In casting production, we often find castings with blisters, either on the surface or inside, which are light in weight and generally can be left untreated or repaired and repaired, which can lead to scrapping. The blisters often appear along with other defects, or the trachoma is often an intuitive phenomenon of other defects, such as sand washing, sand dropping, rat tailing, sand-sanding, and crusting of paint.

Definition and identification of trachoma

A hole in the interior or surface of a casting that engraves sand or sand, called a sand hole. According to the position of the blisters, it can be divided into surface blisters and internal blisters. For the blisters on the surface of the casting, it can be identified by inspection with the naked eye; for the blisters inside the casting, it should be inspected by ultrasonic or radiographic inspection.

In general, the appearance of the blisters and the slag inclusions is similar and sometimes confusing. To distinguish the difference between the two, it is necessary to analyze all aspects, to combine various factors to judge, sometimes to determine by gouging, processing or splitting. Eliminate the defects of sand holes and slag inclusions, and consider them comprehensively.

Cause Analysis

It can be seen from the figure that the defects of the sand hole are related to the processes, the model, the sand mixing, the core molding, the molding, the lower core combination, the pouring, and the like, and the relationship with the process design, the lower core combination and the casting is relatively large. When considering the solution of sand hole defects, we must start from these aspects and comprehensive analysis.

Specific reasons for analysis:

Due to the expansion of the sand or sand core, the design of the casting system is unreasonable and the pouring operation is improper, resulting in sand (core) cracking, type (core) sand falling off, resulting in sand washing, sand loss, rat tail and sand-sanding, cores falling off Sand forms a trachoma in the casting

Modeling, core mixing and sand mixing, unreasonable sand use

Due to improper operation of the molding, lower core and combined type, collapse, crushing, sanding, crushing sand or sand core occurs.

Before the box is assembled, the floating sand in the cavity is not purged before the mold is assembled.

After the mold is formed, it is dropped into the sand or sand by the pouring system or the riser.

Poor paint, or sand type, paint is not dry, the coating peels off during casting, forming a coating interlayer while causing paint crusting.

The cause of sand slag inclusion

If the pouring time is short, the amount of sanding is increased by the sanding of the mold. If the pouring time is long, the amount of sanding will increase due to the heat influence. In order to reduce the amount of blisters, it is important to adjust the sprue flow rate during the optimum pouring time.

For products of different sizes, different structures and different weights, it is necessary to tailor the casting time.

Preventing the occurrence of blisters and slag inclusion measures

1、Low temperature casting;

2、using a filter cake in the sprue portion;Low temperature casting;

3、setting the optimal runner flow rate and runner ratio;

4、Control the flow rate of the gate;
5、The runner section is rounded;

6、Select the appropriate product and the angle of the gate;

7、Optimizing the pouring riser plan;

8、Pay attention to the pouring temperature and speed to avoid the metal liquid flow rate is too fast and the impact force is too large.

 The causes of casting defects are manifold. In order to improve the quality of products, on the one hand, we must improve the quality of operators, and operate in accordance with the operating procedures to reduce the human factors that affect product quality; on the other hand, after problems arise, we must comprehensively consider the problems and find the reasons. Take countermeasures, improve in time, and reduce negative effects to a minimum.

Quality is the life of an enterprise. The negligence and contempt of any job will have different degrees of impact on the overall quality of the company. To ensure the intrinsic and external quality of the product is the integrity of the foundry enterprise and the future of the foundry enterprise.

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