Plastics and composite materials exhibit a wide range of mechanical properties. Variations between components are inevitable during production, leading to manufacturing defects. These defects can be attributed to multiple factors such as fluctuating heat treatment temperatures, molding defects, or inconsistent raw plastic material quality.
Ensuring raw plastic quality and material integrity requires a specific set of evaluation and plastic inspection methods. The suitability of a plastic product can be determined by examining the chemical and physical properties of the raw plastic resin or fiber. Raw plastics typically undergo a series of screening tests, and post-production quality inspection relies on various detection methods.
Raw Material Inspection
Most plastic raw material quality inspection involves either resin or fiber testing. For resins, key indicators include viscosity, moisture content, and color. Continuous fibers are typically checked for tensile strength.
Infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy are commonly used to identify the basic chemical structure and contamination levels. Other standard methods include chemical analysis to reveal the major acid or epoxy groups in a plastic compound, and liquid or gel chromatography to determine the average molecular weight and molecular weight distribution in resin molecules.
Chromatography testing demands particularly high precision, as molecular weight and its distribution directly affect the viscosity characteristics and mechanical properties of plastic materials. In polyester, lower molecular weight generally yields higher viscosity levels, resulting in a slower thickening rate. This can make the plastic more difficult to process or require extremely long periods to thicken adequately. Moisture levels also influence the thickening rate.
Ultrasonic Testing
Ultrasonic inspection uses high-frequency sound waves to track internal defects within a plastic material. An electric transducer generates sound waves, which are then applied to the plastic through a coupling medium such as water. The sound waves travel through the material, and their energy levels shift according to the defects they encounter. A receiving transducer converts the reflected sound waves into electrical signals displayed on a screen. Results can then be compared with predetermined design characteristics to identify any internal defects.
Radiographic Testing
Radiography involves directing a radiation beam through a plastic component, then recording and measuring its intensity upon exiting the opposite side. The difference between the beam’s initial intensity and its post-penetration intensity helps determine defects within the object. The standard method uses X-rays recorded on photosensitive film, while gamma rays are more effective for thicker materials as they provide a higher degree of penetration. Defects in the plastic appear as tonal variations or spots in the film image. Radiography is commonly used to detect internal voids, inclusions, and structural inconsistencies.
Acoustic Emission Testing
Similar to ultrasonic technology, acoustic emission testing uses sound waves to determine material defects. However, rather than sending sound waves and receiving reflections, acoustic emission relies on sound released from elastic stress at microscopic damage zones. In reinforced plastic composites, even a small degree of stress can cause emissions at damaged locations, enabling acoustic emission testing to map defect positions within molded products.
References
- Wikipedia: Nondestructive Testing (NDT): Overview of NDT methods including ultrasonic, radiographic, and acoustic emission testing
- Wikipedia: Ultrasonic Testing: Principles and applications of ultrasonic inspection for material defects
- ASTM E317 ??Ultrasonic Testing Standards: Standard practice for evaluating ultrasonic pulse-echo testing systems
- ISO 9712 ??NDT Personnel Qualification: International standard for qualification and certification of NDT personnel
- ASNT ??Nondestructive Testing Society: Professional society for nondestructive testing standards and certification