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Solutions to ten common problems in foundry coatings (Part 1)

Time：

2022-09-21

source：

Weiye New Materials

Some problems often occur in the spraying process of casting coatings. Such as insufficient coating thickness and peeling off. This article will analyze the causes of ten common problems in casting coatings, and introduce solutions.
1、 Coating accumulation
reason:
1. The flow of the coating is slow. As the coating is a thixotropic fluid, there is a network structure and a yield value. The yield value and viscosity are the two main factors that cause the coating to accumulate. Too high yield value and viscosity of the coating cause poor fluidity of the coating.
2. The flow marks generated during the flow of the paint flow along the sand mold, and the accumulation will occur when encountering grooves, resulting in unclear edges and corners of the sand mold.
3. The sand mold inclination angle is inappropriate.
4. The flow is small, and the coating cannot flow down, causing accumulation.
5. The flow rate is slow due to insufficient pressure, resulting in accumulation.
take steps:
1. Considering the field operation, the Baume degree of the coating should be reduced. Practice has proved that when the Baume degree of the flow coating is between 22-26, the fluidity of the coating is good. The yield value and viscosity of the coating shall be reduced in consideration of its own factors.
2. Remove the flow marks by blowing with air duct or dipping with a brush with thinner.
3. Requirements for placing angle of sand mold: use the crown block to lift the sand mold to the top of the coating tank at an angle of 75-90 degrees with the horizontal for flow coating.
4. Increase the sectional area of the flow coating rod head and hose to increase the flow rate. Generally, the flow coating rod head and hose are 4-branch pipes. If the sectional area is increased, 4-branch pipes and 6-branch pipes can be used respectively or 6-branch pipes can be used for both.
5. Raising the air pressure can increase the flow rate. In order to obtain a proper coating thickness, the velocity of coating flowing out of the flow coater should be 100-200 mm/s, and the air pressure is generally 0.4 × 105Pa-0.6 × If it is too large, it is easy to splash between 105 Pa.
2、 Insufficient coating thickness
reason:
1. The coating flows directly without forming enough coating thickness.
2. The coating is completely penetrated into the molding sand, resulting in insufficient coating thickness.
3. The sand mold surface is stuck with release agent, which reduces the permeability of the coating and directly affects the size of the coating thickness.
take steps:
1. Improve the viscosity of the paint (the maximum value shall not exceed 7 s), so as to improve the coating performance and avoid excessive flowing of the paint.
2. Improve the sand mold compactness, which can effectively avoid excessive penetration of paint. The sand mold compactness is appropriate between 45% and 55%,
3. The mold release agent on the surface of the pattern can be fully dried before production. The part of the sand mold partially stained with the mold release agent shall be rubbed off with fine sandpaper before flow coating.
4. Thickness requirements for sand mold wet coating of iron castings:
Thin wall casting 0.15mm-0.30mm
Medium castings 0.30mm-0.75mm
Thick wall casting 0.75mm-1.00mm
Extra thick casting 1.00mm-2.00mm
3、 Coating surface falling off
During the assembly process, when the operator blows the floating sand in the mold cavity with the air duct, the surface of the coating layer will occasionally fall off.
reason:
1. The coating strength is low.
2. The coating layers are not fully combined to form a whole.
take steps:
1. The small amount of binder in the coating will easily lead to insufficient coating strength, so it is necessary to increase the proportion of binder in the coating.
2. The coating is not fully burnt, which may affect the combination between layers. For castings above 3 tons, the coating surface is easy to fall off, so long as the ignition time is reasonably controlled, this problem can be solved. Generally, it is better to ignite within 3-5 seconds after the box flow coating and 5-7 seconds after the box flow coating. Gas fire baking can also be used, but the time should not be too long, otherwise the coating will crack.
4、 Sand adhesion of castings
The refractory of the coating is not enough, and the coating or sand mold contacts with the high temperature metal liquid to cause chemical reaction, which makes the casting surface form a kind of material that is difficult to clean, usually called sand sticking, and the flow coating method will also produce sand sticking.
take steps:
1. Change the composition of the coating aggregate and improve the fire resistance of the coating. Select high alumina powder, zircon powder and other refractory fillers.
2. The coating thickness shall be increased, but the thickness shall not exceed the maximum value required by the coating thickness, and excessive thickness will cause casting defects such as coating skin.
3. Improve Baume degree of flow coating, which shall not exceed 28, or the fluidity will be reduced.
4. Some castings are overheated locally, and the flow coating is very easy to stick sand. Before the flow coating, paint the hot spot with high fire resistance, which can effectively prevent sand sticking.
5、 Severe flow marks
reason:
The paint has poor fluidity and high viscosity, and cannot drip when flowing downward, resulting in serious flow marks; When the coating flows out, the pressure is too high, the distance between the flow coating rod head and the cavity surface is too close, and the coating liquid impacts the coating surface, resulting in uneven marks; The coating flow is small and unstable, and flow marks will form on the surface of the cavity.
take steps:
1. During flow coating, large flow rate shall be used to flow rapidly from top to bottom, and it is not allowed to stay on the sand mold surface for a long time.
2. Improve the fluidity and leveling of the paint and reduce the viscosity.
3. Increase the distance between the flow coating rod head and the cavity surface, generally 18-25mm.
4. Apply sector flow coating on the rod head.