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.


H:D: Height-to-diameter ratio; a measure of draw ratio.
Heat capacity: A measure of the amount of energy required to raise a polymer’s temperature a specific amount.
Heat transfer coefficient: A measure of the effectiveness of energy transport between a flowing fluid and a solid surface.
Heavy-gauge: Commonly, a seet with a thickness greater than 120 mils (0.120 inches or 3 mm).
Homopolymer: A polymer made from a single monomer, such as PS.
Infrared radiation: Electromagnetic energy transmission at wavelength above visible wavelengths.
In-line trimming: In thin gauge, roll-fed forming, trimming that takes place in a separate station after the plastic exits the forming station.
In-place trimming: Trimming that takes place while the formed sheet is still within the forming station.
Linear draw ratio: The ratio of the length of a line scribed on a formed part to the length of a line scribed on the unformed part.
Linear expansion: Increase in polymer direction on heating.
Loft: Expansion of fiber reinforced sheet during heating.
MDF: Medium density fiberboard.
Machine direction: In the extrusion direction.
Male mold: A mold over which the heated sheet is stretched; also called positive mold.
Mark-off: A mark on the formed part that is attributed to contact with plug.
Matched die forming: The process of forming sheet between to mold halves, commonly used in foam forming.
Melt temperature: The temperature range above which a crystalline polymer changes from a rubbery solid to an elastic liquid.
Moat: A continuous groove around mold cavity periphery.
Modulus: The initial slope of the stress-strain curve for a given polymer.
Molding area diagram: Pressure and temperature restrictions overlaid on stress-strain curve for given polymer.
Neat: Polymers that contain no additives, fillers, or reinforcing fibers.
Negative mold: See “Female mold.”


Orientation: The amount of residual or frozen in strain or stretch in a plastic sheet, usually in a given direction.
Pattern heating: The practice of selectively shielding portions of heaters to achieve a specific energy input pattern on a heating sheet; also called zoned or zonal heating.
Peripheral seal: Region around periphery of twin-sheet part.
Pin-chain: Chain that has spikes or pins regularly spaced along its length to pierce and hold thin-gauge sheet.
Plastic: A mixture of polymers and various additives.
Plug or Plug Assist: A mechanical device used to aid or assist sheet stretching prior to total contact with the mold.
Positive mold: See “Male mold.”
Pressure forming: Commonly, differential pressure across the sheet in excess of 30 lbs/in2, (0.2 MPa or 2 atm).
Punch-and-die: A trimming assembly for thin-gauge forming. Sometimes referred to as matched metal, or high die.
Pusher: See “Plug.”
Radiation: Electromagnetic energy transfer or interchange between hot and cold surfaces.
Reflector: A shaped surface that refocuses radiant energy from a round heater.
Replication: Faithful imaging of the mold surface by the hot formed sheet.
Residual stress: Frozen-in orientation in sheet.
Roll-fed: Thin-gauge sheet, fed continuously into the thermoformer.
Rotary press: Heavy-gauge thermoforming machine in which sheet is conveyed from station to station in a carrousel fashion.
Router: A device for trimming heavy-gauge parts.
Sag: Distortion of a sheet, under its own weight, while heating.
Set temperature: The temperature below which a part can be removed from the mold without appreciable distortion.
Shrinkage: Temperature-dependent volumetric change in the polymer.
Shuttle press: Heavy-gauge thermoforming machine in which sheet or oven move in a to-and-fro fashion.
Skeleton: Residual plastic after finished parts have been removed from the sheet in thin-gauge forming.
Soak time: See “Equilibrium.”
Specific heat: See “Heat capacity.”
Specific volume: Volume per unit weight; reciprocal of density.
Spring-back: Elastic recovery of sheet after forming forces are removed.
Strain: Stretch; the polymer’s response to applied stress.
Stiffness: The product of polymer modulus and part geometry.
Steel rule die: Sharpened metal band used in compression cutting.
Stress: Applied load or force on a sheet, per projected area.
Surge tank: The tank between the vacuum pump and the mold assembly to allow near-uniform differential pressure to be applied to the sheet during forming.
Syntac: See “Syntactic foam.”
Syntactic foam: A mixture of sintered inorganic or organic foam spheres and plastic matrix, commonly epoxy or polyurethane, used for plugs, fixtures, and prototype molds, sometimes referred to as syntac.


Tab: An uncut portion or a trimmed part, used to retain the part in its web in thin-gauge
Terpolymer: A polymer with three sets of monomers, such as ABS.
Thermal conductivity: A measure of the time-independent energy transmission through a material.
Thermal diffusivity: A material property measure of the rate of energy transmission.
Thermoforming window: Temperature range over which sheet is sufficiently soft to allow stretching.
Thermoplastics: Two-dimensional organic molecules that can be reprocessed.
Thermosets: Three-dimensional organic molecules that cannot be reprocessed.
Thick-gauge: See “Heavy-gauge.”
Thin-gauge: Commonly, sheet thickness less than 60 mils, (0.060 in or 1.5 mm).
Trapped sheet forming: Conduction heating of sheet.
Trim: That portion of the formed sheet that is not part of the final product.
Vacuum hole: See “Vent hole.”
Vent hole: Small diameter hole drilled through mold surface to allow exhaustion of cavity air; also called vacuum hole.
View factor: A measure of the fraction of radiant interchange that occurs between primary heaters and energy absorbers.
Virgin polymer: Unprocessed polymer.
Watt density: Heater output in W/in2 or kW/m2.
Wavelength: A measure of the nature of incident electromagnetic radiation.
Web: In forming, a fold of plastic that cannot be stretched flat against a mold surface.
Web: Excess trim in thin-gauge thermoforming.
Zoned heating: See “Pattern heating.”