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Background |
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A
typical rotational molding unit is shown in figure 1. In this process, molds
carrying the plastic powder are mounted on the arm of the machine, which
rotates about a central axis. The mold, in turn, rotates about its own
(perpendicular) axis, which results in a biaxial rotational system. While
in the uniformly heated furnace, the plastic melts and sticks to the inner
surface of the mold cavity. After this stage, the frame housing, on which
the molds are mounted, is moved to a cooling chamber, where ambient air
initially cools it, followed by forced-convective cooling. The solidified
plastic part retains the shape of the inner surface of the mold cavity.
This process is unique because the plastic melts in the mold instead of
molten plastic being forced into the mold cavity. The use of biaxial
rotation of the mold during heating and cooling results in uniform heat
transfer.
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Motivation |
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Rotational molding is a
very popular choice for the manufacture of many plastic parts. There is a
potential to modify this process to make it more energy efficient. Any
energy saving would make a big impact because of the popularity of the
rotational molding process. |
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Past Work |
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From preliminary analysis and from previous work it is apparent that a major
fraction of the energy input is wasted in the auxiliary housing surrounding
the molds. However, this issue of energy consumption by the auxiliary
housing has not been sufficiently addressed. The present work attempts to
address this issue and present novel ideas to minimize this energy wastage. |
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Specific Objectives |
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Detailed analysis of the transient rotational molding
operation with emphasis on the cycle times and energy consumption during
various phases of the operation
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Predict the fraction of the
energy wasted on the auxiliary mass and propose novel designs to
minimize this wastage.
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Approach |
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Figure 2 Mold geometry
Figure 2 shows the actual geometry of the
part to the manufactured. This was simplified as shown in the figure to ease
calculations. |
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Figure 3
Calculation methodology
To solve the transient
energy equations and obtain the heating and cooling times and the energy
consumption, the mold wall and the solid and molten plastic layers were
divided into small segments. The heat transfer equations were solved
iteratively using constant time steps. The physical properties of material
in a segment were calculated at the conditions at the center of the segment.
This is depicted in figure 3. |
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Sample Findings |
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Figure 4 Mold temperature profile
The temperature profile inside the mold is
shown in figure 4. The heating and cooling times were then varied to
optimize the energy consumption. |
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Figure 5 Mold and auxiliary mass
energy consumption
The energy and fuel consumption for the
baseline and the optimized case is shown in figure 5. It is seen that the
auxiliary mass in the mold consumes over 90% of the input energy. A
significant amount of energy can be saved if the auxiliary mass energy
consumption were minimized. |
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Figure 6 Alternate heating and cooling
system
Figure 6 shows a proposed alternate
heating and cooling system. Heating and cooling of the plastic in the mold
is achieved with a fluid flowing in jackets integral to the mold. Since the
heat flowing to the plastic does not have to conduct through the mold
casing, the energy wasted on heating the auxiliary mold casing is reduced.
Also the small channels provide a high heat transfer area and heating by a
liquid also leads to high heat transfer coefficients. This results in lower
heating and cooling times. |
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Figure 7 Alternate system with
recuperation
Figure 7 shows a sample heating and
cooling loop. Innovative schemes are used to recuperate part of the heat
rejected by the molds in the cooling stage and use it to heat the next batch
of molds. Besides additional heat is recovered in the form of hot water,
which can be used for other processes in the plant.
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Figure 8 Energy and fuel consumption
for alternate systems
The energy and fuel consumption with
these alternate heating and cooling schemes is shown in figure 8.
Additionally about 150 to 200 kg of hot water at about 44 to 48oC
is obtained from every batch of molds. |
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Important Implications |
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A detailed analysis of
heating and cooling in a rotational molding operation was carried out.
Special emphasis was laid on tracking the heat flow, heating and cooling
times and energy consumption. As seen from figure 8, use of these alternate
heating and cooling schemes could lead to significant direct benefits in
terms of savings in energy costs. Besides, savings in energy also have
attached benefits in the form of reduced environmental pollution and
reduction in the demand of our non-renewable energy sources. |
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Ongoing Work / Future Directions |
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Some of the directions for future work in this area
include,
 | Experimental verification of the computationally
obtained results in the present work |
| Similar analysis of other types of
molding operations and inventing energy saving alternate schemes for them. |
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Sponsors |
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Schafer
Systems Inc, Adair, IA. |
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Papers |
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Ghosh, K.
and S. Garimella (2004). Dynamic Modeling of Thermal Processes in
Rotational Molding. Proceedings of the ASME HeatTransfer/Fluids
Engineering Summer Conference 2004, HT/FED 2004, volume 3, p 1107-1118
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