What are the common defects and causes in precoated sand shell casting process?

Release time: 2020-07-14 14:21:44.000


Coated sand casting has a long history in foundry field. Nowadays, with the continuous improvement of raw materials, manufacturing equipment and manufacturing technology, the quality of coated sand has been continuously improved, and the production cost has been reduced.The application of coated sand has been developed more rapidly,product types continue to increase, and has formed a series. Coated sand castings have superior quality, low price and are favored by customers.However, do you know what are the defects in the production process of coated sand? What are the main reasons for the defects and what are the measures to avoid them?



Common defects, main causes and solutions in the use of coated sand.
1. Shelling


Unreasonable mold design, core box temperature uneven, so that low-temperature parts of the low strength and shelling; Coated sand has low melting point, slow curing speed and low thermal strength.
Improve the mold structure, so that the temperature distribution is uniform; Select high melting point, high curing speed, high thermal strength resin.


2. Loose surface of Mold(core)

Core pressure is too high or too low; Mold exhaust is not smooth;The mold runs out of sand because of the large gap of the box surface. Coated sand has poor fluidity or permeability.
Select reasonable sand injection pressure, improve the exhaust system to prevent choking; Use materials with small deformation to make core box; Select coated sand with good fluidity and permeability.


3. Deformation and fracture of mold (core)

Mold heating uneven, or core wall thickness differences result in the contraction of inconsistent cooling; The core fork is deformed or the sand core is stored unevenly; The high temperature performance of coated sand is poor; The pouring pressure is too high.
Improve the mold structure, make the temperature distribution uniform. Store sand core in a molding tray; adopt resin with low curing shrinkage; Adopting high temperature resistant and low expansion coated sand;Improve gating system (using pressureless type)


4. The local strength of the core is low or loose.

The crusts are thin. 
Improve the mold structure, so that temperature distribution is uniform; Select high melting point, curing speed, high thermal strength resin; Adjusting the sand shooting pressure; Improve the exhaust system.


5. The air exhaust of the hollow core of the casting is not smooth.

Resin sand has large gas generation capacity or gas generation speed is not appropriate.
Improve the exhaust system, improve the exhaust effect; Select high concentration or coarse sand.Adopting low gas evolution resin coated sand

6. Sand penetration of castings and low SiO2 content of raw sand


Core surface is not dense.
Adjust the pressure of shooting sand, improve the exhaust effect of core box, and make the surface of core more dense.Use high temperature resistant coated sand or zircon coated sand.

7. Shrinkage porosity inside the casting


Resin in coated sand burns at high temperature to produce heat, which slows down the solidification rate of molten iron and leads to shrinkage porosity.
Chilled resin coated sand is used; An inner chill is placed in the shell core.


8. Poor surface of castings

Phenolic resin in the formation of high temperature bright carbon floating in the molten iron surface solidification casting surface wrinkle.Adding about 2% ferric oxide powder, adopting raw sand with high thermal conductivity;Paint the surface of the shell core; adding special excipient to coated sand.
As a kind of coated sand, ceramic foundry sand have obvious advantages, which can be used to make both mold and sand core; The casting expansion defects can be effectively prevented by using part of ceramic foundry sand in the molding sand and core sand. In recent years, some manufacturers of coated sand in China have added some ceramic foundry sand into raw sand, so that the shell mold and shell core can have high temperature resistance, low expansion, easy collapse, high strength and low gas generation.Especially for the complex shape of the core, it can also solve the problem that the sand is not easy to compact.


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