Руководство по проектированию прецизионных деталей методом цинкового литья под давлением
замак 5, Литье под давлением с цинкомZinc die casting is a flexible production method which entails an injection of zinc alloy molten matter into a specially designed mold with high pressure in order to create complex and high-precision parts. Such an approach is especially appropriate for precision parts because zinc has high fluidity, a low melting point, and can be machined to tight tolerances without much post-processing. Zamak die casting is used in industries such as automotive, electronics, consumer goods, and medical devices for parts, including connectors, gears, housings, and complex mechanisms that require dimensional tolerances and longevity. Most zinc alloys are processed using hot-chamber machines, which enable quick cycle times and a steady quality. Accurate parts benefit from zinc’s ability to produce thin walls, complex geometries, and net-shape parts, reducing material waste and assembly time. Nevertheless, high success rates are achieved when specific design rules are followed, and material properties, shape, form, and processing parameters are considered. These rules help reduce defects, improve manufacturability, and enhance precision. This article identifies major guidelines supported by industry standards set by organizations to help engineers design zinc die-cast precision parts. Advantages of Zinc Die Casting of Precision Parts The use of zinc die casting offers several benefits, making it one of the best materials for precision applications. Selection Material: Zinc Alloys Selecting the correct zinc alloy is important for precision parts because it affects mechanical properties, castability, and tolerances. The Zamak die-casting series (2, 3, 5, 7) and the ZA series (8, 12, 27) have specific compositions and performance properties. The table below will summarize the kinds of zinc alloy used in the manufacture of precision parts: Types of Zinc Alloy Tensile strength (ksi/MPa) Elongation % Hardness (BHN) Density g/cm³ Yield strength (MPa) Melting point °C Zamak 2 52/359 7 100 6.6 283 379-390 Zamak 3 41/283 10 82 6.6 269 381-387 Zamak 5 48/328 7 91 6.6 283-269 380-386 Zamak 7 41/283 13 80 6.6 310-331 381-387 ZA 8 54/372 6-10 100-106 6.3 359-379 375-404 ZA-12 59/400 4-7 95-105 6.03 145 377-432 ZA-27 62/426 2.0-3.5 116-122 5.3 N/A 372-484 These properties can be used to ensure that zinc alloys meet precision specifications and that complex parts have high fluidity scores (1-2 on a 1-4 scale; 1 best). To be more exact, manufacturers would want to choose alloys such as Zamak 3 or ZA-8 due to their stability and ability to withstand tight tolerances. Key Design Guidelines Good zinc die casting is designed to facilitate flow and ejection while maintaining strength at minimal cost. Wall Thickness The wall thickness should be uniform to avoid porosity and distortion. For precision parts, keep it within +/-10% and use a minimum thickness of 0.040 inches (1 mm) for a good surface finish, and as fine as 0.020 inches (0.5 mm) in miniature models. Ratios of thick to thin sections should be kept to less than 3:1 to minimize the chance of porosity; preferably the inscribed sphere diameter ratios should not exceed 6:1. Thickening walls enhances flow but raises the cycle time and consumption. To distances away from the ingate, minimum thickness changes: below 0.5 mm in areas less than 50 mm, up to 2 mm at 200 mm. Different simulations can be used to check designs. In zinc casting, specify a minimum of 0.025 inches (0.635 mm) in selected locations, but 0.040 inches (1.016 mm) in best-as-cast locations. Draft Angles Draft angles help eject parts and prevent damage to the die. For zinc, the minimum drafts are 0.5 ° -1 on outside surfaces, 1- -1 on internal surfaces, and 0.1 ° -1 on round holes. Short features that use moving parts (die elements) can be printed in zero, but it is more expensive. For ribs, a 5-10 taper is used when it is not parallel to the shrinkage. Different standards give draft calculations: standard tolerances are 50 (draft angle approximately 1.9 degrees at a 1-inch depth) for inside walls and 100 for outside walls. There are also precision tolerances that permit 60 and 120 constant drafts. Fillets and Radii Fillets and radii should always be added since sharp edges lead to stress concentrations and die erosion. Minimum radius of inside fillet is 0.016 inches (0.4 mm), and outside 0.031 inches (0.8 mm). Radii of larger size (up to 0.063 inches or 1.6 mm) enhance flow and strength. Minimum radii should be used (0.060 inches (1.5 mm) for bosses and ribs). Radius at high stress should not be less than 1 mm; standards suggest ±0.08/±0.04 inches (±2/±1 mm) in fillets. This improves the part’s life and reduces cracking. Ribs and Bosses Ribs enhance reinforcement without adding bulk. Create shallow, rounded ribs (height-to-thickness ratio no more than 3:1) and evenly space them so they are not distorted. Adhesion ribs to walls with fillets to avoid porosity in the intersection. The height of bosses used to mount or thread must be equal to their diameter, and when the diameter is large, they must have ribs. Keep 0.25 inches (6.5 mm) distance between bosses. Tolerances of critical heights can be contained in ribs, such as in the precision designs of ±0.001 inches (±0.025 mm) ribs. Cored Holes and Threads Holes made in the core make it lighter and allow elements such as threads. Maximum depths are around 3 mm diameter, 9 mm blind or 24 mm through; up to 12 mm diameter with larger openings. At least 0.25 inches (6 mm) in diameter, L/D ratio not more than 4:1 in small holes. Parting Line Considerations It is best to place the parting line in the largest section possible to produce the least amount of flash and to allow easy trimming. Vertical or complicated lines should be avoided; die motion plane right angles are best. Tolerances on parting lines are the sum of linear ones, by projected area. Projected area Tolerance (+in) Upto 10 +0.0045 11-20 +0.005 21-50 +0.006 51-100 +0.009 Tolerances of Precision Parts Zinc die casting has even higher precision tolerances than standard, and sometimes 65% of those special controls are
