THERMOFORMING

THERMOFORMING

Thermoforming is the process of heating a
plastic material in sheet form to its particular
processing temperature and forming the hot and
flexible material against the contours of a mold
by mechanical means (e.g., tools, plugs, solid
molds, etc.) or pneumatic means (e.g., differentials
in air pressure created by pulling a vacuum
or using the pressures of compressed air).
When held to the shape of the mold and allowed
to cool, the plastic retains the shape and
detail of the mold. Because softening by heat
and curing by the removal of heat are involved,
the technique is applicable only to thermoplastic
materials and not to thermosets.
Advantages of thermoforming over most
other methods of processing plastics include
lower tooling and machinery costs, high output
rates, the ability to use predecorated plastic
sheet, and good-quality physical properties in
finished parts.
Its disadvantages include the need to begin
with sheet or film rather than less costly basic
resins, trimming material used to clamp sheet
for forming, and the problem of trim scrap reclamation.

METHODS OF THERMOFORMING
There are different technique used in thermoforming some
of them are
1.VACUUM THERMOFORMING

To vacuum form a thermoplastic sheet into a
female mold without prestretching, the ratio of
depth to minor dimension of a given section
should not be greater than 1 : 1, and no sharp
inside radii are required. The sheet stock is
locked in a frame around its periphery only, is
heated to a predetermined temperature, and
then is brought into contact with the edge of
the mold. This contact should create a seal so
that it is possible to remove the air between the
hot plastic and the mold, allowing atmospheric
pressure (about 14 psi) to force the hot plastic against the mold

2.PRESSURE FORMING

In this metod a positive air pressure
is applied against the top of the sheet to force
it into a female mold, and, at the same time,
full vacuum also is applied. As contrasted to
vacuum forming, pressure forming offers a
faster production cycle (the sheet can be formed
at a slightly lower sheet temperature), greater
part definition, and greater dimensional control.

PLUG ASSIST FORMING

Straight cavity forming is not well adapted to
forming a cup or box shape. The sheet, drawn
down by vacuum, touches first along the side
walls and then at the center of the bottom of
the box-shaped mold and starts to cool there,
with its position and its thickness becoming
fixed. As the sheet continues to fill out the
mold, solidification continues in such a way as
to use up most of the stock before it reaches the
periphery of the base; hence this part of the article
will be relatively thin and weak.

To promote uniformity of distribution in such
shapes, designers use the plug assist-any type
of mechanical helper that carries extra stock toward
an area that otherwise would be too thin.

Note that it is also possible to reverse
mold and plug assist from the bottom
platen to give better material utilization around
the edges.

4.DRAPE FORMING


This method also is adaptable to either machine
or manual operation. After framing and heating,
the stock is mechanically stretched over a
male mold to allow the framed edge to make a
seal with the periphery of the mold. This
stretching serves to redistribute or preform the
sheet prior to application of the vacuum. It has
the disadvantage of allowing the stock to touch
prominent projections, freeze there, and perhaps
rob other areas of sufficient mass to make
acceptable articles. Careful control of the temperature
of the mold, plus selective heating of
the sheets, can alleviate some difficulties.

5.MATCHED MOLD FORMING

In this operation, the plastic
sheet is locked into the clamping frame and
heated to the proper forming temperature. A
male mold is positioned on either the top or the
bottom platen with a matched female mold
mounted on the other one.

The mold then is closed, forcing the plastic
to the contours of both molds. The clearance
between the male and female molds determines
the wall thickness. Trapped air is allowed to
escape through both mold faces. Molds are held
in place until the plastic cools and cures.
Matched mold forming offers excellent reproduction
of mold detail and dimensional accuracy.
Internal cooling of the mold is desirable
in this technique.