chemical foaming and physical foaming in details

cables use foaming extrusion

chemical foaming→ normal SCSI core wire and computer cables.

physical foaming(nitrogen foaming)→ high frequancy data transmission cables.
 

advantages and disadvantages of foaming extrusion：
advantages：

A. lighten the cable, save raw material.

B. small dielectric coefficient, less attenuation, stable characteristic, water proof, good bendability, small resistance, and longer service life. 

disadvantages:
bad adhesive force and strength
 

description of chemical foaming:

1.advantages of chemical foaming: used on normal extruder line, the line speed can be fast, used on USB,VGA、DVI、SATA、HDMI and other normal frequency transmission cables. 

2.disadvantages of chemical foaming: hard to test the uniformity of foaming, high sensitive on heat, Max foaming degree is around 40%.

chemical foaming process？

chemical foaming use heat effect foaming agent in the PE to foaming. high temperature makes the heat effect foaming agent decomposite, get the bubbles in the PE extrusion. the foaming agent is very sensitve to the temperature( ±1℃), the stability of the temperature control system have a great influence on the foaming degree. the foaming agent need to be stored in dryer environment, water in the foaming agent can makes the extrusion capacitance and outer diameter unstable.         

PE foaming degree and electricity characteristics:

capacitance need to reduce the effective dielectric constant to reduce the diameter of the core wire, so that in a certain size cable can have more core wires, that is a big advantage for multi core cable. the degree of thin insulation is about 20-30%, thick insulation about 50%.
 

discription of physical foaming: 

1.advantage of physical foaming: foaming degree is easy to control, Max foaming degree can reach 80%, small dielectric coefficient, can used on high frequency data transmission. 

2.disadvantage of physical foaming: need to used special faoming device, machine cost is higher.

physical foaming process？

compressed air star nitrogen injecting pump,pressure maintaining valve control nitrogen injection. nitrogen under the high pressure go into PE insulation like fog, get coring with necleating agent, foaming formed after extrusion.
 

related terms of foaming： 

foaming degree testing: 
foaming degree can use densitometry to test, foaming degree means the percent of gas. 
 

foaming extrusion:

foaming agent of foaming PE

A.high density use Sulphonylureas
B.low density use carbonic acid 
most decomposition product of foaming agent absorb water, the gas also have water, so generally the foaming extrusion line need to have dryer.

process of foaming extrusion: 
A. feeding foaming material from hoppers of extruder.
B. material heating in the screw barrel, screw rotating plastify the material, move the material to the crosshead
C. through tip and die get the core wire insulation, extrusion at the same time foaming.
D. solidify in the water flume.

extruders for foaming:

L/D of screw around 20-28, with longer material feeding parts, compression rate is about 2.0-2.5,the following aspects need to be noticed. 
A. small compression rate screw
B. anti-pressure device control pressure
C. reduce resistance force between die and crosshead.
D. reduce the width of spiral

conditions of foaming extrusion：

foaming wire extrusion influenced by extruder size, screw design, and foaming agent quantity. the most important thing in foaming extrusion is to make the material melt quickly, decomposited gas melt in the material. 

extrusion condition：when pressure, temperature, and time changes, the extrusion product character can also change a lot.extrusion temperature change, foaming can change a lot. so the temperature need to be within a certain range, so that the foaming degree keeps stable, get good foaming wires.

extrusion time: the material stay in the screw barrel time, we call it extrusion time. extrusion time have a big influence to the foaming. 

foaming process：

A.extrusion temperature influence: when use foaming agent for foaming, the extrusion temperature control is the key part of foaming quality. low extrusion temperature makes less bubbles, temperature increase bubble increase, when reach a certain temperature, we get the max. foaming degree with small bubbles; if the temperature still goes up, then the bubbles become big, some bubbles will break, cause the nonuniformity of foaming. this experience can only get by test running.

B.screw rotating speed: screw rotating speed also influence the foaming conditions. higher speed, more and small bubbles, when reach a certain limit, the bubbles goes dwon. 

foaming dies：

foaming dies design need to consider the expensibility.

foaming insulation cooling:

foaming layer also influenced by cooling system. the bubbles expanded after dies, in order to expand fully, it will need a centain length air cooling or warm water cooling, sudden cooling will reduce the foaming degree and smoothness on the surface.

how to reduce foam PE distorsion in extrusion process: 
A. uniform qaulity material 
B. same material feed volume
C. stable extrusion temperature
D. stable screw rotating speed
E. maintain the temperature on conductor
F. maintain stable line speed