Sofia Martinez
Plastics are susceptible to stress factors that can affect their behaviour and performance over time. This is known as environmental stress cracking (ESC) or environmental stress crazing, and it can lead to premature failure of plastic parts. A basic understanding of the factors that contribute to plastic stress is necessary for product designers and engineers to effectively utilise plastic components in various applications. These factors include both external and internal variables that influence the mechanical properties of polymers. External factors include temperature, pressure, strain rate, type of loading, time of loading, and environment, while internal factors encompass intramolecular structure, secondary bonds, and chemical or physical crosslinking, among others. Additionally, the design of moulds, including gate size and cooling channels, plays a significant role in managing stress within moulded plastic parts. By comprehending these stress cycle factors, it becomes possible to enhance the durability and functionality of plastic materials in different contexts.
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What You'll Learn
Plastic fatigue
Plastic parts have elastic limits, and if they are stressed below those limits for a short time, they will return to their original shape. However, if the part is stressed beyond its limit or even below its limit but for a longer period, there is a possibility its shape will deform. This phenomenon is known as "creep". The amount of stress applied, the duration of that stress, and the temperature are factors that affect creep.
The evaluation of the fatigue strength of plastics is difficult. This is due to the fact that the determination of such data is very time-consuming, and fatigue strength properties are often not available. S/N curves (Woehler curves) have to be generated from individual measurements for different stress levels with the corresponding failure load cycles. In addition, dependencies of the S/N curve on temperature and, if necessary, moisture content, and in the case of fiber-reinforced materials, on the fiber orientation, must be taken into account.
The time and financial costs involved in testing for fatigue strength properties are significant, so alternative procedures for determining these properties are desirable. An approach has been proposed that makes it possible to perform an initial estimation of fatigue strength limits based on often-existing static properties. This approach is implemented in the software MatScape, a material database and software for Converse and S-Life Plastics.
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Plastic deformation
In the context of materials science, plastic deformation specifically refers to the breaking of a limited number of atomic bonds by the movement of dislocations. This process requires an enormous force to break the bonds of all the atoms in a crystal plane. Plastic deformation can be observed in many metal-forming processes, such as forging, pressing, rolling, and swaging. It is also relevant in ceramics, where high shear stresses are required due to the presence of covalent atomic bonds.
It is important to note that plastic deformation is not the same as elastic deformation. In elastic deformation, the object returns to its original shape once the applied force is removed. However, in plastic deformation, the object remains permanently deformed even after the removal of the force. This distinction is crucial in understanding the behaviour of materials under stress and designing parts and products accordingly.
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Plastic mould design
One of the critical aspects of plastic mould design is the selection of appropriate materials. Different materials like ABS, Polyethylene, Polycarbonate, and Polypropylene possess unique properties that impact the durability, aesthetics, and functionality of the moulded part. For instance, ABS is known for its toughness and resistance, making it ideal for products requiring high strength. Additionally, understanding the basics of injection moulding, such as draft, radii, and wall thickness, can enhance mouldability and reduce potential production issues.
The design of the mould's gates, sprue, and runners is another significant factor. These components play a crucial role in managing the pressure and flow of molten plastic into the cavity. Edge gates, for example, inject at the portion line where the two mould halves meet, leaving faint witness marks. Hot tip gates, on the other hand, are ideal for circular or conical shapes requiring consistent flow and are often hidden in dimples or around logos. Pin gates are preferred for aesthetic purposes as they inject material from the interior, resulting in a smoother exterior.
Furthermore, the gate's design and type are influenced by the aesthetic and dimensional criteria of the final product. Considerations such as gate size, location, and quantity must be carefully determined to prevent exceeding recommended ranges for shear heating and stress on the material. This includes taking into account dimensions, tolerances, performance criteria, material limitations (temperature, shear stress, and shear rate), and processing parameters (flow rates, pressures, and cooling).
Additionally, it is essential to set the proper flow rate during the injection phase. A shear rate that is too high can degrade the polymer and additives by causing a significant increase in melt temperature. Understanding the orientation and stretching of polymer chains during the filling phase is crucial to preventing warpage, sinks, voids, and assembly issues. The pack pressure and time are also relevant factors, as overpacking can occur if not carefully controlled.
In conclusion, plastic mould design requires a comprehensive understanding of the material's behaviour, the injection moulding process, and the specific application requirements. By carefully considering these factors, designers can create precise moulds that produce strong, aesthetically pleasing, and functional plastic products while minimising potential issues during manufacturing.
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Plastic fracture mechanics
Theoretically, the stress ahead of a sharp crack tip becomes infinite and cannot be used to describe the state around a crack. Plastic fracture mechanics is used to characterise the loads on a crack, typically using a single parameter to describe the complete loading state at the crack tip. A number of different parameters have been developed, including the J-integral and the crack tip opening displacement (CTOD).
In the case of plastics, standard fracture specimens often show highly nonlinear stress-displacement curves due to excessive crack tip plasticity. This has led to the development of elastic-plastic fracture mechanics (EPFM) theories, which explicitly account for plastic deformation and provide protocols for fracture testing of ductile specimens. EPFM is particularly relevant when the crack-tip plastic zone is comparable in size to the crack length or specimen dimensions.
The ductility of the material gives rise to a relatively large plastic zone at the tip of the crack, where the material locally yields and deforms, absorbing energy and increasing toughness. This is in contrast to brittle fracture, which involves little plastic deformation and results in a smooth fracture surface with ridges.
Additionally, the choice of mould design, material selection, and processing parameters can impact the stress within a moulded plastic part. For example, gate size, location, and quantity must be properly designed to avoid exceeding recommended ranges for shear heating and stress. Understanding these factors can help reduce excessive stress and prevent premature functional failures in plastic parts.
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Plastic stress testing
Stress Factors in Plastics:
Plastic parts experience stress from various factors, including repeated actions, temperature changes, and mechanical loading. One significant factor is fatigue, which occurs when stress is repeatedly applied to a specific area. Over time, even small loads can lead to breakdown and failure. Additionally, plastic parts have elastic limits, and stressing them beyond these limits, even for a short time, can cause permanent deformation, known as creep. The amount and duration of stress, as well as temperature, influence the occurrence of creep.
Mechanical Testing:
Mechanical testing plays a vital role in evaluating the performance of plastic materials under specified stress conditions. This type of testing can be fundamental or imitative. Fundamental testing is performed on standardised plastic pieces, while imitative testing involves subjecting finished plastic products to specific mechanical stress to simulate their intended use. Cyclic testing, where loads are applied and removed repeatedly, is recommended for assessing long-term performance. Mechanical testing helps identify potential shortcomings, ensure compliance with standards, and verify batch consistency.
Residual Stress Testing:
Residual stress testing is essential for identifying and addressing inherent stresses in plastic parts. ASTM D4093 is a standard test method that uses a polariscope, often in conjunction with a compensator, to measure retardation/birefringence and residual stress in transparent plastic parts. This nondestructive and fast procedure enables manufacturers to make real-time adjustments and improve product quality and consistency. Residual stress testing can help detect issues such as cracking and crazing, which are indicators of excessive residual stress, and prevent field failures.
Mitigating Stress in Plastic Parts:
To reduce excessive stress in plastic parts, manufacturers can focus on several key factors. These include improving their understanding of part design, mould design, and material selection. Proper mould design, specifically gate size, location, and quantity, is crucial to managing stress within moulded parts. Additionally, flow rates, pack pressure, and time must be carefully considered to avoid overpacking and warpage. By addressing these factors, manufacturers can minimise the risk of part failure and improve the overall performance of plastic components.
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Frequently asked questions
Thermosetting plastic is a resin or plastic compound that, in its final state, does not melt or flow when reheated. Thermoplastics, on the other hand, can be repeatedly softened by heating and hardened by cooling.
ESC is a phenomenon where a polymer component cracks when exposed to a chemical agent while under tensile stress. It is similar to stress corrosion cracking (SCC) in metals.
External factors include temperature, pressure, strain rate, type and time of loading, and the environment. Internal factors include intramolecular structure, secondary bonds, molar mass and its distribution, chain branching, and copolymerisation.
