Alexei Volkov
Plasticity, or plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. Plastic part failure is generally related to four key factors: material selection, design, process, and service conditions. For instance, in applications that demand high-impact resistance, a high-impact material must be specified. Plastic failure can also occur through mechanical failure, where external forces exceed the yield strength of the material, causing it to deform, crack, or break. Stress cracking is another common cause of plastic failure, where stress-cracking agents in cleaning agents, lubricants, and greases can cause plastic products to fail over time. Understanding the expected lifetime of a product and the mechanical capabilities of the selected materials is crucial to preventing plastic failure.
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Stress-cracking agents
Environmental stress cracking (ESC) is a common cause of plastic failure, accounting for 25% of plastic failure in commercial usage. It occurs when plastic under stress or tension is in contact with a fluid, causing the force to travel along the polymer chains. This results in the deformation and eventual separation of molecules, leading to stress cracking.
ASTM D 1693 describes a test for evaluating the stress-crack resistance of ethylene plastics in environments containing stress-cracking agents, such as soaps, wetting agents, oils, or detergents. The test involves placing strips of plastic with controlled defects in a bending rig and exposing them to these agents. The critical strain test, performed using a Bergen elliptical strain rig, determines the minimum strain required to initiate crazing in the presence of a stress-cracking agent.
Additionally, fracture mechanics testing involves machining a well-defined flaw or crack into a plastic specimen. The growth of the flaw in the presence of a stress-cracking agent is monitored until failure. Another method, outlined in ISO 4599, positions strips of plastic under fixed flexural strain and exposes them to a stress-cracking agent for a predetermined period. After exposure, the strips are visually examined for changes and tested for properties like tensile strength.
The presence of stress-cracking agents in various substances underscores the importance of understanding the expected lifetime of a product and the mechanical capabilities of its materials.
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Mechanical failure
The mechanical failure of plastics can be influenced by factors such as material selection, design, process, and service conditions. Hasty material selection can lead to failure, particularly when specific requirements, such as high-impact resistance or outdoor use, are not adequately considered. Proper material selection involves careful planning, a thorough understanding of plastic materials, and reasonable prototype testing. Design-related failures are also common, as the criteria for designing plastic parts vary depending on the material and application. Therefore, designers must follow the guidelines provided by material suppliers to ensure the successful performance of the plastic product.
Stress-cracking is another significant factor contributing to mechanical failure in plastics. Stress-cracking agents are prevalent in everyday substances such as cleaning agents, lubricants, greases, and protective coatings. When plastics are exposed to these agents, they can undergo stress cracking, leading to product failure over weeks or months. This highlights the importance of considering the expected lifetime of a product to ensure the mechanical capabilities of the selected materials are suitable.
Additionally, mechanical failure can occur due to the extension of a product's warranty without reassessing the long-term capabilities of its materials. Plastics, unlike metals, exhibit variable behaviour when subjected to changes in time, stress, and temperature. Crazing, which represents the initial stages of failure, particularly in amorphous polymers, can progress to outright failure if it occurs in the presence of stress-cracking agents. Therefore, it is crucial to periodically evaluate the materials used in products with extended warranties to prevent unexpected failures.
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Thermal failure
The mechanical strength of plastic materials is contingent on temperature. Temperature is one of the key factors influencing the long-term performance of plastics, alongside time and stress. An increase in temperature necessitates a decrease in product lifetime and applied stress. This is because, as the environment becomes hotter, the performance of plastics decreases.
The mechanical strength of plastics is also dependent on the design of the part. For instance, sharp corners are likely to add stress to plastic, leading to functionally weak parts. Additionally, the rate at which creep strain increases is related to the slope of the stress-strain curve at the applied stress.
The investigation of the fatigue behaviour of ABS and PC-ABS polymers at different temperatures has been the subject of recent studies. These studies have examined the effect of high and low temperatures on the fatigue life of these polymers. The mechanical strength of plastic materials used in automotive applications is influenced by temperature, with components installed in vehicles working at temperatures that vary according to working and environmental conditions.
Stress-cracking is the most common cause of plastic product failure. Stress-cracking agents can be found in cleaning agents, lubricants, greases, and protective coatings on metal parts. Crazing, which represents the initial stages of failure in a material, particularly in amorphous polymers, can quickly lead to outright failure if it occurs in the presence of certain chemicals that act as stress-cracking agents.
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Material selection
The mechanical properties of polymers play a crucial role in determining the performance of plastic components under stress. When selecting materials for plastic parts, it is essential to consider four key factors: material selection, design, process, and service conditions. Hasty material selection can lead to failures in plastic parts, and proper selection techniques are necessary to choose the best material for any application.
The mechanical characteristics of materials include strength, stiffness, hardness, and toughness. Strength is the measure of a material's resistance to external stress, and it is an important indicator of how well a material can withstand plastic deformation. Stiffness refers to the material's resistance to deformation, while hardness is its ability to resist deformation under concentrated compressive loads. Toughness, on the other hand, is the measure of the material's energy absorption capacity during impacts.
To evaluate and compare these properties between different materials, standardised test methods can be employed. For instance, the tensile properties of plastics, such as tensile strength and rigidity, can be determined through a tensile test, as outlined in DIN EN ISO 527. Additionally, the impact strength of thermoplastics can be assessed using Charpy or Izod impact tests, where a small rectangular rod is struck by a pendulum at high speed, and the energy absorbed during breakage is measured.
When selecting materials for plastic parts, it is crucial to consider the specific application requirements. For instance, in applications requiring high-impact resistance, a high-impact material must be specified. Similarly, for outdoor use, a UV-resistant material is necessary. Other factors such as thermal, environmental, electrical, and chemical properties should also be taken into account.
Furthermore, it is important to refer to the basic rules and guidelines provided by material suppliers when designing plastic products. Design criteria can vary depending on the material and application. Proper material selection and design are essential, but they do not guarantee success. Poor processing practices can still lead to failures in plastic parts.
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Design
When designing plastic parts, it is important to consider the unique properties of plastics and how they respond to stress, especially over long periods. Plastics can undergo both elastic and plastic deformation, and understanding these behaviours is crucial for effective design.
Firstly, the choice of material is critical. Each type of plastic has distinct characteristics, such as shear rate limits, that must be considered. The presence of additives and fillers can also impact the performance of the plastic part. For instance, ultraviolet (UV) stabilizers, heat stabilizers, and lubricants are commonly added to plastics, but their shear rates should be evaluated as they can degrade before the polymer chains, compromising the part's performance.
Secondly, the design of the plastic part itself plays a significant role in managing stress. Sharp corners and abrupt changes in thickness or direction can increase stress concentrations in the plastic. Smooth, curved corners are preferable as they help the plastic flow during moulding and reduce pressure drops, minimizing disturbances in the molten plastic flow.
Additionally, the wall thickness of the plastic part should be carefully considered. Thick areas are more prone to sink or void issues, so redesigning these sections to achieve a more uniform wall thickness can mitigate these problems. If reinforcement is required, adding ribs can strengthen the part without significantly increasing weight or cost, similar to the use of 'I' and 'T' sections in steel girders.
Furthermore, the boss, a cylindrical design element used for mounting, locating, or reinforcing, is a valuable feature in plastic moulding. It must be designed to withstand various forces, including tension, compression, and torsion. While the boss provides reinforcement, its tubular form may not always be strong enough, and alternative designs may be necessary to avoid sink marks and mould-sticking issues.
Computer-aided design (CAD) systems and flow simulation programs are invaluable tools for designers, allowing them to model and analyse stresses during the moulding process and make iterative improvements. By utilizing these technologies, designers can optimize the plastic part's performance and longevity, reducing the chances of failure due to tension or other stresses.
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Frequently asked questions
Plastic failure, or plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces.
Plastic failure is generally related to four key factors: material selection, design, process, and service conditions. Material selection failures are common in the plastics industry due to hasty decisions. Design-related failures are the most common type of failure.
There are two main types of plastic failure: mechanical failure and thermal failure. Mechanical failure occurs when external forces exceed the yield strength of the material, causing it to deform, crack, or break. Thermal failure happens when products are exposed to extremely hot or cold environments, causing warping, twisting, melting, or brittleness.
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