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Vacuum Snapback Forming Vacuum snapback forming is often used when the desired part
varies from a true "surface tension" shape. This method
is based on the tendency of hot formed Chemcast® GP to return to
its original flat sheet form. Like vacuum forming, snapback forming is done in a vacuum pot.
After the heated sheet is drawn into the pot, a male form which
reproduces the inside contour of the desired part is lowered and
locked inside the bubble formed by the Chemcast® GP sheet. Since
the sheet is still hot, it has a tendency to resume its flat
sheet form. Therefore, as the vacuum is gradually released, the
Chemcast® GP "snaps back" slowly against the form. In vacuum snapback forming, all stretching is done by pressure
differentials. Less mark~off is produced by this method than by
mechanical stretch forming because the Chemcast® GP is stretched
before it comes in contact with the form instead of being
stretched as it is drawn across the form. In snapback forming, the Chemcast® GP sheet will not snap
back into reverse curves and will not follow rapid changes of
contour very accurately. Nevertheless, it is possible to obtain
contour tolerances of + 1/8" or closer in snapback
forming compared with + ¼"/ to + 1/2"
for free forming. An integral flange is formed where the part has been clamped
to the vacuum pot. This flange is usually at least ¾" wide
and provides a strong and simple mounting. The contour of the
flange can be held to fairly close tolerances regardless of the
tolerances maintained on the rest of the part. The form should be constructed with a draft of at least 3° to
permit easy removal of the formed part. Small vents should be
provided in the male form to permit escape of air which may be
trapped between the form and the sheet. Reverse curves can be
produced by connecting the vacuum line to vents in depressed
areas on the forms. |
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Copyright © Plastiglas de Mexico 1999
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