Sprues and Air Vents — Side Pour vs Top Pour, Filling Without Bubbles
Where the sprue and vents go, how big they should be, and pouring technique that keeps bubbles out of a closed two-piece mold.
To fill a closed two-piece mold you need a way in (the sprue) and a way out for the air (the vents). Put them in the wrong place and the cavity does not fill, or air gets trapped and leaves holes in the cast.
Side pour (upright)
Stand the shell on its side and pour into the funnel on the face that is now on top. Silicone rises from the bottom and pushes air up ahead of it, so even viscous silicone fills with few bubbles. This is the recommended mode for most jobs.
The mold generator places the sprue automatically in this mode — not just at the end of the model, but by evaluating many candidate lines along the model's long axis (and the short axis for flat models) and choosing one where the channel:
- removes the least material from the model,
- lands on a broad face rather than a pointed tip, and
- does not graze any other part of the model.
Where the channel meets the cavity it starts as a narrow gate and widens outward, like an injection-mold gate. The mark left on the cast is minimal and the sprue snaps off cleanly.
The Sprue line in the stats panel shows the gate diameter and how far it intrudes into the shape (usually 0.3 mm). Other spot ↻ cycles through the candidates — choose one that lands on the back or underside of the model.
Two vents are placed at the high points where air collects when the shell stands upright. Silicone rising in a vent means the cavity is full.
Top pour (flat)
The shell lies flat and you pour through a hole in the top half. Convenient for thin resin or wax, and the shell needs no stand. Air can get trapped in every concave spot of the cavity ceiling, so the generator drills a vent at each local high point of the ceiling.
Bowl- or lampshade-shaped models whose centre dips below the parting plane need the pour hole to continue through the bottom shell to reach the cavity; the generator handles that automatically.
Sprue size
| Setting | When |
|---|---|
| Small | Small figures, detail-critical areas, thin resin |
| Normal | Most jobs |
| Large | Viscous silicone (Shore A 30+), big shells, when you need to pour fast |
Smaller leaves a smaller mark but pours slower; with a short-pot-life silicone it may start gelling mid-pour.
Keeping bubbles out
- Thin and high — pour from 20–30 cm above the funnel in a thread-like stream; bubbles pop on the way.
- One spot — keep a single steady stream rather than moving around.
- Stir slowly — no whisks. Scrape the bottom with a stick for 2–3 minutes.
- Tap the shell — after pouring, tap it on the bench to release bubbles clinging to the walls.
- Vacuum degas — for clear resin or face detail, 2–3 minutes in a vacuum chamber after mixing. A pressure pot (40–60 psi) shrinks whatever is left to invisibility.
- Thin release — a thick coat of release agent looks like surface bubbles on the cast.
After demolding
The sprue stub snaps off easily because the gate is narrow; sand the mark with 400 grit if visible. Vent stubs are thin enough to peel with a fingernail.
Related: choosing silicone · troubleshooting
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