Mold Venting Design Guide: Prevent Burns and Short Shots

Every shot of plastic has to push air out of a closed steel cavity — and most tools have nowhere near enough room for it to leave. When the air cannot escape, it heats up under compression, and the result shows on the part as burns, short shots, weak weld lines and inconsistent fill.

Figure 1. Mold Venting Design Guide: Prevent Burns and Short Shots.

Venting is one of the cheapest features to design correctly and one of the most expensive to retrofit. This guide covers vent depth by resin, vent location and the vent types that actually work in production.

1. What Poor Venting Looks Like

Poor venting rarely announces itself as "a venting problem". It shows up as a cluster of defects that engineers chase one at a time:

• Burn marks / diesel effect — dark brown or black streaks at the last point to fill, usually at a rib, boss or the end of a long flow path. The trapped air is compressed and ignites the polymer.
• Short shots — the cavity cannot fill because back-pressure from trapped gas resists the melt front.
• Weak weld lines — gas trapped at a weld line prevents the two melt fronts from knitting, so the joint fails in service.
• Gloss / surface variation — matt patches where air sat against the cavity wall and slowed local cooling.
• Higher injection pressure needed — a tool with bad venting is often run at 10–20% more pressure than it should need, which in turn causes flash and over-packing.

If a tool shows two or more of these at once, measure venting before you touch the process.

2. How Much Air Is Actually Trapped?

It helps to size the problem. A cavity of 200 × 150 × 3 mm has a volume of roughly 90 cm³. At atmospheric pressure that is about 0.09 liters of air — but the melt front does not sweep it out evenly. The air is compressed into the last-fill pockets, and at typical injection pressures those pockets can see several bar of pressure.

A practical rule of thumb: plan for a vent cross-section capable of passing the cavity volume in a few hundred milliseconds, distributed across the parting line rather than concentrated at a single point. A single 10 mm × 0.02 mm vent will not evacuate a 90 cm³ cavity — that is why real tools have vents every 25–50 mm around the perimeter.

3. Vent Depth by Resin

Vent depth is a balance: deep enough to pass air, shallow enough that the melt does not flash through it. Depth scales with melt viscosity.

Important: the vent land must be a short flat land (typically 0.5–1.0 mm long) at the vent depth, then open out to a full-depth relief channel 0.3–0.5 mm deep that leads to atmosphere. A vent that stays at vent depth for its whole length will clog and never pass air properly.

Vented depth is measured from the cavity surface, and in a tool with a textured cavity the vent must be cut relative to the texture-free steel.

4. Where to Put Vents

Place vents where the air actually ends up, not where it is convenient to machine.

Mandatory vent locations:

• Last point to fill — every flow-front termination. Confirm from a fill simulation or a short-shot study, not from intuition.
• Weld line locations — a vent here both removes gas and improves knit strength.
• Blind ribs, bosses and deep pockets — air cannot escape downward, so vent at the top of each.
• Around the perimeter of the cavity — every 25–50 mm, on the parting line.
• End of each runner — including the cold slug well area.
• Under the sprue — a shallow vent or a vented sprue bush.

A short-shot study is the fastest way to find them: run the tool at progressively lower fill, and mark where each shot stops. Those flow-front positions are your vent locations on the final shot.

5. Vent Types and When to Use Each

For LSR and very low-viscosity materials, standard parting-line vents are effectively useless — the silicone flashes through any vent you can machine reliably. These tools rely on vacuum venting plus carefully lapped shut-offs.

6. Symptoms, Causes and Fixes

Note that "burn marks" also appear when the melt is simply too hot or the screw is decompressing poorly — always confirm the location matches a last-fill point before blaming venting.

7. Maintenance Matters More Than Design

A vent is a consumable feature. It will fill with oil, vaporized additives and degraded resin, and its effective depth will drop below what the drawing says.

• Clean parting-line vents on a defined schedule — every 5,000–10,000 shots for commodity resins, more often for glass-filled or flame-retardant grades.
• Record baseline vent depth on the tool drawing and measure it annually. A 0.025 mm vent that has closed to 0.012 mm is the reason for the burns that appeared last month.
• Use a vent-cleaning schedule tied to the preventive maintenance plan, not to defects appearing.
• Keep a spare set of vent inserts for high-wear locations.

Figure 2. Quick reference: the key checks and reference values from this guide.

8. Design Checklist

• [ ] Vent locations confirmed from a fill simulation or short-shot study
• [ ] Vent depth specified per resin, on the drawing, with a tolerance
• [ ] Vent land length 0.5–1.0 mm, followed by a full-depth relief to atmosphere
• [ ] Perimeter vents at 25–50 mm spacing
• [ ] Vents at every weld line, rib top, boss top and blind pocket
• [ ] Ejector-pin vents specified with flats for deep pockets
• [ ] Vacuum venting considered for LSR, thin-wall and large parts
• [ ] Vent maintenance interval included in the PM schedule
• [ ] Baseline vent depth recorded for future wear checks
• [ ] Relief channels lead to a clean, unobstructed exit — not into a closed plate gap

Conclusion

Venting is a design decision, not a tryout adjustment. Size the vent to the resin, place it where the air actually goes, and give the gas a clear route to atmosphere. Do that and you remove a whole category of defects — burns, short shots, weak welds — before the tool is even cut, and you stop fighting the process on every batch.

Send us a part and we will review your venting plan against the fill pattern and resin before the cavity is machined.