Casting production process
Casting production is a complex multi process combination process, which includes the following main processes:
1) Production process preparation: prepare the production process plan and process documents and draw the casting process drawing according to the parts drawing, production batch and delivery date to be produced;
2) Production preparation, including preparation of materials for melting, 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;
5) Main processes such as sand falling cleaning and casting inspection.
Casting production is a metal forming method that heats and melts the metal to make it fluidity, then pours it into the mold cavity with a certain shape, fills the mold cavity under the action of gravity or external force (pressure, centrifugal force, electromagnetic force, etc.), cools and solidifies into a casting (or part).
Fig. 1 casting process
The casting is generally processed as a blank to become a part.
However, there are also many castings that can meet the requirements of design accuracy and surface roughness of parts without cutting, and can be 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) include strength, permeability, fire resistance, yield, fluidity, compactness and collapsibility.
2. Composition of molding sand
Molding sand is composed of raw sand, adhesive and additives.
The raw sand for casting shall be sea sand, river sand or mountain sand with small mud content, uniform particles and round and polygonal shapes.
The adhesives used for casting include clay (ordinary clay and bentonite), water glass sand, resin, synthetic oil and vegetable oil, which are respectively called clay sand, water glass sand, resin sand, synthetic oil sand and vegetable oil sand.
In order to further improve some properties of mold (core) sand, some additives, such as coal, sawdust and pulp, are often added to the mold (core) sand.
The structure of molding sand is shown in Fig. 2.
Fig. 2 Schematic diagram of molding sand structure
Casting process characteristics
Casting is one of the main methods to produce parts blank, especially for parts blank of some brittle metals or alloy materials (such as various cast iron and non-ferrous alloy castings), casting is almost the only processing method.
Compared with other processing methods, the casting process has the following characteristics:
1) The casting may not be limited by the metal material, size and weight.
Casting materials can be various cast iron, cast steel, aluminum alloy, 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 tons;
The wall thickness of the casting can be from 0.5 mm to about 1 m;
The casting length can range from several millimeters to ten meters.
2) Casting can produce all kinds of blanks with complex shapes, especially for parts with complex internal cavities, such as various boxes, cylinders, blades, impellers, etc.
3) The shape and size of the casting can be very close to the parts, which not only saves metal materials, but also saves machining hours.
4) Generally, the raw materials used for casting are widely sourced and the casting cost is low.
5) The casting process is flexible and the productivity is high.
It can be produced manually or mechanically.
Analysis of common defects in casting
The casting process is complex, and there are many factors affecting the casting quality.
Often, various casting defects will occur due to poor control of raw materials, unreasonable process plans, improper production operations, and imperfect management systems.
The names, characteristics and causes of common casting defects are as follows.
|Defect name||features||Main causes|
|Stoma||There are smooth holes of different sizes inside or on the surface of the casting.||① The furnace charge is not dry or contains many oxides and impurities;|
② Pouring tools or additives in front of the furnace are not dried;
③ Too much water is contained in the molding sand or too much water is brushed during mold lifting and trimming;
④ Insufficient core drying or blocked core vent;
⑤ The spring sand is too tight and the air permeability of molding sand is poor;
⑥ The pouring temperature is too low or the pouring speed is too fast.
|Shrinkage cavity and porosity||The shrinkage holes are mostly distributed at the thick section of the casting, with irregular shape and rough inside.||① The structural design of the casting is unreasonable, for example, the wall thickness difference is too large, and no riser or cold iron is placed at the thick wall;|
② The position of pouring system and riser is wrong;
③ Pouring temperature is too high;
④ The chemical composition of the alloy is unqualified, the shrinkage rate is too large, and the riser is too small or too few.
|Sand holes||There are holes filled with molding sand inside or on the surface of the casting.||① The strength of the molding sand is too low or the compactness of the sand mold and the core is not enough, so the molding sand is flushed into the mold cavity by the molten metal;|
② The sand mold is partially damaged when the box is closed;
③ The pouring system is unreasonable, the direction of the ingate is wrong, and the sand mold is damaged by the molten metal;
④ The loose sand in the mold cavity or gate is not cleaned up when the box is closed.
|Sticky sand||The surface of the casting is rough with a layer of sand.||① The fire resistance of raw sand is low or the particle size is too large;|
② The fire resistance of molding sand decreases when the mud content is too high;
③ Pouring temperature is too high;
④ The content of pulverized coal in the molding sand is too small in the process of green casting;
⑤ The dry type is not painted obliquely or the coating is too thin.
|Sand inclusion||A layer of molding sand is sandwiched between the metal sheet protrusion and the coupling part.||① The hot wet tensile strength of the molding sand is low, and the surface of the mold cavity is heated and baked to expand and crack;|
② The local compactness of the sand mold is too high, the water is too much, and the surface of the mold cavity is cracked after the water is dried ten times;
③ Improper selection of pouring position causes the cavity surface to expand and crack due to high temperature molten iron baking for a long time;
④ The pouring temperature is too high and the pouring speed is too slow.
|Wrong type||The casting has relative displacement along the parting surface.||① The upper mold half and the lower mold half of the pattern are not aligned;|
② When the box is 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 sufficiently pressed, and the wrong box is generated during pouring.
|Cold barrier||There are gaps or pits on the casting where the cards are completely fused, and the joints are smooth.||① The pouring temperature is too low and the alloy fluidity is poor;|
② The pouring speed is too slow or the flow is interrupted during pouring;
③ The position of the pouring system is improper or the cross-sectional area of the ingate is too small;
④ The casting wall is too thin;
⑤ The height of sprue (including sprue cup) is insufficient;
⑥ When pouring, the amount of metal is insufficient and the cavity is not full.
|Insufficient pouring||The casting is not fully filled.|
|Crackle||The casting is cracked and there is oxide film on the metal surface at the crack.||① The casting structure design is unreasonable, the wall thickness difference is too large, and the cooling is uneven;|
② The retreat of sand mold and core is poor, or the spring sand is too tight;
③ Premature sand falling;
④ Improper gate position leads to uneven shrinkage of all parts of the casting.
Common casting defects and their preventive measures
|Order||Defect title||Defect characteristics||Preventive measure|
|1||stoma||In the interior, surface or near the surface of the casting, there are smooth holes of different sizes. The shapes are round, long and irregular, single or aggregated.The color is white or with a dark color, sometimes covered with a layer of oxide.||Reduce the gettering amount of metal during smelting.Reduce the gas emission of the sand mold during the pouring process, improve the casting structure, improve the permeability of the sand mold and the core, and enable the gas in the mold to be discharged smoothly.|
|2||contraction cavities||Inside the thick section of the casting, the inside of the two interfaces and the inside or surface of the junction of the thick section and the thin section, the shape is irregular, the hole is rough and uneven, and the grain is coarse.||Castings with small and uniform wall thickness shall be solidified at the same time. Castings with large and uneven wall thickness shall be solidified from thin to thick, and the cold iron of riser shall be placed reasonably.|
|3||shrinkage porosity||The small and discontinuous shrinkage holes in the casting are gathered in one or more places, and the particles are coarse. There are small holes between each particle, and water seepage occurs during the hydrostatic test.||The hot joints shall be minimized at the joints between walls, and the pouring temperature and pouring speed shall be minimized.|
|4||slag blowhole||Holes with irregular shape inside or on the surface of the casting.The holes are not smooth and filled with slag in whole or in part.||Increase the temperature of molten iron.Reduce slag viscosity.Improve the slag retaining ability of the gating system.Increase the internal fillet of the casting.|
|5||sand holes||There are holes filled with molding sand inside or on the surface of the casting.||Strictly control the molding sand performance and molding operation, and pay attention to cleaning the mold cavity before closing the mold.|
|6||thermal cracking||There are penetrating or non penetrating cracks on the casting (Note: if it is curved), and the metal skin at the crack is oxidized.||Strictly control the content of S and P in molten iron.The wall thickness of the casting shall be as uniform as possible.Improve the yield of molding sand and core.The pouring riser shall not hinder the shrinkage of the casting.Avoid sudden changes in wall thickness.The opening cannot be too early.Castings cannot be quenched.|
|7||cold crack||There are penetrating or non penetrating cracks (mainly straight) on the casting, and the metal skin at the crack is oxidized.|
|8||sand burning||The surface of the casting is completely or partially covered with a layer of metal (or metal oxide) mixed with sand (or coating) or a layer of sintered molding sand, resulting in rough surface of the casting.||Reduce sand gap.The pouring temperature of the metal shall be appropriately reduced.Improve the fire resistance of molding sand and core sand.|
|9||sand inclusion||On the surface of the casting, there is a layer of metal tumor or sheet, and a layer of molding sand is sandwiched between the metal tumor and the casting.||Strictly control the properties of molding sand and core sand.Improve the pouring system to make the molten metal flow smoothly.Large plane castings shall be poured obliquely.|
|10||cold barrier||There is a kind of incomplete fusion gap or pit on the casting, and its boundary edge is smooth.||Improve pouring temperature and pouring speed.Improve the gating system.The flow shall not be interrupted during pouring.|
|11||unable to pour||Lack of flesh in the casting due to incomplete filling of the cavity with molten metal.||Improve pouring temperature and pouring speed.Do not cut off the flow and prevent fire.|
Casting – casting of molten metal
In production, the principle of high temperature tapping and low temperature pouring shall be followed during pouring.
Because increasing the tapping temperature of molten metal is conducive to the complete melting of inclusions and the floating of molten slag, which is convenient for slag cleaning and degassing, and reduces the slag inclusion and porosity defects of castings;
Lower pouring temperature is beneficial to reduce the gas solubility, liquid shrinkage and baking of high-temperature molten metal on the surface of the mold cavity, and avoid defects such as porosity, sand sticking and shrinkage.
Therefore, on the premise of ensuring that the mold cavity is filled, the pouring temperature should be as low as possible.
The operation of pouring molten metal from the ladle into the mold is called pouring.
Improper pouring operation will cause casting defects such as insufficient pouring, cold shut, air hole, shrinkage hole and slag inclusion, and cause personal injury.
In order to ensure casting quality, improve productivity and achieve safe production, the following operation essentials shall be strictly observed during pouring:
(1) The ladle, pouring tool, inoculant and spheroidizing agent for furnace pretreatment must be fully dried before use, and can be used after drying.
(2) Pouring personnel must wear work clothes and protective glasses as required, and the working site shall be unobstructed.
The molten metal in the pouring ladle should not be too full to avoid spilling and injuring people during transportation and pouring.
(3) The pouring speed shall be properly selected, that is, the pouring shall be slow at the beginning, so as to align the gate, reduce the impact of molten metal on the sand mold and facilitate the gas discharge;
Followed by rapid pouring to prevent cold insulation;
The pouring shall be carried out slowly before the full pouring, i.e. the principle of “slow, fast and slow” shall be followed.
(4) For the parts with relatively high liquid shrinkage and solidification shrinkage, such as medium and large steel parts, the pouring shall be conducted from the gate or riser in time after pouring.
(5) During pouring, the gas emitted from the casting mold shall be ignited in time to avoid gas holes caused by the gas holding in the casting mold, as well as damage to human health and air pollution due to incomplete combustion of the gas.
8 basic steps of sand casting
Sand mold casting technology is a casting method using sand as the main molding material to prepare the mold.
Sand casting is the most traditional casting method.
Due to its own characteristics (not limited by the shape, size, complexity and alloy type of parts, short production cycle and low cost), sand casting is still the most widely used casting method in casting production, especially single piece or small batch casting!
The basic process of the traditional sand mold casting process includes the following steps: sand preparation, mold making, core making, molding, pouring, sand dropping, grinding and inspection.
1. Sand mixing stage
Molding sand and core sand are prepared for molding.
Generally, the sand mixer is used to mix the old drawing and a proper amount of clay.
2. Mold making stage
Molds and core boxes are made according to the part drawings. Generally, single pieces can be made of wood molds, and plastic molds or metal molds (commonly known as iron molds or steel molds) can be made in mass production.
Large quantities of castings can be made of template.
Now the molds are basically made with engraving machines, so the production cycle is greatly shortened. Generally, it takes 2 to 10 days to make molds.
3. Molding (core making) stage
Including molding (forming the mold cavity of the casting with molding sand), core making (forming the internal shape of the casting), and mold matching (putting the cement core into the mold cavity and closing the upper and lower sand boxes).
Modeling is the key link in casting.
4. Melting stage
Prepare chemical composition according to the required metal composition, select appropriate melting furnace to melt alloy materials, and form qualified liquid metal (including qualified composition and temperature).
Cupola or electric furnace is generally used for smelting (due to environmental protection requirements, cupola is now basically banned and electric furnace is basically used).
5. Pouring stage
Use molten iron ladle to inject molten iron melted in the electric furnace into the finished mold.
When pouring molten iron, pay attention to the pouring speed so that the molten iron can fill the whole mold cavity.
In addition, the pouring of molten iron is dangerous, and attention should be paid to safety!
6. Cleaning stage
After the molten metal solidifies after pouring, take a hammer to remove the gate and shake off the sand of the casting, and then use a sand blasting machine to sand blast, so that the surface of the casting will appear very clean!
The casting blanks with less strict requirements can basically be delivered after inspection.
7. Casting processing
Simple processing may be required for some castings with special requirements or some castings that cannot meet the requirements.
Generally, grinding wheel or grinder is used for processing and grinding to remove burrs and make the casting more smooth.
8. Casting inspection
Generally, the casting inspection is in the process of cleaning or processing, and the unqualified ones have been found out.
However, some castings have individual requirements and need to be inspected again.
For example, some castings need a 5cm shaft to be inserted into the central hole, so you need to take a 5cm shaft and try it on.
After the above 8 steps, the casting is basically formed. For the casting requiring high precision, it needs to be machined.
Because it does not belong to the casting scope, it will not be described here.
With the progress of casting technology, the traditional sand casting is either improved or replaced by other casting methods.
Continuous innovation is the eternal theme and the intrinsic quality and foundation that a foundry must have!
What equipment is needed for sand casting?
Sand casting is the most common and traditional casting process in the foundry industry, and it is the casting method that small foundry manufacturers give priority to when starting.
What are the basic equipment and tools required by the sand casting process?
According to the production process sequence, sand mold casting needs: sand mixing equipment, mold making equipment, core making equipment, molding equipment, melting equipment, pouring equipment, cleaning equipment and casting processing equipment.
1. Sand mixing equipment
Equipment required for this step: sand mixer; tools required: shovel and sieve.
Sand mold casting cannot be separated from sand, which is a special molding soil.
The molding soil can be reused, but it needs to be screened with a sieve before use.
The loam is finer, and then it needs to be mixed evenly with a sand mixer.
The small sand mixer is only 2000 pieces.
2. Mold manufacturing equipment
Equipment required in this step: engraving machine.
Materials required: Wood (wood mold), plastic mold (special plastic plate), metal mold (metal block).
The mold should be made according to the samples or drawings provided by the customer.
The quality of the mold directly affects the shape accuracy of the casting!
In the past, traditional wood molds were generally made by carpenters.
Now, due to the promotion of engraving machines, engraving machines are used for wood molds, plastic molds and metal molds.
A small engraving machine capable of engraving wood and plastic molds can be bought for thousands of yuan.
If you can carve metal, you need tens of thousands to hundreds of thousands of engraving machines.
The material range, accuracy and automation of the engraving machine are the main aspects that determine the price!
3. Core making equipment
Equipment required in this step: oven, materials required: coated sand, thick gloves.
Some castings with internal cavities often need to make a core (commonly known as the lower core) and there are many methods for making the core.
There are two common methods.
First, generally, the mold is made first (usually made together with the core mold in the first step), and then manually made with molding soil mixed with sand (a bit like molding, commonly known as core beating).
Second, the aluminum core box is filled with coated sand and fired in an oven.
The oven can be dedicated for casting or self-made.
One can be welded with iron sheet and baked on the furnace.
Because the core box is very hot, you should wear gloves.
Here I recommend the second method.
The core baked with coated sand should not be fired during pouring, and the inner surface of the casting is relatively clean.
4. Modeling equipment
Equipment required for this step: molding machine, tools required: sandbox, bottom plate, scraper, shovel, dip pen, V-shaped iron sheet for gate opening, brush, air hole needle, etc.
Molding is the most tiring and critical step in sand casting and also a process with the most industrial characteristics.
In the past, molding workers used pure manual molding, but now most of them use molding machines, greatly reducing the labor of molding workers.
There are many tools used in modeling, including expansion sandbox, bottom plate, scraper, shovel, dip pen, V-shaped iron sheet for gate opening, brush, air hole needle, etc.
Most of them are small tools made by stylists according to their own habits.
In short, make a good model!
5. Melting equipment
Equipment required in this step: electric furnace and transformer;
Required materials: iron, aluminum, copper and other raw materials, tin and iron (batching).
The main purpose of this step is to melt iron into molten iron.
The difficulty is to mix and control the temperature of molten iron.
Because cupola has been basically banned, most of the large and small factories use electric furnaces as melting equipment.
Electric furnaces have different sizes, and the price difference is large.
The small ones of 200 kg are in the range of 10000 to 20000, and the large ones are even in the range of hundreds of thousands to millions.
In addition, the electric furnace needs to apply for a special transformer, and the transformer equipment needs to be purchased by itself.
In addition, the use of the transformer needs to be applied to the local power bureau, and the application costs are different.
Small foundries have been set up for several hundred thousand at least.
Of which the application fee accounts for half.
6. Pouring equipment
Equipment required in this step: molten iron ladle and aerial crane.
Molten iron can be divided into large and small packages.
The molten iron melted in the electric furnace is poured into the large package first, and then the molten iron is transferred from the large package to the small package.
The workers carry the small package and pour the molten iron into the finished mold.
This step has a high risk factor, and workers should take protective measures.
In addition, large factories pour large castings and use large equipment such as aerial cranes.
7. Cleaning equipment
Equipment required for this step: sand blasting machine.
After pouring, after the casting is cooled, the worker will pick the casting out of the soil.
It is necessary to shake off most of the soil on the surface and smash the gate with a hammer.
Then throw it into the sandblasting machine for sandblasting.
8. Casting processing
Common equipment in this step: grinding wheel, polishing machine, cutting machine and lathe.
Casting processing belongs to the scope of machining.
Most of the general machining equipment can be used, but it has exceeded the scope of casting.
Generally, foundries have the most basic tools, such as grinding wheels, polishing machines, cutting machines, etc. it is better to have a lathe.
Any work needs to be coordinated with each other. Small foundries that have just started cannot have all the casting processes, and may not have their own mold opening and processing capabilities.
They can cooperate with specialized molds and machining, and they are only responsible for casting blanks.
Casting is a traditional industry and an industry worthy of admiration.
Although small traditional foundries have been banned by environmental protection, they have been engaged in casting all their lives, and have been unemployed or professional since then!
Here I salute the older generation of foundry workers!