How to extend the life of die casting dies

The service life of die-casting dies is closely related to the material, design, manufacture, and maintenance. In the actual application, we found, affect the die-casting dies life of the proportion of each element is rough as follows: mold design and manufacturing about 80%; including mold materials about 10% ~ 15%; mold use and maintenance and maintenance of about 5% ~ 10%. Take the following measures is to avoid die-casting mold failure, improve its life effective method.

1. Mold material

 

Die cavity material should be selected with the excellent overall performance of the hot work mold steel. Try to use finer grain, isotropic good, good heat treatment performance of the material as the mold cavity material. At present, the domestic common use SK61, DACS5, 8407, and other imported materials as mold cavity materials. 

Before the mold material is put into processing, the appearance inspection, metallographic inspection, and ultrasonic flaw inspection should be used to check the material to avoid using defective material, which will cause early scrap of the mold.

2. Mold design

 

(1) Pouring system design

The pouring system design is the core part of die casting mold design, especially the position and size of the inner sprue. When choosing the location of the inner sprue, try to avoid the frontal impact of the aluminum liquid on the core, in order to reduce the generation of erosion. In the case of ensuring that the casting is well-formed, increase the cross-sectional area of the inner sprue, so that the aluminum liquid enters the cavity in the form of laminar flow, thus reducing the speed of the inner sprue and reducing the impact of the aluminum liquid on the cavity.

(2)Forming parts design

As far as possible to avoid sharp corners, in order to prevent the corresponding parts of the mold due to stress concentration and cracking. The unavoidable vulnerable parts, such as the slender core, should be used in a reasonable patchwork structure, so as to facilitate timely replacement when damaged.

(3)Formwork design

Determine the appropriate thickness of the template, so that it has sufficient rigidity and strength to effectively resist bending deformation. In the template, especially the structure design of the moving mold template, the integral template instead of the traditional support plate and through the template, is conducive to improving the overall bearing capacity of the die casting mold.

(4)Design of guiding parts

 

First of all, each guide part should choose the correct tolerance fit and surface roughness level. Second, improve the structure of the guiding parts, such as with the slider with the side and bottom surface into easy to replace the guide slip, while the guide slip chooses to wear resistance and thermal expansion of better alloy materials.

(5) Easy to replace the design of maintenance

For the easily damaged core and sliding fit parts, design the structure to facilitate rapid replacement and maintenance. Such as the core from the step-type fixed structure to the screw-type connection structure.

(6) Maintain the thermal balance of the mold

Reasonable design of cooling system to ensure the overall thermal balance of the mold. If conditions permit, you can choose to configure the mold temperature balancing system.

(7)Apply CAD/ CAE technology

CAE simulation analysis is carried out to determine the distribution of temperature field, flow field, and stress field of the mold after the preliminary design has been completed. And on this basis, the mold design optimization, in order to improve the overall quality of mold design, so as to improve the die casting mold life to provide a strong guarantee.

3. Mold manufacturing

 

(1)Develop reasonable processing technology

The machining process of the mold is the link that directly affects the life of the mold and needs to pay attention to the details of the process. For example: choose the appropriate processing benchmark, it is best to unify the parts of the processing benchmark, in order to reduce the cumulative error; two-sided butt method of processing longer cooling water hole, in order to ensure the smooth surface of the water hole at the same time, should also pay attention to ensure its coaxial, to avoid stress concentration and cracking; comprehensive use of CAD / CAM technology, improve the accuracy of processing, etc.

(2)Electric discharge machining

EDM is widely used in mold manufacturing. After EDM, a hardened layer appears on the surface of the cavity. The thickness of the hardened layer is determined by the current intensity and frequency of machining, which is deeper for rough machining and shallower for finishing.2 No matter how deep or shallow the hardened layer is, it will produce large stress on the surface of the mold. Therefore, the hardened layer should be eliminated or stress relieved by high-frequency EDM and other technological measures.

(3) Grinding

The large amount of frictional heat generated by grinding may soften the surface of the mold and reduce its ability to resist thermal fatigue, thus causing early failure of the mold. Therefore, when grinding, the amount of coolant should be moderate, and avoid excessive grinding volume and feed to avoid fine cracks in grinding.

(4)Weld repair

Welding is a common means of repairing mold processing errors. When welding, the surface is thoroughly cleaned and dried, the mold is preheated to 440~460℃ together with the welding rod, and after the surface of the mold is at the same temperature as the heart, the welding is carried out under protective gas. After the repair is completed, temper once at a temperature 50~80 below the original tempering temperature to eliminate stress.

(5)Polishing

Focus on polishing the narrow seams and deep grooves in the cavity to ensure that the overall roughness of the cavity reaches 0.2um, so as to reduce the flow resistance of the cavity surface and improve its resistance to galling and fatigue.

4. Heat treatment of the mold

 

(1)Quenching and tempering

Vacuum quenching can effectively prevent the cavity surface decarburization and carbonization, reduce the early cracking of the mold caused by inadequate dissolution of carbide during heat treatment. After vacuum quenching, 2~4 times tempering is required. In general, the larger the mold, the higher the quenching temperature, and the lower the tempering temperature.

(2)Stress relief tempering

The new mold in the mold after passing the test should be tempered to eliminate stress.

(3)Nitriding

Nitriding treatment of cavity surface can improve the ability of mold surface to resist wear, adhesion, and thermal fatigue. After the new mold is qualified, the first nitriding is carried out. In the process of use, the timing of nitriding is determined according to the specific maintenance procedures and the condition of the mold. When nitriding, the hardness of the nitriding substrate should be between 35~43HRC, and the thickness of the nitriding layer should not exceed 0.15mm, and attention should be paid to the cleanliness of the nitriding surface to ensure the uniformity of the nitriding layer.  When the cracking of the mold is serious, nitriding is not suitable.

5. Use and maintenance of the mold

 

(1)Reasonable setting of die-casting process parameters

Under the premise of ensuring the quality of the castings, try to reduce the pouring temperature of the aluminum liquid, injection speed and injection pressure, shorten the high temperature holding time, especially to eliminate the practice of relying solely on increasing the injection pressure to improve the quality of the castings. In addition, you can use the parameter tester to seek the best combination between the parameters.

(2) Evenly preheat the mold

Avoid using the gas heating method to preheat the mold to prevent different thermal expansion of each part due to uneven preheating. It is better to preheat the mold with the oil temperature of the mold temperature machine. In the case of no mold temperature machine, can use the method of several molds before test pressing, rely on aluminum liquid preheat mold cavity.

(3)Spraying evenly

Focus on spraying the deep cavity of the mold, and at the same time, pay attention to avoid long-time spraying.

(4)Keep the heat balance of the mold

Regularly check and clean the cooling system to prevent the cooling water channel from being blocked, so as to ensure a stable mold temperature.

(5)Timely mold inspection

Pay attention to check whether there are cracks, crazing, and erosion on the mold surface. If these conditions appear, it is necessary to use the method of welding repair in time. If the nitriding layer on the surface of the mold cavity is partially damaged, the nitriding treatment should be carried out again.

(6) Periodic stress relief tempering

This is an effective method to retard cracking. According to the size and complexity of the mold to determine the time interval of stress-relieving tempering. In general, the aluminum alloy die casting mold in the first batch production 5000 ~ 8000 times, after every 10000 ~ 15000 times for stress relief treatment. However, if the mold cracking is serious, it is not suitable to carry out stress relief tempering, so as not to intensify the expansion of cracks.

(7)The mold maintenance when stopping

During the production, when stopping temporarily, you can reduce the cooling water flow and let the mold cool slowly. If it is stopped for a long time, the mold must be thoroughly cleaned and maintained and put into storage after passing the inspection.

(8) Operation training

Pay full attention to the operator's work training, to eliminate the mold damage caused by improper operation.

Conclusion:

Improving the life of aluminum alloy die-casting mold is a comprehensive technical issue. In the actual application, comprehensive consideration of the role of the influencing factors, and take targeted measures, it is possible to improve the service life of aluminum alloy die-casting dies.