Your processing questions answered.
Q: What is the maximum head pressure an extruder should run? How can a screw design reduce head pressure but achieve high output?
—Global packaging films supplier
A: It depends on resin viscosity—more viscous resins run at higher head pressures. Optimum head pressure is between 2500 and 3500 psi. Anything higher will affect the output and melt temperature, and use of a melt pump should be considered. Rather than the screw, downstream adapter and die design should be evaluated to reduce head pressure.
TWWomer & Associates LLC, Edinburg, PA
(724) 355-3311 • twwomer.com
Q: How do processors decide which melt-filtration method is best for their application?
—Southeast film/sheet processor
A: The first criterion is the required level of filtration. When the filtration level is equal to 40 microns (325 mesh) or larger, a screen changer is generally indicated. Then determine how often screens will need to be changed, and whether the line needs to be stopped for other reasons more often than screens will need to be changed. If empirical information is available to allow extrapolation of a screen blinding rate, the required rate of extrusion can be mathematically compared to the known rate of extrusion and known blinding rate to indicate the projected screen area that will be needed. For instance, if you have a 100-mm screen changer with an extrusion rate of 1000 lb/hr that blinds in one hour with a known material, then you will need a screen area exactly two times that size for the same results with a 2000-lb/hr line.
If you will normally need to stop the line for other reasons more often than you expect the screens to need changing, a simple discontinuous screen changer might well suffice. A discontinuous screen changer requires that the line be shut down before changing the screen. If you will need to change screens more often than you plan to shut down, you will need a continuous or semi-continuous screen changer. A continuous unit allows screen changes without shutting down the line or producing off-spec product. A semi-continuous screen changer allows screens to be changed without stopping the line, but may produce some off-spec or scrap material in the process. In cases where the product is very dirty, a self-cleaning screen changer may be needed to extend the screen life, reduce operator intervention, and significantly reduce screen cost. This type of screen changer is able to reverse the flow through the screens to clean the screens and extend the time before a new screen is needed.
Dan Smith, sales leader Americas
Maag Pump Systems Inc, Charlotte, N.C.
(704) 716-9081 • maag.com
Almost all extrusion processes pass melt through wire-mesh screens on the way to the die to provide filtering and improved mixing.
Roll finishes often cost more than the roll itself. The right finish improves film or sheet quality and raises output. Yet most processors don't know how to measure surface roughness or to specify it properly.
Here are some tips and techniques for extruders and injection molders to refer to when using CFAs.