Venting is one of the least visible parts of a liquid silicone rubber (LSR) mold. It is also one of the main reasons two molds that look the same on a drawing can perform very differently in production.
LSR presents a conflict. Air must escape from the cavity, or you get burns, voids and incomplete filling. But LSR has such low viscosity that it flows into almost any gap air can pass through. A vent that works well for a thermoplastic mold can produce flash on every shot with LSR.
In thermoplastic molding, melt viscosity is high enough that vents of a few hundredths of a millimeter release air without letting much plastic through. LSR before curing behaves much more like a liquid. As a general guide in the industry:
So venting an LSR mold is not just “making smaller vents”. It needs a different approach.
| Problem | Typical Cause | What You See on the Part |
|---|---|---|
| Vents too shallow or blocked | Air cannot escape | Short shots, voids, bubbles at end of fill |
| Air compressed at end of fill | Trapped air heats up | Burn marks, white or brown spots |
| Vents too deep | LSR enters vent | Flash at vent locations, extra trimming |
| Vents in wrong place | Air trapped in dead corners | Repeated defects at the same feature |
| Worn or contaminated vents | Cured residue, wear | Defects that slowly increase over a production run |
The last row deserves attention. A mold can pass trials and still develop venting problems after weeks of production as vents fill with cured silicone residue.
Shallow vent channels are ground into the parting line at the last-to-fill areas. For LSR they must be very shallow and placed accurately using filling analysis or short-shot trials.
Works best for: simple geometry, parts with clear end-of-fill locations.
Limitations: limited air flow per vent; difficult for deep ribs, thin walls and closed features.
Air can also escape through carefully fitted inserts, cores and ejector clearances. The fit must be controlled within a few microns.
Works best for: deep features or areas far from the parting line.
Limitations: requires precise machining and close maintenance; clearances can wear and start flashing.
The cavity is sealed and air is evacuated before or during injection. A vacuum seal (typically an O-ring groove around the cavity area) and a vacuum pump remove air so there is little left to trap.
Works best for: multi-cavity molds, thin-walled parts, complex geometry, medical and babycare parts where voids are not acceptable.
Benefits:
Things to manage: the sealing surfaces and O-rings must be maintained, and the vacuum timing must be set correctly within the cycle.
For high-cavitation LSR molds, vacuum is often the most practical way to combine reliable filling with flash-free parts.
Vent location matters as much as vent size. Good practice includes:
Venting placed only by experience, without trials, often leads to repeated mold corrections.
When a defect appears, a structured check saves time:
Venting is not “done” when the mold is approved. Include in the mold maintenance plan:
This is especially important if the mold may later be moved to another molder.
Before you approve an LSR mold design:
A supplier that answers these clearly is likely to deliver a mold that stays stable after trial approval.
In LSR molds, venting is a matter of microns. Too little and you get burns and short shots; too much and every part has flash. Good LSR molds combine simulation, trial verification and, for demanding parts, vacuum systems, followed by disciplined maintenance during production.
TYM designs and builds LSR molds in-house, including vacuum-assisted, valve-gate cold-runner multi-cavity molds for medical, babycare, automotive and consumer parts. Send us your part drawing for a mold design review.
