Common problems and solutions of flow coating process
The reasons include the following aspects.
① Since the paint is a thixotropic fluid, there is a network structure in it, and the flow is slow. Yield value and viscosity are the two main factors causing paint accumulation. The yield value of the coating is too high and the viscosity is too high, resulting in poor fluidity of the coating.
②The flow marks generated during the flow of the paint flow along the sand mold, and accumulate when they encounter grooves, resulting in unclear edges and corners of the sand mold.
③ The inclination is not suitable.
④ The flow rate is small, and the paint will not flow down to cause accumulation.
⑤ Due to insufficient pressure, the flow rate is slow, resulting in accumulation.
Solution :
From the perspective of on-site operation, it is necessary to reduce the Baume degree of the paint. Practice has proved that when the Baume degree of the flow coating paint is 22~26, the fluidity of the paint is better. Considering the paint's own factors, it is necessary to reduce the yield value and viscosity of the paint. Blow with an air duct or use a brush dipped in thinner to remove the flow marks. When placing the sand mold, use the crane to lift the sand mold to the top of the coating tank and form an angle of 75°~90° with the horizontal for flow coating. The purpose of increasing the cross-sectional area of the flow coating rod and hose is to increase the flow rate. The commonly used flow coating rod and hose are 4-point tubes. If the cross-sectional area is increased, you can use 4-point tubes and 6-point tubes or both. Use 6 tubes. Increasing the wind pressure can increase the flow rate. In order to obtain a suitable coating thickness, the flow rate of the coating from the flow coater should be 100-200 mm/s. The wind pressure is generally (0.4-0.6)×10 5 Pa. If it is too large It is prone to splashing .
Coating thickness is not enough
There are three reasons:
①The paint flows directly without forming a sufficient coating thickness.
② The paint completely penetrates into the molding sand, resulting in insufficient coating thickness.
③ The surface of the sand mold is stuck with a release agent , which reduces the permeability of the coating and directly affects the thickness of the coating.
solution:
Increase the viscosity of the paint (the maximum value is not more than 7 s), so as to improve the hangability and avoid excessive flow of the paint; improve the compactness of the sand mold, which can effectively avoid excessive penetration of the paint; the release agent on the surface of the pattern is fully dry before production , The part of the sand mold that is partially stained with the release agent is rubbed off with fine sandpaper and then flow coated. Wet coating thickness requirements for iron castings: general thin-walled castings are 0.15-0.30 mm, medium-sized castings are 0.30-0.75 mm, thick-walled castings are 0.75-1.00 mm, and extra-thick castings are 1.00-2.00 mm.
Coating surface peeling off
During the assembly process, when the operator uses the air duct to blow the floating sand in the cavity, the surface of the paint layer will occasionally fall off. The reason is that the coating strength is low or the coating layer is not fully combined to form a whole.
Solution: The amount of binder in the coating is small, which may easily lead to insufficient coating strength, so the proportion of binder in the coating should be increased; the coating is not fully burned, which is likely to affect the bonding between layers. For castings over 3 t, the surface of the coating is prone to peeling off. As long as the ignition time is reasonably controlled, this problem can be solved. Generally, it is better to ignite 3 to 5 s after flow coating in the upper box, and ignited in 5 to 7 s after flow coating in the lower box. good. Baking can also be used, but the time should not be too long, otherwise the coating will crack.
Casting sticky sand
The refractoriness of the coating is not enough, and the coating or sand mold will react chemically with the high-temperature molten metal, which will form a very difficult to clean substance on the surface of the casting, which is usually called sticky sand [1], and the flow coating method will also produce sticky sand .
Solution: Change the coating aggregate composition to improve the refractoriness of the coating. Choose high alumina bauxite powder, zircon powder and other refractory fillers. Increase the coating thickness, but the thickness cannot exceed the maximum required coating thickness. Increase the Baume , the highest is not more than 28, otherwise the fluidity will be reduced. Some castings have local overheating phenomenon, which is very easy to stick to sand. Before flow coating, paint , which can effectively prevent sand from sticking.
Severe flow marks
The reason is that the fluidity of the paint is poor, the viscosity is high, and the paint cannot drip when it flows downwards, resulting in serious flow marks; when the paint flows out, the pressure is too high, and the distance between the flow coating rod head and the surface of the cavity is too close, which will cause serious flow marks on the coating surface. Impact, there are uneven marks; the flow small, the flow is unstable, and flow marks will be formed on the surface of the cavity.
Solution: When flow coating, use a large flow rate to quickly flow from top to bottom, and do not stay on the surface of the sand mold for a long time. Improve paint fluidity, leveling , reduce viscosity. Increase the distance between the flow coating rod head and the surface of the cavity, and the general distance is 18-25 mm. Fan-shaped flow coating rod head is used.
lamination
Lamination is the superimposed texture produced by two or more flow coatings from top to bottom or from left to right during flow coating on the surface of the cavity [2]. The main reason is that the temperature of the sand mold is high, the viscosity of the paint is high, the flow rate of the flow coating is small, and it is caused by multiple flow coatings.
Solution: The temperature of the sand mold that has just been made is high, so do not flow coat it immediately. Reduce the Baume degree of paint and improve fluidity. Increase flow and avoid multiple flow coats. Reasonably control the flow rate by selecting a Diaphragm Pump. When selecting a pump , its head and flow rate should be slightly higher, and the liquid flow pressure is high. The liquid flow can be controlled by controlling the discharge of the switch to achieve the required application pressure and pressure. flow rate. By adjusting and comprehensively considering the shape of the sand mold cavity, a 40QB Diaphragm Pump is selected, with a flow rate of 2~14 m 3 /h and a lift of 0~60 m.
paint splatter
A paint splatter, is a splash of paint drops . This defect is mainly caused by excessive flow coating outlet pressure .
Solution : reduce flow coating outlet pressure. The thickness, length, surface roughness and outflow position of the paint flow pipeline will have a great influence on the pressure of the flow coating , and the outflow pressure of the paint should not be less than 0.4×10 5 Pa. Do not flow vertically between the flow coating rod head and the cavity surface to avoid paint splashing.
Sand off the surface of the sand mold
If the pattern is used for a long time or the mold is not stable , the surface of the sand mold will often lose sand. After flow coating, the surface of the sand mold is not flat enough, and the surface is sunken, which has a great impact on the appearance quality.
Solution: After the sand-lost part of the sand mold is repaired with paint paste , flow coating is carried out. The disadvantage of this method is that it needs to be left for a long time after flow coating, otherwise the part repaired with paint paste will foam. After flow coating, use paint paste to repair the sand-lost parts, then use thinner to brush the paint paste flat, and finally ignite. This method is currently widely used, saves manpower, and makes up for the shortcomings caused by tooling and pre-order operations.
uneven coating
During flow coating, the sand mold is often prone to the situation that the coating is thin at the top and thick at the bottom. When the rotor speed of the viscometer is fixed, the apparent viscosity of the paint decreases with the increase of the shear time, and can reach a constant value for a long time. If it is turned into a static state, its apparent viscosity increases with the static time And gradually pick up, this is the thixotropy of the coating [2]. The thixotropy of the coating is too strong, which is good for leveling, but it is easy to cause excessive flow, making the coating appear thin at the top and thick at the bottom. Poor fluidity and small inclination angle will also cause uneven coating thickness. For water-based zircon powder coatings , the thixotropy rate should be controlled at 9%~12%.
