I formed my transparent fuel guages from Lexan (polycarbonate) and
they worked perfectly first time.
First the caveats. Acrylic (perspex)is easier to form, is harder,
more prone to cracking and is much less strong. It will blow or
vacuum form with ease, provided you follow some simple rules. It is
also available in a multitude of colours - which lexan is not.
If you use a mold, male or female, the quality of finished product is
entirely dependent upon the finish of the mold. You want a mirror
finish? The mold must have one.
If you blow or vacuum form the final shape is determined by two factors :
1. the plan view of the "holder" i.e. circular results in a
hemisphere, pear shape results in a teardrop, etc.
2. the pressure differential which determines the finished height.
Because there is nothing touching the surface, you get a mirror
finish and good optical qualities by default.
For acrylic, the trick is to hold the blank sheet - typically 3 mm
securely in a frame whilst it is heated. The exact temperature can be
found from the manufacturer's web site or the product sheet. It is
evenly and slowly heated until it sags under its own weight. The
frame is then sealed and clamped to the air / vacuum source and the
pressure briskly increased/ decreased until the desired hieght / depth
The pressure is held until the part cools. The frame is then
unclamped, the final shape released from the frame and the excess cut
away with a dremel and grinding wheel. It is a good idea to mask the
edge of the cut with tape. Support both sides by frequent taping acoss
the cut. Warm everything before cutting.
For stretch forming, which is most appropriate for lexan, the first
trick is to realise that it is very prone to absorbing moisture which
manifests itself as bubbles in the finished product. For this reason
it vital to preheat and prepare the material for some time - typically
30 minutes prior to preforming.
I used metal tools because it was easier to get the clearances and the
finish I wanted. These I also put in the temperature controlled oven
and brought them up to temperature. Again the temperature is
important and is such that the blank is like uncooked pastry - floppy.
When I was ready, armed with the leather gloves, I removed the bottom
form onto a nearby bench, quickly laid the blank over the form - it
draped well, and then quickly forced the external ring down over the
bottom form and the blank to get the sharp transitions I required.
It cooled quickly, is very tough - hit it with hammer tough. and
Hope this helps
--- In Q-LIST@..., "Joseph Snow" <1flashq@...> wrote:
atop the vertical stabilizer. I will be carving the wing tips and top
of the vertical stabilizer from blue foam; then, covering with a 2
layers of BID. I then want to make custom clear plastic lens where
the position light will be installed.
It seems the clear plastic is heated to about 300 deg., then forced
over the part or a mold of the part to stretch and form the clear
plastic. After cooling, the formed plastic is trimmed and installed.
to a detailed description, perhaps in Q-talk, on the Web, a book or
article? What type of plastic?
How thick originally? Temperature? Etc.