Abstract: This paper presents factors affecting die casting quality from aspects such as die casting design, mold structure, machining accuracy, mold material selection, shrinkage rate of casting materials, formulation and execution of die casting processes. It summarizes relationship between die casting quality and mold design from aspects such as determination of number of cavities, gating system design, venting system design, mold temperature, determination of dimensions of formed parts, determination of parting surface position, prevention of mold deformation.
With development of science and technology, requirements for safety and aesthetic appearance of die casting products are constantly increasing. Evaluation of part quality varies depending on intended use. Specifically, if a part meets usage requirements in terms of mechanical properties, geometry, dimensional accuracy, shrinkage cavities, porosity, and roughness, it is a qualified product; if part is slightly inferior to quality required by drawing but still usable, it is a defective product. If it completely fails to meet usage requirements, part is scrap. Producing high-quality parts is of great significance for saving materials and energy, shortening manufacturing time, and improving economic efficiency.

 

Many factors affect quality of die castings, such as type and quality of die casting machine, rationality of die casting's geometric structure and technical requirements, mold structure, and operator's skill level.
1.1 Die Casting Design
Designers should first fully understand user's requirements and working conditions, stress on die casting, then select appropriate materials based on usage requirements and working environment, understanding material's die casting performance. During design process, special attention should be paid to keeping die casting structure as simple as possible while meeting usage requirements. Wall thickness should be appropriate and uniform, with necessary draft angles; otherwise, defects such as pits, porosity, shrinkage cavities, under-casting marks, cracks, and deformation may occur in die casting.
Dimensional accuracy requirements for die castings should be reasonable; otherwise, it will cause unnecessary trouble for mold design, mold processing, process conditions formulation and management, and result in a large number of defective products.
1.2 Selection of Mold Structure, Machining Accuracy, and Mold Material
Die castings are produced using molds, so design, machining accuracy, material selection of mold are undoubtedly closely related to product quality. An unreasonable mold structure makes it difficult to achieve product quality regardless of technological measures. Furthermore, mold material, machining accuracy, surface roughness, machining marks, micro-cracks from heat treatment, nitriding layer thickness, improper mold assembly all affect product quality and mold life.
1.3 Shrinkage Rate of Casting Materials
When shrinkage rate of casting materials is given as an average percentage or a percentage with a certain range of variation, average shrinkage rate is usually chosen. For high-precision die castings, special attention should be paid to material shrinkage rate when designing mold; if necessary, a test mold should be made first. After obtaining required data from test mold, mold for mass production can then be designed and manufactured. To calculate working dimensions of various parts of a die-casting part using different shrinkage rates, basic calculation formula is as follows:
Cavity dimension Y + δ = (YO + KYO - n△) + δ
Core dimension Y + δ = (YO + KYO - n△) + δ
Position distance dimension Y ± δ = (YO + KYO) ± δ
Where Y is calculated model dimension (mm), Yo is limit dimension (maximum or minimum) of this part of casting (mm), K is comprehensive calculated shrinkage rate, N is mold dressing system, △ is tolerance of the nominal dimension of die-casting part (mm), δ is mold manufacturing tolerance (mm).
1.4 Die-casting process formulation and execution
Formulation and execution of die-casting process are related to quality of mold and die-casting equipment, as well as technical level of operators. Under current conditions of die-casting equipment in China, it is still difficult to achieve stable, reliable, and precise control of die-casting process parameters. Achieving basic control over die-casting process involves combining and utilizing elements such as die-casting equipment, die-casting materials, and molds. Failure to strictly adhere to process and key parameters can lead to shrinkage porosity, deformation, under-casting, and dimensional inaccuracies in die-cast parts.

Molds are primary tool for die casting. Therefore, when designing molds, attention should be paid to ensuring a rational overall structure and structure of mold parts, facilitating manufacturing, ease of use, safety and reliability. Mold should not deform during die casting, molten metal should flow stably within mold, castings should cool evenly, and fully automated die casting should be possible without malfunctions. Furthermore, appropriate mold materials should be selected based on production volume and material characteristics.
2.1 A Rational Mold Structure and Mold Part Structure are Essential
From a strength perspective, designing mold parts as a single unit is preferable, as it is sturdy, durable, less prone to damage and deformation during use. However, if die-cast part has a complex shape, mold parts are also complex, it will make mold processing difficult and result in lower processing accuracy. If mold parts are made in a modular fashion, machining process is greatly simplified, high machining accuracy is easily achieved, and high-quality die castings can be obtained.
2.2 Determining Number of Cavities
Determining number of cavities requires considering equipment capacity, mold machining difficulty, production batch size, precision requirements of castings. Especially for multi-cavity molds, due to difficulty of mold machining, large dimensional accuracy errors, and difficulty in achieving a balanced runner configuration, performance of castings from different cavities will be inconsistent. For die castings requiring high precision and with complex geometries, it is best to use a single cavity per mold. For small castings, choice depends on specific circumstances.
2.3 Design of Gating System
Gating system is not only channel for liquid metal to fill die casting mold, but also plays a regulatory role in transmission of molten metal flow rate and pressure, as well as venting conditions and thermal stability of die casting mold. Therefore, designing gating system requires analyzing structural characteristics of casting, technical requirements, alloy type and its properties, also considering type and characteristics of die casting machine. Only in this way can a reasonable gating system be designed.
Currently, there is no unified calculation method for gating system. Design is largely based on experience, with trial molding and adjustments. Experience suggests following:
Gate dimensions are determined based on cross-sectional area of sprue, i.e., sprue cross-sectional area: runner cross-sectional area = 1:3-1:4. Sprue thickness: runner thickness = 1:5-1:8.
2.4 Venting System Design
Mold should have overflow channels and venting passages with sufficient overflow range, which is crucial for ensuring product quality. Phenomenon of overflow channels becoming prematurely blocked by incoming molten metal is often overlooked. Using structure shown in Figure 1 allows molten metal to flow into deeper part of overflow channel first, ensuring venting holes remain open for the longest possible time. In addition, overflow channel should have ejector pins to remove metal from overflow channel.
2.5 Mold Temperature
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 castings (such as defects like porosity, shrinkage cavities, looseness, film adhesion, and coarse grains), but also impacts dimensional accuracy and can even cause deformation. It can lead to mold cracking and formation of difficult-to-remove mesh-like burrs on casting surface, affecting appearance quality of die casting. Taking aluminum alloy as an example, alloy temperature is typically 670-710℃ when poured into mold. Long-term production practice has shown that optimal mold temperature should be controlled at 40% of pouring temperature, resulting in an aluminum alloy die casting mold temperature of 230-280℃. A mold temperature within this range is beneficial for obtaining high-quality and high-yield castings.
Molds generally do not use gas or electric heating, but rather preheating and cooling devices. These devices use oil as a medium for preheating and cooling mold, as required.
2.6 Determining Dimensions of Molded Parts
When calculating the dimensions of die casting parts, shrinkage rate of selected die casting material must be accurate; otherwise, produced products will be substandard. If necessary, dimensions of die casting should be calculated after actual measurement using a test mold. For high-precision products, thermal expansion of die-casting parts and impact of storage and usage environment on product's dimensional accuracy must be taken into account.
2.7 Determining Parting Surface Location
Location of the parting surface affects mold processing, venting, and product demolding. Parting surface usually leaves a mark line on product, affecting its surface quality and dimensional accuracy. Therefore, when designing parting surface location, in addition to considering product demolding, mold processing, and venting, parting surface can be placed in a location where product's surface quality requirements or dimensional accuracy requirements are not high.
2.8 Mold Deformation Prevention
Often, unreasonable mold structure or inappropriate mold material selection can cause cracks and deformation during use, leading to product defects. Therefore, appropriate measures must be taken during mold design to ensure product quality. Typically, during die casting, internal pressure of mold is 70-100 MPa. To prevent mold deformation and misalignment, cavity must be sufficiently thick, plates and backing plates for installing core must also be sufficiently thick. If necessary, additional supports can be added under backing plates. Core and cavity must be reliably installed, surface roughness of core and mounting hole sides must be appropriate. Roughness should not be too low, and cores with through holes should be fixed on both sides to prevent uneven wall thickness on one side of product. For cores with blind holes on product, measures should be taken to ensure balanced stress on core by considering location and quantity of feed inlet and reinforcement of core. For die-casting molds, strength of cavity and backing plate should be checked, strength and rigidity of cavity wall thickness should be checked. Rigidity of backing plate should also be checked. In addition to taking certain guarantee measures in mold structure, mold materials with low deformation and high strength must be selected. Furthermore, gaps between mold guide pillars and guide sleeves, or wear and tear from separation during use, will affect product quality. Especially for products with high dimensional accuracy, to ensure product accuracy, a moving and fixed mold conical surface mating part can be set on parting surface, or 24 positioning rods can be set at appropriate locations around cavity for positioning and reinforcement to prevent misalignment of moving and fixed molds. This is especially important for molds used in large-scale, high-volume production.
Ejection system should be equipped with guide pillars to prevent uneven operation of push rods and other components, thus ensuring uniform force on product and preventing deformation during ejection.

See Table 1 for common causes and troubleshooting methods for die-casting molds.
Table 1. Several Faults and Troubleshooting Methods for Die Casting Molds

Currently, mold design is mainly based on analogy, relying on experience, intuition, trial and error. In the future, experience should be accumulated in production, especially experience gained from mold design based on die-casting characteristics of materials. This experience should be input into computers and applied to mold design; molds should be classified and standardized; computer-aided design should be used for free-form surface design, flow design, temperature determination, and strength calculation. CAE analysis should be used to improve speed of mold design and make mold structure more rational.