EPOXY RESINS, THE BASICS.
The resin to be used has two or three basic ingredients; the resin itself and the catalyst or hardener
and sometimes filler sand. These ingredients will have to be purchased from a supplier specialising in
these industrial chemicals. Consult your local Yellow/Pink Pages (look under resins or plastics) for one
near you. Contact him or her for information regarding cost and usage of the ingredients.
Before you order anything you will have to work out approximately how much resin you will need. This
is a trick that will require some maths you may not have used since high school!
Working out the volume of the mould box is fairly straight forward
(L x W x H), but the volume of the plug
is somewhat more nebulous, with the difference in volume between the two equal to the quantity of resin
needed... in theory!
There are several approaches. You can break down the volume of the plug into calculable parts and add
contents up, or simply take a stab at it and assume that the plug is 1/2 or 3/4 of the box volume. With
the resin being fairly expensive it is best to be as accurate as you can. Failing that, have another
non-urgent project along side it where you can use any excess resin. Pour resin over coins, kids teeth,
insects or flowers etc. stuck inside old film containers. That will take care of next year's
Christmas presents with a novelty paperweight each!
So roughly how does it work? The epoxy resin is mixed at a certain ratio with hardener; say 3:1 depending
on the brand you get. After mixing slowly, this gives you a liquid with the consistency of honey.
Use a ROUND stick to stir as they cause less air entrapment in the liquid. Once the hardener is added, at
room temperature (20-25 C) you usually have about an hour to work with the liquid. Higher
temperature = less time, lower temp = more time, but do not use it below 10C. Check with the supplier on
specifics. Once the curing process starts the temperature of the liquid rises considerably. This will
make it momentarily thinner before it sets. At this stage it will find leaks in the mould box that you
have overlooked! I used duct tape to slow the flow on mine.
From here on the whole project is in the hands of the modellers' gods as the whole kit and caboodle now
needs at least 24 hrs to harden out, preferably at room temperature.
Should you find, after pouring all the prepared resin into the mould box, that you do not have enough,
there are certain things you can do to save the situation. First of all don't
panic! You have got at
least an hour. More than likely in the mould box there are corners or other spots near the top liquid
level where resin is the only thing there. At these spots try and push a small block of wood into the
resin thus replacing it with the timber. The timber will float in the resin so it should be kept down
and in place with a weight or clamp, away from your plug. Do not use stone as it will sink and make
contact with the plug. Keep adding timber until your top level is right.
To reduce the amount of resin needed, for larger projects you can replace a certain volume of resin
with special filler sand, much like the gravel in concrete. You have to do your own cost analysis if
this is worthwhile or not. Don't use beach sand, as it is way too fine. The filler sand is specially
sized for this purpose. You can use anything from a resin /sand ratio of 1:1 to 1:2 or more.
Again, check with your supplier.
When you use the sand filler method and realise your resin level is too low after the pour, do not panic.
Simply add more sand by sprinkling it thinly over the surface of the resin. The sand will settle to
the bottom of the mould thus leaving a clear layer at the top. As long as you can see a clear layer
of liquid resin at the top, within that first hour, you can gradually keep adding thin layers of sand.
Give the sand time to be absorbed by the resin and start the slow journey to the mould bottom before
adding more, that way pushing out any airbubbles from between the sand grains.
Of course you also have the wood block method as an option.
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In an ideal modeller's world the resin mould will come away from the plug and the baseboard with the
minimum of fuss. However, reality is quite different! After unscrewing my base board from the box,
I had to chop the board away carefully with hammer and chisel. Then it was onto the plug, which had
to be removed from the mould with the care of a palaeontologist excavating a million year old fossil.
See Pic. G. After hours of careful scraping, scrubbing and sanding, I was however left with an exact
and 99.9% perfect replica of the plug I made, inside a sturdy wooden case. See
Pic. H. It seems to
me that something wasn't quite right with the way I used the release agent. Probably the U-shaped cross
section of my mould didn't help either.
In the event that the mould gets damaged, it is possible to repair the surface with the two-component
epoxy glue available from your hardware shop. Just mix the glue according to instructions, keeping in
mind that the working time is now 2 minutes instead of an hour. I have successfully repaired air
bubble entrapment and other minor scratches in this way. This glue can be moulded in its last stages
of hardening by pressing on it with a wet finger tip to obtain a hollow shape. The mould surface
can also be sanded. Do not use too coarse grit and use a minimum of pressure on the surface. As
the surface gets warm while sanding, it becomes sticky again. Either cool the whole mould down
(put it in a chest freezer for an hour or so) or use wet and dry emery paper to work on imperfections
on the mould surface.
Watch this space as more interesting titbits come bubbling to the surface of what is the modeller's
morass and let me know how you got on with your project. I hope this helps you. Good luck!!!
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