Die-casting mold is one of three major elements of die-casting production. A mold with a correct and reasonable structure is a prerequisite for smooth progress of die-casting production, and plays an important role in ensuring quality of castings. Due to characteristics of die-casting process, correct selection of each process parameter is decisive factor in obtaining high-quality castings, and is prerequisite for correct selection and adjustment of each process parameter. Mold design is essentially a comprehensive reflection of various factors that may occur in die-casting production. If mold design is reasonable, there will be fewer problems encountered in actual production and casting pass rate will be high. On the contrary, mold design is unreasonable. For example, when designing castings, wrapping force of moving and fixed molds is basically same, pouring system is mostly in fixed mold, and is produced on die-casting machine where punch cannot feed material after injection. Normal production cannot occur and castings always stick to fixed mold. Although finish of fixed mold cavity is very smooth, there is still a phenomenon of sticking to fixed mold due to deep cavity. Therefore, when designing mold, it is necessary to comprehensively analyze structure of casting, be familiar with operation process of die-casting machine, understand possibility of adjusting die-casting machine and process parameters, master filling characteristics under different circumstances, consider mold processing method, drilling and fixing forms before designing a mold that is realistic and meets production requirements.
As mentioned at the beginning, filling time of molten metal is extremely short, specific pressure and flow rate of molten metal are very high. This is extremely harsh working conditions for die-casting mold. Coupled with impact of alternating stress from cooling and heating, it has a great impact on service life of mold. Service life of mold usually refers to number of molds that can be die-cast (including number of scraps in die-casting production) before it can no longer be repaired and scrapped due to natural damage that occurs under normal use conditions and good maintenance through careful manufacturing.
In actual production, there are three main forms of mold failure:
① Thermal fatigue cracking damage and failure
② Fragmentation failure;
③ Dissolution failure.
There are many factors that lead to mold failure, including external factors (such as casting temperature, whether mold is preheated, amount of aqueous paint sprayed, whether tonnage of die-casting machine matches, die-casting pressure is too high, inner gate speed is too fast, cooling water is not opened in synchronization with die-casting production, type of casting material and level of Fe composition, size and shape of casting, wall thickness, coating type, etc.). There are also internal factors (such as metallurgical quality of mold material itself, forging process of blank, rationality of mold structural design, rationality of gating system design, internal stress generated during processing of mold machine (electrical machining), heat treatment process of mold, including various fit accuracy and smoothness requirements, etc.). If mold fails early, it is necessary to find out internal or external causes so that it can be improved in the future.
①Mold fails due to thermal fatigue cracking. During die-casting production, mold is repeatedly subjected to effects of cold and heat shock, causing deformation of molding surface and its interior. They are involved in each other and cause repeated cycles of thermal stress, resulting in damage to organizational structure and loss of toughness, triggering appearance of micro-cracks and continuing to expand. Once crack expands, molten metal squeezes in, and repeated mechanical stress accelerates expansion of crack. For this reason, on the one hand, mold must be fully preheated when die casting starts. In addition, during die-casting production process, mold must be maintained within a certain operating temperature range to avoid early cracking failure. At the same time, it is necessary to ensure that there are no internal problems before mold is put into production and during manufacturing. Because in actual production, most mold failures are thermal fatigue cracking failures.
②Fragmentation failure. Under action of injection force, mold will develop cracks at the weakest point, especially if scribing marks or electrical machining marks on molding surface have not been polished, or clear corners of molding will first appear fine cracks. When there is a brittle phase at grain boundary or grains are coarse, it is easy to break. In brittle fracture, crack expands very quickly, which is a very dangerous factor for mold to break and fail. For this reason, on the one hand, all scratches and electrical machining marks on mold surface must be polished, even if they are in pouring system, they must be polished. In addition, mold materials used are required to have high strength, good plasticity, impact toughness and fracture toughness.
③ Melting failure. As mentioned before, 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, and they have good affinity with mold materials.  In particular, Al is easy to bite mold. When hardness of mold is higher, corrosion resistance is better, but if there are soft spots on molding surface, corrosion resistance is unfavorable. However, in actual production, corrosion is only limited to local parts of mold. For example, corrosion is prone to occur in areas where gate is directly washed (core, cavity), and aluminum alloy mold sticking is prone to occur in softer hardness areas.
Points to note about common mold problems encountered in die-casting production:
1. Examples of pouring system and overflow system
(1) Requirements for mold sprues on cold chamber horizontal die casting machines:
① Size of inner diameter of pressure chamber should be selected based on required specific pressure and fullness of pressure chamber. At the same time, inner diameter deviation of gate sleeve should be appropriately larger than deviation of pressure inner diameter by a few wires, so as to avoid problem of stuck or serious wear of punch due to non-coaxiality of sprue sleeve and inner diameter of pressure chamber, and wall thickness of sprue sleeve should not be too thin. Length of sprue sleeve should generally be smaller than delivery lead of injection punch to facilitate paint to escape from pressure chamber.
② Inner holes of pressure chamber and gate sleeve should be finely ground after heat treatment, and then ground along axis. Surface roughness should be ≤ Ra0.2μm.
③ Depth of diverter and cavity forming paint is equal to depth of lateral runner, its diameter matches inner diameter of sprue sleeve, and there is a 5° slope along demoulding direction. When a coating-introduced sprue is used, filling degree of pressure chamber can be increased because effective length of pressure chamber is shortened.
(2) Requirements for mold runner
① Entrance of cold horizontal mold runner should generally be located above 2/3 of inner diameter of upper part of pressure chamber to prevent molten metal in pressure chamber from entering runner prematurely under action of gravity and starting to solidify in advance.
② Cross-sectional area of lateral runner should gradually decrease from sprue to inner gate. In order to enlarge cross-section, negative pressure will occur when molten metal flows through, which will easily inhale gas on parting surface and increase eddy current entrainment in flow of molten metal. Generally, exit cross-section is 10-30% smaller than entrance.
③Horizontal runner should have a certain length and depth. Purpose of maintaining a certain length is to stabilize flow and guide flow. If depth is not enough, molten metal will cool down quickly. If depth is too deep, condensation will be too slow, which will not only affect productivity but also increase amount of recycled material.
④Cross-sectional area of cross runner should be larger than cross-sectional area of inner gate to ensure speed of molten metal entering mold. Cross-sectional area of main runner should be larger than cross-sectional area of each branch runner.
⑤ Both sides of bottom of runner should be rounded to avoid early cracks, and the two sides can have a slope of about 5°. Surface roughness of the runner part is ≤ Ra0.4μm.
(3) Inner gate
① Parting surface should not be closed immediately after molten metal enters mold, overflow groove and exhaust groove should not impact core head-on. Flow direction of molten metal after entering mold should be as close as possible to cast ribs and heat sinks, filling from thick wall to thin wall, etc.
②When selecting location of inner gate, try to keep flow of molten metal as short as possible. When using multi-strand ingates, 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 entrainment and oxidation inclusions.
③ Inner gate of thin-walled parts should be appropriately smaller for thick parts to ensure necessary filling speed. Inner gate should be set up to facilitate removal without causing defects in casting body (eating meat).
(4) Overflow tank
① Overflow groove should be easy to remove from casting and try not to damage casting body.
② When opening an exhaust slot on overflow tank, attention must be paid to position of overflow opening to avoid premature blocking of exhaust slot, which will render exhaust slot ineffective.
③ Several overflow openings or a very wide and thick overflow opening should not be opened in same overflow tank to prevent cold liquid, slag, gas, paint, etc. in molten metal from returning to mold cavity from overflow tank , causing casting defects.
2. Casting fillets (including corners). Casting drawings often indicate requirements such as unspecified rounded corners R2. When making molds, we must not ignore role of these unspecified rounded corners. We must not make clear corners or too small rounded corners. Casting fillets can make filling of molten metal smooth, allow gas in cavity to be discharged sequentially, reduce stress concentration, and extend service life of mold. (Castings are also not prone to cracks or various defects due to uneven filling).
3. Draft angle. It is strictly forbidden to have any artificial side concavity in demoulding direction (often casting sticks in mold during mold trial, and when it is handled in incorrect ways, such as drilling, hard chiseling, etc., causing local concavity).
4. Surface roughness, molding parts and pouring system should be carefully polished as required, and polished along demoulding direction. Because the entire process of molten metal entering pouring system from pressure chamber and filling mold cavity only takes 0.01-0.2 seconds. In order to reduce resistance to flow of molten metal and minimize pressure loss, it is necessary to have a high smoothness on surface through which it flows. At the same time, heating and erosion conditions of gating system are relatively poor. The worse finish, the easier it is to damage mold.
5. Hardness of aluminum alloy in molding part: When processing, mold should try to leave a margin for repair, make upper limit of size, and avoid welding.
Technical requirements for die-casting mold assembly:
1. Requirements for parallelism between mold parting surface and mold plate plane.
2. Requirements for verticality of guide posts, guide sleeves and templates.
3. Plane of movable and fixed mold inserts on parting surface is 0.1-0. higher than movable and fixed mold cover plates.
4. Push plate and reset rod are flush with parting surface. Generally, push rod is recessed by 0.1mm or according to user requirements.
5. All movable parts on mold move reliably, without any sluggishness or pin movement.
6. Slider is positioned reliably, keeping a distance from casting when core is pulled out, more than 2/3 of matching area between slider and block is after mold is closed.
7. Sprue roughness is smooth and seamless.
8. When closing mold, local gap on parting surface of insert is <0.
9. Cooling water channel is smooth, entrance and exit signs are marked.
10. Molding surface roughness Rs=0.04, no minor damage.