1. Coating Accumulation

Causes

1. Coating's Own Properties: Coatings are thixotropic fluids with a network structure and yield value inside. If the yield value is too high and the viscosity is too large, the fluidity of the coating will be greatly reduced, making it difficult for the coating to flow evenly and thus forming accumulation.

2. Impact of Construction Flow Marks: Flow marks generated during the flow of the coating will flow along the surface of the sand mold with the coating. When encountering concave parts such as grooves on the sand mold, the coating carried by the flow marks is easy to accumulate here.

3. Improper Placement Angle of Sand Mold: If the inclination angle of the sand mold is too small, the flow speed of the coating on the surface of the sand mold slows down, and it is easy to stay and accumulate in low-lying areas.

4. Insufficient Flow Rate: During flow coating, the output flow rate of the coating is too small to meet the requirement of uniform coverage of the coating on the surface of the sand mold, and the excess coating is easy to accumulate locally.

5. Low Pressure: Insufficient air pressure of the flow coating equipment leads to slow outflow speed of the coating, and the coating stays on the surface of the sand mold for too long and gradually accumulates.

Measures Taken

1. Adjust Coating Performance and Baume Degree: From the perspective of on-site operation, reduce the Baume degree of the coating. Practice shows that when the Baume degree of the flow coating is controlled between 22 and 26, the fluidity is the best; from the perspective of the coating formula, optimize the composition to reduce the yield value and viscosity and improve the fluidity.

2. Remove Flow Marks: If flow marks have been generated, use an air duct to blow air at the flow marks to smooth the excess coating with the air flow; or dip an appropriate amount of thinner with a brush and gently wipe the flow marks to make the coating evenly distributed.

3. Standardize the Placement Angle of Sand Mold: Use a crane to lift the sand mold above the coating tank, adjust the angle between the sand mold and the horizontal plane, and keep it in the range of 75-90 degrees for flow coating to ensure the coating flows smoothly and reduce accumulation.

4. Increase Flow Rate: Increase the flow rate by increasing the cross-sectional area of the flow coating rod head and the hose. Conventional flow coating rod heads and hoses are mostly 4-inch pipes. Both can be replaced with 4-inch and 6-inch pipes respectively, or both can be replaced with 6-inch pipes to expand the coating delivery channel.

5. Adjust Air Pressure: Increase the air pressure of the flow coating equipment to increase the coating flow rate. Generally, the air pressure is controlled between 0.4×10⁵Pa and 0.6×10⁵Pa. At this time, the outflow speed of the coating from the flow coating machine can reach 100-200mm/s, which can not only ensure the appropriate coating thickness but also avoid coating splashing caused by excessive air pressure.

2. Insufficient Coating Thickness

Causes

1. Coating Flows Too Fast: The coating has low viscosity and poor adhesion. After construction, it flows along the surface of the substrate without forming a sufficient thickness, resulting in insufficient overall thickness of the coating.

2. Excessive Penetration of Coating: The substrate (such as sand mold) has insufficient compaction and large internal pores. After the coating is applied, it will penetrate into the substrate in large quantities and cannot form a coating of sufficient thickness on the surface.

3. Substrate Surface Contamination: The surface of the sand mold is adhered with a release agent, which will form an isolation layer on the surface of the sand mold, reducing the adhesion and penetration of the coating to the sand mold and directly affecting the coating thickness.

Measures Taken

1. Increase Coating Viscosity: On the premise of ensuring the fluidity of the coating, appropriately increase the coating viscosity, with the maximum value not exceeding 7s, so as to enhance the coating adhesion, avoid excessive flow of the coating, and ensure that the coating can form a sufficient thickness on the substrate surface.

2. Improve Sand Mold Compaction: By optimizing the sand mold manufacturing process, control the sand mold compaction between 45% and 55%, reduce the internal pores of the sand mold, and effectively prevent excessive penetration of the coating.

3. Clean the Substrate Surface: Ensure that the release agent on the mold surface is fully dried before making the sand mold; if the local part of the sand mold is adhered with the release agent, use fine sandpaper to polish off the part adhered with the release agent, and then carry out the flow coating construction.

4. Clarify Coating Thickness Standards: For the wet coating of cast iron sand molds, set the thickness requirements according to the type of castings:

• Thin-walled castings: 0.15mm-0.30mm

• Medium-sized castings: 0.30mm-0.75mm

• Thick-walled castings: 0.75mm-1.00mm

• Extra-thick castings: 1.00mm-2.00mm

3. Coating Surface Peeling

Causes

1. Insufficient Coating Strength: The amount of binder in the coating is too small to provide sufficient cohesion for the coating, resulting in low coating strength and easy peeling under external forces (such as air flow impact).

2. Poor Bonding Between Coating Layers: During multiple flow coatings, the previous coating layer is not fully bonded with the subsequent coating layer, and there is a delamination phenomenon between the coating layers, which does not form a whole and is prone to interlayer peeling under the influence of external forces.

Measures Taken

1. Increase the Proportion of Binder: According to the requirements of the coating formula, appropriately increase the amount of binder in the coating to enhance the cohesion of the coating, improve the coating strength, and avoid peeling due to insufficient strength.

2. Optimize Ignition and Baking Processes: Insufficient combustion of the coating will affect the interlayer bonding. For castings over 3 tons, it is necessary to reasonably control the ignition time: ignite 3-5 seconds after the upper box flow coating, and ignite 5-7 seconds after the lower box flow coating; gas fire baking can also be used, but the baking time must be strictly controlled to prevent coating cracking caused by excessive baking.

4. Casting Sand Adhesion

Solutions

1. Optimize Coating Aggregate Composition: Replace the aggregate in the coating, select fillers with high refractoriness such as high bauxite powder and zircon powder, improve the overall refractoriness of the coating, and enhance the resistance of the coating to high-temperature molten metal.

2. Reasonably Increase Coating Thickness: On the premise of not exceeding the maximum value of the coating thickness, appropriately increase the coating thickness to improve the protective effect of the coating on the sand mold, but it should be noted that excessive thickness may cause casting defects such as coating skins.

3. Adjust Coating Baume Degree: Increase the Baume degree of the flow coating to enhance the coating coverage, but the maximum Baume degree does not exceed 28 to avoid reducing the fluidity of the coating due to excessively high Baume degree and affecting the construction quality.

4. Targeted Coating of High-Temperature Parts: For castings with local overheating, before flow coating, pre-coat a special coating with higher refractoriness on the hot spot (the part of the casting prone to overheating) to focus on protecting the part prone to sand adhesion.

5. Severe Flow Marks

Causes

1. Poor Coating Fluidity: The coating has high viscosity, poor fluidity and leveling property, and cannot drip smoothly when flowing downward, forming obvious flow marks on the substrate surface.

2. Improper Construction Pressure and Distance: The pressure when the coating flows out is too high, and the distance between the flow coating rod head and the cavity surface is too close. The coating liquid strongly impacts the coating surface, destroying the flatness of the coating and causing uneven marks.

3. Unstable Flow Rate: The output flow rate of the coating is small and fluctuates greatly, resulting in uneven coverage of the coating on the cavity surface and forming intermittent flow marks.

Solutions

1. Optimize Flow Coating Operation Method: During flow coating, use large flow output, and quickly complete the flow coating from top to bottom during the operation process, avoiding the flow coating rod head staying on the sand mold surface for a long time to reduce the generation of flow marks.

2. Improve Coating Fluidity: Reduce the coating viscosity by adjusting the coating formula or adding an appropriate amount of thinner, improve the fluidity and leveling property of the coating, so that the coating can evenly cover the substrate surface.

3. Adjust Flow Coating Distance: Increase the distance between the flow coating rod head and the cavity surface, generally controlling it at 18-25mm, to avoid excessive impact of the coating liquid on the coating surface.

4. Replace Flow Coating Rod Head: Use a fan-shaped flow coating rod head. The fan-shaped nozzle can make the coating spray in a wider and more uniform way, reduce local coating accumulation, and reduce the probability of flow marks.

6. Laminations

Causes

1. Excessively High Sand Mold Temperature: The sand mold just made by the sand mixer has a high temperature. If flow coating is carried out immediately, the high temperature will accelerate the surface drying of the coating, and the coating applied in subsequent flow coatings cannot be fully integrated with the previous coating, forming laminations.

2. Excessively High Coating Viscosity: The coating has high viscosity and poor fluidity. During multiple flow coatings, the subsequent coating is difficult to cover the traces of the previous coating, and it is easy to form superimposed textures.

3. Insufficient Flow Rate and Multiple Flow Coatings: The flow coating flow rate is small, and a single flow coating cannot meet the requirements of coating thickness, so multiple flow coatings are required. Boundaries are easy to form between the coating layers of each flow coating, forming laminations.

Solutions

1. Control Sand Mold Temperature: Do not carry out flow coating immediately on the sand mold after flowing out of the sand mixer. According to the actual temperature of the sand mold, place it in the air to cool for a period of time, and carry out flow coating after the temperature drops to a suitable range (generally close to the ambient temperature).

2. Reduce Coating Baume Degree: Reduce the coating Baume degree by adding a thinner to improve the coating fluidity, so that the coatings applied in multiple flow coatings can be better integrated and reduce lamination marks.

3. Optimize Flow Rate and Number of Flow Coatings: Make flow coating machines of different specifications, reasonably control the flow rate according to the coating thickness requirements, select a pump with a slightly higher head and flow rate, and realize controllable liquid flow through the discharge at the control switch to ensure that a single flow coating can reach the required coating thickness and avoid multiple flow coatings.

7. Coating Splashing

Cause

Solutions

1. Reduce Flow Coating Outlet Pressure: Considering factors such as the thickness, length, surface roughness of the coating flow pipeline and the outflow position, control the coating outflow pressure P within a reasonable range of not less than 0.4×10⁵Pa to avoid splashing caused by excessively high pressure.

2. Adjust Flow Coating Angle: During flow coating, avoid the flow coating rod head being perpendicular to the cavity surface. The flow coating rod head can be tilted appropriately to make the coating contact the cavity surface at a gentle angle and reduce coating splashing.

8. Sand Mold Surface Sand Scratching (Hairing)

Solutions

1. Repair Before Flow Coating: Fill the sand-scratched part of the sand mold with coating paste to make it flat. Do not carry out flow coating immediately after repair. It is necessary to place it for a period of time to allow the coating paste to dry fully to avoid foaming at the repaired part after flow coating. This method is suitable for cases where the sand-scratched area is large and the damage is serious.

2. Repair After Flow Coating: First, carry out flow coating on the sand mold. After the coating is dry, repair the sand-scratched part with coating paste, then use a thinner to smooth the coating paste, and finally carry out ignition treatment. This method is widely used at present, is easy to operate, can save labor, and effectively makes up for the sand mold surface defects caused by tooling and previous operations.

9. Uneven Coating

Causes

1. Excessively Strong Coating Thixotropy: The coating has excessively strong thixotropy, and its apparent viscosity decreases with the increase of shear time during the flow process, and it can reach a constant value after a long time. If it is converted to a static state, its apparent viscosity gradually increases with the increase of static time. During construction, the coating is easy to flow excessively, resulting in excessive loss of coating on the upper part of the sand mold and forming a situation where the top is thin and the bottom is thick.

2. Problems with Fluidity and Inclination Angle: The coating has poor fluidity and cannot evenly cover the sand mold surface; or the sand mold inclination angle is small, and the coating flow speed is slow, which is easy to accumulate at the bottom, resulting in uneven coating thickness.

3. Properties of Coating Type: Different types of coatings have different thixotropy requirements. For example, for water-based zircon powder coatings, if the thixotropy rate is not properly controlled, the problem of uneven coating is easy to occur.

Solutions

1. Control Coating Thixotropy: For water-based zircon powder coatings, control the thixotropy rate M within the optimal range of 9%-12%, which can not only ensure that the coating has good leveling property but also avoid excessive flow and reduce the situation where the coating is thin at the top and thick at the bottom.

2. Improve Coating Fluidity and Adjust Inclination Angle: Improve the coating fluidity by optimizing the coating formula or adding a leveling agent; at the same time, adjust the sand mold inclination angle to ensure that the coating can flow evenly on the sand mold surface and reduce the thickness difference.