Casting Technology of Lost Foam Grinding Ball


1 Foreword

lost foam castingThe advantages and process characteristics, especially suitable for the casting of grinding balls, it gives full play to the characteristics of investment casting can be grouped string pouring, and does not need crust, has a higher process yield, higher production efficiency, product quality and other advantages. Lost foam casting grinding ball is a relatively simple type of lost foam casting process.

lost foam castingGrinding balls must meet national standards for grinding balls. As well as the Ministry of Power, the Ministry of Building Materials and other standards. At the same time, the production of grinding ball manufacturers in accordance with the above standards, combined with the requirements of users, according to the local raw material conditions, appropriate adjustment, such as near the production of rare earth mining area or other mineral materials with rare earth elements, in the casting of grinding ball when the appropriate amount of people can improve the mechanical properties and performance of the grinding ball.

2 white mold making and stringing

Selection of 2.1 foam pattern materials

lost foam castingFoam plastic is selected as the pattern material. There are many types of foam plastic. Foam plastic required for the pattern of lost foam plastic requires certain conditions, so few can meet the requirements. Because when foam plastics are applied to the field of lost foam casting, the following conditions must be met:

(1) Low density and high strength;

(2) easy to vaporize, high temperature decomposition of the residue less;

(3) easy to shape, can obtain a smooth surface appearance.

EPS (polystyrene) has high strength, gas volume is small, the main decomposition products for the larger molecular weight of aromatic hydrocarbons, gas more, more carbon, cheap, more widely used. EPS (polystyrene) is used more in China, and it has good foaming properties from 00 to polystyrene. Therefore, when using foaming molding and mass production, polystyrene or polystyrene is selected. Copolymer with polymethyl methacrylate (PMMA). In addition, the foamed PMMA has high hardness and brittleness, and it is not easy to obtain a low density pattern, which also makes PMMA not suitable for foam molding. Therefore, suitable materials can be selected according to the actual situation of the company.

2.2 white mold forming

Take EPS (polystyrene), which is used more in China, as an example:

(1) Pre-foaming:

Before molding, the resin particles (raw material particles) are first pre-sent to a suitable density of about 18g/L to 22g/L, generally by steam heating or electric heating.

(2) Pre-foaming process parameters (see Table 2-1):

 

Table 2-1 Operating parameters of SJ-KF-450 intermittent steam pre-generator

Item Name

Preheating temperature

Steam pressure

foaming temperature

EPS

80-85

0.10-0.12

85-90

PMMA

90-95

0.12-0.15

95-105

 

EPS is adopted, the preheating temperature is 80 ℃, the steam pressure is 0.1MPa, the foaming temperature is 90 ℃, the feeding time is usually set to 3-5s, the heating time is 150-200s, and the holding time is 15-30s. After cooling, the discharge beads need to be matured and dried. The best aging temperature of the beads is 23-25 ℃, and the time is 12-24h.

2.3 molding process

The pattern material shrinks, and the recommended shrinkage rate is 0.3 to 0.4 when EPS is used.

lost foam castingGrinding balls, like investment casting, can string EPS white mold balls, and bonded sprue (E P S rod) can be square, pentagonal, hexagonal, polygonal and circular. The size, length, number of E P S balls in each row and the bonding of upper and lower layers shall be determined according to the pouring and grinding process of each factory and the arrangement of several groups of EPS balls (size, number, weight and gating system) in each sand box determined by the capacity of smelting equipment. For example: each layer of 6 balls uniform. The production rate of multi-vertical string process is more than 80%.

Design of 2.4 Gating System

Because the piece belongs to small and medium-sized castings, the pouring time of the pouring system, the cross-sectional area of the inner runner and the cross-sectional size of each component of the pouring system are calculated as follows:

∑ F straight: ∑ F horizontal: ∑ F inside = 1.4:1.2:1.

(1) Pouring time

T = G½ G⅓

T = 10.6s(T is the pouring time);

(2) Calculation of the cross-sectional area of the inner runner

by hydraulics calculation formula:

 

According to the above formula, the section size of each component in the gating system is calculated respectively. The top injection method with high process yield is used for pouring.

2.5 white mold bonding

The appearance is divided into two parts and played by a molding machine, so the two parts should be combined. After the combination, the gate cup, the inner gate, the runner and the foam pattern parts are bonded with cold glue.

3 Selection of coatings

The yellow zone is the process of preparing the lost-form coating. lost-mode coating islost foam castingAn important part of it is one of the key factors. Different from other casting coatings applied to the cavity, the lost foam coating is applied to the foam model, so a special coating is required. Lost foam coatings are generally made of refractory materials, binders (solvents), surfactants, suspending agents, thixotropic agents and other additives.

3.1 paint composition

(1) Refractory materials

Refractory material determines the refractoriness, chemical stability, and thermal insulation properties of the material, which is the backbone of the lost foam coating. The castings are cast iron, and bauxite, kaolin clinker, quartz powder and brown corundum powder are often used as refractory materials.

(2) Binder

Reasonable selection of organic binder and inorganic binder can ensure the high strength and high permeability of the coating. Among them, bentonite, silica sol, water glass, phosphate and the like are commonly used as inorganic binders. This time calcium bentonite was used as the inorganic binder. Silica sol and white latex can be selected for the organic binder.

(3) Carrier liquid

The commonly used carrier liquid of lost foam coating is water or alcohol. Considering environmental protection and cost, water is mostly used as carrier liquid, and the coating made is water-based coating.

(4) Surfactant

In order to improve the coating of the coating to add a surfactant, it is both hydrophilic and lipophilic. The commonly used surfactant is JFC, full name polyoxyethylene alkyl alcohol ether.

(5) Suspending agent

In order to make the refractory and binder fully mixed, so that it can be in suspension, the need to add a suspension agent. The commonly used suspending agent is calcium-based bentonite, and CMC can also be used as a suspending agent for water-based coatings. This time, bentonite and CMC are mixed as suspending agent.

3.2 coating process design

There are four ways to apply the vanching mold coating: brush, dip, shower and spray. Because the castings are small and medium-sized thin-walled castings with complex shapes in batch production, spraying method can be used. The paint for lost foam casting is a water-based paint, and the viscosity of the paint will decrease during the stirring process, and the viscosity will increase after the stirring. Therefore, in order to obtain a uniform thickness, it is necessary to apply while stirring. At the same time, the speed of mixing the coating should be normal to avoid precipitation and the generation of gas. Apply evenly and do not expose.

The water-based paint needs to be dried after application. The coating is dried at low temperature or room temperature due to the limitation of the softening temperature of the EPS model. Usually 40-60 ℃, drying time is 2-10h. The humidity should also be controlled during the drying process, generally less than 40% humidity and ventilation to achieve better drying effect. Drying equipment is commonly used in drying chamber, drying oven and continuous drying kiln, heat source is electricity, heating, hot air and so on. And in the drying should pay attention to the following matters.

(1) The drying process requires the placement and support of the model to prevent the model from deforming.

(2) must be dried through.

(3) After drying, it is necessary to place the appearance in a place with low humidity to prevent moisture absorption.

4 Modeling process

4.1 buried box modeling related design

The sand is jewel sand. On the one hand, because the jewel sand has good fluidity, it is easy to fill up when vibrating, the compaction effect is good, the quartz sand has poor fluidity and cannot be filled in some places, which leads to iron-clad sand during pouring, thus reducing the casting accuracy. On the other hand, the breakage rate of jewel sand is low, and in the long run, it is appropriate to use jewel sand.

The mesh number of molding sand is about 20~40 mesh. The sand is too fine and the air permeability is not good, which has a great influence on the surface quality of castings. The sand is too coarse and easy to cause sand sticking. It is selected according to the actual production situation. Adopt rain sand. There is a quantitative material box at the bottom of the sand hopper. After the air cylinder pulls the valve plate, the sand flows into the sand box through the small holes evenly distributed on the valve plate. The size of the material box is basically similar to that of the sand box. The sand is added evenly and the impact force is minimum.

4.2 vibration parameter design

The vibration parameters include vibration time, vibration direction, vibration frequency and amplitude. When packing, the direction and time frequency and amplitude of vibration should be selected according to the geometry of the foam pattern.

Usually high frequency and low amplitude vibration is selected, and the commonly used vibration frequency is about 50Hz to ensure that the dry sand is firm and does not damage the foam plastic model. The amplitude is generally 0.5~1.5mm, and the vibration acceleration is 10~20m/s².

5 Casting process

5.1 gating system

Top injection or bottom injection is selected, the process yield of top injection is higher than that of bottom injection, and the slag collection advantage of bottom injection is higher than that of top injection.

5.2 negative pressure (vacuum degree)

Low, medium and high chromium alloy cast iron is -0.04-0.05MPa, and other alloy grinding balls are selected within this range.

5.3 pouring temperature

Ductile iron 1400-1450 ℃, even 1480 ℃; Alloy cast iron 1450-1550 ℃, high chromium grinding ball 1520-1550 ℃.

5.4 pouring speed

Check that the condition of the sand box before pouring is correct. The pouring speed should be slow at the beginning, then turn fast, and the final package should be slow at the end. Start to discharge the EPS gasification and cracking products to the gate cup, and prevent them from splashing. Later EPS gasification to form a channel, sealing the sprue, so that the mold is sealed, maintain its vacuum, smooth pouring. Finally, in order to make the gasification residue and impurities have sufficient time to float to the top, so that the gas can escape and turn to slow pouring, but the pouring stream is fine-coarse-fine and cannot be interrupted.

5.5 voltage stabilization and unpacking

After pouring, the vacuum pump continues to pump negative pressure for 3~5 m in. However, when pouring low chromium grinding balls, in order to achieve the cooling speed of metal mold casting and cast hard layer, some factories also use normalizing in austenitic zone (under red heat of as-cast state) by blowing, blowing and spraying, blowing and spraying fine water (rotating ball string) to improve hardness and increase wear resistance.

6 Possible defects and prevention

6.1 collapse box

Cause:

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

Preventions:

(1) Reasonably control the pouring speed to ensure that the gate cup is always filled with metal liquid and cannot be cut off during the pouring process.

(2) Appropriately increase the negative pressure size to make the sand more compact.

(3) Choose a low-density appearance to reduce gas generation.

(4) choose high strength, good air permeability coating.

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

6.2 cold insulation, insufficient pouring

Cause:

The filling temperature is too low; the pouring system, structure and pouring operation process are improper, and the negative pressure during the filling process is too large.

Preventions:

(1) Improve the pouring temperature of metal liquid.

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

(3) the use of appropriate negative pressure.

6.3 backspray

Cause:

Insufficient negative pressure is the root cause of backspray. There are many reasons that affect the degree of negative pressure, including the pattern density is too large, the dry sand has poor air permeability, the coating has poor air permeability, the pouring temperature is too high or the pouring speed is too fast, etc.

Preventions:

(1) select the appropriate dry sand molding, in order to control the permeability of dry sand, do not mix different grades of dry sand mesh.

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

(3) reasonable control of the coating thickness, improve the permeability of the coating, so that the gas is discharged in time after the gasification of the pattern.

(4) Reasonable design of the gating and riser system to ensure that the metal liquid fills the cavity smoothly and quickly during pouring.

6.4 stomata

(1) The main reason for the production of a kind of pores in castings is: high-temperature molten metal is involved in air when it is injected, and the involved foam debris will be vaporized by the molten metal to generate gas. The air cannot be effectively discharged from the casting mold, thus remaining in the casting mold. This kind of pore treatment method is to reduce the pressure in the sand box appropriately.

(2) casting another porosity caused by the main reason is because of the coating: poor permeability of paint, and coating layer is too thick, so that the pattern after vaporization of the gas can not be timely and effective from the mold completely excluded, the residue in the mold.

6.5 shrinkage porosity

Under the action of liquid metal at high temperature, the white mold vaporizes and disappears, and the molten metal occupies the space of the white mold. When the high-temperature liquid metal cools down and becomes solid, volume shrinkage will occur. When the high-temperature solid metal cools to room temperature, there will be solid shrinkage. The solid shrinkage of the metal will cause the change of the outer contour of the casting. After the liquid metal solidifies and shrinks, the new liquid metal will be blocked and cannot enter.

7 Conclusion

lost foam castingAlthough the grinding ball does not have the shell of metal mold casting to resist wear, the output of the ball mill will not be affected by the loss of circle at the gate of the metal mold casting grinding ball due to the uniform cooling speed. Based on the experience of many manufacturers, the diameter of the lost foam casting grinding ball should be 30-120mm in diameter. If the diameter is less than 30mm, carbon deposition, wrinkled skin and slag inclusion (carbon black) are easily caused due to insufficient heat of alloy liquid for gasification and cracking of white mold. If the diameter of grinding ball is greater than 120 m m m, it is easy to cause loose and hollow center. In short, the lost foam casting grinding ball process is constantly developing to meet the needs of development.

Hebei Ruiou Lost Foam Technology Co., Ltd.

Telephone/Fax:0311-89652816

Website:www.roepc.cn

Address: Ecological Street, Xinhua District, Shijiazhuang City, Hebei Province

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