Water Filter Housing
Note: the following analysis and its write-up are property of The Madison Group and cannot be copied and/or distributed in anyway without prior permission from The Madison Group. This analysis in no way suggests that any or all plastic pipe failures occur in the manner described. Each plastic failure is unique and should be treated as such.
A water filter housing was received for failure analysis. The filter housing shows a failure located at the housing bottom cap. The failure appears as a circumferential crack that completely separated the bottom cap from the housing. This failure caused extensive water damage in the property where it was installed.
Analyzing the stresses and forces on the filter housing during operation, two main sources of stresses are identified. There is a stress originating at the threads of the housing, and a stress caused by the internal water pressure. The stress at the threads is produced while tightening the filter housing to seal it to the filter base. A structural analysis (see Figures) was performed to determine which areas are exposed to high stress. The maximum stress (reddish color) occurs at the inner corner of the bottom cap. This is the exact place where the failure occurred on the analyzed housing. The higher stress is at the smallest radius (stress concentration point) and where the housing wall is thinnest.
Further inspection of the housing reveals defects that originated during molding. As seen in the picture, there are molding ripples and poor material mixing. The molding ripples are generated during mold filling and they can be the source of stress concentrations that can lead to crack initiation. Poor mixing during processing leads to material inhomogeneities that can be the source of stress concentrations, local material weaknesses and local material degradation. The molding defects identified here are contributing factors for failure to occur. Other important factor is material selection. There are thousands of material grades to choose for a given application. It is important to have the right knowledge about the application to choose the appropriate material. A balance between material cost and performance must be obtained to avoid failure. A material as the one used to mold this filter can include fillers (such as talc). The use of fillers reduces material costs, but at the same time reduces material strength. Material processing is also an important issue. Molders prefer to use low mold temperature and low viscosity materials to obtain the fastest processing time possible. A material with low viscosity allows faster mold filling, but it is usually of lower molecular weight that reduces the mechanical performance of the part. A colder mold can be the cause of numerous filling problems. It is usually the combination of several factors that lead to failure of a plastic part. It is important to consider all these when properly designing a plastic product.