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Theory of Operation/ General
General mixing theory indicates that dispersion is best accomplished at high viscosity. In high viscosity regions the interparticulate shear is greatest, causing maximum break-up of agglomerates and dispersion of the phases. Thus, a mixing device which melts the resin "instantly", and maintains the entire mass at uniform high viscosity while mixing makes an ideal mixer.
The extruder is designed to melt resin by mechanical energy being converted directly to heat by interparticular friction within the resin. This causes a rapid melting of the resin, minimizing the temperature variation within the melting mass. Thus, the viscosity is uniform throughout the mass, maximizing the dispersive mixing.
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Although the melting is done by the conversion of mechanical energy to
heat within the resin, the barrel temperatures are still important for several
reasons.
When the extruder is clean, the heated barrel causes surface melting of the
resin introduced during start-up. This allows the resin particles to begin to
stick to one another, and for the mechanical agitation to create shear forces
resulting in frictional heat, which then becomes the primary melting force.
When starting the extruder with residual resin from a previous trial, the
residual resin must be melted before the agitators can be rotated. This heat
is transferred from the barrel to the resin and agitators.
• Turn off the water to the chill knife
• If screw cooling is fitted it must be turned off during warm-up.
With some feed materials, especially finely powdered resins, the extruder
barrel temperature can have a pronounced effect on the feeding
characteristics.
Generally, warmer temperatures of the barrel first zone aid feed conveying.
Cool water should be circulated around the first 7.5D of barrel length when
low melt temperature resins are used to prevent blockage and poor
dispersion resulting from resin melting in the feed port area.
Certain resins show two different characteristics, depending upon barrel
temperature. When the barrel is cool enough, the molten resin does not
adhere to the surface, thus there is minimal shear at the barrel wall. The
resulting discharge temperature is much lower than when the barrel
temperature is increased to the point where the molten resin adheres. As
resin sticks to the barrel wall, the shear between the paddle tips and the
wall generates additional heat seen as a sharp increase in melt
temperature.
In most cases cooling the barrel will increase the melt viscosity; increasing
the shear, and thereby increasing the dispersion within the product.
Since in most cases the conveying capacity of the feeding section of the
extruder exceeds the melting capacity; a metering device must be used to
feed the twin-screw compounder. This metering device should be capable of
a continuously adjustable range of feedrate that is uniform for a given
setting. (Uniform feed means that rate checks on several consecutive 10
second samples are within 1-2% of the average). The uniformity of feed
rate is important because of the short residence time and limited back mix
within the extruder. Typical residence time will be approximately 10-30
seconds for a 15D barrel, and 15-40 seconds for a 20D barrel, i.e. longer
barrel machines will have a proportionally longer residence time.
Additionally the filled length will vary with throughput; other parameters
being constant causing variation of the input energy resulting in possible
product property variations.
Typical meter feeding devices are:
• Volumetric screw feeders
• Volumetric vibratory feeders
• Gravimetric screw feeders
• Gravimetric belt feeders
Single Stage Feeding
In single stage compounding all feed ingredients are metered into one feed
port. The materials are conveyed by the feedscrew to paddles where
melting and compounding occurs. The filled length is controlled by the
agitator configuration. Product is discharged by the camelback discharge
screw.