Answers to seven common questions: A complete guide to the extrusion process of magnesium hydroxide cable materials

In the battlefield of low-smoke halogen-free cable manufacturing, magnesium hydroxide is like a "firefighter" in white armor, who must not only extinguish the raging flames, but also protect the bones of the material. However, the journey of this environmental guardian is not smooth - from the roar of the screw to the game of the mold, from the temperature to the surface finish, every detail may become a stumbling block to the flame retardant revolution. From the perspective of a front-line engineer, this article disassembles the solution to the seven core problems.

1. Screw selection: Why does the compression ratio become the "life and death line"?

When highly filled magnesium hydroxide powder (often with a content of more than 60%) flows into the extruder, the compression ratio of the screw is the same as the regulating valve: if it exceeds 1:2.5, the shear heat will cause the melt to "spontaneously combust", resulting in a cluster of bubbles; if it is lower than 1:1, the material cannot be fully plasticized. The golden rule is to use a stepped screw with a compression ratio of 1:1~1:2.5 and a length-to-diameter ratio of 20:1.

According to actual measurements by a Zhejiang enterprise, the optimized screw reduces extrusion torque by 30% and increases production capacity by 40%.

Practical skills: Installing an air cooling system in the fourth zone of the screw can instantly remove the 30°C temperature rise caused by friction and prevent magnesium hydroxide from decomposing and producing gas at the critical point of 170°C.

2. Temperature control: the balance between ice and fire

The "sensitive constitution" of magnesium hydroxide makes temperature control like walking on a tightrope: if the die exceeds 190°C, the flame retardant will decompose and produce surface pitting; if it is lower than 150°C, the melt viscosity will surge and the extrusion pressure will exceed 25MPa. The optimal temperature curve is: 160-170°C in zone 1, 175-185°C in zone 2, and 180-190°C in die head. The temperature difference is controlled at ±5°C through zone cooling.

Fatal misunderstanding: Do not blindly increase the die temperature in pursuit of surface finish! A cable factory in Jiangsu once caused the oxygen index to plummet from 38 to 29 due to the die head heating to 210℃.

3. Die Game: Extrusion VS Semi-extrusion

The extrusion of the insulation layer and the sheath can be described as "ice and fire": the insulation layer needs to use a full extrusion die to ensure density, and the die sleeve size is 8%-12% smaller than the finished product; the sheath is suitable for a semi-extrusion die, the core is retreated 1-3mm, and the stretch ratio is controlled at 2.5-3.2

. A submarine cable project in Qingdao adopted this solution, and the tensile strength of the sheath was stable at 14MPa, and the elongation at break exceeded 250%.

Invisible killer: When the length of the die sleeve corridor section is greater than 1mm, the shear stress increases by 50%, causing melt fracture. The solution is to use a "short-necked die sleeve" with a corridor section ≤0.8mm.

4. Surface "eye mucus": annihilation war of precipitates

The precipitates at the die mouth are like stubborn stains, and their essence is the compatibility defect between magnesium hydroxide and the matrix resin. Double tactics can be used to crack it:

Physical obstacle removal: Install a hot air knife (temperature 80-100℃) at the die lip and blow continuously at a wind speed of 0.5m/s;

Chemical modification: Use zinc stearate + silane coupling agent to coat the powder surface, so that the contact angle is reduced from 110° to 65°, and the precipitates are reduced by 70%.

Advanced solution: A company in Hebei has developed a "nano-protrusion mold" with 0.2mm micro-bumps designed on the inner wall of the die mouth, successfully reducing the average monthly shutdown cleaning times from 15 times to 2 times.

5. Chronic bubble disease: systematic treatment from drying to screw structure

The three main culprits of bubble generation - material moisture absorption, shear overheating, and screw design defects, need to be treated in a trinity:

Raw material pretreatment: 80℃ hot air circulation drying for 4 hours, moisture content ≤0.15%;

Screw optimization: add a counterflow groove in the compression section to disperse the shear stress;

Process control: extrusion speed is controlled at 12-18m/min, and vacuum is maintained at -0.08MPa.

Data verification: A laboratory in Shanghai found through CT scanning that after adopting the above solution, the pore density dropped from 15/cm³ to 0.3/cm³.

VI. Jacket cracking: crack prevention code from formula to cooling

The brittleness problem caused by high filling requires the construction of a "rigid and flexible" system:

Formula optimization: Add 5%-8% POE elastomer, and the elongation at break is increased to 300%;

Gradient cooling: The water temperature of the first section of the water tank is 50-60℃, the second section is air cooling, and the final section of water cooling temperature drops sharply to 20℃;

Stress release: 0.1mm glass fiber mesh is embedded in the jacket layer, and the tear strength is increased by 3 times.

Case warning: A subway project caused jacket stress cracking due to direct water cooling. After changing the gradient cooling process, the laying failure rate dropped by 90%.

7. Process collaboration: from single-point breakthrough to system optimization

A true craftsman knows how to make equipment, materials, and parameters dance together:

Intelligent temperature control: Using PID algorithm to dynamically adjust the heating power, the melt temperature fluctuation is compressed to ±1.5℃;

Particle size ratio: 0.5-1μm main particles + 30nm enhanced lamellar "concrete structure", tensile strength exceeds 16MPa;

AI early warning: Monitor the screw load through the vibration sensor, and predict the risk of blocking 15 minutes in advance.

In a smart factory in Guangdong, this system has increased the comprehensive yield of magnesium hydroxide cable materials from 82% to 98.7%, and the production cost per kilometer has dropped by 1,200 yuan.

From the metal jungle of the screw to the microscopic battlefield of the mold, from the hot line of temperature to the undercurrent of stress, the extrusion process of magnesium hydroxide cable materials is a never-ending technical symphony. Every parameter adjustment and every structural innovation are reshaping the safety boundaries and efficiency limits of green cables. As China's intelligent manufacturing has carved its own coordinates on this white battlefield, the ultimate victory rules of this battle have become clear: only systematic thinking can control complexity; only by respecting details can excellence be achieved.