Die Casting Leak Testing and Impregnation: How to Specify Pressure-Tight Parts

01“Can you leak test it?” is not the right question

A die-cast housing can look sound, pass dimensional inspection, and still leak through a microscopic connected pore. This is why pressure-tightness cannot be treated as a visual finish requirement or added as a final inspection after the quotation is approved. The part design, alloy, gating and venting, die temperature, machining plan, test method, and impregnation decision all influence the result. For a buyer, the key question is not simply “Can you leak test it?” It is “What pressure must the part contain, what leak rate is acceptable, where will the part be machined, and what happens when a casting fails?” A supplier that answers these questions early can design a realistic process. A supplier that only promises 100% testing may be detecting an avoidable process problem after value has already been added. This article explains pressure-tight die castings, leak-test methods, impregnation, purchasing mistakes, and supplier evidence.

02What pressure tightness means in die casting

According to the 2024 NADCA Product Specification Standards, pressure-tightness requirements add to die design and casting costs and should be specified only when they are necessary for the application. The customer should state the pressure the part must withstand and the testing method at the quotation stage. Pressure tightness is not a universal property of an alloy or a surface appearance; it is the result of a specific part, process, and acceptance method. Die-castings solidify from the outside toward the center. NADCA describes a typical dense skin about 0.015 to 0.020 inch (0.38 to 0.50 mm) thick. The center of a wall can contain gas or shrinkage porosity. Removing too much material during machining can break through this relatively dense skin and expose a connected leak path. This is why a part that passes as-cast testing may fail after drilling, boring, tapping, or milling.

03How leak paths form

Gas porosity is associated with entrapped air, vapor, and gas in the metal. Shrinkage porosity forms when liquid metal cannot feed a solidifying heavy section. Oxide films, cold shuts, parting-line defects, and cracks can also create connected paths. The important distinction is connectivity: an isolated pore may not leak, while a small chain of connected pores can carry gas or liquid through the wall. According to NADCA, zero porosity is virtually impossible to achieve in die casting. The customer and die caster should therefore agree on the size, nature, and location of permissible porosity. For pressure-tight areas, the practical target is not an unrealistic claim of “zero pores,” but a controlled process and a measured leakage result that meets the application requirement.

04Main pressure and leak-test methods

NADCA identifies several common methods, shown in the table. According to NADCA Guideline G-6-1, pressure-tightness testing for die castings is generally specified in the range of 5 to 40 psi (about 0.03–0.28 MPa). Higher pressures require special consideration and depend strongly on part design. The exact pressure, medium, stabilization time, test volume, temperature, fixture, and leak-rate limit must be written into the purchasing specification.

Common pressure and leak-test methods (NADCA)
Method	What it shows	Typical use
加压气泡法 / Pressurized air bubble test	液槽中可见泄漏的位置 / The location of a visible leak in a liquid bath	快速排查、定位泄漏通道 / Fast troubleshooting and locating a leak path
气体压降法 / Gas pressure-decay test	规定时间内的压力损失 / Pressure loss over a defined time	有可重复泄漏限值的量产筛选 / Production screening where a repeatable leak limit is defined
质量流量法 / Mass-flow test	以单位时间流量表示的泄漏量 / Leakage as flow rate per unit time	定量验收与自动化测试 / Quantitative acceptance and automated testing
氦检探头 / Helium detection probe	极低泄漏率与局部泄漏 / Very low leak rates and localized leakage	密封要求严苛的关键零件 / Critical parts with demanding sealing requirements

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05What impregnation does

Impregnation is a post-casting sealing process used when a part contains interconnected porosity that cannot be economically eliminated through design and process control. A liquid sealant is drawn into the leak path, commonly by vacuum and pressure, then excess material is removed and the sealant is cured or set. NADCA notes that anaerobic and methacrylate systems can produce sealed, pressure-tight aluminum castings with smooth surfaces. Impregnation is not a substitute for sound casting design and will not repair a large crack, grossly open passage, severe flash, or dimensional defect. Resin compatibility, cure condition, fluid exposure, coatings, and service temperature must be validated for the application.

06A leak test is a process control, and machining changes the result

If testing is performed only at the end of production, it may reject parts after machining, washing, coating, and assembly value have been added. A high reject rate can also hide drift in metal temperature, die temperature, fill conditions, vacuum, vents, or machining depth. The best programs use leak testing to monitor the process. NADCA recommends minimizing machining stock on pressure-tight castings because deep cutting can expose porosity. Machining both sides of the same pressure boundary is especially risky. A hole that is cored rather than drilled after casting may preserve more of the dense skin and reduce the chance of opening a leak path. The die caster needs the final machining locations, depths, and datums before die design is completed. Temperature, fixture leakage, dirty sealing faces, trapped water, unstable pressure, and insufficient stabilization can create false results. Test equipment needs calibration, master leak checks, and a borderline-result procedure. Impregnation adds cleaning, curing, re-testing, traceability, and sometimes masking. It can recover a controlled population, but should not hide poor yield. The buyer should know whether it is included, when it is applied, and how treated parts are identified.

07From our experience: locate the cause from where it leaks

Mark the leak paths on the part drawing and the cause usually announces itself. It is the first thing we ask for when a complaint comes in: a photograph or a marked-up drawing showing where it leaks, not just a bare “3% leak rate”. Position carries far more information than proportion. The table shows the correspondences we use when investigating. For control, in our experience there are three routes. The gating system: gate area calculated from what the fill needs, not chosen for easy break-off, with overflows placed at the last-to-fill regions and where flow fronts meet. Local squeeze: a pin at the hot spot that re-pressurises that region late in solidification, aimed squarely at shrinkage porosity. And impregnation — a remedy, not a design tool. On impregnation our position is that it must be declared, never done quietly: it goes into the control plan and onto the shipping documents. In our experience it is appropriate when the pores are micron-scale and scattered, do not affect mechanical performance, and the customer has accepted it in the technical agreement; it is not appropriate when the voids are larger, sit in a loaded region, or the customer forbids it.

From our troubleshooting: leak location, likely cause, countermeasure
Where it leaks	Most likely cause	Countermeasure
厚大部位 / 筋位交叉处 / Heavy sections, rib junctions	缩松（热节补缩不足）/ Shrinkage — hot spot not fed	结构掏空 + 局部挤压 + 增压提前 / Core out, local squeeze, earlier intensification
末端充填区 / Last-to-fill regions	卷气（排气不足）/ Entrapped air, insufficient venting	加溢流槽与排气道 / Overflows and vents
内浇口附近 / Near the gate	喷射与回流卷气、冲蚀 / Jetting, back-flow, erosion	改浇口方向与截面积 / Re-aim and resize the gate
分型面沿线 / Along the parting line	飞边根部未熔合、夹渣 / Unfused flash root, inclusions	检查锁模力与分型面磨损 / Check clamp force and parting-face wear
螺纹孔 / 深孔周边 / Around threaded or deep holes	型芯周边包气或冷隔 / Gas or cold shut around the core	调整型芯冷却与进浇顺序 / Adjust core cooling and fill sequence

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08Common mistakes

Saying “leak-proof” without a test definition. A supplier cannot quote a meaningful process without pressure, medium, hold time, temperature, fixture, and allowable leak rate. Specifying a very high test pressure because it sounds safer. NADCA notes that higher pressures require special consideration. The test should represent the service requirement and safety margin, not create an unrelated production barrier. Testing only before machining. Drilling, tapping, boring, or face milling can expose porosity that was hidden by the as-cast skin. The final test condition must match the delivered part. Treating X-ray as a direct leak test. Radiography can reveal internal discontinuities, but it does not directly measure whether a connected path leaks at the specified pressure. Use it as a complementary method with pressure testing. Using impregnation to cover a design problem. Heavy sections, abrupt wall changes, poor gating, inadequate venting, and uncontrolled thermal conditions should be corrected where practical before resin sealing is considered. Failing to distinguish a local leak from a structural defect. A small interconnected pore may be sealable. A crack, broken wall, or large shrinkage cavity may require machining, welding evaluation, insert replacement, or rejection.

09Our typical approach

An experienced supplier treats pressure tightness as a chain of controls: sound design, stable casting, controlled machining, validated testing, and a documented response to failure. Testing and impregnation are selected together with part function and service pressure. An impregnated part is clearly identified as a recovered part, not confused with a naturally sound casting. First, define the pressure boundary. We identify every cavity, passage, sealing face, threaded hole, machined surface, and fluid-contact area that must contain pressure. We confirm service pressure, test pressure, fluid or gas, operating temperature, and expected life. Second, review the part and die before quoting. We look for heavy sections, abrupt transitions, deep machining, parting lines across seals, and features that should be cored instead of drilled. We review gating, overflows, vents, cooling, and the location of critical porosity zones. In our experience half of a pressure-tight part's fate is decided on the drawing: keep sealing faces within one die half, keep heavy sections and rib junctions out from under sealing faces, and avoid running threaded holes through a sealed cavity. Third, plan machining and testing as one sequence. We define whether testing occurs as-cast, after trimming, after machining, after washing, or after coating. For a pressure-tight component, the final test should normally be performed after the operations that could expose the leak path. Fourth, select the test method to match the risk. Bubble testing is useful for finding a leak. Pressure decay or mass flow is better for numerical acceptance. Helium is reserved for very low leak-rate requirements. We specify test pressure, stabilization, test time, fixture, calibration, and reaction limits. Fifth, qualify impregnation before production use. If impregnation is part of the plan, we validate resin compatibility, vacuum and pressure parameters, drain and cure conditions, cleaning, masking, and retest performance. We also establish when a part is rejected rather than repeatedly impregnated. Sixth, use data to improve the process. We trend leak rate by cavity, machine, shift, alloy lot, machining operation, and repair status. A sudden change is investigated through porosity samples, sectioning, X-ray, and die or process checks rather than solved only by adding resin.

10From our projects: a motorcycle water pump housing

A motorcycle aluminum water pump housing we make is leak-tested on every finished part at 0.25 MPa for 30 seconds, with no leak allowed. During development the leak rate sat at 6.2% for a long time, with the leaks concentrated at the thin water-passage walls and the root of the mounting flange: near-surface gas porosity connected with shrinkage channels in the heavy sections and, under pressure, formed a through-path — invisible on the surface and exposed only on the test rig. We now control entrapped gas through optimised gating and venting, with local squeeze at the heavy sections; cooling is zoned to match the thick and thin regions; internal soundness is checked by X-ray sampling, and every finished part is leak-tested. In stable production the leak rate has fallen from 6.2% to below 0.5%.

11Questions buyers should ask

1. What service pressure and test pressure are required, and why were those values selected? 2. Is the acceptance criterion pressure loss, mass flow, bubble location, or helium leak rate? 3. What test medium, temperature, stabilization time, hold time, and fixture will be used? 4. Will testing be performed after all customer-specified machining and cleaning operations? 5. Which surfaces and passages are pressure boundaries, and where is machining most likely to expose porosity? 6. What porosity standard applies to critical areas, and is there an agreed X-ray or sectioning reference sample? 7. What design and process measures are used before impregnation: wall-section review, gating, venting, vacuum, thermal control, or machining limits? 8. Is impregnation included in the quotation? If so, what resin family, process, cure, masking, and retest are included? 9. How many impregnation cycles are allowed before rejection or engineering review? 10. How are test equipment calibration, master leaks, borderline results, and retests controlled? 11. What production records will be supplied for leak rate, cavity, shot, machining lot, impregnation status, and traceability? 12. Who owns the cost and decision when a part fails before machining, after machining, or after impregnation?

SOURCES AND NOTE

[1]North American Die Casting Association (NADCA). NADCA Product Specification Standards for Die Castings: Aluminum, Aluminum-MMC, Copper, Magnesium, Zinc and ZA Alloys, 12th edition, revised for 2024. Section 6, Guideline G-6-1-24, Pressure Tightness in Cast Parts, pp. 6-2 to 6-3; Section 7, Porosity and Pressure-Tight Castings, pp. 7-12 to 7-16.
[2]North American Die Casting Association (NADCA). NADCA Product Specification Standards for Die Castings: Aluminum, Aluminum-MMC, Copper, Magnesium, Zinc and ZA Alloys, 12th edition, revised for 2024. Section 3, Aluminum Alloy Data, p. 3-8: impregnation using anaerobic and methacrylate systems.
[3]North American Die Casting Association (NADCA). Gating Manual, Publication #512, 2006. Step 1, Determine the Casting Quality Requirements: leak-test requirements, porosity planning, and joint customer-supplier specification.
[4]North American Die Casting Association (NADCA). Introduction to Die Casting. Chapters 3 and 8: vacuum die casting, porosity, pressure-tight applications, and pressure testing.
[5]E. A. Herman. Die Casting Process, Engineering and Control. Sections on thermal control, die spray, porosity sources, and process monitoring.

This is a technical introduction and design guide, drawing on the sources listed above and on Jinxiong's many years of production practice. It is for reference only and is not a commitment for any specific project. For a specific project, the technical agreement between both parties governs.

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