Key points of die casting mold design

Core Tip: The most widely used materials for die-casting molds are H13, SKD61, 8407, etc. However, due to the enlargement of die-casting parts, the required molds have also been developed to large-scale. In order to suppress the quenching deformation and cracks of the mold, it is necessary to develop a low toughness. Deformation rapid cooling technology improves the toughness level of the mold material.

1. Material of die-casting mold
The most widely used materials for die-casting dies are H13, SKD61, 8407, etc. However, due to the increase in the size of die-casting parts, the required molds have also been developed. In order to suppress the quenching deformation and cracks of the molds, low-deformation rapid cooling to improve toughness needs to be developed. Technology to improve the toughness of mold materials. Improving the production efficiency and reducing the cost of die casting requires high-cycle operation. Compared with the previous mold, high-cycle operation increases the heat load due to insufficient cooling, which causes the surface temperature of the mold to rise and cause thermal cracks to occur earlier. Due to the increase of internal cooling holes and closer to the cavity, the temperature difference between the internal cooling holes and the cavity surface will increase the tensile thermal stress load and reduce the rigidity, and it will easily cause stress corrosion cracking, which will connect water leakage to the cavity and reduce the mold. life. Therefore, it is necessary to develop a new material that can increase the life span more than SKD6l. DAC is similar to SKD61 and is widely used, but its flexibility is better than SKD61. The DAC-S developed in order to improve its toughness is 60% higher than that of DAC. In order to evaluate the toughness of die-casting mold materials, the North American Die-casting Association (NADCA) set the Charpy impact value of AISI H13 (equivalent to SKD61). In order to stabilize the mold life, higher grades of Prenium H13 and Superion were set. Charpy impact value of H13. DAC-P is equivalent to the former, and DAC-S is equivalent to the latter. The hot cracking countermeasures of die-casting molds must have high temperature strength. Generally speaking, toughness and strength have the opposite relationship. DAC-P and DAC-S have the same strength as DAC at room temperature and high temperature due to their high-definition purification and tissue densification. At room temperature of 20 degrees and high temperature of 600 degrees, it has 0.2% higher endurance than DAC. As mentioned above, it is necessary to have a mold material with higher grade than SKD61, greatly improving the life, high temperature strength and toughness. As high-performance die materials for die-casting dies: DACIO, DAC55, DAC45 and the latest development ZHD435. DACI0: It has superior strength and toughness equivalent to DAC in the 600 degree temperature region. It is suitable for precision die-casting parts with high precision and complex shapes for electrical appliances and communication equipment. DAC45: It has higher strength than other materials at 700 ° C and has the characteristics of resistance to softening, but it has poor toughness compared to DAC. Special attention should be paid to crushing fire cooling. Suitable for small and medium size die casting molds. DAC55; It is characterized by high toughness, which can increase the hardness at the beginning, is a material with balanced toughness and strength, and its service life is 1.4 times higher than SKD6l. Applicable to automobile cylinder block, cylinder head and drive system components. ZHD435. It has the same properties as DAC55 and can be used for large parts. It has higher high temperature strength than DAC55 and is more advantageous than DAC55 in thermal cracking and life.


Second, the machining of die casting mold
The processing of die-casting molds is an important aspect to ensure quality and reduce costs. CAD / CAM / CAE production systems, five-axis processing machines, the latest cutting tools and other processing molds are used to continuously improve the mold quality and production efficiency. Its movement has three aspects: three-dimensional model-based processing methods, cutting machining center machining instead of EDM, and hole processing automation. In terms of three-dimensional models, the three-dimensional models are mainly used in mold design, the simplification of NC digital compilation, and the construction of quality assurance system after processing. EDM is replaced by a machining center. The main reason is that electrode machining requires more work than cutting. Therefore, EDM is mainly because the mold has a deep groove shape like a rib. Now deep groove shapes may also be used. Cutting processing to solve, so that mold production efficiency is more improved. Automation of hole processing: The mold has a large variety of holes, which can be roughly divided into shallow holes with a depth of less than 100mm but accuracy requirements of H7 to H8 and deep holes of 100 to 600nun for cooling and deflation, but the accuracy There is no requirement for shallow holes. Hole processing takes up a large part of the time in mold processing. Coupled with the hardness of the mold material (HRC48 level), it is more difficult to make holes. The quality engineering evaluation technique used in the machining center is to find the best target based on the study of the hole machining conditions and the best data used in the machining center. As a result, the surface roughness is increased by 1/3, and the hole diameter is improved. The deviation is increased by 1/5, and the life of the cutting tool is increased by more than 5 times. At the same time, the machining of holes on curved surfaces and inclined surfaces is also automated.

Third, the surface treatment of die-casting mold
There are several methods such as diffusion treatment, coating, nitriding + coating. Although there are various trade names for the diffusion treatment method, mainly nitrogen, soft nitriding, sulfur soaking, oxidation, nitriding + oxidation treatment, such as N, C, S, 0 alone or composite diffusion, are representative nitrogen During the chemical treatment, a compound layer (white layer) is formed, which has good melt loss resistance, overburn resistance, and adhesion resistance, but has poor crack resistance. Conversely, if the compound-free layer is formed of only a diffusion layer, it has good thermal cracking resistance, but poor melt loss resistance, overburning resistance, and adhesion resistance. Recently, the diffusion treatment trend, oxidation treatment or nitridation + oxidation composite treatment is the mainstream, which is suitable for various surface treatment methods of die-casting molds.


1.Diffusion treatment

Nitriding, soft nitriding, sulphur nitriding, oxynitriding, oxynitriding + oxidation

1 Soft nitriding 2 Gas soft nitriding 3 Gas soft nitriding 4 Gas oxynitriding 5 Ion nitriding, plasma nitriding 6 Overexcited nitriding 7 INIX 8 High vacuum gas compounding 9 EH 10 Phosphate

2, coating

PVD CVD TD

TiN TiCH TiAIN CrN TiN TiCH TiC AL2O3 VC

3.Nitriding + coating

Nitriding + PVD low temperature + ID

TiN TiCH TiC TiAIN CrN Cr (CN)

4.Diffusion treatment + coating

PCVD

Diffusion treatment: N, C, B, 0 Coating: l-Si-B-C-N-O



Coatings include PVD, CVD and ID processes. The ceramic magnetic coating film formed by the hard film coating method has excellent resistance to dissolution loss, overheating resistance, and adhesion resistance, but thermal cracking is rarely improved. In such a hard film coating method, the CVD method and the ID process method are better in the adhesion of the film, but the processing requires a high temperature of 1000 degrees, and the mold is prone to deformation and size change. The PVD method can suppress mold deformation due to low temperature processing, but the film is inferior to the CVD method in terms of adhesiveness. Nitriding coating, the representative method of which is nitriding. A method in which only a nitrided diffusion layer is formed in a nitriding furnace, and various coatings of the mold are performed in another PVD device after taking out. Such nitriding + PVD has the highest surface layer film having resistance to dissolution loss, overburning resistance, and adhesion resistance, and the nitrogen diffusion layer has heat crack resistance. However, the film formed by the PVD method has a certain range of limits when it is applied to a mold with a complicated shape. (Nitriding + oxidation) composite treatment. The surface treatment of die-casting molds has a worldwide trend, from the past nitriding and sulfur soaking nitriding treatment to the application of oxidation treatment. Compared with iron nitrides formed by nitriding, iron oxides with high high temperature stability can be used in die-casting molds to exert their effects in terms of dissolution loss, overburning, and adhesion. (Nitriding + oxidation) compound processing can be continuously processed in one device, and the time can be greatly reduced. In addition, continuous oxidation treatment that can form a compound layer without a nitrided diffusion layer by processing in a controlled atmosphere can improve melt loss resistance, overburn resistance, adhesion resistance, and thermal cracking resistance. Plasma CVD method (PCVD). The raw materials are all gases. Because it is a film formed by plasma chemical reaction, it is a film with excellent low-temperature adhesion and denseness, which can cover complex products well. In addition, the PCVD method can easily produce a so-called composite process of diffusion hardening of a hard film such as nitridation in one device without breaking the vacuum in one process. Therefore, PCVD can be applied to the processing of die-casting molds for three-dimensional solid objects. PCVD of boron-containing gas systems creates new boron diffusion-treated (boronized) boron-containing films, such as: TiB, TiBN, TiBCN, TiBIBN, TiAIBCN0, TiAISiBCN0, BN, BCN and other hard films. For example, the TiAIBN film has a hardness of more than 4000 Hv, which is called an ultra-high-grade coating. Selecting a combination of different elements can achieve a high degree of wear resistance, oxidation resistance, mold release, and lubricity that were not available in the past, so that the performance of die casting molds that meet various requirements can be greatly improved. Examples of current applications include: magnesium alloy die-casting molds for mobile phone parts. In the past, mold release agents were used, which required welding and repair due to corrosion about 30,000 times, and the environment was polluted by the smoke emitted from the release agent. Edge burrs make it difficult for the product to maintain accuracy. PCVD nitrided diffusion hardened layer + TiN / TiAN / TiAISiCN0 multilayer film is applied to this mold without mold release at all, and can be used for 300,000 times of die casting, extending the life of the mold, improving the environment, and suppressing burrs. Generates and shortens the die-casting cycle and improves production efficiency. In the past, when molding machine parts, mold release agents were applied every time. The melt had poor fluidity, produced wrinkles, caused problems in product processing, and had a high reject rate. After the nitrided diffusion hardened layer + TiN / TiAN / TiAISiCN0 multilayer film is used for this mold, the amount of release agent is only less than 1/8 of the past, the fluidity is improved, no wrinkles appear, and beautiful products are processed. Reduced consumption and improved production efficiency. For the molding of automobile parts, a large amount of spray molding was used in the past with mold release agents, which were repaired by welding about 60,000 times, and there were many casting defects and a high defect rate. After the nitrided diffusion hardened layer + TiN / TiAN / TiAISiCN0 multilayer film is used, the release agent is reduced to less than 1/2, and the casting is increased to more than 120,000 times. Not only extends the life of the mold, but also suppresses casting defects, reduces consumption, and improves the production environment. Due to the combined effect of the method, heat crack resistance, overheat resistance, and adhesion resistance are improved. In aluminum die-casting, there are already die-casting molds that do not require mold release agents and are cast more than one million times.

Fourth, the life and heat treatment of die-casting molds
The reason for the low life of the die is related to many factors. Materials, processing, heat treatment, maintenance, repair and other complex factors will affect the life of the mold. During the casting process, the thermal fatigue of the mold is related to the toughness, ductility, and crack propagation speed of the mold steel, which are related to the characteristics of the material. The mechanical stress generated during the operation is also the cause. On the other hand, the dissolution phenomenon is also very different due to the reactivity and contact friction of the mold material and the molten metal. The reaction morphology is also different due to the different types of die-casting metals (AI, Mg, Zn, etc.). In addition, the stability and the improvement of the life of the mold during the operation are also greatly related to its maintenance and repair technology, such as the improvement of the electro-degradation layer. , Selection of welding conditions, adjustment of deformation, etc. The heat treatment of mold steel is a very important basic technology to effectively play the material's function and performance. There is a significant difference in the performance of heat treatment. The best steel does not have proper heat treatment. During the operation of the mold, there will be many failures such as thermal cracking, cracking, and reduction of corrosion resistance and wear resistance. In recent years, the vacuum steel pressurization heat treatment furnace has been used for the heat treatment of mold steel, mainly because it can improve the operating environment, Bright heat treatment of molds, high pressure during cooling, large heat treatment furnaces can be manufactured, etc. Vacuum and pressurized gas cooling methods, traditional water quenching, oil quenching, polymer quenching, salt bath quenching, etc. The same cooling rate can be achieved with less soft spots. In recent years, with the enlargement of the mold, development and improvement Steel type for large molds with quenching performance. The heat treatment of large molds has a large difference in the cooling rate between the surface layer and the center. Unbalanced heat treatment can cause problems such as deformation, scaling, and cracking. Therefore, it is well controlled during the cooling process. The difference between the internal and external temperature is very important. No matter how good the steel is, the unstable heat treatment is difficult to exert its characteristics. The North American Die Casting Association has formulated standards for the quality of die casting materials and heat treatment. The main items are: 1. Material composition 1. Hardness (annealing, quenching-tempering treatment) 3. Cleanliness of the material (determined by ASTM, A-681, Sec6) 4. Ultrasonic flaw detection test (internal defect inspection), impact test (flexible, (Determination of ductility) 6, crystal grain size 7, micro-segregation 8, annealed structure, etc. All countries, regions, suppliers, manufacturers, mold steel, mold processing, heat treatment, etc. can be uniformly recognized in all aspects, It is good for improving the quality and production of the die.