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The main defects of gray iron castings caused by moulding and core making and their causes analysis and prevention methods

Views: 144 Update date: Jul 10,2021




(1) Holes


Features and discovery methods:

Local holes: In local parts of the casting, single holes or honeycomb holes with a clean and smooth surface of the cavities appear

It can be found by visual inspection, machining or magnetic inspection.

Cause Analysis:

1. The setting of the gating system is unreasonable, so that the exhaust is not smooth or the vortex is generated, and the gas is involved.

2. The compactness of the sand mold is too high, which reduces the air permeability

3. Poor exhaust of sand core, or blockage of air passage

Prevention method:

1. The setting of the gating system should consider the smooth exhaust in the cavity and the smooth flow into the mold

2. The sand mold needs to be evenly compacted, and should not be too tight

3. The sand core exhaust is required to be unblocked. When closing the box, pay attention to sealing the gap between the core and head to prevent the molten iron from drilling people and blocking the air passage

4. At the highest point of the casting, a vent hole or a vent piece, etc. can be set

5. It is not advisable to brush too much water when drawing and repairing the mold

6. For large flat castings, inclined pouring can be used, and the vent hole is slightly higher to facilitate exhaust

7. The core support and cold iron must be clean and rust-free


(2) Sand hole

Features and discovery methods:

There are sand particles in the cavities of the casting

It can be found by external inspection, machining or magnetic inspection

Cause Analysis:

1. The position of the pouring system is inappropriate, such as directly facing the sand core, or the gate is too small, and the iron liquid has a large scouring force. Destroy local sand mold

2. Because the model structure design is not good enough, 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 part or protruding part to fall off

4. The falling sand when modeling and closing the box, the sand is not cleaned up

Prevention method:

1. The position and size of the pouring system are appropriate

2. Reasonably choose the draft angle and rounded corners, and the rounded corners can be pressed out when sculpting by hand. In batch production, the pattern should be painted with parting agent to avoid sticking to the mold, and attention should be paid to repair the damaged parts.

3. Shorten the residence time of 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, you must clean the falling sand inside the mold, close the box carefully, and cover the pouring riser in time to avoid falling into the sand again


(3) Sand inclusion

Features and discovery methods:

On the surface of the casting, a layer of molding sand is sandwiched between a layer of iron and the casting

It can be found by visual inspection or mechanical processing

Cause Analysis:

After the molten iron enters the sand mold. Make the moisture in the surface layer migrate inward, forming a high-moisture zone at 3~5mrn from the surface. The strength at this place is greatly reduced, and it is easy to cause the molten iron to sneak in, or the surface layer swells due to the stress of the high temperature expansion of the silicon sand grains, and the molten iron penetrates, forming sand

inclusions.

1. Sand compaction is too hard or uneven compaction

2. Improper pouring position; for horizontally pouring large flat castings, sometimes the molten iron intermittently covers a part of the large flat surface, resulting in sand inclusions

Prevention method:

1. The sand mold compactness should not be too tight, uniformity is required, and air permeability should be strengthened

2. When sculpting by hand, nails can be inserted to strengthen the weak spots

3. Try to make the large plane face down or on the side to reduce the baking area and baking time of the molten iron on the upper plane

4. For large flat castings, the pouring system can be distributed, and the inner gate section should be appropriately enlarged to shorten the pouring time or tilt the 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. The sand mold compactness is uneven or too small

2. The paint is too thin

Prevention method:

1. Appropriately increase the compactness of the sand mold, reduce the gap between sand grains (to ensure air permeability requirements), and tamping evenly

2. Choose appropriate paint (mostly graphite powder water paint) and brush with a certain thickness, which can not only improve the fire resistance, but also prevent the molten iron from penetrating into the sand.


(5) Thermal cracking

Features and discovery methods:

Cracks, with dark or almost black oxidized surface

Use visual inspection, light transmission method, magnetic flaw detection, pressure test, kerosene penetration and other methods to find

Cause Analysis:

1. The retreat of sand core and sand mold is poor, and casting shrinkage is hindered

2. The core bone eats too little sand or the sand box belt is too close to the casting, which prevents the casting from shrinking

3. The inner runner is set to be too concentrated, local overheating, increasing stress

4. The flash of the casting is too large, and the crack at the flash extends to the casting

Prevention method:

1. Appropriate requirements for sand mould compaction, and appropriate amount of sawdust can be added to the moulding sand

2. Use a smaller core bone to make the amount of sand eaten appropriately, and use a reasonable sand box

3. The layout of the inner runners should be properly dispersed

4. At the junction of thick and thin castings, shrink ribs can be added

5. Correctly choose the position of the parting surface, and try to close the box as much as possible when closing the box


(6) Deformation

Features and discovery methods:

Long or flat castings are recessed on the side close to the wall thickness, forming a curved shape

Use visual inspection, marking and other methods to find

Cause Analysis:

Due to the uneven wall thickness of the casting and the sequential cooling, thermal stress is generated. When the value is greater than the yield limit of the material, deformation and bending will occur

Prevention method:

1. Set cold iron at the thick wall or open the inner gate at the thin wall to create simultaneous solidification conditions

2. Leave pre-deformed curvature or add stiffening ribs on the pattern

3. Improve casting structure


(7) mismatch

Features and discovery methods:

Relative displacement of the casting along the parting surface

It can be found by visual inspection or cross-line measurement

Cause Analysis:

1. The size of the shape is wrong or deformed

2. Inaccurate positioning of sand box or parting plate

3. Inaccurate closing

4. The position of the pattern on the template is offset

Prevention method:

1. Check and trim the appearance

2. Check, repair or change the sand box and parting plate

3. Pay attention to closing the box accurately

4. Check and adjust the position of the pattern on the template


(8) Protrusions

Features and discovery methods:

Irregular burrs, drape or protrusions on the casting

Found by visual inspection

Cause Analysis:

Due to the pressure of the molten iron, the cavity is locally swollen. Mostly appear in the lower mold, such as insufficient or uneven sand mold compaction, partial looseness, etc.

Prevention method:

Appropriately increase the compactness of the sand mold and require uniform tamping


(9) Shrinkage

Features and discovery methods:

Concentrated holes with irregular shapes and rough surfaces are produced at the hot joints of the casting

It can be found by visual inspection, machining or magnetic inspection

Cause Analysis:

1. Insufficient feeding of castings

2. Improper setting of cold iron

3. Improper position of the inner gate

4. Insufficient compactness of the sand mold, which will be produced after bulging

Prevention method:

1. Appropriately increase the size of the riser

2. At the thick wall, install cold iron to create simultaneous solidification conditions or when used in conjunction with risers, create sequential solidification conditions

3. Correct selection of pouring position and pouring system to cause simultaneous solidification or sequential solidification

4. Appropriate sand compaction is required


(10) Box-lifting

Features and discovery methods:

The shape of the casting does not match the drawing, and it is found by visual inspection

Cause Analysis:

1. The weight of the pressure box is not enough

2. The force is uneven or too loose when the clamp box is tightened

Prevention method:

1. Sufficient pressure box weight or evenly tightened with bolts

2. The parting surface should be flat, and pay attention to tightness when closing the box to avoid leakage of molten iron


(11) Iron beans

Features and discovery methods:

There are small iron beads in the pores; it can be found by inspection of the casting section and machining

Cause Analysis:

1. Moist sand

2. The inner runner is too high from the lowest part of the casting. When pouring,

the molten iron splashes to form iron beans. After the molten iron is full, the iron beans cannot be melted, making it encased in the casting together with gas.

3. Poor air permeability of sand core

Prevention method:

1. Do not brush too much water when repairing

2. Reasonably determine the location of the gating system

3. Strengthen the ventilation of the sand core


(12) Slag hole

Features and discovery methods:

There is slag in the cavities outside or inside the casting

It can be found by visual inspection, machining or magnetic inspection

Cause Analysis:

Poor slag retention in the gating system

Prevention method:

Reasonably select gating system and strengthen slag retaining measures


(13) Cold barrier and insufficient pouring

Features and discovery methods:

There are gaps or partial lack of meat on the castings that are not completely fused, with rounded edges around them

It can be found by visual inspection

Cause Analysis:

1. Improper setting of the gating system, or the gate section is too small

2. The local wall of the casting is too thin

3. Improper selection of cold rail location

4. Hanging the core, the dislocation when closing the mold makes the wall of the casting part too thin, or even no wall thickness at all

Prevention method:

1. Appropriately increase the size of the pouring system

2. For long castings, two-end pouring can be used; for tall parts, step pouring or scattered gates can be used, etc.

3. The inner runner should not be too far away from the thin wall of the casting, or the thickness of the thin wall can be appropriately increased

4. When lifting the core, check the size at any time and pay attention to the accuracy of the fitting


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