Chapter V Quality Control, Defect Analysis and Prevention of Lost Foam Casting


5.1.1 Wrinkling (carbon deposition) on the surface of cast iron parts and prevention

Fig. 1 Large Area Wrinkle on Upper Surface

1.Cause:

When liquid iron (gray cast iron, ductile iron, alloy cast iron) is cast into the mold,EPS or EPMMA, STMMA mold rapidly decomposes, forming a gap between the mold and the molten iron, and the mold is pyrolyzed to form a gas phase, a liquid phase and a solid phase. There is often a viscous asphalt-like liquid. This liquid decomposition remains on the inner side of the coating, part of which is absorbed by the coating, and part of which forms a film between the casting and the coating. This film forms fine flake or dander-like, corrugated crystalline residual carbon, namely bright carbon, under the reduction (CO) atmosphere. This low density (loose) bright carbon has poor wettability with molten iron, deposits (wrinkles) are thus formed on the surface of the casting.

2、Influencing factors

(1)Foam appearance

AppearanceEPS is easier to form wrinkled skin than EPMMA and STMMA, because EPS has higher carbon content than the latter two, EPS has 92% carbon content, STMMA (styrene methyl methacrylate copolymer resin) has 69.6 carbon content, EPMMA (expandable polymethyl methacrylate) has 60% carbon content, the higher the pattern density, the greater the influence, the more liquid phase products after decomposition, and the easier it is to produce wrinkled skin.

(2)Effect of composition of casting materials

Cast iron with carbon content (alloy cast iron), the carbon in the decomposition product of the pattern can be partially dissolved in it, and it is not easy to produce wrinkled skin. Cast iron with high carbon content (ductile iron) is most likely to produce wrinkled skin defects.

(3)Effect of Gating System

Fig. 2 the design of gating system is unreasonable, the casting is 5mm thin, and there are too many castings with one gate belt

  

Figure 3 Reduce the number of castings with a gate, casting quality is very good

The gating and risking system has a significant impact on the flow field and temperature field of the molten iron filling, which directly determinesThe pyrolysis products of EPS(EPMMA/STMMA) pattern and its flow direction; After the cross-sectional area of the straight, horizontal and inner runner is increased, wrinkled skin (the amount of pattern increases) is easy to produce. The top injection is less likely to wrinkle skin than the bottom injection, and the top riser is beneficial to reduce and eliminate wrinkle skin.

(4) Influence of casting structure

The volume of cast iron is related to the ratio of surface area (modulus). The smaller the ratio, the smaller the ratio is, the smaller the discharge of the pyrolysis products of the pattern, and the smaller the tendency of wrinkled skin defects.

5) Influence of pouring temperature

 

Fig. 4 Crumple skin cold insulation caused by low pouring temperature, pouring temperature 1340 ℃

Figure 5 The pouring temperature is raised to 1440°C, and the defects of wrinkled skin and cold shut are removed.

Under the same conditions, with the increase of pouring temperature, the wrinkle defect is reduced or eliminated, because the temperature increases, the pyrolysis of the pattern is more thorough, the proportion of gas phase products increases, and the liquid phase and solid phase products decrease, which is beneficial to reduce and eliminate the wrinkle phenomenon.

6) The influence of coating layer and sand permeability

The higher the permeability of the coating and molding sand, the more conducive to the discharge of the pyrolysis products of the pattern and the reduction of the tendency to form wrinkled skin. Therefore, the thinner the coating, the coarser the aggregate, and the coarser the particle size of the molding sand, the more conducive to exhaust and reduce wrinkled skin.

  (7) Negative pressure influence

The higher the degree of negative pressure, the more conducive to the pattern pyrolysis products through the coating layer to the sand suction, the more conducive to reduce the formation or appearance of wrinkled skin.

2、Preventions

(1)Use low-density EPS or EPMMA as the pattern material.

For larger castings or sprue, hollow pattern and sprue can be used (to reduce gas emission), pattern density16~18 grams is appropriate.

2) Design a reasonable pouring system.

It should be ensured that the molten iron flows smoothly, balanced and quickly fill the mold, so as to ensure that the foam residue and gas escape from the mold cavity or are sucked into the coating and dry sand gap, so as to minimize the heat loss of the molten iron flow during the pouring process, so as to facilitateTo accelerate the appearance of gasification. Although top injection and side injection are not easy to appear wrinkled skin defects, they will produce internal carbon-rich defects (because the falling iron flow is easy to involve the residues of pattern decomposition); bottom injection can reduce the internal carbon-rich defects of the casting, which is easy to cause wrinkled skin on the top surface, especially in thick parts.

Figure 6 Steady, balanced and rapid filling of the mold when the molten iron flows.

3) Increase the pouring temperature and pouring speed.

4) reasonable control of negative pressure.

 Due to negative pressure and lack of oxygen under vacuum conditions, the pouring pattern will mainly undergo gasification and rarely burn, thus greatly reducing the gas emission (104g foam generates 1000L of gas when burning at 1000 ℃ in air; Only 100L of gas is produced under anoxic conditions). At the same time, the gas product is pumped out through the dry sand mold in time, the gap pressure between the molten iron and the mold is reduced, and the filling speed of the molten iron is accelerated, which is conducive to the decomposition of the mold.

5) Improve the air permeability of the coating and select the corresponding coating.

5.1.2 Carburization of steel castings and prevention

1. Causes

foam plastic moulding compoundEPS (or EPMMA, STMMA) in the role of high-temperature steel decomposition, cracking of its products and the role of steel, at the same time in the role of paint and dry sand is a complex physical and chemical metallurgical reaction process to form carburizing (carburization).

2. Preventions

1) Use low-density EPS or EPMMA as the pattern material.

2) Design a reasonable pouring system.

The mold and pouring process design should be able to accelerate the gasification of the mold material, reduce and stagger the liquid phase and solid phase contact and reaction time in its decomposition products, and can reduce or avoid carbon absorption of steel parts.

3) Suitable pouring temperature and pouring speed.

4) reasonable control of negative pressure.

The control of negative pressure must be matched with the speed of the whole pouring process. If the negative pressure is too large, sand sticking and other defects will easily occur. If the negative pressure is too small, the products of pyrolysis will not be easily discharged, resulting in carbon increase.

(5) Improve the air permeability of the coating and select the corresponding coating.

6) Additives (decarburizing agent) are added to the pattern to prevent carburizing of steel castings.

7) Use anti-carburizing paint.

Adding some anti-carburization catalysts such as alkali metal salts and limestone powder in the coating, the coating can decompose a sufficient amount of carbon dioxide gas to absorb carbon after pouring; adding oxidants in the coating to promote the decomposition of carbon and hydrogen into neutral gas,Prevent or reduce the penetration of decomposed carbon into steel castings.

5.1.3 Backspray and prevention

1. Causes

1) the degree of negative pressure is not enough is the root cause of back spray.

There are many reasons that affect the degree of negative pressure: the density of the pattern is too large; the permeability of dry sand is poor; the permeability of the coating is poor; the pouring temperature is too high and the pouring speed is too fast; the design of the gating system is unreasonable and so on.

2. Preventions

(1)EPS pattern density control in 0.013~0.022 g/cm3The appearance should be dry, the coating should be dried thoroughly, and the air content (moisture) and air emission should be reduced.

2) Increase the permeability of the coating and adjust the thickness of the coating so that the gas can escape in time after the pattern is cracked.

3) control the permeability of dry sand, do not mix different grades of dry sand mesh, hinder its permeability. Dry sand dust should be removed as much as possible.

4) Control the pouring temperature and pouring speed, that is, the filling speed to ensure the gasification of the pattern.

5) The reasonable design of the gating and risking system shall ensure that the liquid metal flows smoothly, balanced and quickly filled with the mold pattern during filling, so as to ensure that the cracking gas of the mold pattern escapes from the mold cavity and is sucked and discharged.

5.1.4 Air holes and prevention

The process of gassing during the cracking of the lost foam, the gas enters the casting and produces pores.

1、 Cause:

(1) Turbulent flow is generated during the filling process, or under the condition of holding or side injection, part of the pattern is surrounded by metal liquid and then cracked (decomposed). If the generated gas cannot be discharged from the metal, pores will be generated, which are large and numerous (clustered) and accompanied by carbon black.

2) Porosity caused by poor drying of appearance and coating layer.

3) Porosity caused by excessive pattern binder.

4) Involved in air to form air holes during pouring.

5) The pouring temperature is too low, the appearance cannot be completely vaporized, and the pores are formed.

Figure 8 low pouring temperature, the formation of pores.

6) Slag pores formed by flushing slag or coating during pouring.

 

Fig. 9 Slag pores formed by rushing into sand.

 

(7)Reactive pores caused by coating (insufficient refractoriness, reaction with components in molten iron, etc.).



Fig. 10 Insufficient Refractoriness of Coating

Fig. 11 Insufficient Refractoriness of Coating

2. Preventions

1) reasonable process, so that the pouring filling process layer by layer replacement, no turbulence, improve the pouring temperature, improve the degree of negative pressure (if the turbulence caused by pores, can reduce the degree of negative pressure); Improve the permeability of coating and sand.

2) the model must be dry, the coating must be dry.

3) Select a low-gas model binder; under the premise of ensuring adhesion, the less the binder, the better.

4) The closed pouring system is adopted, and a certain amount of metal liquid is kept in the gate cup during pouring to ensure that the sprue is in a full state.

5.1.5 Box collapse and prevention

During the pouring process or solidification process, part or part of the casting mold collapses, and the mold collapses so that the casting cannot be formed or is locally fleshy.

Fig. 12 Collapsed Box

1、 Cause

Excessive buoyancy of poured liquid metal; Excessive gas pressure in the mold; Hollow or insufficient strength in the pattern; Internal flow and impact of liquid metal; Local damage of sand box moving mold, etc.

2、 Preventions

 (1) Increase the amount of sand on the top surface or place pressure iron on the top of the mold.

 (2) Select low-density mold materials to make shapes to reduce gas emission.

 (3) Under negative pressure pouring, select a reasonable vacuum degree and match the pouring speed.

 (4) choose high strength, high fire resistance, good air permeability, good coating performance coating, with reasonable coating hanging process.

 (5) the use of coarse sand, timely removal of sand dust.

 (6) The vibration molding process parameters should be reasonable to ensure that the dry sand everywhere is uniform and compact.

 (7) The pouring system and pouring process should be reasonable.

5.1.6 Sand sticking and prevention

Part or the entire surface of the casting is clamped with difficult to clean sand.

1、Cause

(1)The coating is too thin, falling off or cracking; the pouring temperature is too high; the negative pressure is too large, etc.

Fig. 13 high temperature overheating zone, the coating is too thin, forming sticky sand.

  (2) Dead angle of sand filling or iron-clad sand caused by insubstantial vibration.

   

Fig. 14 Iron Clam Sand

2、 Preventions:

1) The selected coating should have good performance, can be firmly bonded to the pattern, and have sufficient strength and refractoriness.

2) modeling tight strength can not be too large, so as not to damage the coating.

3) Select the appropriate degree of negative pressure.

4) The pouring temperature should not be too high.

5.1.7 Cold insulation (against fire), insufficient heavy skin pouring and prevention

There is a gap on the casting that is not completely fused, and the bite of the junction edge is smooth. This gap is cold insulation (fire-to-fire), and there is an obvious mark of fire on the surface, which forms heavy skin in severe cases. The part is not full, the casting is short of meat, and the end is arc-shaped, which is called insufficient pouring.

Fig. 15 Insufficient Pouring

1、Cause: the filling temperature is too low; the filling speed is slow; the pouring system, structure, pouring operation process is improper, and the negative pressure during the filling process is too large.

2、Preventions

1) Increase the pouring temperature of the metal liquid.

2) Improve the gating system and increase the filling speed.

3) control the appropriate degree of negative pressure.

 

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