Bego (usa) Dental Restoration Model

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a die model of the teeth by utilizing a commonly used plaster known as Dental Stone, which is essentially Calcium Sulphate Dehydrate (CaSO4.2H2O). Wax is then poured over the die model and carved into the shape of the various teeth to form the complete wax pattern of the patient’s teeth (Krishnan, 2011).

Figure 5: Wax model of a tooth (Source: doctorspiller.com)

2.1.1.3. Attaching of Sprue and Cavity Creation

After the wax pattern of the teeth is done, a sprue, which is a vertical extension from the bottom of the wax pattern, is attached to it. It acts as a channel for wax to be removed as well as for

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molten metal to move from the crucible into the restoration chamber. At the same time, a bulb on the sprue works as a reservoir to help to compensate for shrinkage of the molten metal flowing into the restoration during casting (Spiller, 2016).

Figure 6: Attachment of Sprue on tooth (Source:

Figure 7: Investing of Wax pattern in ceramic (Source: pocket

doctorspiller.com)

dentistry.com)

The wax pattern and sprue are then invested in a ceramic material. The invested ceramic piece would then be placed in a very hot oven to melt away all the wax remnants inside. All that would be left is a cavity in the shape of the original wax pattern.

2.1.1.4. Casting Process (Low Pressure Casting)

For the casting process, a low pressure casting machine such as the Cascom SE Vacuum and Pressure Casting Machine is used, with KDF Denken-Highdental being the supplier. KDF is a reputable company with a history of producing dental equipment with excellent quality. It also offers a whole range of dental equipment and materials from impression materials to investment materials.

The Cascom SE casting machine is able to achieve high yields (around 85%) as there are no risers required (The Library Of Manufacturing, 2016). Castings done using this method also have better mechanical properties as at constant pressure, there is little turbulence when filling the mould. At the same time, it utilizes the chamber inverting casting layer, so that no oxidation layer forms in the material.

The machine is able to melt the Co-Cr alloy within minutes. After the Co-Cr alloy is melted, pressured Argon gas at 0.5-0.8MPa is forced through an opening into the air tight chamber. This pressurizes the top surface of the molten Co-Cr alloy and pushes the molten Co-Cr alloy

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through the refractory pouring tube into the mould (Figure 8). Typical pressure exerted during this process is 4.40 g/m2 (The Library Of Manufacturing, 2016), while the pouring temperature is approximately 1,823K, 100K above the melting point of the alloy. The air pressure is maintained until the metal casting has hardened within the mould, after which the mould is opened and the part is ready for removal. This constant pressure applied prevents any shrinkage in the cavity, as well as creates intricate casting results, with high performance and good quality parts produced.

Figure 8: Schematic of Low Pressure Casting (Source:

Figure 9: Cascom SE Vacuum and Pressure Casting

Degarmo, Black, Kohser, & Ronald, 2003)

Machine (Source: Cascom)

After the casting process is completed, the ceramic around the casting is then knocked off, leaving the remaining metal casting and the attached sprue. The sprue is then removed to leave the completed teeth casting. The casting is then polished before use.

Figure 10: Final casted tooth (Source: doctorspiller.com)

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2.1.2. Manufacturing Cost

The cost of the impression generation largely comes from the hydrocolloid impression material and skilled labour cost for creating the negative image.

Solid Co-Cr alloy costs approximately SGD 210 per kilogram. To estimate the amount of Co-Cr to be used, the team has re-engineered a set of 3-unit Bridge from the BEGO case study and the volume of the model is 5 cm3. Based on ASTM F75, Co-Cr true density is ranging from 8.4 to 8.8 g/cm3, in this study we are taking the average of 8.6 g/cm3.Therefore, the estimated cost of the raw material per model would be SGD 9.

Figure 11: Re-engineered 3-unit Bridge

It is noted that the material cost is relatively low as compared to the costs of the other steps in the Lost-Wax Casting technique. In fact, the cost of patterning and moulding is significant