Injection Molded Plastics Design for Venting

Many a frustrated engineer has suffered through the nightmare of injection molded plastics not coming out as designed. Sometimes injection molded plastics display short shots, aesthetic defects like burn marks, or reduced part strength. Identifying the cause can consume valuable time and resources. Design for manufacturing offers a shortcut in advance and even after the fact, especially with advances in MoldFlow simulation.

Small, isolated pockets in the injection mold can form high pressure areas where gas becomes trapped. This gas can cause short shots or ignite to produce burn marks. The clamp force of the two halves of the injection mold also plays a role. Higher clamp forces increase the risk of air being unable to escape. This can cause splay and other aesthetic concerns.

Injection molded plastics simulation software offers insights into these risks prior to mold construction. These simulations predict where plastic melt fronts will converge within the injection mold. Those areas pose high risks of dead pockets, which result in non-fill, v-notches, and reduced part strength. You can read more about “V” notches in injection molded plastics here.

Vents do bring tradeoffs. Vents can leave marks on injection molded plastics, so the engineers should choose vent locations deliberately.

Sometimes the development of injection molded plastics turns into an iterative process where some iterations happen after the injection mold is built. Design for venting is a good example of this. Defining vent locations before injection mold build offsets the risks. When these defects appear after injection mold build, fine-tuning venting can still mitigate those defects. An example case study illustrates this point.


A medical customer had enlisted Natech to develop the injection molded plastics for a unique diagnostics test. The design included multiple nozzles. Each nozzle had a main channel and a side channel running parallel. The side channel protruded just beyond the tip of the main channel to prevent cross-contamination of the two streams.


During preliminary production runs of the part, the team observed the injection molded plastics nozzles were not filling. This prevented formation of the tips of the nozzles and the key snorkel geometry. Because the nozzles were buried in a single steel insert, the team theorized it was a venting issue.

Plastic Injection Molding Venting

The non-filling of the nozzles would impact functionality. The input channel could cross-contaminate with the waste channel. This would compromise the integrity of the results.


The Natech engineers planned to retroactively run MoldFlow simulation. In the best-case scenario, they could adjust the injection molded plastics process. Altering temperature, clamp force, ejection velocity, and cycle time could resolve the non-fill. This would avoid costly injection mold changes.

The back-up plan would be to modify the injection mold. If venting proved to be the issue, parting lines would offer the simplest vent locations. Venting on ejector pins would take advantage of pre-existing features. Adding vent inserts or sintered inserts in dead pockets would allow gas to escape.


The MoldFlow simulation confirmed the venting issue. The injection molded plastics melt fronts met within an insert. This area trapped the gas, causing the nonfill.

Design for Plastic Injection Molding Mold Flow Analysis Venting

The team redesigned the insert into a three-piece insert. The parting lines where the three plates of the insert met provided venting opportunities. The engineers placed one of these parting lines at the plane of the tip of the nozzles.


The new parting lines in the injection mold allowed the gas trapped in the insert to escape. The nozzles filled properly with reliability.

Thinking about venting when developing injection molded plastics reduces risks that might only emerge later. MoldFlow simulation tools provide deep insight into potential filling issues. Simulation tools also help identify and address molding issues after the fact. Both of these uses of MoldFlow simulation improve the workflow, cost, and risk with injection molded plastics.

For a free preliminary review of your design for manufacturing readiness, schedule a meeting with a Natech Engineer.