Die casting parts are parts formed by filling the mold cavity with liquid metal at a high speed under the high pressure of a die casting machine and solidifying under pressure. Due to the extremely fast filling speed of liquid metal, it is difficult to timely discharge the gas in the mold cavity. Therefore, there are often pores and oxide inclusions in die castings, which affect the quality of the castings. Reducing porosity defects in die-casting parts, especially for thick walled die-casting caster supports, is a major challenge in die-casting production. Due to its structure with multiple ribs, thick and uneven walls, die cast aluminum caster brackets are prone to defects such as porosity and shrinkage. Taking the aluminum die-casting caster bracket as an example, by analyzing the structure of the caster bracket, designing a reasonable pouring system to facilitate exhaust and shrinkage of the casting, mastering the key points of aluminum liquid melting process in production, and summarizing some production experience related to solving porosity problems in thick walled aluminum die-casting castings. At the beginning, traditional die-casting technology was used, but the results were not ideal, with a scrap rate of 50%. After analyzing the process of the caster bracket parts, a slow injection process was adopted, with a thickened inner gate of 4mm and an injection speed of 0.4m/s, reducing the scrap rate to 20%. Further improve the quality of the alloy, improve the exhaust method, and ultimately reduce the scrap rate to 1%.

Graphic and textual results

The caster bracket is an important component on the chassis of an electric wheelchair, as shown in Figure 1. The material is ADC10, and the bearing holes at the front end and the two pin holes at the tail end of the component are machined by CNC machining, with reserved machining allowance for the hole positions. Defects such as porosity and shrinkage are not allowed on the machined surface of the casting, and the surface of the casting is finally treated with powder coating. The external dimensions of the aluminum die-casting caster bracket are 260mm × 220mm, with an average wall thickness of 8mm. During the filling process, it is prone to air entrapment and the formation of porosity defects. The thickest part of the blank is located on the inner edge of the caster bracket near the bearing hole (see Figure 1), with a thickness of about 15mm, which is prone to shrinkage defects.

The technical points of using slow injection die casting process: 1. The cross-sectional area of the sprue and exhaust groove should be appropriately increased compared to normal die casting molds; 2. Adjust the slow injection rate so that it can inject slowly at the lowest possible speed while ensuring internal and external quality; 3. Find the transition point for pressurization, so that the metal liquid is immediately pressurized after filling the mold cavity, in order to ensure maximum gas discharge from the mold cavity and make the casting shrink under pressure; 4. Increase the alloy pouring temperature and mold temperature appropriately.

To reduce porosity defects in die-casting production, it is first necessary to choose a reasonable pouring system and control the quality of aluminum melting during the production process to ensure the effectiveness of aluminum refining, degassing, and slag removal. At the same time, a reasonable die-casting process should be adopted, and the injection speed should be adjusted in a timely manner according to the structural characteristics of the casting. The transition point between slow pressure, fast pressure, and pressure increase should be identified to ensure that the aluminum liquid can smoothly advance in the mold cavity without generating high-speed air entrapment. For thick walled castings, the reasonable use of slow injection die casting technology can effectively reduce porosity and improve casting quality. Setting up exhaust slots and overflow slots on the die-casting mold, and opening exhaust plugs in the deep cavity of the mold, using a inlay form can significantly increase the exhaust effect.

Author of this article:
Zhou Zhijun
This article is from the magazine "Special Casting and Nonferrous Alloys",
Strategic Partner of Die Casting Weekly