How do explosion-proof electrical equipment address the causes of uneven wall thickness in castings and the solutions for it

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In production practice, the wall thickness of castings for explosion-proof electrical equipment is sometimes uneven, but such defects cannot be detected from the appearance of these castings. This poses a significant potential risk to the safe operation of explosion-proof electrical equipment. For example, during an explosion-proof inspection, it was found that the cover of the explosion-proof motor (made of gray cast iron) had been damaged by the explosion pressure ； The same fragment of the cast for explosion-proof electrical equipment showed a thickness of about 3 millimeters in the thinner parts and about 14 millimeters in the thicker parts. The sample appears normal on the outside, but it has serious internal defects, which represents a potentially severe quality issue for flameproof electrical equipment. 1. Reasons for uneven wall thickness in castings of explosion-proof electrical equipment. The main reason for uneven wall thickness in such castings is essentially an imperfect casting process, especially in sand casting. The following process factors may cause this defect: (1) Misalignment – severe misalignment occurs during the alignment step. In other words, during the molding process, the top mold, bottom mold, core, etc., are severely out of alignment. In the case of flame-proof electrical control cabinets, this situation often results in uneven thickness of the side walls; such unevenness makes it difficult to break them apart based on their sled-like appearance. The mechanical strength of the natural box will decrease, and in severe cases it will not be used. (2) The core is not properly fixed and/or is not securely fastened. When determining the pouring position, the large flat surface of the castings for explosion-proof electrical equipment should, in principle, be located at the bottom of the mold. In such a case, if the core is not properly fixed – for example, if it is tilted – the planar thickness of this explosion-proof electrical casting may become uneven, with some areas being thicker and others thinner. Another situation is that the core is not firmly fixed, which also affects the uniformity of the wall thickness ； Pouring molten metal at high temperature causes the core side to arch, resulting in thick and thin sections in the castings of explosion-proof electrical equipment. These defects are not visible to the naked eye. Core fixation is an important factor affecting the wall thickness uniformity of castings for explosion-proof electrical equipment. The effects of improper and/or weak fixation are sometimes independent and sometimes combined, and should not be underestimated in sand casting. 2. To prevent uneven wall thickness in the castings of explosion-proof electrical equipment, appropriate measures should be taken at the manufacturing stage to ensure that the dimensional requirements are met, especially for such equipment. (1) Controlling the error type value: Generally, the basic principle for error types is that their values must fall within the tolerance range; in other words, the error type value cannot be more than half of the tolerance value. This requirement essentially pertains to the dimensional tolerances of the positioning devices used in processing equipment, as well as the operational procedures involved. It can be controlled by appropriately selecting the dimensional tolerances of these positioning devices, and by regularly checking their dimensional accuracy and condition of use. It must also be noted here that incorrect type values are neither positive nor negative, and cannot be added to the tolerance values. (2) Proper and secure fixation of the core: Whether the core is properly and securely fixed depends on the support, positioning, and fixation of the core tip. The supporting function of the core tip ensures that the core remains in its proper position under the effect of the gravity and buoyancy of the poured molten metal. The positioning function of the core tip is to enable the core to be placed and fixed correctly within the sand mold, through the cooperation between the core tip and the core holder; therefore, it is very important to select the various parameters of the core tip appropriately. People should determine the shape and size of the core, as well as the clearance between it and the core holder, based on the type of sand used in the core (such as wet sand, dry sand, or self-hardening sand). (See JB/T 5106-1991 for basic dimensions of cores for explosion-proof electrical castings). For flameproof electrical equipment, it is advisable to choose a smaller value in order to ensure uniformity in the wall thickness of the castings of such equipment, thereby enabling them to withstand the impact of explosive pressures. Furthermore, in some cases, a core holder may be required to secure the core. As for other possible defects in the castings of explosion-proof positive-pressure cabinets, they pose little threat to explosion-proof electrical equipment, so they will not be discussed in detail here.