Your pouring basin decides what enters the mold.
If your pouring basin is acting like a funnel, it’s doing two things you don’t want: aspirating air, and collecting and delivering oxides/inclusions straight into the sprue.
A good pouring basin isn’t a “nice-to-have.” It’s a flow conditioning device.
Best practice:
Offset pouring basin (not centered over the sprue entrance
Basin geometry that matches the sprue entrance
A controlled path that encourages quiet, full flow into the sprue
Flow rate matching between the sprue entrance and sprue exit
Zero features that suck surface defects and inclusions into the system
Conical / funnel-shaped basins are basically a self-loading chute for oxides and anything floating on the surface. They aspirate and they funnel every extraneous inclusion into the mold.
This is one of the simplest tooling choices that separates “we hope it works” from “this process is stable.”
Simulation video courtesy of Joseph Rawding joseph@rawding.co.uk
The basin + sprue entrance must be designed as one system.
When the basin outlet isn’t correctly matched to the sprue entrance, you get:
- separation and reattachment of the stream,
- air entrainment,
- turbulence right where you need calm,
- and inconsistent sprue running conditions.
The fix is not complicated — it’s just intentional:
- the outlet area and shape should match the sprue entry geometry
- the metal should enter the sprue without free-fall or stream breakup
- and the system should promote a sprue running full, from the first metal onward
Downstream features can’t “filter out” turbulence you designed in at the top.
The sprue isn’t a downpipe — it’s an anti-aspiration device.
Once the metal leaves the pouring basin, your #1 job is simple:
keep the sprue running full from first metal to last.
What ruins stability fast:
- straight or poorly tapered sprues (separation + aspiration),
- a “stream” entering the sprue instead of a full transition,
- designs that let the metal free-fall and break up.
If the sprue aspirates, you don’t just entrain air — you create oxide, move oxide, and seed defects that show up downstream as “random” scrap.
Velocity is not a quality strategy. Control is.
After the basin and sprue are right, the next mistake is designing runners to “move metal fast.”
High velocity + sharp geometry = turbulence
Turbulence = oxide + entrainment
Oxide + entrainment = defects
A stable runner system:
- avoids high-speed 90° turns and hard impingement,
- keeps flow calm and predictable,
- and makes results less sensitive to small changes in head/temp/pour.
If your process only works when everything is perfect… the gating is doing you no favors.
Your ingates decide where the defects end up.
You can have a decent runner and still ruin a casting with ingates that:
- jet into thin walls,
- slam cores,
- wash sand/ceramic,
- or create free surface turbulence inside the cavity.
The highest-leverage ingate principles:
- Bottom fill -always
- Use flow rate and gravity to prevent turbulence
- Control the velocity as metal enters the mold
- Avoid meandering flow that can randomly converge flow
Most “mystery” defects aren’t mysterious — they’re delivered.