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Introduction
I realise that as a modeller, one tends to use materials lying
around the workshop. However, this can be costly in both wasted
time and wasted plastic, if you haven't prepared the mould properly.
It is very frustrating for me as well. Moulds I come across are
made from various materials such as gelcoat and fibreglass, plaster
and fabricated timber. It is rare to be handed a mould constructed
from the materials and method I will be suggesting in part two
of this article.
Fibreglass Moulds
These moulds have usually been taken off pre-existing plastic
canopies obtained from original kits, which are no longer available.
The moulds are usually filled with stone and epoxy or plaster.
However, in some cases they were left hollow. This can result
in the moulds collapsing under pressure if the glass is not thick
enough and/or there is no base support.
The combined problem that arises with this method of reproduction,
is that the glass backed moulds usually haven't had enough resin
impregnated into the fibreglass backing and as well, the glass
has not been rolled out properly to make sure that all the air
has been extracted. (Of course this can't be achieved if there
isn't enough resin in the glass in the first place.) The end result
is that when the plastic is formed over the mould, the combination
of heat and pressure causes pockets of air to expand. I can leave
the rest up to your imagination as to how the canopy ends up looking.
Solution: If you choose to use this method, mix the minimum amount
of catalyst into the gelcoat; two coats are preferable. Porosity,
which can be caused by too much catalyst, usually isn't a problem
unless you decide to sand the surface. If you can get your hands
on some 'Plug White', (a mixture of gelcoat and filler obtainable
from Synthetic Resins in Moorabbin), you will be pleasantly surprised
at its post sanding characteristics. It is relatively inexpensive
as well. One is able to sand the mould more easily than normal
gel coat. In fact it's like sanding hard plaster. Porosity isn't
as much of a problem either with the addition of the filler.
Another trick is to use 'Surface Tissue' after you have applied
the gel coat (or 'Plug White'). This reduces the risk of air bubbles,
as it provides a hard sealed surface prior to laminating the thicker
glass backing. “Surface tissue” is a fine grade of
fibreglass, which looks and feels like toilet paper. (But don't
use it instead of toilet paper, will you now!)
Note: Most people believe that the smoother a mould is presented,
the better the finish will be on the final part. Not Necessarily
So! If the mould is finished off with say 600 w&d then polished,
the plastic will seal so well over the mould, that air locks will
form, resulting in slight lumps protruding over the final part.
I recommend that you go no higher than 400 w&d and definitely
no polishing.
I do try to keep the heat to a minimum when forming. The theory
is that the less heat one uses, the less definition that shows
on the part, which means that there is a less likely chance of
imperfections showing up on the part. I use PETG in most cases.
Although much more expensive than PVC, it is by far a much easier
material to form. As well, PETG gives a excellent finish and is
considerably stronger than PVC.
Plaster Moulds
Presenting me with plaster moulds usually isn't a problem
if the plaster has been applied properly. However, if the brew
is too porous, this will show on the final part. Also, you can't
repair or touch-up plaster very well. Putting new plaster over
existing plaster to fix or repair, may result in de-lamination.
Using body-fillers over plaster isn't very successful either.
Because the materials are so different in density, when the part
is formed, the heat causes the different materials to expand and
contract at different rates, resulting in marks showing on the
final part. Cheap plaster moulds can also be brittle. Sometimes
just a small jolt in the vacuum forming process can result in
the mould breaking in half. This has happened to me several times.
Adding PVA glue to the plaster mix (a known trick of the trade
to prevent this problem), may help a little if done correctly,
but I still don't like plaster.
Solution: Use plaster only as a last resort.
Timber Moulds
Fabricated moulds presented to me in many cases collapse on forming.
The modeller usually has no idea of the heat and vacuum power
of the equipment used in the process.
Mould Construction
| The following part
suggests the two best methods (complete with instructions)
of making a mould. The first method will be making a mould
from scratch, whilst the second method will be making a mould
from an existing canopy (or what ever else you plan on making).
In the meantime, have a think about the diagrams below. They
are three of the most commonly known canopy profiles used. |
Click on the diagram above for the full-size version. |
These instructions are for a pattern profile similar to diagram
three (D3) above.
Make profile templates to suit, using the thinnest material possible.
(See templates T1, T2, T3 & T4.) The number of templates may
vary depending upon the length of the pattern and how accurate
you want the shape the section. The templates can either be made
as full profiles as shown in T2, T3, & T4 or half profiles
as shown in T1, in which case you will need to scribe a centreline
along the middle of the pattern as a guideline. Half profile templates
are easier to “home in” on to the finished shape as
you can come in from any direction.
When selecting the timber to use for the pattern, you can't go
past GELUTONG. Patternmakers prefer it as their number one timber
for general pattern work. It is an absolute pleasure to carve,
it has a close grain and has no sap. One is able to finish a pattern
to very accurate tolerances. Gelutong is available from Matthews
Timber in Rooks Road, Vermont or Britton Timbers in Hammond Road,
Dandenong. Otherwise, if you only need a small amount, look in
the yellow pages under “patternmakers” and I'm sure
one of the local companies will give or sell you an off cut or
two.
When gluing the gelutong, is better to glue the layers in the
horizontal plane rather than the vertical plane. This horizontal
glue line will have a better resistance to the heat of the vacuum
former. When the timber has been glued up to the appropriate thickness,
square all sides on an accurate vertical disc sander if possible.
This is important for best results when the time comes to put
the finishing touches with the templates.
Mark out the plan view on the bottom of the timber block and
the side view on one of the two sides. (D5 shows the plan view
marked out on the top for viewing purposes only). Now cut out
the side view using a band-saw, then sand to the line with a vertical
disc sander, Placing the pattern on the bench the right way up,
temporarily fix a block of wood at one end (see D1). Router the
top of the temporary block till it matches the highest point at
the other end. Now flip the block upside down and proceed to band-saw
and sand the plan view shape (which, just to remind you, should
previously been marked on the bottom side. If you are preparing
a pattern for a profile similar to diagram D2, you will need to
repeat this procedure with the base pattern. For the base section,
you will need to make the template identical to the fuselage,
fore and aft of the area to which the canopy will eventually be
attached - carefully matching (or marrying) the two parts together,
whilst still in their respective square block form.
Sometimes it is preferable to fit the pattern (still in block
form) into the pre-made fuselage. In this way, you can mark all
views accurately from the fuselage to the pattern. Of course,
when you take the pattern out for final shaping, you will then
need to fix additional pieces on the back and base for vacuum
forming purposes. The VARMS U2 was done using this method. In
this particular case, there was no need to paint the additional
pieces, as they were to be cut off the plastic later anyway. Now
you are ready for carving.
START SHAPING. Shape using either a 1” to 1½”
chisel or electric sander. Continue to shape the plug using either
a rasp or sanding block. Use a very rough grit sandpaper. i.e.
40-grade heavy duty. To check how you are going, a good quality
profile gauge is quite a time-saver, although they aren't cheap
to buy. Otherwise just stick with your made-up templates.
When you are within a millimetre or so, start using 80-grade
free-cut (white or grey in colour) sandpaper. This type of sandpaper
is much more resilient than the more common yellow coloured papers.
Different brands give different results, so try different hardware
stores when re-stocking.
Finish off the final shape using 100-grade free-cut, followed
by 120-grade free-cut. Some stockists only have 150-grade instead
of 120-grade. In this case use your discretion. Wet and dry sandpaper
will also do the job successfully, although it is considerably
dearer.
Now, evenly apply a coat of 'Plug White' to the bare surface.
Room temperature for best results. Brush only, as spraying will
not get right into the grain. When dry, sand back or through to
the timber using 80 to 100-grade sandpaper and possibly finishing
off with 120-grade. (NOTE: The first coat is meant to be a grain-filling
exercise, sort of like when using spray putty on a car.)
Apply the 2nd coat of 'Plug White'. Spraying will give a more
even finish for the next process. The solvent to thin the 'Plug
White' is 'wax in styrene', not acetone. If brushing, you will
probably need to put apply an additional coat after the second
coast has dried. Do this by brushing at right angles to the second
coat. When dry, apply a light coat of spray paint of any kind
in a contrasting colour. This will enable you to rub back through
to the 'Plug White' and at the same time have control on the evenness
of your sandpapering. The 'Plug White' must be kept to an even
thickness of at least .38 - .5 mm (15 - 20 thou or .015”
-.020”). When dry, rub back with 100-grade sandpaper followed
by 120 grade.
When you have rubbed all of the spray paint off and got rid of
any high spots, continue on with 180 grade, then if you choose,
use wet and dry sandpapers of say 280-grade, 320-grade and finally
400 grade. Do not polish plug, as this will create airlocks during
the vacuum forming process.
If you haven't the facilities or equipment to prepare the initial
stages of the pattern, don't despair, help is at hand. I can accurately
prepare for you, a squared-up block of wood ready for shaping.
Cost, (which includes gelutong and postage), may vary depending
on size and complexity.
Cost: $40 - single pattern. $70 - main pattern & base - matched.
Prices are in Australian Dollars and do not include postage.
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