On Thursday April 30, 2020 ARM held a discussion on Leak Testing, moderated by Education Committee Chairman Ron Cooke (ExxonMobil) and Sandy Scaccia (Norstar Aluminum Molds). The session was very well attended and the discussion between everyone on the call was excellent. There was so much useful information exchanged that we decided to try to capture all the salient points.
We began by talking about the methods that are commonly used for leak testing of rotomolded parts, then discussed some more unusual methods that had been tried. Then we talked about whether leaks can be rectified and what can be done to stop the leaks happening in the first place. This led us into a very interesting detailed discussion about issues with metal inserts, which is one of the most common causes of leak problems.
Air Pressure Testing
Several molders described the set-ups they use; this may be to check the air-tightness of rotomolded cases or to detect leaks in large tanks.
Typically all orifices of the part are sealed and air pressure is applied at a very low level; usually 1 psi or less. Safety issues around any type of air pressurization were immediately stressed; the provision of appropriate control valves, pressure relief valves, and emergency venting facilities is essential.
The part is held at pressure for a period of time (15-60 minutes in many cases) and the outside is sprayed with a solution of dish detergent. Vigorous bubbling will provide evidence of leaks, even small ones.
Another version of the Air Pressure Test is to take the part up to pressure and close off the air source. If the part holds its pressure over time, you can be confident that leaks are not occurring. You should do this on a fully cooled part, to avoid part shrinkage complicating the issue.
“Dunk Tank” Testing
This type of testing is typically carried out for rotomolded fuel tanks. The tanks are pressurized with air and the assembly is submerged in a tank of water. In many cases, slightly higher air pressures (up to 3 psi) and shorter pressurization times (2 – 10 minutes) are used, compared to Air Pressure Testing.
Leaks will be detected by visual observation of bubbles escaping from the outside surface.
Submersion depths of 2 inches under the water surfaced were mentioned. Mostly single “dunks” are employed, rather than multiples. Due to tank buoyancy, submersion will require considerable force and various mechanical systems can be employed to do this.
Sometimes there can be false positives; air bubbles can form on the part for other reasons. If in doubt, the questionable area can be subjected to a confirmatory test using air pressure and detergent (see above).
Practical issues raised included the commercial availability of suitable dunk tanks, the need for different sized tanks for different sized parts, water filtration (typically standard pool filters systems are used), and the difficulty of observing bubbles (some tanks have side windows to facilitate this).
Testing very large parts would necessitate an excessively large tank; in these cases Air Pressure Testing with detergent spray would be more practical.
Typical leak problem areas are similar to Air Pressure Testing.
Coastguard Regulations specify a 3 psi pressurization; this can create problems with the walls of the tank being over-stretched and possibly failing.
Comments & Other Methods of Testing – Tried or Currently Used
There was a caution that any detergent sprayed onto part surfaces should be washed off afterwards; it’s possible that the detergent could cause stress cracking problems later in the life of the part.
One contributor shared that they had traced leak problems back to dirty tools and poor powder quality.
One question was how much the part should be allowed to cool before testing. Theoretically, immediate contact with cold air or water after molding might change the morphology of polyethylene in subtle ways that could change stiffness and impact strength. In most industrial environments, it seems that leak testing is carried out after other post-mold assembly operations have taken place; maybe 24 -36 hours after molding. In the case of nylon fuel tanks, post-treatment by hot water immersion is often used (to improve impact strength).
One molder related an experience with natural crosslink PE tanks, where the parts passed the initial tests, but leaks were detected later. This was traced back to the presence of small voids around metal inserts. Initially, a thin membrane of PE was present that acted as a seal, but with use this membrane became degraded. An additional test was implemented, where a 100W inspection light was used to intensely illuminate the tank from the inside. This was sufficient to highlight voids to visual inspection. This obviously wouldn’t work with black or highly pigmented tanks.
One interesting technique was described, using a thermal imaging device. Cold air was blown through the hot part immediately post-molding (using a transvector) and cold spots / possible leak areas could be identified.
A high-frequency spark tester, and even ultrasound have also been used to test for leaks, although both were reported to be extremely labor-intensive options.
It was pointed out that a presentation at a recent ARM meeting (by a rotomolder member, Floteks in Turkey) had described a leak detection method that employed a dunk tank fitted with hydrophones to detect bubble release.
Leak Protection & Rectification
Typical leak problem areas are parting line faults, excessive part porosity, inserts that have not been fully encapsulated, badly applied spin-weld fittings and gaskets that have not been sufficiently tightened down. Some of these “bad actors” can be addressed after testing, some cannot be.
Depending on client requirements and specifications, it may be allowable to fix small leaks after testing. Military specifications tend not to allow for any rectification.
Parts made from crosslink PE will be less able to be retrospectively fixed.
Minimizing Scrap Through Preparation / Handling of Inserts
It was generally agreed that problems around molded-in metal inserts are a major cause of leak testing failures.
One molder described a test that could be used as an initial screen on parts immediately after molding. A suction cup was applied to the area of the part around the insert. If the cup was able to stay in place, this indicated that there was no air leakage path around the insert. If the suction cup fell off, there was a potential problem.
One way to avoid problems with molded-in inserts may be to avoid using them! One option mentioned is to mold an empty pocket into the part and then apply an insert after molding.
Voids around metal inserts are often due to contamination by liquids (such as cutting oils) during their manufacture. Cleaning of inserts before use in rotomolding is recommended; acetone was suggested as a suitable cleaning solvent.
Standard polyethylene (PE) will not chemically bond to metal, so inserts depend on full encapsulation of the insert by the PE during molding. Poor PE powder quality may stop this happening, creating a void. So may insufficient heat getting to the insert.
In the past, inserts have been sandblasted, to make contact surfaces rougher. Another intriguing option would be to powder coat inserts in a PE that had been specially treated to promote adhesion. One molder said that he had used a rotolining PE grade to do this, with good results. Although this would add unit cost to the insert, it could save considerable scrap costs and may be economic in many cases.
Our participant survey showed that this was a very useful session and hopefully the above notes will enable us to share the information more widely.
One of the real benefits of ARM is that so many members are willing to share both their good and bad experiences, which helps to improve the rotomolding industry for all of us.
ARM holds sessions like this every two months. Sometimes they focus on a specific topic, sometimes they cover a more general “What’s Your Problem” format. Either way, I try to attend as many as I can; I never finish listening to one without learning several valuable new lessons. This is one of the many free-to-member services that ARM provides.
Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has more than 30 years of experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.