Choice
of Plastic, Glue &
What
to do about thickness?
I've
been comparing Styrene
with PVC (type 1) and Expanded PVC as building materials but don't know
enough to make a choice.
Expanded PVC is sold as Sintra, Celtex and
Komatex (brand names). In checking with a few suppliers,
Expanded PVC is typically cheaper than Styrene.
Here
are the material specifications I've been able to find
|
|
sp-2010 |
Celtex |
typical |
|
|
hips |
expanded pvc |
type 1-pvc |
| density |
gm/cm3 |
1.04 |
.55-.70 |
1.4 |
| Tensile-strength |
psi |
3500 |
2253-3000 |
7400 |
| Tensile-modulus |
kpsi |
239 |
144-232 |
|
| Flex-strength |
psi |
6900 |
3329 |
12000 |
| Flex-modulus |
kpsi |
277 |
144 |
400 |
Based on this, if I'm reading the numbers correctly:
- type 1 pvc is about 40% heavier and about twice
as strong as styrene.
- Expanded PVC is 30% to 45% lighter than styrene
but has only half of the flex strength.
It's been pointed out that strength is an issue with Dave
Everett's
Styrene droid plans. I would not consider substituting expanded pvc for
styrene for the critical parts. For non critical parts, like
the
skirt or the shoulder horseshoes, its worthwhile. From experience, Expanded PVC cuts
cleaner than styrene.
Glue
There are a number of cements that IPS recommends for styrene/pvc. From experience, either Weldon #3 or Weldon
#16 produce excellent bonds between styrene-styrene, pvc-pvc and
pvc-styrene. Weldon #3 is clear, water thin, very fast curing solvent cement. Weldon
#16 is clear syrupy (medium bodied) cement for high strength joints.
Thickness
The Styrene Droid plans published by
Dave Everett call for sheet Styrene in metric sizes, 1mm, 2mm and 3mm.
Sometimes however your local plastics supplier only caries 'English'
dimension material .040, .125 and .250 inches. As many folks, on
various web sites, have pointed out the difference in dimensions is
small
| metric |
English |
matches |
difference |
| 1mm |
.0394 |
.040 |
.0006 |
| 2mm |
.0787 |
.080 |
.0013 |
| 3mm |
.1181 |
.125 |
.0069 |
| 6mm |
.2362 |
.250 |
.0138 |
If you are substituting .125 for 3mm
then the difference is only 7 thousand's of an inch. If you are cutting
by hand that difference will be lost in the accuracy of your cuts.
However, I'm going to be using a 3d cad package to design my parts.
Since small difference play havoc with mating parts the computer parts
are all designed assuming English measurements (.040, .125 and .250
thicknesses.) Layouts will be done using the part centerline so that dimensions remain true to R2BC drawings.