1. Coating build-up
Die Ursachen:
1. The paint is slow to flow. As paint is a thixotropic fluid, there is a reticular structure and yield value. The yield value and viscosity are the two main factors causing paint to build up. If the yield value of the paint is too high and the viscosity is too great, the fluidity of the paint will worsen.
2. Flow marks produced during the flow of the paint, flowing along the sand mould and building up when it encounters grooves, causing the edges of the sand mould to become unclear.
3. The angle of inclination of the sand mould is not appropriate.
4. The flow rate is low, and the paint cannot flow, causing it to accumulate.
5. Insufficient pressure causes the flow rate to be slow, resulting in accumulation.
Measures to be taken:
1. Considering on-site operation, the Baume degree of the paint should be reduced. Practice has proved that the flowability of the paint is best when the Baume degree of the flowable paint is between 22 and 26. Considering the factors of the paint itself, the yield value and viscosity of the paint should be reduced.
2. Use an air hose to blow off or use a brush dipped in thinner to remove flow marks.
3. Sand mould placement angle requirements: Use a vehicle to lift the sand mould above the paint tank at an angle of 75–90 degrees to the horizontal to flow coat the product.
4. Increase the cross-sectional area of the flow coating rod and hose to increase the flow rate. Generally, a 4-mm diameter pipe is used for the flow coating rod and hose. If the cross-sectional area is increased, either a 4-mm or 6-mm diameter pipe can be used, or both pipes can be 6-mm diameter pipes.
5. Increasing the air pressure can increase the flow rate. In order to obtain a suitable coating thickness, the speed at which the paint flows out of the flow coater should be 100-200mm/s. The air pressure should generally be between 0.4×105Pa and 0.6×105Pa. If it is too high, it will easily cause splashing.
2. Insufficient coating thickness
Die Ursachen:
1. The coating does not form a sufficient coating thickness and flows directly.
2. The coating permeates completely into the sand mould, resulting in an insufficient coating thickness.
3. The surface of the sand mould is stuck with a release agent, which reduces the permeability of the coating and directly affects the thickness of the coating.
Measures:
1. Increase the viscosity of the coating (maximum value not exceeding 7 s) to improve the coating properties and avoid excessive coating flow.
2. Improve the compactness of the sand mould, which can effectively prevent excessive penetration of the coating. A sand mould compactness of between 45% and 55% is suitable
The mould surface should be allowed to dry thoroughly before production
3. The parts of the sand mould that will be coated with release agent should be sanded with fine sandpaper before coating.
Wet coating thickness requirements for cast iron sand moulds:
Thin-walled castings 0.15mm-0.30mm
Medium-thick castings 0.30mm-0.75mm
Thick-walled castings 0.75mm-1.00mm
Extra-thick castings 1.00mm-2.00mm
3. Coating surface peeling
During assembly, when the operator blows the floating sand in the cavity with an air hose, the surface of the coating layer will occasionally peel off.
Die Ursachen:
1. The coating has low strength.
2. The paint layers are not sufficiently bonded together to form a single whole.
Measures:
1. The binder content in the paint is too low, which makes the coating less strong.
2. Insufficient combustion of the paint can affect the bonding between layers. For castings weighing more than 3 tons, the coating surface is prone to peeling. This problem can be solved by reasonably controlling the ignition time. Generally, it is appropriate to ignite the upper box 3-5 seconds after the flow coating, and it is best to ignite the lower box 5-7 seconds after the flow coating. Gas fire baking can also be used, but the time should not be too long, otherwise the coating will crack.
4. Castings with sand adhesion
The coating is not fire-resistant enough, and the coating or sand mold comes into contact with the high-temperature molten metal, causing a chemical reaction that forms a substance on the surface of the casting that is extremely difficult to clean, commonly known as sand adhesion. The flow coating method can also cause sand adhesion.
Measures:
1. Change the composition of the coating aggregate to improve the fire resistance of the coating. Choose refractory fillers such as high-alumina bauxite powder and zircon powder.
2. Increase the coating thickness, but the thickness should not exceed the maximum value required for the coating thickness. If it is too thick, it will cause casting defects such as a coating skin.
3. Increase the Baume degree of the flow-coat paint, but the maximum should not exceed 28, otherwise the fluidity will decrease.
4. Some castings are partially overheated, and flow-coating is extremely prone to sand sticking. Applying a high-refractoriness paint to the hot-spot position before flow-coating can effectively prevent sand sticking.
5. Serious flow marks
Cause:
The paint has poor fluidity and high viscosity, so when it flows downwards it cannot drip, resulting in serious flow marks; the paint flows out with excessive pressure, and the distance between the flow coat rod tip and the surface of the cavity is too close, so the paint liquid impacts the coating surface, leaving uneven marks; the paint flow rate is low, the flow is unstable, and flow marks form on the surface of the cavity.
Measures to be taken:
1. When flow coating, use a large flow rate to quickly flow from top to bottom, and do not allow the coating to remain on the sand mold surface for a long time.
2. Improve the fluidity and leveling of the coating to reduce its viscosity.
3. Increase the distance between the flow coating rod tip and the cavity surface. A distance of 18-25 mm is generally appropriate.
4. Use a fan-shaped flow coating rod tip.
VI. Laminations
Laminated texture is produced when the flow coating is applied to the surface of the cavity from top to bottom or from left to right twice or more.
Reason:
Mainly caused by the high temperature of the sand mould, high viscosity of the coating and small flow rate of the flow coating.
Measures:
1. Do not apply flow coating immediately after the sand mould has just been discharged from the mixer as it is still hot. Air cooling should be used according to the specific situation.
2. Reduce the Baume degree of the coating to improve its fluidity.
3. Increase the flow rate to avoid multiple flow coating. The flow rate can be reasonably controlled by making flow coating machines of different specifications. When selecting a pump, the head and flow rate should be slightly higher. If the fluid pressure is high, the fluid flow can be controlled by controlling the switch and other places to achieve the desired application pressure and flow rate.
7. Paint splashing
Paint splashing is the splashing of paint droplets on a smooth coated surface.
Die Ursachen:
This defect is mainly caused by excessive pressure at the flow coating outlet.
Measures:
1. Reduce the pressure at the flow coating outlet. The thickness, length, surface roughness and outflow position of the paint flow pipeline will have a significant impact on the flow coating pressure. The outflow pressure P of the paint must not be less than 0.4×105Pa.
2. Do not apply flow coating vertically to the surface of the cavity to avoid paint splashing.
8. Sand removal from the sand mold surface
This is commonly known as ‘hairline’, and it often occurs when the mould has been in use for a long time or when the moulding is unstable. The surface of the sand mould is not flat enough after flow coating, and there are depressions, which has a significant impact on the appearance quality.
Measures:
Method 1: Repair the sanding surface with putty before flow coating. The disadvantage of this method is that the sanded surface needs to be left for a long time after flow coating, otherwise the repaired area will blister.
Method 2: Repair the sanding surface with putty after flow coating, then use thinner to flatten the putty, and finally set fire. This method is widely used at present, saving manpower and making up for the shortcomings caused by tooling and previous operations.
9. Uneven coating
When flow coating, the sand mould often tends to have a thin top layer and a thick bottom layer. With a fixed rotor speed in the viscometer, the apparent viscosity of the coating decreases with increasing shear time and reaches a constant value for a long time. If it is allowed to stand, the apparent viscosity gradually increases with increasing standing time. This is the thixotropy of the coating. A strong thixotropy of the coating is good for leveling, but it is easy to cause excessive flow, resulting in a thin top layer and a thick bottom layer. Poor fluidity can also cause uneven coating thickness with a small tilt angle. For water-based zirconium powder coatings, a thixotropic rate of M=9%-12% is considered good.
10. Poor adhesion to the substrate and paint peeling
During the coating and spraying process, paint peeling often occurs due to poor interlayer adhesion between the substrate and the paint, resulting in a high rate of defective products and seriously damaging quality and the production cycle.
Measures:
The current general practice is to use an adhesion promoter, which is a special treatment agent that can improve the adhesion between the coating and the substrate. It has special functional groups that can effectively combine with the polar groups on the surface of the material to produce a highly adherent interlayer adhesion, which plays a very good role as a primer.
1. The flow coating process is nearly ten times more efficient than the original brush coating process, and is very suitable for assembly line operations.
2. After flow coating, the surface of the mould is smooth, the coating thickness is uniform and dense, and the contours are clear. After casting, the surface of the casting is smooth, the surface roughness can reach Ra25um or more, and the dimensional accuracy of the casting is high, reaching CT9 or above in GB 6414-1999 ‘Castings: Dimensional Tolerances and Machining Allowances’.
3. Due to the even flow coating, the paint that runs off can be recycled. According to on-site measurements, the flow coating process can save about 25% of the paint compared to the original method.
4. After many experiments, it was found that when the Baume degree of the flow coating paint is between 22 and 26, the paint has the best fluidity, the coating thickness is appropriate, and the casting has the fewest defects.
5. It reduces pollution in the work environment, and the use of flow coating completely solves the problem of paint dust polluting the air.
The adhesion problem between the coating and the substrate can be quickly solved by using an adhesion promoter.
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