Extrusion Methods
By
plasticizing, there are dry extrusion and wet extrusion processes and by
pressurizing there are continuous and intermittent extrusion processes
Continuous
production yields a high efficiency, simple operation for a wide range of
applications, such as tubing extrusion, sheet/film extrusion etc.
Profile
Extrusion
The Profile
Extrusion process, such as drinking straws and medical tubing, is
manufactured the same as a regular extrusion process up until the die.
Placing a pin or mandrel inside the die extrudes hollow sections, and in most
cases positive pressure is applied to the internal cavities through the pin.
Tubing with
multiple lumens (holes) must use tooling made for specialty applications. For
these applications, the tooling is made by placing more than one pin
in the center of the die to produce the number of lumens necessary. In most
cases, these pins are supplied with air pressure from different sources. This
way, the individual lumen sizes can be adjusted by adjusting the pressure of
the individual pins.
Co-Extrusion
Co-Extrusion
is the extrusion of multiple layers of material simultaneously. This type of
extrusion utilizes two or more extruders to melt and deliver a steady
volumetric throughput of different viscous resins to a single die and extrude
the resin in the desired form. This process can be used in a variety of
extrusion processes (blown film, profile or sheet). The layer thicknesses
are controlled by the relative speeds and sizes of the individual extruders
delivering the materials.
There are a
wide range of reasons why a manufacturer may choose co-extrusion over single
layer extrusion. One example is in the vinyl fencing industry, where
co-extrusion is used to tailor the layers based on whether they are exposed to
the weather or not. Usually a thin layer of compound that contains expensive
weather resistant additives are extruded on the outside while the inside has an
additive package that is more suited for impact resistance and structural
performance.
Extrusion
Coating
Extrusion
coating is used in a blown or cast film process to coat an additional
layer onto an existing roll stock of paper, foil or film. For example, this
process can be used to improve the characteristics of paper by coating it with
polyethylene to make it more resistant to water. The extruded layer can also be
used as an adhesive to bring two other materials together.
Compound
Extrusion
Compound
extrusion is a process that mixes one or more polymers with additives to create
a plastic compound. The feed stock may be pellets, powder and/or liquids, but
the product is usually in pellet form and used in other plastic-forming
processes such as extrusion and injection molding. Machine size varies from
tiny lab machines to the biggest extruders in the industry, running as much as
20 tons per hour, as used by the chemical companies that make base resins.
Usually twin-screw extruders are preferred because they offer superior mixing
at lower melt temperatures. Most of these have screws and barrels comprised of
smaller segments (mixing, conveying, venting and additive feeding) so that the
design can be changed to meet the production and product needs. Single-screw
extruders can be used for compounding as well, especially with appropriate
screw design and static mixers after the screw. Selection of the components to
be mixed (viscosities, additive carriers) is as important as the equipment.
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