Almost all extrusion processes use a melt-through wire mesh screens that filter-out any contaminants before passing through to a die. The screens are held together by breaker plates, which have holes or slots and these form a seal between the extruder and the die.

Wire mesh screens can also improve mixing and protect gear pumps from being damaged by hard contaminants. Fine meshes make the most suitable shields for gear pumps that have small clearances.

Typically, screen packs will place the coarsest screen against the breaker plate and the finest screen facing the screw. This prevents the screen from breaking or blowing contamination through the breaker plate holes and being torn or twisted from the rotating motion of the melt. 

A screen pack may have two screens of the same mesh at either end. For example: a pack may contain screens in the following order: 20 mesh / 100 mesh / 60 mesh / 20 mesh. This type of pack can look the same from both sides, so processors must ensure that it’s not inserted backwards. 

Although some processors will argue that in doing so, the coarsest screen will catch the larger particles on the upstream surface. Advocates of this approach claim it allows more lateral melt circulation and less degradation on the upstream surface of the breaker plate.

A fear that some processors hold is that screens can introduce new variables to the process which can raise back pressure, increase melt temperature and sometimes even reduce output. This means that more stabilisation is needed, which adds to material costs.

This is often the case for extruding rigid PVC, so some processors avoid using screens altogether. However, in doing so, plastics are vulnerable to contamination which causes bigger problems further down the line.

The solution to this is using a relatively coarser pack or special screen changers for plasticised PVC.

If you’re extruding PVC it’s also handy to remember not to leave unprotected screens on top of the extruder, as they are exposed to the hydraulic acid in the air near the die, which causes corrosion. The best way around this is to add a fume hood over the die head and keep spare screens stored away. 

To find out more about what screen is best for you click here.