I. Advantages and Disadvantages of Metal Casting
1. Advantages
Comparing metal mold casting with sand mold casting, there are numerous technical and economic advantages:
(1) Castings produced in metal molds have superior mechanical properties than those cast in sand molds. For the same alloy, tensile strength can increase by approximately 25%, yield strength by approximately 20%, and there are significant improvements in corrosion resistance and hardness.
(2) The precision and surface smoothness of castings are higher than those made with sand molds, and the quality and dimensions are more stable.
(3) The yield of the casting process is higher, reducing the consumption of liquid metal, generally saving 15-30%.
(4) The use of sand is eliminated or minimized, generally saving 80-100% of mold materials.
Furthermore, metal die casting has a high production efficiency; the causes of casting defects are reduced; The process is simple, easy to mechanize and automate.
2. Disadvantages
Despite the advantages of metal mold casting, there are also disadvantages, such as:
(1) The production cost of metal molds is high.
(2) Metal molds are not breathable and have no tolerance, which may lead to casting defects such as insufficient casting, cracking or white mouth in cast iron parts.
(3) During metal mold casting, factors such as mold working temperature, pouring temperature and alloy speed, the time the casting remains in the mold and the type of coating used can significantly affect the quality of the casting. and require rigorous control.
Therefore, when deciding to use metal mold casting, it is necessary to comprehensively consider the following factors: the shape and weight of the casting must be suitable; there must be sufficient batch size; and the production deadline must allow it.
II. Characteristics of the metal mold casting process
There are significant differences between metal and sand molds in terms of their properties. For example, sand molds are breathable while metal molds are not.
Sand molds have low thermal conductivity, while metal molds excel in this aspect. Sand molds are shrinkable, but metal molds are not. These characteristics of metal molds determine their unique principles in the casting process.
The influence of changes in the state of the gas inside the mold cavity on the formation of the casting: During metal filling, the gases inside the mold cavity must be expelled quickly. However, the metal's lack of breathability means that slight negligence in the process can negatively affect the quality of the casting.
Features of heat exchange during casting solidification: Once the molten metal enters the mold cavity, it transfers heat to the metal wall of the mold. The liquid metal loses heat through the mold wall, causing solidification and shrinkage.
Meanwhile, the mold wall expands as it heats, creating a “gap” between the casting and the mold wall. Until the “casting-gap-mold” system reaches a uniform temperature, the casting can be considered to be cooled within the “gap”, while the mold wall is heated through the “gap”.
The impact of the metal mold preventing the casting from shrinking: Metal molds or molds with a metal core do not retract during the casting process, making it difficult for the casting to shrink – another unique feature of them.
III. Metal Casting Process
1. Preheating the metal mold
Non-preheated metal molds cannot be used for casting due to their high thermal conductivity. If the liquid metal cools too quickly, its fluidity decreases drastically, leading to casting defects such as cold closing, insufficient casting inclusions, and porosity.
Non-preheated metal molds are prone to damage from thermal shock and stress build-up during casting. Therefore, metal molds must be preheated before use.
The appropriate preheat temperature (i.e., operating temperature) depends on the alloy type, structure, and size of the casting and is usually determined through testing. As a general rule, the preheating temperature of a metal mold should not be less than 1500°C.
Methods for preheating metal molds include:
(1) Preheating with torch or gas flame.
(2) Using a resistance heater.
(3) Use an oven for heating, which provides a uniform temperature, but is only suitable for small metal molds.
(4) Preheat the metal mold in a furnace, and then melt the liquid metal to heat the mold. This method is only suitable for small molds as it wastes some liquid metal and can shorten the life of the mold.
2. Metal mold pouring
The pouring temperature for metal molds is generally higher than that for sand casting and can be determined based on the type of alloy, its chemical composition, and the size and thickness of the casting through testing. The following data can be used as a reference.
Pouring temperatures for various alloys:
- Aluminum and tin alloy: 350-450°C
- Brass: 900-950°C
- Zinc alloy: 450-480°C
- Tin bronze: 1100-1150°C
- Aluminum alloy: 680-740°C
- Aluminum bronze: 1150-1300°C
- Magnesium alloy: 715-740°C
- Cast iron: 1300-1370°C
Given the rapid cooling and non-porous nature of metal molds, the pouring speed must be slow initially, then fast, and finally slow again. It is essential to maintain a constant flow of liquid during the pouring process.
3. Timing of Casting Removal and Core Removal
The longer a metal core remains inside a casting, the stronger the adhesion to the core will be due to shrinkage of the casting, thus requiring greater force to pull out the core.
The ideal duration for a metal core to remain inside a casting is when the casting has cooled to a plastic deformation temperature range and has sufficient strength, at which point it is the best time to remove the core.
If the casting remains in the metal die for a long time, the die wall temperature increases, requiring more cooling time and reducing the productivity of the metal die.
The most appropriate time to remove the core and casting is normally determined through experimental methods.
4. Adjustment of the working temperature of the metal mold
To ensure the stability of metal mold quality and normal production, it is crucial to maintain a constant temperature change in the metal mold during production.
Therefore, after each casting, the metal mold must be opened and left for a certain period of time until it cools down to the specified temperature before the next casting.
If relying on natural cooling, the time required is longer, which reduces productivity, so forced cooling is commonly used. There are generally several cooling methods:
1. Air cooling: Blowing air around the outside of the metal mold to increase convective heat dissipation. Although the structure of an air-cooled metal mold is simple, easy to manufacture and low in cost, the cooling effect is not particularly ideal.
2. Indirect water cooling: Installation of a water jacket at the back or in a specific part of the metal mold. Its cooling effect is better than that of air cooling and is suitable for casting copper parts or forged cast iron parts. However, intense cooling to cast thin-walled gray iron castings or ductile iron castings may increase casting defects.
3. Direct water cooling: Directly make a water jacket on the back or a specific part of the metal mold and cool it with water flowing through the jacket. This method is mainly used for casting steel parts or other alloy castings where strong mold cooling is required. Due to its high cost, it is only applicable for large-scale production.
If the wall thickness of the casting varies greatly, when using a metal mold for production, a common method is to heat one part of the metal mold while cooling another part to adjust the temperature distribution of the mold wall.
5. Coatings for metal molds
During the metal mold casting process, it is common to apply a coating to the working surface of the metal mold.
The coating functions regulate the cooling speed of castings, protect the metal mold from erosion and thermal shock caused by high-temperature metal liquid, and facilitate the release of gas through the coating layer.
Depending on the alloy, the coating can have different formulas and is generally composed of three types of substances:
1. Refractory powder materials (such as zinc oxide, talcum powder, zircon sand powder, diatomaceous earth powder, etc.);
2. Binders (normally glass of water, syrup or liquid waste paper pulp, etc.);
3. Solvent (water). Specific formulas can be consulted in relevant manuals. The coating must meet the following technical requirements: it must have a certain viscosity to facilitate spraying, be able to form a thin and uniform layer on the surface of the metal mold; after drying, the coating should not crack or peel off and should be easy to remove; must have high refractoriness; it must not generate a large amount of gas at high temperatures; must not react chemically with the alloy (exceptions for special requirements).
6. Resin sand metal molds (resin sand iron molds)
Although coating can reduce the cooling speed of castings in the metal mold, there is still a certain difficulty in producing ductile iron parts (such as crankshafts) with metal molds that use coatings, as the cooling speed of castings is still very high. quickly, and castings tend to turn white.
If a sand mold is used, the casting will have a slower cooling speed, but shrinkage or porosity will easily occur at the hot junction.
Applying a 4-8mm layer of sand to the metal mold surface can result in satisfactory ductile iron castings.
The sand layer effectively regulates the cooling speed of the casting, on the one hand preventing the occurrence of a white mouth on the cast iron body, and on the other hand making the cooling rate faster than sand casting.
Metal molds do not disintegrate, but a thin layer of resinous sand can adequately reduce the shrinkage resistance of castings. Furthermore, the metal molds have good rigidity, effectively limiting the expansion of spheroidal graphite, achieving riserless casting, eliminating looseness and improving the compaction of castings.
If the sand layer of the metal mold is made of resinous sand, it can generally be covered by sandblasting. The temperature of the metal mold must be between 180-200°C. Resin sand metal molds can be used to produce ductile iron, gray iron or steel castings, and its technical effects are significant.
7. Service life of metal molds
Ways to improve the life of metal molds include:
1. Choice of materials with high thermal conductivity, low coefficient of thermal expansion and high resistance for manufacturing metal molds;
2. Appropriate coating technology, strictly following process specifications;
3. The metal mold structure must be reasonable, and residual stresses must be eliminated during the manufacturing process;
4. The grains of the metal mold material should be small.
4. Process design for metal mold castings
To ensure casting quality, simplify the structure of the metal mold, and fully exploit its technical and economic benefits, an initial analysis of the casting structure must be carried out and a reasonable casting process must be established.
1. Process Analysis of Casting Structure
The process design quality of a metal mold casting structure is a prerequisite for ensuring casting quality and exploiting the advantages of metal mold casting. A reasonable casting structure must adhere to the following principles:
(1) The cast structure must not impede demolding or shrinkage;
(2) The thickness variation should not be too large to avoid significant temperature differences, leading to shrinkage cracks and porosity in the casting;
(3) The minimum wall thickness of metal molds should be restricted.
Furthermore, the precision and smoothness of the unmachined surfaces of the casting must be required appropriately.
2. Casting pouring position in metal mold
The casting position of the casting is directly related to the number of cores and parting surfaces, the liquid metal introduction position, the riser feeding effect, the degree of exhaust smoothness and the complexity of the metal mold.
The principles for selecting the pouring position are as follows:
1. Make sure the metal liquid flows smoothly during filling, allowing easy ventilation and preventing air entry and metal oxidation;
2. Promote sequential solidification and good shrinkage to ensure the acquisition of dense structure castings;
3. The number of cores should be minimized and they should be easy to place, stable and easy to demold;
4. It facilitates the simplification of the metal mold structure and the ease of demolding the casting.
3. Selection of parting surface in moldability
Parting surface shapes are generally vertical, horizontal and combined (vertical, mixed horizontal or curved). The principles for selecting the parting surface are as follows:
1. In order to simplify the structure of the metal mold and improve the casting accuracy, the shape of the simplest casting should be arranged within the half mold, or most of it should be arranged within the half mold;
2. The number of separation surfaces should be minimized to ensure the aesthetic appearance of the casting and to facilitate demolding and core placement;
3. The selected parting surface should ensure that the adjustment of the gate and risers is convenient, allowing smooth flow of metal during filling and facilitating the expulsion of gas from the mold cavity;
4. The parting surface should not be selected on the machining reference surface;
5. Avoid curved parting surfaces as much as possible to reduce the number of disassembled parts and moving mold components.
4. Casting System Design
The following factors must be considered when designing the casting system due to the specific characteristics of metal mold casting: the metal casting speed is high, exceeding that of sand molds by about 20%.
Furthermore, the gas in the mold cavity must be able to be expelled smoothly when the liquid metal fills the mold. Its flow direction should be as consistent as possible with the liquid flow direction, effectively pushing the gas toward the riser or vent riser.
Additionally, care must be taken to ensure that the liquid metal flows smoothly during the filling process without creating turbulence, impacting the mold wall or cores, or causing splashes.
Metal die casting system generally falls into three categories: top channel, bottom channel and side channel.
(1) Top gating: This method has reasonable heat distribution, which is beneficial to sequential solidification and can reduce liquid metal consumption. However, the flow of liquid metal is unstable, which can cause inclusions. When the casting height is high, it may impact the mold bottom or cores. If used for casting aluminum alloy parts, it is generally only suitable for simple parts with a height of less than 100 millimeters.
(2) Lower Gating: Liquid metal flows more smoothly, which is beneficial to ventilation. However, the temperature distribution is unreasonable, which is not conducive to the smooth solidification of the casting.
(3) Side gating: This method has the advantages of the two methods mentioned above. The liquid metal flows smoothly, which makes slag collection and ventilation easier. However, the consumption of liquid metal is high and there is a large workload for cleaning the gates.
The structure of the metal mold casting system is basically similar to that of sand mold casting.
However, because the metal mold wall is not breathable and has strong thermal conductivity, the structure of the casting system should facilitate the reduction of liquid metal flow speed, ensure smooth flow, and reduce its impact on the mold wall.
In addition to ensuring that the gas in the mold cavity has enough time to be expelled, it must also ensure that no splashes occur during the filling process.
When casting ferrous metals with metal molds, due to the high cooling speed of the casting and the rapid increase in viscosity of the liquid flow, a closed passage system is often used. The ratio of the cross-sectional areas of its various parts is: F_inner: F_transverse: F_vertical = 1: 1.15: 1.25
5. Riser Project
Risers in metal mold casting have the same functions as those in sand mold casting: they compensate for shrinkage, collect slag and ventilate. The design principles for risers in metal molds are the same as those for risers in sand molds.
Because metal molds cool faster and risers often use insulating coatings or layers of sand, the size of risers in metal molds may be smaller than those in sand molds.
Editorial Section: Metal Mold Casting Process Parameters
Due to the characteristics of the metal mold process, the process parameters of its castings are slightly different from those of sand mold castings.
The linear shrinkage rate of metal mold castings is not only related to the linear shrinkage of the alloy, but also to the structure of the casting, the obstruction of shrinkage in the metal mold, the demolding temperature of the casting, the expansion and to the change of size of the metal mold after heating, etc. Its value also needs to consider leaving room for size modification during the test casting process.
To remove the metal core from the mold and the casting, an appropriate draft must be made in the direction of core removal and demolding of the casting. Consult the relevant manuals for the casting project of various castings of different alloys.
The precision of metal mold castings is generally higher than that of sand mold castings, so the machining margin can be smaller, generally between 0.5 and 4 mm.
After determining the parameters of the casting process, the design of the metal mold casting process can be drawn. This design is basically the same as the design of the sand mold casting process.
V. Metal mold design
After the casting process diagram is drawn, metal mold design can proceed. The project mainly involves determining the structure, dimensions, core, exhaust system and ejection mechanism of the metal mold.
The design of the metal mold should aim for simplicity in structure, convenience in machining, appropriate material selection and ensure safety and reliability.
1. Structure of metal molds
The structure of the metal mold depends on the shape and size of the casting, the number of parting surfaces, the type of alloy and the production volume. Based on the position of the parting surface, there are several forms of metal mold structures:
1. Integral metal mold: This mold has no parting surface and a simple structure, suitable for simple shaped castings without parting surface.
2. Horizontal split metal mold: This mold is suitable for thin-walled wheel castings.
3. Vertical split metal mold: This type of mold is convenient for establishing passage and exhaust systems, easy to open and close, and suitable for mechanized production. It is often used to produce simple small castings.
4. Composite metal parting mold: It consists of two or more parting surfaces, or even movable blocks, generally used for the production of complex castings. It is convenient to operate and widely used in production.
2. Metal mold main body design
The main body of a metal mold refers to the part that forms the mold cavity and is used to form the outer shape of the casting. The main structure of the body is related to the size of the casting, its casting position in the mold, parting surface and alloy type.
The design must seek precise dimensions of the mold cavity; facilitate the establishment of gate and exhaust systems, ease of ejection of the casting and sufficient strength and rigidity.
3. Design of metal mold cores
Depending on the complexity of the casting and the type of alloy, different materials can be used for the mold core.
Generally, sand cores are used to cast thin-walled complex parts or high melting point alloys (such as stainless steel, cast iron), while metal cores are mainly used to cast low melting point alloys. fusion (such as aluminum, magnesium alloys). Sand cores and metal cores can also be used together in the same casting.
4. Metal mold exhaust
When designing a metal mold, an exhaust system is essential. The following methods can be used for exhaustion:
(1) Use the gap between the parting surface or the combined surface of the mold cavity for exhaust.
(2) Create an exhaust groove on the parting surface or the combined surface of the mold cavity, the core seat or the ejector rod surface.
(3) Install exhaust holes, which are generally located at the highest point of the metal mold.
(4) Exhaust plugs are commonly used in metal molds.
5. Ejector mechanism design
The uneven parts of the metal mold cavity can make it difficult for the casting to shrink, causing resistance when the casting is demolded. An ejector mechanism must be used to eject the casting.
When designing the ejector mechanism, the following points must be observed: avoid damage to the casting, that is, prevent the casting from being deformed or crushed by ejection; prevent the ejector rod from getting stuck.
The clearance between the ejector rod and the ejector hole must be appropriate. If the gap is too large, metal can easily enter; if it is too small, it may cause binding. Experience suggests using D4/dC4 level matching.
6. Mechanisms for positioning, guiding and locking metal molds
When a metal mold is being assembled, precise positioning of the two halves is necessary. This is generally achieved in two ways: pin positioning and “stop” positioning. For vertical cuts with a circular cutting surface, “stop” positioning can be used, while pin positioning is mainly used for rectangular cutting surfaces.
The positioning pin must be located within the contour of the parting surface. When the metal mold itself is large and heavy, to ensure convenient positioning during opening and closing the mold, an orientation format can be adopted.
7. Selection of materials for metal molds
Based on the analysis of the causes of metal mold failure, the materials used in the manufacture of metal molds must meet the following requirements: good heat resistance and thermal conductivity; no deformation or damage when repeatedly heated; certain strength, toughness and wear resistance; good machinability.
Cast iron is the most commonly used material for metal molds. It has good machinability, is cheap and can be manufactured in general factories. Furthermore, it is heat and wear resistant, which makes it a suitable metal mold material. Carbon steel and low alloy steel are only used when high requirements are required.
The use of aluminum alloys in the manufacture of metal molds has attracted attention abroad. The surface of aluminum molds can undergo anodic oxidation treatment, resulting in an oxide film composed of Al2O3 and Al2O3·H2O.
This film has a high melting point and hardness and is resistant to heat and wear. It is reported that such aluminum metal molds, when using water cooling measures, can not only cast aluminum and copper parts, but also be used to cast ferrous metal castings.
