Slush Casting
|
| Slush Casting Process |
Slush casting can be defined as the variation of permanent mould casting used to produce hollow castings such as ornaments, jewellery, components, slush casting pewters, trophies and metal toys such as miniature cars and soldiers.
It is a special coreless casting process used to produce lightweight thin hollow casting.
Which pattern is used in slush casting?
No pattern is used in slush casting for making a hollow casting.
The slush casting process starts by pouring molten metal into the mould cavity and allowing the metal to solidify until thin shell metal is formed around the mould.
Once the metal skin of the desired thickness is solidified, unsolidified molten metal is drained out of the mould.
What remains in the mould is the hollow casting with a fine surface finish.
Slush casting can be done in open or closed mould and is an extension of metallic mould casting.
The final casting has a shiny and smooth exterior structure.
This casting process is suitable for products where external quality is of primary importance.
The process has numerous advantages making it an exceptional use to produce the hollow casting.
I have listed the advantages of the slush casting process that make the process special.
This slush-casting process was first invented by William Britain in 1893 for the production of lead toy soldiers.
What is slush casting also known as?
Slush casting is also known as hollow casting or hollow cast.
What is the difference between slush casting and slush moulding?
Slush casting is mostly associated with metal products while slush moulding is associated with liquid materials such as resin, silicone, latex, thermosetting plastic and rubber.
Slush Casting Animation
 |
| Slush Casting Process Animation |
Shows above slush casting process animation from pouring operation to ejection of casting from the metal permanent mould.
Slush Casting Process Step-By-Step Explained With 7 Casting Operations And Detailed Diagrams
 |
| Slush Casting Flow Chart |
Step 1: Melting Ingots In Furnace
Low-temperature, non-ferrous metal ingots are desirable for this casting process.
Metal is heated above the melting point keeping it ready for pouring operation.
Step 2: Inspection And Pre-Heating Permanent Mould
Mould must be first opened and inspected for any abnormalities and defects in mould.
The mould cavity is coated with die lubricant to improve the surface finish and reduce defects in the final casting.
Permanent mould is pre-heated to the desired temperature as shown in diagram (a), before pouring molten metal.
|
| (a) Preheating Hollow Mould |
The mould here is designed such a way that mould has only single gate without any sprue, pouring basin, runner or riser.
Step 3: Pouring Operation
Care must be taken that molten metal is free from dross, slag and other impurities before pouring molten metal.
|
| (b) Pouring Operation In Slush Casting Animation |
Molten metal is poured till the cavity is full with the help of a ladle as shown in diagram (b) above.
This casting only has one gate which is used for pouring metal in and draining metal out remaining metal.
This casting process is a variation of gravity die casting when it comes to pouring the liquid metal into the mould cavity.
No external force is used here to pour liquid metal in the die cavity.
Step 4: Solidification Operation
After the molten metal is poured into the mould, the metal is not allowed to solidify completely in the mould.
|
| (c) Solidification Operation Animation |
In this process, only the layer of thin thickness is allowed to solidify around the mould and the rest molten metal is drained out.
As shown in diagram (c) above, a thin layer of the shell is solidified around the mould which I have marked dark red. This shell is the actual casting which is solidified around the mould.
Metal is at the lowest temperature at the wall of the mould (T1) and at the highest at the centre mould (T3) as shown in diagram (d) below.
|
|
(d) Solidification Zone of The Molten Metal Diagram |
Metal starts solidifying at the surface of the mould wall first and then slowly starts solidifying inwards.
During the solidification molten metal gets into a slushy state.
The "Slushy" state is the state where molten metal is between a solid and a liquid state.
The thickness of casting depends on how long the metal is in contact with the mould wall.
The more time metal is in contact with the wall more the thickness of casting will be.
Thickness of casting ∝ Solidification time.
What is the formula of the slush casting process?
The slush casting formula for calculating solidification time is as follows:
`t_{1}=C_{1}+\sqrt{T_{1}}+C_{2}`
`t_{2}=C_{2}+\sqrt{T_{2}}+C_{2}`
`t_{1}` and `t_{2}` are the thickness of the casting.
`T_{1}` and `T_{1}` are solidification times for casting.
`C_{1}` and `C_{2}` are constants.
Step 5: Draining Out Excess Metal From Mould
Once the desired thickness is achieved non-solidified metal is drained out as shown in diagram (e) below.
|
| (e) Drain Metal Out of The Mould Animation |
Metal that is drained out is unwanted metal which has no significant importance.
This drained metal is reused making this process a very economical process.
This is the only casting process where unsolfidied metal is poured out by inverting the entire mould.
Step 6: Ejecting Casting From Mould
This is the final stage of slush casting where mould is opened, and hollow casting is ejected out of the permanent mould.
|
| (f) Ejecting final slush casting. |
As permanent mould is used here, this process is repeated multiple times for mass production by reusing the mould.
The final casting that we get is hollow, thin, shiny, and has a higher surface finish, as shown in diagram (f) above.
Step 7: Secondary Manufacturing Operation
After the casting is taken out, the edges are trimmed and shaved.
Other coating elements are coated to polish the surface and to avoid long-term corrosion of the casting.
Properties And Consideration Of Manufacturing By Slush Casting
- All permanent moulding principles apply to the slush casting process.
- The casting produced is hollow and has lower weight as compared to a casting produced by sand casting.
- Controlling internal geometry is difficult, casting takes the shape of the die walls.
- A high-quality smooth exterior surface can be achieved with this casting process as compared to die casting and sand casting.
- A mechanism to invert the hollow mould is necessary to drain excess molten metal out.
- Strength, toughness, hardness and other mechanical properties are difficult to achieve for these mechanical properties squeeze casting needs to be used.
- The manufacturing process is straightforward compared to the sand casting process and is very cost-effective as multiple casting can be manufactured using permanent metal moulds.
Slush Casting Vs Sand Casting Process
What is the difference between sand casting and slush casting?
| Slush Casting | Sand Casting Process |
|---|---|
| 1) Slush casting is used for manufacturing casting which is hollow from the inside without core. | 1) The sand casting process is used for manufacturing solid casting. |
| 2) This process can produce thin lightweight casting with magnificent outer surface finish and texture. | 2) This process cannot produce lightweight casting with high-quality surface texture. |
| 3) Molten metal in this process is partially allowed to solidify along the walls of the mould and the remaining metal is drained out. | 3) Molten metal here is fully solidified to form solid casting. |
| 4) The yield of this casting process is higher than sand casting due to the absence of a gating system. | 4) The yield of this casting process is low because of the use of riser, runner, gate and sprue. |
| 5) The number of casting produced in a given time is more usithe ng slush casting process because of using permanent expandable dies. | 5) Sand dies are here in this process which needs to be produced every time for new casting reducing production quantity. |
| 6) Products produced from this process are mostly used for aesthetic value, few products only have functional engineering value to them. | 6) Most of the products produced from this process have engineering functional value to them. |
| 7) Better control over process parameters such as pouring rate, solidification rate, and molten metal temperature are possible in this casting process. | 7) Less control over process parameters is possible in sand casting once the molten metal flows frthe om pouring basin to the riser. |
| 8) No riser is used for direction solidification in this process, the majority of molten metal is drained out of the die once the metal solidifies along the die walls. | 8) The riser is an important element of sand casting to take care of proper liquid solidification of casting. The secondary |
| 9) The secondary manufacturing process required for these casting components is less as only sides and edges need to be trimmed. | 9) The secondary manufacturing process is required for these casting components more as casting needs to be further machined, drilled, reamed and treaded. |
| 10) This casting is not prone to blowholes, pinhole porosity and other surface casting defects as the casting is made in a metal die and is free from moulding sand. | 10) Casting is more prone to porosity, pinholes and surface defects because of the use of moulding sand in the casting process to produce casting products. |
Slush Casting Process Defects
- Drip marks- Rough texture formed due to the molten metal getting frozen in the interior side of casting while draining molten metal.
- Warping- Uneven solidification causes the shell to warp while the molten metal is drained out in the thinner section of the casting. This will happen when the mould is a bit tilted on one side.
- Scab- Impurities found in molten metal settling in the final casting.
- Cracking- Cracked formed due to uneven cooling while draining out molten metal.
Why Interior Of Slush Casting Have a Rough Texture?
Once the required thickness of casting is solidified in the mould, un-solidified molten metal is drained out.
With passing time, temperature of draining metal starts dropping. Metal starts solidifying internally giving rough texture to the casting.
Slush Casting Process Products Interior Vs Exterior Texture
| EXTERIOR TEXTURE | INTERIOR TEXTURE |
|---|---|
| 1) The texture of the metal outside the casting is smooth, fine and shiny. | 1) The texture of the metal inside the casting is rough because of drip lines due to the draining operation. |
| 2) The exterior is glowing and aesthetically appealing. | 2) The Interior is rough and not pleasant to see. |
| 3) Having a good exterior structure is an essential requirement and a need. | 3) Having a good interior structure is never a requirement. |
Slush Casting Process Summary
As explained below in diagram (g) five-point quick summary to conclude the process.
This casting is used to produce thin hollow casting with decorative aesthetic value with appealing surface finish texture which cannot be produced with shell casting, sand casting, investment casting and die casting process.
It is a special type of traditional method of producing a wide variety of hollow casting products without using a core that cannot be produced by another casting process such as sand casting.
Cores are not used in this process to produce large hollow casting as this is a coreless casting process to produce thin casting components.
Permanent moulds used in this process can produce multiple castings in large quantities as compared to sand casting with one single die mould.
Control over the internal surface of the mould is not possible as the die is full of molten metal during the solidification process.
I have written the difference between slush casting vs sand casting for a better understanding of this part.
The slush casting process is also popular among hobbyists.