What Is Venting System?
I've been in this trade going on twenty years now. Whenever a young engineer asks me what gets overlooked most in mold design, my answer is always the same. Venting.
Not gating. Not cooling. Venting.
The Basic Idea
Here's the thing. Plastic comes in, the air that was sitting in the cavity has to get out. Air has nowhere to go, it gets compressed right at the last spot to fill. Temperature spikes. Burns the plastic. I've seen this more times than I can count. Yellow marks all over the part surface. Customer loses it. Production shuts down. Turns out the vents were clogged the whole time.
Vent Depth and Material
A vent is just a shallow channel cut into the parting line. How shallow? When I'm running ABS I usually go 0.03 to 0.04. Nylon and other crystalline stuff needs shallower, maybe 0.015 to 0.02, otherwise you get flash. PC and PMMA can handle a bit deeper since they don't flow as easy.
Placement
Where do you put them? Wherever the plastic gets to last. Anyone who's done a short shot study knows this. Air piles up at the end of flow. Corners. Ribs. Bottom of bosses. Nine times out of ten these spots need venting. Some guys like to space vents evenly around the parting line. That works but it's lazy.
A Job From Last Year
We had a job last year. Automotive interior trim. High cosmetic requirements. Mold kept burning the parts. Tried three different resins. Nothing worked. Finally I told them to send the mold over. Took one look and the vents were completely plugged up. The resin had flame retardant in it. Few thousand shots and the vents were carboned over. Cleaned them out. Problem gone.
When Parting Line Can't Reach
Some spots the parting line can't reach. Deep ribs. Blind holes. You grind a flat on the ejector pin and let the gas escape through the pin bore. Another option is porous steel inserts. Sintered metal that lets gas through but blocks plastic. Expensive though. And you have to clean them constantly. Not for every application.
Vacuum Venting
I've done vacuum-assisted venting a few times too. Hook a vacuum pump to the cavity. Pull the air out before you shoot. Thin-wall parts and big surface areas see the most benefit. Cycles get noticeably faster. But the equipment costs money and sealing is a pain. Only makes sense on high-volume jobs.
Keeping Vents Clean
Maintenance is straightforward. Keep the vents clean. PVC and flame retardant grades need more frequent attention. Regular stuff maybe every twenty or thirty thousand shots. Vents wear down over time and start flashing. When that happens you weld and recut.
Venting looks simple on paper. Get it wrong and you're in for a bad time.