Typically
in all the extrusion processes, involving plastic materials, the melt is
passed through wire mesh screens on their way to the die. This provides
filtering as well as improved mixing. But some times the screens are known
to introduce process variables, elevating back pressure and melt temperature
along with case of a reduction in output. Screens are held together by
breaker plate with holes or slots, that forms a seal between the die and the
extruder. A tiny amount of pressure is added by the clean screens, to the
resistance of the head. The amount of the contamination trapped by them
provides the big pressure variable. The image of extruder screens are shown
here:
Material for Extruder screen
When it comes to choosing the material for extruder screen one needs to be
very careful. Steel is a popular material used in extruder screens. Though
Stainless steel costs more but is used on some plastics extrusion for
avoiding typical problems associated with rust. Nickel alloys are also used
in screens to avoid the problems with regard to corrosion from
fluoropolymers and PVDC.
What's dirtying your Extruder screens?
Dirty screens are a good indicator of what goes through an extruder.
Once a processor is able to trap contaminants or gels, a question may be
asked from where they have came from in the first place. It’s easy
to blame the resin supplier. But gels, that are elastic, and slide
through the finest screens, can be produced in resin plant or by an
extrusion process. If there is many gels at start-up and then they
quickly decrease, they may probably be home-grown, and not at all a
resin maker’s fault. However, in case they start up right away and
stays consistent for considerable hours, there is chance that they have
come in with the resin. |
Tips for Better performance of screens
- Steel screens are prone to rusting in storage, soit is better to keep
them away from moisture.
- A rusty screen can break easily and let through all the collected
contamination. It is advised to pack screens in a poly bag or better in
rust-preventing paper.
- A regular replacement of screens is vital to avoid the problem of
excessive pressure drops that develops across the screen pack, and
results in loss of flow.
- If PVC is extruded it is not advisable to leave unprotected screens
lying on top of the extruder. There can be corrosion by Hydrochloric
acid in the air near the die.
- Screens can suddenly become “blind” if confronted by mass
of contamination, like paper. This can cause a rapid pressure spike in
the extruder. So a safety system in place like rupture disc, shear pins,
an alarm that is connected to the pressure gauge can come handy.
Types of Extruder Screens
Extruder screens typically comes in a vast range of shapes and sizes. Some
of the most popular shapes are described here:
