The key to physical foaming insulation materials is to inject gas (nitrogen) intothe ex truder by high pressure injection and mix it with plastic.Therefore, the design of the extrusion screw must ensure that the gas and plastic can get the most uniform mixing effect.Screw used for physical foami ng extrusion, the 1 ength diameterratio (L/D) is 32:1, the injection point of gas is about 16Dof the length of thescrew, so it can be divided into two parts.The first section of the 16D length ratio of compress ion screwis 2, its effect is the polymer melt; second screw is used forfixing the gas insulation material and thoroughly mixed, the structure is part of the screw is conducive to reducing the melting temperature, improve melt vi scosity. The basicprinciple is shown in figure 2.PE insulation is extruded through the screw to the nose andexposed to atmospheric pressure at the exit of the nose, thusforming a bubble. If only the double nose extrusion of the foamskin is used, the insulating material will be exposed to a tmospheric pressure before the die exit. As a result, the gasis released between the conductor and the hole of themoldanda long bubble is formed along the surface of the conductor.
To solve this problem, we must use three layers of C0 extrusion head, a layer of thin skin (thickness generally 0.02~ 0. 05mm)squeezed into the inner layer, to prevent the gas along the surface of the conductor release. That is, a three – 1 ayerextrusion unit with skin Foam skin coextrusion insulation(as shown in Figure 3). For the choice of inner skin, it is necessary to conform to the requirements of forming thin wallunder high speed. LLDPE can meet this requirement. The innerthin surface greatly improves the extending per formance of the insulating layer (frac ture extensi on) and ensures that theinsulating layer is adhered to the conductor properly.
