Alexei Volkov
Plastic is a versatile material with a wide range of applications, from pharmaceuticals to everyday products. The process of creating plastic involves converting long-chain hydrocarbons into simpler molecules through a process called cracking, which involves high temperatures and pressure. The effect of pressure on plastic is a fascinating area of study, with pressure having the ability to significantly alter the mechanical properties of plastic, including its elasticity. This is evident in the everyday example of a plastic bottle, where changes in temperature and altitude can cause the bottle to expand and contract, affecting the internal pressure and potentially leading to leaks. Additionally, the right amount of pressure can give plastic sheets vitality, transforming them from flat surfaces into accurate three-dimensional shapes.
| Characteristics | Values |
|---|---|
| Plastic deformation | Pressure increases the elastic properties of plastics |
| Plastic formation | Pressure turns plastic sheets from flat expanses into accurate, three-dimensional parts |
| Cracking of hydrocarbons | High temperature and pressure break down complex hydrocarbons |
| Polymerisation | Pressure, heat, and catalysts join hydrocarbon monomers into long, repeating carbon chains |
| Atmospheric pressure changes | Lowering ambient pressure and increasing temperature cause internal pressure to rise |
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What You'll Learn
Plastic deformation
In crystalline materials, plasticity is facilitated by the presence of dislocations, allowing planes of atoms to slip past each other and resulting in a permanent change of shape. This phenomenon is more common in metals due to the higher occurrence of dislocations within their crystal structure. On the other hand, amorphous materials, such as polymers, lack long-range order but can still undergo plastic deformation. When subjected to tension, the bending moment can exceed the fully plastic moment, leading to deformation.
The plasticity of a material is directly related to its ductility and malleability. Ductile metals, for instance, exhibit elastic behaviour under tensile loading, returning to their original size when the load is removed. However, once the load surpasses the yield strength, the extension becomes more rapid, and upon unloading, some degree of extension persists, indicating plastic deformation.
Perfect plasticity describes the ability of materials to undergo irreversible deformation without any increase in stresses or loads. However, plastic materials that have been pre-deformed, such as through cold forming, may require higher stresses for further deformation. Additionally, the rate of deformation is influenced by the deformation speed, with higher stresses needed to increase the rate.
In ceramics, plastic deformation occurs under specific loading conditions, such as contact loading by a blunt or sharp indenter. The failure modes include the development of a cone crack due to tensile stresses or damage caused by compressive stress. Understanding plastic deformation is crucial in engineering to manage the transition from elastic behaviour to plastic behaviour, known as yielding, and ensure the proper functioning of products.
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Pressure forming
During the pressure-forming process, a single sheet of thermoplastic material is clamped into a pressure box and heated to a forming optimal temperature. The heated sheet is then positioned over a custom mold, with the non-mold side of the sheet creating a seal between the pressure box and the mold. Air pressure is applied to the non-mold side, while a vacuum is pulled on the mold side, ensuring the plastic conforms tightly to the mold. This combination of air pressure and vacuum technology results in a final product with a high level of surface detail, precision, and quality.
One of the key advantages of pressure forming is its ability to create complex geometries and detailed surface features that are challenging to achieve with other manufacturing processes. It offers enhanced forming pressure compared to traditional vacuum forming, resulting in sharper and more precise details. Additionally, pressure-formed plastic parts can be a good alternative to sheet metal, fiberglass, or wood, offering weight advantages and design flexibility.
The versatility of pressure forming is further enhanced by the range of thermoplastic materials available, each with unique properties to meet specific requirements. These materials include ABS, PC/ABS, HDPE, TPO, HIPS, and PVC/Acrylic, among others. By selecting the appropriate thermoplastic material, pressure forming can be tailored to meet industry standards for strength, durability, impact resistance, flammability, and weather resistance.
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Effect of temperature
The effects of temperature on plastic are varied and complex. Temperature plays a crucial role in the formation, deformation, and overall behaviour of plastic materials.
During the production of plastic, temperature and pressure are applied to transform hydrocarbons into the desired plastic resins. For instance, in the case of ethylene monomer, when subjected to heat, pressure, and a catalyst, it forms long, repeating carbon chains, resulting in a plastic resin known as polyethylene (PE). This process involves melting the ethylene into a thick liquid, which is then cooled and hardened into a solid plastic product.
The choice of plastic for specific applications, such as pressure forming, depends on various factors, including temperature tolerance. Different plastics have distinct temperature ranges within which they maintain their structural integrity. Exceeding these temperature thresholds can lead to deformation or degradation of the plastic material.
In the context of plastic bottles containing liquids, temperature plays a significant role in altering the internal pressure. As the temperature increases, the liquid inside the bottle expands, leading to a rise in internal pressure. This phenomenon is evident when observing bottles stored in varying temperature conditions, such as in a car that experiences fluctuations from 30°F at night to 90°F during the day. The heat causes the liquid to expand, and the subsequent increase in internal pressure can result in leaks.
Additionally, temperature changes can affect the air pressure inside a sealed plastic bottle. When a bottle is filled and capped, the pressure of the trapped air can be influenced by the temperature of the liquid. As the bottle is transported to higher elevations, the external air pressure decreases, allowing the warmer air inside the bottle to expand and exert pressure on the liquid, potentially causing it to leak.
Understanding the effects of temperature on plastic is crucial for designing plastic products and ensuring their integrity under various environmental conditions. The behaviour of plastic materials in response to temperature fluctuations is an important consideration in applications ranging from plastic manufacturing to everyday use.
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Plastic under pressure in cars
Plastic is now a common feature in modern cars, especially in the engine bay. There are several reasons for this, including the fact that plastic is lighter and cheaper than metal. Plastic is also impermeable, protecting complex electronics from moisture. In addition, plastic covers can protect an engine from corrosive dust, reduce noise, and protect wiring and other components.
Plastic panels under cars can also serve several functions. Firstly, they can aid in cooling by directing airflow towards the turbo or radiator. This helps to prevent overheating. Secondly, plastic panels can improve aerodynamics, leading to better fuel mileage and high-speed stability. They can also help to keep grime and dirt out of the chassis and drivetrain, protecting sensitive parts.
However, there are some concerns about the use of plastic in cars. Plastic can be difficult to repair and service, and it can be prone to damage from road debris and the elements. In addition, plastic refuses to break down in the environment, contributing to the microplastics problem. Despite these concerns, the use of plastic in cars is driven by consumer demand for more features, better fuel economy, and lower prices.
Overall, while plastic in cars has its advantages, such as weight reduction and cost-effectiveness, there are also environmental and maintenance concerns associated with its use. The automotive industry is constantly under pressure to balance these factors to meet consumer demands and regulatory requirements.
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Plastic's response to stress concentration
Plastics respond to stress concentration differently from metals. A change in direction can cause plastic to fail, and the forming of a range of plastics can put a lot of stress on a point. This is especially true for the point of a cone, which would most likely fail under pressure.
Residual stresses are introduced by nearly all techniques used for polymer manufacturing. They are formed as a result of extrusion, stretching, drawing, molding, casting, joining, or other strain-inducing processes. Strains can be introduced by differential shrinkage, uneven cooling, or nonuniform flow. Residual stresses and molecular orientation throughout a product provide important information about how that product will perform.
The presence or absence of residual stresses is a major concern in the production of plastics for applications such as medical devices, packaging, instrument manufacturing, and optical components. Measuring residual stress in transparent plastics can be done through destructive and non-destructive methods. For non-transparent plastics, choices are limited to destructive tests such as layer removal, section removal, and stress-corrosion tests.
One of the simplest ways to observe stress in transparent plastics is to place a sample of the material between two illuminated polarized sheets. When these are properly aligned, the stress pattern becomes visible. Birefringence, or the change in the index of refraction in a strained material, can be measured using a "compensator," a calibrated, graduated wedge that is placed in series with the measured item. This method is a standard ASTM test procedure and is particularly effective for quality control of clear plastics.
The effect of pressure on plastic properties is a well-recognized phenomenon. Hydrostatic pressure loading can significantly affect the mechanical properties of solids, increasing their elastic properties.
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Frequently asked questions
Plastic is made from petrochemicals, which are extracted from the Earth's source of oil. Crude oil is a mixture of hundreds of hydrocarbons. The long-chain hydrocarbons are converted into simpler hydrocarbons, which can be turned into chemicals used to prepare plastics.
The process of converting hydrocarbons involves high temperature and pressure. This process is called steam cracking, which breaks the hydrocarbon chains without a catalyst. Pressure is also used to form plastic sheets into accurate, three-dimensional parts.
When a plastic bottle containing liquid is transported to higher elevations, the air pressure decreases, allowing the air inside the bottle to expand and causing pressure on the liquid. This pushes the liquid up and out of the bottle.
Pressure can significantly impact the mechanical properties of solids, including plastic. One of the most notable effects is the increase in elastic properties.
