Glossary of Common Thermoforming Terms

Many of the definitions below were obtained from “Understanding Thermoforming” by James L. Throne and published by Hanser Gardner, Cincinnati, 1999. Terms and definitions are used with the author’s permission. (January 2005)

Amorphous polymer: A polymer that has no melting temperature.
Absorption: The amount of radiant energy absorbed by plastic.
Angel hair: Fine fibers caused by improper trimming technique.
Areal draw ratio: The ratio of surface area of the formed part to that of the sheet used to form the part.
Billow: Pre-stretching sheet by inflation with air pressure.
Blend: Physical melt-mixing of two or more polymers.
CMM: Coordinate Measuring Machine
Cavity isolator: See “Grid.”
Chamfer: Bevel
Chill mark: A mark on the formed part that is often attributed to contact with a cold mold or plug.
Coining: Localized pressing of heated sheet between two portions of mold.
Conduction: Energy transfer by direct solid-to-solid contact.
Contact heating: Heating of sheet by conduction.
Convection: Energy transfer by moving or flowing fluids.
Coordinate Measuring Machine: Accurate three-dimensional electronic ruler used in quality control.
Co-polymer: Polymer with two sets of monomers such, such as HIPS.
Cross-machine direction: At right angles to the extrusion direction; also known as transverse direction.
Cut-sheet: Usually heavy-gauge sheet, fed one at a time into a rotary or shuttle thermoformer.
Cutter: Mechanical bit with tip designed to cut specific types of plastics.
Dam: Continuous ridge around mold cavity periphery.
Density: Weight per unit volume.
Diaphragm forming: Stretching heated sheet using an elastic membrane.
Differential pressure: The difference in the pressure on each side of a sheet.
Dimensional tolerance: Part-to-part variation in local dimension.
Draft: Mold angle from vertical.
Draw ratio: Measure of the extent of sheet stretching; a measure of the area of the sheet after being formed to that before forming.
Elastic liquid: A molten polymer that has both fluid and solid characteristics; sometimes called a viscoelastic polymer.
Elastic modulus: See “Modulus.”
Enthalpy: A thermodynamic measure of the intrinsic heat content of a polymer.
Equilibration: Allowing a sheet to approach uniform temperature throughout, after the heating source is removed.
Endothermic foaming agent: A chemical that requires heat to decompose to produce gas for agent foaming. Sodium bicarbonate is a common endothermic foaming agent.
Energy dome: When energy input to sheet is uniform, temperature at the center of the sheet is higher than at the edges and corners.
Exothermic foaming agent: A chemical that gives off heat when decomposing to produce gas for agent foaming. Azodicarbonamide (AZ) is a common endothermic foaming agent.
Extrusion: The process of producing plastic or foam sheet.
Extrusion/forming line: A process where the extruder output feeds directly into the thermoforming machine.
FEA: Finite Element Analysis, a mathematical method for determining stress distribution when an object is mechanically deformed.
FEM: Finite Element Method, See FEA.
FFS: Form, Fill and Seal; in-line thin-gauge process used in food and medical device packaging.
Free surface: The sheet surface that is not in contact with the mold surface.
Foaming agent: Additive that produces gas during extrusion to produce foamed sheet.
Female mold: A cavity into which the heated sheet is stretched; also known as a negative mold.
Gel: Hard resinous particle in plastic sheet.
Glass Transition Temperature (Tg): The temperature range above which a brittle or tough polymer becomes rubbery.
Grid: A mechanical frame that presses hot sheet against a multi-cavity mold; also known as a cavity isolator or hold-down grid.