lost foam castings common problems of negative pressure system
Insufficient or unstable negative pressure of lost foam castings
Equipment failure: wear of the impeller of the vacuum pump, aging or damage of the seal, etc., will lead to decreased or unstable air extraction capacity; motor failure and loose transmission belt will also affect its normal operation.
Pipeline leakage: There are leaks in the exhaust pipeline, sand box connecting pipeline and other parts, such as poor sealing at the pipeline interface, aging and rupture of the pipeline, etc., which will cause air to enter the system and cause negative pressure fluctuations.
Valve failure: If the valve on the pipeline is poorly sealed, the valve core is worn or stuck, it will affect the circulation and control of the gas, resulting in unstable negative pressure.
Poor sealing of the sand box: The box wall, box door, pouring cup and other parts of the sand box are not well sealed. If the sealing strip is aged, damaged or improperly installed, air will enter the sand box when the negative pressure is extracted, causing negative pressure fluctuations.
Uneven permeability of molding sand: Uneven particle size distribution of molding sand, inappropriate binder content or uneven sand mixing will lead to large differences in the permeability of molding sand. During the process of vacuuming negative pressure, the speed of gas passing through the molding sand is inconsistent, resulting in negative pressure fluctuations.
Casting defects of lost foam castings
Blowholes: Insufficient or unstable negative pressure will prevent the gas generated by the pyrolysis of the foam mold from being discharged in time, resulting in the formation of pores; poor air permeability of the coating will also hinder the discharge of gas and form pores.
Box collapse: Low or unstable negative pressure, insufficient compactness of the molding sand in the sand box, prone to box collapse; The mismatch between the speed of molten metal filling and the speed of white mold disappearance will cause abnormal pressure changes in the mold, resulting in box collapse; low strength and inappropriate thickness of the coating, which cannot effectively support the mold, will also cause box collapse.
Sand sticking: Excessive negative pressure will aggravate the penetration of molten metal into the sand and cause the wall attachment effect, which is not conducive to the adsorption of liquid foam by the coating and generates many pores.
Carbon defects: Insufficient negative pressure will not be able to remove the foam vaporization in time, which will cause the plastic foam melt product to remain on the casting, causing carbon defects.
Other problems of lost foam castings
Pouring back spray and black smoke: The pouring cup is wet, the white mold is dense, the adhesive glue and surface repair are too much, the coating is not breathable, the negative pressure in the sand box is too low, the model is not dried, the dust in the molding sand is too large, the negative pressure is unstable, etc., which may cause back spray and black smoke during pouring.
Coating surface peeling: Low coating strength, insufficient bonding between coating layers The combination will cause the coating surface to fall off and affect the quality of the casting.
Methods to solve common problems of lost foam casting negative pressure system:
Insufficient or unstable negative pressure
Check and maintain the vacuum pump: regularly check the impeller, seals, etc. of the vacuum pump, and replace worn or damaged parts in time; clean the air intake pipe and filter to prevent debris from entering the pump body and affecting the air extraction effect.
Check the pipeline system: carefully check the air extraction pipe, sand box connecting pipe and other parts, seal the leaking points or replace the aging and broken pipes; check the sealing performance of the valve to ensure that the valve core is not worn or stuck, and ensure the normal flow of gas.
Check the sand box and molding sand: replace the aging and damaged sealing strips of the sand box, and ensure that the box wall, box door, pouring cup and other parts are well sealed; ensure that the particle size distribution of the molding sand is uniform, the binder content is appropriate, and the sand mixing is uniform, so as to improve the air permeability of the molding sand.
Optimize process operation Operation: Control the speed and amount of sand addition to avoid uneven compaction of the sand mold; adjust the vibration compaction parameters to improve the compactness and air permeability of the sand mold; adjust the process parameters appropriately according to the changes in ambient temperature and humidity.
Casting defects
Blowholes: Select the appropriate negative pressure to ensure that the gas generated by the pyrolysis of the foam mold can be discharged in time; use a coating with good air permeability, and ensure that it is evenly applied and of appropriate thickness; increase the pouring temperature so that the metal liquid at the front of the filling can fully decompose and vaporize the foam mold; optimize the design of the pouring system to avoid gas from being involved in the metal liquid.
Collapse box: Adjust the appropriate negative pressure and repair the leakage and blockage of the negative pressure system; ensure that the coating has high strength and appropriate air permeability, and dry it in place; control the metal liquid filling speed to match it with the disappearance speed of the white mold; ensure that the top sand intake is sufficient and take anti-splashing measures.
Sand sticking: Control the negative pressure to avoid excessive negative pressure; select appropriate coatings to improve their refractoriness and anti-permeability; optimize the pouring process to reduce the pouring temperature and speed of the molten metal.
Carbon defects: increase the negative pressure, such as thickening the exhaust pipe, adding negative pressure exhaust pipes, etc.; strengthen the sand box seal, and clean the pouring garbage on the top of the sand box in time; ensure that the foam mold is fully dried.
Other problems
Pouring back spray and black smoke: bake and dry the pouring cup, reduce the density of the white mold, reduce the adhesive used for bonding, improve the air permeability of the coating, increase the negative pressure in the sand box, reduce the humidity in the drying room, regularly screen out the dust in the molding sand, stabilize the negative pressure flow, and control the flow at the beginning of pouring.
Coating surface shedding: improve the strength and bonding performance of the coating, ensure that the coating layers are fully combined; control the drying temperature and time of the coating to avoid cracking and shedding of the coating due to excessive drying.
