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.
