Checklist: Verifying Porosity Prevention in Supplier Quotes

What process controls does the quote specify?

Process control is the core of porosity prevention. A credible bid will name the variables they monitor, not just state that they “maintain quality.” Ask for specific numbers or ranges for injection pressure, mold temperature, and melt temperature.

A supplier that cannot provide these ranges is likely using a generic template. Generic language is a red flag. It suggests they have not tailored their quality plan to your part. For instance, if you are casting a thin-walled connector, the injection pressure profile differs significantly from a thick-bodied housing. The pressure curve must match the geometry to avoid cold shuts or gas entrapment. If the quote lists a single static pressure value, it lacks the nuance required for complex geometries.

Control Variable	Typical Check Point	Red Flag
Injection Pressure	Pressure profile recorded per shot	No mention of pressure data
Mold Temperature	Preheating schedule documented	No temperature range given
Melt Temperature	Furnace log kept for each batch	“Standard temperature” only
Gating System	Venting and gating design reviewed	No drawing or CAD file provided

If the quote lacks these details, request them before awarding the contract. A supplier that resists providing process data is likely hiding a lack of control. Look for evidence of data collection. Do they use a machine data historian? Can they pull a report for a specific heat number? If the answer is no, they are relying on intuition rather than measurement. Intuition fails when variables drift.

Melt temperature is particularly sensitive. Aluminum alloys, for example, have a narrow window where the liquid is fluid enough to fill the mold without absorbing excessive oxygen. If the furnace runs too hot, the melt oxidizes. If it runs too cool, the viscosity increases, making it harder to vent gas bubbles from the cavity. Ask for the target range and the tolerance. A range of plus or minus ten degrees is a reasonable starting point for many alloys, but it depends on the specific grade.

Mold temperature stability is equally critical. If the mold is too cold, the metal solidifies too quickly at the gate. This traps gas in the part. If the mold is too hot, the part may solidify slowly, leading to excessive shrinkage. The preheating schedule must be documented. Do they use a thermocouple probe? How often do they calibrate it? These are practical questions that separate a disciplined shop from one that runs by habit.

The gating system is the final variable. Gating and venting design must be reviewed for every new part. If the supplier provides a generic CAD file that has not been updated for your specific geometry, the vents may be insufficient. Gas has to go somewhere. If the vents are too small or blocked by oxide, porosity forms. Ask for the gating drawing. If they cannot provide it, they are not controlling the process.

How does the supplier quality audit cover die casting?

Many buyers skip the supplier quality audit because it feels like an extra cost. It is the cheapest way to prevent expensive defects. A quality audit should verify that the supplier’s internal controls match their bid promises.

Do not rely on the supplier’s self-report. Ask for their last internal audit report or a summary of corrective actions taken in the past year. If they have no audit history, that is a significant gap. A self-report is a statement of intent. An audit report is evidence of execution. The difference matters when a defect occurs. You need to know what they checked and when.

The audit should cover the following areas:

1. Mold Maintenance: Are inserts and cavities inspected for wear and porosity buildup?
2. Melt Handling: How do they prevent oxide formation and gas absorption?
3. Operator Training: Do operators know how to adjust the machine when porosity spikes?
4. Documentation: Is there a paper trail for every production run?

A supplier with a strong audit trail will welcome these questions. A supplier that is vague or dismissive is a risk. Look for specific findings in the audit report. Did they find a mold cavity with a crack? Did they find an operator bypassing a check? The details matter. A supplier that treats audits as a formality will have vague findings. A supplier that uses audits to drive improvement will have specific, actionable findings.

Operator training is often overlooked. A skilled operator can adjust the machine when porosity spikes. They can identify a change in the sound of the machine or a change in the appearance of the part. But only if they are trained to do so. Ask how often they train their operators. Do they use simulation or on-the-job training? Do they have a list of common defects and their causes? If not, their operators are flying blind.

Documentation is the backbone of the audit. Is there a paper trail for every production run? Do they keep records of machine settings, melt temperatures, and mold temperatures? If they do not keep records, they cannot prove their process was under control. You need a record that shows what happened when a defect occurred. Without it, you are stuck guessing.

What is the defect acceptance standard?

You need a written agreement on what counts as an acceptable porosity level. Without a defined standard, disputes are inevitable. The quote should reference a specific defect class or a written acceptance criterion.

For example, a buyer might specify that no isolated gas porosity larger than two millimeters is acceptable on a critical surface. Or they might require a maximum total porosity area per square inch. The standard must be measurable. “No porosity” is not measurable. “Less than 1% porosity by volume” is. The difference is critical.

If the quote does not define the acceptance standard, you are setting yourself up for a negotiation later. Define the standard in the contract. Make it clear which surfaces are critical and which are non-critical. A critical surface is one that affects function or safety. A non-critical surface is one that affects appearance only. The standard for each should be different.

Do not assume that “zero defects” is a standard. It is not achievable. Use a measurable standard. A clear standard also helps the supplier understand where their process needs to improve. If the standard is too loose, the supplier will not push for improvement. If it is too tight, the supplier will not be able to meet it and will scrap every part. You need a balance.

The standard should also define how porosity is measured. Do you measure it by area or by volume? Do you use X-ray or ultrasonic? The method matters. X-ray shows internal defects. Ultrasonic shows deeper defects. Dye penetrant shows surface defects. The method must match the defect type you are concerned about.

How do they handle root cause analysis?

When a porosity issue occurs, the supplier must have a process to find the root cause. A good quote will mention their corrective action process. Look for references to 8D reports, fishbone diagrams, or Pareto analysis.

If the quote only says they will “fix the problem,” that is not enough. You need to know how they will prevent the problem from recurring. Ask for examples of past root cause analyses. A supplier that has a history of porosity issues should be able to show how they solved them. If they cannot, they are likely to repeat the same mistakes. Request a sample 8D report or a summary of a past corrective action.

This also helps you understand their engineering capability. A supplier that only has quality inspectors but no process engineers is a weak link. You need people who can change the process, not just inspect the part. A process engineer can analyze the data and find the root cause. A quality inspector can only see the symptom.

The root cause analysis should be specific. Did the melt temperature drift? Did the mold vent block? Did the operator change the settings? The answer should be specific. “The process was not under control” is not a root cause. “The furnace controller failed and the melt temperature rose 15 degrees” is.

What testing methods are included in the bid?

Final inspection is a backstop, not a prevention tool. But it must be part of your verification. Ask what testing methods the supplier will use to detect porosity. X-ray, ultrasonic, and dye penetrant are common options.

The quote should specify which method is used and how often. For example, “100% X-ray on first article and 10% random sampling on production runs.” If the quote does not mention testing, assume it is not happening. You need a clear plan for detecting defects before they ship. The testing plan should match the criticality of the part.

High-value or safety-critical parts require more rigorous testing. Lower-risk parts may have different sampling rates. Make sure the bid reflects the risk profile of your specific part. A part that holds a seal requires different testing than a part that holds a screw. The seal part cannot have internal porosity. The screw part can.

The testing method must be validated. Do they have a calibration plan for the X-ray machine? Do they have a standard for the ultrasonic probe? If not, the test results are meaningless. A test without a standard is just a guess.

How do they manage mold changes and maintenance?

Molds are the heart of die casting. A well-maintained mold produces clean parts. A worn mold produces porosity. Ask about their mold maintenance schedule.

A credible supplier will have a schedule for cleaning, inspecting, and repairing molds. They should know the wear life of their inserts. They should have a plan for when a cavity starts to show porosity.

If the supplier does not have a mold maintenance plan, that is a major red flag. It suggests they are running the mold until it breaks. This leads to inconsistent quality and frequent production stops. Ask how they track mold usage. Do they count shots? Do they track hours? A simple shot counter is a basic but necessary control. Without it, they cannot manage mold life.

Mold changes are also critical. When a mold is changed, the process must be re-validated. The new mold may have different thermal properties. It may have different venting. The supplier must know this. If they change a mold without re-validating the process, they are introducing risk. Ask for their mold change protocol. Do they run a first article inspection? Do they check the melt temperature? Do they check the injection pressure?

What is their plan for rejected lots?

Every process produces some defects. The question is how they handle them. The quote should have a clear plan for what happens when a lot is rejected.

Do not accept a supplier that simply says they will “replace the parts.” You need to know the process. Do they sort the parts? Do they rework them? Do they scrap them?

A clear rejection plan protects your production line. It prevents defective parts from reaching your warehouse. It also shows the supplier takes quality seriously.

Ask for their scrap rate. A low scrap rate is a good sign. A high scrap rate may indicate a process that is not under control. Do not be afraid to ask for this number. It is a direct measure of process stability.

A supplier that is transparent about their scrap rate is more likely to be reliable than one that hides it. Transparency is a form of quality. Ask for the scrap rate for the specific part you are buying. Do not accept a general company scrap rate. The scrap rate for a thin-walled connector is different from the scrap rate for a thick-bodied housing. The specific number tells you more about the process than a general one.