Agriculture Reference
In-Depth Information
Extrudates coming out of the die must be cooled and many systems are provided
by machinery suppliers. In general, water, air or other means are used for cooling,
which should be sufficient to prevent warp. Profiled extrudates sometimes take
the form or complex or unsymmetrical cross-sections. In such cases, the dies must
be designed carefully to achieve uniformity. Extrusion systems have electronically
controlled methods that are fully computerised to control wall thickness with
an image of the cross-section on a computer for easy monitoring and adjustment
(if necessary).
Guidelines for Extruder Shutdown
Different extrusion systems may have different shutdown procedures. However,
it is important to work out a sensible and trouble-free shutdown procedure for
your system to save time and money. Shutdowns can be due to various reasons,
such as power failures, machinery breakdowns, temporary repair, or a shortage
of material. For example, during a stoppage due to a temporary repair, one
can periodically pump the extruder by passing material at a minimum screw
speed without allowing molten resin mix to build up inside the barrel. Also, the
temperatures can be lowered so that the material inside will not heat up and
degrade.
If the shutdown is at the end of a run and the machine has to be emptied due to an
unstable material (e.g., PVC), the following steps are recommended:
• Lower temperatures on cylinder heaters.
• Keep die heaters on.
• Put barrel cooling on maximum.
• Purge barrel of material at low speed.
• Watch the pressure gauge (a rise will indicate cooling of the material).
• When the material is relatively cool, close the hopper.
• Using a purging material such as PE or PS to purge the screw dry.
• If no material is coming out of the die, slow then screw speed and then slow it
to zero.
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