Extruded Profile

• Innovation in Rubber Extrusion Process
• Sponge Extrusion

Innovation in Rubber Extrusion Process
The advantages of extrusion via a gear pump is a well known fact in the domain of the plastic industry, where the services of gear pumps have been effectively utilised. Some of the benefits of the gear pumps include an effective pressure build-up, constant throughput and dimensional accuracy of the extrudate. However it is only in recent years that the benefit of the gear pumps have been extended to the rubber extrusion process. Though this implementation in the rubber industry was not smooth because of the vulnerability of the gear pump as well as high compound viscosities. The following is a schematic of the extruder-gear pump combination.


The plasticizing attributes of the extruder are used for proper masticating of the compound and to feed the gear pump. The gear pump generates high die pressures without excessive temperatures. The gear pump and the extruder perfectly complement each other. Actually the extruder screw can plasticize, but does not have the conveying efficiency on the other hand the gear pump though highly efficient material conveyor, but lacks the ability to plasticize.


Advantages of the Gear Pump
Using the gear pump in the rubber extrusion process offers the following advantages in an extrusion line.

• Increase in the extruder efficiency
  As the gear pumps are used for pressure build-up, there is considerably less demand put on the extruder. This translates into an increase in throughput for extruder. The extruder can be applied to achieve effective mastication as well as ensuring sufficient feed to the gear pump. As less energy is wasted for pressure build-up, the extruder is now free for processing more material.
  
  
• More output
  Results indicate that a combination of the extruder-gear pump delivers more output than standalone extruders.
  
  
• Reduction in pressure fluctuations
  A gear pump can compensate for, as well as reduce the pressure fluctuations in an extruder system.
  
  
  
• Improved conveying efficiency
  The superior chamber conveyance of gear pumps results in displacement of small volumes of material along with exertion of minimal friction on the compound, thus conveying efficiency is significantly improved.
  
  
• Low temperature operations
  At equal back pressure, the temperature increase due to pressure build-up is considerably less in gear pumps, this facilitates the whole system to function at lower temperatures also giving rise to more throughput of the extruder.

Sponge Extrusion
Sponge extrusion is a relatively new addition to the rubber extrusion process. They are closed-cell cellular profiles that are made up of high quality rubber often Silicone, EPDM or Neoprene. Sponge extrusion offers diversified shapes and sizes like cords, tubes rectangular profiles etc. Sponge rubbers are applied for use in seals and gaskets where compression forms the main sealing method.

They can be used in applications where there is a chance of accommodating large tolerances. Some of the typical shapes available are shown here.

Advantages of Sponge extrusions

• A closed cell structure enabling low absorption
• Low compression set
• Excellent thermal insulating properties
• Fire-resistant grades also available
• Shows high temperature resistance
• Ozone-resistant
• Good elasticity

Processing Silicone Sponge
A standard silicone extrusion equipment is used for processing of silicone sponge. Let's discuss about some aspects of the production methods.

• Extrusion Die
  The extrusion die shape must be able to reflect properly the linear expansion rate of silicone rubber sponge. There is a need for adjustment of the Die geometry in order to counterbalance the die swell of the solid material. As the following diagram shows side openings are typically made convex The reason being to facilitate the formation of straight sides as the silicone rubber swells upon leaving the die.
  
  Another important factor in the Die geometry is the fast heat transfer rates. The fast heat transfer rates, it has been seen to affect the straight areas of the profiles more as compared to the less corner areas. As the image shows areas that are intended to be straight on the profile cross section comes with additional convex shape.
  
  
• Stock temperature
  Stock temperature at the screw tip should remain around 25 – 50°C. So a slight cooling is needed
  
  
• Curing
  Curing is another important aspect of silicone sponge extrusion. The cure temperatures is strongly dependant upon the heating equipment used. As a general rule, cure temperatures of a new food grade sponge should be higher than the temperatures let's say for a conventional and chemically blown sponge. It has been seen there is an inverse relationship between cure temperature and density. Higher the cure temperature level, lower the density. For getting good silicone sponge, the temperature at the entry of the curing tunnel needs to go beyond 210°C.