For silicone production it is important the design of:
- Extrusion cross head
- Extruder
- Vulcanization process and mode.
The critical parameters for the extrusion of silicone or silicon rubbers are:
- Moisture temperature into the extruder and cross head
- Working back pressure that can generate compound over-heating due to the viscous friction.
- Material stagnation into the extruder, collar, head.
- Reticulation system and line speed
Too high temperature of the compound generates pre-reticulation and loss of vulcanization agent. The extruder must be designed in terms of barrel, feeding system of the silicon stripes, screw and cooling system.
The extruder barrel is normally Bi-metallic without grooves on the feeding zone and cooled by water. The thermocouples can be installed to detect the barrel or water temperature.
The extrusion screw is in nitrided steel with lowest roughness and with Colmonoy or TIG 5 coating on the threads for wear and corrosion resistance. The screw normally used to process silicone is a simple compression screw with compression ratio 1,15<C.R.<1,50 and 12<L/D<15.
The compression can be generated by:
- Increasing of screw core diameter (Radial compression)
- Reduction of the screw pitches (Axial compression).
The screw is cooled by water flow in order to avoid compound over-heating or pre-reticulations. The screw can even be equipped and designed with special gripping system on the feeding threads in order to improve the stripe feeding. The extruder feeding is carried out by the motorized cooled rolls.
The roll can be motorized in different ways:
- Mechanically connected with screw rotation
- Electrically driven and nor mechanically connected with screw rotation.
The extruder can work, in terms of feeding mode, in: “Full mode” or “Starving mode”.
In case of direct connection between screw and roll the extruder, feeding is “Full Mode”. It means that the screw rotation is driving the feeding. This mode, due to the drag effect on the extruder, can generate “Regurgitation effect” so the extruder is not able to take all stripes given by roll creating an extruder over-feeding. This effect can create moisture over-heating due to the increasing of friction between the molecules of the compound. The proper strips dimensions can compensate this effect.
In case of using of electrically driven roll, the differences between the screw speed and roll speed can be electrically adjusted by rpm motor variation. In this case is possible to feed the extruder less than its capacity (Starving mode) without changing the strip dimensions.
In order to reduce the material friction and its over-heating it’s important the thermal treatment, surface coating of the screw, barrel and cross-head.
The clamp/jaw must be cooled by water as well.
In conclusion, regarding the silicone’s extrusion it is important is the control of the moisture temperature in order to avoid pre-reticulations and defects on the insulation. It means that the friction between moisture and mechanical components must be kept under control as well as the extruder back pressure avoiding material stagnation into the barrel and flow path into the head and collars.
The compound temperature is influenced by:
- Working pressure, that generates localized over-heating and localized pre-reticulations.
- Mechanical friction into the extruder and feeding roll.
- Mechanical friction on the flow distributor and tools chamber.
Excessive moisture temperature generate defect on the cable and loss of reticulation agent acting on the cable quality.
Due to the above explanation, the design of the extrusion screw and cross head is of vital importance in terms of:
- Screw diameter on the feeding and pumping zone, screw pitches on the feeding, metering and pumping zone, Compression Ratio, surface treatments.
- Flow distributor design and flow path dimensioning
- Flow distributor cooling system
- Cooling system into the head, die holder and ring for skin.
- Water flow, cooling system of the head, flow distributor and die holder.
The flow distributor must be design with correct geometric profile and correct kinetic-dynamic design in order to avoid over-heating and material stagnation. The feeding and distributor channel section/dimensions of the flow distributor, their path variation must be accurately calculated, as well as the leakage gap. The feeding section of the flow distributor is crucial for the extruder working back-pressure, for the impact pressure and for the correct and efficient material distribution.
Through the feeding channels is better to realize a proper section variation in order to generate and increasing of material speed guaranteeing the continuous channel cleaning. Eventual and undesirable reduction of speed reduction of the compound could generate localized stagnation.
The material over-heating can be even reduced by proper surface treatments of the flow distributor and inner head surface. At the main time the treatments must guarantee a high wear and corrosion resistance.
Moreover, due to the conicity of the distributor as well as of the inner head, is obvious that the channel section, on each point who belonged to the material flow path, is not square but trapezoidal. The trapezoidal shape of the channel section mainly on the distribution path, generate a speed variation among the material layer. Assuming we divide the compounds, contained into the distributor channel section, in different layers, is comprehensible that the external, internal and middle layers have different speed. These speed variations create an internal mechanical torque, mechanical friction among molecules and consequently material over-heating. For these reasons it’s important to define the correct geometry of the channels and the proper cooling system.
The head cooling and the water circulation mode is fundamental for the correct extrusion process. The head, front head, collar, die holder must be cooled down as well as the flow distributor. The cooling mode acts on the heat-exchange efficiency. The design of the tools (tip and die) as well as the tools chamber have a huge effect on the extruder back pressure and material over-heating.
HP EXTRUSION has developed and designed a performing cooling circuit in order to guarantee and highest efficient heat – exchange and optimal temperature distribution.
The flow distributor, the head and ring for skin have been calculated in order to avoid material stagnation, material over-heating through distributor channels and leakage zone. Moreover, the flow distributor, head body, rings undergo additional and proper surface treatments in order to:
- Reduce the friction coefficient.
- Increase the level of fluidity of the compound avoiding over-heating, stagnation.
- Increase the wear and corrosion resistance.
