7/28/2019 Extrusion Screens and Screening

1/2

19.02.13 Extrusion Screens and Screening

www.ides.com/articles/processing/2007/griff_screens.asp 1/2

Login to Prospector

UL IDES >Articles > Processing

Extrusion Screens and Screening

What are screens and why do we use them?

Wire-mesh screens are used in almost all extrusions, to filter out contaminants and also to build up pressure in the extruder. Pressure is both

good and bad: it helps mixing, especially if screw speed is high and the plastic isnt in the system very long. However, more pressure may

increase melt temperature coming out of the extruder, which could limit production rate.

When to change screens

The most sensible way of doing this is to watch the pressure gauge, which should be sensing pressure at the tip of the screw just before the

screens, and set a limiting pressure, so that when it gets that high the screens are changed. This is done manually with small equipment,

putting the screen into the end of the barrel such that they dont block the seal line ; otherwise leakage will occur during operation. The changing

should be manually managed even with automated changing equipment, as the changing time may need to be adjusted to match ends of runs,

shutdowns or color changes.

Ideally, the pressure gauge should be set to flash a light or other alarm as the pres sure is getting close to the limit, but this is seldom done, and

the operators still need to watch the gauge.

Sometimes there is no pressure gauge or the existing gauge isnt working, so until one is installed or it is fixed, screen changing must depend

on something else, such as the motor, which will reflect higher pressure by drawing more current, which can be seen on the motor ammeter.

It is common to set a regular time, such as once a day, or once a week, to change screens, but this may create problems if the contamination

level varies substantially with time.

What sizes are used

Typically, the screens are supported on a breaker plate, a perforated steel disk that also serves as the seal between the head section (die and

adapter) and the extruder barrel. They are usually square mesh that is, same number of wires per inch or cm in both directions. The exception

is Dutch weave, where one direction uses fewer but thicker wires than the other. That makes a stronger screen with rectangular openings, and

only one of these is usually enough. In most extrusions, though, a screen pack is used, which is made up of several screens a 20-mesh (20

wires per inch) is put up against the breaker plate, then perhaps a 40-mesh, then an 80 if that much filtration is wanted. Some very critical

applications such as fibers, filaments and thin film use even finer screens, such as 200-350 mesh. Beyond this, there are three-dimensional

media such as sand beds, sintered metal, and other specialized systems that are common in synthetic fiber spinning but seldom used in

conventional extrusion and are beyond the scope of this article. Dont forget the wire gauge. The same mesh size can have bigger holes if thinner

wire is used, so once a size of screen is found to work well, make sure the same wire gauge is reordered as ell as the mesh. Sometimes

screens are doubled up two 40s or two 80s to get a tighter pack. If that is done, make sure that the doubles are set at 45 degrees to each

other, so that maximum and consistent filtration capacity is achieved.

Although most screen packs are assembled with the heaviest screen next to the breaker plate, a few operations do it the other way round, and

that works too. Proponents of the first way, heaviest next to the plate, say that the heavier screens support the finer ones and prevent their

blowing through from the differential pressure. The others note that if the fine screen is next to the plate, the coarser ones will catch most of the

contamination and the pressure drop across the finest one will stay low, hence no blow-through.

Some packs are actually sandwiches, with a coarse screen on both sides, partly to protect the finest screen that otherwise might be exposed to

the rotating flow of the melt just coming off the screw, and partly to make a symmetrical pack which can be put in either way with the same

result.

When to use a screen changer

There are many devices sold to help change screens, basically reducing the downtime required to do this job. For a small machine (2.5

diameter or less) it is a relatively simple and quick job to open the head and replace the screens maybe 30 minutes to an hour, and the line is

back in production again, depending more on the time to stop and re-thread the line rather than the time to do the actual changing. For bigger

machines, or lines where opening the head is not so easy, a screen changeris preferred. And where recycled material is being used and

contamination levels are high, the changes are much more frequent and the value of a changer is higher.

Many changers are the slide-plate or lever type, where a new breaker with screens is pushed into position and the old one comes out. In such

cases it is good to have at least two plates, so the line doesnt stay down while the old one is cleaned.

Some changers require the line to be stopped during the change, but many can be operated on the fly, with some production time lost to

Plastics UL Yellow Cards Generics Properties More

http://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://search.ides.com/search.aspx?query=pressure+gaugehttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=141&CampaignID=938&AdvertiserID=123&BannerID=1110&SiteID=1&RandomNumber=1091425495&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://www.ides.com/http://www.ides.com/http://plastics.ides.com/materialshttp://plastics.ides.com/yellowcardshttp://plastics.ides.com/genericshttp://plastics.ides.com/genericshttp://www.ides.com/property_descriptions/http://www.ides.com/articles/processing/2007/griff_screens.asp#http://www.ides.com/articles/processing/2007/griff_screens.asp#http://prospector.ides.com/loginpage.aspxhttp://www.ides.com/http://www.ides.com/articles/processing/2007/griff_screens.asp#http://www.ides.com/property_descriptions/http://plastics.ides.com/genericshttp://plastics.ides.com/yellowcardshttp://plastics.ides.com/materialshttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=137&CampaignID=1171&AdvertiserID=142&BannerID=1417&SiteID=1&RandomNumber=2109335413&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://search.ides.com/search.aspx?query=screen+changerhttp://search.ides.com/search.aspx?query=wire+gaugehttp://search.ides.com/search.aspx?query=Dutch+weavehttp://search.ides.com/search.aspx?query=breaker+platehttp://search.ides.com/search.aspx?query=pressure+gaugehttp://search.ides.com/search.aspx?query=Wire-mesh+screenshttp://services.ides.com/Advertising/a.aspx?Task=Click&ZoneID=141&CampaignID=938&AdvertiserID=123&BannerID=1110&SiteID=1&RandomNumber=1091425495&Keywords=%2farticles%2fprocessing%2f2007%2fgriff_screens.asphttp://twitter.com/ul_ideshttp://www.linkedin.com/groups?gid=1813581http://www.ides.com/articles/processing/2007/griff_screens.asp#http://www.ides.com/register/prospector.aspxhttp://www.ides.com/articles/processing/http://www.ides.com/articles/http://www.ides.com/http://prospector.ides.com/loginpage.aspxhttp://www.ides.com/

7/28/2019 Extrusion Screens and Screening

2/2

Products Advertising About Contact UL and the UL logo are trademarks of UL LLC 2013

http://www.ides.com/articles/processing/2007/0727_griff_screens.asphttp://search.ides.com/search.aspx?query=fluoroplasticshttp://search.ides.com/search.aspx?query=PVDChttp://support.ides.com/http://www.ides.com/about/http://www.ides.com/advertise/http://www.ides.com/products/