Mold Making Question - How To Make A Negative From A Positive - I'm Confused!

[Holophote]

I've made several two piece plaster molds from varies objects
I needed to copy. Haven't had any problems until now!

I've got an object which is part of a plastic (fiberglass
acutally) auto body panel. Its called a "dog leg molding."
Funny name, I know. Its curved in several directions and
has numerous grooves and ridges. I lost the one on the
passenger or right side of the car. No one has a replacement.
I've checked junk yards, car dealers, etc.

I can easily remove the remaining panel (its about 3 inches
wide and 12 inches long) and make a two part plaster cast
from the panel. The problem is I need to make a piece
the exact opposite from the one I'm casting. In other
words all the curves, grooves, and ridges must be a negative
of the original positive so that it will fit on the left
side of the car instead of the right side.

I've never done anything like this before and I'm confused!
In effect, I have to create a negative mold from the dog
leg panel. I think I have to start by making a conventional
plaster mold and then somehow create a second negative
mold from the original positive mold.

If anyone has ever done anything like this before I would
appreciate a reply.

Thanks guys!

Holophote

[Chek]

I think it does get confusing because of the way we humans see things, but
actually a mirror image of an object is completely different to its, well-
mirror image.
What you can do is get an engineering shop to pantograph it into a CAD
program and then reverse the axis directions for you, but its not a home
workshop project. Easy enough for a computer, but not a by hand job
(depending how exact you wanna be).

Chek

Post by Holophote
I've made several two piece plaster molds from varies objects
I needed to copy. Haven't had any problems until now!
I've got an object which is part of a plastic (fiberglass
acutally) auto body panel. Its called a "dog leg molding."
Funny name, I know. Its curved in several directions and
has numerous grooves and ridges. I lost the one on the
passenger or right side of the car. No one has a replacement.
I've checked junk yards, car dealers, etc.
I can easily remove the remaining panel (its about 3 inches
wide and 12 inches long) and make a two part plaster cast
from the panel. The problem is I need to make a piece
the exact opposite from the one I'm casting. In other
words all the curves, grooves, and ridges must be a negative
of the original positive so that it will fit on the left
side of the car instead of the right side.
I've never done anything like this before and I'm confused!
In effect, I have to create a negative mold from the dog
leg panel. I think I have to start by making a conventional
plaster mold and then somehow create a second negative
mold from the original positive mold.
If anyone has ever done anything like this before I would
appreciate a reply.
Thanks guys!
Holophote

[EmilA1944]

Post by Holophote
I've got an object which is part of a plastic (fiberglass
acutally) auto body panel. Its called a "dog leg molding."
Funny name, I know. Its curved in several directions and
has numerous grooves and ridges. I lost the one on the
passenger or right side of the car. No one has a replacement.
I've checked junk yards, car dealers, etc.
I can easily remove the remaining panel (its about 3 inches
wide and 12 inches long) and make a two part plaster cast
from the panel. The problem is I need to make a piece
the exact opposite from the one I'm casting. In other
words all the curves, grooves, and ridges must be a negative
of the original positive so that it will fit on the left
side of the car instead of the right side.
I've never done anything like this before and I'm confused!
In effect, I have to create a negative mold from the dog
leg panel. I think I have to start by making a conventional
plaster mold and then somehow create a second negative
mold from the original positive mold.
If anyone has ever done anything like this before I would
appreciate a reply.
Thanks guys!
Holophote

You don't say what make of car, but have you tried scoping out "Hemmings Motor
News"?

Over a period of say, a year, if you can't find a part through Hemmings, likely
it may not be found, in my experience.

Short of that, I am afraid your only answer is to either make a pattern, using
your original part as a reference, either by hand, or by having a pattern-shop
do it. I wouldn't be daunted by doing it by hand, as before computers, that's
the way it was done.

Art Anderson

[Don Stauffer]

Are we talking a model or a full sized car here? For model work, one
can use silicone resin to make the molds, and one can remove the
original part (the pattern) relatively easily. For plaster molds, NO
undercutting is allowed, one must even have a slight positive clearance
angle. So only specially designed patterns generally work with plaster.

Problem is, in translating this to duplicating a full-size part, that
the cost of that much RTV gets prohibitively expensive.

If you then MUST use plaster, you must make the mold in many pieces,
using clay dams, so that you avoid undercutting, or even parallel sides.

Post by Holophote
I've made several two piece plaster molds from varies objects
I needed to copy. Haven't had any problems until now!
I've got an object which is part of a plastic (fiberglass
acutally) auto body panel. Its called a "dog leg molding."
Funny name, I know. Its curved in several directions and
has numerous grooves and ridges. I lost the one on the
passenger or right side of the car. No one has a replacement.
I've checked junk yards, car dealers, etc.
I can easily remove the remaining panel (its about 3 inches
wide and 12 inches long) and make a two part plaster cast
from the panel. The problem is I need to make a piece
the exact opposite from the one I'm casting. In other
words all the curves, grooves, and ridges must be a negative
of the original positive so that it will fit on the left
side of the car instead of the right side.
I've never done anything like this before and I'm confused!
In effect, I have to create a negative mold from the dog
leg panel. I think I have to start by making a conventional
plaster mold and then somehow create a second negative
mold from the original positive mold.
If anyone has ever done anything like this before I would
appreciate a reply.
Thanks guys!
Holophote

[Umineko]

Can't be done... using the piece from the opposite side as a master will
only give you a copy of that piece... best advice is to scratchbuild a
replacement part using the opposing piece as a guide for the curves etc. the
part can be made from any material then used as a master to remould the part
in Fibreglass.

Post by Holophote
I've made several two piece plaster molds from varies objects
I needed to copy. Haven't had any problems until now!
I've got an object which is part of a plastic (fiberglass
acutally) auto body panel. Its called a "dog leg molding."
Funny name, I know. Its curved in several directions and
has numerous grooves and ridges. I lost the one on the
passenger or right side of the car. No one has a replacement.
I've checked junk yards, car dealers, etc.

Post by Holophote
I can easily remove the remaining panel (its about 3 inches
wide and 12 inches long) and make a two part plaster cast
from the panel. The problem is I need to make a piece
the exact opposite from the one I'm casting. In other
words all the curves, grooves, and ridges must be a negative
of the original positive so that it will fit on the left
side of the car instead of the right side.
I've never done anything like this before and I'm confused!
In effect, I have to create a negative mold from the dog
leg panel. I think I have to start by making a conventional
plaster mold and then somehow create a second negative
mold from the original positive mold.
If anyone has ever done anything like this before I would
appreciate a reply.
Thanks guys!
Holophote

--
Don Stauffer in Minnesota
webpage- http://www.usfamily.net/web/stauffer