When going to market with a new product one needs to be confident the part will perform as expected over its life expectancy. The underlying reason why plastic materials have extremely attractive properties is also the reason why plastic parts fail unexpectedly. The molecular structure of plastics makes the plastic part extremely time and temperature dependent. The structural response of a part today while under load may be much different a few months later. The resistance to chemicals of a plastic part while in a free state can be much different when exposed to stress. The environment a plastic part is exposed to can dramatically alter its mechanical properties. Though these are concerns seen at times with metals, there are concerns that should be applied to every new and existing plastic part.
The Madison Group has years of experience in designing and failure analysis of plastic parts. We are dedicated to plastics! We have several Ph.D.’s and Master degreed engineers on staff that have expertise in processing, design, and failure of plastics. To fully understand how a plastic part will behave one must understand the intricacies of plastic. One must know what properties are important and what testing needs to be completed to fully qualify a part. The qualifications for plastics will be different than when testing metal materials. Common causes of failure include:
Environmental Stress Cracking (ESC)
It is well known that many plastics and chemicals are not compatible. Cracking of the plastic will occur shortly after exposure. Typically on can go to a chemical resistance table to determine compatibility. However, that it is not the end of the story when dealing with plastics and chemicals. One must be aware of a phenomenon called environmental stress cracking (ESC). Though a rare occurrence with metal, it is common in plastics. This leading cause of failure is overlooked so often that one study showed that 25% of plastic part failures were the result of ESC. This occurs when a plastic is exposed to a chemical while under stress. Together, the stress and the chemical work together to cause cracking. It is important to note that classical chemical attack with molecular degradation does not occur making detection of this type of failure difficult for the inexperienced. With ESC the chemical penetrates the molecule structure and breaks the binding forces that bind the molecular chains, leading to molecular disentanglement. The tensile stress allows or accelerates chemical penetration into the plastic.
Because stress and a chemical, which can be any foreign substance including water, are both needed for cracking to occur, it can be difficult for an engineer to predict and conclude cause of failure. Failure by ESC is shown in the figure below. The part performed for years without any problems. The use of a different screw adhesive resulted in failures several months after being in the field. The stress was from the molding, but was found to not have changed from parts that did not crack.
It is important to note that environmental stress cracking can be resolved by reducing stress, eliminating the chemical and/or improving the quality of the plastic, i.e. molecular weight.
Plastic Additives
Plastic materials are processed at extremely aggressive conditions with high temperatures and shear rates. Likewise, the plastic material has to survive severe conditions in the field where it is exposed to high temperature, sunlight, and humidity. This all has to take place without changes to the plastic’s appearance and mechanical properties. To ensure this occurs, additives are used. Likewise, additives are used to obtain the correct coloring, protect against fire and microbials; provide antistatic and antifogging; to name a few. 
Failures with additives can occur for a variety of reasons. One of the most common additives used in the plastic industry is antioxidants. Plastic materials such as polypropylene and polyethylene, and butadiene based rubbers oxidize relatively easy without an additive to prevent its occurrence. When plastics oxidize they lose mechanical properties and change in appearance. This phenomenon is sometimes referred to as “aging”.This is the result of the chain scission of the plastic’s molecules. Oxidation failure occurs when the incorrect amount or wrong antioxidant is used. The figure below shows a polyethylene part that was exposed to sunlight while in the field after a year of being in the field the part literally was crumbling apart. Using Fourier Transformation Infrared (FTIR) it was determined that significant oxidation had taken place. Using Differential Scanning Calorimetry (DSC) it was shown that the polyethylene part did not contain any antioxidant. Being in the environment it was exposed to, it was imperative that this part have adequate amount of antioxidant. Simple DSC testing prior to the product release would have ensured this and prevented a product recall. Techniques such as Gas Chromatography Mass Spectroscopy (GCMS) can be used for additive identification.
 Reinforcements, such as glass, wood or carbon fiber, are commonly added to plastics to increase mechanical properties, make them more conductive or increase chemical resistance. Failures of fiber reinforced products can entail the incorrect amount of fiber, the wrong fiber size, inappropriate bonding between fiber and polymer, along with many others causes. The photo shown below shows a fiber reinforced material that has been chemical attacked and in the process the bonding between the fiber and polymer as been lost. Once bonding between the fiber and polymer no longer exists, the fiber no longer acts as a reinforcement, but rather a stress concentration and reduces bulk mechanical properties. Using material testing and microscopy, The Madison Group can determine the amount, type and condition of the fiber-reinforced part.
 Rules used for design of a plastic part are different than one made out of metal. Direct conversion from metal to plastic commonly leads to part failure. Changes in part thickness and features are needed to assure the part can be manufactured and stresses are kept to a minimum.
Design of a plastic part can be play an important role in avoiding failure. The figure below shows a part made up of a top cap and bottom canister. The two parts are assembled using a plastic welding process. Cracking, via a creep mechanism, occurred in the leg of the top cap. As a result of improper design of the assembly the stress in the top cap was too high.
The Madison Group has vast experience with the design verification of plastic parts. The knowledge of how and why a plastic fails assists us in improving a plastic part design.
 Inexperience with designing, specifying or manufacturing of plastics can lead to part failure. There are hundreds of different plastics, with hundreds of different additives and reinforcements that can be used. This allows the engineer to essentially create a very unique material, with unique mechanical properties tailored to a specific product. Unfortunately, this creates a larger window for the incorrect decisions to be made, which may eventually lead to part failure. The figure below illustrates what can happen when the wrong material is used. Severe chemical attack is seen at the inside surface.
The use of a different material could have prevented this and numerous other failures like it.
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