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surface detailing on both sides of the part. Usually, one of the moulds is made from a
softer material (e.g., rubber) to lessen the need for monitoring and realignment. The
softer surface is not shaped exactly to the part but is used simply to force the sheet
against the opposite mould face. The sheet is heated to a significantly lower
temperature compared to conventional thermoforming, saving heating costs, but
tooling investment is higher because the process requires two moulds.
ADVANCED THERMOFORMINGS : Here, the heated sheet is pre-stretched prior to
forming. This promotes even material distribution and uniform wall thickness.
Advanced methods are used for applications with deep draw ratios.
PLUG-ASSIST VACUUM FORMING:
In plug-assist forming the clamped, heated sheet is pre stretched using a
mechanically driven plug into a female mould, simulating the drape forming effect on
wall thickness. Vacuum is then applied from the mould side to suck the sheet against
A
B C D
Seal Thick
Even
Walls
Mold
Clamp
Formed Part
Vacuum
the mould surface. Some slippage between hot sheet and plug may be required to
achieve the required wall thickness uniformity, but the plug must be very smooth to
avoid marking the sheet
PLUG-ASSIST PRESSURE FORMING:
Plug-assist forming can also employ pressure instead of a vacuum. After the plug
pushes the sheet into the mould, pressure from the plug side makes the sheet billow
up around the plug. The sheet contacts the mould and the part is formed.
A B
C D
Air
Air on
Air on
Thick
Seal Even
Walls
Mold Clamp Formed Part
Vent
Vent Plug-assist Pressure Forming.
VACUUM SNAP-BACK FORMING:
In the vacuum snap-back process, the heated sheet is sealed to the vacuum box. Then
a vacuum is slowly applied to the vacuum box and the sheet is pulled down into it
causing a hemisphere shape to form. Then the mould on the top platen is lowered into
the hemispheric shape of the sheet in the vacuum box. At this point the vacuum is turned
off in the vacuum box and a vacuum is applied to the mould chamber. This causes the
sheet to snap up against the mould surface, hence the term “snapback”. Then the
vacuum box is dropped away from the mould to allow the plastic to cool on the mould.

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