By Vishu Shah, Consultek
Part failures due to impurities and contamination of virgin material are common. Material contamination usually occurs during processing. A variety of purging materials are used to purge the previous material from the extruder barrel before using the new material. Not all of these purging materials are compatible. Such incompatibility can cause the loss of properties, brittleness, and delamination. In the vinyl compounding operation, failure to add key ingredients, such as an impact modifier, can result in premature part failure. Simple laboratory techniques cannot identify such impurities, contamination, or the absence of a key ingredient. More sophisticated techniques; such as Fourier Transform infrared (FT-IR) analysis and gel permeation chromatography (GPC) must be employed. These methods can not only positively identify the basic material, but also point out the type and level of impurities in most cases.
Once the part failure resulting from poor molding practices or improper material usage through visual examination and material identification is ruled out, the next logical step is to carry out an experimental stress analysis. Experimental stress analysis is one of the most versatile methods for analyzing parts for possible failure. The part can be externally stressed or can have residual or molded-in stresses. External stresses or molded-in stresses or a combination of both can cause a part to fail prematurely. Stress analysis is an important part of failure identification. Detection of residual stresses has a different meaning than evaluation of stresses due to applied forces. It is possible of course to see failure due to poor design, or underestimating of forces. These failures are usually detected in proof testing, or in early production. Residual stresses are altogether different: A molding process can generate residual stress just about anywhere, anytime. Here, ongoing photoelastic inspection can prove extremely helpful, allowing detection of defective molded parts or identification of failures in clear plastic products. Experimental stress analysis can be conducted to determine the actual levels of stress in the part. Five basic methods are used to conduct stress analysis.
- Brittle coatings
- Strain gauge
- Heat reversion
The photoelastic method for experimental stress analysis is quite popular among design engineers and has proved to be an extremely versatile, yet simple technique.
If the parts to be analyzed are made out of one of the transparent materials, stress analysis is simple. All transparent plastics, being birefringent, lend themselves to photoelastic stress analysis. The transparent part is placed between two polarizing mediums and viewed from the opposite side of the light source. The fringe patterns are observed without applying external stress. This allows the observer to study the molded-in or residual stresses in the part. High fringe order indicates the area of high stress level whereas low fringe order represents an unstressed area. Also, close spacing of fringes represents a high stress gradient. A uniform color indicates uniform stress in the part. Next, the part should be stressed by applying external force and simulating actual-use conditions. The areas of high stress concentration can be easily pinpointed by observing changes in fringe patterns brought forth by external stress.
Another technique known as the photoelastic coating technique can be used to photoelastically stress-analyze opaque plastic parts. The part to be analyzed is coated with a photoelastic coating, service loads are applied to the part, and coating is illuminated by polarized light from the reflection polariscope. Molded-in or residual stresses cannot be observed with this technique. However, the same part can be fabricated using one of the transparent plastic materials. In summary, photoelastic techniques can be used successfully for failure analysis of a defective product.
The brittle-coating method is yet another technique of conveniently measuring the localized stresses in a part. Brittle coatings are specially prepared lacquers that are usually applied by spraying on the actual part. The part is subjected to stress after air drying the coating. The location of maximum strain and the direction of the principle strain are indicated by the small cracks that appear on the surface of the part as a result of external loading. Thus, the technique offers valuable information regarding the overall picture of the stress distribution over the surface of the part. The data obtained from the brittle coating method can be used to determine the exact areas for strain gauge location and orientation, allowing precise measurement of the strain magnitude at points of maximum interest. They are also useful for the determination of stresses at stress concentration points that are too small or inconveniently located for installation of strain gauges. The brittle-coating technique, however, is not suitable for detailed quantitative analysis like photoelasticity. Sometimes it is necessary to apply an undercoating prior to the brittle coating to promote adhesion and to minimize compatibility problems. Further discussion on this subject is found in the literature.
About the Author
Consultek Consulting Group
|Vishu H. Shah is President of Consultek Consulting Group, a technical and management consulting firm for plastics professionals. His 35 years of extensive practical experience in plastics Industry includes positions as president and cofounder of Performance Engineered products, Senior Plastics Engineer of Rain Bird Corporation and Nibco Inc. His areas of expertise include product design, processing, automation, materials, rapid prototyping, tooling, failure analysis and testing. His industry related experience ranges from developing and training personnel, writing standard operating procedures, obtaining certifications, negotiating major contracts, and developing business plans for growth and expansion as a custom Injection Molder, Providing technical services to engineering and quality assurance staff in the area of material selection, product development, processing, tooling design, and inspection techniques, setting up and running PVC compounding operation, developing formulations, and automation projects to serving as expert witness for legal community.|
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