This mould is the special material to house the amputated knee or simply it acts as a major connection between knee and the foot. Many prosthetics that have been in the market lately have used carbon fibre as the base. Although carbon fibre is strong the major factor to be considered is that the prosthetic should not be an added weight which may result in excessive stress during movements. It should be light and must satisfy the criteria it is essentially strong to withstand the weight of the whole body. You can even 3D print the mould but that's gonna cost atleast 35$! If you would still like to 3D print the module I have attached the STL file and you could make some modifications and 3D print the mould.

You can even check out in this website http://3dprintingpricecheck.com/

Why silicone rubber?

The mould for housing the knee region can be made from any material keeping in mind the important factor- it doesn't deform in size or shape and is able to withstand the thrust of the athlete. Some of them that are used are mostly in the form of carbon fiber polymer. Carbon fibre polymers are of high cost but with great properties. Since we decided to develop the whole prototype within 50$ we founded an alternative in the form of Silicone Rubber. 

It is far cheaper than other materials and has almost similar properties to that of the carbon fiber and that makes it as the best choice. Silicone rubber is an elastomer (rubber-like material) composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost. Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures from -55 °C to +300 °C while still maintaining its useful properties. Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including: automotive applications; cooking, baking, and food storage products; apparel such as undergarments, sportswear, and footwear; electronics; medical devices and implants; and in home repair and hardware with products such as silicone sealants. To see the full properties of Silicon Rubber Polymer 

https://www.azom.com/properties.aspx?ArticleID=920

We used this silicone rubber mould making material from amazon http://www.amazon.in/MoldSil-15-Mold-Making-Silicon/dp/B06WGMCSSZ?tag=googinhydr18418-21

We used water bottle as the base to make the mould and and a solid to make the mould hollow. The process is so simple and could be done within minutes. It takes few hours for the liquid to turn into a perfect solid resulting in Silicon Rubber. Before it dries up attach one end of the laces preferably velcro in the mould so that we could tie the bionic leg to our thigh region.

While making mould make sure to measure the diameter of knee and add few millimeters for placing the cushiony  material. The dimensions of the mould that we made has a radius of 5cm, height of 10cm, volume of 215 cub.cm and a surface area of 36 sq.cm. Make sure that the mould is completely dried up so that it retains its properties. Drill up a hole underneath the mould so as to place the stainless steel rod. In the hollow region of the mould place some cotton material that acts as a cushion.

Further we would be adding an adjustable vacuum suction part such that it becomes easy for the athlete to attach the amputated region to the socket easily!