Analysis of the disadvantages of green sand process from China Bestech company

Green sand is a type (core) sand based on green (pottery green) as a polishing agent. Because of its exquisite shape, artificial sand is suitable for the production of demonstration sand factories, so it can now be used as a method of casting production. In the process of recycling, the thermal part of each group in the group changes in physical properties, such as digital regeneration, it is used, and the quality of the used sand is stable. According to statistics, 30-40% of the casting waste rate is the quality of molding sand. Therefore, the quality of molding sand is leading and the quality control plays a role in the modeling of green sand. With the increasing requirements of the times for castings, the requirements for the quality of sand are getting higher and higher.
 
Common defects related to sand quality in green sand modeling
 
1 Stomas, no pouring, cold partition
 
The composition of green sand molding sand is mostly old sand (85~95%). Due to repeated thermal shocks during the recycling process of old sand, some components will undergo thermal decomposition and change in physical properties:
 
In the area where the temperature of the sand mold is higher than 500 degrees, the bentonite crystal structure is completely destroyed, and it becomes invalid green with no wet cohesive force. It exists in the old sand in the state of dust and becomes one of the old sand and mud components;
 
The pulverized coal is carbonized into decayed material;
 
Unstable sand (containing impurities) will pulverize.
 
Together, these derivatives become fine powder of old sand. If the content of the micropowder exceeds a certain limit, the micropowder will block the voids of the sand and the sand will have poor air permeability. In addition, ineffective green has a stronger water absorption capacity than effective green, and it takes effective water from effective green. Therefore, when the content of ineffective green is large, the amount of water required for mixing must be increased. Increasing the amount of water added to the molding sand also increases the amount of gas generated by the molding sand to a certain extent. Due to the increase in gas generation and the decrease in air permeability, the resistance received by the liquid increases during casting, which will inevitably lead to the formation of intrusive pores, inadequate pouring, and cold barriers.
 
2 Poor surface finish (trachoma, burr, sand and scarring, etc.)
 
A large amount of ineffective green causes the molding sand to have poor tensile strength, low toughness and poor air permeability. Increase the brittleness of the molding sand, make the molding sand easy to collapse and fall off, and the sand particles are easy to fall during casting to form defects such as sand holes, burrs, slag inclusions and scars, which will affect the quality of castings.
 
3 Sticky sand
 
The ineffective green (dead green) accounts for about 2 to 5% of the weight of the entire sand mold. A part of the ineffective green is wrapped on the surface of the sand grain under the action of high temperature and sintered to form a firm film, which cannot be washed off with water and becomes a part of the sand grain. This film is also called an inert film. After numerous cycles of mixing and casting, the molding sand is heated, and the inert film wraps multiple layers of overlap. This process is also called the phenomenon of fish-pebbleization. Proper pebbleization reduces the quartz content, reduces the thermal expansion of the molding sand, and reduces expansion defects. When the pebble is excessively petrified, the inert film is too thick, due to the low melting point of the inert film, the overall sio2 content of the molding sand is reduced, and the refractoriness of the molding sand (only 1150 degrees) is reduced. If the casting temperature is higher than 140℃~145℃, it is easy It will cause sand sticking on the surface of the casting, and produce mechanical sticking sand, which will cause the surface of the casting to be unsmooth and clean.
 
4 Sand drop and box collapse during modeling
 
Dead green has no cohesive force, and a large amount of dead green causes poor moldability of molding sand. The molding sand is easy to collapse and fall off.
 
5 Sticky mold
 
Due to the increase in dead green content, this type of molding sand is not only unfavorable for transportation, but also easy to demold during molding.
 
6 There is more and more porcelain in the old sand
 
Due to the influence of high temperature, the green coated with sand particles can be porcelainized into a film. The thickness and total amount of the film are beneficial to the green sand within a certain range, but exceeding a certain limit will affect the particle size distribution and uniformity, and affect the strength .
 
7 Simple manual sand screening
 
Many companies use double sand system, simple manual screening, adding a large amount of new sand and more auxiliary materials to mix the surface sand, the back sand is screened regularly.
 
There are many problems:
 
The mud content of the old sand seriously exceeds the process requirements (mostly ≥20%)
 
The amount of water added exceeds the standard
 
Uneven particles, low concentration