deburring the canopy edges

15 May 2004 deburring the canopy edges

–> RV-List message posted by: Doug Gray

Here is a post from 5 years ago. It was a keeper then and I repost for
the benefit of this discussion.

Do not archive … because it is already there!
Doug Gray
RV-6 fuselage

Dear list,

Motivated by my recent bad experience with my canopy and vinylester
resin I have done some research into plastics. These are some of the
things I have learned.

1. Plexiglass is a trade name for one version of acrylic plastic. Our
canopies are acrylic, whether they are “plexiglass” per se I am not sure.

2. Acrylic plastics are long chain polymers. You can reduce them to
short chain polymers and monomers by “cracking” them with heat and a
solvent. In this sense “cracking” has the same connotation as “cracking”
in a petrochemical refinery.

3. The crack resistance (in a mechanical sense) of acrylics depends on
their being long polymers. When you reduce them to short polymers they
will then lose their crack stopping properties and allow cracks to
propagate from any tiny surface imperfection.

4. Both vinylester and polyester resins may or may not contain solvents
that will depolymerize acrylics. The composition of either is quite
variable and dependent on the local manufacturer. This explains why some
people have got away with both polyester and vinylester resins on occasion.

5. The more aromatic (lighter fraction in general) the solvent the worse
it hates plexiglass. If you have to clean plexi it is best to use heavy
(non-aromatic) hydrocarbon fractions. The safest are kerosene or mineral
turpentine. There is a fraction called X95 which is slightly lighter and
still safe. This is the chief component of WD40 and most other
penetrating water displacement agents. Anything based around the benzene
ring (benzine, toluine etc) is bad, bad, bad.

6. In Australia the manufacturer DOES add a small quantity (less than
.5%) of silicone to WD40. In Austalia this means it does not have to be
mentioned on the label. I cannot speak to the US formulation. This is
bad for paint.

7. High heat of its own is enough to depolymerise acrylics. Thus “flame
finishing” gives a lovely look but is a bad idea as it leaves a non
crack-resistant edge.

8. Edges should be sanded after cutting in a longitudinal fashion. Any
surface imperfections are less likely to propagate cracks. This is the
natural tendency when finishing a long edge anyway. Holes can be
finished with a cone shaped grinding stone. This again leaves any
scratches aligned with the surface rather than at 90 degrees.

9. It is good to work on warm plexiglass. It is bad to make it locally
very hot. Van’s recommendation of several partial thickness passes with
the cutoff wheel is good practice.

10. There will be a tendency for the plexi to “learn” a new shape with
the passage of many temperature cycles. This means it is bad to store it
under stress because it will slowly deform to the stresses. On the other
hand it means if you install the canopy with some local stresses and get
away with it (no cracks) the likely hood of cracks will go down with the
passage of time.

11. UV will gradually depolymerise the acrylic. Over a long period of
time it becomes more brittle. Protecting your canopy from UV is a good
idea. I hope the above is helpful to someone, it is garnered from a
variety of sources but I think is all correct. I welcome any further input.

Cheers, Leo Davies