Although the finish of the fixed mold cavity is very smooth, the phenomenon of sticking to the fixed mold still occurs due to the deep cavity. Therefore, when designing the mold, it is necessary to comprehensively analyze the structure of the casting, be familiar with the operation process of the die-casting machine, understand the possibility of adjusting the die-casting machine and process parameters, master the filling characteristics under different conditions, and consider the mold processing method, drilling and fixing form, in order to design a practical mold that meets production requirements. As mentioned at the beginning, the filling time of the molten metal is extremely short, and the specific pressure and flow rate of the molten metal are very high, which is an extremely harsh working condition for the die-casting mold. In addition, the impact of the alternating stress of chilling and heating has a great impact on the service life of the mold. The service life of the mold usually refers to the number of molds (including the number of scraps in die-casting production) that are die-cast before they can no longer be repaired and scrapped due to natural damage caused by careful design and manufacturing under normal use and good maintenance.
In actual production, there are three main forms of mold failure: ① thermal fatigue cracking failure; ② fragmentation failure; ③ corrosion failure. There are many factors that cause mold failure, including external factors (such as casting temperature, whether the mold is preheated, the amount of water-based paint sprayed, whether the tonnage of the die-casting machine is matched, the die-casting pressure is too high, the gate speed is too fast, the cooling water is not opened synchronously with the die-casting production, the type of casting material and the level of Fe composition, the size and shape of the casting, the wall thickness, the type of coating, etc.). There are also internal factors (such as the metallurgical quality of the mold material itself, the forging process of the billet, the rationality of the mold structure design, the rationality of the pouring system design, the internal stress generated during the mold machine (electrical processing), the heat treatment process of the mold, including various matching accuracy and finish requirements, etc.). If the mold fails early, it is necessary to find out which internal or external factors are so as to improve it in the future. ① Failure of mold due to thermal fatigue cracking. During die casting, the mold is repeatedly subjected to the effects of cold and hot shocks, and the molding surface and its interior are deformed. The mutual involvement causes repeated cycles of thermal stress, which leads to damage to the organizational structure and loss of toughness, causing the appearance of microcracks, and continues to expand. Once the cracks expand, the molten metal liquid is squeezed in, and the repeated mechanical stress accelerates the expansion of the cracks. For this reason, on the one hand, the mold must be fully preheated at the beginning of die casting. In addition, during the die casting production process, the mold must be kept in a certain operating temperature range to avoid early cracking failure. At the same time, it is necessary to ensure that there are no problems with the internal factors before the mold is put into production and during manufacturing. In actual production, most mold failures are thermal fatigue cracking failures. ② Fragmentation failure. Under the action of the injection force, the mold will initiate cracks at the weakest point, especially when the marking marks or electrical machining marks on the molding surface of the mold are not polished, or the corners of the molding will first appear fine cracks. When there is a brittle phase or coarse grains at the grain boundary, it is easy to break. During brittle fracture, cracks expand very quickly, which is a very dangerous factor for the failure of mold fragmentation. For this reason, on the one hand, all scratches and electrical machining marks on the mold surface must be polished, even if it is in the pouring system. In addition, the mold material used is required to have high strength, good plasticity, good impact toughness and fracture toughness. ③ Melting failure, as mentioned earlier, commonly used die-casting alloys include zinc alloys, aluminum alloys, magnesium alloys and copper alloys, and pure aluminum die-casting. Zn, Al, and Mg are more active metal elements, which have good affinity with mold materials, especially Al is easy to bite the mold. When the mold hardness is higher, the corrosion resistance is better, and if there are soft spots on the molding surface, it is not conducive to corrosion resistance. However, in actual production, dissolution is only a local part of the mold. For example, the part directly washed by the gate (core, cavity) is prone to dissolution, and the aluminum alloy is prone to sticking to the mold at the soft hardness.
Formulating the correct die-casting process, the correct and skilled operation of the die-casting workers and the high-quality mold maintenance are crucial to improving production efficiency, ensuring the quality of die-casting parts, reducing the scrap rate, reducing mold failures and extending the life of the mold.
Formulating the correct die-casting process
The die-casting process is a reflection of the technical level of a die-casting factory. It can correctly combine the production factors such as the die-casting machine characteristics, the mold characteristics, the casting characteristics, and the die-casting alloy characteristics to produce die-casting products that meet customer requirements at the lowest cost. Therefore, it is necessary to attach importance to the selection and training of die-casting process engineers. The die-casting process engineer is the technical director of the die-casting production site. In addition to formulating the correct die-casting process and revising the die-casting process in a timely manner according to changes in production factors, he is also responsible for the training and improvement of mold installation and adjustment workers, die-casting operators, and mold maintenance workers.
(1) Determine the most reasonable production rate and specify the cycle time of each injection cycle. Too low a production rate is certainly not conducive to improving economic benefits. Too high a production rate often sacrifices the mold life and the casting qualification rate, and the economic benefits of the general account and detailed account may be worse.
(2) Determine the correct die-casting parameters. Under the premise of ensuring that the castings meet the customer's quality standards, the injection speed, injection pressure and alloy temperature should be kept to the lowest. This will help reduce the load on the machine and mold, reduce failures and increase the life. According to the isosceles triangle of the die-casting machine characteristics, mold characteristics, casting characteristics and die-cast aluminum alloy characteristics, determine the fast injection speed, injection pressure, boost pressure, slow injection stroke, fast injection stroke, punch follow-up distance, push-out stroke, holding time, push-out reset time, alloy temperature, mold temperature, etc.
(3) When using water-based paint, a strict and detailed spraying process must be formulated. Paint brand, paint and water ratio, spraying amount (or spraying time) and spraying sequence of each part of the mold, compressed air pressure, distance between the spray gun and the molding surface, spraying direction and the angle between the molding surface, etc.
(4) Determine the correct mold cooling plan based on the actual die-casting mold. The correct mold cooling plan has a great impact on production efficiency, casting quality and mold life. The plan should specify the cooling water opening method, start cooling after a few die castings, and open the cooling water valve to the specified opening several times after a few die castings. The cooling intensity of the spot cooling system should be adjusted on-site by the die-casting process engineer, and spraying should be used to achieve thermal balance of the mold.
(5) Different lubrication frequencies should be specified for different sliding parts, such as punches, guide pins, guide sleeves, core pulling mechanisms, push rods, reset rods, etc.
(6) Formulate die-casting operation procedures for each die-casting part, and train and supervise die-casting workers to operate according to the procedures.
(7) Determine the appropriate mold preventive maintenance cycle based on the complexity and age of the mold. The appropriate mold preventive maintenance cycle should be the number of die-casting molds that are about to fail but have not yet failed. It is not a recommended method to repair the mold when it has failed during use and cannot continue production.
(8) Determine the module stress relief cycle (generally once every 5,000 to 15,000 molds) and whether surface treatment is required based on the complexity, age and risk of mold sticking.
Notes
Common problems with molds in die casting production Notes:
1. Casting system, overflow system
(1) Requirements for the straight runner of the mold on the cold chamber horizontal die casting machine: ① The inner diameter size of the pressure chamber should be selected according to the required specific pressure and the filling degree of the pressure chamber. At the same time, the inner diameter deviation of the gate sleeve should be appropriately enlarged by a few wires compared with the inner diameter deviation of the pressure chamber, so as to avoid the problem of punch jamming or severe wear caused by the misalignment of the gate sleeve and the inner diameter of the pressure chamber, and the wall thickness of the gate sleeve should not be too thin. The length of the gate sleeve should generally be less than the delivery stroke of the injection punch so that the coating can be removed from the pressure chamber. ② The inner holes of the pressure chamber and the gate sleeve should be finely ground after heat treatment and then ground along the axial direction, and the surface roughness should be ≤Ra0.2μm. ③ The concave depth of the diverter and the cavity forming the coating is equal to the depth of the cross runner, and its diameter matches the inner diameter of the gate sleeve, with a 5° slope along the demolding direction. When using a coating-introduced straight runner, the filling degree of the pressure chamber can be improved by shortening the volume of the effective length of the pressure chamber.
(2) Requirements for the runner of the mold: ① The entrance of the cold horizontal mold runner should generally be located at more than 2/3 of the inner diameter of the upper part of the pressure chamber to prevent the molten metal in the pressure chamber from entering the runner too early under the action of gravity and starting to solidify in advance. ② The cross-sectional area of the runner should be gradually reduced from the straight runner to the gate. In order to expand the cross section, negative pressure will appear when the molten metal flows through, which is easy to inhale the gas on the parting surface and increase the vortex entrainment in the flow of the molten metal. Generally, the cross section at the outlet is 10-30% smaller than that at the inlet. ③ The runner should have a certain length and depth. The purpose of maintaining a certain length is to stabilize the flow and guide the flow. If the depth is not enough, the molten metal will cool down quickly. If the depth is too deep, the condensation will be too slow, which will affect the productivity and increase the amount of recycled materials. ④ The cross-sectional area of the runner should be larger than the cross-sectional area of the gate to ensure the speed of the molten metal entering the mold. The cross-sectional area of the main runner should be larger than that of each branch runner. ⑤ The bottom of the runner should be rounded on both sides to avoid early cracks, and the two sides can be inclined at about 5°. The surface roughness of the runner is ≤Ra0.4μm.
(3) Ingate: ① The parting surface should not be closed immediately after the molten metal enters the mold, and the overflow groove and the exhaust groove should not directly impact the core. The flow direction of the molten metal after entering the mold should be along the cast ribs and heat sinks as much as possible, and fill from the thick wall to the thin wall. ② When selecting the location of the ingate, the molten metal flow should be as short as possible. When using multiple ingates, it is necessary to prevent several strands of molten metal from merging and impacting each other after entering the mold, thereby causing defects such as vortex gas inclusion and oxidation inclusion. ③ The ingate of thin-walled parts should be appropriately smaller than that of thick parts to ensure the necessary filling speed. The setting of the ingate should be easy to cut and should not cause defects (eating meat) in the casting body.
(4) Overflow groove: ①: The overflow groove should be easy to remove from the casting and try not to damage the casting body. ② When opening the venting groove on the overflow groove, pay attention to the position of the overflow port to avoid blocking the venting groove too early and making the venting groove ineffective. ③ Several overflow ports should not be opened on the same overflow groove or a very wide and thick overflow port should not be opened to prevent the cold liquid, slag, gas, coating, etc. in the molten metal from returning to the cavity from the overflow groove, causing casting defects.
2. Casting fillet (including corner): The casting drawing often indicates the requirements of unfilled corners R2 and other requirements. When making the mold, we must not ignore the role of these unmarked fillets and must not make them clear or too small. Casting fillets can make the molten metal fill smoothly, discharge the gas in the cavity in sequence, reduce stress concentration, and extend the service life of the mold. (It is also not easy for the casting to crack at this place or have various defects due to improper filling).
3. Demolding slope: It is strictly forbidden to have artificial side concave in the demolding direction (often when the casting is stuck in the mold during the mold trial, and when it is handled by incorrect methods, such as drilling, hard chiseling, etc., it causes local concave).
4. Surface roughness: The molding part and the pouring system should be carefully polished as required, and should be polished along the demolding direction. Since the entire process of the molten metal entering the pouring system from the pressure chamber and filling the cavity takes only 0.01-0.2 seconds. In order to reduce the resistance of the molten metal flow and minimize the pressure loss, the surface finish of the flow needs to be high. At the same time, the heating and erosion conditions of the pouring system are relatively poor. The worse the finish, the easier it is to damage the mold.
5. The hardness of the mold molding part, aluminum alloy: about HRC46°; copper: about HRC38°; during processing, the mold should leave a margin for repair as much as possible, make the upper limit of the size, and avoid welding.
Technical requirements for die-casting mold assembly: 1. Requirements for the parallelism between the mold parting surface and the template plane. 2. Requirements for the verticality of the guide column, guide sleeve and template. 3. The plane of the movable and fixed mold inserts on the parting surface is 0.1-0.05mm higher than the movable and fixed mold sleeves. 4. The push plate and reset rod are flush with the parting surface. Generally, the push rod is recessed by 0.1mm or according to user requirements. 5. All movable parts on the mold are reliably movable without stringing. 6. The slider is reliably positioned, and the core is kept away from the casting when it is pulled out. The matching parts of the slider and the block after the mold is closed are more than 2/3. 7. The runner roughness is smooth and seamless. 8. The local gap of the insert parting surface is <0.05mm when the mold is closed. 9. The cooling water channel is unobstructed, and the import and export marks are marked. 10. The molding surface roughness Rs=0.04, without micro-scratches.
Die-casting molds have developed rapidly, and are second only to stamping molds and plastic molds in production output and quantity. Die-casting molds have accounted for about 8% of the total output of various molds in China.
Installation:
(1) The mold installation position meets the design requirements, and the distance between the mold expansion force center and the die-casting machine is minimized as much as possible, so that the die-casting machine bar is subjected to more uniform force.
(2) Regularly check whether the mold lifting eyebolts, screw holes and lifting equipment are intact to ensure the safety of personnel, equipment and molds during heavy lifting.
(3) Regularly check the force error of the die-casting machine bar and adjust it if necessary.
(4) Thoroughly wipe the machine installation surface and mold installation surface before installing the mold. Check whether the length of the ejector rod used is appropriate and whether all ejector rods are of equal length. The number of ejector rods used should be no less than four and placed in the specified ejector rod holes.
(5) The pressure plate and the pressure plate bolts should have sufficient strength and precision to avoid loosening during use. The number of pressure plates should be sufficient, and it is best to press on all four sides, with no less than two places on each side.
(6) Large molds should have mold brackets to avoid sinking, dislocation or falling of the mold during use.
(7) Molds with larger cores or molds that need to be reset may also need to install the movable and fixed molds separately.
(8) The cooling water pipes and installation should ensure sealing.
(9) Adjustment of the mold after installation. Adjust the mold tightness. Adjust the injection parameters: fast injection speed, injection pressure, boost pressure, slow injection stroke, fast injection stroke, punch follow-up distance, ejection stroke, ejection reset time, etc. After adjustment, put soft objects such as cotton in the injection chamber, simulate the entire injection process twice, and check whether the adjustment is appropriate.
(10) Adjust the mold to the appropriate distance between the moving and fixed molds, stop the machine, and put the mold preheater in. (11) Set the insulation furnace to the specified temperature and configure the scoop spoon of the specified capacity. (12) Confirm the integrity of the mold before production. For molds with neutrons, correctly connect the neutron oil pipe and control switch circuit, confirm that the metal of the conductive part is not exposed, and select the control program before operation. (13) For molds with reverse pull devices, the reverse pull rod must be installed, and the ejector must be retracted after ejection, otherwise the mold cavity will be damaged. (14) For double-opening oblique core-pulling molds, when opening the mold, the front half of the rear mold must be ejected first, otherwise the mold core will be damaged. (15) Molds with sliders on the top and left and right of the mold must be fixed with appropriate springs. (16) Molds with slider cores, core pulls, and complex structures that are easy to jam should be fully preheated before production (all parts of the mold cavity must be oiled before preheating the mold). (17) For molds with core orientation requirements or shared cavities, the correctness of the core must be confirmed. (18) Confirm that each cooling water channel is unobstructed.
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