Metal Injection Molding vs Investment Casting

Investment casting is also known as lost-wax casting, it is one of the diverse casting methods in metal parts manufacturing. Comparing to other casting technologies (sand casting or die casting), investment casting provides high dimensional accuracy, smooth surface finish, design complexity and fast production.

IC (investment casting) apply wax-like materials to create expendable patterns, then manufacture complex metal parts with high precision. In addition, ceramic molds with elite thermal quality result in incredible surface finish. The main manufacturing process:

The master pattern is the replica of final metal parts with the same details, there is also an allowance for thermal contraction or shrinkage. This mater pattern is reusable for numerous casting cycles as a standard. Common material for mater pattern dies are wax, wood, metal, and silicone rubber. Different material choice depends on various factors like pattern life, design complexity, thermal expansion or contraction, surface quality. Operators will apply the master pattern to produce wax patterns for every casting cycle.

Conventionally, wax patterns are made from die casting. Nowadays, new technologies like 3D printing is popular in pattern manufacturing, in reason of its decent accuracy and low cost.

All wax patterns should be assembled into a single investment casting mold, this combined pattern is known as ‘tree’. Operators will assemble the patterns with a sprue/runner/riser network, this gating network provides the pathway for liquid metal, in order to flow smoothly into all parts of the mold.

This assembly process is manual or robotic. For low volume production, or each casting with different design, it is optimal for manual assembly. Otherwise, for repetitive casting, it is wise to apply robotic assembly units.

Operators apply a coat of refractory materials onto the wax pattern, this coating process is in multiple dipping cycles. The ‘tree’ is dipped repeatedly into a bath of ceramic slurry and sand stucco with gradual and rotary motion. The ceramic will increase the temperature-resistance of mold, and stucco will enhance its strength.

In each dipping cycle, the thickness of ceramic shell is increasing. It is critical to apply the initial layer (primary core), as it has significant effect on quality and geometry of the final parts. The dipping process is repeated until achieve the required shell thickness. Depending on the casting size, weight and materials, the typical shell thickness range from 9.5 mm to 13.0 mm. For heavy metal parts or high melting temperature metal, we should increase the shell thickness to bear the temperature and stress.

The wax is removed from the ceramic shell by melting, normally, we apply autoclaves to fire furnaces to heat and melt the wax. The mold will be place upside down, so molten wax will flow out easily. In autoclave, the majority of wax will flow out at 90℃-175℃temperature for 12 hours. Then bake the mold at high temperature of 1000℃for a few hours, in order to burn off the remaining was and evaporate moisture content.

Dewaxing is a crucial stage in the whole casting process, such intense temperature will give rise to many potential defects in the mold shell, such as cracking. In this process, heat treatment also has some positive effect on refractory materials. It improves the strength of shell, and decrease its reactivity with molten metal.

The mold is preheated at a specific temperature, in order to minimize its thermal shock once in contact with high-temperature liquid metal. Then liquid metal will be poured into the mold in a controlling manner. The pouring rate is critical to minimize turbulence-related casting defects. In addition, it is suggested to create a vacuum environment once molten metal tends to react with air. After the pouring process, the molten metal will cool down and solidify inside the mold, then create the metal casting.

Nearly any alloy can be manufactured by this casting process. Air melting or vacuum melting are dictated by the alloy chemical properties. Vacuum melting is utilized for alloys with reactive elements in air.

The mold shell will be broken down in a knockout operation, once the casting metal cools and solidifies sufficiently. These ceramic molds are knockoff by hammer or water jetting. After that, the casting parts will go through several semi-finishing operations, in order to achieve required dimensions and properties.

Firstly, the gates and runners (gating system) are cut off. Meanwhile, grinding or blasting technology are able to flat rough surfaces, jagged edges, and other defects. Moreover, machining like milling and drilling can finish holes and surfaces. In addition, other surface finish treatments can be applied to achieve high surface quality.