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A plastic product can exhibit a range of mechanical properties, and while most variations between components are intentional results of the production process, there may be some deviations that can be traced to a defect in fabrication. Flaws can derive from a number of different factors, such as fluctuating heat treatment temperatures, molding imperfections, or inconsistencies in the quality of the plastic material. Resolving problems in heat treating and molding typically involves close control of manufacturing parameters and monitoring through each stage of a product’s fabrication.
However, ensuring the quality of raw plastic stock or the integrity of the plastic within a product requires a specific set of evaluation and plastic inspection methods. Both chemical and physical characteristics can be examined to determine whether a batch of raw plastic resin or fiber is suitable for fabrication or to assess the material accuracy of a molded plastic product. While raw plastic typically undergoes a series of tests to screen for forming viability, post-production quality inspection usually relies on methods that rate a product according to its closeness to an intended design.
Raw Material Inspection
Most raw plastic materials are quality inspected in resin or fiber form. For resins, the main criteria include viscosity, moisture levels, and color, while continuous fiber is often examined for its tensile strength. Infrared or nuclear magnetic spectroscopy is typically used to identify basic chemical structures and contamination levels. Other standard methods include wet chemical analysis, which reveals the main acid or epoxide groups in a plastic compound, and liquid or gel chromatography, which determines the average molecular weight and weight distribution among resin molecules.
Chromatography techniques can be particularly useful because molecular weight and distribution are major influences on a plastic material’s viscosity and mechanical characteristics. In polyester, lower molecular weights generally yield higher levels of viscosity, resulting in a slower rate of thickening. This can make the plastic more difficult to handle or prohibitively long to thicken to the appropriate degree. Moisture levels can also affect the thickening rate.
Ultrasonic inspection employs high-frequency sound waves to trace internal flaws within a plastic material. An electric transducer generates these sound waves, which are then applied to the plastic through a separate medium, such as water. The waves move through the material and their energy levels shift according to the defects they encounter. A receiving transducer converts reflected sound waves into electrical signals displayed on a screen. The results can then be compared with the intended design characteristics to identify any internal flaws. Some of the common defects ultrasonic inspection can spot include:
• Large voids
• Groupings of small voids
• Gaps in lamination
• Contaminants or other foreign materials
Radiography involves firing a beam of radiation through a plastic component, and then recording and measuring its power when it exits through the other side. The difference between the beam’s initial intensity and its intensity after passing through the material helps to determine internal defects in the object. Standard methods employ x-rays that are then recorded as images on photographic film, while gamma rays are more effective for thicker materials because they provide a higher degree of penetration. Defects in the plastic appear as shades or spots on the film image. Radiography is commonly used to detect the following flaws:
• Large voids
• Translaminar fractures
• Uneven fiber distribution
• Contaminants or other foreign materials
• Misaligned fiber patterns and wrinkles
Like ultrasound technology, the acoustic inspection method uses sound waves to pinpoint material defects. However, rather than transmitting reflected sound waves, acoustics relies on sound to release elastic stress emissions from areas of microscopic damage. In reinforced plastic composites, a small degree of applied stress will result in emissions at or near the damaged site, enabling acoustic emission inspection to map defect locations within a molded product. Some of the typical defects that can be spotted through the acoustic method include:
• Microscopic cracking
• Fiber detachment
• Loss of bonds in the fiber matrix
• Fiber breakage
• Local delamination
Acoustic emission can be picked up by a series of highly sensitive transducers placed at specific locations on the material’s surface. When stress waves result in surface disturbances, the transducers record an electrical signal output and store the data for analysis.
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