The casting industry is critical for the manufacturing sector. This includes the medical investment industry. We need extremely precise and specialized manufacturing in medical equipment and this is where casting comes in. With the growth of medical industry in India, we are also looking at reciprocal growth for the casting industry. Medical investment castings is also important for bringing in innovation in medical equipment.
What Is Investment Casting?
The word ‘investment’ here is misleading. Or rather, it refers to the more traditional meaning of the word. In old English invested also stood for being clothed in garments. Here it means a pattern surrounded (or invested) with a refractory material. It is based on one of the oldest forms of metal casting method — lost-wax casting. Traditionally the pattern used was beeswax. Today we use more advanced waxes. The metal used to invest can also vary. For instance, certain alloys are popular because of their specific properties. Investment casting is used to produce both small and large components.
There are two popular methods in medical investment castings: water glass casting and silica sol investment. While there are a few differences, the basic method is the same.
Creating master pattern: We start with creating a 3D pattern. This is created by an artist or a designer. A pattern is created with materials like clay, wood, wax, plastic. Today patterns created by 3D printing are also gaining popularity. These use casting wax filament or standard PLA filament. The pattern has the details of the finished part. However, we do have to keep in mind that there would be shrinkage.
Creating a mould: Once we have the pattern, we have to create mould. Also known as a master die, the mould fits in the master pattern. In case of steel master patterns, the master die can be cast directly using a metal with low melting point. Similarly, rubber moulds are also cast directly from the master pattern. With proper machines and casting, a master die can even be machined without a master pattern.
Making wax patterns: Wax patters were traditionally made of beeswax. Today modern waxes like ceramic or soluble waxes are used more often. Other materials like plastic and frozen mercury are also used. There are two ways of making wax pattern:
- The wax is poured into the mold. It is then moved around till it coats the inner surface. The wax is moved around till we get the desired thickness.
- The molten wax is poured into the mould, filling it. It is then cooled till it solidifies.
Assembling wax patterns: Once we have wax patterns, we have to assemble these. We create a pattern cluster, which is also known as a tree. Patterns are attached using a heating tool. When applied to the wax pattern surface, it causes the wax to melt slightly. The melted surfaces are pressed against each other, forming a seal. The attached pattern is again left to cool and the seal to harden. A tree can contain as many as thousands of patterns. We can also add finishing touches like parting lines. The final step is to give it a polished finish.
Apply investment materials: Next comes the application of the investment. Usually a ceramic mould, the investment is applied using multiple steps that coast the pattern and then ensure that it is firmly attached. It involves steps like coating, stuccoing and hardening. During coating the pattern is dipped in slurry of refractory material. Later a uniform surface coating is applied. In stuccoing we dip the patterns in a fluidized bed. Coarse ceramic particles are applied by hand. Finally comes hardening where we allow the coatings to firm up and cure. The steps are repeated till we get the thickness we are looking for. The shell should be strong enough to take on the medical investment castings.
Removing the wax: Now that we have the ceramic shell, we need to remove the wax. We melt the wax by placing the assembly in a steam autoclave. Any wax that is stuck to the shell is melted in a furnace. As all the wax is removed we are left with the hollow shell. This treatment under high temperature also makes the shell strong enough for later processes.
Casting: This is the typical metal casting where the mould is heated and filled with a molten metal. We often use alloys as material and air melting or vacuum melting for the procedure.
Finishing: After the casting cools, the mould shell is removed. Methods like sandblasting and machining are used to attain the desired dimensions. The product also goes through rigorous testing.