15
Contemporary Machining Processes
The main characteristic of the LFC process is that patterns made of polymers
remain in the cast until the molten liquid metal is introduced. Contact with liquid
metal causes intensive decomposition of the polymer pattern, which evaporates over
a relatively short time simultaneously with metal crystallization (Prstić et al., 2014).
During the process, the decomposed liquid products are pushed toward the molding
cavity upper surface, then the liquid metal front. In order to achieve high-quality
products, it is crucial to reach balance in the system of evaporable polymer pattern–
liquid metal–refractory coating–sandy mold during the metal inflow, decomposition
and evaporation of polymer pattern, and solidification of casting. Important factors
influencing the process of decomposition and evaporation of the patterns in LFC are
as follows: temperature, pattern density, type and thickness of the refractory coat
layer the evaporable pattern is covered with, type and size of sand grain, respective
permeability of sand, gating, and other structural details. The pattern density and
permeability of the refractory coating and sandy cast determine polymer evaporation
velocity. In order to achieve desired quality and predicted characteristics of a part,
critical process parameters and the type of alloy for casting should be determined for
each particular pattern material (Prstić et al., 2014). Among the advantages of LFC,
the following can be listed (Fan et al., 2014):
1. Low surface roughness Ra 6.3 to 12.5 μm, and high dimensional precision from CT5 to CT7. The time of further mechanical processing can be
decreased by 40% to 50% compared to parts obtained by the traditional
sand casting method.
2. The structure design of LFC casting is flexible, with no drawing pattern
inclination and a final casting can be obtained whole. As a consequence,
the core can be omitted and the pore structure can also be directly manufactured, resulting in great cuts in time and cost of processing.
3. Loose-sand compact modeling is adopted in the LFC process, and the sand
has no binder. Therefore, the production process of castings is simplified
and the sand can be fully reused, saving production costs.
4. The LFC process has potential for cleaner production, since the polystyrene (EPS) does no harm to the environment at low temperatures.
Compared with the traditional casting process, the harm from noise,
CO, and silica dust is significantly decreased, resulting in an improved
environment.
When the melt fills the mold cavity, complex physical and chemical phenomena take
place, including heat transfer, filling flow, chemical reactions, cooling, and solidification. These influence each other, so that the liquid metal filling process is very
complex and ultimately affects the casting quality. In recent years, researchers have
carried out extensive simulation and experimental research into the liquid metal filling process flow and heat transfer of LFC (Xie et al., 2015). Some drawbacks of
the LFC motivate further development of several novel LFC technologies, such as
lost foam casting under vacuum and low pressure, vibration and pressure solidification conditions, expendable shell casting technology, and preparation technology of
bimetallic castings based on the LFC process (Jiang and Fan, 2018).
Contemporary Machining Processes
The main characteristic of the LFC process is that patterns made of polymers
remain in the cast until the molten liquid metal is introduced. Contact with liquid
metal causes intensive decomposition of the polymer pattern, which evaporates over
a relatively short time simultaneously with metal crystallization (Prstić et al., 2014).
During the process, the decomposed liquid products are pushed toward the molding
cavity upper surface, then the liquid metal front. In order to achieve high-quality
products, it is crucial to reach balance in the system of evaporable polymer pattern–
liquid metal–refractory coating–sandy mold during the metal inflow, decomposition
and evaporation of polymer pattern, and solidification of casting. Important factors
influencing the process of decomposition and evaporation of the patterns in LFC are
as follows: temperature, pattern density, type and thickness of the refractory coat
layer the evaporable pattern is covered with, type and size of sand grain, respective
permeability of sand, gating, and other structural details. The pattern density and
permeability of the refractory coating and sandy cast determine polymer evaporation
velocity. In order to achieve desired quality and predicted characteristics of a part,
critical process parameters and the type of alloy for casting should be determined for
each particular pattern material (Prstić et al., 2014). Among the advantages of LFC,
the following can be listed (Fan et al., 2014):
1. Low surface roughness Ra 6.3 to 12.5 μm, and high dimensional precision from CT5 to CT7. The time of further mechanical processing can be
decreased by 40% to 50% compared to parts obtained by the traditional
sand casting method.
2. The structure design of LFC casting is flexible, with no drawing pattern
inclination and a final casting can be obtained whole. As a consequence,
the core can be omitted and the pore structure can also be directly manufactured, resulting in great cuts in time and cost of processing.
3. Loose-sand compact modeling is adopted in the LFC process, and the sand
has no binder. Therefore, the production process of castings is simplified
and the sand can be fully reused, saving production costs.
4. The LFC process has potential for cleaner production, since the polystyrene (EPS) does no harm to the environment at low temperatures.
Compared with the traditional casting process, the harm from noise,
CO, and silica dust is significantly decreased, resulting in an improved
environment.
When the melt fills the mold cavity, complex physical and chemical phenomena take
place, including heat transfer, filling flow, chemical reactions, cooling, and solidification. These influence each other, so that the liquid metal filling process is very
complex and ultimately affects the casting quality. In recent years, researchers have
carried out extensive simulation and experimental research into the liquid metal filling process flow and heat transfer of LFC (Xie et al., 2015). Some drawbacks of
the LFC motivate further development of several novel LFC technologies, such as
lost foam casting under vacuum and low pressure, vibration and pressure solidification conditions, expendable shell casting technology, and preparation technology of
bimetallic castings based on the LFC process (Jiang and Fan, 2018).