Discussion on the Extrusion Process of Low-Smoke Halogen-Free Cables

With the development of flame retardant technology for cable materials, new types of flame retardant cables have been continuously emerging, evolving from the original ordinary flame retardant cables to low-smoke low-halogen flame retardant cables and low-smoke halogen-free flame retardant cables. This indicates that the requirements for flame retardant cables have been getting higher and higher in recent years.

In countries such as Europe, the United States, and Japan, environmentally friendly wire and cable products have become the mainstream of all cable varieties. The governments strictly prohibit the use or import of non-environmentally friendly cables. Ordinary flame retardant materials contain a large amount of halogen. When burning, they will produce a large amount of smoke and toxic corrosive hydrogen halide gas. Halogen-free flame retardancy is mainly achieved in polyolefins. Therefore, low-smoke halogen-free cables will be the main development trend in the future. So, the extrusion of low-smoke halogen-free cable materials will be discussed from the following aspects.

1. Extrusion Equipment
   A. The main component of wire and cable extrusion equipment is the screw, which is related to the application range and production efficiency of the extruder. To meet the needs of different plastic processing, there are many types of screw designs. Low-smoke halogen-free flame retardant cable materials contain highly filled magnesium hydroxide or aluminum hydroxide. Therefore, for the selection of screws, ordinary screws are generally used, and their compression ratios should not be too large, usually between 1:1 and 1:2.5 is more appropriate.
   B. Another important factor affecting the extrusion of low-smoke halogen-free cable materials during the extrusion process is the cooling device of the extruder. Due to the special nature of low-smoke halogen-free materials, a large amount of heat is generated due to friction during the extrusion process. This requires that the extrusion equipment has a good cooling device to control the process temperature. This is a problem that cannot be ignored. If the temperature is too high, large pores will be formed on the surface of the cable; if the temperature is too low, the overall current of the equipment will increase, and the equipment is prone to damage.
2. Extrusion Molds
   Due to the high filling materials in low-smoke halogen-free cable materials, there are significant differences in the melt strength, draw ratio, and viscosity between it and other cable materials in the molten state. Therefore, the selection of molds is also different. First, in the choice of extrusion methods of molds. For the extrusion of low-smoke halogen-free cable materials, the extrusion mold for insulation should be of the extrusion type, and during the extrusion of the sheath, the semi-extrusion type should be used. Only in this way can the tensile strength, elongation, and surface finish of the material be fully guaranteed. Second, in the selection of die sleeves. When using extrusion molds, due to the high viscosity of the material, the pressure at the die head is large, and the material will expand when it exits the mold. Therefore, the die sleeve should be slightly smaller than the actual size. Finally, the mechanical properties of low-smoke halogen-free materials are not as superior as those of ordinary cable materials and low-smoke low-halogen materials. Its draw ratio is small, only about 2.5 to 3.2. Therefore, when selecting molds, its drawing properties should also be fully considered. This requires that the selection and matching of die sleeves should not be too large, otherwise the surface of the cable will not be dense, and the extrusion coating will be loose.
   One additional point: The motor power of the main machine should be large enough. Due to the relatively high viscosity of LSHF materials, insufficient power will not work.
   One point of disagreement: The length of the gallery section of the extrusion mold should not be too long, usually less than 1 mm. If it is too long, the shear force will be too large.
   1. For halogen-free materials, using a screw with a low compression ratio for processing is fine. (A large compression ratio will cause severe heat generation inside and outside the plastic, and a large length-to-diameter ratio will result in a long heating time for the plastic.)
   2. Due to the addition of a large amount of flame retardant in low-smoke halogen-free materials, there are great difficulties in the extrusion process. The shear force of the screw on the halogen-free materials is large. The most effective way currently is to use a special extrusion screw for halogen-free materials.
   3. During extrusion, a material like eye discharge appears at the outer die opening. When there is more of it, it will attach to the wire and form small particles, affecting its appearance. Have you ever encountered this? Do you have any good solutions? It is a precipitate attached to the outer die opening. Lowering the temperature of the die opening and adjusting the mold to have a little stretching will improve the situation a lot. I also often face this problem and haven’t found a fundamental solution. I suspect that it is caused by the poor compatibility of the material components. It is said that using a blowtorch to bake it can work, but the temperature should not be too high, otherwise the insulation will be damaged. If the temperature of the die head is high, lowering the temperature a little will solve the problem. There are two solutions to this problem: 1) Use an air gun to blow, preferably with hot air; 2) Change the design of the mold by making a small protrusion at the die opening. The height of the protrusion is usually about 1 mm. But I don’t know if there are any domestic manufacturers that can make such molds. For the problem of precipitates at the die opening during the extrusion of low-smoke halogen-free materials, installing a hot-air slag removal device at the die opening can solve this problem. Our company is currently using this method, and the effect is very good.
      One additional point: When producing low-smoke halogen-free materials, it is best to use a semi-tubular extrusion mold for tubular extrusion. In addition, the surface finish of the mold should be high to avoid the appearance of eye discharge-like deposits at the outer die opening.
   4. Question: When producing low-smoke halogen-free materials currently, the temperature in the fourth zone of the barrel keeps rising. After the speed is increased, the temperature will rise by about 40 degrees, causing the material to foam. Are there any good solutions? For the phenomenon of bubbles appearing during the extrusion of low-smoke halogen-free materials, according to the conventional analysis: One is that low-smoke halogen-free materials are easily affected by moisture. Before extrusion, it is best to do a drying treatment; Two is that the temperature control during the extrusion process should be appropriate. The shear force of halogen-free materials during the extrusion process is large, and natural heat will be generated between the barrel and the screw. It is recommended to relatively lower the set temperature; Three is the quality reason of the material itself. Many cable material factories simply add a large amount of filler to reduce costs, resulting in an excessive material specific gravity. For the phenomenon of bubbles appearing during the extrusion of low-smoke halogen-free materials, according to the conventional analysis: One is that low-smoke halogen-free materials are easily affected by moisture. Before extrusion, it is best to do a drying treatment; Two is that the temperature control during the extrusion process should be appropriate. The shear force of halogen-free materials during the extrusion process is large, and natural heat will be generated between the barrel and the screw. It is recommended to relatively lower the set temperature; Three is the quality reason of the material itself. Many cable material factories simply add a large amount of filler to reduce costs, resulting in an excessive material specific gravity. If it is a pin-type screw head, can it also produce low-smoke halogen-free materials? No, the shear force is too large, and there will be all bubbles. 1) Determine the compression ratio of your screw and the shape and structure in the fourth zone, whether there are diversion sections or reverse flow sections. If so, it is recommended to replace the screw. 2) Determine the cooling system in the fourth zone. You can use a fan to blow air towards this zone to cool it down. 3) Basically, this situation has not much to do with whether the material is affected by moisture or not. However, the extrusion speed of halogen-free sheath materials should not be too fast.
   5. The following points should be noted when extruding low-smoke halogen-free materials: 1) The temperature during extrusion is the most important. The temperature control must be accurate. Generally, the maximum temperature requirement is between 160 – 170 degrees. It should not be too high or too low. If the temperature is too high, the aluminum hydroxide or magnesium hydroxide in the material is prone to decomposition, resulting in a non-smooth surface and affecting its performance; if the temperature is too low, the shear force is too large, the extrusion pressure is large, and the surface is not good. 2) It is best to use a tubular extrusion mold during extrusion. When matching the mold, there should be a certain stretching. During extrusion, the mandrel should be 1 – 3 mm behind the die sleeve. The extrusion speed must not be too fast, and it should be controlled between 7 – 12 m. If the speed is too fast, the shear force is too large, and the temperature is difficult to control. (Although LSZH is not easy to process, it is definitely not that slow (as mentioned by Little Bird, 7 – 12 M). Anyway, it is also a speed of 25 or more, and the outer diameter is about 6 MM!!)
   6. The extrusion temperature of low-smoke halogen-free materials will vary depending on the size of the extruder. The extrusion temperature I tested with a 70-type extruder is as follows for your reference. Section 1: 170 degrees, Section 2: 180 degrees, Section 3: 180 degrees, Section 4: 185 degrees, Die head: 190 degrees, Machine eye: 200 degrees. The maximum can reach 210 degrees. The decomposition temperature of the above-mentioned flame retardant should be 350 degrees, so it will not decompose. The larger the melt index of the halogen-free material, the better its fluidity and the easier it is to extrude. Therefore, a 150-type screw can also extrude it as long as the fluidity of the halogen-free material is good enough. (I would like to ask you whether the highest temperature you mentioned is the displayed temperature or the set temperature? When we do it, the set temperature generally does not exceed 140 degrees.) Yes, the flame retardant performance will decline when the temperature exceeds 160 degrees.
   7. Successful production using a BM screw with a compression ratio of 3.0. I am also concerned about this. May I ask all the experts: Why can’t screws with a high compression ratio (>1:2.5) be used for production? The shear force is too large, and bubbles will be formed. Our company has been using 150 to produce low-smoke halogen-free cables, and the effect is very good. We use equidistant and equal-depth screws, and the heating temperature of each section should be well controlled, otherwise bubbles or old glue problems will occur. However, it is very troublesome. Each time, the screw and pulley need to be changed, and the pressure in the barrel and die head is also large.
   8. I think it is better not to vacuum during extrusion to allow a relative sliding in the radial direction and not be prone to cracking.
   9. However, attention should be paid to preventing the expansion of materials at the feeding opening.
   10. Our company used ordinary halogen-free materials before, which were prone to whitening. Now we use GE materials, which are more expensive but do not have the whitening problem. I would like to ask if your halogen-free materials have the whitening problem?
   11. Due to the addition of a large amount of flame retardant in low-smoke halogen-free materials, this is the main factor why the speed cannot be increased, resulting in great difficulties in the extrusion process. During extrusion, a material like eye discharge appears at the outer die opening. When there is more of it, it will attach to the wire and form small particles, affecting its appearance. As mentioned above, it can be baked with a blowtorch. The temperature should not be too high, otherwise the insulation will be damaged. This is the most difficult control point in the process. For halogen-free materials, using a low-compression-ratio and hollow screw for processing has no problem in terms of processing speed. From the perspective of small extrusion machine equipment (with a screw diameter of 100 mm or less) and the extrusion of low-smoke halogen-free wires using vinyl acetate copolymer as the base material, the appearance and performance are not significantly affected when using ordinary PVC screws and special screws for low-smoke halogen-free materials for production. The most critical factors affecting the extrusion performance and appearance are still the formulations and proportions of various flame retardants, other filling materials, and base materials. When using PVC and PE material extrusion screws to produce low-smoke halogen-free materials, due to the high viscosity of such materials, and the compression ratio of ordinary PVC material extrusion screws is about 2.5 – 3.0. If such compression ratio screws are used to produce low-smoke halogen-free materials, during the extrusion process, the mixing effect within the screw will not reach the best during the time the material stays in the screw, and the material will adhere to the inner wall of the barrel, resulting in insufficient glue output, inability to increase the extrusion speed, and at the same time increasing the motor load. Therefore, it is not advisable to use them. If mass production is carried out, it is best to use a special screw with a low compression ratio. It is recommended that the compression ratio be below 1.8:1. In addition, the motor power needs to be increased and a suitable power inverter needs to be selected to achieve the best extrusion effect and wire performance.
   12. The overall problems of low-smoke halogen-free materials are: 1) There are pores in the extruded product; 2) The surface finish is poor; 3) The glue output is small; 4) The frictional heat of the screw is large.
   13. When extruding halogen-free low-smoke flame retardant materials, since the temperature cannot be too high, the viscosity of the material is high. The extrusion machine screw should be selected as 20/1, and the compression ratio should not be greater than 2.5. Due to the large shear force, the natural temperature rise is large. It is best to use water to cool the screw. Baking with a blowtorch on a low fire is more effective for the eye discharge at the die opening and will not break the insulation.
   14. Seeking help for the ratio of low-smoke halogen-free extrusion molds. The draw ratio is 1.8 – 2.5, the draw balance degree is 0.95 – 1.05. The draw ratio is a little smaller than that of PVC. Try to make the mold matching compact! The draw ratio is about 1.5. The mandrel does not need to carry the wire. Use the semi-extrusion method. The water temperature of the first water tank is 70 – 80°. Then air cooling is used, and finally water cooling.