Screw Essentials

More on what you need to know about the screw to prevent molding issues and maintain a reliable process.

The screw provides the reciprocating or starts and stops rotation during the molding process.
 
The barrel is typically a steel cylinder that houses the screw and valve assembly, and onto which the end cap bolts.
 
There are many types of valve assemblies, so your customer should be working with their screw supplier to have the most efficient application engineering done up front in the design phase.
 
Coatings and application-specific options include chrome plating, carbide encapsulation and specialty steels. Your customer should be aware of these and know how to configure for optimal performance. 
 
L/D Ratio means the larger the ratio, the greater the shear heat that can be produced uniformly without degrading the polymer being processed.  You have greater opportunity for mixing, resulting in a homogeneous melt and an improved opportunity to control conductive and shear-related heat, resulting in faster cycle times. 
 
The profile is described in turns or diameters along the screw’s length, which make up the three zones of an injection molding screw.  The feed zone is where the plastic first enters the screw and is conveyed along a constant root diameter.  The transition zone is where the plastic is conveyed, compressed and melted along a root diameter that increases with a constant taper. The meter zone is where the melting of the plastic is completed and the melt is conveyed forward along a constant root diameter reaching a temperature and viscosity to form parts. If a screw is designed for a specific resin, its profile is designed to mimic the melt curve of a semi-crystalline material or the glass transition profile of an amorphous material.  With this option, energy can be applied at a given rate that the material can accept without any uncontrolled degradation, and in turn provide an optimum melt.
 
The channel depth in the meter section is determined by the polymer being processed.  The channel depth in the feed and transition sections are dependent upon the selected compression ratio and screw profile. 
 
Compression ratio is the ratio between the feed zone channel depth to the meter zone channel depth. A GP style screw may range in CR from 2.5 – 2.8 depending on the supplier and application. A screw designed for a shear-sensitive amorphous material could range from 1.5 – 2.5. A semi-crystalline material could range from 3.0 – 5.0.  
 
Generally, injection molding screws are designed as a square pitch or 17.66° (the distance between flights is equal to the diameter of the screw).  As materials and designs mature, a reduction in the helix angle (to mimic a longer L/D)  has shown advantages.  By reducing the angle it can convey stiffer materials with greater ease and less torque. It can also improve the homogeneous melt quality due to the decrease in the axial melting length. It simply has more time to transition.
 
The 80-20 rule is a rule of thumb. It’s a generalized range that says: If the total material (shot) needed for one cycle falls somewhere between 20 to 80 percent of the total shot volume required (parts and runners combined), it should work. 
 
Shot volume (or capacity) is a machine calculation based on the volume of plastic in front of the screw when it is fully retracted. The machine shot volume is calculated using the density of PS (Poly Styrene) of 1.05, for example.  When molding other resins with different densities (0.9 for example), the shot volume expected could be incorrect (delta of 1.05 to .9). This miscalculation can lead to using false data to establish a process. Then if issues arise, the problems may be misunderstood, potentially sending the team in the wrong direction when seeking root cause solutions. One of which could be a mold issue that does not really exist.
 
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