The key to coaxial cable physical foaming is to inject gas (nitrogen) into the extruder through high pressure and mix it with the plastic. Therefore, the design of the extrusion screw must ensure that the gas and plastic can get the most uniform mixing effect.
The screw used for coaxial cable physical foaming extrusion has a length to diameter ratio (L/D) of 32:1. The injection point of the gas is about 16D of the length of the screw, so it can be divided into two parts. The compression ratio of the first 16D length screw is about 2, and its role is to melt the polymer. The second-stage screw is used to thoroughly mix the gas and the insulating material. The design structure of this part of the screw is helpful to reduce the melting temperature, that is, increase the viscosity of the melt.
The coaxial cable PE insulation material is extruded through the screw to the machine head and is exposed to atmospheric pressure at the outlet of the machine head, thus forming bubbles. If only a bubble-skin double-layer extruder head is used, the insulating material will be exposed to atmospheric pressure before the die exit. As a result, the gap between the gas and the hole of the mold is released, forming an elongated bubble along the surface of the conductor.
To solve this problem, a three-layer coextruded handpiece must be used to squeeze a thin layer (generally 0.02 to 0.05 mm) into the inner layer to prevent gas from being released along the surface of the conductor. That is a three-layer extrusion unit with skin-bubble-skin coextrusion insulation. For the selection of inner thin skin, it must meet the requirements of extruding into thin walls under high-speed conditions. LLDPE can meet this requirement. This inner thin skin greatly improves the elongation performance (breaking elongation) of the insulation layer and ensures that the coaxial cable insulation layer can adhere well to the conductor.