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Green Sand Moulding Process Introduction

Green Sand Moulding

What basic one must know before studying the green sand moulding process?

Casting produced from this mould is called green sand casting because the mould used in the sand casting process is made from green sand. The entire process of making green sand casting is called green sand moulding.

When we say green sand casting that does not mean casting is green in colour. Casting is produced in green sand that is why it is called green sand casting.

Green sand is sand which is high in moisture content as compared to other sand such as dry sand which is dried in the oven.

Green sand moulding processes can produce small-size simple shape casting at a lower cost as compared to dry sand mould and skin-dried sand moulding processes.

The cost of production, the cycle time, floor space in the foundry, labour requirement and the steps involved in producing green sand moulds are less as compared to dry sand moulding and skin-dried moulding.

These green moulds are made from green sand which has the following characteristics making this sand most suitable for producing simple shape casting at lower volumes:

Higher cohesiveness (Sand can hold its shape during green sand moulding).
Adhesiveness (Sand can cling/stick to the flask, moulding box and core box)
Green strength (Ability of sand to have tensile and compressive strength during the moulding process)
Hot strength (During the casting process sand can withstand the high temperatures of the metal during the pouring operation escaping all unwanted gases and retaining the shape of the mould to produce high-quality sand casting)

As we have covered the basics need of this process let's study this process with advantages, disadvantages and differences between green sand moulding and dry sand moulding which will help students understand this process better.

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Green Sand Mould

What is green sand moulding?

Greensand mould is a freshly prepared mould made from silica sand, clay and moisture used to produce green sand casting in the sand casting process.

The green sand is called green because of the moisture content in it and is freshly prepared without curing benefiting rapid production of casting.

The green sand mould is dependent on moisture and clay to hold the sand particles together forming one complete mould.

Green sand moulds indicate that it has a high amount of moisture content up to 5% and the mould is not baked in the oven or dried as we do it in dry sand moulding. This mould is fully wet and suitable for producing small-size casting.

Shows below green sand mould is made up of green sand in diagram (a) perfectly able to go through the moulding process.

(a) Green Sand Moulding

Before we start casting products in the mould, green sand moulds are dusted with coatings and carbonaceous materials such as coal dust called blacking to improve the surface finish of the final sand casting. This mineral coating is applied wet or dry to the sand mould directly.

Along with minerals and coating adhesives are used to hold the moulding sand together with refractory materials such as coke, talc, charcoal, graphite, charcoal, mica, black lead and gas carbon.

Coal dust (facing sand) prevents the molten metal from fusing into the sand mould. Facing sand is used near the pattern in the mould cavity to prevent system sand or moulding sand from getting burned.

In other moulds (dry sand and skin-dried moulds), it is expected that the actual casting will start after the mould has dried but in green sand, mould molten metal is poured directly when the mould is still wet and the metal is solidified eventually.

Care is taken to use course grain here for better permeability to avoid porosity and blowhole defects but control over sand erosion is beyond the control of the operator as mould is still fresh and not dried.

Green compression strength and green shear strength are done on sands used in the green sand moulding process to check for green strength and shear strength during the mould-making, pouring molten metal and solidification processes respectively.

Green sand mould has a time duration until which green sand mould is to be used as the probability of this mould drying in a hot foundry environment increases with time reducing its adhesive property of the sand.

Mould should be used for moulding before moisture in the mould starts drying up as greensand moulding is made by using green sand and has a good-quality moisture content that binds sand particles together with the help of clay.

Moisture content in this mould is more than 5% and clay is up to 15% in the green sand moulding process along with binders and additives.

Before starting the casting process, the mould is sprinkled with facing sand or mould wash to prevent penetration of molten metal into the mould's course structure, giving an uneven surface finish and increasing surface roughness.

Green Sand Moulding Methods

How many types of green sand moulding techniques are present for making green sand casting?

There are three types of green sand moulding methods.

Open sand method.
Beded-in method.
Turn around method.

Open Sand Mould Method

Step-1 Before pouring sand on the floor, a proper horizontal uneven floor with a flat surface is selected. The floor surface is checked using the sidebar and bevelgear to see any unevenness in the floor.
Step-2 Clean the floor and pour system sand on the floor followed by backing sand and facing sand.
Step-3 Lightly do sand ramming making sure sand is compact for the further geen sand casting operation.
Step-4 Now place the pattern on the facing sand and lightly do sand ramming making sure the sand is compact for the further geen sand casting operation.
Step-5 Remove the sand once a desirable impression is created in the sand by the pattern.
Step-6 Pour molten metal into the cavity, allow the metal to solfiy in an open mould and take out solidified casting.
This is an open green sand casting method used for producing casting having smaller depth, height, size, complexity and weight.
Application of the open sand floor green sand casting method is for slabs, grills, plates, small ingots, section bars and prototypes using solid patterns.

Bedded In Mould Method

Bedded Method Introduction: The method is used for casting having greater height, thickness, complexity and change in design as compared to the open sand method. Following are the steps to make bedded in mould green sand casting.
Step-1 Create a cavity/pit in the floor of the foundry according to the size, shape and design of the final sand casting.
Step-2 Pour system sand in the pit followed by backing sand and facing sand.
Step-3 Cut the gating system in the bedded mould.
Step-4 Do sand ramming manually and compact the sand evenly.
Step-5 Place the pattern on the facing sand and press it against the sand creating an impression on the mould.
Step-6 Pour molten metal into the bedded mould cavity to form the final green sand casting.
Step-7 Take out casting from the bedded mould after casting has solidified and processed for the secondary manufacturing process.

Turn Over Mould Making Method

This is a classical green sand mould-making process used for making complex shape casting using match plate patterns, three-piece pattern, cope and drag pattern, split pattern and multiple-piece pattern.
I have written a separate article explaining the green sand mould-making step-by-step process that I suggest students read for further understanding of the turn-over mould-making method.
This method is best for casting that is split along the parting line giving more flexibility over design, complexity, shape and size.

Advantages of Green Sand Mould

What are green sand moulding/casting advantages?

Low production cost- Economical, budget-friendly and low-cost casting production method for smaller to medium size casting. Sand used in this process is easily available, cheap and can be directly used for making mould.
Low production steps- No baking, curing or drying of the sand mould is required, resulting in mass production of the green mould. The reusability of the flask is faster as the green sand moulding process takes less time.
Machine moulding- Fast moulding process using horizontal and vertical automated machine moulding methods.
Low production time- The time required to produce green sand mould is less as no baking/curing of moulding sand is involved in the process.
Hot tearing- Hot tearing in the final green casting is less due to higher moisture content compared to other sand moulds.
All metal friendly- Both ferrous and non-ferrous metals can be used for producing green casting.
Less manpower required- As the steps involved in this method are less, less manpower required to make a mould is less as compared to fully dry mould.
Less hot cracks- Hot cracks formed in this process are less as compared to skin skin-dried and dry casting process.
Pattern suitability- This pattern is best suited for split piece patterns, segmental patterns, single piece patterns, skeleton patterns and loose piece patterns.
Less tooling requirement- The tooling cost of making this mould is less than the dry and skin-dry sand casting process.

Disadvantages of Green Sand Mould

What are the green sand moulding/casting disadvantages?

Weak Mould- The green sand moulding process is weak for producing large-size complex casting as compared to the dry sand moulding process due to the presence of moisture in it.
High gas-related defects- Casting produced from green sand mould is more prone to gas-related defects such as blowhole defects, open-hole casting defects, pinhole porosity and air inclusions.
Lower dimensional accuracy- Overall dimensional accuracy is not good compared to dry and skin-dried sand mould. Difficult to produce a better surface finish because of the inability to use finer grain near the surface of the mould as this mould requires a higher permeability property to escape gases and moisture.
Small-size casting- Limitation over the size of the casting, process is most suitable for producing small-size casting only.
Sand erosion- Molten metal when travelling to a mould cavity tends to erode sand. Erosion in sand causes defects such as
Mould handling and damage- Handling green sand mould in a foundry becomes difficult and prone to mould damage during handling green sand mould.
Escape of gases from mould- Requires more vents as compared to dry moulds to escape carbon dioxide, steam and hydrogen out of the mould from pouring to solidification operation.
Molten metal flow rate- Requires high control over the flow rate of molten metal entering the mould cavity to reduce turbulence of molten metal.
Mould defects- More prone to mould defects such as scabs, sponginess defects and dirt which are formed when loose sand gets mixed with the molten metal.
Pouring and soldification rate- As the velocity of molten metal increases possibility of cuts and washes of green sand mould increases making this mould not suitable for large-size casting which requires a mould to be filled at a higher rate taking into consideration the solidification and pouring rate of the molten metal.
Machining operations- Green sand casting requires more machining and secondary manufacturing operations to improve the quality of casting as compared to dry sand casting and skin dry castings.
Thin casting difficulty- It is difficult to cast thin sections casting as compared to the dry sand casting process.

Green Sand Mould Applications

Casting bars and studs.
Irrigation system and equipment.
Boat engine components.
Showerheads and housing fixtures.
Heat exchanger internal components.
Small valves and pump sets.
Oil drillers and housing.
Small pump and compressor housing.
Aircraft and helicopter parts.
Cookware pans, gas burners and manhole covers.
Antique statues, idols and symbols.
Hubs, rotors and covers.
Aircraft landing gear suspension components.
Used by hobbyists for casting small ornamental casting objects.

Green Sand Moulding Vs. Dry Sand Moulding

What is the difference between green sand moulding and dry sand moulding?

Green Sand Moulding	Dry Sand Moulding
1) In the green sand moulding process green sand moulds are fully made from moisture green sand using clay as a binding agent.	1) In the dry sand moulding process the dry sand moulds are made from dried sand using resin as a binding agent.
2) Incapable of producing large-size casting due to weakness in structure as mould is still wet and weaker for heavy complex thick casting.	2) Capable of producing large-size casting as structure (mould) is cured, dried and strong because of the higher dried binding agent and sand into it.
3) Distortion in the green mould is less.	3) Distortion in the dry mould is high.
4) Prone to all moisture, pinhole defects, open hole defects, metal penetration in sand and sand erosion-related defects such as porosity.	4) Not prone to moisture, pinhole defects, metal penetration of metal in sand mould and erosion-related defects as mould is dried.
5) The shelf life of green sand mould is low as mould needs to be used before it dries up.	5) The shelf life of dry sand mould is high if kept dry.
6) The surface quality of green sand casting is low increasing the chances of rejection during quality inspection.	6) The surface quality of dry sand casting is better reducing the chances of rejection during final quality inspection.

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