Investment Casting is considered the quickest and most robust method available to produce complex metal prototype parts. The technique of using a disposable pattern (the investment) as the basis for casting a metal object dates back to the 15th century. In the modern era, World War II provided an urgent need for accurate, finished, complex metal parts that traditional machining could not address.
The initial discovery that a lost art provided a practical metalworking technique caused great enthusiasm. Investment casting was found practical for many wartime needs, and during the postwar and modern periods it expanded into many applications where complex metal parts were needed.
Growth of the investment casting industry has continued. The process has an excellent reputation for reliability, value, accuracy, versatility, integrity and finish. The marriage of Investment Casting and Rapid Prototyping techniques as a means to rapidly produce the investment pattern has resulted in even greater economies and timesaving in the manufacture of small quantities of complex metal parts.
Using RP Patterns
The investment casting process using RP patterns begins as the pattern to be invested in the casting process is grown using the appropriate RP process. At NECO Inc., we have standardized on using SLS patterns grown using “Castform” polystyrene material. The Castform polystyrene, infiltrated with investment casting wax material, has proven over time to be a consistent high-quality performer for the casting of both ferrous and non-ferrous metals.
SLS Castform Polystyrene Material Properties
|Thermal Properties||Test Method||Units||Cast Form|
|Glass Transition: Tg (Polystyrene)||DSC||oC||89|
|Melt Point: Mp (Wax)||oC||<63|
|Tensile DTUL, 0.45 MPa||ASTM D648||oC||33|
|DTUL, 1.82 MPa||ASTM D648||oC||40|
|Flash Point (Polystyrene)||Cleveland Open Cup||oC||350|
|Flash Point (Wax)||Cleveland Open Cup||oC||>200|
|Autoignition Point (Polystyrene)||oC||410|
The completed SLS Castform RP pattern is assembled to a pattern gating system. The gating system serves as a conduit for molten metal to quickly fill the mold. The gating and runner system is attached to a pour cup to comprise a complete investment pattern assembly.
Assembled investment patterns are dipped into ceramic slurry, drained, and then coated with fine ceramic sand. After drying, this process is repeated several times using progressively coarser grades of ceramic material, until a strengthened shell has been formed. The finished mold shells are heated in order to both melt out the RP pattern, and also to harden the ceramic mold shell. This leaves a ceramic shell cavity of the actual casting shape, ready to accept a molten metal pour.
Ceramic shell molds are preheated then filled with molten metal alloy for solidification in the shell. After the poured molds have cooled, the casting’s ceramic shell material is removed, gates and runners are removed, and the resulting casting is finished, ready for shipment.