Analysis on the Causes of Demagnetization of Magnetic Boxes After Use

Analysis on the Causes of Demagnetization of Magnetic Boxes After Use

With the vigorous development of domestic building industrialization, an increasing number of PC component factories have adopted magnetic boxes to fix side formworks. However, many component factories have reported that magnetic boxes exhibit obvious demagnetization after a period of use. The insufficient magnetic suction fails to fix the formworks stably, resulting in unqualified component quality, which makes many customers question the application effect of magnetic boxes. Meanwhile, magnetic boxes suffer from severe wear and tear and show no advantages over traditional screw fixing methods in terms of labor cost, which seriously restricts the application prospect of magnetic boxes.

Based on field investigations of numerous component factories and professional knowledge of magnetic materials and magnetic components, this paper analyzes the potential causes of demagnetization of magnetic boxes after use. It also proposes production processes to improve magnetic box quality and avoid magnetic attenuation, as well as maintenance suggestions for magnetic box operation, for industry reference. Comments and suggestions are warmly welcomed.

1. Operating Temperature

Magnetic boxes are generally made of sintered neodymium-iron-boron (NdFeB) permanent magnet materials. Sintered NdFeB materials have a wide range of applications, including ordinary magnetic suction parts, permanent magnet motors, voice coil motors, high-grade speaker horns, and mobile phone vibration motors. NdFeB materials with different performance grades can be selected according to diverse application scenarios.

As verified by relevant parameters, the maximum operating temperature of sintered NdFeB materials can reach 230°C, and even the lowest-grade materials have a maximum operating temperature of 80°C. For this reason, most conventional magnetic boxes adopt N or M-grade NdFeB materials. When the curing temperature in PC component production is lower than 80°C, it exerts no adverse impact on the magnetism of magnetic boxes. For operating environments with temperatures of 80°C or above, higher-grade NdFeB materials can be adopted for magnetic box production.

2. Uneven Magnetic Surface Leading to Poor Fitting with the Mold Table

The magnetic suction force weakens with the increase of the distance from the magnetic pole. In operation, the bottom suction surface of the magnetic block fits tightly against the steel mold table to achieve stable and firm fixation. Nevertheless, foreign matters such as concrete residues, iron filings and adhesive films attached to the bottom of the magnetic block will prevent close fitting with the mold table, leading to a significant drop in suction force.

Taking the company’s SX-1000B magnetic box as the research object, this paper studies the correlation between suction force and the gap between the magnetic block bottom and the mold table. In the test, 0 to 10 pieces of A4 paper (each 0.09mm thick) were placed between the magnetic block bottom and the mold table to measure the suction force under different gap conditions.

The test results show that the gap between the magnetic block bottom and the mold table significantly affects the suction performance of the magnetic box. Accordingly, standardized maintenance measures are recommended for customers as follows: Check the cleanliness and flatness of the magnetic block bottom and the mold table before use. Foreign matters on the magnetic box bottom can be removed with a stainless steel scraper (iron scrapers will be adsorbed by magnets and cannot work effectively). For stubborn attachments such as concrete residues, a grinder equipped with a steel wire polishing disc can be used for polishing and cleaning.

In addition to residual impurities, poor flatness of the magnetic surface may also result from defective production processes. The magnetic block is assembled with NdFeB magnets and iron strips connected by bolts. Uncontrolled dimensional accuracy and positioning of screw holes will cause uneven surface of assembled iron strips, which requires flattening treatment by a grinding machine.

During operation, the longitudinal screw penetrating the middle iron strip tensions the entire magnetic block. Excessive dimensional tolerance of screw holes and gaps between longitudinal and transverse connecting screws will cause dislocation of iron strips, resulting in overall unevenness of the magnetic surface and a substantial reduction in the working magnetic force of the magnetic block.

3. Damage to the Internal Magnet Body of the Magnetic Block

Sintered NdFeB materials are hard, brittle, fragile and corrosion-prone. Therefore, the magnets shall avoid external impact and damage to the surface coating. Broken magnets or damaged magnet bodies will directly lead to decreased suction of the magnetic block.

During the assembly of magnets and iron strips, the magnet surface is designed to be slightly lower than the plane of the iron strips to effectively avoid impact damage. Meanwhile, reliable protective layers are essential to prevent magnet body damage and corrosion. To better protect the magnet body, Saixin Company adopts an advanced process to equip each magnet with a stainless steel protective ring.