Five precautions for using die-casting molds

When using die-casting molds, it is important to pay attention to the following aspects: the characteristics of die-casting mold usage, the temperature of the alloy melt, the operating temperature of the mold, the lubrication of the die-casting mold, and the adjustment content of the die-casting mold.
1. Characteristics of Die Casting Die Usage: In the die casting production process, the forming conditions for the components of the die casting die are extremely harsh. They are subjected to repeated mechanical wear, chemical corrosion, and thermal fatigue.
1) The molten metal enters the mold cavity under high pressure and high speed, causing intense friction and impact on the surface of the mold cavity, resulting in erosion and wear of the mold surface.
2) During the pouring process, it is inevitable for molten slag to be carried into the metal liquid, which exerts complex chemical effects on the surface of the molded parts. Compounds of aluminum and iron act like sharp wedges, accelerating the formation and development of cracks in the die-casting mold.
3) Thermal stress is the primary cause of cracks on the surface of molded parts. In the production process of each die-casting part, in addition to being subjected to high-speed and high-pressure erosion from the molten metal, the surface of the molded part also absorbs heat released during the solidification process, resulting in heat exchange. Furthermore, due to the thermal conductivity of the mold material, the surface layer of the molded part experiences a sharp increase in temperature, creating a significant temperature difference with the interior, which in turn generates internal stress. When the molten metal fills the mold cavity, the surface layer of the cavity first reaches high temperatures and expands, while the inner mold temperature remains lower, resulting in a smaller relative expansion and generating compressive stress on the surface layer. After the mold is opened, the surface of the cavity comes into contact with air and is chilled by compressed air and coating, generating tensile stress. This alternating stress increases as production continues, and when it exceeds the fatigue limit of the mold material, it causes plastic deformation and cracks on the surface layer of the mold.
To maintain the durability of the molding surface, it is required to possess thermal fatigue resistance, wear resistance, mold non-stickiness, and ease of part removal. Therefore, the forming parts are made of the currently well-applied 4Cr5MoSiV1 (H13) material.

II. Temperature of alloy melt During the production process of die-casting molds, in order to better fill all the recesses and deep areas of the die-casting mold and ensure the metal flows smoothly and melts together, the pouring temperature of the metal should be correctly selected when using the die-casting mold. The pouring temperature of alloy die-casting liquid is as follows: Material Name Die-casting Liquid Temperature/℃ Zinc Alloy 420-500 Aluminum Alloy 620-690 Magnesium Alloy 700-740 Copper-Zinc Alloy 850-960 Selection principles for die-casting alloy temperature: 1) The lower the temperature of the poured metal, the longer the life of the die-casting mold; 2) Using low-temperature die-casting can reduce the increase in the depth of the exhaust slot and the risk of metal liquid splashing; 3) Adopting low-temperature die-casting can reduce the chance of the die chamber and the ejector pin meshing tightly; 4) Adopting low-temperature die-casting can reduce the occurrence of shrinkage holes and cracks in the castings.
In summary, when process conditions permit, it is better to choose low-temperature die-casting for the temperature of the die-casting alloy.

III. Mold Operating Temperature

The operating temperature of die-casting molds varies depending on the die-casting alloy. Below are recommended values for several alloy molds for selection:

Mold Name Operating Temperature (℃)
Zinc alloy mold 150-180
Aluminum alloy mold 180-225
Magnesium alloy mold 200-250
Copper-zinc alloy mold 300

Principles for selecting the operating temperature of die-casting molds:

1) If the mold temperature is too low, the internal structure of the casting will be loose, making it difficult for air to be expelled and hindering molding.
2) If the mold temperature is too high, the internal structure of the casting will be dense, but the casting is prone to "welding" to the mold cavity, making it difficult to demold. Additionally, excessively high temperatures can cause the mold body itself to expand, affecting the dimensional accuracy of the casting.
3) The mold temperature should be selected within an appropriate range, and generally, after being tested to be suitable, constant temperature control is preferred.

IV. Lubrication of Die Casting Molds 1. Purpose of Lubrication: Lubrication serves as a parting agent between the die casting mold and the die-casting parts, facilitating the removal of the die-casting mold; it also acts as a lubricant for the moving parts of the die-casting mold and press, reducing friction and enhancing the service life of the die-casting mold. Furthermore, it can serve as a coolant for the die-casting mold, reducing thermal fatigue due to long-term operation and extending the mold's lifespan.
2. Requirements for lubricants: The selection of lubricants should meet the following needs: a. They must not cause die-casting parts to adhere in the mold cavity; b. They must not corrode the steel material of the mold surface; c. They must not produce toxic gases; d. They must not generate ash or slag when heated; e. After lubrication, they should adhere evenly to the mold cavity and working surfaces and not be washed away by high-pressure metal.
3. Preparation of lubricant: a. For total loss system oil, use 85%-90% + graphite 10%-15%; b. For heavy oil, use 100%; c. For paraffin, use 30% + yellow wax 30% + Vaseline 14% + graphite 26%; d. For graphite, use 25% + glycerin 20% + water glass 5% + water 50%. 4. Precautions for using lubricant: a. Lubricant can be used on the surface of the mold cavity and movable parts; b. The amount of lubricant sprayed should be small and evenly distributed each time, and it is best to form a thin film on the mold surface after spraying.

V. Adjustment of Die-Casting Molds After the die-casting mold is completed, it needs to undergo mold testing for adjustment. Only by selecting the correct die-casting conditions and process parameters can stable die-casting be achieved and qualified castings be produced.
Before conducting a mold test, the test personnel should conduct a preliminary inspection of the alloy raw materials used for die-casting, understand the characteristics of the alloy materials and the die-casting properties; they should also be familiar with the structure of the mold, the performance of the die-casting machine, the die-casting conditions, die-casting processes, and operation methods.
Correctly selecting die-casting forming conditions is crucial for mold adjustment. Often, even if the mold design and manufacturing are impeccable, improper selection of die-casting forming conditions can still prevent the production of qualified castings. Conversely, in some cases, adjusting the die-casting forming conditions can help overcome the deficiencies of the mold and produce qualified castings. Therefore, mold testers must be familiar with the effects and interrelationships of various die-casting forming conditions, as well as the operating principles of the mold, in order to correctly select and reasonably adjust these conditions.
The adjustment of die-casting forming conditions includes: material melting temperature, mold temperature during injection, and melt temperature; determination of injection pressure, clamping force, and mold opening force of the die-casting machine, as well as the required injection ratio and injection speed based on the part conditions. Finally, the die-casting molded product needs to be trimmed to obtain a perfect die-casting part.