What is Molding Process in Casting?
In casting, Molding process is a type of manufacturing process in which a sand mold of desired shape is produced by using a pattern, and the molten metal is poured into the mold cavity which on solidification gives the final casting.
Let’s discuss different types of molding process in casting.
Classification / Types of Molding Process in Casting
Molding Processes can be classified into following types.
- According to the method used
- Floor molding
- Bench molding
- Pit molding
- Machine molding
- According to mould materials
- Green sand molding
- Dry sand molding
- Loam sand molding
- Core sand molding
- Other moulding process
- Shell molding
- Permanent molding
- Carbon dioxide molding
Let me tell you a few important things about all these types of molding processes.
#1) Floor molding
- Floor molding is mainly used for preparing the molds for large sized and heavy jobs.
- In this type of molding, the floor acts as a drag.
- Floor molding is beneficial for bigger castings in which the upper surface finish has no importance.
#2) Bench molding
- The bench molding is done on the workbench, which is at the height preferred by the worker.
- Bench molding is used for manufacturing the small and light castings of non-ferrous metals.
#3) Pit molding
- For some large size castings which cannot be moulded in the mold boxes, the pit molds are used.
- In pit moulding, the pit itself acts as a drag.
#4) Machine molding
- The main purpose of using machine molding is that it can be used for mass production of identical jobs.
- In machine molding, the operations such as ramming, rolling, gate cutting, etc are performed by the molding machine itself.
- Because of this, machine molding saves much time and it is also more efficient than hand moulding.
#5) Green sand molding
- The main ingredients in green sand are silica, 10 % to 15 % clay and 6 % to 7 % is water.
- All these ingredients are properly mixed and then the molding of sand is done.
- There are 3 main methods for green sand moulding.
- Open sand method
- Bedded in method
- Turn over method
(For detailed information on these methods, refer to the main article on: Green sand molding.)
#6) Dry sand molding
- Dry sand molding is exactly similar to the green sand molding, but the only difference is in their composition.
- In dry sand molding, the binders are added to the sand which gives more strength and adhesiveness to the sand mold.
- In dry sand molding, the mould is dried completely before pouring the molten metal.
- Large size castings like engine parts, flywheel, etc. can be prepared using dry sand molding.
- Dry sand molding is little more costly as compared to green sand molding.
#7) Loam sand molding
- Loam sand molding is used for making extremely large size castings which are to me manufactured in very less numbers.
- The major constituents of loam sand are coarse grain sand, clay, coke, horse manure and water.
- In loam sand molding, the mold preparation is done by making a rough structure using bricks and loam sand.
- After this, the structure is finished using sickle and sweep.
- The mold surface is dried and finally the casting work is done in the mold.
#8) Core sand molding
- In core sand molding, the mixture of silica sand, olivine, and chamotte sands is prepared.
- The sand having more than 5% clay content is not suitable for core sand molding.
- The mixture is filled in the core box and then it is rammed properly by hand only.
- The process of core sand molding is simple and it takes a short time.
#9) Shell molding
- Shell molding is generally used for mass production of castings with smooth surface finish.
- Shell moulding is also known as the carning process which is basically used for accurate thin castings having tolerance of ±0.02 mm.
- In shell molding, the molding sand is prepared by mixing dry thermosetting plastic powder and fine sand in the ratio of 1:20.
(For detailed information on shell molding, refer to the main article on: Shell molding.)
#10) Permanent molding
- Permanent molding is a type of molding process in which the permanent molds are used which are made from metal.
- The permanent mold is also called reusable mold, as it can be used again and again.
- Permanent molding is used for casting of metals like aluminum, magnesium, and copper alloys.
- In general, the molten metal is filled in the mold due to gravity. Sometimes gas pressure or vacuum pressure is also used to fill the mold cavity.
(For detailed information on Permanent molding, refer to the main article on: Permanent molding.)
#11) Carbon dioxide molding
- Carbon dioxide moulding is used to harden the mold prepared from the green sand.
- While preparing the mold, a small amount of starch is added to the sand which improves its green compression strength.
- Other constituents like coal dust, sea coal, wood floor, pitch, graphite and sugar are added to improve the collapsibility of the moulding sand.
- The main purpose of passing carbon dioxide gas in the mould is to improve the strength of the mould.
(For detailed information on Carbon dioxide molding, refer to the main article on: Carbon dioxide molding.)