Improve performance of die-casting molds and extend service life of die-casting molds

Pressure casting (die casting for short) has characteristics of high production efficiency, short production process, high casting finish and strength, small machining allowance, and saving of metal materials. China’s die-casting industry has developed rapidly in recent years, with total output increasing significantly, and it has become a veritable die-casting power. Molds, die-casting machines and die-casting materials are the three major elements of die-casting production. Only high-quality molds can produce high-quality castings stably and efficiently. Working environment of die-casting molds is very harsh: During die-casting production process, mold cavity is in direct contact with high-temperature, high-pressure, and high-speed molten metal, and is directly washed by molten metal. It is prone to wear, high-temperature oxidation, and various corrosions: high-efficiency production causes mold temperature to rise and fall dramatically periodically, and working surface is prone to thermal fatigue cracks: when metal is forced to deform, it rubs against surface of cavity, easily wearing out mold and reducing its hardness. Mold cost is high, production cycle is long, and repair is difficult. Service life is also particularly important. Therefore, research on factors that affect mold performance and service life is beneficial to improving casting quality and reducing economic losses caused by early scrapping of molds. Generally speaking, factors that affect performance and service life of die-casting molds include mold materials, mold design and manufacturing, surface treatment technology and specific usage of mold.

Die-casting molds are very expensive special precision machinery, which require mold maintenance workers not only to have superb skills and meticulous work style, but also to be serious and responsible. Mold maintenance workers should be familiar with technical standards of die-casting molds as follows:
1. It is all about cleaning metal cracks and scales everywhere in mold to reveal true color of mold.
2. Refer to the last die-cast product sent for repair together with mold to carefully check problems with mold. Is there any strain, sticking to mold, pressure or loss of flesh? Is there any small core that is bent or broken? Is there any movable core that is inserted incorrectly in position? Is there a broken push rod or a change in length of push rod? Is there any insert that is not positioned correctly? Is there fastening bolts are loose, etc. Depending on damage, repair or replacement is determined.
3. For cavity collapse, cracks, falling pieces, etc. that cause slight strain on casting, partial welding repairs can be performed. Welding repairs should be performed strictly according to welding repair process, otherwise a lot of mold life will be lost. Above failures of smaller molded parts are more serious or mold is damaged.
4. If molding surface of larger molded parts is seriously collapsed, cracked, or dropped, it can be repaired locally by welding. Welding repair should be performed strictly according to welding repair process, otherwise a lot of mold life will be lost. Above failures of smaller molded parts are more serious or mold is damaged.
5. Sliding parts such as core pulling mechanisms, guide devices, etc. should be thoroughly cleaned, carefully inspected, and repaired. Re-lubricate with high-temperature grease before assembly.
6. If there is hydraulic core pulling, hydraulic part and mold should be repaired at the same time. When repairing hydraulic part, pay special attention to cleanliness to prevent contamination, otherwise the entire hydraulic system of die-casting machine will be polluted.
7. When mold fails or is damaged during production process, repair plan should be determined according to specific situation. Perform comprehensive repairs as above if necessary.
8. After maintenance of mold has been completed, rust-proof molding surface, parting surface, and installation surface, close and set mold, and place it on backing plate according to installation direction of mold on machine. Mold accessories are placed with mold.

01 Mold material

Performance and service life of die-casting mold are closely related to material of mold. Good die-casting mold manufacturing materials generally have following characteristics: good machinability and forgeability; high wear resistance and corrosion resistance: high strength at high temperatures, high red hardness, high temperature oxidation resistance, impact toughness and tempering stability at high temperatures: good thermal conductivity and fatigue resistance; small thermal expansion coefficient: small heat treatment deformation rate and good hardenability.
In the past, 3Cr2W8V hot work die steel was commonly used in China, and life of die-casting mold was about 50,000 mold times. H13 hot work die steel was introduced in the 1990s, and die-casting molds produced have a service life of 150,000 to 200,000 molds. It is currently a widely used die-casting mold material. 3Cr2W8V hot work die steel has high strength and hardness, good cold and heat fatigue resistance, and good hardenability, but has poor toughness and plasticity, short service life, high alloying degree, and high cost. H13 has good comprehensive properties at medium temperature (~600℃), high hardenability (can be hardened in air), low heat treatment deformation rate, and its performance and service life are higher than 3Cr2W8V.
Material selection of die-casting mold should not only be based on temperature of casting metal and type of casting metal, but also impact and wear of various parts of die-casting mold by casting metal. The higher temperature, the higher thermal fatigue performance and high temperature performance material should have. Parts that are more severely worn should have higher hardness. Working conditions of die-casting molds are becoming increasingly demanding, and requirements for metallurgical quality, performance, and lifespan of mold materials are constantly increasing. In particular, requirements for material purity and isotropy are higher. Some high-alloy, high-quality, and optimized mold materials are constantly emerging. In turn, it also promotes development of die-casting industry.

02 Mold design and manufacturing

Reasonable mold design is an important prerequisite for extending service life of die-casting molds. Reasonable wall thickness and cooling water channel design can ensure strength and thermal balance of mold. When designing mold, special attention should be paid to the areas where stress concentration and greater abrasion occur during work. Accuracy of each selected part must be reasonable: if gap is too large, heat conduction will be poor, leading to thermal fatigue damage; if gap is too small, extrusion force and tensile stress will occur. Internal stress is easily generated during mold manufacturing process, and internal stress has a great impact on service life of mold. Therefore, in process of manufacturing and processing molds, internal stress should be avoided and eliminated in time. For example, after rough machining, timely stress relief and tempering can be performed, and electric pulses can be used instead of electric sparks to reduce surface tension of mold.

03 Mold surface treatment technology

Through rigorous and reasonable technical treatment of surface of die-casting mold, its performance and life can be greatly improved. Die-casting mold surface treatment technology can be roughly divided into three categories: traditional heat treatment process improvement technology; surface modification technology, such as surface laser treatment technology: coating technology.
(1) Improvement technology of traditional heat treatment process. Traditional die-casting mold heat treatment process is quenching-tempering. So-called improvement technology of traditional heat treatment process is to combine quenching-tempering with advanced surface treatment technology. Such as NQN (i.e. carbonitriding-quenching-carbonitriding composite strengthening), surface hardness of mold is higher, internal strength is increased, hardness gradient of casing layer is reasonable, tempering stability and corrosion resistance are improved, the overall performance and service life are greatly improved.
(2) Surface modification technology. Surface modification technology refers to use of physical or chemical methods to change properties of mold surface. Generally speaking, there are two types: surface heat, expansion and penetration technology and surface laser treatment technology. Surface heat, expansion and carburizing technologies include carburizing, nitriding, boronizing, carbonitriding, sulfur carbonitriding, etc. Carburizing helps strengthen surface hardness of mold. Carburizing process methods include solid powder carburizing, gas carburizing, vacuum carburizing, and ion carburizing. Vacuum carburizing and ion carburizing have fast carburizing speed, uniform carburizing layer, gentle carbon concentration gradient and small deformation of workpiece. Nitriding process is simple, and nitrided layer of mold has high hardness, good wear resistance, and good mold sticking resistance. Boriding improves surface properties most obviously, and mold hardness, wear resistance, corrosion resistance and adhesion resistance are significantly improved, but process conditions are harsh.
Laser treatment of mold surfaces is a technology that has emerged in the past thirty years. It improves surface performance of molds in two ways. One is to melt mold surface with laser and then combine it with carburizing, nitriding, plating and other processes. Another method is to combine laser treatment surface technology with some metal auxiliary materials with better physical properties to integrate them into surface of die-casting mold.
(3) Coating technology. Coating technology is to put a layer of protective clothing on mold by coating the surface, such as polytetrafluoroethylene composite plating. Main purpose is to enhance wear resistance, corrosion resistance and cold and heat resistance of mold.

04 Mold use

Choosing a reasonable die-casting process and maintenance are crucial to service life of mold. Most mold damage is caused by improper use and lack of scientific maintenance. First of all, special attention should be paid to temperature control of mold. Mold should be preheated before production and an appropriate temperature range should be maintained during production to prevent surface cracks or even cracking caused by excessive temperature gradients between inner and outer layers of cavity. Secondly, choose a high-quality die-casting release agent with a moderate thickness and evenly coat mold surface, which plays an important role in protecting mold material. Finally, in order to reduce accumulation of thermal stress and avoid cracking of die-casting mold, it is necessary to regularly use techniques such as tempering to eliminate thermal stress.

05 Mold material conclusion

Die-casting mold materials, mold design and manufacturing, mold surface treatment technology and mold usage comprehensively affect performance and service life of mold. Combining these factors and taking effective measures can effectively improve performance of die-casting molds and extend service life of die-casting molds.

Relationship between aluminum alloy die casting quality and mold design

With development of science and technology, requirements for safety and aesthetic appearance of die-casting products are constantly increasing. Quality of parts is evaluated differently depending on use. Specifically, if part meets usage requirements in terms of mechanical properties, geometric shape, dimensional accuracy, shrinkage cavities, pores, roughness, etc., it is a qualified product; if quality of a part is slightly worse than drawing requirements, but it can still be used, part is considered a defective product. If it completely fails to meet usage requirements, part is discarded. How to produce high-quality parts is of great significance to saving materials and energy, shortening manufacturing hours, and improving economic benefits.

1. Factors affecting quality of die castings

There are many factors that affect quality of die-casting parts, such as type and quality of die-casting machine, rationality of geometric structure and technical requirements of die-casting parts, structure of mold and technical level of operator, etc.

1.1 Design of die castings

Designers should first fully understand user’s usage requirements and working conditions, as well as stress conditions of die castings, then select appropriate materials based on usage requirements and working environment, understand die-casting properties of their materials. When designing, special attention should be paid to making die-casting structure as simple as possible while meeting usage requirements. Wall thickness should be appropriately uniform and have necessary draft angle. Otherwise, defects such as pits, pores, shrinkage and under-casting marks, cracks, deformation and other defects will occur on die-casting parts.

Requirements for dimensional accuracy of die castings should be reasonable, otherwise it will cause unnecessary trouble in mold design, mold processing, formulation and management of process conditions, and result in a large number of substandard products.

1.2 Mold structure, processing accuracy and mold material selection

Die-casting parts are made from molds. There is no doubt that design, processing accuracy, and selection of mold materials are closely related to product quality. If mold structure is unreasonable, it will be difficult to make product qualified no matter what measures are taken from process. In addition, mold materials, mold processing accuracy, surface roughness, processing marks, tiny cracks in heat treatment, nitride layer thickness, improper mold assembly will all affect product quality and mold life.

1.3 Shrinkage of casting materials

When shrinkage of casting materials is generally given as an average percentage or as a percentage with a certain range of variation, average shrinkage of material is usually selected. For high-precision die castings, special attention should be paid to shrinkage rate of materials when designing mold. If necessary, a test mold can be made first. After obtaining required data on test mold, design and manufacture of mold for mass production begins. Different shrinkage rates should be used to calculate working dimensions of each part of die casting. Basic calculation formula is:

Cavity size Y+δ=(YO+KYO-n△)+δ

Core size Y+δ=(YO+KYO-n△)+δ

Position distance size Y±δ=(YO+KYO)±δ

where Y——calculated model size, mm

Yo – limit size of this part of casting (maximum or minimum), mm

K——Comprehensive calculated shrinkage rate

N——Mold dressing system

△——Tolerance of nominal dimensions of die castings, mm

δ——Mold manufacturing tolerance, mm

1.4 Formulation and execution of die-casting process

Formulation and execution of die-casting process is related to quality of mold, die-casting equipment, and technical level of operators. Under conditions of existing domestic die-casting equipment, it is difficult to achieve stable, reliable and precise control of die-casting process parameters. Achieving basic control of die-casting process is a process of combining and using elements such as die-casting equipment, die-casting materials, and molds. Failure to strictly implement process and main parameters will cause shrinkage, deformation, under-casting, and substandard dimensions of die-casting parts.

2. Relationship between die casting quality and mold

Mold is main tool for die castings. Therefore, when designing mold, you should try your best to make the overall structure of mold and mold parts structurally reasonable, easy to manufacture, easy to use, safe and reliable. In order to prevent mold from deforming during die-casting, molten metal flows stably in mold, casting can be cooled evenly, and die-casting can be fully automatic without failure. In addition, appropriate mold materials should be selected reasonably based on production batch size, material conditions, etc.

2.1 Mold structure must be reasonable, and structure of mold parts must also be reasonable.

From perspective of strength, mold parts are designed to be well integrated, strong and durable, and not easily damaged or deformed during use. However, if shape of die casting is complex and mold parts are also complex, it will be difficult to process mold and processing accuracy will not be high. If mold parts are made into combinations, processing will be greatly simplified, high processing accuracy can be easily obtained, and high-quality die castings can be obtained.

2.2 Determination of number of cavities

To determine number of cavities, you must consider equipment capabilities, difficulty of mold processing, production batch size, precision requirements of castings, etc. Especially for multi-cavity molds, due to difficulty of mold processing and large dimensional accuracy errors, it is not easy to achieve a balanced runner configuration, and performance of each cavity casting is inconsistent. Die castings require high precision, and when geometry is complex, it is best to use one mold and one cavity. Small castings are subject to availability.

2.3 Design of gating system

Gating system is not only a channel for liquid metal to fill die-casting mold, but also regulates factors such as flow rate and pressure transmission of melt, exhaust conditions, and thermal stability of die-casting mold. Therefore, when designing gating system, structural characteristics, technical requirements, alloy types and characteristics of casting must be analyzed, type and characteristics of die-casting machine must also be considered, so that a reasonable gating system can be designed.

Currently there is no unified calculation method for sprue systems. Most of design and trial mold adjustments are based on experience. Experience is:

Size of sprue is determined based on cross-sectional area of inner gate, that is, cross-sectional area of inner gate: cross-sectional area of sprue = 1:3-1:4. Inner gate thickness: sprue thickness=1:5-1:8.

2.4 Exhaust system design

Mold should be equipped with an overflow groove and exhaust channel with sufficient overflow range, which is very important to ensure product quality. People often ignore phenomenon that overflow channel is blocked prematurely by incoming molten metal. Using structure shown in Figure 1, molten metal can flow into deeper part of overflow tank first, ensuring that exhaust hole remains open for the longest time. In addition, overflow tank should be equipped with a ejector rod to remove metal from overflow tank.

2.5 Mold temperature

Temperature of die-casting mold is an important factor affecting quality of casting. Improper mold temperature not only affects internal and external quality of die castings (such as defects such as pores, shrinkage cavities, looseness, mucous membranes, and coarse grains in castings), but also affects dimensional accuracy and even deformation of castings, causing cracks in die-casting mold and formation of network-like burrs on the surface of castings that are difficult to remove, affecting appearance quality of die-casting parts. Taking aluminum alloy as an example, alloy temperature is poured into mold at 670-710℃. It has been concluded from long-term production practice that optimal temperature of mold should be controlled at 40% of pouring temperature of mold. Temperature of aluminum alloy die-casting mold is 230 ~ 280℃. Mold temperature within this range is conducive to obtaining high-quality and high-yield castings.

Molds generally do not use gas or electric heating, but use preheating and cooling devices. These devices use oil as medium to preheat and cool mold as required.

2.6 Determination of size of molded parts

When calculating size of die-cast parts, shrinkage rate of die-cast material must be realistic, otherwise products produced will be unqualified. If necessary, calculate size of die casting after actual measurement on test mold. For high-precision products, it is even necessary to take into account thermal expansion of mold die-casting part material, impact of product’s storage and use environment on dimensional accuracy of product after die-casting.

2.7 Determining location of parting surface

Position of parting surface will affect mold processing, exhaust, product demoulding, etc. Usually parting surface will leave a trace line on product, affecting surface quality and dimensional accuracy of product. Therefore, when designing position of parting surface, in addition to taking into account issues such as product demoulding, mold processing, exhaust, etc., position of parting surface can be placed where product surface quality requirements are not high or dimensional accuracy is not high.

2.8 Mold cannot be deformed

Often due to unreasonable mold structure or improper selection of mold materials. It will cause cracks and deformation of mold during use, which will lead to unqualified products. For this reason, appropriate measures must be taken when designing mold to ensure quality of product. Usually during die-casting, pressure inside mold is 70-100MPa. In order to prevent mold from deforming and dislocating, cavity must be sufficiently thick, plate and backing plate for installing core must be sufficiently thick. If necessary, support pads can be added under backing plate. Core and cavity must be installed reliably, side roughness of core and mounting hole must be appropriate. Roughness cannot be too low, through-hole core should be fixed on both sides to prevent product from having thick walls on one side and thin walls on the other. For core of blind hole on product, we should also think of ways to balance stress on core from location, quantity and core reinforcement of inlet. For die-casting mold, strength of cavity and backing plate can be checked, strength and stiffness of cavity wall thickness can be checked, stiffness of backing plate can be checked. In addition to taking certain guarantee measures on mold structure, mold materials with small deformation and good strength must also be selected. In addition, quality of product will be affected if there is a gap between mold guide post and guide bushing or if guide post or guide bushing is stored and worn away from body during use. Especially for products with high dimensional accuracy, in order to ensure product accuracy, matching parts of movable and fixed mold cones can be set on parting surface or 24 positioning rods can be set at appropriate places around cavity for positioning and reinforcement to prevent misalignment of movable and fixed molds. This is more important for large-scale, mass-production molds.

Ejection system should be equipped with guide posts to prevent push rod from being unstable due to unilateral wear, ensuring that product is evenly stressed and that product is not deformed during ejection.

3. Common causes and troubleshooting methods of die-casting molds

Several faults and troubleshooting methods of die-casting molds

Common malfunctions	Cause	Suggestions for Improvement
Coarse cracks	1. Unreasonable design, sharp edges and corners2. Mold preheating is not good3. Poor heat treatment4. Surface hardness of cavity is too high and toughness is poor.5. Improper operation causes greater stress in mold	1. Modify design and add it to arc as much as possible2. Increase preheating temperature3. Reheat treatment4. Tempering treatment5. Operate according to correct operating procedures
Crack	1. Low mold temperature and insufficient preheating2. Surface hardness of cavity is low3. High stress on cavity surface4. Partial decarburization of the cavity	1. Increase preheating temperature2. Cavity quenching and nitriding improve hardness3. Tempering eliminates seat force4. Remove decarburization layer and then nitriding
Erosion	1. Surface hardness of cavity is low2. Surface decarburization3. High residual stress on cavity surface4. Injection speed is too fast5. Melting temperature is too high	1. Cavity quenching and nitriding improve hardness2. Remove decarburized layer and then nitriding3. Tempering to eliminate stress4. Reduce injection speed within process range5. Reduce material temperature within process range
Strain	1. Unreasonable design and mold materials2. Insufficient heat treatment hardness3. Cavity surface is rough4. Iron content of alloy liquid is greater than 0.6%5. Release agent is unqualified6. Injection speed is too fast	1. Improve design and mold materials2. Reheat treatment to improve hardness3. Finely polish cavity surface, and polishing texture direction is consistent with mold.4. Reduce iron content5. Re-select release agent6. Reduce injection speed within process range

See Table 1 for common causes and troubleshooting methods of die-casting molds.

4. Conclusion

At present, mold design basically adopts analogy method, relying on experience, intuition and heuristic methods. In the future, we should accumulate experience in production, especially mold design experience based on characteristics of material such as die-castability, input it into computer, then apply it to mold design; classify molds to standardize and standardize them; use computer to design, carry out free-form surface design, flow design, temperature determination, and strength calculation. Use electronic computers to perform CAE analysis to increase speed of mold design and make mold structure more reasonable.