Skin Dried Moulding | Advantage | Disadvantage | Skin Dry Mould | Mechgrace |

Skin Dried Moulding Introduction

Skin Dried Moulding

What basic one must know before learning the skin-dry moulding process?

At times in the foundry there is a need for producing large-size casting with less production cost, more dimensional tolerance and fewer surface defects, this is where skin dried moulding process is used.

Skin dry moulds are made partly of green sand and partly of dry sand. This moulding process provides the advantages and benefits of both green sand moulding and dry sand moulding processes.

This mould has two layers of moulding sand. One is green sand and the other is dry sand. As the name says only the inner layer of the skin of the mould is baked and dried remaining entire part of the mould is surrounded by green sand which is a mixture of silica and moisture.

This gives the added advantages of a high surface finish, reduced gas defects, improved castability and reduction in the direct cost of the mould as a major part of the mould is made from low-cost green sand and only the skin of the mould is made from dry casting.

The following process explains the skin dry moulding process with advantages, disadvantages, applications and major differences between skin dry mould and dry sand moulding process with diagram.

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Skin Dry Mould

What is skin dry mould in the sand casting process?

Fully dry sand moulding takes a lot of time, energy, material handling, labour and costs for casting a larger casting. To overcome this issue skin-dried mould is made with a combination of green sand and dry sand as shown below.

Products made from this moulding process are also called skin-dry casting.

Mould made from a combination of green sand and dry sand is shown below in diagram (a) which is used in the sand moulding process.

(a) Skin-Dried Sand Mould

To reduce production cost and time for producing mould inner skin of the mould is dried with heating elements and the entire outer layer of the mould is as same as green sand mould.

As shown in diagram (a) above the dry part of the inner mould is where molten metal comes with is 20mm-25mm thick and made from the system sand.

This mould cavity is coated with mould wash to reduce the penetration of molten metal in the mould to improve the surface finish of the casting.

Turbulence and erosion of the sand mould are reduced as compared to green sand moulding reducing defects such as sand inclusion defects, sand mould drop defects and the presence of dirt.

The pattern is used to make the mould cavity in the mould, only after that mould is baked and cured for the casting process.

Skin-dried mould can be called mould made up of a combination of green sand mould and full dry mould for producing sand casting.

Skin-dry sand mould can handle both hot strength and dry strength in the moulding process.

This mould costs less than fully skin dry mould but is more expensive than green sand mould for making the same size as the casting.

For producing casting of larger sizes these moulding sand should have higher refractory, hardness, durability and cohesiveness sand properties.

The surface of the mould is dried by heating it directly with the flame torch, hot air gas flames for medium-size casting moulds and large electrical ovens for large-size casting moulds.

As hot air passes through the skin of the mould it starts drying and starts increasing its strength. Both green sand and dry sand have different values of clay and moisture content. The green sand part of the mould has a higher amount of moisture and clay as compared to skin-dry mould.

Casting made from aluminium, bronze, manganese, iron and brass are mainly manufactured using this mould. 

Advantages Of Skin-Dry Sand Mould

What are skin-dry sand mould advantages?

• Skin dry mould has the added advantage over green sand mould as this mould has better hardness properties.
• Better dimensional accuracy, surface finish and tolerance than green sand mould for medium-size casting.
• As molten metal comes in contact with the dry section of the mould gas defects such as blowholes are eliminated.
• Better sand casting output than green sand mould in terms of dimensional accuracy.
• Can produce larger size casting than green sand moulding process.
• Better surface finish than green sand mould due to the use of more amount of fine sand.
• The strength, toughness, longevity and durability of this mould are better than green sand moulding.
• Easy to handle as compared to green sand moulds as this mould has better dry strength in foundry.
• Surface defects reduce significantly as molten metal coming in contact with the inner surface of the mould is dry.
• The hot strength of the moulding sand improves the quality of the casting.
• The flowability of molten metal in the skin-dried mould is better than in green sand moulds.
• Best for medium to heavy weight simple shape casting.
• Covienet to dry mould directly with the ambient air, torch or flames for medium-size casting moulds.
• Mould washes can be applied directly on the inner side of dry mould giving a better surface finish.
• Small moulds can be heated up and can be made dry easily with heat lamps.
• Control over process parameters is better than green sand moulding but less than fully dry sand moulding.
• The mechanical properties of mould achieved from the skin-dried moulding process are better than the green moulding process.
• Strong mould as compared to green sand mould reducing runout and drop defect.

Disadvantages Of Skin-Dry Sand Mould

What are skin-dried sand mould disadvantages?

• The equipment and tools required are more than green sand mould but less than dry sand mould.
• The time required for mould production is higher than for green sand mould and dry sand mould as both green sand and baked dry sand parts of the mould need to be produced.
• The cost of production of skin-dry sand mould is less than dry sand mould.
• The process of manufacturing large-size casting is more economical than dry sand mould.
• Not economical to produce smaller-size casting as two layers of sand must be set to make mould.
• For the same casting size and shape, fully dried sand mould gives better sound casting.
• The mould needs to be completely dried before pouring molten metal to reduce gas defects in the final casting.
• Excess baking can cause cracks in the mould, an experienced and skilled operator is required for the drying layer of the mould.
• Reduce rejection of final casting due to pinhole defects, shrinkage and moisture-related defects.

Skin Dry Mould Application

What are the applications of the skin-dried moulding/casting process?

• Railway wheels and track parts.
• Crusher jaws.
• Cast iron anchors.
• Mechanical brackets.
• Spur gear, bevel gear and helical gear blanks.
• Cast iron and stainless steel ingots.
• Cast iron machine beds of milling machine, shaper machine, drilling machine and planning machine.
• Machine pillars and chucks of CNC machine.
• Rotating shafts of windmills and impulse turbines.
• Artistic and aesthetic symbols and structures.
• Robotic arms, frame and supporting structure.
• Automobile handles, brakes, clutch plates and covers.

Skin Dry Mould Vs. Fully Dry Sand Mould

What is the difference between skin dry mould and dry sand mould?

Skin Dried Mould	Dry Sand Mould
1) Skin-dried mould has only skin which is baked and dry. The entire mould is not green or dried.	1) The dry sand mould is completely dried.
2) The cost of production is lower than complete dry mould as the majority part is made from green sand.	2) The cost of production is higher than skin-dry mould as the entire mould needs to be dried.
3) Does have green sand as backing sand.	3) Does not have green sand as backing sand only dry sand is used.
4) The production time, floor space, equipment, tools and labour of mould making is less.	4) The production time taken and labour required for mould making is more.
5) Comparatively smaller size casting can be produced than fully dried mould.	5) Comparatively larger size casting can be produced as compared to skin-dried mould.
6) More moisture-related defects are observed in this mould.	6) Less moisture-related defects are observed in this mould.
7) Baking time is less as only the skin of the mould is to be dried. Skin can be dried by a portable heat torch and elements.	7) Baking time and energy is longer for this mould.