Common Die Casting Defects: Porosity, Flash, Cold Flows & More | Wochang Metal

4 Common Die Casting Defects - and How to Prevent Them Before They Happen

No die casting comes out perfect on the first shot. But some defects show up on 90% of new molds, and they're almost always preventable if you catch them during DFM review.

After 20 years in this business, we see the same four issues over and over. Here's what they look like, what causes them, and how to avoid them.

1. Porosity (Gas Porosity / Shrinkage)

What it looks like: Small holes or voids inside the casting - sometimes visible on the surface, sometimes only after machining or X-ray.

What causes it:

Gas porosity: trapped air or gas in the metal as it solidifies. Usually from poor gating design or too-fast injection

Shrinkage porosity: the metal shrinks as it cools, and there isn't enough feed metal to fill the gap. Common in thick sections or at junctions

How to prevent it:

Uniform wall thickness - avoid thick blocks that don't feed properly

Proper gating and venting design (this is a mold design issue, not a material issue)

Vacuum-assisted die casting for critical parts

If you're machining into the part, tell the foundry where the machined surfaces will be - they can design the gates to keep porosity away from those areas

2. Flash

What it looks like: Thin feather-like edges of metal that squeeze out at the die parting line or around ejector pins.

What causes it:

Too much injection pressure

Worn die or insufficient clamping force

Die parting line not seating properly

How to prevent it:

Flash is normal in small amounts and gets trimmed. But if you're seeing heavy flash, it usually means the die needs refurbishing or the process parameters need tuning

Design your part so the parting line falls on a non-critical surface where flash doesn't matter

Good foundries trim flash automatically as part of the process

3. Cold Shuts (Cold Flows)

What it looks like: Lines or seams on the casting surface where two fronts of metal meet but didn't fuse properly. Often looks like a crack but isn't.

What causes it:

Metal temperature too low

Die temperature too cold

Injection speed too slow - the metal starts solidifying before it fills the cavity

How to prevent it:

This is mostly a process parameter issue. If you see cold shuts on T1 samples, the foundry can raise metal/die temperature or increase injection speed

Design-wise, avoid placing gates where two flow fronts meet at a thin section

Ribs and bosses should be designed so the metal flows through them without stagnating

4. Die Soldering (Sticking)

What it looks like: Aluminum (or zinc) metal adheres to the die surface, leaving rough patches on the casting and requiring die repair.

What causes it:

Aluminum is particularly prone to soldering because it chemically reacts with steel

Die temperature too high

Incorrect die steel or insufficient heat treatment

Lack of proper die lubrication (release agent)

How to prevent it:

Use the right die steel (H13 is standard for aluminum die casting) and proper heat treatment

Maintain correct die temperature

Apply release agent consistently

Sharp internal corners on the part are the worst offenders - add generous fillets and radii on all internal corners

The Real Fix: DFM Before Tooling

Here's the thing about all four of these defects: most of them are preventable if you review the design before cutting the die.

That's why we always do a DFM (design for manufacturing) review before quoting. We look at wall thickness, gating, draft angles, and parting line placement, and we flag issues that would cause defects down the line. It costs nothing, and it saves weeks of mold revision later.