What Are Die Casting Cold Partitions and How to Prevent This Defect?

What Exactly Are Die Casting Cold Partitions?

Basic definition

Cold partitions are defects where molten metal splits into multiple streams during filling. Each stream cools independently. When they meet again, they are too cold to bond properly. A distinct separation line remains on the casting surface.

Unlike surface scratches, cold partitions are not just cosmetic. They often extend into the casting interior, creating weak planes that compromise structural integrity.

What they look like

On the surface, cold partitions show as:

• Irregular linear seams—often curved or zigzagged with smooth, rounded edges
• Dull, matte appearance along the seam with no metallic luster
• Localized depressions or grooves adjacent to the seam

Under microscopic examination, you see:

• Incomplete fusion between metal streams—a visible gap
• Concentrated pores or shrinkage voids near the partition line
• Reduced density along the seam compared to normal areas

Metallographic analysis with 5% nitric acid etching reveals these features clearly. Ultrasonic testing can detect internal extensions of the partition.

Why they matter

Cold partitions create serious risks:

Mechanical degradation—Tensile strength drops 25-40% along the partition. Fatigue life shortens 50-70%. An aluminum suspension bracket with a cold partition may crack under normal driving loads.

Functional failure—For pressure components like hydraulic cylinders, cold partitions cause leakage. Incomplete fusion creates tiny channels that let fluids escape.

Production losses—Even a 5% defect rate increases costs by 15-20% from material waste, rework, and delays.

Reputational damage—Defective parts reaching customers can trigger recalls. A 10,000-part recall costs millions in replacement, legal fees, and lost trust.

What Causes Cold Partitions?

Temperature control failures

Low pouring temperature below the alloy’s liquidus increases viscosity. Metal loses fluidity quickly and cannot fuse when streams meet.

Insufficient mold preheating lets the mold absorb heat from the metal, cooling stream surfaces rapidly.

Excessive heat loss in runners—long, uninsulated runners allow metal to cool before reaching the cavity.

A real example: An ADC12 plant set pouring temperature at 650°C—10°C below liquidus. Cold partitions jumped from 2% to 15% in a week. Defective parts failed tensile tests.

Mold and gating design flaws

Unreasonable runner layout with sharp bends or sudden cross-section changes disrupts flow, splitting metal into multiple streams.

Improper inner gate placement causes chaotic flow and conflicting directions. Streams fill at different times and cool separately.

Inadequate exhaust traps gas that acts as a barrier between metal streams, preventing fusion even if they remain hot.

A zinc toy manufacturer had a 90° sharp bend in the main runner. Cold partitions formed at the bend in 30% of castings. Redesigning with a 15mm radius and adding an auxiliary gate cut defects to 1.5%.

Process parameter mismatches

Slow injection speed prolongs filling time. Metal contacts cold mold and runner walls longer, cooling stream fronts excessively.

Insufficient injection pressure cannot push cooled streams together for fusion.

Excessive release agent creates a thick insulating film that reduces heat transfer between merging streams.

An automotive supplier used 1.8 m/s injection speed for 2mm aluminum brackets. Cold partitions appeared in 22% of castings. Increasing speed to 4.2 m/s and cutting release agent 30% eliminated the defect.

Material issues

Poor alloy fluidity—low silicon in aluminum alloys means metal cools quickly and loses fusion ability.

Contaminated raw materials with oxides, impurities, or moisture create barriers between merging streams.

High return material ratio—oxidized, cold return material lowers overall melt temperature.

A magnesium plant mixed 40% unscreened return material with oxide scales into new ingots. Cold partitions rose by 18%. Reducing return to 20% and adding a 50μm ceramic filter cut defects to 3%.

How Do You Prevent Cold Partitions?

Step 1: Optimize temperature control

Molten metal temperature: Set pouring temperature 10-20°C above liquidus. For ADC12 aluminum, that is 680-700°C. For ZAMAK 5 zinc, 450-470°C.

Use a double-furnace system. Main furnace at higher temperature ensures complete melting. Holding furnace maintains precise pouring temperature. Install online infrared thermometers with ±2°C accuracy. Trigger alarms if deviation exceeds 5°C.

Mold preheating: Heat molds to recommended ranges—180-250°C for aluminum, 120-180°C for zinc, 220-280°C for magnesium. Use zone-specific heating to keep deviation under ±10°C.

Add extra heating elements in cold spots—deep cavities, thin-walled sections—to prevent localized cooling.

Runner insulation: Insulate runners with ceramic sleeves of ≤0.5 W/m·K thermal conductivity. For runners over 300mm, add electric heating tapes to maintain temperature 50-80°C below pouring temperature.

Step 2: Redesign mold and gating system

Runner optimization: Use streamlined designs with gradual transitions—taper angle 1-3°. Large-radius bends of ≥10mm prevent flow disruption. Reduce cross-sectional area gradually from main runner to inner gate at a 1:0.8 ratio to maintain consistent velocity.

For multi-cavity molds, use balanced runner layouts so metal reaches each cavity simultaneously. Verify with CAE simulation (MAGMA, AnyCasting).

Inner gate design: Position gates to create a single, continuous filling stream. Avoid placing gates opposite each other—that causes conflicting flows. Keep gate length ≤5mm to minimize heat loss. For thin walls under 2mm, use fan-shaped gates to distribute metal evenly.

Exhaust enhancement: Add serpentine exhaust grooves at last-filling positions—depth 0.1-0.15mm, width 5-8mm. Total exhaust area should be at least 1/3 of inner gate area.

For complex castings, use vacuum exhaust with vacuum over 90kPa to eliminate gas barriers between streams.

Step 3: Adjust process parameters

Injection speed: Use a two-stage profile. Initial slow at 1-2 m/s fills runner without splashing. Then fast at 4-6 m/s fills cavity quickly. Total filling time should be ≤2 seconds for parts under 200mm.

Injection pressure: Set specific pressure to 80-120MPa. For complex castings with multiple flow paths, increase by 10-15%. Maintain pressure until metal at the inner gate solidifies—holding time 5-10 seconds.

Release agent: Use low-volatile, high-temperature resistant types like graphite-based. Apply thin, uniform film of 5-10μm with automatic spray systems. No visible droplets or thick layers.

Step 4: Control material quality

Alloy selection: Choose alloys with good fluidity. For aluminum, silicon at 11-13% (ADC12) or 7-9% (A380). For zinc, ZAMAK 5 with 4% aluminum flows well. Add 0.1-0.2% rare earth elements to aluminum to improve fluidity by 15-20%.

Test every batch with spectral analysis. Reject batches with composition deviations over ±0.5%.

Raw material management: Store ingots in dry environment with humidity under 60%. Preheat to 120-150°C before melting to remove moisture.

Screen return material with 1mm mesh to remove oxide scales and impurities. Limit return ratio to ≤30% mixed with 70% new ingots.

Molten metal refining: Use argon rotary degassing for 15-20 minutes at 2-3 L/min to remove hydrogen and oxides. Filter through ceramic filters of 50-80μm pore size before pouring.

How Do You Diagnose Cold Partitions Quickly?

Step 1: Visual inspection

Look for linear seams with dull edges. Tap the area with a small hammer. A dull, hollow sound indicates cold partition. A clear, resonant sound means normal metal.

Step 2: Microscopic verification

Take a small sample from the suspected area. Polish and etch with 5% nitric acid. Under 100x microscope, a cold partition appears as a distinct gap or incomplete fusion line between metal grains.

Step 3: Parameter review

Check recent data:

• Did molten metal temperature drop below set range?
• Was injection speed or pressure lower than normal?
• Did mold temperature in defect area fall below target?
• Was there a change in raw material batch or return ratio?

What Emergency Actions Work?

If cold partitions appear during production:

Adjust temperature: Increase pouring temperature by 10-15°C within safe range. Raise mold preheat by 20-30°C. Test 10-20 samples to verify.

Tweak parameters: Increase injection speed by 0.5-1 m/s up to maximum safe limit. Raise pressure by 10-15%. Reduce release agent by 20-30%.

Clean mold and runner: Remove residual oxide scales or cold metal fragments. For molds with sharp bends, temporarily add auxiliary exhaust holes of 0.5-1mm diameter at last-filling positions.

Adjust material: If return ratio is high, reduce to 20% and add new ingots. If composition is off-spec, add necessary elements to restore fluidity.

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.

Industry Experience: Fixing Cold Partitions

An automotive supplier had 18% cold partitions in brake calipers. Temperature logs showed mold temperature dropping to 160°C in deep cavities—well below the 220°C target. Adding zone-specific electric heaters to those cavities raised temperature to 230°C. Defects dropped to 2%.

An electronics manufacturer struggled with cold partitions in 1.5mm phone frames. Runner design had a sharp 90° bend. Redesigning with 15mm radius and adding an auxiliary gate cut filling time by 40%. Cold partitions disappeared.

A hydraulic component maker used 40% unscreened return material. Oxide inclusions blocked stream fusion. Installing a 50μm ceramic filter and reducing return to 25% cut defects from 15% to 2%.

Die casting cold partitions are preventable with systematic control. They form when temperature drops, mold design disrupts flow, process parameters mismatch, or material quality suffers. Fix them by optimizing temperature control—set pouring temperature 10-20°C above liquidus, preheat molds properly, insulate runners. Redesign gating systems with streamlined runners, proper gate placement, and adequate exhaust. Adjust process parameters—two-stage injection speed at 4-6 m/s, pressure at 80-120MPa, thin release agent films. Control material quality with proper alloy selection, screened return material, and thorough refining. With these steps, you can hold cold partition defects under 2%.

Our solution for Die Casting Cold Partitions

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 Partitions

The short version

Die Casting Cold Partitions 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.