Mold sticking phenomenon in aluminum alloy die-casting process has always been a difficult problem for die-casting practitioners. Because of mold sticking, many casting defects will directly extend, such as casting quality problems such as product surface strain, lack of thickness, internal shrinkage and leakage caused by damage to surface dense layer.
Therefore, die-casting practitioners need to do a good job in all aspects of prevention from die-casting mold process design stage, mold surface treatment, production and maintenance, and die-casting production process control, in order to minimize and control occurrence of sticking. Following is a specific analysis from above aspects combined with actual cases.

When designing inner gate, aluminum alloy liquid should be prevented from entering mold cavity at a relatively large angle, so as to reduce direct flushing of cavity wall by alloy liquid. This kind of sticky castings will be scrapped due to casting quality inspection, which will lead to excessive wall thickness of castings and lack of thickness.
As shown in Figure 1 below, casting cavity is relatively deep. In order to facilitate mold filling, development process of casting cavity including fixed mold side is adopted. Fixed mold uses a false slider to ensure demoulding of follower mold, and runner can only be designed on fixed mold side.

 

Figure 1 shows improvement of previous sprue design process. Due to relatively large angle of ingate, runner is in the shape of direct flushing and ingate directly flushes cavity wall of mold, resulting in a serious sticking problem. Due to reason of mold sticking, mold has to be polished frequently, and protruding surface of fixed mold forms an undercut, which further aggravates occurrence of sticking and drawing problems. Figure 2 shows improved design process, runner angle is adjusted from 180° to 120°-130°, ingate surface is parallel to inner wall surface of cavity, then ingate feeding angle is adjusted between 60°-75° . After on-site die-casting production verification, this kind of mold sticking problem has been significantly improved.

 

For hot spots, islands in cavity, thick and large parts of casting, a cooling process should be added inside mold to eliminate mold sticking caused by excessive local temperature of mold. When temperature of mold becomes very high, causing aluminum alloy and mold to react, when compound on contact surface becomes a mixture of aluminum and iron, mold sticking phenomenon will occur. With increase of die-casting spraying cooling and heating cycles and injection molding times, sticking modulus also increases, accumulation on mold surface increases at the same time, which intensifies attenuation of thermal conductivity of hot spot of mold island, sticking becomes more and more serious. More serious results can cause casting to stick to mold and make it difficult to separate casting from mold. This type of sticky mold is different from sticky mold characteristics of Case 1. This kind of sticky mold can be observed with naked eye as if a layer of white powder-like coating is adhered to surface of mold cavity, which directly destroys surface dense layer of die-casting itself and leads to quality defects such as leaks in air-tightness test of casting.
Following is an engine oil pan integrated with installation position of oil filter. During solidification analysis in the early process design, it was found that there were thick and large heat joints in installation part of the oil filter, as shown in Figure 3. Combined with analysis of mold sticking problem that occurred in the early stage caused by similar isolated island heat festivals, part was changed from the overall model to mosaic structure design, and interior was cooled by "U"-shaped circulating water, as shown in Figure 4. It has been verified by production practice that this solution eliminates mold sticking problem caused by island hot spots in mold cavity, dense layer on the surface of casting is excellent, and it also solves quality problem of air tightness detection leakage here.

 

Sticky mold and drawing mold are particularly prominent on thin cores. Implementation of this case draws on high-pressure thin core point cooling process.

In current automated die-casting production process, robots and manipulators have replaced manual spraying. Due to structure of casting, there are inevitably some spraying dead spots inside mold cavity. To solve this kind of mold sticking problem caused by insufficient spraying and lubrication, in addition to designing profiling spraying module according to mold cavity on automatic spraying module, designing internal spraying structure inside mold can also prevent sticking problem of dead angle of spraying.

 

In Fig. 5, casting forming part of mating surface of slider and movable mold core is a spraying dead spot, which is a dead spot where effective spraying cannot be achieved by spraying manipulator and profiling spraying module. In order to solve this kind of mold sticking problem caused by insufficient spraying, when designing mold, two nozzle designs in mold were added. Nozzle is connected with pipeline of sprayer system to achieve effect of synchronous spraying with spraying manipulator, spraying angle and spraying distance are relatively good, which well solves sticking problem of spraying dead point.

Steel of mold core is usually H13 steel, steel is quenched and tempered to make steel hardness reach 46-50HRC. Under normal circumstances, mechanical properties of H13 steel can not only meet requirements for thermal fatigue resistance, but also have certain thermal stability. Use special materials with a higher melting point to coat surface of mold cavity to form a coating layer to avoid mold sticking, such as molybdenum-based alloys, which can be mixed with iron and can be bonded on the surface of mold where sticking occurs; it is also possible to use various materials to prevent mold sticking at sticking position to treat mold surface, such as metal tungsten; surface treatment of the mold by physical means (PVD) can effectively prevent the occurrence of mold sticking, such as Cr N+W, ON, (Ti Al)N and CrC.

 

Figure 6 is a mold cavity insert coated with metal tungsten by cold welding. Insert is located on the side of gate. Due to structural limitations, internal cooling cannot be achieved, due to high-speed and high-pressure erosion of melt, protective film formed by spraying and lubricating external release agent will be quickly washed away during injection filling, and mold is frequently stuck. After being coated with metal tungsten, surface is relatively rough, which can effectively lock release lubricant on the surface of insert molding and not be washed away by aluminum liquid, which can ensure that sticking problem within 2000 shots is well improved. In terms of mold maintenance, it is necessary to prepare several sets of inserts, which are designed as a quick-change structure. Once coating layer is washed out, inserts that have been recoated can be replaced quickly through maintenance in the shortest time.

If iron content in aluminum alloy is insufficient, it is easy to bond in the cavity of H13 material mold. Iron is a harmful element in aluminum alloy melt, so as iron content increases, mechanical properties decrease, especially impact toughness and plasticity decrease, tendency of thermal cracking increases, it also makes aluminum alloy appear hard spots, and processing performance deteriorates. However, iron can reduce tendency of aluminum alloy to stick to mold, which is convenient for die casting. Therefore, content of iron in die-casting aluminum alloy should be controlled within a certain range. Practice has proved that controlling content of iron in die-casting process at 0.7-1% has a certain effect on improving stickiness of mold.

Too high temperature of molten aluminum, too high gate speed, and too high injection specific pressure will also lead to mold sticking problems. Higher molten aluminum temperature leads to higher mold temperature, chemical reaction occurs between mold and molten metal, resulting in mold sticking.
A higher gate speed will make casting structure porous and mechanical properties will be significantly reduced. Therefore, for castings with high requirements on internal quality, mechanical properties and compactness, it is not suitable to use a large gate speed, and it is also used to prevent occurrence of sticking phenomenon. At the same time, according to performance requirements of casting, casting pressure, holding pressure and cooling time of mold should be reasonably calculated and set, so as to effectively feed shrinkage before molten metal starts to solidify.

High-quality die-casting release agent can reduce occurrence of sticking phenomenon through reasonable proportioning. At present, die-casting industry has not issued a unified standard for release agent, and international community, including the most authoritative North American Die Casting Association, has not published a unified inspection standard for application of release agent. Technicians should strictly check brand of release agent, that is, select suppliers with good reputation, strong financial and technical strength, high technical service level and stable quality. Once mold sticking problem is effectively prevented after selection and trial use, do not easily change brand of release agent.

In addition to measures to prevent mold sticking in above three aspects, in structural design of die castings, it should also fully consider that castings have a large enough casting slope and uniform wall thickness transition, and bosses should try to avoid forming thick islands form.

Die-casting sticking problem is result of many factors, it is difficult to completely avoid, but through innovation of die-casting mold process design, application of new technology of mold surface treatment, combination of production process control and reality, problem of sticking mold can be well prevented and controlling.