Reasons for sticking die casting and preventive measures

201710250923309627

The sticky mould phenomenon in the process of die casting of aluminum alloy has always been a difficult problem for die casting workers. Because of the sticky mould, many casting defects will be directly extended, such as surface strain, lack of meat, internal shrinkage and leakage caused by the damage of the dense layer on the surface of the casting quality problems.

Therefore, die casting practitioners need to take full precautions from the process design stage of die casting die, mold surface treatment production and maintenance, die casting production process control and so on, in order to minimize and control the occurrence rate of sticky die. The following aspects from the above combined with the actual case for specific analysis.

1.Die casting mold process design

(1)Design of inner gate Angle

When designing the inner gate, the aluminum alloy liquid should not enter the mold cavity at a relatively large Angle, so as to reduce the direct flushing of the alloy liquid on the cavity wall. The castings with this kind of sticky mould will be discarded due to poor wall thickness and lack of meat after casting quality inspection.

(2)Local cooling design

Hot joints and islands in the cavity and most of the thick parts of the casting should be added in the mold to eliminate the sticky mold caused by the high local temperature of the mold. When mould temperature is high, the reaction of aluminium alloy and mould, and in the contact area of the compound into a mixture of iron, aluminum, sticky mold phenomenon will occur with the die casting coating cold heat circulation and increase injection mold time, viscous modulus also increases, the mold on the surface of the accumulated content increased at the same time, increase the attenuation performance of thermal conductivity of mold heat island section parts, mold is more and more serious. More severe results can make the casting stick to the die and make it difficult to separate the casting from the die. This kind of sticky mold is different from the sticky model characteristic of Case 1. It can be observed by naked eye that the surface image of the mold cavity is adhered to a layer of coating similar to white powder, which directly destroys the surface dense layer of the die casting itself and leads to quality defects such as leakage in the air tightness detection of the casting.

(3)Internal spray structure is designed for the mold

In the current die casting production process automation, robot, manipulator replaced manual spraying, due to the casting structure and form of the mold cavity internal inevitably exists some spray dead Angle, to solve such sticky mold problem caused by spraying lubrication does not reach the designated position, in addition to the automatic spraying module to copying spraying module is designed according to the mold cavity, By designing the inner spray structure in the mold, the sticking mold problem in the dead corner can also be prevented.

2.mold surface treatment

The steel of the mold core is usually H13 steel, and the steel is quenched and tempered to make the steel hardness reach 46-50HRC. Under normal circumstances, the mechanical properties of H13 steel can not only meet the requirements of heat resistance and fatigue, but also have a certain thermal stability. Special materials with higher melting point are used to coat the surface of the mold cavity to form a coating layer to avoid the occurrence of sticky die, such as molybdenum-based alloy, which can be mixed with iron and can be bonded to the position of the mold surface where the sticky die occurs; Can also be used in the sticky mold position to prevent the occurrence of a variety of materials to the mold surface treatment, such as metal tungsten; The surface treatment of the mold by physical means (PVD) can effectively prevent the occurrence of sticky molds, such as Cr N+W, ON, (Ti Al)N and CrC.

3. die casting production process control

(1) Fe content in aluminum alloy Aluminum alloy has a strong affinity with the iron in the mold

If there is not enough iron in the aluminum alloy, it is easy to bond in the cavity of the H13 material mold. Iron is a harmful element in aluminum alloy molten liquid, so with the increase of iron content, mechanical properties decline, especially impact toughness and plasticity decrease, hot cracking tendency increases, and will make aluminum alloy hard point, processing performance deteriorates. However, iron can reduce the tendency of aluminum alloy sticking to die, which is convenient for die casting. Therefore, the content of iron in die-casting aluminum alloy should be controlled in a certain range. Practice has proved that the iron content in the process of die-casting is controlled at 0.7-1%, which has a certain effect on the improvement of sticky die.

(2) Process parameters

Too high temperature of molten aluminum, too high gate velocity and too high injection specific pressure will also lead to mold sticking. Higher liquid aluminum temperature will be higher die temperature, die and liquid metal between the chemical reaction, resulting in mold adhesion.

The higher inner gate velocity will make the casting microstructure appear porous and the mechanical properties decrease obviously. Therefore, for castings with high requirements for internal quality, mechanical properties and compactness, it is not appropriate to choose large internal gate speed, but also to prevent the occurrence of sticky mould phenomenon. At the same time, according to the performance requirements of the casting, the casting pressure, pressure retention and cooling time of the mold are reasonably calculated to effectively feed the metal molten liquid before it begins to solidify.

(3) release agent

High quality die casting release agent can reduce the occurrence of mold sticking by reasonable proportion. At present, the die casting industry has not issued a unified standard for release agent, and the international including the most authoritative North American die casting Association has not published a unified inspection standard for the application of release agent. Technicians should strictly check the brand of release agent, that is, choose those suppliers with good reputation, strong capital and technical force, high technical service level and stable quality. Do not change the brand of release agent easily once it is effective to prevent sticky mould problems after trial selection.

(4)other

In addition to the above three measures to prevent the occurrence of sticky mold, the structural design of die casting should also fully consider that the casting has a large enough casting slope, and the transition of uniform wall thickness, and the boss should try to avoid the formation of thick large island shape.

4.endnotes

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