Alexei Volkov
Plastic grain structure stops shrinkage when the temperature and moisture content stabilize. Plastic shrinkage is the contraction of a plastic moulding as it cools after injection. It is caused by the loss of water through evaporation or suction during the plastic state of the concrete, leading to volume reduction and surface cracking. The rate of shrinkage depends on the type of plastic and the conditions in which it is produced, such as temperature and pressure. The degree of shrinkage is also influenced by the molecular weight of the plastic. Additionally, crystalline plastics tend to have higher shrinkage rates than amorphous plastics.
| Characteristics | Values |
|---|---|
| Cause of shrinkage | Loss of water through evaporation or suction |
| When shrinkage occurs | When plastics melt and cool |
| Factors influencing shrinkage | Temperature, pressure, mold design, processing conditions, part geometry, type of material, presence of filler or fiber reinforcement |
| Types of shrinkage | Volumetric, linear |
| Volumetric shrinkage | Caused by thermal contraction and/or crystallization for semi-crystalline polymers |
| Linear shrinkage | Greater in the direction perpendicular to flow |
| Mold temperature influence | Higher mold temperature results in larger shrinkage, lower mold temperature results in smaller shrinkage |
| Injection pressure influence | Higher injection pressure results in lower shrinkage rate |
| Packing pressure influence | Higher packing pressure is required for highly branched polymers to compensate for cavity pressure drop and prevent higher shrinkage |
| Plastic type influence | Semi-crystalline polymers have higher shrinkage rates than amorphous polymers |
| Plastic material shrinkage range | 0.000 inch/inch to approximately 0.050 inch/inch |
| Low shrinkage range | 0.000 to 0.005 inch/inch |
| Medium shrinkage range | 0.006 to 0.010 inch/inch |
| High shrinkage range | Above 0.010 inch/inch |
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What You'll Learn
- Plastic shrinkage is caused by water loss through evaporation or suction
- Crystalline plastics have higher shrinkage rates than amorphous plastics
- The temperature of the plastic affects the amount of shrinkage
- Mould temperature also impacts the degree of shrinkage
- Packing pressure can be used to minimise plastic shrinkage
Plastic shrinkage is caused by water loss through evaporation or suction
Plastic shrinkage is a phenomenon observed in cement-based materials, such as concrete, during their plastic state. It is characterised by a reduction in volume due to the loss of water through evaporation or suction, leading to surface cracking. This occurs when water evaporates from the surface of the concrete or is drawn out by the suction of dry concrete below.
The rate of evaporation is influenced by various factors, including temperature, humidity, and wind velocity. High temperatures, low humidity, and strong winds can accelerate evaporation, increasing the likelihood of plastic shrinkage. Additionally, the rate of evaporation should exceed the rate of "bleeding," a process where water rises to the top of the concrete after it has been placed and the particles within it begin to settle.
To mitigate plastic shrinkage, it is essential to control the evaporation rate and ensure proper curing practices. This can be achieved by covering the concrete surface, using evaporation retardants, employing temporary windbreaks, and placing the concrete during cooler times of the day. Using recycled aggregates with higher water content can also act as internal curing agents, reducing the effects of plastic shrinkage.
The shrinkage behaviour of plastic materials, such as amorphous plastics and glass fibre-reinforced plastics, is influenced by factors such as orientation, molecular structure, and mould structure. The direction of shrinkage can vary, with amorphous plastics shrinking more in the flow direction and glass fibre-reinforced plastics shrinking more in the direction perpendicular to the melt flow.
Understanding and managing plastic shrinkage are crucial to preventing long-term cracking in concrete structures, reducing repair costs, and maintaining the durability and integrity of the construction projects.
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Crystalline plastics have higher shrinkage rates than amorphous plastics
The shrinkage of plastic materials depends on various factors, including the type of material, processing and part design factors. When plastic materials melt and cool, shrinkage starts at the molecular level. All plastic materials have a distinct shrinkage rate, and this rate is used to predict the difference between the moulded product and the final product after it has cooled to room temperature.
On the other hand, crystalline materials tend to have high shrinkage rates, and the shrinkage is greater in the direction of flow than across the direction of flow, known as "anisotropic" shrinkage. An exception to this rule is when using reinforced materials, which will shrink less in the direction of flow due to the orientation of the reinforcement fibres. Crystalline materials, such as polytetrafluoroethylene (PTFE), isotactic polypropylene, and high-density polyethylene, have significantly higher shrinkage rates than amorphous materials. As these polymers cool, the molecules form crystalline regions, allowing the material to become denser and shrink more.
The shrinkage rate of plastic parts in the vertical direction is generally smaller than that in the horizontal direction. Additionally, thicker walls result in slower cooling, higher crystallinity, and higher shrinkage. Adjusting processing conditions, such as temperatures, pressures, and cooling times, can help mitigate shrinkage.
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The temperature of the plastic affects the amount of shrinkage
The temperature of the mould also affects the amount of shrinkage. A hot mould will result in less shrinkage than a cold mould. This is because a cold mould causes the plastic "skin" to solidify sooner, which means the plastic begins to shrink before full injection pressure is applied. On the other hand, a hot mould allows the molecules to continue moving and be compressed by the injection pressure before solidifying, resulting in less shrinkage because the molecules are not allowed to move as much after solidifying.
The temperature of the plastic and the mould are not the only factors that affect the amount of shrinkage. The injection pressure also has a direct effect on shrinkage rates. Higher injection pressure results in lower shrinkage rates because the plastic molecules are packed more tightly together.
The cooling rate of the plastic product also affects shrinkage. For example, parts needing annealing can be annealed at 40-50º C on a fixture in an air oven.
Additionally, uneven shrinkage can lead to warping. This occurs when the shrinkage in the thickness direction is larger than in the plane direction due to the mould structure restriction in the plane direction. Warping must be prevented during the product design stage, as it is challenging to fix once it has occurred.
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Mould temperature also impacts the degree of shrinkage
The temperature difference within the mould can also lead to uneven shrinkage. When one side of the mould is significantly warmer than the other, the warmer side will exhibit larger shrinkage, while the cooler side will show smaller shrinkage. This uneven shrinkage can lead to warpage, especially in the corners of the plastic part, where the wall thickness is often greater than the basic wall thickness.
The type of plastic material used also influences the shrinkage rate. For instance, amorphous plastics tend to shrink more in the direction of the flow, whereas glass fibre-reinforced plastics shrink more in the direction perpendicular to the melt flow due to the molecular orientation of their crystalline parts. Additionally, certain plastics, such as fluoroplastics and low-density polyethylene (LDPE), are known for their higher shrinkage rates, which can reach up to 5-6%.
To manage shrinkage effectively, manufacturers can adjust the moulding parameters, including temperature, pressure, and holding time. Selecting appropriate plastic materials and understanding their unique shrinkage characteristics are also crucial steps in the process. By considering these factors during the mould design phase, manufacturers can implement a well-designed cooling system to optimise the cooling conditions and effectively control the degree of shrinkage.
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Packing pressure can be used to minimise plastic shrinkage
Plastic shrinkage is a common issue in concrete and cement-based materials, leading to surface cracking and long-term structural damage. This phenomenon occurs due to water loss through evaporation or suction during the plastic state of the material, resulting in volume reduction. While it is impossible to eliminate shrinkage entirely, it is crucial to minimise it to ensure the accuracy and durability of moulded parts.
Packing pressure plays a significant role in mitigating plastic shrinkage. By applying pressure to liquid plastic within a mould, the molecules can be compressed into a smaller volume. This compression directly counteracts the shrinkage, as the greater the cavity pressure, the less the shrinkage. Packing pressure is particularly effective in thicker areas of the mould, where it forces more molten material into these sections, reducing the impact of differential shrinkage.
The filling and packing times are critical factors in this process. Sufficient time is required for the plastic to reach all areas of the mould cavity and pressurise them adequately. The holding time, which refers to the period after the gate freezes, is also essential to maintaining cavity pressure and minimising shrinkage. However, it is important to note that excessively high holding pressures can lead to the material expanding within the mould, making it difficult to remove the final part.
Additionally, the temperature and cooling rate of the mould can influence shrinkage. Non-uniform cooling can cause variations in shrinkage and subsequent warpage, so even cooling is crucial. While increasing the cooling rate can help mitigate shrinkage, it may compromise part performance by reducing crystallinity. Therefore, a balance must be struck between minimising shrinkage and maintaining the desired material properties.
By adjusting processing conditions, including packing pressure, temperature, and cooling rates, mould engineers can effectively minimise plastic shrinkage and improve the quality and accuracy of the final moulded parts.
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Frequently asked questions
Plastic shrinkage is the reduction in a plastic part's dimensions as it cools from its molten processing temperature to room temperature. This phenomenon is caused by the loss of water through evaporation or suction during the plastic state, leading to surface cracking.
The amount of shrinkage varies depending on the type of plastic and the conditions in which it is produced. Factors such as mould design, processing conditions (temperature, pressure, cooling rate), and part geometry also play a role in the final shrinkage. The degree of shrinkage is also influenced by molecular weight, with higher molecular weights resulting in higher viscosity and pressure drop, requiring higher packaging pressure to compensate.
Temperature has a significant impact on plastic shrinkage. A higher temperature generally results in greater expansion and shrinkage, while a lower temperature leads to lower expansion and shrinkage. A 10% change in barrel temperature can result in a 10% change in shrinkage rate. Additionally, a hot mould will create less shrinkage than a cold mould, as the plastic molecules can move more freely before solidifying.
Crystallization plays a crucial role in plastic shrinkage, especially in crystalline plastics. During cooling, highly organized and closely packed "polymer crystals" are formed in crystalline polymers, such as HDPE. Amorphous polymers, like PVC, have negligible crystalline shrinkage due to the formation of secondary crystals during annealing. The presence of side chains in highly branched polymers can also inhibit crystallization and lower shrinkage rates.
