Technical requirements for the performance and quality of magnesium oxide powder

Different heating elements require vastly different quality and technical specifications for magnesium oxide powder under varying operating environments and technical parameters. For example, two heating elements with the same load capacity, one for dry-burning and the other for water heating, would be unreasonable to impose identical quality requirements on the magnesium oxide powder used in both cases. This also reflects the technical expertise of the heating element manufacturer. Therefore, heating element manufacturers first need to categorize their products. For example, they need to differentiate between various types of heating elements, such as dry-burning, water heating, air conditioning, fryer, copper, aluminum, and iron heating elements, based on their surface operating temperature range and the type of magnesium oxide powder used. The basic requirement is that the magnesium oxide powder must meet the technical requirements for electric heating and relevant safety regulations. Currently, magnesium oxide powder is generally categorized into high-temperature magnesium oxide powder, medium-low temperature magnesium oxide powder, and modified magnesium oxide powder. Of course, different requirements may exist, and heating element manufacturers can communicate directly with magnesium oxide powder manufacturers to procure and produce according to specific requirements.

When selecting magnesium oxide powder, we need to focus on several key aspects:

① Mesh Size: Mixtures with different mesh sizes have completely different flow rates, directly affecting the powder density.

a. Definition of Mesh Size: Mesh size refers to the particle size or fineness of a material. Generally, it refers to the number of openings in a 1-inch x 1-inch sieve that magnesium powder particles can pass through. For example, 80 mesh means that the magnesium powder particles can pass through a 1-inch x 1-inch sieve with 80 openings. Similarly, a larger mesh size indicates finer magnesium powder particles, and vice versa.

Adding a plus or minus sign before the mesh number indicates whether the particles can pass through that mesh size. A negative number indicates that the particles can pass through the mesh size, meaning the magnesium powder particle size is smaller than the mesh size; a positive number indicates that the particles cannot pass through the mesh size, meaning the magnesium powder particle size is larger than the mesh size.

b. Differentiation of Magnesium Oxide Powder Mesh Size and Optimal Proportion:

Coarse Powder: 40-100 mesh 40%

Medium Powder: 100-200 mesh 40%

Fine Powder: 200-325 mesh 20%

c. Powder Density: Different powder densities affect the elongation and resistance of the tube. Therefore, each batch of magnesium powder needs to be tested for confirmation. Generally, the compacted density after tapping should be ≥2.3 g/cm³, and the compacted density after tube shrinking should be ≥3.05 g/cm³.

d. Influence of Mesh Size on Heating Tubes: Too high a mesh size results in small particles that easily absorb moisture, affecting the lifespan of the heating tube and causing serious pollution to the powder-adding environment. Too low a mesh size results in larger particles, which will damage the surface of the heating wire after tube shrinking, affecting the lifespan of the heating tube. It also easily causes problems such as misaligned or disordered fine wires and voids in the heating wire coil core during powder addition. (The distinction between fine wire specifications and defects such as sieving caused by voids in the heating wire coil core will be explained in detail in the resistance wire section.)

② Electrical Performance: The electrical performance testing items for heating tubes are basically limited to four indicators: normal insulation, withstand voltage, and hot insulation and withstand voltage. There are several common issues that require coordination and standardization in testing methods between heating tube manufacturers and magnesium oxide manufacturers.

a. Safe Creepage Distance: It is well known that the distance from the outer diameter of the heating wire to the inner surface of the tube has a significant impact on electrical performance testing. Without a unified standard for this requirement, both parties cannot make standardized judgments when using magnesium powder. Based on the current state of equipment and machinery used in the design, manufacturing, and application of heating tubes, the safe creepage distance is ≥1.2mm, a parameter that is widely used.

b. Density: After establishing the requirement for safe creepage distance, it is necessary to determine the parameters of magnesium powder at the appropriate filling density. Since different densities yield completely different results when testing withstand voltage, the commonly used requirement in the design and manufacturing of electric heating tubes is a compacted powder density of ≥2.3 g/cm³. Therefore, this is used as a reference item to unify the testing standards between the two parties.

c. Insulation Test Standards for Hot and Normal Conditions: According to the relevant standards of the International Electrotechnical Commission (IEC), the insulation strength tester for these products must be set to DC 500V. However, in actual testing, many electric heating tube manufacturers often use the non-standard DC 1000V setting, resulting in significant measurement errors. This aspect will be resolved through coordination and standardization.

d. Withstand Voltage Test Standards for Normal and Hot Conditions: Also according to relevant standards, the leakage current setting is 0.5mA, and the test time is 1 second. Many electric heating tube manufacturers have excessively high requirements in practical applications, such as constantly raising the basic withstand voltage index, setting excessively low leakage current, and increasing the withstand voltage test time. These are all non-standard and unreasonable practices.

Regarding electrical performance, each of the above four basic requirements must be considered. These are test conditions that both parties should agree upon; otherwise, their respective so-called enterprise standards lack universally applicable conditions.

③ Moisture resistance: Strictly speaking, the moisture resistance of magnesium powder is limited to its moisture resistance in its unprocessed state. The basic moisture resistance requirement is: a moisture test is conducted in a constant temperature and humidity chamber under the following conditions: temperature 60±2℃, humidity 100%, and time 168 hours. The result is that the insulation withstand voltage index remains constant before and after the test to be considered合格 (qualified). This is the basic moisture resistance performance requirement for magnesium powder. However, many electric heating tube manufacturers conduct moisture tests on the tubes after shrinking or annealing and require that the performance remain unchanged. This is an unreasonable requirement, mainly because changes caused by some production processes lead to a decrease in moisture resistance. To be objective and fair, the decline in electrical performance due to moisture after tube manufacturing is a specific manifestation of insufficient production capacity of electric heating tubes. If it is due to the magnesium powder itself experiencing weight loss upon burning, then it is a quality problem of the magnesium powder manufacturer.