The main defects of gray cast iron parts in modeling and core making and their causes analysis and prevention methods
(1) Stomata
Features and discovery methods:
Partial pores: In the partial parts of the casting, there are single pores or honeycomb pores with relatively clean and smooth surface
With the method of visual inspection, mechanical processing or magnetic flaw, detection can be found.
Cause Analysis:
1. Poor setting of the pouring system makes the exhaust gas unblocked or produces vortex, which is involved in the gas
2. The sand compactness is too high, reducing the permeability
3. Poor exhaust of sand core, or blocked air passage
Prevention method:
1. The setting of the pouring system should consider the smooth exhaust and smooth flow into the mold
2. The tightness of the sand mold requires uniformity, and should not be too tight
3. The exhaust requirements of the sand core are smooth. When close the box, pay attention to seal the gap of the core head, so as to prevent the molten iron from drilling and blocking the air passage
4. At the highest part of the casting, a vent or a vent can be set
5. It is not advisable to brush too much water when lifting and reshaping
6. For large plane castings, oblique casting can be used, the vent hole is slightly higher to facilitate exhaust
7. The core support and cold iron must be clean and free of rust
(2) Trachoma
Features and discovery methods:
The casting cavity contains sand
With external inspection, mechanical processing or magnetic flaw, detection can be found.
Cause Analysis:
1. The position of the pouring system is not suitable, such as straight to the sand core, or the gate is too small, and the erosion force of the molten iron is large. Destruction of local sand mold
2. Because the model structure design is not good enough, the mold sticking occurs, and the sand mold is not repaired, or the corners of the casting are not rounded
3. The residence time of the wet type before pouring is too long, causing the dry bath part or the protruding part to fall off
4. Falling sand during styling and boxing, uncleaned sand
Prevention method:
1. The location and size of the pouring system are appropriate
2. Reasonable selection of draft angle and rounded corners. When hand-shaped, rounded corners can be pressed out. In batch production, the pattern should be coated with a parting agent to avoid sticking the mold and intended to repair damaged parts.
3. Shorten the residence time of the wet type before pouring
4. Reasonably choose the gap between the core head and the core seat to avoid crushing when closing the box
5. Before closing the box, the sand falling inside the mold must be cleaned, close the box carefully, and cover the riser in time to avoid falling into the sand
(3) Sand
Features and discovery methods:
On the surface of the casting, there is a layer of sand between the layer of iron and the casting
Can be found by visual inspection or mechanical processing
Cause Analysis:
After the molten iron enters the sand mold. Moisture of the surface layer migrates inward, forming a high-moisture zone at the 3 ~ 5mrn from the release surface. The strength of this place is greatly reduced, and it is easy to cause the molten iron to infiltrate, or the surface layer will bulge due to the stress of the high-temperature expansion of the silicon sand grains, and the molten iron will drill in, forming sand inclusions.
1. The tightness of the sand mold is too hard or uneven
2. Improper pouring position; for large plane castings that are cast horizontally, sometimes sand is generated due to the molten iron intermittently covering somewhere on the large plane
Prevention method:
1. The tightness of the sand mold should not be too tight, it needs to be uniform, and strengthen the ventilation
2. When hand-molding, local weak points can be inserted to strengthen
3. Try to make the large plane face down or put it on the side to reduce the baking area and baking time of the molten iron on the upper plane
4. For large plane castings, the pouring system can be distributed, and the cross section of the inner gate can be appropriately increased to shorten the pouring time or oblique pouring
(4) Sticky sand
Features and discovery methods:
The molten iron and sand on the surface of the casting stick together to form a rough surface, which can be found by visual inspection
Cause Analysis:
1. Sand compactness is uneven or too small
2. The paint is too thin
Prevention method:
1. Properly increase the compactness of the sand mold, reduce the sand gap (to ensure the breathability requirements), and tamping evenly
2. Choose the appropriate paint (mostly graphite powder water paint), and brush to a certain thickness, not only can improve the fire resistance, but also can prevent molten iron from drilling into the sand
(5) Hot crack
Features and discovery methods:
Cracks, with dark or almost black oxidized surfaces
Found by visual inspection, light transmission method, magnetic flaw detection, pressure test, kerosene penetration, etc.
Cause Analysis:
1. Poor yielding of sand core and sand mold, casting shrinkage is hindered
2. The amount of core sand is too small or the sand box is too close to the casting, which hinders the shrinkage of the casting
3. Inner sprue is set too concentrated, local overheating, increase stress
4. The burr of the casting is too large, and the crack at the burr extends to the casting
Prevention method:
1. The compactness of the sand mold is suitable, and an appropriate amount of sawdust can be added to the molding sand
2. Switch to smaller core bones to make the amount of sand suitable and select a reasonable sand box
3. The layout of the inner runner should be properly dispersed
4. At the junction of thick and thin castings, shrinkage ribs can be added
5. Correctly select the position of the parting surface and make it as close as possible when closing the box
(6) Deformation
Features and discovery methods:
Long or flat castings are recessed on the side near the wall thickness and are curved
Found by visual inspection, line drawing, etc.
Cause Analysis:
Due to the uneven wall thickness of the casting, the cooling has a sequence, which generates thermal stress. When the value is greater than the yield limit of the material, deformation and bending occur
Prevention method:
1. Set cold iron at the thick wall or open the gate at the thin wall to create simultaneous solidification conditions
2. Leave the pre-deformation curvature on the pattern or add reinforcement ribs
3. Improve the casting structure
(7) Wrong box
Features and discovery methods:
The relative displacement of the casting along the parting surface
It can be found by visual inspection or line measurement
Cause Analysis:
1. Wrong or deformed shape
2. Inaccurate positioning of sand box or parting board
3. Improper packing
4. The position of the pattern on the template is offset
Prevention method:
1. Check and trim
2. Check, repair or replace the sand box and parting board
3. Pay attention to closing the box accurately
4. Check and adjust the position of the pattern on the template
(8) Fleshy
Features and discovery methods:
The casting has irregularly shaped burrs, sacks or protrusions
Found by visual inspection
Cause Analysis:
Due to the pressure of the molten iron, the cavity expands locally. Most of them appear in the lower mold, such as the sand mold is not tight or uneven, and the local is too loose
Prevention method:
Properly improve the compactness of the sand mold and require uniform compaction
(9) Shrinkage
Features and discovery methods:
Concentrated holes with irregular shape and rough surface at the hot joint of casting
Visual inspection, mechanical processing or magnetic flaw detection can be found
Cause Analysis:
1. Insufficient casting shrinkage
2. Improper setting of cold iron
3. Improper location of the gate
4. The compactness of the sand mold is not enough, which will occur after the expansion
Prevention method:
1. Increase the riser size appropriately
2. Set cold iron at thick walls to create simultaneous solidification conditions or create sequential solidification conditions when used in conjunction with risers
3. Correctly select the pouring position and pouring system to cause simultaneous solidification or sequential solidification
4. It is required that the sand compactness is suitable
(10) Lifting the box
Features and discovery methods:
The shape of the casting does not match the drawing, it is found by visual inspection
Cause Analysis:
1. The weight of the press box is not enough
2. Uneven or too loose force when clamping the clamp
Prevention method:
1. Sufficient weight of pressure box or evenly tighten with bolts
2. The parting surface should be flat, pay attention to the close when closing the box, so as to avoid the leakage of molten iron
(11) Iron Bean
Features and discovery methods:
There are small iron beads in the pores; by casting section inspection, mechanical processing can be found
Cause Analysis:
1. Sand type is wet
2. The inner runner is too high from the lowest part of the casting. When pouring, it will cause molten iron to splash and form iron beans. After the molten iron is filled, it will not melt the iron beans, so that it is wrapped in the casting together with the gas
3. Poor sand core permeability
Prevention method:
1. Do not brush too much water when repairing
2. Reasonably determine the location of the pouring system
3. Strengthen the ventilation of the sand core
(12) Slag eye
Features and discovery methods:
There is slag in the cavity outside or inside the casting
Visual inspection, mechanical processing or magnetic flaw detection can be found
Cause Analysis:
Poor slag blocking of pouring system
Prevention method:
Reasonable selection of pouring system and strengthen slag blocking measures
(13) Insufficient cold insulation and pouring
Features and discovery methods:
There are gaps or partial lack of meat on the casting, and the edges are rounded
Can be found by visual inspection
Cause Analysis:
1. Improper setting of the pouring system, or the gate section is too small
2. The local wall of the casting is too thin
3. Improper selection of cold iron location
4. Hanging core, the misalignment during closing makes the wall of the casting part too thin, or even no wall thickness at all
Prevention method:
1. Properly increase the size of the pouring system
2. For long castings, two-end casting can be used; for tall parts, step casting or dispersion gates can be used
3. The inner runner should not be too far from the thin wall of the casting, or the thickness of the thin wall can be increased appropriately
4. When hanging the core, check the size at any time, and pay attention to the correct type
