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Introduction and Disclaimer
This is a step-by-step outline of the method I used to make a mould using epoxy resin.
The method is used to make a mould of a model boat hull for scratch building, in this case
the tanker VLCC Esso Nederland, scale 1:350. All terms and references involved are typical
of model boat building. You will have to decide what is applicable to your project so first
read all the information to get an overview before you start doing anything. Consider this
information a suggestion only and no responsibility is accepted for any
failures or disasters, but please, attribute all successes to my methods!
You will need a base board, timber for reinforcement of the base board, woodscrews, drawings
to scale, drawing materials, small sheet of three-ply
(600mm x 900mm), wood glue, scraps of Styrofoam,
one (or more) cans of Polyurethane spray foam, plaster of Paris (or any other French city), lacquer
or varnish as a sealant, sand paper and the usual hobby tools.
Instructions
STEP 1.
Reduce all your acquired drawings down to the same scale as needed for the model so you can check
sizes and distances directly against the drawing without calculations.
A plan printing office or architect can help you with the reductions for a small fee.
Based upon your general arrangement drawing (or similar) work out the approximate size of
the baseboard you will be needing. For example, if your hull to scale is 550 mm long overall
and the beam is 120 mm, add 10 mm all around for the mould thickness plus (in my case) 19mm
for the mould box timber. Your baseboard should thus be minimum 608 mm long and 178 mm wide.
Later on you will make the mould box from the 19 mm timber, but first it will be used to reinforce
the baseboard from underneath keeping it straight, while you work on your model or "plug",
to use the appropriate trade term. The height of the mould box needs to be 10-15 mm higher than
the highest point of your plug so keep that in mind when you buy your timber. If the highest point on
your plug, measured from the baseboard, is 115 mm, the height of the resin level in the mould box
should be 125-130 mm which should be the minimum size of the box timber.
Designate one side of your baseboard as your reference point for future measurements. Along this
straight side you draw a parallel line at a distance of 58 mm from this edge. This is the keel line.
Perpendicular (look it up!) to this line, you draw the lines indicating the position of the frames
at their appropriate spacings as per your plans. Along the keel line, nail a strip of timber or
beading as straight as possible. This will be used to align the frames when they are being glued down.
On your gen. arrangement drawing find the centre line, usually marked
C.L. This is the largest frame
midship, i.e. the same distance from the bow as the stern. Mark it on your baseboard.
Screw the reinforcement timbers underneath the baseboard, i.e. the side without the drawings.
See Pic. A. This will keep the baseboard straight and rigid while you are working on it. Later on
these timbers will be removed and attached to the same side as the frames to form part of the mould
box utilising the same screw holes.
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STEP 2.
Photocopy the frames on the line plan, as many times as the number of frames you will be using.
Depending on your scale you may not be using every frame in the drawing. Cut out the two (starboard
and portside ) halves of each frame accurately and number them as per your plan. Trace each frame
onto the plywood and cut it out with a scroll saw, making sure its centreline (deck to keel) is straight.
Sand all edges and curves smooth as per the corresponding paper cut out.
STEP 3.
Between ALL the frame markings drill a 3mm hole approx. 10-15 mm in from the keel line. You need to
create an air passage from what will be the sealed space between the frames to the outside air. Since
the resin heats up during curing, you need to avoid air pressure building up inside the plug. Ommitting
drilling these holes will damage the plug when the resin cures and ruin the mould, as happened to yours truly!
Glue the plywood frames to the baseboard starting with the C.L. frame. Glue the C.L. frame
ON the marked
line using P.V.A glue or similar. Working towards the bow and the stern, glue the other frames on the C.L.
side of their corresponding markings. This way your hull will have the correct profile on the marked spot,
as the thickness of the plywood will not interfere with that profile. The frames have to be square to
the baseboard as well as to the keel line. Use squares and square cut blocks of timber to check and support
them during the building process.
STEP 4.
With all frames in place, fill the space between them with scraps of timber to strengthen the structure,
making sure not to cover the air vents drilled previously.
Refer Pic.B. top hull. Add pieces of Styrofoam in
the bigger sections, leaving approx. 20-30 mm space from the edge of the frames.
Refer Pic. B bottom hull.
Remove the timber beading along the keel line.
STEP 5.
Remove all dust from your baseboard. Buy a spray can of polyurethane foam (used to fill cracks and gaps in
walls and timberwork) and read the manufacturer's instructions. Apply the foam to the spaces between the frames.
Remember that the foam expands 2 or 3+ times from the quantity that you apply! When all the spaces are filled,
the foam fully expanded and hardened, your hull should look like something from Startrek or the top hull in
Pic. C. Add more foam if the spaces did not fill to the top of the frames on the first pass. Cut away most
excess foam with an old hacksaw blade. Sand back the rest until frames are just showing, following the
curvature of the hull indicated by the frames. See Pic. C.
bottom hull. The trick here is to put not too
much downward pressure on the foam, as it will give way to the hand pressure.
STEP 6.
Stabilise the face of the foam, which should now look like honeycomb, with one or two layers of varnish or lacquer.
Sand between layers to maintain the same frame height. Paint the baseboard surface as well. This will make it
easier to remove it from the cured resin after it is poured into the mould box. When the foam is sufficiently
stable, use a putty knife to first fill the largest holes with plaster of Paris, again sanding between
applications. Then apply several layers of thin plaster slurry with brush or cloth to fill most irregularities
and build up skin thickness. Work the surface of the hull until an acceptable level of smoothness has been
achieved. Check if the point where the hull edge and the base board meet is sharp and well defined all around
the hull. See Pic. D. for inspiration. Make sure not to distort the outline of the hull by adding plaster
unevenly. During this process measure the outlines and curves of the hull as accurately as possible by
means of rulers etc. or paper templates. Thus ensuring the plug corresponds with the original design
and /or drawing
STEP 7.
This is your last chance to fix any irregularities on the plug by means of adding plaster in places. When you
are happy with the smoothness, surface appearance and the measurements are OK, you can seal the plug with
lacquer, a varnish or other sealant available that will give a hard, smooth surface. (Keep in mind the release
agent you are going to use. They need to be compatible. See Step 8). Sand lightly between layers. If your
project looks like Pic. E., you are doing well!
STEP 8. When the sealant is dry, apply a release agent to the surface of the plug and the baseboard.
Careful what you use! I used a water based varnish and a silicon spray, the combination of which I tried out
on a scrap piece of wood to check for unwanted chemical reactions.
Remove the reinforcement timbers one at the time, maintaining one timber fastened to the baseboard at
all times. Using the same screw holes, attach it to the same side as the plug, maintaining approx 10 mm
space between plug and timber, as per Pic. F. Visually inspect the box for water tightness, treating
any gaps with a smear of P.V.A. glue on the outside to seal them. Turn the box upside down to stop dust
settling on the surface of the plug and check if all the little holes are functioning by poking through
them with a nail. Redrill those that are blocked. Now you are ready for the big
pour!
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