Casting production is a complex multi-process combination process, which includes the following main processes:

1) production process preparation: according to the part drawing to be produced, production batch and delivery date, make production process plan and process documents.

◆ Draw the casting process diagram;

2) Production preparation, including preparation of materials for smelting, materials and patterns for molding and core making, core boxes, sand boxes and other process equipment;

3) modeling and core making;

4) melting and pouring;

Forming principle

Casting production is to heat and melt the metal to make it have fluidity, and then pour it into the mold cavity with a certain shape.

Under the action of gravity or external force (pressure, centrifugal force, electromagnetic force, etc.). ), cavity filling, cooling and solidification into castings (or parts).

A metal forming method based on.

Figure 1 casting process

Castings are generally cut into parts as blanks. But there are also many castings that can meet the design of parts without cutting.

Precision and surface roughness requirements, directly used as parts.

Properties and composition of molding sand

1, properties of molding sand

The main performance requirements of molding sand (including core sand) are strength, permeability, fire resistance, concession, fluidity, compactness and collapsibility.

Sex, etc.

2. Composition of molding sand

Molding sand consists of raw sand, binder and additives. Raw sand for casting requires less mud content, uniform particles, roundness and multiform.

Angular sea sand, river sand or mountain sand. Binders for casting include clay (common clay and bentonite), sodium silicate sand, resin,

Synthetic oil and vegetable oil are called clay sand, sodium silicate sand, resin sand, synthetic oil sand and vegetable oil sand respectively. In order to enter

In order to further improve some properties of mold (core) sand, it is often necessary to add some additives, such as coal, sawdust,

Pulp, etc. The structure of molding sand is shown in Figure 2.

Fig. 2 Schematic diagram of molding sand structure

technical characteristics

Casting is one of the main methods to produce blank parts, especially for some brittle metals or alloy materials (such as various iron castings,

Non-ferrous alloy castings, etc. ), casting is almost the only processing method. Compared with other processing methods, foundry workers

Art has the following characteristics:

1) castings are not limited by metal materials, size and weight. The casting materials can be cast iron, cast steel and aluminum alloy.

Gold, copper alloy, magnesium alloy, titanium alloy, zinc alloy and various special alloy materials; Castings can be as small as a few grams and as large as hundreds of grams.

Tons; The wall thickness of the casting can be from 0.5 mm to about 65438 0 m; The casting length can be from a few millimeters to more than ten meters.

2) Casting can produce all kinds of blanks with complex shapes, especially suitable for producing parts blanks with complex cavities, such as various boxes.

Body, cylinder, blade, impeller, etc.

3) The shape and size of the casting can be very close to the parts, which saves both metal materials and cutting time.

4) The raw materials commonly used for casting are widely available, and the casting cost is low.

5) The casting process is flexible, and the productivity is high, and it can be produced manually or mechanically.

Manual modeling of castings

Main methods of manual modeling

Sand casting is divided into manual modeling (core making) and machine modeling (core making). Manual modeling refers to the main work of modeling and core making.

Manual completion; Machine modeling refers to the main modeling work, including sand filling, compaction, mold lifting and box closing, which is completed by the molding machine.

Botou foundry workers and measuring tools friends introduce the main methods of manual modeling;

Manual modeling is flexible, adaptable, simple in process equipment, and widely used in single machine.

Small batch production. However, manual modeling has low productivity and high labor intensity. There are many methods of manual modeling, and the following are commonly used.

Several kinds:

1. Integral mold modeling

For castings with simple shape, flat ends and the largest cross section, integral modeling should be adopted. The whole mold modeling operation is simple and convenient.

When molding, the whole appearance is placed in the sand box, and there will be no box-misplacing defects. The whole mold modeling is suitable for simple shapes and maximum cross sections.

Castings with end faces, such as gear blanks, bearing seats, covers, shells, etc. (Figure 2).

Tuzhengmo modeling

2. Typing modeling

When the maximum cross section of the casting is not at the end of casting, in order to facilitate modeling and demoulding, the model should be divided into two halves or parts.

This kind of modeling is called parting modeling. When the largest section of the casting is in the middle of the casting, two boxes should be used for parting molding (Figure 3), and the mold should be

Divide the sample into two halves from the largest cross section (located with a needle). When modeling, the model is placed in the upper and lower sand boxes respectively, and the parting surface (mold

The joint surface between samples) coincides with the parting surface (the joint surface between sand molds). Two-box parting mode is extensive.

It is used to produce castings with complex shapes, such as porous castings such as water pipes, shaft sleeves and valve bodies.

Fig. 3 Two-box molding process of shell parting.

The shape of the casting is big at both ends and small in the middle, such as pulley, grooved wheel, lathe square tool rest and so on. In ord to ensure smooth demoulding,

Three-box parting molding should be adopted (Figure 4). At this time, the parting surface should be selected at the smallest cross section of the model, and the parting surface should still be selected during the second casting.

At the maximum cross section of the end, because the three-box molding has two parting surfaces, the dimensional accuracy in the height direction of the casting decreases and the separation increases.

It is not suitable for machine modeling because of the heavy workload and low productivity of burr cleaning at the contour. So, three boxes

Molding is only used to produce castings with complex shapes, and cannot be divided into two boxes.

Fig. 4 Three-box parting modeling example

3. Movable block mold modeling

Parts (such as bosses, ribs, etc.). ) is made into a movable block, and the movable block is connected with the die main body through a pin or a dovetail structure.

Forming a detachable connection. When drawing the mold, take out the pattern main body first, and the movable block mold remains in the mold, and then take out the movable block from the side after drawing the mold.

This modeling method is called slipper modeling (Figure 5). Movable block mold modeling is mainly used for castings and plates with protruding parts that hinder mold lifting.

Small batch, hand modeling occasions. If this kind of casting is large in batch and needs machine modeling, sand cores can be used to form obstacles.

That part of the mold outline.

Fig. 5 the process of forming angle steel with movable block die.

4. Sand excavation modeling

When the outer contour of the casting is bent (such as handwheel, etc.). ), the largest cross section is not at the end, and the shape is not suitable for being divided into two halves.

Make the pattern into a whole, and dig out the part of the molding sand that hinders the removal of the pattern when modeling. This modeling method is called dredging modeling. dig

The parting surface of sand mold is curved surface. In order to ensure the smooth demoulding, the sand must be dug to the maximum cross section of the pattern (Figure 6).

Because it is hand-dug sand, it has high technical requirements and low production efficiency, and is only suitable for single piece and small batch production.

Fig. 6 sand digging and molding process of handwheel.

Manual core making

The core is used to form the inner cavity or local shape of the casting. Because the core surface is surrounded by high-temperature molten metal, it floats for a long time.

Force effect and baking effect of high temperature molten metal; When the casting is cooled and solidified, the sand core often hinders the free shrinkage of the casting; Sha Xinqing

And harder to manage. Therefore, the core sand for core making has high strength, air permeability, high temperature resistance, concession and

Collapse

Manual core-making is widely used because it does not need core-making equipment and the process equipment is simple. According to the size and complexity of the sand core,

Hand-made core boxes include integral core boxes, split core boxes and detachable core boxes, as shown in Figure 7.

Fig. 7 schematic diagram of core box manufacturing

Relationship among Parts, Models, Core Boxes and Castings

Patterns are used to form the outer contour of castings, and core boxes are used to make sand cores and form the inner contour of castings. Use the mold alone when modeling.

Samples and core boxes are used to make molds and cores. Figure 1 shows the relationship among parts, patterns, core boxes and castings respectively. Manufacturing mode and core box

The selected materials are related to the casting size, production scale and molding method. Commonly used in small batch production and manual modeling.

Wood is used to make patterns and core boxes, and metal materials (such as aluminum alloy, cast iron, etc. ) or hard plastics are often used in mass production and machine modeling.

Material production mode and core box.

Figure Relationship among Parts, Models, Core Boxes and Castings

Analysis of common defects in castings

The casting process is complex, and there are many factors affecting the quality of castings, which are often caused by lax control of raw materials and unreasonable process plan.

Improper production operation and imperfect management system will cause various casting defects. Names of common casting defects,

The characteristics and reasons are shown in the table.

Common casting defects and their causes

Defect name

air hole

trait

The main reason for this situation.

There are smooth holes of different sizes inside or on the surface of the casting.

(1) The charge is not dry or contains many oxides and impurities; ② pouring tools

Or the additives in front of the furnace are not dried; ③ Too much water in molding sand or too much brush water when demoulding and repairing; (4) core or core drying is not sufficient.

The vent hole is blocked; ⑤ Spring sand is too tight, and the permeability of molding sand is poor; ⑥ The pouring temperature is too low or the pouring speed is too fast.

Shrinkage and porosity

Shrinkage holes are mostly distributed in the thick section of castings, with irregular shapes and rough holes. ① The structural design of castings is unreasonable, such as wall thickness difference.

There is no riser or cold iron for the wall that is too large and thick; (2) the gating system and the riser are not in the right position; ③ The pouring temperature is too high; ④ Alloying

The chemical composition is unqualified, the shrinkage rate is too large, and the riser is too small or too little.

sand hole

There are holes filled with molding sand inside or on the surface of the casting.

(1) The strength of molding sand is too low or the density of sand mold and core is not enough, so it is molded.

Sand is washed into the cavity by molten metal; (2) The sand mold is partially damaged when the box is closed; (3) the gating system is unreasonable, the inner gate is in the wrong direction, and the metal

The liquid washed away the sand mold; (4) When the box is closed, the loose sand in the cavity or gate is not cleaned.

Burning sand

The surface of the casting is rough and a layer of sand is stuck. ② Molding sand has high mud content and high fire resistance.

Decline; ③ The pouring temperature is too high; ④ The content of pulverized coal in molding sand is too little during green mold casting; ⑤ During dry casting, the mold is not inclined or brushed

The paint is too thin.

sand inclusion

Metal flake protrusions are produced on the surface of the casting, and a layer of molding sand is sandwiched between the metal flake protrusions and the casting.

① Molding sand fever

The wet tensile strength is low, and the cavity surface expands and cracks when heated; ② If the local compactness of sand mold is too high and there is too much water, the water will dry up.

The surface of the back cavity is cracked; (3) Improper pouring position and long-time baking with high-temperature molten iron will cause the cavity surface to expand and crack; ④ Pouring

The injection temperature is too high and the pouring speed is too slow.

Type of error

The relative position of the casting moves along the parting surface ① The upper half and the lower half of the pattern are not aligned; (2) When the boxes are closed, the upper and lower sand boxes are misaligned;

(3) The upper and lower sand boxes are not clamped or the upper box is not pressed enough, which leads to the wrong box during pouring.

religious sutra/canon

There are incomplete fusion notches or pits on the casting, and the joint is smooth.

① The pouring temperature is too low and the fluidity of the alloy is poor;

② The pouring speed is too slow or the flow is cut off during pouring; ③ The position of gating system is improperly set or the cross-sectional area of inner runner is too small; ④ Casting

The wall is too thin; ⑤ The gate (including the gate cup) is not high enough; ⑥ When pouring, the amount of metal is not enough, and the cavity is not full.

Underwater irrigation

The casting is not filled.

crack

The casting cracked and there was an oxide film on the metal surface at the crack.

① The structural design of castings is unreasonable, the wall thickness is too different, and the cooling is uneven.

Even; (2) The sand mold and core have poor concession, or the spring sand is too tight; ③ premature sand falling; (4) The improper position of the gate leads to poor casting in all parts.

Uneven local shrinkage

Common casting defects and their preventive measures

Order defect name

Defect characteristics

preventive measure

1

air hole

Inside, on or near the surface of the casting, there are smooth holes with different sizes, which are round and long.

And irregular, some are single, and some are gathered into blocks. The color is white or with a dark color, sometimes covered with an oxide layer.

Skin.

Reduce the gas absorption of gossip metal during smelting. Reduce the gas generation of sand mold during pouring and improve the casting knot

Structure, improve the permeability of sand mold and core, and make the gas in the mold discharge smoothly.

2

shrinkage cavity

Inside the thick part of the casting, inside the interface between the two parts and inside or on the surface of the joint between the thick part and the thin part,

Irregular shape, rough holes and coarse particles.

Castings with small and uniform wall thickness should be solidified at the same time, and the wall thickness is large.

Uneven castings are solidified from thin to thick, and riser chills are placed reasonably.

three

shrinkage porosity

Tiny and discontinuous shrinkage cavities in castings gather in one or more places, and the grains are coarse and exist in them.

It's a very small hole, which oozes water during the water pressure test. Minimize the hot spots at the joints of the wall, and minimize the pouring temperature and pouring speed.

Degree.

four

Irregular holes in or on the surface of castings. The holes are not smooth and are completely or partially filled with melt.

Slag.

Raise the temperature of molten iron. Reduce the viscosity of slag. Improve the slag blocking ability of gating system. Enlarge the fillet of the casting.

five

sand hole

There are holes filled with molding sand inside or on the surface of the casting.

Strictly control the performance and molding operation of molding sand,

Pay attention to cleaning the cavity before installation.

six

Hot tearing

There are penetrating or non-penetrating cracks on the casting (pay attention to bending), and the metal skin at the crack is oxidized.

Strictly control the content of S and P in molten iron. The wall thickness of castings should be as uniform as possible. Improve the concession of molding sand and core. Casting head

The shrinkage of castings should not be hindered. Avoid sudden changes in wall thickness. You can't open the mold too early. Castings cannot be cooled.

seven

eight

Cold crack

Burning sand

There are penetrating or non-penetrating cracks (mainly straight cracks) on the casting, and the metal skin at the crack is oxidized.

The surface of the casting is completely or partially covered with a layer of metal (or metal oxide) and sand (or paint).

Due to the mixture or a layer of sintered sand, the surface of the casting is rough.

The pouring temperature of the genus. Improve the fire resistance of molding sand and core sand.

Reduce the gap between grains of sand. Reduce gold appropriately

nine

sand inclusion

There is a layer of metal tumor or thin film on the surface of casting, and a layer is sandwiched between metal tumor and casting.

Molding sand. Strictly control the properties of molding sand and core sand. Improve the gating system to make the molten metal flow smoothly. Large plane castings should be inclined.

come down in torrents

10 keep cold

There are incomplete fusion notches or pits on the casting, and its boundary edge is smooth. Improved pouring

Temperature and pouring speed. Improve the gating system. Continuous flow during pouring.

1 1 Because the mold cavity is not completely filled with molten metal, the casting cannot be poured without meat. Improve pouring temperature and pouring speed.

Don't cut off the water flow to stop the fire from spreading.

Pouring of molten metal in castings

In production, pouring should follow the principle of tapping at high temperature and pouring at low temperature. Because increasing the tapping temperature of molten metal is beneficial to inclusions.

The complete melting of materials and the floating of molten slag facilitate slag removal and degassing, and reduce slag inclusion and porosity defects of castings; Use low pouring.

Temperature is beneficial to reduce the gas solubility and liquid shrinkage in molten metal, as well as the baking of cavity surface by molten metal at high temperature.

Avoid porosity, sand sticking, shrinkage cavity and other defects. Therefore, under the premise of ensuring the fullness of the mold cavity, try to use a lower one.

Pouring temperature.

The operation process of pouring molten metal from ladle into mould is called pouring. Improper pouring operation will lead to insufficient pouring, cold preservation, porosity,

Casting defects such as shrinkage cavity and slag inclusion, personal injury.

In order to ensure casting quality, improve productivity and realize safe production, the following operation points should be strictly observed during pouring:

(1) Ladles, pouring tools, inoculants and nodulizers used for furnace pretreatment must be fully dried before use, and can only be made after drying.

Use.

(2) Pouring personnel must wear work clothes and protective glasses as required, and the workplace should be unobstructed. The gold in the spoon

The liquid should not be too full, so as to avoid spilling and injuring people during transportation and dumping.

(3) Choose the pouring speed correctly, that is, pouring should be slow at the beginning, which is convenient for aligning the gate and reducing the impact of molten metal on the sand mold.

Conducive to gas discharge; Then pour it quickly to prevent cold preservation; Before it is almost full, you should pour it slowly, that is, follow the slow, fast,

The principle of slow.

(4) Castings with large liquid shrinkage and solidification shrinkage, such as medium and large steel castings, should be poured from the gate or in time.

Fill your mouth.