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Pressure testing, or leak testing, CNC machined, cast parts is a common functional test performed on parts and assemblies for the automotive, aerospace and medical industries. Read on to learn our best tips for pressure testing, including how it works, when to pressure test, pressure testing specifications and pressure testing methods.
What is Pressure Testing?
通过密封测试样品（通常将软管连接到入口并密封出口）并提供加压流体（以固定的质量流速）来进行压力测试，以填充测试标本的内部空腔。液体的压力作用于内部腔内的墙壁，并压力材料。这是一项非破坏性测试，主要用于铸件。
铸造是一种制造过程，将熔融金属倒入霉菌腔中，从模具中冷却并提取。听起来可能很简单，但是铸造是一个相对复杂的过程和铸件的零件，以及随后的制造过程（例如焊接）或CNC加工的零件，必须在使用或释放给客户之前进行一系列质量保证测试。铸件容易受到几种潜在的缺陷，这些缺陷可以归类为表面，包含，成型，冷却或倒入缺陷。压力测试往往会暴露于成型，倒入或冷却缺陷类别中的问题。
Some Types of Casting Defects Detected with Pressure Testing:
- Pinholes and blowholes:由于气体陷入毒气，在冷却时会留下空隙和/或气泡；当气体试图逃脱时，倾向于将空隙留在模具的顶部或零件的外部。
- 收缩孔隙率：Caused by a lack of molten metal volume in the mold; results in small holes in the casting that are typically not as smooth as pinholes or blowholes.
- Shrinkage cavities:更大的收缩孔隙度可能导致重大的结构问题；通常从铸件的内部传播到外部，因为铸件从外部冷却。
- 热眼泪或裂缝：Defects due to rapid cooling; like shrinkage cavities but they are less jagged; cause mechanical performance issues.
When Do You Pressure Test?
The first place one should look for pressure testing requirements is in the customer requirements or specifications. Typically, a purchase order or Request for Quote (RFQ) will contain all the required function testing a component or assembly must pass before it passes all quality assurance requirements. With that said, there are instances where a part or assembly should be pressure tested:
- When a hermetic seal is required, or the system is hydraulic.
- When a part or assembly is safety critical.
- 当一个部分或组装has an internal cavity to hold or allow for the transfer of a fluid.
- When a part or assembly experiences high pressures.
- 当一个组件或组装已被接受post-casting CNC machining or welding which may have exposed sub-surface defects.
Types of Pressure Testing
There are several different pressure tests which may be specified for new product introduction (NPI) or production quality assurance testing. Types of pressure testing may be classified based on the purpose of the test and test methodology. Purpose of the test can include pressure range, leak, proof and burst testing. Test methodology may include a number of different test fluids, and can typically be classified by pneumatic (air/nitrogen/gas as test fluid) or hydrostatic (water as test fluid).
Pressure Range or Leak Testing
Pressure range or leak testing is often considered to be functional testing intended to ensure the assembly or component can properly perform over the entire operating pressure range, without leaks. This testing often involves exposing the test specimen to an operating environment (primarily temperature) and a lower and upper limit of pressure values. The purpose of this test is to ensure the component or assembly being tested maintains its structural integrity and there are not any casting, machining or metal joining defects. A pressure range or leak test is considered an Acceptance Test Procedure (ATP) performed on every production part but may also be part of a more extensive Qualification Test Procedure (QTP). This type of pressure testing can also be utilized to assess the integrity of any additional components assembled on the test specimen, such as gaskets or seals.
Most of the assemblies or components which undergo pressure testing are designed such that the fluid flowing through them only experiences a set amount of pressure drop (aka air decay). This is something that is determined during NPI and is often a measure of success during a pressure range or leak test. For pressure testing where the test specimen is suspended in fluid, with a gas flowing through it, leak testing works by visually looking for air bubbles during the testing. For example, with pneumatic pressure testing, if gas bubbles are present in the fluid during testing, the test specimen is not airtight and it does not pass the test.
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Proof Testing
The proof pressure is the maximum pressure level which the component or assembly may be supplied before permanent deformation and/operational function damages are seen. This type of pressure test typically involves subjecting the test specimen to increasing pressure, until the proof pressure is reached and then held for a set amount of time. During the test, the operator will watch for leaks and monitor pressure drop. In some instances, the test specimen’s dimensions will be monitored to ensure an excess amount of deformation is not occurring.
证明测试通常是在产品NPI期间执行的QTP。
Burst Testing
The burst pressure is the maximum amount of pressure a test specimen can be subjected to without ‘bursting’ or causing rupturing of material, resulting in an irreversible, catastrophic failure. Typically, the burst pressure is based off the maximum operating pressure multiplied by a Factor of Safety, depending on the industry and/or specification requirements. Burst pressure testing involves increasing the pressure on a test specimen, beyond the proof pressure, until failure occurs.
Burst testing is most often a QTP, performed during the product’s NPI.
我们的压力测试的主要技巧
Tip One:Ensure all parts and components have reached a stable temperature before testing. Parts which have been brazed together or welded should not be immediately pressure tested before they are cooled entirely first. Sometimes cleaning or drying operations can raise part temperature as well.
提示二：确保满足所有测试要求。检查所有规格，法规和/或客户RFQ/采购订单（PO）语言的要求。并非您需要遵循一次压力测试所需的所有要求；众所周知，客户会更改或增加PO Notes中的测试要求。
提示三：测试前确保测试样品清洁。在进行任何压力测试之前，应清洗或从部分中清除所有CNC加工金属芯片和碳氢化合物。碳氢化合物可以击败，导致虚假负压测试，甚至在测试钻机设备本身上堆积。这些碳氢化合物通常是在切割或研磨液中发现的。
Tip Four:Design or purchase test stands or pressure testing machines which are simple to use, ergonomic, and allow for a tight seal between the test specimen fluid inlets/outlets and the test equipment hoses or piping. Leaking at these interfaces will complicate the testing and may show inaccurate results.
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