With development of science and technology, requirements for safety and aesthetic appearance of die-casting products are constantly increasing. Quality of parts is evaluated differently depending on use. Today, I will tell you how mold design affects quality of aluminum die castings. How to produce high-quality parts is of great significance to saving materials and energy, shortening manufacturing hours, and improving economic benefits.

 

There are many factors that affect quality of die-casting parts, such as type and quality of die-casting machine, rationality of geometric structure and technical requirements of die-casting parts, structure of mold and technical level of operator, etc.

Designers should first fully understand user's usage requirements and working conditions, as well as stress conditions of die castings, then select appropriate materials based on usage requirements and working environment, and understand die-casting properties of their materials. When designing, special attention should be paid to making structure of die-casting parts as simple as possible while meeting usage requirements. Wall thickness should be appropriate and uniform, and a necessary draft angle should be left. Otherwise, defects such as pits, pores, shrinkage porosity, undercasting, pull marks, cracks, deformation and other defects will occur on die castings.
Requirements for dimensional accuracy of die castings should be reasonable, otherwise it will cause unnecessary trouble in mold design, mold processing, formulation and management of process conditions, and result in a large number of substandard products.

Die-casting parts are made from molds. There is no doubt that design, processing, and selection of mold materials are closely related to product quality. If mold structure is unreasonable, it will be difficult to make product qualified no matter what measures are taken from process.
In addition, mold materials, mold processing accuracy, surface roughness, processing marks, heat treatment micro cracks, nitride layer thickness and improper mold assembly will all affect product quality and mold life.

 

When shrinkage of casting materials is generally given as an average percentage or as a percentage with a certain range of variation, average shrinkage of material is usually selected. For high-precision die-casting parts, special attention should be paid to shrinkage rate of material when designing mold. If necessary, a test mold can be made first. After obtaining required data on test mold, mold can be designed and manufactured for mass production.

Formulation and execution of die-casting process is related to quality of mold, die-casting equipment, and technical level of operators. Under conditions of existing domestic die-casting equipment, it is difficult to achieve stable, reliable and precise control of die-casting process parameters. Achieving basic control of die-casting process is a process of combining and using elements such as die-casting equipment, die-casting materials, and molds. Failure to strictly implement process and main parameters will cause shrinkage, deformation, under-casting, and substandard dimensions of die-casting parts.

Mold is main tool for die castings. Therefore, when designing mold, you should try your best to make the overall structure of mold and mold parts structurally reasonable, easy to manufacture, easy to use, safe and reliable. In order to prevent mold from deforming during die-casting, molten metal flows stably in mold, casting can be cooled evenly, and die-casting can be fully automatic without failure. In addition, appropriate mold materials must be reasonably selected based on production batch size, material conditions, etc.

From perspective of strength, mold parts are designed to be well integrated, strong and durable, and not easily damaged or deformed during use. However, if shape of die casting is complex and mold parts are also complex, it will be difficult to process mold and processing accuracy will not be high. If mold parts are made into combinations, processing will be greatly simplified, high processing accuracy can be easily obtained, and high-quality die castings can be obtained.

To determine number of cavities, equipment capabilities, difficulty of mold processing, production batch size, casting accuracy requirements, etc. must be considered. Especially for multi-cavity molds, due to difficulty of mold processing, large dimensional accuracy errors, and difficulty in achieving balanced flow channel configuration, performance of each cavity casting is inconsistent. When die castings require high precision and complex geometric shapes, it is best to use one mold and one cavity. Small castings are subject to availability.

 

Gating system is not only a channel for liquid metal to fill die-casting mold, but also regulates factors such as flow rate and pressure transmission of melt, exhaust conditions, and thermal stability of die-casting mold. Therefore, when designing gating system, structural characteristics, technical requirements, alloy types and characteristics of casting must be analyzed, type and characteristics of die-casting machine must also be considered, so that a reasonable gating system can be designed.

Mold should be equipped with an overflow groove and exhaust channel with sufficient overflow range, which is very important to ensure product quality. People often overlook phenomenon that overflow channel is blocked prematurely by incoming molten metal. A reasonable structure should be adopted so that molten metal flows into deeper part of overflow tank first to ensure that exhaust hole remains open for the longest time. In addition, overflow tank should be equipped with a ejector rod to remove metal from overflow tank.

Temperature of die-casting mold is an important factor affecting quality of casting. Improper mold temperature not only affects internal and external quality of die casting (such as defects such as pores, shrinkage holes, looseness, mucous membrane, coarse grains, etc.), but also affects dimensional accuracy of casting, and even deforms casting, causing cracks in die casting mold and forming network-like burrs on the surface of casting that are difficult to remove, affecting appearance quality of die-casting. Taking aluminum alloy as an example, alloy temperature is poured into mold at 670-710℃. In long-term production practice, it is concluded that optimal temperature of mold should be controlled at 40% of pouring temperature of mold. Temperature of aluminum alloy die-casting mold is 230-280℃. Mold temperature within this range is conducive to obtaining high-quality and high-yield castings.
Molds generally do not use gas or electric heating, but use preheating and cooling devices. These devices use oil as medium to preheat and cool mold as required.

When calculating size of die-cast parts, shrinkage rate of die-cast material selected must be realistic, otherwise products produced will be unqualified. If necessary, calculate dimensions of die casting after actual measurement with test mold. For high-precision products, even thermal expansion of mold die-casting part material, impact of storage and use environment on product's dimensional accuracy after die-casting must be taken into account.

Position of parting surface will affect mold processing, exhaust, product demoulding, etc. Usually, parting surface will leave a trace line on product, affecting surface quality and dimensional accuracy of product. Therefore, when designing position of parting surface, in addition to taking into account issues such as product demoulding, mold processing, and exhaust, parting surface position can be placed where product surface quality is not high or dimensional accuracy is not high.

 

Often due to unreasonable mold structure or improper selection of mold materials, mold will crack and deform during use, which will lead to unqualified products. For this reason, when designing mold, appropriate measures must be taken to ensure quality of product.
Usually, during die-casting, pressure inside mold is 70-100MPa. In order to prevent mold from deforming and dislocating, mold cavity must be sufficiently thick, plate and backing plate for installing core must be sufficiently thick. If necessary, support pads can be added under backing plate. Core and cavity must be installed reliably. Side roughness of core and mounting hole must be appropriate, and roughness cannot be too low. Through-hole core should be fixed on both sides to prevent product from having a thick wall on one side and a thin wall on the other. For core of blind hole on product, we should also find ways to balance stress on core from location, quantity of feed port and reinforcement of core.
For die-casting molds, strength of cavity and backing plate can be checked. Check strength and stiffness of cavity wall thickness, and check stiffness of backing plate. In addition to taking certain guarantee measures on mold structure, mold materials with small deformation and good strength must also be selected. In addition, there is a gap between mold guide pin and guide bush, or wear of guide pin and guide bush during use will affect quality of product. Especially for products with high dimensional accuracy, in order to ensure product accuracy, matching parts of movable and fixed mold cones can be set on parting surface, or 2-4 positioning rods can be set at appropriate places around mold cavity to position and strengthen mold to prevent misalignment of moving and fixed molds. This is more important for large-scale and mass-production molds.
Ejection system should be equipped with guide posts to prevent push rod from working unevenly and from being worn on one side, ensuring that product is evenly stressed and that product is not deformed during ejection.