dc.description.abstract |
Investment casting is known as a “near net shape” process as it can produce
parts that need very little secondary machining operations. The investment
casting process is one of several metal casting processes and it has an
advantage in that very complex designs can be produced with good accuracy
and surface finish. The investment casting process begins with the fabrication
of a sacrificial pattern, typically made of foundry wax, with the same basic
geometrical shape as the finished cast part. Normally, the sacrificial wax
patterns are produced by injection moulding where hot wax is injected into the
desired patterns. Runner and gates systems are assembled and attached
onto the completed fabricated wax patterns. Next, the wax patterns with
runners and gates are repeatedly dipped into a ceramic slurry, with a drying
period in between dipping, to form layers that create a shell. Investment
casting sacrifices a pattern and ceramic shell mould for each metal part that is
made. The investment casting process suffers long lead times when a new
part is designed, due to the fabrication of initial tooling. Cost and lead-time to
produce tooling can be prohibitively high and complexity is limited by what is
possible with injection moulding in the chosen time. These factors have lead
investment casting foundries as well as the companies purchasing the
castings to explore alternate methods to create investment cast parts without
the cost and time burden associated with permanent tooling.
Additive manufacturing patterns provides an alternative method for producing
investment casting patterns that can provide dramatic time and cost savings.
It also gives the designers freedom to rapidly modify and redesign a product
without significant increase in the total development time and cost. Nowadays,
the foundries are able to play around with different designs, test them, and
reach the optimum design very quickly. It is relatively expensive and timeconsuming
to do this using conventional investment casting. Furthermore, by
using additive manufacturing, patterns can be made as complex as needed
without any impact on the cost. This study determined the difference in
dimensional accuracy between PrimeCast® and PMMA patterns produced for investment casting by two different additive manufacturing technologies as
well as their corresponding castings.
PrimeCast® and PMMA patterns were built at the same time at Central
University of Technology, Free State and Vaal University of Technology,
respectively. Metrology was performed on all patterns just after manufacturing
using a micro-computed X-ray tomography scanner to compare dimensional
accuracies of different features of the patterns. Aluminium alloy A356 was
cast in the moulds made from both types of patterns. Similar metrology was
performed on all the castings to compare dimensional accuracies of different
features of the castings from the two types of patterns. The patterns had
features such as thin walls, cavities and angles that pose challenges to these
additive manufacturing technologies and the investment casting process.
From the results of this study, it was found that both technologies provided
good dimensional results on simpler shapes. The PrimeCast® pattern had a
better dimensional accuracy than the PMMA pattern. However, the casting
from the PMMA pattern had relatively better dimensional accuracy than the
casting from the PrimeCast® pattern. |
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