Oct 27, 2018
Fabrication of optical cable filler rod by microcellular foaming extrusion technology
Microporous foamed materials have light weight, 5% - 95% lighter density, 5 times higher impact strength, 3 - 5 times higher rigidity - mass, 5 times longer fatigue life, higher thermal stability, lower dielectric constant and lower thermal conductivity than unfoamed materials.
And in the process of microporous foaming materials, chemical and fluorine, chlorine and other foaming agents are not used, no environmental pollution problems.
Due to the uniform distribution of very small size bubbles in microporous foamed materials, a large number of crazes and shear bands are initiated around the micropores, which can absorb energy for toughening. As a result, many mechanical properties of microporous foamed materials are obviously superior to those of ordinary foamed and non-foamed materials. Therefore, microporous foamed materials are considered as the 21st century New materials.
1,Principle and properties of Microcellular Plastics Foaming
Microcellular foaming is divided into two ways: chemical foaming and physical foaming.
The so-called chemical foaming:
The foaming agent is added in high-density PE, and its main components are generally ammonia compounds.It will decompose nitrogen at high temperature, and use smaller external mold to make the head get greater pressure.After extrusion, the nitrogen inside the air is expanded into pores and released into PE by pressure release.
Chemical foaming is called chemical foaming because of the addition of foaming agent and the use of chemical reaction to achieve foaming effect. It is characterized by simple operation of foaming degree, which is beneficial to control, but easy to produce chemical residue.
Physical foaming is to mix HDPE, LDPE and nucleating agent in a certain proportion. In the extrusion process, nitrogen or carbon dioxide gas at appropriate pressure is injected into the screw chamber and mixed with polyethylene in the melting state. The foam is stirred evenly by the screw, and then expands to form light pore at a higher pressure instantly after extrusion. PE.
Physical foaming eliminates the disadvantage of chemical foaming, that is, there is no residue or water produced by chemical foaming in PE, which is not easy to produce impurities in the spiral chamber to reduce the probability of breaking rope in production. The foaming degree is easy to control, so it is more suitable for the production of filling rope, but for the operator to increase the operation steps for beginners have a certain degree. The difficulty. The physical foaming and extrusion process of filling rope generally needs four stages, namely gas injection, bubble formation, bubble growth and bubble finalization.
2,Microcellular foaming extrusion process and equipment
The extrusion process of microcellular foams consists of three steps：
1) formation of homogeneous gas / polymer system.
2) bubble nucleation;
3) bubble growth and control.
These three steps are the basis for the processing of microcellular foams.
The temperature and pressure of the single phase melt formed by liquid polymer and gas supercritical liquid state must be controlled accurately to prevent the melt from foaming in advance.
The nucleation and growth of foam micropores, the condensation process during injection and the final size and shape of micropores depend on the technological conditions of extrusion.
Homogeneous system is formed to make the dissolved gas in polymer reach or exceed the solubility of gas in polymer at this temperature and pressure as far as possible, so as to facilitate the use of thermal dynamic imbalance to nucleate polymer when pressure release or temperature rise.
At the same time, in order to meet the requirements of industrial production, homogeneous system should be formed as soon as possible to ensure the formation of polymer melts and gases in the nucleation state before the formation of homogeneous system.
The following requirements are required for the formation of polymer / gas homogeneous systems:
(1) sufficient gas must be provided for the dissolution process.
(2) polymer and gas must be fully mixed to form homogeneous system in the shortest possible time.
(3) the physical blowing agent gas must have good diffusivity and high solubility.
(4) the process parameters such as pressure and temperature must be well controlled during the dissolution process.
It mainly includes 65 extruder, high pressure nitrogen injection system, 35 extruder, cooling water tank, tractor, blower, diameter measuring instrument, wire rack.
The take-up requirement can carry at least 300 m / minute high speed. The equipment is different from the ordinary filling rope production line in that it adds a high-pressure nitrogen injection system and a skin extruder.
High pressure nitrogen injection system consists of four parts:
The first part: high pressure nitrogen storage device, high pressure liquid nitrogen tank or high pressure nitrogen cylinder.
The second part: pressure regulating device, generally use nitrogen pressure relief meter or nitrogen injection metering pump.
The third part: the connecting device is a high pressure resistant metal tube.
Part 4: One-way nitrogen injection device connected with the screw chamber - one-way valve; (one-way valve is installed in the plasticizing zone of the screw chamber); the simplest configuration is nitrogen cylinder, nitrogen pressure reducing meter, sealed copper pipe, one-way valve.
Extruder head: The extruder head adopts the foam co-extrusion head. After installation, the physical foam extrusion can be used at the same time with the skin extrusion to realize the foam co-extrusion.
Raw material composition: LDPE with a small amount of nucleating agent mixed with HDPE or directly mixed with HDPE nucleating agent, PBT as skin extrusion, the foaming agent used is nitrogen.
Squeezing effect of foam filling rope: In order to ensure the mechanical strength and surface smoothness of foam filling rope, a layer of PBT is extruded on the surface of the filling rope, and the skin is extruded. At the same time, the occurrence of air hole explosion on the surface of the foam material after the extrusion die mouth can be restrained, and the residual explosion holes around the die mouth can be avoided. Material, easy to control the outside diameter, effectively reduce the production of rope breakage probability; and in the extrusion of skin material mixed with color masterbatch, production of color filling rope, that is, to make the color more distinct and can save the amount of master batch.
3，Strength and tensile properties of foaming filler rope
After testing, we found that the influence of the rope tension is the diameter of the rope and the density of the rope. It is proved that the formula for calculating the pulling force deduced from the filled rope theory is established: F= (1- E) PcS
F is the maximum tensile force; S is the cross-sectional area of the rope; epsilon is the degree of foaming; Pc is the tensile strength constant of the material.
Generally speaking, the tension of filling rope is between 3N and 8N. From the above formula, it can be seen that the tension of filling rope after foaming can meet the needs of cable formation. And when the outside diameter of the filling rope is the same, the bigger the foaming degree is, the smaller the density is, and the less the material is needed. However, when the outside diameter of the filling rope is too small, the foaming degree of the filling rope can not be too large. In the production process, rope breakage is easy to occur, which affects the production efficiency. Generally speaking, the foaming degree of foamed filling rope should be controlled below 50%, and the foaming degree of 2.2 mm or more filling rope can be controlled below 60%.
4，Cost and productivity advantage
Assuming that the foaming degree of the filling rope is 40%, the same quantity and specifications of the filling rope can be produced, and the amount of foaming filling rope material can be saved by 40% compared with the unfoamed filling rope.If the price of ordinary PE material is 10 000 yuan / ton, the price of foam material is usually 12 000 yuan / ton, and the unit price of material is generally increased by 20%. If the foaming degree is controlled at 40%, the material cost will be saved by 28%.
The production speed of the filling rope depends mainly on the extrusion time per unit time of the extruder.General 65 extruder screw revolution in 80 rpm to achieve a higher stability and speed balance, the general production of 1.8 mm filling rope limit speed in 180 m / min.Because the foam filling rope is filled with nitrogen during the production process, the same extrusion revolution can extrude a larger volume of foam filling rope material. The production line can also reach 250 meters per minute with the production of 1.8mm specifications. Production efficiency increased by more than 40%.
In summary, the microporous foaming technology is applied to the production of fiber-optic cable filling rope. The foaming filling rope has the advantages of high production speed and low material consumption. Foaming filling rope can reduce the weight of filling rope on the basis of meeting the mechanical properties, thereby reducing the cost of materials, and opening up a space for cost-saving for the fiber optic cable industry.