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Manual stretch forming is often used when the compound
curvature is not great, when optical distortion is not
objectionable and the number of parts to be made does not warrant
setting up mechanical equipment. The sheet is heated to forming
temperature and clamps are fastened to the edges, six to ten
inches apart. Holding the Chemcast® GP with these clamps, the forming crew
draws the sheet down over the form. For some shapes, one edge of
the sheet may be clamped to the form, and the sheet stretched
over the form from the other edge. The sheet should be stretched
as uniformly as possible. Use slow, steady tension and let the
Chemcast® GP stretch gradually. After the sheet has been stretched, a clamping ring can be
clamped in position around the edges, leaving the crew free to
work on other forms. Plug and Ring Forming A modification of stretch forming, known as the
Plugandring method, operates on a principle similar to the
familiar embroidery hoop. The hot Chemcast® GP is clamped over a ring and the center of
the suspended material is pushed in with a tapered Plug. The ring
is made larger than the outside of the male form or Plug, with
allowance, of course, for the thickness of the sheet. Plugandring forming usually has the disadvantage of
producing excessive markoff, particularly at the inside corners
of the formed part where markoff is most difficult to remove.
This disadvantage is not important for parts where optical
properties are not important and for forming translucent sheets. Vacuum Assist Plug and Ring Forming Deeply drawn parts (ratio of width of opening to depth of part
is one or less) with little thickness can be made if vacuum is
used to assist the draw. The ring support is made into an
airtight box to which the heated Chemcast® GP sheet is clamped.
Vacuum is applied to draw the sheet to about half the desired
depth. The Plug is then forced into the drawn sheet to secure the
desired shape. When the Plug is ``home," air is vented into
the box to allow the Chemcast® GP to shrink onto the Plug. This
method produces more uniform final part thickness than by either
method above or by vacuum snapback. |
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Copyright © Plastiglas de Mexico 1999
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