Precision Stainless Steel Investment Casting: Advanced Production of Complex Parts

Why should we choose investment casting for stainless steel parts?

Some stainless steel products with complex structures are not able to be manufactured by bending and other processings, while we can choose the investment casting, from wax mould to sand mould, and then liquid stainless pouring, to produce such irregularly-shaped hollow or solid parts.

Stainless steel precision casting Advantages:

Various parts can be manufactured from different wax mould.

Stainless steel precision casting Disadvantages:

Lower Precision & High Surface Roughness of the casting blank.

Stainless Steel Investment Casting Process

1.Customizing the mould according to the structure of the products. The investment casting mould is classified into upper concave and lower concave. The mould is completed via CNC turning, plaing, milling, etching, EDM and several processing and the shape and dimension of the concave is consistent with the half of the desired parts which are divided by parting line.

The economical aluminum alloy material is chosen for the mould with low melting point, low hardness and lighter weight since it is mainly used for industrial wax pressing.

2.Normally, one industrial wax solid core model can only generate one casting blank so we need to produce a large number of models by the aluminum alloy mould.

3.The wax mould will be attached to the mould head (also known as the tree-like assembly.) after further repairment and deburring. The mould head is one kind of industrial wax solid core model. (shaped like a tree)

4.The first layer of fine sand (a kind of refractory sand with high temperature resistance, most commonly used is silicon sand) will be coated on the wax moulds which have been sticked to the mould head by industrial glue. The sand particles are very small and fine, thus the final blank surface can be guaranteed to be as smooth as possible.

5.Air dry the wax mould with fine sand under stated indoor temperature (or constant temperature) and meantime the shape of the internal wax mold cannot be affected. The drying time depends on the internal complexity of the product it will take approximately 5-8 hours normally.

6.After the first layer sand have been dried, the second layer with larger and rougher particles will be coated after dipping into the industrial glue (a slurry of fine-grained silica) and then air drying the wax mould again under constant temperature.

7.The samely consecutive 5 sand blastings will be operated followingly.

Requirement:

• Optimizing the sand blasting times according to the surface requirements, volume size, weight of the part being cast,. The normal frequency is 3-7.
• The latterly-blasted sand partles during each cycle is always bigger and rougher than the previous ones and their air drying time is also different. Generally, the whole production cycle of the complete sand blasting is 3~4 days.

8.Before the heating the wax mould for dewaxing, particles of sand (a slurry of silica) are dropped onto the surface of the wet tree assembly so as to to stick and solidify the sand mould and seal the mould.

9.The dewaxing process

• The wax pattern with the refractory slurry is turned upside done, placed into a sealed oven (commonly used is steam-dewaxing autoclave or flash fire furnace). During this heating process, the wax mould melts and drains away from the gate due to it’s low fusion point of 150, then a hollow ceramic shell is achieved.
• The molten wax can be reused to create new patterns only after filtration or there will be the casting defects like gas porosity or pitting and also the shrinkage rate of the casting will be affected.

10.Heating the ceramic shell – The ceramic mold must be heated around 1000°C before pouring the liquid stainless steel. The heating process further strengths and solifies the mold.

11.Pouring-The molten stainless steel is poured into each and every mold cavity on the tree (including the head) until filling.  

12.The foundry must inspect the material percentage and optimize the material proportion as needed since there will be left other metal material exist on the furnace.

13.Shakeout – Once the liquid stainless steel solidifies, the ceramic molds will be removed by manual or mechanical knockout and the wax mould pattern which is the casting blanks will show up. Then we separate the the individual casting blanks from the tree assembly and roughly grind them.

14.Inspection of the blank – The casting blank with sand hole and gas holes on the surface must be repaired by argon arc welding, the much more defected blanks will be casting again.

15.Cleaning of the blank: The qualified blanks must be cleaned before further processing.

16.Further machining will be needed to achieved the final product.