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| Common defects in solid casting and their solutions |
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I wrinkled skin
Features and positions: there are surface defects such as corrugated wrinkled skin, slag - sandwiched wrinkled skin, and cold - separated wrinkled skin. It is usually located on the upper surface or side of iron castings. This defect is unique to iron castings.
Ii. carbon film
Features and locations: the upper surface of castings is covered with a thin bright carbon film. This defect is unique to iron castings.
Cause analysis:
1. the air permeability of the mold is poor;
2. the density of foamed plastics is high.
Prevention methods:
1. improve air permeability of molding sand;
2. thinning the coating thickness;
3. choose the shape material with small density;
4. take exhaust measures for internal and external ventilation passages.
Iii. slag pores
Features and locations: deep closed or semi - closed, distributed on the upper surface of the casting, rarely appearing on the side. The hole or surface is usually covered with a bright carbon film. This defect is often found in cast iron and bronze castings.
Cause analysis:
1. pouring speed is too fast;
2. foam density is too high;
3. unreasonable distribution of gating system;
4. the air permeability of the mold is poor.
Prevention methods:
1. controlling suitable pouring speed;
2. using low density foam plastics;
3. the distribution of the pouring system should ensure the smooth flow of metal;
4. improving the air permeability of the mold;
5. set air outlet riser, hidden riser or slag collecting bag.
Iv. sand sticking
Features and positions: it often appears at the bottom or lower side of castings, as well as at hot spots and places where the mold is not compacted with sand.
Cause analysis:
1. appearance surface quality is poor;
2. the pouring temperature is too high;
3. the mold is not compacted or is not evenly compacted;
4. the grain size of the molding material is too coarse;
5. the coating is brushed too hard, or the composition of the coating is not ideal.
Prevention methods:
1. improving cutting tools and processing methods;
2. choosing suitable pouring temperature;
3. firm the mold to maintain a sufficiently uniform compactness;
4. choose the best coating formula, the coating should be evenly brushed and the thickness should be appropriate;
5. use fine sand instead, or adjust the particle size.
V. cold separation and slag inclusion
Features and locations: cold compartments are often covered with carbon films or residues of polystyrene decomposition products. This defect is a common defect in solid casting.
Cause analysis:
1. the pouring temperature and speed are low;
2. there is a pause in the pouring process;
3. the surface of the foam plastic mold is rough and has deep pores;
4. the density of foamed plastics is too high;
5. pouring paint or loose sand into the mold;
6. unreasonable distribution of gating system.
Prevention methods:
1. increasing pouring temperature and speed;
2. continuously pouring;
3. reduce the surface roughness of the pattern;
4. choose low density foam plastics;
5. choose a better coating and use a direct runner made of refractory brick.
Vi. surface crust
Characteristics and location: there are often large pores or black carbon residue and carbon film under the crust. This defect often occurs in large planes or in the plane parts of iron castings where the surface area and volume are relatively large.
Cause analysis:
1. pause during pouring, so that the cold iron liquid injected first contacts the mold wall and a thin metal crust is easily formed there;
2. a small amount of molten iron first flows into the mold due to careless pouring, i.e. the mold wall is cooled to form a thin crust.
Prevention methods:
1. operate carefully according to the pouring process, and ensure uninterrupted metal flow during pouring;
2. properly raise the pouring temperature of the molten metal.
Vii. carburizing
Features and locations: only appear on the surface layer of low carbon or low carbon alloy steel castings.
Cause analysis:
1. the carbon content in the alloy is low ( ω c < 0.4 % );
2. the air permeability of the mold is poor;
3. improper pouring speed and method;
4. the density of appearance materials is large;
5. incomplete gasification of appearance.
Prevention methods:
1. improving the air permeability of the mold;
2. pouring should be smooth to maintain a certain delta gap;
3. using low density shape materials;
4. improve the gasification conditions of the pattern, such as adopting hollow structure patterns and internal and external ventilation channels.
Viii. nodal tumor
Features and positions: some irregular protruding parts appear on the surface of the casting. This defect often appears on the surface of iron castings, accompanied by cold insulation and wrinkled skin.
Cause analysis:
1. mold compactness is low or uneven;
2. anti-sticking sand coating is too thin;
3. due to careless pouring operation, some molten iron may first flow into the mold and then remain on the surface of the casting.
Prevention methods:
1. the mold is evenly ground;
2. increase the thickness of the coating and brush it evenly;
3. strictly follow the pouring principles and methods.
Ix. crystalline reticulation
Features and locations: the concave seam at the joint of foam molding particles is copied on the surface of the casting. This defect is a surface defect of the foam molding model, which only affects the appearance and does not affect the quality of the casting.
Cause analysis: the foam polystyrene particles in the foam molding die are poorly bonded, resulting in concave gaps between the foam lattices. After pouring and cooling, the molten metal will be coated on the surface of the casting to form a mesh surface defect.
Prevention methods:
1. improve the forming and foaming process to improve the surface quality of the pattern;
2. coat the concave seam with bright and clean paint on the surface of the pattern.
X sand collapse
Features and locations: the larger plane on the upper part of the mold, and the local damage of the sprue or mold resulting in the collapse of the mold wall. This defect often occurs in dry sand casting without binder.
Cause analysis:
1. the strength and compactness of the casting mold are low, and the casting mold cannot withstand the scouring of molten metal with high casting speed;
2. negative pressure appears at the gate;
3. casting speed is too slow ( mainly in dry sand without binder );
4. the strength of anti-sticking sand coating is not enough.
Prevention methods:
1. increase the content of binder ( or hardening agent ) in the molding sand and compact the mold;
2. improve the structure and distribution of the gating system;
3. appropriately increasing the pouring speed of the molten metal;
4. increasing the thickness and strength of the coating;
5. for molding materials without binder or molding sand with low strength, refractory bricks or ceramic pipes should be used as the direct runner.
Eleven, casting deformation
Features and locations: geometry of castings destroyed.
Cause analysis:
1. the strength of the model is low or the structure is unreasonable, causing deformation in the modeling process;
2. improper modeling operation, or excessive force when grinding sand;
3. the weight of pressed iron is not enough, or the compactness of the mold is low.
Prevention methods:
1. use foam plastic with appropriate strength;
2. take measures to enhance the shape strength and deformation resistance, such as strengthening ribs;
3. extra care should be taken in modeling, or auxiliary tools ( such as padding plates ) should be used for operation.
4. the mold should be fully compacted or the weight of compacted iron should be increased.
Xii. slag dissolution and sintering
Features and locations: molding materials and castings are fused together, mostly appearing in thick wall, hot joint and bottom of castings. This defect is a common defect of binder-free molding materials.
Cause analysis:
1. the pouring temperature is too high;
2. the high temperature performance of the coating is poor, the coating is too thin and the strength is insufficient;
3. the particles of molding material are too large;
4. molding materials have poor high temperature resistance.
Prevention methods:
1. lowering the pouring temperature;
2. choose a good coating to increase the strength and thickness of the coating
( b ) to: 3. selecting fine particles or molding materials with appropriate particle size composition;
4. choose high melting point molding materials, for example, for steel castings, silica sand can be replaced with zirconium sand, etc.
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