The Practical Processor: August 2010

Your Processing Questions Answered

Q. How do I know how much screen (filtration) area will be required for my application? —Northeast film extruder

A. When specifying a screen changer of any type, there are several ways to determine the proper amount of screen area:

  1. If you have a line running the same material, you can simply measure the rate and existing pressure with the desired screen configuration. Scale-up is linear. For instance, if you are running 100 lb/hr at 500 psi pressure drop, you will need exactly twice the screen area for 200 lb/hr and 500 psi. Contaminant holding capacity is also a factor, particularly in the case of recycled material. Even if you have enough screen area for an acceptable clean-screen pressure drop, you may not have enough screen area to last for an acceptable length of time until the screen blinds. Since most of the pressure increase occurs in the last 20% of screen life, there may be little time to react after pressure increase begins, so contamination level will be as important a consideration as initial pressure drop. Contaminant type and distribution are also important, since intermittently high contamination levels can skew performance. For example, if a normally clean product randomly has cardboard boxes ground up by accident, the result can be an unacceptable screen life at times when the cardboard contamination passes through, while screen life may be acceptable at other times.
  2. Send rheology curves along with desired rate and pressure drop to the manufacturer. Most reputable manufacturers will have computer software to predict pressure loss.
  3. If no empirical data is available, find a laboratory to run your material with a known filter medium, screen size, and rate to develop such data.
  4. If your material is virgin and clean, you may be able to simply size the screen changer based on the manufacturer’s guidelines, derived from extruder barrel size and rate. However, if you change resins, and if you plan to run recycled content, you may want to conduct a more thorough evaluation as described above.

As with most other types of equipment, there may not be a clear choice, so you must be able to compare the various options based on performance, initial cost, labor input, expected maintenance required, as well as future plans for the extrusion line.
Dan Smith, sales manager

Q. I work for a company that runs bumper molds for vehicles. All our molds have multiple hot-runner nozzles. When we torque down a manifold, should we torque it down cold or at operating temperature? Currently, we are heating them up to operating temperature and torquing them down per bolt torque spec. I disagree with this method and believe they should be torqued down at room temperature. What is the correct procedure? —Ohio appliance molder

A. Torque specifications are recommended to be applied at room temperature only. Applying torque at elevated temperature places excess stress on the threads when the tool returns to room temperature.

Bob Starr, director of marketing
DME Company LLC, Madison Heights, Mich.
(800) 626-6653 •