Die casting accuracy, surface roughness and machining allowance
The precision, surface roughness, and machining allowance of die-cast parts are important indicators of their quality, directly impacting the product’s assembly performance and performance. Precision encompasses dimensional, shape, and positional accuracy. Surface roughness reflects the microscopic unevenness of the casting surface, while machining allowance represents the thickness of the material reserved for subsequent machining. These three interrelated factors require comprehensive consideration within the design and production process to achieve a balance between product quality and production costs.
The precision of die-casting parts is influenced by a variety of factors, primarily mold accuracy, the die-casting process, and the properties of the casting material. Mold accuracy is fundamental to ensuring die-casting precision. The mold cavity size, positioning accuracy, and guiding accuracy directly determine the dimensional and positional accuracy of the casting. Therefore, mold manufacturing requires the use of high-precision machining equipment (such as CNC machining centers and wire-cutting machines). Cavity dimensional tolerances are controlled within ±0.01mm, and the clearance between locating pins and locating holes is ≤0.005mm. Die-casting process parameters significantly impact precision. Excessive injection speeds can cause flash in the casting, affecting dimensional accuracy. Excessive mold temperature fluctuations can cause uneven shrinkage in the casting, reducing shape accuracy. For example, for aluminum alloy die-castings, flatness errors can increase by 50% if mold temperature fluctuations exceed ±10°C. Regarding material properties, shrinkage rates vary among alloys, ranging from approximately 0.5-1% for zinc alloys, 1-1.5% for aluminum alloys, and 1.5-2% for copper alloys. During mold design, dimensional compensation based on shrinkage rates is necessary to ensure casting accuracy. The aluminum alloy motor end cover produced by a certain factory did not take the alloy shrinkage rate into consideration, resulting in a smaller bearing hole size. After adjusting the mold size (adding 1.2% shrinkage compensation), the dimensional accuracy met the requirements.
Surface roughness is a key indicator of die-casting surface quality, primarily determined by the mold surface quality, die-casting process, and alloy properties. The surface roughness of the mold cavity is directly replicated on the casting surface, so the mold cavity must be finely polished to a roughness of Ra ≤ 0.8μm to ensure a casting surface Ra ≤ 1.6μm. During the die-casting process, the molten metal filling speed and mold temperature significantly influence surface roughness. Too slow a filling speed can cause the molten metal to solidify prematurely on the cavity surface, resulting in a rough surface. Too low a mold temperature can cause a cold shut on the casting surface, increasing roughness. For example, when producing zinc alloy decorative parts, the mold temperature must be controlled between 150-180°C and the injection speed set at 6-8m/s to achieve a mirror-like finish of Ra ≤ 0.8μm. Regarding alloy properties, high-silicon aluminum alloys typically have a higher surface roughness than pure aluminum castings due to the presence of silicon particles. To reduce roughness, the silicon content can be reduced or surface treatments (such as anodizing) can be performed. A factory reduced the surface roughness of aluminum alloy die-castings from Ra3.2μm to Ra1.6μm by optimizing the mold polishing process (to achieve a mirror effect) and die-casting parameters, meeting the appearance requirements of parts.
The determination of machining allowance requires comprehensive consideration of the die-casting’s precision, surface finish, and subsequent machining requirements. A reasonable machining allowance can compensate for dimensional errors and surface defects while avoiding excessive allowances that lead to material waste and increased machining costs. The size of the machining allowance varies depending on the size of the casting. For small castings (<100mm), the allowance per side is typically 0.3-0.5mm; for medium-sized castings (100-300mm), it's 0.5-1mm; and for large castings (>300mm), it’s 1-2mm. For areas requiring high precision (such as bearing holes), the allowance should be increased appropriately to ensure the designed accuracy after machining. For example, a bearing hole with a single-side allowance of 0.8-1.2mm can achieve a dimensional tolerance within ±0.01mm after machining. Castings with poor surface roughness require increased machining allowance to remove surface defects (such as scale and pinholes). For example, a casting with a surface roughness of Ra3.2μm requires a machining allowance of ≥0.5mm to achieve a surface quality of Ra1.6μm. A factory optimized the machining allowance of aluminum alloy housing castings, reducing the allowance on non-mating surfaces from 1mm to 0.5mm and on mating surfaces from 1.5mm to 1mm. This increased material utilization by 10% and reduced machining time by 20%.
The matching of die-casting precision, surface roughness, and machining allowance is crucial to ensuring product quality. For castings with high precision and low surface roughness, machining allowance can be reduced or even eliminated (e.g., zinc alloy toy parts). For castings with low precision and high surface roughness, machining requires increased allowance. For example, the die-casting of automotive engine blocks requires high precision (dimensional tolerance ±0.05mm) and a surface roughness Ra ≤ 3.2μm. The machining allowance is set at 1-1.5mm, and after milling and grinding, it reaches Ra 0.8μm and a dimensional tolerance of ±0.01mm. For die-cast parts that require no machining (e.g., decorative parts), the die-casting process must be strictly controlled to ensure that the required precision and surface roughness are met directly, avoiding subsequent machining. An electronics company, by improving mold precision and optimizing the die-casting process, achieved a dimensional tolerance of ±0.03mm and a surface roughness of Ra 1.6μm for magnesium alloy mobile phone midframes. This allows for direct assembly without machining, increasing production efficiency by 30%.
Technical measures to improve die-casting precision, reduce surface roughness, and optimize machining allowances include mold optimization, process improvements, and the application of new materials. For molds, hot runner systems are used to minimize the impact of gate marks on surface quality, and wear-resistant materials (such as H13 hot-work die steel) are used to extend mold life and ensure long-term precision stability. For processes, vacuum die-casting is used to minimize the impact of porosity on surface quality, servo-controlled die-casting machines are used to improve shot accuracy, and mold temperature controllers are used to precisely control mold temperature. For new materials, high-purity alloys are used to minimize the impact of impurities on surface quality, and new release agents are developed to reduce surface defects in castings. After introducing vacuum die-casting technology, a die-casting plant reduced the porosity of its parts from 5% to 1%, improved surface roughness by 40%, reduced machining allowance by 0.3mm, and reduced overall costs by 15%. With the continuous advancement of precision die-casting technology, the precision and surface quality of die-castings will be further improved, machining allowances will be further reduced, and “near-net-shape” production will be achieved.
