Alexei Volkov
Residual stress is an inherent result of plastics manufacturing processes, which can cause field failures and reduce product quality. While destructive testing methods are available for nontransparent plastics, such as layer removal and stress-corrosion tests, these methods are time-consuming and may not provide quantitative results for comparison. Birefringence, or optical anisotropy, is a phenomenon observed in stressed plastics that can be quantified using a compensator, as outlined in ASTM D 4093. This standard test method is nondestructive and provides fast results, making it a popular choice for quality control in clear plastics.
| Characteristics | Values |
|---|---|
| Calculating stress in plastics | Requires knowledge of residual stress, which is an inherent result of plastics manufacturing processes |
| Residual stress | Occurs when molten polymer is cooled and shaped |
| Birefringence | An orientation in the index of refraction that can be used for quality control; can be quantified using a compensator |
| Sources of residual stress | Injection molding, extrusion, vacuum forming, machining, drawing, casting, joining, differential shrinkage, uneven cooling, nonuniform flow rate, material contaminants, mechanical loading, thermal cycling during transport and storage, inadequate or non-uniform annealing, and sharp coring |
| Impact of residual stress | Reduced product quality, increased scrap and rejects, reduced impact strength, environmental stress cracking, diminished performance at high or cycling temperatures, crazing, reduced optical clarity, stability, and resistance to environmental stress factors, distortion, shrinkage, creep |
| Stress calculation methods | Destructive methods such as layer removal, section removal, and stress-corrosion tests; non-destructive methods such as placing a sample between two illuminated polarized sheets, using a full-field polariscope or strain viewer, ASTM D 4093 (nondestructive procedure for transparent plastics), computer-based instrumentation, spectrophotometric analysis |
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What You'll Learn
Understanding the causes of residual stress in plastics
Residual stress in plastics is a critical concern in the production of plastics for various applications, including medical devices, packaging, and optical components. It refers to the stresses that remain in a material after the original cause has been removed. These stresses can be introduced during polymer manufacturing through processes such as extrusion, stretching, drawing, molding, and casting.
One of the primary causes of residual stress in plastics is non-uniform cooling. When a plastic part is cooled, the outer portion solidifies first, contracting and compressing the hotter inner material. This results in thermal contraction and non-uniform stress, with the surface and interior of the plastic experiencing different cooling rates. Additionally, differential shrinkage, uneven cooling, and non-uniform flow during manufacturing can induce residual stresses.
Another factor contributing to residual stress is the heterogeneity of chemical or crystallographic order. Treatments such as nitriding or case hardening can introduce residual stresses into the plastic. Similarly, surface treatments like enameling, electroplating, and coating processes (PVD and CVD) can also be responsible for causing residual stresses in plastics.
Furthermore, residual stresses can arise from non-uniform plastic deformation during heating or cooling. This occurs when plastic deformation is not consistent throughout the cross-section of an item during processes such as bending, drawing, rolling, or extruding. The regions that were elastically deformed attempt to recover, leading to residual tension in some areas and residual compression in plastically deformed regions.
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Methods for measuring residual stress
When it comes to measuring residual stress in opaque plastics, there are a variety of methods available, each with its own advantages and limitations. Here is an overview of some commonly used techniques:
Destructive Methods
Destructive testing involves permanently altering or damaging the plastic sample to measure residual stress. While these methods can be time-consuming and may not provide quantitative data for comparison, they are often the only option for opaque and nontransparent plastics. Some common destructive tests include:
- Layer Removal: This method involves removing and examining individual layers of the plastic to identify stress patterns and distributions.
- Section Removal: Similar to layer removal, this technique focuses on evaluating residual stress in specific sections or areas of the plastic.
- Stress-Corrosion Testing: Exposing the plastic to corrosive agents or environments to observe how it responds to stress.
Nondestructive Methods
Nondestructive testing is preferable as it does not damage the sample and can often provide faster and more accurate results. However, these methods are typically designed for transparent plastics due to their reliance on light transmission and birefringence observations. Some nondestructive methods include:
- Polarized Light Transmission: Using polarized sheets to illuminate a sample, allowing stress patterns and birefringence to be visualized.
- Polariscope Evaluation: Employing a polariscope, which consists of a white light source and polarizing elements, to evaluate stress distribution in transparent parts.
- Computer-Based Instrumentation: Utilizing PC-based instruments and computerized vision systems to quantitatively assess retardation, birefringence, and residual stress in transparent plastic films or parts. This method offers high accuracy through spectrophotometric analysis.
It is important to note that the choice between destructive and nondestructive methods depends on the opacity of the plastic and the specific requirements of the application. While destructive methods may be necessary for opaque plastics, nondestructive methods are often preferred for quality control and process optimization due to their speed, accuracy, and non-damaging nature.
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Interpreting stress patterns
Understanding Residual Stress
Residual stress is inherent in plastics manufacturing due to the cooling and shaping of molten polymers. It can be introduced by various strain-inducing processes such as extrusion, stretching, drawing, molding, and casting. These stresses can significantly impact the performance and applications of the plastic product, especially in critical areas like medical devices, packaging, and optical components.
Destructive vs. Nondestructive Methods
When dealing with opaque plastics, the evaluation of residual stress is limited to destructive methods. These methods involve physically altering the plastic, such as layer removal, section removal, or stress-corrosion tests. Destructive methods can be time-consuming, challenging to execute, and may not provide quantitative comparative data. However, they are necessary for opaque materials since nondestructive methods, which are more convenient and precise, rely on the transmission of light through the material.
Visual Interpretation of Stress Patterns
One common approach to interpreting stress patterns in opaque plastics is through visual inspection. When illuminated by polarized light, stressed areas of polymers exhibit multicolored bands or fringes, known as birefringence. Each color band corresponds to a specific degree of stress, and this relationship is defined by Brewster's Law, which states that the change in the index of refraction is proportional to the stresses. Interpreting these color patterns can be challenging and may require a trained eye, as the same color fringe can represent multiple stress levels. However, it is useful for identifying maximum stress areas and comparing stresses between identical parts.
Quantitative Measurement
While visual interpretation provides a qualitative understanding of stress patterns, quantitative measurement tools offer more precise and reproducible results. Instruments like polarimeters and polariscopes can be used in conjunction with a ""compensator" or "measuring wedge" to convert complex stress color patterns into simple black lines that indicate stress levels. These tools are especially useful for materials with very low or very high stresses, complex geometries, and high stress gradients, ensuring accurate stress evaluation beyond empirical techniques.
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Adjustments to reduce stress
While residual stress is an inherent part of the plastics manufacturing process, there are adjustments that can be made to reduce it. Residual stress is caused by the cooling and shaping of molten polymers, and it can negatively impact product quality and performance. High levels of stress can lower a product's impact strength and cause environmental stress cracking and diminished performance at high or cycling temperatures.
One way to reduce residual stress is to minimise differential cooling rates, non-uniform temperatures, or flow rates during the manufacturing process. This can be achieved through careful control of the manufacturing conditions and equipment. Ensuring adequate and uniform annealing can also help reduce stress.
Another factor that can contribute to residual stress is material contaminants. By using high-quality, pure materials and implementing strict quality control measures, manufacturers can reduce the likelihood of contaminants causing stress in the final product.
Mechanical loading and thermal cycling during transport and storage can also induce stress. To mitigate this, manufacturers can optimise the design of products to reduce stress concentration and improve their ability to withstand mechanical and thermal loads.
Additionally, the choice of polymer and manufacturing process can impact residual stress levels. Different polymers have varying susceptibilities to stress, so selecting an appropriate polymer for the specific application can help manage stress. Likewise, certain manufacturing processes, such as extrusion, stretching, drawing, and moulding, are more prone to inducing stress. Manufacturers can explore alternative processes or modifications to reduce the stress induced by these processes.
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Impact of residual stress on product quality
Residual stress is an inherent result of plastics manufacturing processes and occurs when molten polymer is cooled and shaped. It can be introduced by nearly all techniques used for polymer manufacturing, including extrusion, stretching, drawing, molding, casting, joining, or other strain-inducing processes.
The presence of residual stress can have a significant impact on product quality. In most cases, it is an invisible problem that can cause field failures and reduce product quality. High levels of stress can lower a product's impact strength, diminish its high-temperature performance, and increase the likelihood of environmental stress cracking. For example, when styrene parts are dipped in kerosene, stressed areas will quickly craze, leading to a deterioration in product appearance and, ultimately, product failure.
Additionally, residual stress can affect the optical performance of clear plastics used in lenses and optical transparencies. It introduces birefringence, an asymmetry in the index of refraction, which can impair the optical properties and product performance of plastic lenses.
However, residual stress can also be intentionally induced to enhance the mechanical properties of certain products, such as biaxially oriented films. These films have improved strength and formability due to their carefully designed orientation.
To maintain product quality and consistency, manufacturers must evaluate residual stresses at every stage of the product cycle. By monitoring and managing stresses effectively, they can improve productivity and ensure the suitability of various approaches to eliminate undesirable stresses or induce beneficial ones.
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Frequently asked questions
Residual stress is an inherent result of plastics manufacturing processes and occurs when molten polymer is cooled and shaped.
Residual stress can reduce product quality and cause field failures, leading to product-liability claims against the manufacturer. It can also lower a product's impact strength and contribute to environmental stress cracking and diminished performance at high or cycling temperatures.
For non-transparent plastics, choices are limited to destructive tests such as layer removal, section removal, and stress-corrosion tests.
ASTM D 4093, Standard Test Method for Photoelastic Measurements of Birefringence and Residual Strains in Transparent or Translucent Plastic Materials, can be used for translucent plastics. A full-field polariscope, or strain viewer, can also be used for fast inspection of transparent plastic products.
Once the stress profile within a plastic part is known, adjustments in the process, material, or part design may be made to eliminate or reduce these stresses. Proper annealing can also minimize stress and prevent crazing, which is the appearance of many fine microcracks across a material.
