Mold Vent Depth Calculator

Pick a resin and where the vent goes – get a starting vent depth, land length, relief depth and vent width.

Material

Vent location

Grade flow

Part size

Glass / mineral filled grade

Recommended vent depth

0.032 mm0.0013 in

Land length L

1.0 – 2.0 mmstart 1.5 mm

Relief depth h

0.50 mm≥ 10 × d, min 0.5

Vent width W

6 mmsmall part

Venting rules of thumb

As deep as possible without flash. Start at the value shown, sample, then deepen in small steps until flash appears and back off.
Low end of the range near the gate and at the bottom of thin ribs; high end along runners and in thick sections.
Land length about 1–2 mm (never under ~0.8 mm). Shorten it if you see burning or high fill pressure.
Relief channel right after the land, at least 10× the vent depth and no shallower than about 0.5 mm, same width as the vent.
Collector channels further out need a cross-section at least equal to all the vents feeding them (add a 1.5–2× margin).
Vent every last-to-fill spot, every weld-line meeting point, blind ribs and bosses (use vented pins or inserts there), and the runner ends.
Wider is better – about 6 mm on small parts, 12 mm on large ones, so the vent still works if the end of fill shifts.

Material	Group	Vent depth (mm)	Vent depth (in)
PP (Polypropylene)	Crystalline, runny	0.015 – 0.025	0.0006 – 0.0010
PE (HDPE / LDPE)	Crystalline, runny	0.015 – 0.025	0.0006 – 0.0010
PA6 / PA66 (Nylon)	Crystalline, runny	0.010 – 0.020	0.0004 – 0.0008
PA + Glass Fiber	Crystalline, runny	0.015 – 0.025	0.0006 – 0.0010
POM (Acetal)	Crystalline, runny	0.015 – 0.025	0.0006 – 0.0010
PBT	Crystalline, runny	0.015 – 0.025	0.0006 – 0.0010
PET	Crystalline, runny	0.015 – 0.025	0.0006 – 0.0010
TPE (SEBS / TPV)	Crystalline, runny	0.015 – 0.025	0.0006 – 0.0010
PPS	Very low viscosity	0.005 – 0.012	0.0002 – 0.0005
LCP	Very low viscosity	0.005 – 0.012	0.0002 – 0.0005
PS (GPPS / HIPS)	Amorphous, viscous	0.020 – 0.035	0.0008 – 0.0014
ABS	Amorphous, viscous	0.025 – 0.040	0.0010 – 0.0016
SAN	Amorphous, viscous	0.025 – 0.040	0.0010 – 0.0016
PC/ABS	Amorphous, viscous	0.025 – 0.040	0.0010 – 0.0016
PVC (Rigid)	Amorphous, viscous	0.025 – 0.040	0.0010 – 0.0016
TPU	Amorphous, viscous	0.025 – 0.040	0.0010 – 0.0016
PC (Polycarbonate)	Amorphous, viscous	0.030 – 0.050	0.0012 – 0.0020
PMMA (Acrylic)	Amorphous, viscous	0.030 – 0.050	0.0012 – 0.0020
PPO / PPE (Noryl)	Amorphous, viscous	0.030 – 0.050	0.0012 – 0.0020
PSU / PEI	Amorphous, viscous	0.030 – 0.050	0.0012 – 0.0020
PEEK	Amorphous, viscous	0.025 – 0.050	0.0010 – 0.0020

Typical starting values for the primary vent land. Grade, melt flow, fillers, mold temperature and injection speed all shift the right depth – always check the resin supplier's datasheet and confirm on the first mold trial.

Source: Topworks Plastic Mold