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Notes on injection moulding: part design, tooling and cost

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Why does an injection mould need vents, and how deep are they?

A mould needs vents because the cavity is full of air that has to escape as plastic flows in. Vents are very shallow channels, typically about 0.0005 to 0.0030 in (0.013 to 0.076 mm) deep depending on the resin, which let air out but are too thin for the plastic to follow. Without them, trapped air causes burn marks, short shots and flash.

Plastic fills a cavity in a fraction of a second. The air already inside is compressed ahead of the flow front and, if it cannot get out, it heats up and resists the plastic. Plastics Technology lists the results: unfilled or poorly packed parts, burnt plastic, voids, dimensional variation, steel damage at the end of fill and, if cavity pressure rises enough, the mould being forced open slightly, which causes flash.

Published vent depths

Resin typePublished vent depthSource
Easy-flowing crystalline resins (nylon, PE, PP)0.0005 to 0.0010 inPlastics Technology, venting basics
More viscous amorphous resins (PC, TPU, PEEK)0.0010 to 0.0030 inPlastics Technology, venting basics
ABS (published value)0.002 inPlastics Technology, vent depth study
Nylons (published value)0.0005 to 0.0007 inPlastics Technology, vent depth study
Eastman copolyesters, small parts or near gates0.012 to 0.025 mm (0.0005 to 0.001 in)Eastman
Eastman copolyesters, larger parts0.025 to 0.038 mm (0.001 to 0.0015 in)Eastman

Both Plastics Technology articles stress that these are starting points: each grade, filler and additive package can call for a different depth, so the resin supplier's processing guide for your grade takes priority.

Part thickness changes the answer

One study reported in Plastics Technology used a test mould with 18 tabs of three thicknesses and six vent depths. Thin sections flashed at shallower vents because the plastic flows faster and thins out there. With ABS, vents flashed at 0.0030 in on a 0.125 in tab but already at 0.0020 in on a 0.0312 in tab. The author found 0.003 in vents worked on 0.125 in ABS parts, deeper than the published 0.002 in, and nylon tolerated close to 0.0015 in on thick sections against a published 0.0005 to 0.0007 in.

Land, width and relief

  • Land length (the shallow section next to the cavity): Plastics Technology gives about 0.060 to 0.080 in as typical, with a consensus minimum of 0.030 in. A longer land needs more pressure to push air out. Eastman gives 3 to 6 mm for its resins.
  • Width: at least about 0.25 in (5 mm), and vents can run all the way around the part as a ring vent.
  • Relief channel behind the land: at least 0.010 in deep in one article; the venting-basics author prefers at least ten times the vent depth, with a 0.020 in minimum.
  • Finish: vents should be polished in the direction the air flows, because a rough vent collects residue faster.

Where vents go

  1. At the last place to fill, which a fill simulation or short-shot trial will show.
  2. Where flow fronts meet, to reduce weak, visible weld lines.
  3. At the bottom of deep ribs and bosses, sometimes through ejector pins or vented inserts.
  4. Along the runner as well as the part, so air is cleared before plastic reaches the cavity.

Vent dimensions here are published guidance and test results. The resin supplier's figures and the toolmaker's trials set the values for a real mould.

Questions buyers ask

How do I know if a mould is poorly vented?

Typical signs are dark burn marks at the end of fill, short shots in the same spot every cycle, and a need for unusually high injection pressure. Flash can also appear if the moulder pushes pressure up to compensate.

Can vents be added after the mould is made?

Usually, yes. Vents are ground into the parting line or inserts and are a common adjustment during mould trials. Making a vent shallower again is harder.

Why do vents need cleaning?

Gases from the melt leave a sticky residue that builds up over thousands of cycles. Blocked vents bring back the burn and short-shot problems, so cleaning them is part of routine mould maintenance.

Is venting the moulder's or the designer's problem?

Mostly the toolmaker's and moulder's, but part design matters: deep blind features and thin sections ending in dead corners are hard to vent. Flag them in the design review.

Sources