leak testing of die cast parts

Quite often we see the requirement of leak proof parts from our customer. There are some specifications mentioned in drawing & may be some in the product specification sheet that our customer provides at the time of development.
However it has been observed that the die caster is not always very much aware of this process & theory behind the leak proof parts and thus it takes a back seat when actual development work takes place. As a result we encounter customer complain about the leak that takes place while in final assembly & testing at the customer plant or in some cases failure is reported from field.
As a die caster we must have an good understanding about what leak testing is all about & what measures we should take to ensure "0 PPM" parts to customer.
What is Leak

A leak is a flow of gas (or liquid) through the wall of a component (via an imperfection such as a hole, crack or bad seal). Leaks require a pressure difference to generate the flow; they always flow from higher pressure to lower pressure.

Leaks takes place from positive pressure (inside an component) to outside (at atmospheric pressure). Also a leak could be from atmosphere to inside an evacuated component, in both the cases we can see there are some flow of gas or liquid from one stage to another. 

How the leak rate is measured

For leaks of air into atmosphere, units are usually expressed as mm3 or cm3 (cc) per second or minute. So 16.6 mm3/sec = 1 cm 3/min. A bubble under water is about 30 - 50 mm3, so 1 bubble per second is about 30 mm3/sec or 2 cm3/min roughly. A standard unit of leakage which takes account of air pressure is the mbarl/sec. (Millibar-litre per second). A leak into atmosphere of 1 mbarl/sec is equivalent to a volume leak of 1000 mm3/sec.

Leak testing methodology

Key questions at the start of any leak test requirement are:

• What size is the component and what is it’s internal volume?
• What is the leak limit
• Are the parts clean and dry, free from dirt etc:?
• Is there access to inside geometry or is it a sealed unit?
• Is it rigid or flexible?
• Are parts at ambient temperature during final applications?
• What is the surface finish of any sealing surfaces?

Test methods

Considering the various leak rate the test methods also varies to capture leak rate as per the requirement. Following are the methods adopted for capturing leak rate as per the specifications.

Flow rate defined as  -  10^{-9  -} 10^-3

^{By High volume Helium}

Flow rate defined as  -  10^{-2  -}  5

Air decay test - water submersible, Dunk test, Bubble leak test, Dry test

Pressure / vacuum method - The test piece and the reference volume are simultaneously pressurised (or evacuated) to a preset pressure. The air in the system is then allowed to stabilise, with the supply valves all closed. The Differential Pressure Transducer is automatically zeroed. After this stabilisation time, the pressure change in the test piece is compared to the pressure change in the reference volume, using the Transducer. If the test piece is leaking, the difference will increase and be measured, an alarm limit may be set for a pass/fail decision. The sequence is fully automatic, the accuracy and sensitivity of the system is defined by the method of setting the preset pressure together with the quality and type of control valves and Differential Pressure Transducer.

Helium systems. A vacuum pump evacuates the test chamber and test piece simultaneously to a preset vacuum. At this preset level, the chamber and the test piece are isolated and the chamber evacuated further to a very low pressure. A positive pressure variation is therefore created between the test piece and the chamber. Helium gas is then introduced into the test piece, often in a 10% concentration. A Mass Spectrometer analyses a sample from the chamber as the vacuum continues to be drawn. The Mass Spectrometer measures the helium leakage and sets the pass/fail decision. The test piece pressure is often compared to the chamber pressure before dosing with helium, to avoid saturating the Mass Spectrometer in the event of a gross leak.

High pressure air decay testing (over 6 bar)

The differential testing of parts with higher pressures poses additional challenges for leak testing.

Generally, the high pressure is not suitable for making accurate leak test calculations as it is too unstable.In these cases one arranges for the test volume to be on the opposite side of the leak path and the instrumentation is arranged to use the differential pressure rise. Parts are placed into a chamber possibly with infill pieces to reduce the test volume as much as possible. With this setup, the internal volume of the test piece can be pressurised to high pressure whilst the test system monitors the pressure in a volume outside the part

Low Pressure air decay testing (0 to 6 bar)

• Offers the lowest cost solution
• Simple to understand results data
• Direct value results
• Direct calibration
• Temperature dependent process (alternative test method is vacuum
• Best used when component under test is stable

The die caster must consult the leak testing machine manufacturer the detail requirement & repeatability of the machine function must be ensured. Also if required the testing must be conducted in controlled environment. also the cleanliness of  component, the tank condition, water quality, seal conditions must e monitored in periodic manner.

Happy leak testing.