Chloe Zhang
Cavitation is a phenomenon where vapour bubbles or cavities in a fluid grow and collapse due to pressure fluctuations. It can lead to material deformation and damage, such as microcracks and pitting, through repeated cycles of tensile and compressive stresses. Cavitation occurs in fluids with tensile stresses (low pressures) and regions of higher pressure where compressive stresses can cause bubbles to implode. Cavitation has two primary forms: vaporous and gaseous, with the former being more rapid and resulting in shock waves and microjets that can erode surfaces. Cavitation is observed in various materials, including metals and plastics, with plastics exhibiting lower impact loads due to their reduced acoustic impedance. The occurrence of cavitation during plastic extrusion, specifically in polyethylene, has been studied, revealing insights into the factors influencing cavitation and its impact on the final product.
| Characteristics | Values |
|---|---|
| Cavitation occurrence | Observed in the last 1.5 mm of the capillary tube immediately upstream of the exit |
| Cavity shape | Highly irregular |
| Cavity dimensions | Typical length and width of 150 μm |
| Cavity dynamics | Form and disappear over approximately 20 ms |
| Cavitation cause | Reduced pressure and extensional shear stress |
| Cavitation types | Vaporous, Gaseous |
| Cavitation wear | Occurs under vaporous cavitation conditions due to shock waves and microjets |
| Cavitation erosion resistance of plastics | Ranges between half and 30 times that of carbon steel |
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What You'll Learn
Cavitation of polyethylene during extrusion processing instabilities
The study concluded that extensional and shear stress in the exit region were not the direct causes of cavitation. Instead, it was found that cavitation occurred in conjunction with an upstream rupture of the polymer in the contraction region leading into the capillary tube (gross melt fracture). The exit region was still considered to be the initiation point of cavitation due to the reduced pressure and extensional shear stress.
Cavitation is a phenomenon that can occur during the processing of polymers, particularly in the case of polyethylene. It involves the formation and collapse of voids or cavities within the material. This can be influenced by various factors such as processing conditions, polymer characteristics, and the presence of additives.
The occurrence of cavitation during extrusion processing instabilities can have significant effects on the final product's quality and performance. It can lead to defects such as irregular surfaces, dimensional inaccuracies, and weakened structural integrity. Understanding the mechanisms behind cavitation is crucial for developing strategies to mitigate its negative impacts.
Additionally, the presence of cavitation during extrusion processing instabilities can serve as an indicator of underlying issues in the process conditions or material characteristics. By studying and analysing the occurrence of cavitation, valuable insights can be gained into optimising processing parameters, improving material selection, and enhancing overall product quality.
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Cavitation occurs in the contraction region leading into the capillary tube
Cavitation is a mechanism in which vapor bubbles or cavities in a fluid grow and collapse due to local pressure fluctuations. Cavitation damage has several periods of activity: the incubation period, accumulation period, and steady-state period. During the incubation period, microcracks form around grain boundaries and inclusions due to the elastic and plastic deformation of the surface. In the accumulation period, cracks grow and spread in relation to the degree of splitting, shearing, and tearing action on the material. Finally, in the steady-state period, the rate of crack nucleation and propagation remains constant for the remainder of the exposure time.
Cavitation can occur in two principal types: vaporous and gaseous. Vaporous cavitation is an ebullition process that occurs when the pressure level drops below the vapor pressure of the liquid, causing the bubble to grow explosively in an unbounded manner as the liquid rapidly turns into vapour. Gaseous cavitation, on the other hand, is a slower diffusion process that occurs when the pressure falls below the saturation pressure of the non-condensable gas dissolved in the liquid. Gaseous cavitation depends on the degree of convection present. Cavitation wear is exclusive to vaporous cavitation due to the presence of shock waves and microjets that can erode surfaces.
Cavitation surface etching has been observed in meandering capillary tubes with an inner diameter of 1.8 mm. Etching pits were found on the interior face at the heat rejection region after 200 hours of operation. Irregular copper debris was also discovered in the reclaimed operating fluid. Analysis of temperature and acoustic data revealed that spiking temperature differences resulted in highly turbulent two-phase flow, causing rapid condensation and shrinkage of vapour bubbles. This, in turn, led to strong micro jet impingement that damaged the pipe wall.
Cavitation has also been observed inside a capillary die during the extrusion of polyethylene. Specifically, cavitation occurs in the contraction region leading into the capillary tube, known as gross melt fracture. Voids form near the wall, growing to a typical length and width of 150 μm before shrinking and disappearing within approximately 20 ms. The shape of these cavities is highly irregular. While extensional and shear stress in the exit region are not considered the direct causes of cavitation, the reduced pressure and extensional shear stress in this region likely contribute to the initiation of cavitation.
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Cavitation damage and fatigue failure
Cavitation is a phenomenon that involves a considerable local reduction of pressure in a liquid, leading to the disruption of continuity. This reduction in pressure causes the formation of vapor cavities and bubbles, which can expand explosively and cause irreversible damage, such as material erosion. This is known as cavitation erosion, a destructive and complex phenomenon that can lead to fatigue failure in materials.
During cavitation, the liquid reaches its boiling point, and ebullition occurs, forming vapor bubbles. These bubbles have a short lifespan as any increase in pressure causes them to collapse and produce shock waves. These shock waves impinge on adjacent metal surfaces, generating high-impact forces that can cause work hardening, fatigue, and cavitation pits. The damage caused by cavitation is characterised by pittings and a rough profile, with the severity increasing as the surface roughness increases.
To prevent cavitation and its associated damage, it is crucial to minimise tensile stress on the fluid. This can be achieved by increasing the pressure level at throttling valve outlets and supercharging pump inlet suction ports. Additionally, the use of ductile materials with higher fatigue resistance and corrosion resistance can enhance protection against cavitation-induced damage.
In the context of plastic extrusion, cavitation has been observed in the processing of polyethylene. Specifically, it occurs upstream of the exit in the capillary die, in conjunction with an upstream rupture of the polymer in the contraction region. This rupture leads to a reduction in pressure and extensional shear stress, triggering cavitation. While the exit region is considered the initiation point, the extensional stress and shear stress at the exit are not the direct causes of cavitation in this case.
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Cavitation wear and hydraulic issues
Cavitation is a destructive phenomenon that occurs in hydraulic systems when there is a considerable local reduction of pressure, leading to the formation of vapor bubbles in the fluid. This process of bubble formation and collapse, known as cavitation erosion, cavitation wear, or cavitation pitting, results in surface destruction and material displacement. The metal parts begin to wear away, causing accelerated surface wear, premature seal failure, generation of unwanted heat, compromised hydraulic fluid quality, and lack of lubrication leading to additional wear.
Cavitation wear is mechanical in nature and is caused by the application of tensile and compressive stresses. It is characterised by abrasions in a pattern consistent with the movement of metal parts, rather than pitting. The size of the particles generated by cavitation wear depends on the Brinell hardness of the exposed material, with larger particles occurring during the accumulation period and smaller particles produced at higher cavitation intensities.
Many areas in hydraulic systems are prone to cavitation wear, including downstream of control valves with high-pressure differentials, suction chambers of pumps with starved inlet conditions, rapidly moving actuators, leakage paths, and devices where fluid flow is subjected to sharp turns or reductions in cross-sections. When investigating a cavitation problem, it is crucial to identify sources of low pressure, high temperature, and locations where air ingression may occur.
To prevent cavitation, temperature control is essential. Hydraulic fluid that is too viscous can contribute to pressure drops, and high temperatures combined with low pressures can also induce cavitation. Piping losses, such as using too many fittings or a collapsed pipe liner, can further exacerbate the issue. When parts experience surface damage due to cavitation, repairs are often challenging, and replacement of damaged components may be necessary.
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Cavitation erosion resistance of plastics
Cavitation does occur during plastic extrusion. Cavitation erosion tests have been carried out on plastics such as epoxy resin, polypropylene, high-density polyethylene, and polyamide 66. The results show that the cavitation erosion resistance of plastics ranges between half and 30 times that of carbon steel. The cavitation erosion of plastics is caused by fatigue fracture, similar to metals. However, due to plastics' low acoustic impedance, the impact loads from bubble collapse are significantly reduced. This means that the resistance and incubation period of cavitation erosion can be assessed in terms of bubble collapse impact energy and strain energy derived from fatigue strength.
The addition of epoxy mortar as an intermediate layer has been found to enhance cavitation resistance by delaying the formation of cavitation pits on the coating surface. This is due to the material's improved energy absorption capacity and superior adhesive properties. Furthermore, polyurea coatings have demonstrated exceptional elastic-plastic deformation capabilities.
Underdosed and overdosed copper coatings can negatively impact cavitation erosion resistance. While copper improves thermal conductivity, excessive copper weakens interface bonding. The optimal resistance was achieved with a coating containing 8.3% copper, which exhibited a 14% lower volume loss after 10 hours of cavitation erosion compared to pure UHMWPE coating.
The bonding between the reinforcement and the matrix can be improved through various surface treatments. For example, adding B4C particulates to epoxy improves its hardness and marginally enhances shear strength while reducing the plasticization tendency after moisture absorption. However, poor anchoring of B4C particulates can compromise wear resistance, especially when exposed to moisture.
Understanding the cavitation erosion resistance of plastics is crucial in various industries, including manufacturing and engineering. By studying the behaviour of different plastics during cavitation and implementing appropriate coatings or treatments, we can enhance the performance and longevity of plastic components in applications where cavitation may occur.
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Frequently asked questions
Cavitation is a fluid-to-surface type of wear that occurs when a portion of the fluid is exposed to tensile stresses, causing it to boil, and then exposed to compressive stresses, causing vapor bubbles to collapse and produce mechanical shock.
Yes, cavitation can occur during plastic extrusion. It has been observed during the extrusion of polyethylene, for example.
Cavitation during plastic extrusion is caused by high relative motions between the surface and the exposed fluid, which reduce the local pressure of the fluid and cause small vapor cavities to form.
