Extruders (Screw Design)


	Friday, May 09, 2008

Extruders (Barrier Screws)

Extruders (Cooling Systems)

Computing and analyzing the melting capacity of a single screw extruder, enables the barrier melting section of the screw to be designed around its melting capacity. By doing this, Techflow barrier screws assure that the geometry of the screw matches the melting behavior of the polymer. Because the screw geometry is fixed once the screw is manufactured, it is important to predict how the screw melting behavior will be affected by changes in processing conditions. These include changes in resin feed properties, motor speed, back pressure and changes in the temperature profile of the barrel heaters along the screw.

Proper barrier screw design involves determining how large the melt and solids channel should be in order to accommodate the melt that is being generated. The first step in achieving this is to determine the melting capacity of the barrier screw. This can be estimated from the thermal and material properties of the polymer as well as process related variables including screw speed, barrel temperature settings and the surface area where the solid bed is exposed to the barrel. By calculating the melting capacity along the axis of the screw it is possible to determine the volume of melt that is being generated at any point along the screw.

Processing Problems Due to Improper Screw Design

The volume of the melt channel must be large enough to accommodate the polymer melt or a flow restriction will result because the melt will be compressed and forced to occupy the solids channel or leak backwards over the primary flight into a preceding section of the screw. The leaking of material over the main flight will increase the temperature of the exiting polymer. In both cases, this surplus molten material will interfere with the melting process reducing the output of the screw. Furthermore screws whose geometry does not match their melting behavior have higher pressures in their axial profiles. This is the main cause of premature screw wear since the high pressures will cause the flight of the screw to be pushed against the barrel wall. Once the melt and solids channels are sized to match the melting behavior of the polymer various process improvements can be realized. The main advantages include lower pressures along the screw, higher throughput rates due to lower melt temperatures, and lower pressure fluctuations causing stable extruder operation.

Whatever your application, we can work with you to solve your extrusion process problems or improve your current standards at a price you will agree is very competitive. Using our proven scientific approach to screw design we avoid the costly trial and error work that has historically plagued screw design. In a world where process optimization makes the difference in profitability, you need an extruder screw that is able to perform to today's ever increasing demands. We draw from our experience and knowledge base to provide you with an extruder feed screw and barrel to allow you to meet today's and tomorrow's demands.

Standard Feed Screw Construction Features:

Chrome plating in root
Wear resistant full flight width Colmonoy 56, 83, X830, or UCAR
Spiral or straight Maddock mixing section for dispersive mixing
Pineapple, Saxton, or pin mixing section for distributive mixing
Highly polished streamlined surfaces
High quality materials and construction (manufactured from aircraft quality 4140, 4340, stainless steel or Duranickel)

Standard Barrel Construction Features:

Bimetallic construction
Guaranteed straightness of 1 in 20,000
58-64 Rockwell C
Finish to within 8-12 micro-inches
Enhanced abrasion and corrosion resistance

Process Improvement	Effect
Lower pressures along screw axis	Reduced screw wear maximized pressure stability and less power consumption
Less compression of melt	Optimized melt temperature and stability, maximized output and less polymer degradation
Efficient melting	Higher melt quality (no gels), at higher outputs
No flow restriction	Higher output and pressure stability
Improved mixing / melt	Wider blending ranges, uniform melt properties and maximized process flexibility
Streamlined design	Cleaner operation