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Free Forming Free forming is used to form threedimension shapes entirely
by the use of air pressure differentials-vacuum or positive
pressures without the use of male or female forms (Figures 18 and
19). Parts produced by this method usually have excellent optical
properties. In free forming the heated sheet is simply clamped over a
vacuum pot or pressure head and drawn or blown to shape. There is
no possibility of markoff since the sheet does not con tact any
form. Further, when an air pressure differential does the work of
forming, the man power required is reduced, and cooling is
relatively uniform because both surfaces of the Chemcast® GP are
exposed to air. When the opening in the vacuum pot or pressure head is
circular, the finished part approximates a section of a sphere
for shallowdrawn parts. Since the center of the sheet stretches
most, this area thins out first and therefore cools first. The
thicker areas around the sides and the circumference continue to
stretch since they are still hot. Thus deep draws produce a
bulging or fish bowl shape. Even if the opening of the vacuum pot is square or triangular,
the Chemcast® GP tends toward a spherical shape, since a sphere
has the smallest surface area of any shape for any given volume.
An analogy is the blowing of soap films through different shaped
openings. The resultant blown or drawn shapes are often called
"free form" or "natural" shapes. A variety of shapes can be formed by differential air pressure
by altering the shape of the pot opening in the third dimension.
This can materially reduce thinning out of the sheet, depending
on the shape of the part. The choice between the use of vacuum and positive pressure in
forming will usually depend upon the equipment at hand. In
general, vacuum forming is preferred because it is safer, easier
to control, and simpler to seal. It is important that the joints
in the vacuum pot be sealed against air leaks which could cause
uneven cooling of the part. Sometimes the maximum pressure
differential possible with vacuum (14.5 psi maximum) is not
enough and positive pressures must be used. Original tooling
costs may be lower for positive air pressure forming as only a
pressure head, clamps, and clamping ring are required. |
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Copyright © Plastiglas de Mexico 1999
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