Scratchbuilding - Resin Casting Reproduction
How to Mass Produce Your Scratchbuilt Items
Text and Photos by Scott G. Perry
Copyright ® 2021 Scott G. Perry. All rights reserved. Duplication and publication are prohibited.
Click on Photos to Enlarge
Buying a Bachman On30 flat car costs over $70 now that
they are out of production (Pic 01). Building a single flat car from scratch is
a tedious and time-consuming project. If your railroad needs a fleet of flat
cars, say one dozen, you are looking at tons of money or tons of time building
the same thing. If you can’t buy the cars you want, you can build one very good
model and replicate it using resin casting techniques. While this appears to be
a difficult process, it really isn’t and its loads of fun to see your first
part release from the mold! I’m making a fleet of freelanced cars for my Hebard
Cypress Lumber Company and the frames I cast will be used for several logging
flat cars, boxcars, gondolas and logging passenger cars. These will be used earn
my AP Master Builder Cars Certificate in the NMRA. Feel free to make any scale
frame you wish as the process is the same.
The first step is to gather reference materials on the
frame that you want to make (Pic 02). There is a ton of information on making
flat car frames available. I decided to base my car on a plan I found in the
Narrow Gauge and Short Line Gazette magazine (Pic 03). Using their drawing and
the reference materials, I drew my own freelance plan and got to work.
Remember you are building a master for a mold, not the
car itself. Materials are a critical choice in mold making. Styrene works
amazingly well for a master, but wood (Pic 04) is a little trickier. Molding
materials can get into every crack and crevice and can cause the master to
break when you remove it from the set mold. Wood can also float, causing the
mold material to encapsulate it and make it unusable (want to see one)? We are
going to use wood in this case so we can get the grain look on the cast parts.
The silicone is so good at mold production I have even cast my own fingerprint
on a mold just by handling the master.
Since this master is going to make lots of castings, it
should be your best possible work (Pic 05). Take your time to be precise and
build it very sturdy! Start with the basic frame. Using a sawblade and a worn
hobby knife blade I scored grain into each piece of wood. Carefully cut all the
outside boards and put them together with Carpenter’s Glue or Elmer’s White
Glue. Be sure to keep the work square and clean. Next put in the long
stringers.
Once the frame has dried, I use thicker piece of wood to
make the truck bolsters per my drawing and glue them in place (Pic 06). I’m
careful to make sure they are straight and firmly attached with no cracks between
them and the stringers. Clamp them if you need to hold them in place and give
it plenty of time for the glue to dry (Pic 07).
To build the two queen post supports you will need to
measure and notch the wood, so the board is mounted in between the stringers
(Pic 08). Carefully mount these in place
being sure to get the best fit possible (Pic 09). Remember the mold compound
will find every crack and make the mold harder to remove.
I fill in the area on the ends of each car where the
couplers will go and added a small piece of wood to be a mounting pad (Pic 10.)
Once that is done the main frame of the model is complete (Pic 11).
The last step for the master is to make sure that all
spaces are filled. The area between the bolster underneath and the top of the
frame will need to be filled with small pieces of wood. Once the glue is dry
take a wood filler (putty) and fill in all gaps anywhere (Pic 16). Sand each one
smooth and make sure there are no undercuts or gabs that could cause the master
to hold in the mold.
You’ll need to sand the top of the frame flat and smooth.
Wood may not release from the mold compound so I spray a light coat (Pic 17) of
clear sealant (Rustoleam Crystal Clear Enamel) and let it dry thoroughly. It
only has to be a very thin coating so you don’t lose any detail. Be sure to
spray the top of the frame, too.
We need to build a mold box next, so I start with a sheet
of .060 styrene and glue the frame upside down on the plastic using a few drops
of super glue. You want enough CA to keep it from moving, but not enough you
can’t remove the frame (Pic 18) from the base. The mold box needs to be about ¼
inch away from the master on all sides and on top. This gives the mold enough
strength for multiple uses without using too much expensive mold compound. I
just cut some .060 styrene into 1 inch strips and cemented them to the base
using Plastruct Plastic Cement. This attachment is in a pin-wheel fashion (Pic 19)
which allows it to be removed easily. You can’t allow any gaps in the mold box
anywhere because the mold compound will find the tiniest leak! Check your model
one last time for any missing parts like this one NBW that fell out somehow
(Pic 20).
There are tons of mold making and urethane resins out
there, and many more suppliers. I recommend you start with Alumilite products.
They have been long time supporters of model railroading and have great
products and video training. You can also call them up and ask a question
anytime. For mold compound I recommend Alumilite Amazing Mold Rubber Kit High
Strength (Pic 21) which is $26 on Amazon, or possibly at your local hobby shop.
This is a high strength mold rubber which you’ll need for pulling out the long
casting. Be sure to read the directions (Pic 22). I know, you are a model
builder and don’t read directions, but I warn you if you don’t do it exactly
right the frame will be coated in a silicone slime forever. When you mix it be
sure to mix it 10:1, silicone to activator. I use a gram scale for more
precision (Pic 23), but you can use the measuring cups that come with the kit.
You have plenty of time to work the material so stir it thoroughly!
25 Let Cure for 24 Hours or More
Starting in a corner of the mold box (Pic 24) start pouring very slowly. I get a tiny stream going so that the compound can drape around the master and eliminate air bubbles naturally. Pro mold builders will put the compound in a vacuum chamber and remove the air bubbles, but that isn’t really necessary for this project. Fill the mold box up to ¼ inch above the bolster (Pic 25) and let the material cure for 24 hours.
In 24 hours you can tear off the dam and remove the mold
(Pic 26). You’ll notice that a little mold compound will have found its way
over the wood in very thin sheets (called flashing). This is normal. Gently
remove the master (Pic 27) from the rubber mold, taking your time and going
slowly. There is no rush and you want to save the master for making new molds,
or for finishing out a model. Once the frame is removed take a hobby knife or
scalpel with a brand new blade and cut away flashing that is in the way. You now
have a production mold (Pic 28)!
Do not wash the mold in water! Urethane foam reacts
dramatically with water. The mold must always be dry. I recommend that you
start your casting work with Alumilite AlumiRes (RC-3) Tan Casting Resin (Pic
29). You’ll need about 2/3rds of an ounce per frame. This is a fast curing
resin that cures tan in color. I have used this resin for over 20 years with
great results. Many hobby shops have it or you can order online from Amazon. It
is a two-part urethane product (Pic 30) that you mix together at a 1:1 ratio.
Again, please really read the instructions and don’t bother using mold release
for this project. Mark the two mixing cups as A and B parts so you don’t get
them mixed up (Pic 31). I use a small paper cup and a popsicle stick to mix the
two. Pour in the clear A part first as it is thicker and takes more time. Then pour
in the brown liquid and mix quickly for about 15 seconds. Squeeze the cup to
form a narrow lip and pour slowly into the mold. This can be tricky so your
first casting may not come out perfect. You
only have a short time to work with the liquid so get it into the mold as quick
and clean as you can (Pic 32.)
Now the amazing part. These chemicals cause a thermal
reaction when mixed and they get very hot in the mold. Once it reaches a
certain level it begins to cure by turning a tan color (Pic 33). This happens
very quickly, and you’ll have a solid casting in a minute or less (Pic 34). Allow
the casting to cure for longer than the instructions say. I leave them in for
about 30 minutes each to get them very firm. Once cured gently remove the
casting from the mold a little at a time (Pic 35). If the casting bends a
little, don’t worry. Once it is out of the mold you can straighten it if it is
still warm. You may have a little resin flashing on the top but it can easily
be removed with a hobby knife or a sanding block. Put the casting on a hard
flat surface for 12 hours to allow it to fully cure (Pic 36). Ignore the
instructions on this because speed is NOT a friend to a casting.
Inspect the casting for air bubbles and details that didn’t
cast correctly. Small bubble holes can be filled with Squadron Putty. If you have
a lot of small details that do not cast right due to air pockets, put the mold
in the microwave on high for two minutes and get it warm, then immediately pour
it. It should cure most of the problem. Depending on the master, the mold material,
and the resin you can get an average of 25 castings per mold before it wears down
or tears. I have gotten much more than that however (Pic 37).
Give it a try and have fun! If you have questions,
contact me at scottgperry@comporium.net. There is a lot more to casting and by
changing resins or making more complex mold you can make a lot of things.
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