Die Casting
What Is Die Casting Cold Material and How to Eliminate This Defect?
Die casting cold material destroys parts from the inside out. It happens when molten metal cools too much before filling the mold, losing the fluidity needed to reach every corner. The result? Rough surfaces, internal voids, and structural weakness. Scrap rates can hit 20% in severe cases. For automotive engine parts or aerospace components, cold material creates safety risks that cannot be ignored.
What Exactly Is Die Casting Cold Material?
Basic definition
Cold material is a defect where molten metal experiences excessive cooling during melting, transfer, or mold filling. The metal loses heat too quickly, reducing its fluidity. It solidifies prematurely or forms irregular structures that fail to bond properly with surrounding metal.
Unlike surface scratches or dents, cold material is often a hidden threat. It may appear as minor surface marks but hide internal flaws like shrinkage voids or pore clusters deep inside the part.
What it looks like
On the surface, cold material shows as:
- Rough, dull patches with no metallic luster
- Flow lines or layered striations that look like frozen waves
- Localized material shortage—small gaps where metal never reached
Inside the casting, you find:
- Concentrated shrinkage voids of 0.1-0.5mm diameter
- Aggregated pores clustered near cold material areas
- Unmelted solid particles—residues from incomplete melting
X-ray inspection or metallographic analysis reveals these internal defects. A sample etched with 5% nitric acid shows the structure clearly under magnification.
Where it forms
Cold material targets specific areas:
Thin-walled sections far from the gate lose heat fastest. A 1mm aluminum shell 100mm from the gate is 3x more likely to develop cold material than a 5mm section near the gate.
Deep cavity bottoms take longer to fill. Metal sits against cold mold walls, cooling before filling completes.
Low-temperature mold zones act as heat sinks. Areas near cooling channels or without proper preheating suck heat from the metal.
What Causes Cold Material?
Temperature control failures
Incomplete melting leaves unmelted particles in the metal. These act as “cold cores” that reduce overall fluidity.
Holding furnace issues let temperature drop 20-30°C. Cooled metal solidifies at the flow front, blocking further filling.
Unpreheated pressure chambers absorb heat from the first metal. At room temperature instead of 150-200°C, the chamber causes front-end solidification in 2-3 seconds.
A real example: An aluminum ADC12 plant had a faulty holding furnace. Metal temperature dropped from 700°C to 650°C. Cold material defects jumped from 3% to 18% in one week.
Mold design problems
Poor runner design with sudden cross-section changes or sharp bends increases flow resistance. Filling takes longer. Heat loss increases.
Uneven mold temperature from cooling channels too close to the cavity (<5mm) creates cold spots. Metal contacting these areas cools below its liquidus and stops flowing.
Excessive release agent over 0.1mm thick acts as an insulator. It prevents heat transfer from mold to metal, making cooling worse.
A zinc toy manufacturer had a 90° sharp bend in their runner. Cold material formed at the bend in 25% of castings. Redesigning to a 15mm radius cut defects to 2%.
Process parameter mismatches
Slow injection speed under 2 m/s for aluminum extends contact time with cold mold. More time means more cooling.
Incorrect pressure holding timing compresses partially solidified metal, creating layered cold material.
Excessive pouring volume leaves residual cold material in the chamber. This mixes with new hot metal, reducing overall temperature.
An automotive plant used 1.5 m/s injection speed for 2mm aluminum brackets. 30% had cold material. Increasing speed to 4 m/s eliminated the defect.
Material issues
Alloy composition deviation hurts fluidity. Aluminum with silicon below 9% instead of 11-13% loses flow capacity. Silicon is the “flow enhancer.”
Unscreened return material brings oxide scales and impurities. These act as nucleation sites for premature solidification.
A magnesium plant mixed 50% unscreened return material with new ingots. Cold material rose by 12%. Reducing return to 30% and adding a 50μm filter cut defects to 4%.
| Cause Category | Specific Failure | Impact |
|---|---|---|
| Temperature | Furnace drops 20-30°C | Defects 3% → 18% |
| Mold design | 90° sharp bend | 25% scrap |
| Process | 1.5 m/s speed | 30% cold material |
| Material | 50% unscreened return | +12% defects |
How Do You Eliminate Cold Material?
Step 1: Build precise temperature control
Melting stage: Use a double-furnace process. Main furnace at 720-750°C for full melting. Auxiliary furnace at 680-710°C for precise adjustment to the upper process limit.
Install online infrared thermometers with ±2°C accuracy. Trigger an alarm if temperature drops below the lower limit—670°C for ADC12.
Transfer stage: Use heated transfer ladles with 5kW electric heaters. Maintain metal temperature during transport. Keep heat loss under 5°C.
Injection stage: Preheat the pressure chamber to 150-200°C for aluminum, 180-220°C for magnesium. Use electric heating jackets. Never start with a room-temperature chamber.
Mold preheating: Set gradient temperatures. Aluminum molds at 200-250°C. Magnesium molds at 220-280°C. Use zone-specific heating for deep cavities. Keep temperature deviation under ±10°C.
Step 2: Optimize mold design
Runner redesign: Replace sudden cross-section changes with gradual transitions of 1-3° taper. Use smooth curved runners with radius ≥10mm. This cuts filling time by 30% and heat loss by 25%.
Add buffer grooves at thin-walled entrances with volume 1.2× runner volume. They stabilize flow and prevent front-end cooling.
Thermal balance: Embed ceramic heat-insulating inserts in thin-walled or deep cavity areas. Thermal conductivity of 0.5 W/m·K slows heat dissipation by 50%.
Keep cooling channels at least 8mm from cavity surface, not 5mm. This prevents over-cooling.
Surface maintenance: Polish cavity surfaces to Ra ≤0.8 μm with diamond wheels. Smooth surfaces reduce friction and heat loss from metal-mold contact.
Control release agent with mist sprayers. Apply thin, uniform film of 5-10μm. Avoid excessive spraying that insulates the metal.
Step 3: Adjust process parameters dynamically
| Parameter | Adjustment | Target |
|---|---|---|
| Injection speed | Fast-slow-fast three-stage | Fill time ≤2 sec for parts <200mm |
| Pressure holding | Start 0.2-0.3 sec after fill | 80-120MPa, hold 70-80% of solidification time |
| Pouring volume | (Casting + Runner) × 1.05 | No residual in chamber |
Injection speed: Use three-stage control. Initial fast at 4-6 m/s to reach cavity quickly. Middle slow at 2-3 m/s for thick areas. Final fast at 3-5 m/s for thin walls.
Pressure holding: Trigger by cavity pressure sensors, not timers. Start 0.2-0.3 seconds after filling completes. Hold at 80-120MPa for 70-80% of total solidification time.
Pouring volume: Calculate exactly: (Casting Volume + Runner Volume) × 1.05 safety factor. Clean chamber every 50 shots to remove residual cold material.
Step 4: Tighten material management
Alloy composition: Test every batch with spectral analysis. For ADC12, keep silicon at 11-13%. For AZ91D magnesium, keep magnesium at 0.7-1.0%.
Add flow-enhancing elements if needed. For aluminum, 0.1-0.2% rare earth (cerium, lanthanum) improves fluidity by 15-20%.
Return material: Screen with 1mm mesh to remove oxide scales and impurities. Limit return to ≤30% mixed with 70% new ingots. Higher ratios reduce fluidity.
Refining and degassing: Use argon rotary degassing for 15 minutes at 2L/min. Reduce hydrogen to <0.15ml/100g Al. This also removes small oxide inclusions.
Let metal stand for ≥15 minutes after refining. Slag floats to top. Skim it off before pouring.
How Do You Diagnose Cold Material Quickly?
Step 1: Visual inspection
Look for rough, dull patches or flow lines. Tap the area with a small hammer. A dull sound indicates internal cold material. A clear ring means normal metal.
Step 2: Microscopic check
Take a small sample from the defective area. Polish it. Under 100x magnification, look for unmelted particles or layered structures. If present, it is cold material.
Step 3: Parameter review
Check recent data logs:
- Did metal temperature drop below process range?
- Was injection speed slower than usual?
- Did mold temperature in defect area fall below target?
What Emergency Actions Work?
If cold material appears during production:
Adjust temperature: Increase metal by 10-15°C. For ADC12, go from 680°C to 695°C. Test 10-20 samples to verify improvement.
Clean chamber: Stop production. Clean pressure chamber with steel brush to remove residual cold material. Preheat to 200°C before restarting.
Tweak parameters: Increase injection speed by 0.5-1 m/s within safe range. Reduce filling time. Do not exceed 8 m/s—that causes turbulence.
Temporary mold fix: Apply small amount of high-temperature graphite release agent to cold zones. This reduces heat loss until full maintenance.
How to write a spec that gets priced correctly
A drawing is a contract. If it leaves a number open, the shop fills that number in with its own default. The default is rarely the one you had in mind. Put these five lines on every RFQ and most of the back-and-forth disappears before it starts.
- 3D model with the parting line marked
- Alloy grade and the standard it meets
- Wall thickness and draft on every face
- Machining allowance on the faces that will be cut
- Pressure tightness or porosity class
The point is not paperwork. Each line removes a decision the shop would otherwise make for you. A complete RFQ comes back priced. An incomplete one comes back guessed.
What actually drives the price
Buyers usually push on material because it is the line they can see. In practice the material is rarely the biggest number on a quote. These five lines decide most of it:
- die steel and cavity count
- part weight and wall section
- draft and slide count
- porosity requirement
- machining allowance
Ask for the quote split by line. A supplier who cannot break it down is guessing, and you will pay for the guess. The line you can usually renegotiate is the tolerance band, not the raw material.
Problems you will meet, and the fix
Every process has a failure mode it prefers. The same four show up again and again on this kind of work, and none of them are mysteries once you know what to look for.
- Cold shuts where two flow fronts meet
- Porosity that shows up only after machining
- Soldering on a hot spot in the die
- Shrinkage voids in a heavy section
Catch the fault on the first part, not the hundredth. A first-article check costs an hour. A bad batch costs the whole order. Write the check into the purchase order so nobody has to remember it.
What to check on the first article
The first article is the cheapest inspection you will ever buy. Use it properly and the rest of the run becomes boring, which is exactly what you want.
- Every dimension on the drawing, with the measured value written beside it.
- The material certificate, matched to the heat or lot number on the parts.
- The finish against the sample you approved, under the same light.
- Fit against the mating part, assembled rather than measured.
- Packaging, because parts that arrive damaged were never in tolerance.
Check in this order: datums, then mating features, then cosmetic faces. If the datums are wrong, every number after them is noise. Keep the report on file. When a problem shows up in month six, that document is the only thing that tells you what changed.
How to compare two quotes fairly
Two numbers are not comparable unless the scope matches. Line them up before you decide, because a low number with three gaps is not a low number.
Same drawing, same material, same finish, same inspection. Anything else is a different job wearing the same part number. Then look at what each quote leaves out. Freight, packaging, certificates and tooling are the usual gaps.
Ask each supplier to state their assumptions in writing. The one who does is usually the one who has run the process before.
Scaling from one part to a batch
The first part teaches you the process. The hundredth teaches you the process cost.
Watch three things as volume grows: setup amortisation, tool wear, and how much hand work the finish still needs. If the unit price barely moves between ten and a hundred, the cost sits in material or in hand work, not in setup. Ask which, because only one of them is negotiable.
Lead time behaves the same way. A one-off is scheduled around other jobs. A repeat order is scheduled around your forecast. Give the shop a forecast and the lead time gets shorter, not longer.
How to read a supplier's quote
A quote is a short document with a long shadow. Four things in it decide whether the job runs smoothly, and only one of them is the price.
- The scope line. It should name the process, the material and the finish, not just the part.
- The tolerance basis. General tolerance or the drawing, and which standard it follows.
- The exclusions. Tooling, freight, certificates and inspection are the usual omissions.
- The validity date. Material prices move, and an open-ended quote is a promise nobody can keep.
Read the exclusions before you read the total. A quote that leaves out tooling is not cheaper. It is a quote for a different job.
Industry Experience: Fixing Cold Material
An automotive plant had 15% scrap from cold material in engine brackets. Temperature logs showed mold temperature dropping to 160°C in deep cavities—well below the 200°C target. Adding zone-specific electric heaters to those cavities raised temperature to 210°C. Scrap dropped to 3%.
An electronics manufacturer struggled with cold material in 1mm phone frames. Injection speed was 2 m/s. Increasing to 4.5 m/s with a fast-slow-fast profile cut filling time from 1.2 seconds to 0.6 seconds. Cold material disappeared.
A magnesium component maker used 50% return material without screening. Oxide inclusions acted as nucleation sites for cold material. Installing a 50μm ceramic filter and reducing return to 30% cut defects from 12% to 2%.
Die casting cold material is preventable with systematic control. It forms when temperature drops, mold design blocks flow, process parameters mismatch, or material quality suffers. Fix it by building precise temperature control—double furnaces, heated transfer, preheated chambers, zone-specific mold heating. Optimize mold design with gradual runner transitions, thermal inserts, proper cooling channel spacing, and controlled release agent. Adjust process parameters dynamically—three-stage injection speed, sensor-triggered pressure holding, exact pouring volume. Tighten material management with composition testing, screened return material, and thorough degassing. With these steps, you can hold cold material defects under 2%.
Our solution for Die Casting Cold Material
YiguProto runs die casting in house, from a single prototype to a production batch. We quote the process, the material and the finish as separate lines, so you can see where the money goes instead of arguing with one number.
Send a STEP file or a drawing and you get a DFM report in one to two business days. It flags the features that add cost and the features that will not hold tolerance on the process you picked.
Every order ships with an inspection report on the features you call out. If a part misses, we make it again. Ready to price a part? Start with a free quote and DFM review, or read more about our die casting. You can also talk to an engineer before you commit to a drawing.
FAQ about Die Casting Cold Material
Is there a minimum order quantity?
No hard minimum. One part is fine. What changes with volume is which costs dominate, so a single unit is priced mostly on setup and a thousand units are priced mostly on cycle time and material.
How much does Die Casting Cold Material cost?
Price comes from five lines: material, machine or tool time, setup, finishing and inspection. The material itself is usually the smallest of them. Send a drawing and we will split the quote so you can see which line is doing the damage.
Does the process work for prototypes as well as production?
Yes, and that is usually the point. Running the prototype on the production process removes a whole class of surprises, because the part you test is made the same way as the part you sell.
Can Die Casting Cold Material be made in small quantities?
Yes, and small quantities are where a good supplier earns the next order. Setup dominates the price at low volume, so the cheapest route is usually the one with the fewest operations, not the cheapest material.
What files do you need in order to quote?
A STEP file plus a PDF drawing that carries the same revision letter. The model carries the geometry and the drawing carries the intent. When the two disagree, the shop picks one, and it is never the one you meant.
What is the lead time for Die Casting Cold Material?
Expect 20 to 30 business days including die build. Tooling, material stock and the finishing queue all move that number, so we confirm the date in writing before the run starts rather than after.
The short version
Die Casting Cold Material rewards the buyer who reads the data sheet and the drawing together. Pick the option for the property that matters, not for the price per kilo.
Tighten only what mates. Test a sample before the run. Then let the process do the rest.