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Guill Provides Helpful Terms for Extrusion

Guill’s Chief Engineer, Denis Finn,  provides this helpful glossary of useful terms, used in the extrusion business worldwide. He would appreciate any feedback. 

  • Extrusion Die assembly- also known as crosshead, inline tubing die, head, die.  Connected to extruder by feed pipe/flange.  Forms extrudate into desired shape.
  • Tooling- comprised of two components—the tip and the die.  
  • Tip- defines ID of product for a tubing application or locates the substrate in a jacketing application.  Tips are also known as pins, mandrels,
  • Die- defines OD of product.  AKA bushing.  
  • Land length- straight section at end of the tool set. 
  • Gum space- distance between end of tip and back of die land. 
  • Draw down ratio- relationship between ID and OD of tooling vs. the ID and OD of the finished product.  
  • Linear draw- the ratio of the ID of the die and OD of the product.  And the ratio between the OD of the tip vs the ID of the product.  
  • Area draw- ratio between the cross sectional area of the output of the tooling and the cross sectional area of the finished product. 
  • Draw balance- ratio of the outer surface of the draw down cone to the inner surface of the draw down cone.
  • Concentricity- how well centered the OD of the product is to the ID of the product or the jacketed substrate.  
  • Ovality- how round the OD your product is
  • Dead spot- section of the flow where material is allowed to stagnate.  Usually causes burning and degradation issues over time. 
  • Balanced flow- uniform exit velocity over the cross section of the product.  A well balanced flow produces a higher quality product. 
  • Fish eye, gel, unmelt, comet- common terms for unmelted or non-homogenous melt
  • Melt fracture-surface defect in product caused by shear degradation (usually manifests as a grainy/frosted appearance on surface of extruded part.) 
  • Die drool- particle buildup on outside surface of tooling.  Buildup can break off and cause defects on finished product.  
  • Bulk residence time- amount of time polymer spends in the extrusion die assembly 
  • Shear- term to describe a force that the melt experiences as it is forced through the extrusion die assembly. 
  • Viscous dissipation aka shear heat- frictional heat generated as a result of shear. 
  • Viscosity- resistance to flow
  • Cold spot- section of extrusion die assembly that is not adequately heated resulting in imbalanced flow or a thermoplastic freeze off
  • Rheology- science of characterizing a fluid’s behavior
  • Draw down cone- section of molten polymer as it draws down to its final product dimension.
  • Calibrator- device to precisely establish and control the desired product dimensions
  • Stripe-feature to identify or establish orientation of finished product. 
  • Encapsulated-  stripe between outside and inside surfaces of product
  • Embedded- surface stripe with established depth. Typically a percentage of the wall.
  • Surface- stripe only on surface of product.  
  • Window- strip that goes through wall from OD to ID. 
  • Surging- inconsistent melt output that results in product dimension variation..  
  • Coextrusion- any extrusion that uses two or more extruders.  
  • Wire line/pass line- distance between face of extruder to the center line of the crosshead.  
  • Direction of travel- a way to establish production line orientation.  
  • Recommended moisture content- a qualifier of how dry your polymer should be before you process it.
  • Substrate- the material that is being coated in a jacketing process.  Can be wire, bundle of strands, fibers, etc. 
  • Jacketing- covering a substrate with melt.
  • Multi-lumen- a tube with multiple internal cavities.
  • Melt probe- a device to directly measure  pressure and/or  temperature of the melt.
  • Thermocouple- a device to measure temperature of the extrusion die assembly.  Typically used to control heater output. 
  • Heater bands- provide heat to the extrusion die assembly. 
  • Flow distributor- shapes the melt to achieve balanced flow.

For more information, please contact: 

Denis Finn
Guill Tool & Engineering
10 Pike Street 
West Warwick, RI 02893 
(401) 823-7600