate and deeper away from the gate; on the opposite side, the arrangement was reversed.

It also had a checkerboard pattern of squares cut into the cavity surface, with the actual measurement of each line in the squares carefully measured and recorded. This pattern allowed us to measure actual shrinkage in both flow and cross-flow directions on the molded boxes.

The gate was inserted to run different sizes of subgates and tab gates, as well as a central direct sprue. Changeable core inserts enabled us to produce boxes with varying wall thicknesses. With this one mold, we could mimic multiple part configurations to get accurate data on vent depth and shrinkage before the mold was designed.

What Changed?
If a mold that has been operating well begins to show signs of inadequate venting, the most likely culprits are mold wear or hobbing, which can reduce venting depth. Also, a buildup of deposits between mold components or on vent surfaces may contribute to this issue.

It is crucial to have measurement data of vent depths when the mold is new to compare for any changes later. I have occasionally noticed that steel on the opposite half of a mold can become raised due to the constant opening and closing. In one instance with a small P-20 steel mold, I saw such significant hobbing that it not only completely blocked all the vents but also closed off most of the secondary channels. Grinding the surface flat again resolved the issue. However, thorough cleaning of all mold components is often enough to address venting problems that may arise.