Technical Article
Figure 5: Selected transmitted light micrographs of the waxes (A) material A containing two filler forms and (B) containing one filler form (1)
of the shell (see AdBond® Quikset™ at
RemetUK Insights -
http://www.remet. com/uk/remet-insights).
Wax Properties Investment casting pattern waxes are usually made up of paraffin waxes and microcrystalline waxes that form the base wax and further addition of fillers, resin and plasticizer depending on the application. To design a wax good for autoclave de-waxing, parameters such as volumetric thermal expansion, viscosity, melting point, heat capacity and conductivity are critical. Let’s start with filled wax. The main
intention of adding fillers to wax is to improve dimensional performance, reduce shrinkage and better mechanical properties. The two common fillers that are added to the waxes but not limited to are cross linked polystyrene and terephalitic acid (see Figure 4). There are other fillers available and have been used in different waxes including water. Recently there is even development of pattern waxes that do not contain any fillers such as the Fast Form™
. The addition of suitable fillers
is complicated. Addition of fillers changes the rheological properties of the base wax, which usually increases the viscosity. Plastisizers are another additive that is occasionally added to manipulate the rheological behaviour
28 ❘ November 2018 ®
Quicklock®
Boilerclave®
System
of waxes. Addition of filler will also increase the thermal conductivity of pattern wax. The fillers, which are generally better thermal conductors, will cause excessive melting of the bulk wax and thus the pressure build up in the shell moulds during dewaxing. The thermal expansion of wax
can also be affected by the amount of crystallinity in the wax (4). They suggest that crystallinity in wax varies because of shear during wax injection and greater crystallinity in wax increases the
volumetric expansion. Certain resins can be added to reduce the crystalline structure and therefore reduce the expansion and contraction as a result (Figure 5). Example of the volumetric thermal expansion result of commercial filled and unfilled wax is illustrated in the Figure 6. Notice that unfilled wax expands more and this is generally true for most waxes when comparing waxes in that two different categories.
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