Die casting die is one of the three major elements of die casting production. A die with correct and reasonable structure is a prerequisite for the smooth progress of die casting production, and plays an important role in ensuring the quality of castings (off machine qualification rate). Due to the characteristics of die-casting process, the correct selection of process parameters is the decisive factor to obtain high-quality castings, and the mold is the premise to correctly select and adjust the process parameters. In essence, mold design is a comprehensive reflection of the possible factors in die-casting production. If the mold design is reasonable, the problems encountered in the actual production are less, and the qualified rate of the casting is high. On the contrary, the mold design is unreasonable. For example, the wrapping force of the dynamic fixed mold is basically the same during the design of the casting, while the gating system is mostly produced on the fixed mold and placed on the Guannan die casting machine where the punch can not feed after injection, so it can not be produced normally, and the casting has been stuck to the fixed mold. Although the finish of the mold cavity is very smooth, it still sticks to the mold due to the deep cavity. Therefore, in the mold design, it is necessary to comprehensively analyze the structure of the casting, be familiar with the operation process of the die casting machine, know the possibility of adjusting the pressure relief casting machine and process parameters, master the filling characteristics under different conditions, and consider the mold processing method, drilling and fixing form, so as to design a mold that is practical and meets the production requirements.
As mentioned at the beginning, the filling time of liquid metal is very short, and the specific pressure and flow rate of liquid metal are very high, which is extremely bad for the working conditions of the die-casting die. In addition, the impact of alternating stress of cooling and heating has a great impact on the service life of the die. The service life of the die usually refers to the die-casting module (including the number of scrap products in the die-casting production) before it can no longer be repaired and scrapped due to natural damage under normal use conditions and good maintenance through careful design and manufacturing.
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 die failure, including external factors (such as casting temperature, whether the die is preheated, the amount of water spray coating, whether the tonnage of the die-casting machine is matched, the die-casting pressure is too high, the inner gate speed is too fast, the cooling water is not opened synchronously with the die-casting production, the type of casting material and the content of Fe, casting size and shape, wall thickness, coating type, etc.). There are also internal causes (for example, the metallurgical quality of the material of the die itself, the forging process of the blank, the rationality of the die structure design, the rationality of the gating system design, the internal stress generated during the machining of the die machine (electric machining), the heat treatment process of the die, including various matching accuracy and cleanliness requirements, etc.). In case of early die failure, it is necessary to find out the internal or external causes for future improvement.
① During die casting production, the mold is repeatedly subjected to the action of chilling and heating, and the forming surface and its interior are deformed and involved in each other, resulting in repeated cyclic thermal stress, resulting in damage to the structure and loss of toughness, leading to the emergence and continuous expansion of microcracks. Once the crack is expanded, molten metal is squeezed in, and repeated mechanical stress accelerates the expansion of the crack. Therefore, on the one hand, the die must be fully preheated at the beginning of die casting. In addition, in the process of die-casting production, the mold must be kept in a certain working temperature range to avoid early cracking failure. At the same time, it is necessary to ensure that there are no problems with the internal causes before the mold is put into production and during manufacturing. In actual production, most of the die failure is thermal fatigue cracking failure.
② Fragmentation failure under the action of the injection force, the mold will initiate cracks at the weakest part, especially when the scribed marks or electric machining marks on the mold forming surface are not polished, or the fine cracks will appear at the clear corners of the molding first. When there is brittle phase at the grain boundary or the grain is coarse, it is easy to fracture. However, the crack propagates rapidly during brittle fracture, which is a very dangerous factor for the fracture failure of the die. For this reason, on the one hand, all scratches and electric machining marks on the mold surface must be polished, even if they are in the pouring system. In addition, the die materials used are required to have high strength, good plasticity, good impact toughness and fracture toughness.
③ As mentioned earlier, the commonly used die-casting alloys include zinc alloy, aluminum alloy, magnesium alloy and copper alloy, as well as pure aluminum die-casting. Zn, Al and Mg are relatively active metal elements, which have good affinity with mold materials, especially Al, which is easy to bite the mold. When the hardness of the die is high, the corrosion resistance is good, while if there are soft spots on the forming surface, the corrosion resistance is unfavorable. However, in actual production, corrosion is only a local part of the mold. In the case, the parts directly scoured by the gate (core and cavity) are prone to corrosion, and the parts with soft hardness are prone to die sticking of aluminum alloy.
Problems often encountered in die casting production and precautions:
1. Examples of gating system and overflow drainage system
(1) Requirements for mold sprue on cold chamber horizontal die casting machine:
① The inner diameter of the pressure chamber shall be selected according to the required specific pressure and the fullness of the pressure chamber. At the same time, the inner diameter deviation of the sprue sleeve shall be appropriately enlarged by a few wires compared with the inner diameter deviation of the pressure chamber, so as to avoid the punch jamming or serious wear caused by the different axis of the inner diameter of the sprue sleeve and the pressure chamber, and the wall thickness of the sprue sleeve shall not be too thin. The length of the sprue sleeve should generally be less than the delivery lead 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 sprue sleeve shall be finely ground after heat treatment, and then ground along the axis direction. The surface roughness shall be ≤ Ra0.2 μ m。
③ The concave depth of the diverter and the cavity forming the coating is equal to the depth of the runner, and its diameter is matched with the inner diameter of the sprue sleeve, with a 5 ° slope along the demoulding direction. When the coated guide sprue is used, the filling degree of the pressure chamber can be improved because the volume of the effective length of the pressure chamber is shortened.
(2) Requirements for mold runner
① The inlet of the transverse runner of the cold horizontal mold shall generally be located at the position with an inner diameter of more than 2/3 of the upper part of the pressure chamber, so as to prevent the metal liquid in the pressure chamber from entering the transverse runner prematurely under the action of gravity and starting to solidify in advance.
② The cross-sectional area of the runner shall be gradually reduced from the sprue to the inner gate. In order to expand the cross-sectional area, negative pressure will appear when the molten metal flows through, which is easy to suck in the gas on the parting surface and increase the eddy current air wrapping in the molten metal flow. Generally, the section at the outlet is 10-30% smaller than that at the inlet.
③ The runner shall have a certain length and depth. The purpose of maintaining a certain length is to stabilize the flow and guide. If the depth is not enough, the temperature of molten metal will drop quickly. If the depth is too deep, the condensation will be too slow, which will not only affect the productivity, but also increase the amount of returned material.
④ The cross-sectional area of the transverse runner shall be greater than that of the inner gate to ensure the speed of molten metal entering the mold. The cross-sectional area of the main runner shall be greater than that of each branch runner. ⑤ The two sides of the bottom of the runner shall be rounded to avoid early cracks. The two sides can be inclined by about 5 °. Surface roughness of runner ≤ Ra0.4 μ m。
① The parting surface shall not be closed immediately after the molten metal enters the mold, and the overflow groove and exhaust groove shall not impact the core in the front. The flow direction of molten metal into the mold shall follow the cast ribs and fins as far as possible, and be filled from the thick wall to the thin wall.
② When selecting the ingate position, the molten metal flow shall be as short as possible. When using multi strand ingate, it is necessary to prevent several strands of molten metal from converging and impacting each other after molding, resulting in defects such as eddy current aeration and oxidation inclusions.
③ The inner gate of the thin-walled parts shall be appropriately smaller to ensure the necessary filling speed. The setting of the inner gate shall be easy to cut off, and the casting body shall not be damaged (eat meat).
(4) Overflow tank
① The overflow tank shall be easy to remove from the casting and try not to damage the casting body.
② When setting an exhaust slot on the overflow slot, pay attention to the position of the overflow port to avoid blocking the exhaust slot too early and making the exhaust slot ineffective.
③ Several overflow ports or a very wide and thick overflow port shall not be set on the same overflow tank to prevent the cold liquid, slag, gas, paint, etc. in the metal liquid from returning to the mold cavity from the overflow tank and causing casting defects.
2. The casting drawing of casting fillet (including corner) often indicates requirements such as unmarked fillet R2. When opening the mold, we must not ignore the role of these unmarked fillets and must not make clean corners or too small fillets. The casting fillet can make the liquid metal fill smoothly, make the gas in the cavity discharge in sequence, reduce the stress concentration and prolong the service life of the mold. (the casting is not easy to crack or have various defects due to improper filling). For example, there are many clean corners on the standard oil pan mold. Relatively speaking, the brother oil pan mold is the best at present, and there are also many heavy machine oil pans.
3. The demoulding slope is strictly prohibited from man-made side concave in the demoulding direction (it is often that the casting is stuck in the mold during mold test, and local concave is caused by drilling, hard chisel, etc. when incorrect methods are used for treatment).
4. The surface roughness forming part and pouring system shall be polished carefully as required, and shall be polished along the demoulding direction. The whole process of molten metal entering the pouring system from the pressure chamber and filling the mold cavity takes only 0.01-0.2 seconds. In order to reduce the resistance of liquid metal flow and minimize the pressure loss, it is necessary to have a high surface finish. At the same time, the heating and erosion conditions of the gating system are bad. The worse the finish is, the easier it is to damage the mold.
5. Hardness of the mold forming part aluminum alloy: ° about copper: ° about processing, the mold should be left with repair allowance as far as possible, the upper limit of the size should be made, and welding should be avoided.
Technical requirements for die casting die assembly:
1. Requirements for parallelism between the parting surface of the mold and the plane of the formwork.
2. Requirements for perpendicularity of guide post, guide sleeve and formwork.
3. The movable and fixed die insert plane on the parting surface and the movable and fixed die sleeve plate are 0.1-0.
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 the user's requirements.
5. All moving parts on the die shall move reliably without stagnation and pin shall not move in series.
6. The sliding block shall be positioned reliably, the distance between the core and the casting shall be kept when the core is extracted, and the matching part between the sliding block and the block shall be more than 2/3.
7. The sprue roughness is smooth and seamless.
8. Local clearance of insert parting surface during mold closing < 0
9. The cooling water channel is unblocked, and the inlet and outlet signs.
10. The formed surface roughness rs=0.04, without minor damage.
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