Die casting troubleshooting

Why Choose Metal Die Casting China: A Defect-First Guide

Most buyers meet metal die casting china through a drawing and a target price. The real questions show up later: a blister after heat treat, a bore that will not hold size, a tool that arrived late. This page lists seven symptoms we see on die-cast programs, what usually causes them, and how to handle each one.

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On this page

1
Defect symptoms, causes, and what to do
2
Blistering and gas porosity after heat treat
3
Cold shuts and flow lines on thin walls
4
Porosity that only shows up after machining
5
Bores and flat faces that drift out of tolerance
6
Flash, solder, and other tool-side problems
7
What this means when you source in China
8
Step by step: taking a defect from symptom to closed

Symptom map

Defect symptoms, causes, and what to do

Read down the first column until you find your part, then check the cause before you call the tool shop.

Symptom	Likely cause	What to do
Blister or bubble after heat treat	Gas trapped in the melt or shot sleeve	Change gate position, add vacuum assist, degas the melt
Cold shut or flow line	Melt too cold, runner too long, thin wall too far	Raise barrel temperature, move the gate, thicken the wall locally
Porosity on a machined face	Turbulent fill, low intensification pressure	Adjust the shot profile, raise pressure, add a local squeeze pin
Bore will not hold size	Cast skin removed unevenly, residual stress	Leave 0.3–0.5 mm stock, rough machine, stress relieve, finish
Flash at the parting line	Die lock insufficient, tonnage too low	Re-clamp the die, check tie bars, raise locking force
Solder build-up in the cavity	Die too hot, release agent too thin	Drop die temperature, re-check spray pattern and cycle time
Dimensional drift over a run	Die erosion or thermal growth	Track first-off and last-off, recut or re-polish the cavity

Symptom 1

Blistering and gas porosity after heat treat

A blister is a bubble that appears only after the part sees heat. During the shot, gas from the melt, the release agent, or the air in the shot sleeve gets trapped in the metal. It stays invisible until a solution treatment or a paint bake raises the internal pressure and pushes the skin outward.

The fix is rarely a single setting. Vacuum assist on the die helps, but only if the seal is good and the vacuum is pulled before the shot. Degassing the melt with a rotary degasser and checking hydrogen content matters more than most shops admit. A gate that fills the cavity in a straight front instead of a spray also reduces the amount of air that gets folded in.

One more cause gets missed. If the plunger lubricant is applied too heavily, the carrier burns off and leaves gas in the sleeve. Ask what lubricant is used and how much. A short, dry shot at the start of a shift will show you the sleeve condition in seconds.

If the part is going to heat treat anyway, say so on the RFQ. A foundry that knows this will pick a lower-gas alloy and adjust the shot profile from the first trial, not after the first reject.

Aluminium alloys with lower gas content, such as ADC12 for thin walls, behave better here than a general-purpose alloy poured with no control. Ask for the hydrogen reading, not a promise.

Symptom 2

Cold shuts and flow lines on thin walls

A cold shut is where two metal fronts meet and do not fuse. You see a line on the surface, and under a microscope the line is a crack. It appears when the melt loses heat faster than it fills. Thin walls far from the gate are the usual location.

The first lever is temperature. Barrel temperature for aluminium die casting usually sits between 650 and 700 °C, and die temperature between 180 and 250 °C depending on wall thickness. If the die is cold at the start of a run, the first ten parts will show flow lines and then clean up. That is normal. Lines that persist after twenty shots are a design issue.

The second lever is the runner. A short, thick runner keeps the metal hot. A long, thin one does not. If the gate is at one end of a long part with a 1.5 mm wall at the other, no amount of temperature will save it. Move the gate, add an overflow, or accept a thicker wall.

The third lever is fill time. Faster fill reduces heat loss but raises turbulence, which trades a cold shut for porosity. There is a window, and finding it takes a trial or two. Ask the shop to run a short shot study and send you the photos.

If the wall is below about 1 mm over a long distance, die casting is the wrong process. Say that early rather than after the tool is cut.

Symptom 3

Porosity that only shows up after machining

This is the most expensive defect because it appears after you have paid for the casting and the machining. The casting looked fine. Then a face is milled and a cluster of pinholes opens up.

The cause is usually turbulence during fill. A fast shot that splashes metal against the cavity wall folds air into the part. The air sits just under the skin, in the region the machinist is about to remove. A slower fill with a well-placed gate reduces this, but slows the cycle, which is why some shops resist it.

Intensification pressure is the second cause. After the cavity fills, the plunger must push more metal in to squeeze out shrinkage. If the pressure is too low or applied too late, the centre of a thick section will be porous. On a part with a 6 mm boss next to a 2 mm wall, that is almost guaranteed without a squeeze pin or a local chill.

Ask where the machining stock will be removed and how much. If a critical bore will cut 1 mm into the casting, the foundry needs to know before the gate is designed. Send the machining drawing with the casting drawing.

A dye penetrant check on a trial part costs little and tells you a lot. Do it before the tool is hardened, not after.

Symptom 4

Bores and flat faces that drift out of tolerance

A die casting cools from roughly 650 °C to room temperature. That cooling is not uniform, and the part warps as it shrinks. A bore that is round in the die is not round after cooling. Machining then removes the cast skin unevenly, and the part relaxes again.

The practical answer is to leave stock. For a bore that must finish at ±0.005 mm, leave 0.3 to 0.5 mm on the diameter, rough machine it, let it sit, then finish. On a thin-wall housing, a stress-relief step between roughing and finishing removes most of the movement.

Fixturing matters as much as the cutting. Clamping a thin casting hard will distort it before the tool touches it. A soft-jaw or a vacuum fixture that holds the part on a machined face gives a more honest result than three clamps on the flash line.

If the part is a flat cover with a sealing face, flatness is the number to watch. As-cast flatness on a 300 mm cover can be 0.3 mm or worse. Machining one face and using that face as the datum for the rest is the usual route.

Measure after the part reaches room temperature. Checking a warm part on the shop floor is how good parts get scrapped and bad ones get shipped.

Symptom 5

Flash, solder, and other tool-side problems

Flash at the parting line means the die halves are not held together hard enough, or the faces are worn. It is not a metal problem. Check the locking force against the projected area of the part, then check the tie bars for even stretch. A die that flashes on one side only is usually sitting on a worn face, not short of tonnage.

Solder is different. It is aluminium sticking to the steel, and it happens when the die surface is too hot or the release agent is not doing its job. A local hot spot, often where a gate shoots straight at a wall, will solder first. Reposition the gate or add cooling to that insert.

Erosion shows up as a gradual change in a gate or a core over tens of thousands of shots. The gate gets bigger, the fill pattern changes, and dimensions drift. Track the gate size every few months and recut it when it moves beyond the drawing.

None of these need a new tool. They need someone to look at the tool with the right instruments and a record of what changed. Ask for that record before you approve a tool handover.

Sourcing angle

What this means when you source in China

The reason metal die casting china works for a lot of programs is not a single cost number. It is that the tool shop, the casting cell, and the CNC department can sit in one building and argue about the same drawing. When a gate needs to move because a bore will not hold size, the person who cuts the gate and the person who bores the hole are in the same meeting.

That integration is what removes the two most common sourcing failures: a foundry that ships castings nobody can machine, and a machine shop that blames the casting. If both steps are quoted separately by two suppliers, the gap between them is where your schedule dies.

It also shortens the loop on a defect. When a blister appears on a heat-treated part, the fix touches alloy choice, degassing, gate design, and the heat-treat cycle. Four vendors means four conversations and a week of emails. One supplier means one trial.

We are in Dongguan with a second plant in Singapore, 127 CNC machines, and a die casting line that feeds them. That layout is the point. The casting and the machining share a schedule, a quality record, and a person who owns the part.

Certifications matter here, but read them for what they cover. ISO 9001:2015 covers the quality system. IATF 16949:2016 adds the automotive process controls that keep a defect from reaching a customer. ISO 13485:2016 covers medical device work. ISO 27001:2022 covers the drawings and data you send. Ask which one applies to your program, and ask to see the scope statement, not the certificate logo.

How to run the fix

Step by step: taking a defect from symptom to closed

This is the sequence we use on a new die-cast part. It works on a running part too, just start at the step that matches your symptom.

1
Send the casting and machining drawings togetherOne RFQ with both files, plus the alloy, the heat-treat spec, and the surfaces that will be machined. If the machining stock is not on the casting drawing, the gate will be designed blind.
2
Ask for a DFM note before the tool is cutWe return a quotation and a free DFM analysis within 12 hours. Look for wall thickness calls, gate location, and any feature that needs a slide. Push back on anything you do not understand.
3
Run a short shot and a first-off sampleA short shot study shows the fill pattern. A first-off sample shows dimensions. Both are cheap next to a recut tool. Ask for photos of the fill and a dimensional report.
4
Machine a trial part before the tool is hardenedCut the critical bore and face on one casting. If porosity opens up, the gate moves now, not after 10,000 parts. This is the single step most programs skip.
5
Set the process window and record itBarrel temperature, die temperature, fill time, intensification pressure, and cycle time. A window with numbers can be repeated. A window in someone's head cannot.
6
Run a capability check on the critical dimensionsTwenty parts minimum, measured at room temperature. If a bore is drifting, you will see it in the trend before you see a reject.
7
Lock the inspection plan before mass productionRaw material check, in-process monitoring, final inspection, 100% inspection before shipment. Reports on request. Agree on what is measured and how often.
8
Track the tool over the runLog the gate size and any erosion every few months. A tool that is measured is a tool that does not surprise you at shot 80,000.

FAQs

Questions buyers ask before the first tool

Which aluminium alloy should we specify for a die-cast housing?

ADC12 (close to A380) is the common choice for thin walls and general housings. It casts well, machines reasonably, and takes most finishes. A360 gives better corrosion resistance and slightly better strength if the part sees moisture.

If the part will be anodized, say so. Standard ADC12 does not anodize to a cosmetic finish because of its copper and silicon content. A low-copper alloy or a different process is needed, and that decision belongs on the drawing, not in a phone call after the first batch.

How much machining stock should we leave on a casting?

For a face, 0.5 mm is usually enough. For a bore that must hold ±0.005 mm, leave 0.3 to 0.5 mm on the diameter and plan a rough and a finish pass with a stress-relief pause between them.

More stock is not safer. Cutting 2 mm off a casting releases more internal stress than cutting 0.5 mm, and the part moves further. Deep cuts also expose more porosity. Keep stock tight and tell the foundry where it is.

Can you cast and machine the part in one order?

Yes. Die casting, CNC machining, and surface finishing run under one roof, so the casting and the machining share a schedule and a quality record. There is no minimum order quantity, from one prototype to runs over 10,000 parts.

That matters most when a defect appears. The person who can move a gate and the person who can adjust a fixture are in the same building, so a fix takes a trial instead of a shipment.

What lead time should we plan for a new die-cast part?

Quotation and DFM analysis come back within 12 hours. Production can start within 24 hours after the tool and process are approved, and machined parts ship in 3 to 5 days on a running program. Historical late-delivery probability is below 2%.

Tooling time depends on part size and slide count, so treat any single number with suspicion. Ask for the tool schedule in writing with the quotation and hold the shop to the first-off date, not the ship date.

How do you protect our drawings and part data?

Uploads are secure and confidential, and an NDA is available on request. ISO 27001:2022 covers the information security side, which is the certification that applies to your files rather than your parts.

If your program is under a customer NDA, tell us at the RFQ stage. It is easier to set up the data handling once than to unwind a shared folder later.

When is die casting the wrong choice?

Below roughly 1,000 parts a year, or when the wall is under 1 mm over a long distance, the tool cost and the fill risk usually beat the per-part saving. Sand casting or CNC from billet can be cheaper at low volume.

Very large parts with tight tolerances on many faces also fight the process. The casting sets the shape, the machining sets the tolerance, and if every face is critical, you are paying for a casting to make chips.