The main factors affecting the dimensional accuracy of die castings
The dimensional accuracy of die-casting parts is a core quality indicator, directly impacting product assembly performance and operational reliability. During the die-casting production process, multiple factors contribute to the dimensional accuracy of die-casting parts. Understanding the mechanisms by which these factors work is crucial for achieving high-precision die-casting. First, the properties of the alloy material are intrinsic factors influencing dimensional accuracy. The shrinkage rates of different alloys vary significantly. For example, zinc alloys have low shrinkage rates and good stability, while aluminum alloys’ shrinkage rates are significantly affected by composition fluctuations. Improper control of the content of elements like silicon and magnesium in the alloy can lead to unstable shrinkage rates, which in turn can cause dimensional deviations in the die-casting. Furthermore, the alloy’s fluidity and solidification rate can affect dimensional accuracy. Alloys with poor fluidity are prone to underfilling, resulting in locally undersized parts. Uneven solidification rates can induce internal stresses, causing deformation of the die-casting after cooling and compromising dimensional stability.
Mold manufacturing accuracy is a key external factor determining the dimensional accuracy of die-castings. The mold cavity’s dimensional tolerance, surface roughness, and assembly accuracy directly reflect the die-casting. If the cavity dimensional machining error exceeds the design tolerance, the die-casting’s dimensional accuracy is impossible to achieve. The accuracy of the mold’s guide mechanism is equally important. If the clearance between the guide pin and guide sleeve is too large, the cavity will misalign during mold closing, resulting in flash or dimensional deviation in the die-casting. Furthermore, mold wear increases with each production batch, especially wear on areas like the ingrowth and parting surfaces. This can gradually increase the actual cavity size, causing the die-casting to be out of tolerance. Therefore, the selection of mold materials (such as heat-resistant and wear-resistant H13 steel) and surface treatment processes (such as nitriding) are crucial to maintaining dimensional accuracy during long-term production.
The setting of die-casting process parameters has a significant impact on the dimensional accuracy of die-cast parts. Pouring temperature is one of the core parameters. Excessively high temperatures will increase the liquid shrinkage and solidification shrinkage of the molten metal, resulting in smaller dimensions of the die-cast part after cooling. Excessively low temperatures will cause incomplete local molding due to insufficient filling speed, affecting dimensional accuracy. The uniformity of mold temperature cannot be ignored either. If the temperature difference between different parts of the mold is too large, the cooling speed of different areas of the die-cast part will be inconsistent, resulting in uneven shrinkage and causing warping. Die-casting pressure and holding time are also critical. Insufficient pressure or too short a holding time will result in insufficient shrinkage of the molten metal, resulting in shrinkage cavities and dimensional shrinkage. Excessive pressure may cause elastic deformation of the mold, and the die-cast part will change in size due to stress release after the mold is opened. Therefore, it is necessary to optimize the process parameters through trial molds and establish a corresponding relationship between temperature, pressure and dimensional accuracy.
The structural design of die-castings can also restrict dimensional accuracy. Uneven wall thickness is a common cause of dimensional deviation. Thick-walled areas cool slowly and shrink more, while thin-walled areas shrink less. This difference can easily cause die-castings to bend or twist. Due to their poor rigidity, slender die-castings are prone to elastic deformation due to friction during demolding, affecting straightness and perpendicularity. In addition, deep cavities and narrow gaps in complex molds can increase the difficulty of filling with molten metal. If venting is not smooth, localized gas entrapment can lead to insufficient mold dimensions. Therefore, during the product design phase, principles such as “equal wall thickness” and “fewer sharp angles” should be followed. Measures such as adding reinforcing ribs and optimizing draft angles can be used to reduce the negative impact of structural factors on dimensional accuracy.
The production environment and subsequent processing steps may also indirectly affect the dimensional accuracy of die-castings. Drastic changes in ambient temperature can cause die-castings to expand and contract due to heat and cold. This is especially true for precision parts that require extremely high dimensional accuracy, which need to be produced and tested in a constant temperature workshop (usually controlled at 20±2°C). The amount of release agent used and the uniformity of spraying are equally important. Excessive release agent evaporates at high temperatures to produce gas, which may cause bubbles to appear on the surface of the die-casting and accompanied by dimensional depressions; while uneven spraying will cause differences in the lubrication effect of different parts of the mold, resulting in uneven demolding force and dimensional deviations. Post-processing of die-castings, such as shot blasting and electroplating, may cause dimensional changes due to stress release or uneven coating thickness if the process parameters are not properly controlled. Therefore, it is necessary to establish a full-process quality control system, from raw material inspection, mold maintenance to environmental control, to comprehensively guarantee the dimensional accuracy and stability of die-castings.
