Donan’s Component Testing Laboratory (CTL) studies numerous water filters that have failed and resulted in a water loss. One typical water filter received is a nominal 10-inch style with a canister and head assembly component (Figure 1). These components are constructed of plastics and are intended to be installed with a filter cartridge designed to remove tastes and odors from potable water. The cartridge can be installed or removed by unscrewing the canister’s threaded connection with its head assembly.
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Figure 1: Nominal 10-inch Style Water Filter Canister[/caption]
What Causes a Failure?
A common failure mode for these canisters is a circumferential fracture at its molded female threads (Figure 2). The manufacturer will typically attempt to claim that this is the result of overtightening of this threaded connection during installation of the cartridge. Although tightening of the canister will create stress at the threaded connection, there are other very important considerations to investigate before abandoning a subrogation procedure against the manufacturer.
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Figure 2: Circumferential Fracture in Canister Threads[/caption]
Improper Design Practices
Two improper design practices that can create a weak part are often found in the canisters that contain this type of failure mode: thread termination and corner design in the crest and root regions of the threads.
It is best practice for plastic parts to allow a clearance of at least 1/32 inch at each end of threads, and the part should not be designed with threads that terminate at a shoulder., 
If the threads do not exhibit this best practice, they will create an extremely sharp corner, and this sharp corner will lead to a very high stress concentration at the corner. Often, the threads of canisters will terminate at the shoulder in which a groove for a rubber O-ring is present. Figure 3 shows a comparison between a properly and improperly thread termination, and Figure 4 shows a cross section of the improper thread termination. Figure 4 clearly shows the sharp corner that is created when the threads are improperly terminated.
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Figure 3: Comparison of Improper (Top) and Proper (Bottom) Thread Termination[/caption]
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Figure 4: Cross Section of Improper Thread Termination[/caption]
Best Design Practices
The preferred thread design for plastic connections contains rounded corners at the crest and root regions, eliminating sharp corners, which reduces the stress concentration at these points and produces a part with greater structural strength. All inside and outside rounded corners should have as large a radius as possible to reduce stress concentration. Although some rounding of the corners may be present in water filters with this type of failure mode, it may be insufficient to significantly reduce the stress concentration. Figure 5 shows a comparison between the rounded corners in the crest and root regions of the threads that is preferred and the unrounded corners that should be avoided.
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Figure 5: Proper and Improper Corner Design in the Threads [/caption]
Cycling Loading Stress
These water filter canisters are subjected to cyclical loading stress, which is created when the faucet (or other object the filter services) opens to provide filtered water, then closes after use. This use of the filter will decrease then increase the pressure, respectively, in the filter in a cyclical manner. Parts under cyclical loading stress, such as the subject filter canisters, will have a significantly shortened life due to stress concentrations, such as improper thread termination and corner design.
Steps for Subrogation Success
Water filters can be studied for causation of a water loss by a licensed professional engineer. If possible, photographs should be taken before removing the failed component. Not all water filter components contain date markings, so ask the homeowner when the filter was installed, as well as when any service work was performed.
E.I. du Pont de Nemours and Company, General Design Principles for DuPont Engineering Polymers
Michael L. Berins, editor, Plastics Engineering Handbook of the Society of the Plastics Industry, Inc.
, fifth edition (Springer-Verlag New York, 1991).