wiredInUSA Magazine

Guill provides helpful terms for extrusion

Denis Finn, chief engineer at Guill Tool & Engineering, has compiled a glossary of useful terms used in the extrusion business. Mr Finn would welcome 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 comprising 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 or 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.
  1. 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.
  2. Area draw — the 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 of the 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-homogeneous 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 build-up on outside surface of tooling. Build-up 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 — the 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.
  1. Encapsulated — stripe between outside and inside surfaces of product.
  2. Embedded — surface stripe with established depth. Typically a percentage of the wall.
  3. Surface — stripe only on surface of product.
  4. Window — stripe 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 polymer should be before it is processed.
• 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.