Aluminum Die Casting Defects: The 10 other miscellaneous issues

Die casting is a precise manufacturing process, but it is not without its challenges. Achieving a perfect part every time requires rigorous control over materials, machinery, and process parameters. When issues arise, they often manifest as specific, identifiable defects.

Common defects in aluminum alloy die castings are typically categorized into three main groups: surface defects, internal defects, and other miscellaneous issues. This article focuses specifically on 10 of these “other” defects. This guide provides a technical breakdown of these 10 common die casting defects detailing their observable phenomena, root causes, and recommended corrective actions to help engineers and quality control teams troubleshoot and improve production quality.

The 10 Other Miscellaneous Issues Of Die Casting Defects

1.Chemical Composition Not Met

Phenomenon: Spectrometric or metallographic analysis reveals that the alloy’s elemental composition or impurity levels are outside the required specifications.

Common Causes:

• Non-compliant raw materials (ingots) or scrap metal.
• Poor quality control or analysis of recycled materials.
• Improper addition or ratio of alloying elements.
• Incorrect sampling methods for testing.
• Improper melting processes (e.g., incorrect temperature, excessive burn-off, or prolonged holding times) altering element composition.
• Contamination from slag or improperly cleaned melting tools.

Analysis & Solutions:

• Implement strict incoming inspection (e.g., spectrometer analysis) for all raw materials and return non-compliant batches.
• Rigorously manage and analyze all scrap/recycled materials before use.
• Use correct, standardized sampling techniques for analysis.
• Strictly control the melting process, including temperature, and ensure timely slag removal.
• Adhere to strict operating procedures for cleaning and using melting tools.

2. Mechanical Properties Not Met

Phenomenon: The final casting fails to meet specified standards for strength, hardness, or ductility, often discovered during processing or assembly.

Common Causes:

• Incorrect chemical composition (e.g., levels of Si, Cu, Mg, Zn, etc., are off-spec).
• Internal casting defects such as porosity, shrinkage, or slag inclusions.
• Improper heat treatment or testing procedures.
• Unreasonable part structure design.
• Improper melting processes creating oxides or other compounds.

Analysis & Solutions:

• Strictly control the chemical composition, aiming for the median value within the specification range.
• Rigorously adhere to melting and die casting process parameters to avoid and minimize internal defects.
• Conduct process capability and heat treatment tests as required.
• Improve the casting’s structural design to avoid internal stress concentration.

3. Flashing

Phenomenon: After the injection cycle, excess material (flash) of a certain thickness appears on the mold’s parting line, which directly affects the part’s dimensions.

Common Causes:

• The mold’s parting line is dirty, with residual flash or other contaminants.
• Uneven or insufficiently tight mold clamping.
• Insufficient clamping force of the die casting machine.
• Mold deformation or misalignment.
• Uneven stress from the four clamp bars.
• Excessively high injection pressure.
• Worn or improperly fitted sliding blocks.

Analysis &Solutions:

• Ensure the mold parting line is clean and free of debris.
• Check mold for squareness and ensure even clamping.
• Recalculate and apply the correct clamping force based on the part’s projected area; upgrade the machine if necessary.
• Ensure the mold face is clean and the machine platens are parallel.
• Check clamp bar stress and adjust for uniformity.
• Reduce injection pressure to the lowest effective level.
• Repair or replace worn sliding blocks and check clearances.

4. Dimensional Deviation

Phenomenon: The finished part’s dimensions, geometric tolerances, or form do not match the engineering drawings or specifications.

Common Causes:

• Flawed part design that fails to account for alloy shrinkage or thermal expansion.
• Poor mold guidance (e.g., worn guide pins) or improper installation.
• Incorrect pressure-holding time (too long or too short).
• Movement or shifting of cores or slides during injection.
• Wear on sliding blocks or loose gibs.
• Improper mold pre-heating.
• Misalignment of the two mold halves.

Analysis & Solutions:

• Review part and mold design; use actual shrinkage data to modify the mold cavity dimensions.
• Inspect and verify mold guidance components (pins, bushings).
• Check and adhere to the specified holding time.
• Reinforce moving cores or add support pillars.
• Repair or replace worn slides and wear plates.
• Ensure proper mold alignment and pre-heating procedures are followed.

5. Poor Corrosion Resistance

Phenomenon: The die-cast part shows signs of early oxidation or corrosion during assembly or use.

Common Causes:

• Poor alloy composition (e.g., high Cu, low Mg, or high Ni content).
• The part did not receive a required surface protection treatment.
• The protective surface layer (e.g., plating, coating) was damaged during processing or handling.

Analysis & Solutions:

• Use spectrometry to strictly control the chemical composition, keeping it within standard range, ideally near the median.
• Apply an appropriate surface treatment (e.g., shot blasting, chrome plating, nickel plating) based on the part’s working environment.
• Strictly control the quality of any protective layer to prevent peeling, flaking, or omissions.

6. Poor Surface Coating Quality

Phenomenon: The part’s surface treatment (e.g., plating, painting) fails to meet standards, exhibiting peeling, improper texture, or incomplete coverage.

Common Causes:

• Peeling caused by poor pre-treatment or underlying casting defects (e.g., cold shuts, porosity).
• Inconsistent color or texture due to poor surface preparation (e.g., uneven polishing, flow marks).
• Peeling or shedding of paint due to poor paint quality or improper application (time, temperature).

Analysis & Solutions:

• Adjust the die casting process to eliminate underlying surface defects; strictly control all pre-treatment (cleaning, polishing) steps.
• For harsh environments, shot blasting or polishing is mandatory; for less severe environments, other pre-treatments may suffice.
• Change or adjust the paint/coating materials; adjust the application process parameters.

7. Soldering (Sticking to Mold)

Phenomenon: The alloy adheres to the mold cores, cavity, or the entire part sticks within the mold, failing to eject properly.

Common Causes:

• Poor mold manufacturing, especially insufficient draft angles (taper) in deep cavities or on cores.
• Improper or missing mold surface treatment (e.g., nitriding, oxidation).
• Incorrect mixing ratio of release agent (e.g., too much water).
• Insufficient or uneven application of release agent.
• Part design or gating issues that cause the part to stick.

Analysis & Solutions:

• Polish the mold surface or repair sticking areas; modify the mold to ensure a draft angle of at least 1.5°.
• Apply a proper surface treatment (nitriding, etc.) to the mold cavity and cores.
• Change to a higher-quality release agent and ensure correct mixing.
• Ensure release agent is sprayed evenly and sufficiently, especially in hot or sticking-prone areas.
• Adjust spray application or add/increase the size of ejector pins.

8. Core Shift / Hole Misalignment

Phenomenon: The position of a core is incorrect or the core itself is bent, causing misalignment. This can result in “black skin” (un-machined as-cast surface) being visible after machining a hole, or interference during assembly.

Common Causes:

• Errors in mold manufacturing (core position incorrect).
• Incorrect shrinkage rate used during mold design.
• Damage or wear on locating features, or part deformation.
• Bent core pins.
• Errors in the machining program.

Analysis & Solutions:

• Measure and adjust the core’s position in the mold.
• After the process is stable, measure parts to confirm the actual shrinkage rate.
• Control the production process tightly to prevent part deformation; repair locating surfaces.
• Check for and replace any bent cores.
• Review and adjust the machining program.

9. Broken Core Pin

Phenomenon: Cores, especially those for forming small round or complex-shaped holes, fracture or break off.

Common Causes:

• Severe soldering (sticking) on the core, causing high pulling forces.
• Poor core quality, insufficient strength, or improper heat treatment.
• Poor gate location that causes the molten alloy to directly impact the core.
• Part warps during ejection, putting angular stress on the core.
• The part sticks to the mold, and the core is broken during manual removal.

Analysis & Solutions:

• Increase release agent spray and polish the core; monitor for signs of soldering.
• Strengthen incoming inspection for cores; remake cores if necessary.
• Adjust the gate location or modify the gating structure to avoid direct impact.
• Use short, uniform ejector pins to prevent part warping.
• Control the process to prevent sticking.

10. Exposed As-Cast Surface After Machining (“Black Skin”)

Phenomenon: After a machining operation, the machined surface is not fully “cleaned up,” leaving patches of the original, dark as-cast surface.

Common Causes:

• A bent core pin.
• Incorrect or worn-out tooling datums/locators.
• Insufficient machining allowance designed into the mold.
• Part deformation (warping) causing inconsistent location during machining.
• Debris or alloy buildup on mold datums, causing the part to be dimensionally incorrect.
• Damage to the part (e.g., bumps, dings) during handling.
• Fixture failure or incorrect part loading during machining.

Analysis & Solutions:

• Promptly check and replace any bent cores.
• Repair the mold to increase the machining allowance.
• Repair the mold to fill in any low spots or correct datums.
• Increase holding time to reduce part deformation; correct any subsequent warping.
• Periodically check and clean the mold, especially datum surfaces.
• Ensure parts are protected from damage during transport and handling.

Conclusion

Successful die casting hinges on a deep understanding of the interplay between material science, mold design, and process control. By systematically identifying the root causes of die casting defects—from material composition to machine settings—production teams can implement targeted solutions, reduce scrap rates, and ensure the final product meets all quality and performance standards.