Supplier Porosity Defects: Causes, Verification, and Corrective Actions

Inconsistent supplier porosity defects disrupt production, increase scrap rates, and complicate quality planning. When a supplier delivers parts with holes in some batches and clean parts in others, the problem is rarely random. It points to a specific process variable, a maintenance gap, or a change in material flow.

This guide walks through the symptoms, likely causes, and fixes for supplier porosity defects. It also covers how to prevent quality variation before it becomes a recurring issue on your line.

What symptoms should you look for?

Porosity defects do not always look like a single, obvious hole. They can appear in several ways, depending on where the defect forms and how the part is handled. The visual appearance is the first clue you get about the underlying mechanism.

Symptom	Likely cause	What to do
Small, scattered pinholes on flat surfaces	Gas entrapment in the mold or insufficient degassing	Review mold cleaning and alloy degassing logs. Ask for a mold maintenance schedule.
Larger voids near the parting line	Mold wear, debris in the cavity, or slow fill	Request a cavity inspection. Check if the supplier has replaced or reconditioned the die.
Porosity concentrated in one area of the part	Design or flow issue, such as a thin section or cold flow	Review the casting design. Ask if the supplier adjusted the shot parameters or cooling pattern.
Inconsistent defects across batches	Unstable process, material variation, or operator error	Require a root cause analysis. Check if the supplier changed alloy lots, mold operators, or shot settings.
Surface roughness near pores	Gas expansion or poor venting	Ask for a venting inspection. Request a comparison of the part with a known-good sample.

Consider a typical automotive bracket. If you see small pinholes scattered across a flat mounting surface, the issue is likely gas entrapment. The mold may not be clean, or the alloy may have retained too much hydrogen during the melting process. If the holes are larger and sit right at the seam where the two halves of the die meet, the mold itself is the suspect. Wear at the parting line creates a gap that traps gas as the metal fills the cavity.

If porosity is concentrated in a thin section, such as a rib or a flange, the problem is often flow-related. The metal may be cooling too quickly before it fully fills the area, or the gas may be trapped in a cold spot. In these cases, the defect is not random. It follows the geometry of the part.

If you see a mix of these symptoms, the cause is likely more than one factor. A worn mold may allow gas to escape, but if the alloy is not degassed properly, the gas can still become trapped. You must look at the whole system, not just one component.

Is the defect coming from the mold or the material?

The location of the defect often tells you where to start. If pores are clustered near the parting line or in a specific region, the mold is a strong suspect. If the defects are spread across the part, the material or process is more likely.

A worn die can create small gaps at the parting line. Those gaps allow gas to remain in the cavity. When the metal solidifies, the gas becomes trapped. The result is a pore that looks like a small hole or a shallow depression. Over time, the die face wears down. Microscopic pits and scratches accumulate. These surface imperfections act as nucleation sites for gas bubbles. The metal fills the cavity, but the gas has nowhere to go.

Material quality also matters. If the supplier switches alloy lots without notifying you, the gas content can change. Some lots may have more hydrogen or other gases that do not escape during filling. That can cause pinholes even if the mold is in good condition. The alloy comes from different scrap sources or different melting temperatures. Each variable affects the gas content. If the supplier does not control these variables, the quality of the finished part will fluctuate.

Ask the supplier for the following:

1. The mold maintenance log for the last three months.
2. The alloy lot numbers used for each batch.
3. The shot parameters, including pressure, temperature, and timing.
4. Any recent changes to the die, such as reconditioning or cavity replacement.

These documents give you a baseline. Without them, you are guessing. With them, you can correlate the defect with a specific event. Did the defect start after the die was reconditioned? Did it appear when the supplier switched to a new alloy lot? The data will point you to the root cause.

How do you verify the cause with the supplier?

A supplier may tell you the defect is “normal” or “within tolerance.” Do not accept that without evidence. Ask for a sample of the affected part and a sample of a good part from the same die and the same alloy lot.

Compare the two parts side by side. Look for differences in surface finish, color, and shape. If the good part has a smooth surface and the bad part has roughness near the pores, the issue is likely gas-related. The gas expanded after the metal solidified, pushing the surface outward and creating a rough texture. If the good part and bad part look identical except for the pores, the issue may be a localized mold defect. In this case, the gas is trapped in a specific spot because of the mold geometry.

You can also ask for an X-ray or CT scan of the defect. This will show the internal structure of the pore. A small, round pore often points to gas entrapment. The gas bubble formed in the metal and remained there until solidification. A larger, irregular void may indicate a mold cavity issue or a flow problem. The void may have formed because the metal did not fill the cavity completely, or because gas was trapped in a dead zone of the die.

If the supplier cannot provide this data, treat that as a red flag. A supplier that is confident in its process should be able to show you the evidence. If they cannot, they do not understand their own process. You need a supplier that can explain the physics of the defect, not just the result.

What fixes should you require?

Once you identify the cause, the fix depends on the problem. If the mold is worn, the supplier should recondition or replace the die. If the alloy is the issue, the supplier should switch to a lot with lower gas content or increase degassing time. If the process is unstable, the supplier should lock in the shot parameters and train the operators.

Reconditioning a die involves grinding or polishing the die face to remove wear and micro-pits. This restores the surface finish and reduces the likelihood of gas entrapment. Replacing the die is more expensive but provides a longer-term fix. If the die is near the end of its life, reconditioning may only buy a few months of production.

If the alloy is the issue, the supplier must control the melting process. Degassing involves removing dissolved gases from the molten metal. This is usually done with a flux or a rotary degasser. The time and pressure of degassing must be controlled. If the supplier skips this step or reduces the time to save money, the gas content increases. Pinholes follow.

If the process is unstable, the supplier must tighten its controls. Shot parameters include pressure, temperature, and timing. Small changes in these variables can have a large impact on the quality of the part. If the supplier changes these settings without approval, the quality will vary. The supplier should lock in the parameters and require operator sign-off for any changes.

Ask the supplier to document the fix. A corrective action report should include:

1. The defect description.
2. The root cause.
3. The corrective action taken.
4. The verification method.
5. The date of the next check.

Do not rely on a verbal promise. A written corrective action report is your evidence that the supplier has addressed the issue. It creates a record that you can refer to in future audits and quality reviews.

How do you prevent quality variation?

Prevention is easier than troubleshooting. If you want to reduce supplier porosity defects, build quality checks into the purchase process.

Start with the RFQ. In your request for quotation, state the quality requirements clearly. Include the acceptable defect rate, the inspection method, and the documentation you expect. For example, you can require a first article inspection report and a monthly quality summary. The first article inspection report should include dimensional checks, surface finish checks, and any non-destructive tests. The monthly quality summary should show defect rates by part and by supplier.

Next, review the supplier’s quality system. Look for a written quality plan, a mold maintenance schedule, and a process control plan. These documents show whether the supplier has a system in place to keep quality stable. A quality plan should describe how the supplier identifies, measures, and improves quality. A mold maintenance schedule should specify when the die is inspected, cleaned, and reconditioned. A process control plan should define the shot parameters and the frequency of checks.

Finally, visit the supplier if possible. A short audit can reveal issues that are not obvious in documents. Check the mold storage area. Look for debris, rust, or worn parts. Watch a casting cycle. See if the operators follow the standard work instructions. If the operators are not following the instructions, the process is not stable. The supplier needs to retrain its staff.

When should you consider a different supplier?

If a supplier has a recurring supplier porosity defect, ask for a trend analysis. If the defect rate is increasing, or if the same defect appears in multiple batches, the supplier may not have the capability to fix it.

A supplier that can identify the cause, fix it, and prevent it is a good partner. A supplier that blames the design, the material, or your handling is not. Blame shifting is a sign that the supplier does not have control over its process. You need a supplier that takes ownership of the quality of the part it delivers.

If the supplier cannot provide evidence of the fix, or if the defect continues after the corrective action, it is time to evaluate other suppliers. A change of supplier may be costly, but a recurring defect can cost more in scrap, rework, and customer complaints. You must weigh the cost of switching against the cost of the defect. If the defect is causing line stoppages or customer returns, the cost of switching is justified.

How do you document the issue for your team?

When you receive a defective part, document it before you send it back to the supplier. Take photos. Mark the defect location on the part drawing. Record the lot number and the date.

Photos are critical. They show the size, shape, and location of the defect. The part drawing shows where the defect is in relation to the functional features. The lot number and date help the supplier trace the defect to a specific batch.

Send the documentation to the supplier with a clear request. Ask for a root cause analysis and a corrective action report. Set a deadline for the response. Without a deadline, the supplier may ignore the issue or provide a generic answer. A deadline forces them to prioritize the problem.

Keep a log of all supplier defects. This log will help you identify trends. If one supplier has a high defect rate, or if one part has a high defect rate, you can address it before it becomes a bigger problem.

A simple spreadsheet is enough. Use columns for supplier, part number, defect type, lot number, date, and corrective action. Review the log monthly. If you see a pattern, act on it. A pattern is a signal that the supplier needs more support, or that you need to change the process.