Materials for wire and cables have to add a lot of inorganic flame retardant to meet the requirement of low smoke halogen free and flame retardant.As fusant has great viscosity while poor fluidity when extruded.It’s easy for them to themogenesis by friction thus part of the fusant may have a too high temperature and materials will decomposition if stay too long on the machine. Also the decomposition gases may cause pore inside the products.There are cautions for extrusion of low smoke halogen free (LSZH) and flame retardant cable materials:(1)Selection for extruding machine.
As there has to be many additives for LSZH flame retardant cable materials,and have great viscosity, extruding machines with lower screw compression should be adopted.The traditional single screw extruder has low shearing rate and poor mixing ability while twin-screw extruder has higher mixing ability. But the higher fill LSZH and flame retardant cable materials always be decomposed and discoloration for excessive shear.It is better to use the combination of single screw and twin-screw extruder. Reciprocating single screw mixer is the ideal machine for processing LSZH flame retardant materials.
(2)Selection for processing technology
In addition to chose the appropriate extruder,processing technology should also be reasonable.As high temperature makes the auxiliaries discomposed and cause pore inside the products while too low temperature cause bad plasticizing.Thus it should lower the extrusion temperature in the condition of reducing the pressure,not too high screw speed to ensure materials fully plasticized.Also the processing technology should be suitable for different materials and formula.
(3)Materials should be dried before extrusion to avoid pore on the fracture surface.
(4)As materials have poor fluxility and big frictional resistance with equipments,organosilicon can de added to improve the dispersion of the inorganic flame retardants and decrease processing torque.It is also processing agent that can improve cable material flame retardant performance .
In a word,LSZH flame retardant wire and cable material have strict requirements on processing equipments and technological conditions,there should be appropriate extrusion equipment and technological conditions according to the characteristics of raw materials,products performance and extrusion characteristics.