4D MODELSHOP MODEL MAKING GUIDE
SHAPING STYROFOAM
Written by David Neat author of ‘Model-Making:
Material and Methods’ (BP00075)
This is an abridged version of this guide, a full
version available here: davidneat.wordpress.com
Wide range of styrofoam are available: yellow,
grey and white 1mm - 165mm thick
Dark grey has replaced the blue in this guide
Styrofoam is one of the easiest materials to shape by normal means e.g. slicing with sharp knives
or a hot-wire cutter, sawing with serrated blades, rasping with files, and smoothing with sandpaper.
The real challenge lies in controlling the shape and especially, in this case, how one achieves
concave forms.
Here are the methods I've employed for a particular model piece which needed a 'bowl-like' form
and very regular curves.
I've used the standard blue styrofoam for this, in it's most available thickness (2.5cm). There are
other styrofoams, such as orange/pink or white, which are even finer and slightly denser.
I found that blue styrofoam will respond to sanding with even quite a thin strip of sandpaper if this
is supported.
I made a 'sanding former' by gluing the strip onto the edge of a prepared shape, in this case a
half-circle corresponding to the diameter of the 'bowl' I wanted. I have used a 40 grit sandpaper
for this.
4D modelshop The Arches 120 Leman St London E1 8EU t:020 7264 1288 e: info@modelshop.co.uk www.modelshop.co.uk
Please note: Dark grey styrofoam has replaced the blue in this guide
First the outline of the 'bowl' needed to be marked on the surface.
Styrofoam is too soft to hold the compass point securely so the centre area needs to be
strengthened with some tape. This is then removed.
I then sanded by revolving the former on the
central point. If pressed too hard while turning the
former will just tear up the foam surface, but this
may not matter at first.
After a while sanding will get smoother as one gets
used to how the material reacts to the tool.
It didn't take long to reach the marked circle.
I then extended the lines from the circle and
marked the shape and depth of the concave on the
edge ... because here I wanted a 'niche' shape
rather than just a circular bowl.
I started hollowing out 'by eye' using another
'customised' sanding tool ... 60 grit sandpaper fixed
to a piece of wooden dowel.
As I got nearer to the marked depth I returned to
using the previous former to finish off (because it
has the right curve) but this time dragging it
straight.
The finished 'niche' form can be seen in the photo
on the right.
Composite forms in styrofoam (or any rigid material for that
matter) are best achieved by deconstructing the intended form
into parts or layers to be worked on separately.
For the 'half-ring' which encircles the central bowl I first cut a
template shape (using plastic or cardboard) outlining the form.
I secured this onto styrofoam (using double-sided tape) and cut
roughly round it, being careful not to undercut at this stage.
4D modelshop The Arches 120 Leman St London E1 8EU t:020 7264 1288 e: info@modelshop.co.uk www.modelshop.co.uk
Please note: Dark grey styrofoam has replaced the blue in this guide
I use either sharp, thin kitchen knives (i.e. fruit knives) or thin, self-supporting saw blades
('padsaws') to cut styrofoam, but a hot wire will also work well with a bit of practise.
To get a smooth, right-angled edge on the form I've fashioned yet another 'custom' tool .. this time
sandpaper fixed to a short length of PVC pipe which has been sliced at a right-angle.
The sander can be dragged round the form, keeping it flat on the work surface, and will gradually
create a smooth perpendicular edge. The template shape is left on to act as a 'stop' in the sanding.
As long as the plastic (or cardboard) is a little more resilient than the foam the difference will be
felt once the edge is reached.
The second part of creating this half-ring
shape involves fixing similar 'stops' to the top
and base of the blocked shape as shown.
Between these edges I used another curved
sanding former (smaller circle this time) to
create the inside profile.
The other small additions are shaped using much
the same methods, starting with a base template
to create blocks.
I then fixed profile templates either end, and used
a flat sanding block to sand down to the templates.
The forms need to be sanded lengthways with a
longer sanding block that will be guided by the
profile templates.
The rough rock texture around the outside of the bowl was created by slashing with the back of
the scalpel and also breaking down using a wire brush.
The most reliable way of gluing separate styrofoam parts together is to use a special 'foam
friendly' glue such as UHU Por.
Strong double-sided tape can also fix flat surfaces together very well but is not guaranteed to be
permanent.
4D modelshop The Arches 120 Leman St London E1 8EU t:020 7264 1288 e: info@modelshop.co.uk www.modelshop.co.uk
Please note: Dark grey styrofoam has replaced the blue in this guide
MORE ON GLUING STYROFOAM
I’ve bonded pieces of styrofoam together with a strong, double-sided carpet tape for years and
they’re actually much more permanent than I’d first imagined. A few observations are necessary
though! ... the two surfaces have to be smooth and flat against each other; if these surfaces have
been sanded to make them fit, all dust should be removed, ideally vacuumed using a brush
attachment. For the same reason the best bond occurs between the slightly ‘skinned’ surfaces of the
sheet as it comes. When using the tape it’s particularly important to avoid placing it too near to
where the styrofoam will be carved or sanded if one wants a seamless join ... but this applies almost
equally when using most glues.
If it’s impossible to predict, or avoid, gluing in an area which will later be carved through or sanded,
I’ve found some of the following options most suitable because they offer the least resistance.
Spraymount
! Works surprisingly well as a bond between flat styrofoam sheets is spraymount,
by which I mean the permanent spray-glue types from 3M.
It should be sprayed on both sides to be joined (different to how it’s normally
used), and it’s best to wait a little i.e. half a minute, before the two pieces are
firmly pressed together.
Hot-melt glue
If you want to glue pieces of styrofoam together which do not lie completely flat
against each other ... you will need something ‘gap-filling’. There’s hardly
anything more gap-filling and instant that hot-melt glue! Contrary to what you
might have read elsewhere, hot-melt glue will work reasonably well with
styrofoam ... as long as it’s not too hot!
Gorilla glue
Both styrofoam surfaces need a light misting of water first, best with a small
pump spray. The glue needs to be applied thinly to just one of the surfaces
before the two pieces are pressed together.
The glue expands 3-4 times in volume, so the pieces should be clamped so the
excess is forced outwards rather than upwards.
A couple of hours are needed for the glue to cure.
If it is properly cured the glue sands through well . although it is different to the styrofoam it is
much more alike than other glues. Also the bond is as strong as they say it is! I think polyurethane
grabs on the styrofoam particularly well ... at least, I couldn’t pull the blocks apart.
4D modelshop The Arches 120 Leman St London E1 8EU t:020 7264 1288 e: info@modelshop.co.uk www.modelshop.co.uk
Please note: Dark grey styrofoam has replaced the blue in this guide
Epoxy resin
I hadn’t even considered epoxy resin glue with styrofoam because I’d just
assumed it would dissolve the surface as polyester resin does .. one really
shouldn’t assume anything!
I tried it and it works perfectly on either styrofoam or polyurethane foam .. no
damage to the surface, and a very strong bond.
Epoxy glue is very tough though, so it is no good in any area that needs to be cut or sanded.
PREPARING STYROFOAM FOR MOULDMAKING
If a styrofoam shape is being made as a prototype
form intended for casting it doesn’t need to be
made particularly durable ... it only needs to
withstand silicone rubber being either brushed or
poured over the surface.
It does however need to be sealed, because if not
the silicone rubber will grab into the surface too
much and become very difficult to separate.
Vaseline (petroleum jelly) is an ideal temporary
sealant in this case because it can be easily
brushed or rubbed into the micropores without damaging the surface. If care is taken not to use too
much of it the Vaseline will also even out the surface, although I’ve noticed that most of it is
absorbed into the silicone anyway. The only problem is .. it’s very difficult to see where you’re
applying it! The solution is to colour it.
The best way to colour Vaseline is to first mix a little powder pigment,
in this case half a teaspoonful, with roughly the same amount of
Vaseline to make a thick paste not unlike tube oil paint.
I chose the ultramarine here because it’s a strong pigment and finely
ground, combining smoothly with the Vaseline .. some powder
pigments may be grainy or clump a bit, which is not so good!
The half teaspoonful was sufficient to give a strong colour to c. 50g
of Vaseline when I added this to it, but one could use far less
pigment.
For example, the pigment will stain a porous prototype, so you have
to bear this in mind if you want to keep it or if it’s an object of value.
There were a couple of larger scratches in the surface which I needed
to fill and I’ve found that soft modelling wax (this one is the
Terracotta Modelling Wax) is the easiest to use, worked carefully in
with a brush.
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