CUSTOM-DESIGNED SCREWS

Special screw designs are often used to enhance the performance of the molding machine.  The more common types of special screw designs include changing channel depths and flight profiles to match the melting curve of the resin.  Mixing screws or barrier screws can also be used for special applications.  Each of these screw variations is designed to achieve a special processing result and can be very effective in solving processing problems.

Johannaber states in his text Injection Molding Machines that "The standard three-zone screw is not designed for efficient mixing."   There are many mixing screw designs offered in the marketplace that involve the shear and reorientation of the resin after it has been melted.  It is important to remember that mixing cannot be achieved without first melting the plastic.

The selection of the proper screw for a given processing application should be based on the resin or resins to be processed.   Screw design includes the selection of the material from which the screw is to be manufactured in addition to choosing the proper geometry, such as The Eagle™  or The Barrier Eagle™. 

MIXING SCREW DESIGN OBJECTIVES

The following objectives are required to obtain optimal mixing capabilities on a screw:

1.    Proper design of feed and transition zones of the screw to achieve a good melt quality prior to entering a mixing zone.

2.    Minimum pressure drop achieved by avoiding excessive constriction of flow through the mixing section.

3.    Avoidance of hang-up areas in the mixer where melt can gather, overheat, degrade or cause variation in color intensity.

4.    Uniform melt temperature accomplished by the use of wiping lands in the mixer which help recirculate the melt and provide a forward pumping capability.

5.    Reorientation of the melt flow in the mixer combined with uniform shear stress without causing excessive melt temperature or heat override.

At Westland, the design variables (shown above) are chosen to match the resins being processed to optimize the operating performance.  There is no extra charge for customizing the screw design.  Screw profile (length of zones), channel depths, compression ratios and helix angles may be altered, along with the selection of the proper screw material, to produce a design to fit the customer's processing requirements!   Each of these design elements is described in detail within the screw design section under the processing tips header.

The terms dispersive mixing and distributive mixing are used by some authorities to distinguish the mixing of the fluids (distributive) from mixing of a fluid with a solid or unmelt (dispersive).  For the sake of simplicity, however, we prefer to limit the discussion of mixing screws to those factors we consider essential to their design. 

The inclusion of all of these factors in the design of a mixing section helps assure an isothermal melt of uniform viscosity with optimum mixing color.  Heat override and material degradation are likewise avoided.

Further information on Westland Corporation's patented Eagle™ Mixing Screw, go to The "Eagle™"  or The "Barrier Eagle™" pages.