Die casting is a manufacturing process in which molten metal is injected into a metal mold cavity under high speed and high pressure, then solidifies quickly under pressure.

In engineering drawings, purchasing documents, and supplier materials, Al die cast is commonly used as a short form of aluminum die casting or aluminum die-cast parts. It usually refers to the aluminum die casting process or aluminum die cast components.

Aluminum die casting is widely used for medium- to high-volume production of complex parts with good dimensional consistency. It is common in automotive, telecommunications, industrial automation, consumer electronics, medical equipment, and thermal management applications.

Although die cast parts usually offer good surface quality and stable dimensions, defects can still occur when part design, mold design, molten metal condition, or casting parameters are not properly controlled.

Below are 12 common Al die cast defects and their main causes.

1. Flow Marks

Appearance

Flow marks appear as streaks or lines on the casting surface, often following the direction of molten metal flow. In some cases, the surface may show irregular patterns with a slightly different color or texture from the surrounding metal.

These defects usually do not grow over time, but they can affect appearance, painting quality, and surface uniformity.

Main Causes

Mold temperature or molten aluminum temperature is too low
Filling speed is too slow, causing the metal to cool too early
Gate position or flow direction is not ideal
Poor venting
Too much release agent or insufficient drying after spraying

2. Heat Checking Marks

Appearance

The casting surface shows hairline, net-like, or spider-web patterns, sometimes as fine raised or recessed marks. As the die is used repeatedly, these marks may become more obvious and spread.

These marks usually come from thermal fatigue cracks on the die cavity surface, which are then transferred to the casting.

Main Causes

Repeated heating and cooling of the die causes thermal fatigue
Uneven die preheating
Excessive local cooling or large temperature differences
Improper die material, heat treatment, or surface maintenance

3. Cold Shut

Appearance

Cold shut appears as a narrow, irregular, and slightly recessed line on the casting surface. In some cases it remains on the surface only, while in others it extends deeper into the part.

At the cold shut location, two streams of molten metal have met but failed to fuse completely, which can reduce strength and sometimes lead to cracking.

Main Causes

Two metal streams meet after their temperature has dropped too much
Molten metal temperature or die temperature is too low
Filling speed is too slow and filling time is too long
Flow path is too long or the wall section is too thin
Runner and gate design is not suitable

4. Sink Marks

Appearance

Sink marks appear as smooth depressions on thicker sections of the casting, such as bosses, rib roots, or areas with major wall thickness changes.

They are surface shrinkage defects and are different from internal shrinkage porosity.

Main Causes

Large differences in wall thickness
Local hot spots in thick sections
Insufficient intensification pressure or holding time
Local die temperature is too high
Cooling layout is not balanced
Die opens too early

5. Ejector Pin Marks and Parting Line Marks

Appearance

The casting surface may show round ejector pin marks, local raised or sunken areas, step marks, or visible mismatch at the parting line.

These defects may appear at ejector locations or at joints between the moving and fixed die halves, slides, or inserts.

Main Causes

Worn ejector pin tips or inconsistent ejector pin heights
Poor ejector pin layout or insufficient ejector support area
Excessive sticking force between the casting and the die
Poor fit between moving and fixed die halves
Loose slides, inserts, or other moving components
Insufficient die alignment accuracy

6. Die Soldering and Adhesion Marks

Appearance

Small patches of metal or non-metallic material appear attached to the casting surface. After these attached materials fall off, the surface may look shiny, dark gray, rough, or locally torn.

When aluminum sticks to the die cavity surface, the defect is often called die soldering.

Main Causes

Residual aluminum remains on the die cavity surface
Local die temperature is too high
Molten metal repeatedly strikes the same area at high speed
Release agent coverage is insufficient
Die surface is rough or its surface layer is damaged
Molten aluminum contains oxides or other contaminants

7. Lamination and Flakes

Appearance

The casting shows visible layered metal, thin flap-like structures, or local areas that may peel. Some laminated defects are not obvious on the raw casting and only become visible after machining or destructive examination.

Main Causes

The first metal entering the cavity has already partially solidified
Later metal flows over the partially solidified layer
Metal filling is unstable or includes backflow
The shot plunger movement is unstable
Runner system design is not appropriate
There is too much oxide film in the molten metal

8. Drag Marks

Appearance

Drag marks appear as rough streaks, scratches, scuffing, or torn surface areas in the ejection direction.

In severe cases, aluminum may stick to the die surface, causing the same defect to appear repeatedly on later castings.

Main Causes

Draft angle is insufficient
Die surface is rough or has aluminum build-up
Local sticking force is too high
Ejection is uneven
Slide movement is not smooth
Release agent is insufficient

9. Erosion

Appearance

Local areas of the casting surface show pits, rough texture, or raised irregular patterns. These defects usually correspond to areas where molten metal strikes the die surface at high speed.

As production continues, die erosion becomes more severe and the defect tends to become more obvious.

Main Causes

Gate position or direction is not suitable
Molten metal directly impacts the core or die cavity surface
Gate velocity is too high
Local die cooling is insufficient
Molten metal temperature remains too high for long periods

10. Cracks

Appearance

Cracks appear as fine, narrow, straight, or wavy lines on the casting surface. Some cracks continue to grow during handling, machining, or assembly.

Cracks in die cast parts may include hot cracks, cold cracks, ejection cracks, or internal cracks exposed after machining.

Main Causes

Alloy composition or molten metal quality is unstable
Wall thickness changes too sharply
Sharp corners, bosses, or holes create stress concentration
Time in the die is too short
The casting is held too tightly in the die
Ejection positions are not well designed or ejection force is uneven
Temperature difference between die areas is too large

11. Gas Porosity and Blisters

Appearance

Gas porosity appears as round, oval, or irregular pores inside the casting or on machined surfaces.

Some pores are invisible on the raw casting and only appear after CNC machining, polishing, sandblasting, or heat treatment. In some cases, trapped gas expands at high temperature and causes surface blisters.

Main Causes

Air is trapped during mold filling
Venting or vacuum performance is insufficient
Slow shot speed is too high
Metal flow is turbulent
Release agent or moisture has not fully evaporated
Degassing is insufficient
Hydrogen content in the molten aluminum is too high
Runner design causes jetting and turbulence

12. Flash and Burrs

Appearance

Thin excess metal appears at the parting line, slide area, insert joints, ejector areas, or vent locations.

Very thin flash often turns into sharp burrs, which affect appearance, assembly, and post-processing cost.

Main Causes

Clamping force is insufficient
Injection pressure is too high
There is aluminum debris or foreign matter on the parting surface
Parting surfaces, slides, or inserts are worn
Die fitting clearance is too large
Die rigidity is insufficient or the die is locally deformed

Common Inspection Methods for Aluminum Die Casting Defects

Different defects require different inspection methods, but not every part needs advanced testing.

Visual Inspection

This is the most common method for checking flow marks, sink marks, flash, drag marks, ejector marks, and visible cracks.

Dye Penetrant Inspection

This method is mainly used to detect open surface cracks and some surface-connected cold shuts on aluminum die cast parts.

X-Ray or Industrial CT

These methods are suitable for internal defects such as gas porosity, shrinkage porosity, inclusions, and laminations. Industrial CT provides more complete 3D internal information, but at a higher cost.

Leak Testing

This is commonly used for liquid cooling housings, valve bodies, pump bodies, and other sealed cavities to confirm whether through defects such as porosity, cold shuts, or cracks cause leakage.

Sectioning and Metallographic Analysis

These methods are usually used for defect analysis, especially for pores, shrinkage, flakes, cracks, and internal structure evaluation.

Which Defects Matter Most for Different Parts

For standard cosmetic parts, the main concerns are flow marks, drag marks, flash, ejector or parting marks, and heat checking marks.

For parts that require CNC machining, the main concerns are internal porosity, shrinkage, laminations, and surface defects exposed after machining.

For structural parts, the key concerns are cracks, cold shuts, internal shrinkage, and mechanical integrity.

For liquid cooling housings, valve bodies, and other sealed components, gas porosity, cold shuts, and cracks are especially important, and leak testing is often required.

Because of this, defect acceptance standards for Al die cast parts should be based on the actual function, assembly position, and quality requirement of the product, not only on surface appearance.

FAQs

What does Al die cast mean?

“Al” is the chemical symbol for aluminum and is also a common abbreviation for the word aluminum. “Al die cast” usually refers to aluminum die casting or aluminum die-cast parts.

In formal technical writing, more common expressions include aluminum die casting, aluminum die-cast parts, and die cast aluminum parts.

Why do aluminum die cast parts easily develop porosity?

In high-pressure die casting, the filling speed is very fast. If molten metal flow is unstable, venting is poor, or the molten aluminum contains too much gas, air can become trapped inside the casting and form porosity.

Why does porosity appear only after CNC machining?

Some pores are located below the casting surface and cannot be seen on the raw part. After machining removes the surface layer, those internal pores are exposed.

What is the difference between a cold shut and a crack?

A cold shut is formed when two streams of molten metal meet but do not fuse completely. A crack is a material separation caused during solidification, shrinkage, ejection, or later loading.

Does wall thickness affect die casting quality?

Yes. Thin walls can lead to incomplete filling and cold shuts, while thick walls can lead to sink marks and internal shrinkage. Sudden wall thickness changes can also cause stress concentration and cracking.

Conclusion

Common Al die cast defects include flow marks, cold shuts, sink marks, die soldering, cracks, porosity, and flash.

These defects may be related to part design, die design, molten metal quality, injection parameters, venting condition, and die temperature. In many cases, one defect can result from several factors working together, so the cause cannot be judged from the defect name alone.

In actual production, it is important to analyze the defect together with its location, metal flow direction, die structure, and process parameters in order to identify the real source of the problem and maintain stable casting quality.