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Glossary of Common Thermoforming Terms
Many of the
definitions below were obtained from the book by James L. Throne, entitled
Understanding Thermoforming and published by Hanser Gardner, Cincinnati, 1999,
and are used with the author's permission. (January 2005)
Glossary A-D /
Glossary E-H / Glossary I-P / Glossary R-S / Glossary T-Z
- Amorphous polymer: 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 to 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
Glossary A-D / Glossary E-H / Glossary I-P / Glossary R-S / Glossary T-Z / Top of
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- 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 sheet
- Extrusion/forming line: A process where the
extruder output feeds directly into the thermoformer
- 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
Glossary A-D / Glossary E-H / Glossary I-P / Glossary R-S / Glossary T-Z / Top of
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- 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 machine after the
thermoforming machine
- In-place trimming: Trimming that takes place
while the formed sheet is still within the thermoforming machine
- 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 impale or 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
- Pusher: See Plug
Glossary A-D / Glossary E-H / Glossary I-P / Glossary R-S / Glossary T-Z / Top of
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- 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: Trim 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
Glossary A-D / Glossary E-H / Glossary I-P / Glossary R-S / Glossary T-Z / Top of
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- Tab: An uncut portion or a trimmed part,
used to retain the part in its web in thin-gauge
- thermoforming
- 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: Trim in thin-gauge thermoforming
- Zoned heating: See Pattern heating
Glossary A-D / Glossary E-H / Glossary I-P / Glossary R-S / Glossary T-Z / Top of
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