Whenever a screenchanger is added to a new or existing extrusion system, certain precautions need to be taken with respect to the extrusion screw. Typically the screw needs to extend into the screen changer about 2 LD; and the entrance bore of the screen changer must not be hardened like the bore of the bi-metallic barrel. Some screw manufacturers just add a torpedo, or what some refer to as an extended nose, to fill the 2 LD void in the screen changer entrance so that there is not a large slug or inventory of material between the end of the screw and the breaker plate/slide plate.
If the 2 LD screen changer entrance is not filled with the “steel” of the screw, and the void is filled with plastic, several undesirable processing issues can occur. These include:
1) Much longer time to do material or color changeovers.
2) Much longer time for the solid to become molten enough to pass through the breaker plate and screens—and if the slug is not melted it will typically damage or break the breaker plate. If this happens the slide plate will not move because it is jammed, and the whole assemble needs to be removed and disassembled.
3) Degradation of heat-sensitive materials.
4) Flow velocity in the void will be very low and therefore much stagnation will occur.
Using an unflighted extended nose will work, but this is not the optimum modification to the screw. The “torpedo” nose will fill a large portion of the void and reduce the inventory of material, but the resin flow will be annular. The wiping of the bore ID will be non-existent and therefore color change and material change over will still be poor.
The best solution when adding a screen changer to an extrusion line is to have the new portion of the screw that extends into the screen changer to be flighted. But the flights must be undercut so that they do not come into contact with the soft metal bore of the screen changer. The flight is typically undersized to the bi-metallic bore by 0.001 to 0.0015 in. /side per diameteral inch of the screw diameter. Therefore, for the portion of the screw that extends into the screen changer, the flight OD should be undersized approximately twice the normal clearance, or 0.003 in. to 0.0035 in./side per diameteral inch of the screw. By using the design criteria, these two additional turns in the screenchanger can actually be used to generate some “pumping” action and in turn improve the color and material change over time.
One more point, if a screen changer is being added to an existing extruder and the screw is extended into the screen changer as it should be, the 2 LD flighted extension can be made as a removable section, and the existing screw can be faced back so that a female thread is added to the end of the screw and then the new section can be threaded on.
Some processors worry about the flights not lining up, but actually if is best if they don’t. Optimally, if the flights of the new removable section were 180º out of phase, this would be the best. In the past, there were actually screw design patents on this for improved mixing, but they are now “Public Domain.” By misaligning the flights, the new flight splits the melt stream, causing the polymer on the push side to move to the trailing side and the polymer on the trailing side moves to the pushing and actually “flip-flopping” the melt pool.
Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; firstname.lastname@example.org; twwomer.com.