Aluminum die-casting molds are currently widely used molds. Due to the long production cycle, large investment, high manufacturing precision, the cost is relatively high. Therefore, it is hoped that aluminum die-casting molds have a longer service life. However, due to the influence of a series of factors such as materials, machining, use, etc., molds often fail prematurely are scrapped, causing great waste. The following is an analysis of the basic factors affecting the service life of aluminum die corresponding precautions through the selection, design, manufacturing, use of the mold.

1. Material

The influence of mold material on mold life is reflected in the three aspects of whether the mold material is selected correctly, whether the material is good whether the use is reasonable. Statistics show that due to improper material selection heat treatment, the early failure of the mold accounts for about 70%. Aluminum die-casting molds are hot work molds, their use conditions are extremely harsh. The melting point of aluminum is 580-740℃, the temperature of molten aluminum is controlled at 650-720℃ during die casting. When die-casting without preheating the mold, the surface temperature of the cavity rises room temperature to liquid temperature, the surface of the cavity will bear great tensile stress. When the mold is opened, the surface of the cavity bears great compressive stress. After thousands of times of die-casting, defects such as cracks occurred on the surface of the mold. Therefore, the requirements for aluminum die materials are relatively high. The commonly used aluminum die-casting mold material is H13 steel, which corresponds to the domestic grade 4Gr5MoV1Si. It is a widely used mold material belongs to the hot work mold steel with high thermal fatigue resistance, fracture toughness thermal stability. Before the production of die-casting molds, the materials must be inspected to prevent defective materials causing early scrapping of the mold waste of processing costs.

2. Structural design

The mold design manual describes in detail the problems that should be paid attention to in the design of aluminum die-casting molds. It is important to emphasize that the structural design of the mold should try to avoid sharp rounded corners excessive cross-sectional changes. The stress concentration caused by sharp rounded corners can be as high as 10 times the average stress. It is easy to cause early failure of the mold. In addition, pay attention to the deformation cracking of subsequent heat treatment due to unreasonable structural design. In order to prevent heat treatment deformation cracking, the cross-sectional size strives to be uniform symmetrical simple. Blind holes are opened as through holes as much as possible, process holes can be opened if necessary. Avoid geometric gaps in structural design. Including knife marks, included angles, grooves, holes cross-section mutations, etc., to reduce the occurrence of parts structure heat treatment defects.

3. Machining

Incorrect machining is easy to cause stress concentration, insufficient finish machining does completely remove the decarburized layer formed by rolling forging, which may lead to early failure of the material. In addition, in the process of processing molds, thicker templates cannot be superimposed to ensure their thickness. Because the steel plate is twice as thick the bending deformation is reduced by 85%, the lamination can only play a superposition. The bending deformation of the two boards with the same thickness as the single board is 4 times that of the single board. When processing cooling water channels, special attention should be paid to ensure concentricity in the two-sided processing. If the head corners are concentric with each other, the connected corners will crack during use. The surface of the cooling system should be smooth, preferably without machining marks. With the advancement of technology, it is necessary to pay attention to the use of advanced machining technology, which is conducive to the improvement of the machining accuracy of mold parts, thereby extending the service life of the mold.

4. Grinding EDM

Grinding time may cause local overheating of the metal surface, high surface residual stress structural changes, which may lead to grinding cracks. In addition, improper pretreatment of the original structure, carbide segregation, coarse grains, insufficient tempering can cause grinding cracks. Therefore, under the condition of ensuring the material, pay attention to selecting the applicable coolant to control the cooling of the grinding process. Control the grinding speed to reduce cracks.

Electrical discharge machining can form a bright white hardened layer of quenched martensite on the surface of the quenched tempered mold. The thickness of the hardened layer is determined by the current intensity frequency during processing. It is deeper during roughing shallower during finishing. The hardened layer will cause great stress on the mold surface. If the hardened layer is removed the stress is eliminated, the mold surface is prone to cracks, pitting cracking during use. Elimination of hardened layer stress relief can be through: ①Remove the hardened layer with oil stone grinding; ②Under the condition of reducing the hardness, the stress is lowered below the tempering temperature, which can greatly reduce the surface stress of the mold cavity.

5.heat treatment

Improper heat treatment is an important factor leading to early failure of the mold. The deformation of heat treatment is mainly caused by thermal stress structural stress. When the stress exceeds the yield strength, the material will undergo plastic deformation. When the stress exceeds the strength limit, it will cause the quenching of the part. Pay attention to the following points during heat treatment of aluminum die-casting molds

(1) When the forging is cooled to room temperature, spheroidizing annealing is required.

(2) After rough machining before finishing, add quenching tempering treatment. In order to prevent the hardness being too high causing difficulties in machining, the hardness should be limited to 25-32HRC. Before finishing, stress tempering should be arranged.

(3) Pay attention to the heating temperature holding time of the steel during quenching to prevent the austenite coarsening. When tempering, keep the heat at 20mm?h, the number of tempering is generally 3 times. When there is nitriding, the third tempering can be omitted.

(4) Pay attention to the decarburization carburization of the cavity surface during heat treatment.

(5) When nitriding, pay attention that there should be no oil stains on the nitriding surface. The cleaned surface is allowed to be touched directly by hand. Wear gloves to prevent the nitrided surface being stained with oil causing uneven nitrided layer.

(6) Between the two heat treatment processes, when the temperature of the previous process drops to the touch, the next process is carried out, it should be cooled to room temperature.

(7) Use static quenching method, salt furnace quenching, special fixture quenching other methods to reduce heat treatment distortion.

(8) Tracking the use of advanced heat treatment equipment processes, such as heating by vacuum furnace flowing particle furnace, can improve the surface finish of the mold, help control heat treatment deformation, extend the life of parts.

6. production operations

When determining the injection speed of aluminum die-casting molds, the speed should be too high. Too high speed will cause corrosion of the mold increase of deposits on the cavity core; but too low will easily cause defects in the casting. Therefore, the minimum injection speed for aluminum die-casting mold is 18m?s, the maximum injection speed should exceed 53m?s, the average injection speed is 43m?s. During the use of the mold, the casting process must be strictly controlled. Within the scope of the process permit, try to reduce the casting temperature injection speed of molten aluminum, increase the preheating temperature of the mold. The preheating temperature of the aluminum die-casting mold is increased 100-130°C to 180-200°C, the life of the mold can be greatly improved. At the same time, it avoids improper adjustment operation of the machine tool, resulting in early failure of the mold.

7.mold maintenance

1. Clean the mold cavity deposits in time. After the mold is used for a period of time, there will be deposits on the cavity core. These deposits are formed by the combination of release agent, coolant impurities a small amount of die-cast metal under high temperature pressure. Some deposits are very hard adhere firmly to the surface of the core cavity, making it difficult to remove. When removing deposits, grinding mechanical methods can be used to remove them. Take care to damage other surfaces cause dimensional changes.

2. Frequent maintenance to keep the mold in good working condition. After the new mold is tested, pay attention to stress relief tempering when the mold is cooled to room temperature. When the new mold is used to 1?6~1?8 of the design life, that is, the aluminum die-casting mold has 10,000 mold times, the mold cavity mold base should be tempered at 450-480℃, the cavity should be polished nitrided to eliminate internal stress slight cracks on the surface of the cavity. The same maintenance will be carried out every 12,000 to 15,000 mold times. When the mold is used for 50000 times, it can be maintained every 25000～30000 times. Through maintenance, the rate time of cracking caused by thermal stress can be significantly slowed down. Effectively improve mold life casting quality.

3. Repair mold defects in time. In the case of serious erosion cracking, the mold surface can be nitrided to improve the hardness wear resistance of the mold surface. When nitriding, pay attention to the bonding firmness of the substrate the nitriding layer. The thickness of the nitriding layer should exceed 0115mm. The too thick nitriding layer is easy to fall off at the parting surface sharp corners. For mold surface defects, welding can also be used to repair, pay attention to the composition of the welding rod the weldment during welding, the cleaning of the welding surface the drying of the welding rod, the trimming finishing of the cavity after welding, the stress relief tempering after welding.

There are many factors that affect the service life of aluminum die, involving many aspects. As a manufacturing industry, it is necessary to correctly design the mold structure, accurately manufacture the mold parts, formulate a reasonable heat treatment process, use the mold reasonably according to the process specification, repair it in time Maintenance can give full play to the performance of the mold material effectively improve the quality service life of the aluminum die.