Working with EPP
Contents |
Suppliers
- eppHobbies (large blocks only)
- slofly Various thickness of sheets and numerous useful precut shapes.
- Robot Birds UK RC supplier sells EPP sheet in several thicknesses.
Construction
For 3D profile planes up to about 30" 9mm foam is sufficient, and a similar weight to 6mm Depron or FFF/blucore. For built up structures 6mm foam is much easier to bend over formers but still tough enough not to tear. This size is also suitable for ribs and spars and inch or so high.
Box fuselages and wings are much more resistant to twisting than flat plates or cruciform (shockie) fuselages - it's all about the enclosed area; sheeting one quadrant of a cruciform fuselage eliminates tail twist in 3D planes.
Glues
Goop, UHUPOR, Welder and other contact adhesives work well, even the spray-on ones (3M 77, or other permanent, high strength spray adhesive - beware of 're-positionable' photo mount sprays, they just aren't strong enough.) Goop can be thinned with toluol to the consistency of melted chocolate and mixed with paint for a nice tough finish.
Polyurethane glue is good for joints where you want a bit of time to get it all lined up, it's light, foams up to fill gaps and penetrates into the foam for a really strong bond. For large areas you can seal the edges with CA to contain the foaming. It takes longer to set than advertised (because it's supposed to be used on wood, where it can absorb moisture)- the 5-minute stuff sets can be worked for about 20 minutes and is set in an hour or two.
Hot Glue is the king when you're in a hurry though, because PU glue takes an hour or two to dry. Be careful not to overuse it as it can be heavy.
Epoxy works but isn't ideal, as it's heavy and too rigid.
CA isn't good, it's brittle and expensive. It's OK for small repairs though. EPP doesn't require foam safe CA.
Plain old white wood glue (PVA, Elmer's) works, and can be mix with cotton or lite patch plaster for filler.
Four parts PU to one part white glue sets much faster than ordinary PU (or PVA) glue. Pot life is about 10 minutes, it sets in an hour or so on a warm day.
Anything not brittle and made for rubber/ceramic/metal seems to do the trick
E6000 (from craft stores) is expensive and does not work well.
Shaping
You need a sharp knife to cut EPP; use a fresh blade and sharpen it (or replace it) as soon as it starts to snag or it will tear rather than cut.
EPP can be sanded but it helps to use a sanding block to spread the pressure, and to sand in long smooth movements to prevent ripping the beads out. Belt, disk and drum power sanders work well, orbital sanders either tear out chunks or leave lots of stringy bits. Stringy bits left over from sanding can be melted with a hot air gun.
Spars
A single 3mm carbon rod or tube will cope with gentle flying on a 30" wing, but two 2mm rods stuck to the sides of a 9mm thick sheet are much stronger.
Apart slots can be cut with a dremmel router, or melted with a soldering iron. Use a ruler in both cases to get the straight.
A good technique for gluing the spar is to make a small puddle of PU glue on a bit of thick plastic bag, then pull the spar through the puddle, pinching the bag together (around the spar) just after the puddle to scrape off the excess.
At relatively low power, an EPP spar with EPP skins is sufficient.
Mixing with other materials
EPP can be mixed with other materials. You can use Depron control surfaces for a extra rigidity, or Corex for tip fins on flying wings. Others have found that a depron wing is sufficient with an EPP fuselage (or just an EPP nose cone)
Strapping tape is a great way to prevent EPP from tearing. It sticks a lot better if the foam is sprayed with contact adhesive beforehand.
Hinges
Conventional hinges create stress points, and EPP prefers it's stresses spread out. Tape hinges work well (again use spray adhesive to help the tape stick) especially if you tape both sides at each end and by the control horn, but there are some more unusuall methods that work well.
Foam hinge
An accurate hot wire can cut a V shaped notch in a wing or stabiliser that leaves about 1mm of foam to act as a hinge. Multiplex models mould their hinges like this, others are CNC hot wire cut. It's hard to get sufficient accuracy by hand and leaving too much foam in the V makes the hinge stiff. The thin form in these hinges can tear when abused.
Hot glue hinges
- Bevel one or both edges, depending how much movement you require. Make sure it's a nice sharp cut.
- Place the two pieces together so they line up accurately and *just* touch. Use tape to hold them in place.
- Run a thin (<5mm) bead of hot glue along the hinge line for an inch or two (thinner beads can be longer).
- Use a flat spreader (razor blade, credit card, etc) to spread the bead of glue along the hinge while it is still hot.
- Let the glue cool for a few seconds before doing the next bit.
Don't let the glue run down into the bevel or it won't hinge at all. Do spread it as thinly as you can or you or the hinge will resist at large deflections. This method not only gives strong, free hinges but also strengthens the thin beveled edge of the EPP.
Here's a good video of hot glue hinging from the superfly build thread:
Spectra line hinges
This thin cord can be stiffened with CA and glued into slots or holes in the EPP. Break the CA at the hinge to restore the flexibility of the cord.
Covering EPP
Lightly loaded EPP models (i.e. 3D planes) don't need covering, just a bit of re-enforcement in the high stress areas.
Before covering, a light coat of spray adhestive (3m77 or other good contact adhesive. NOT repositionable spray mount!) to help it stick. Beware that this will pick up dirt very easily while it's exposed.
Strapping tape (the stuff with strands of glass fibres in it) is a great way to add strength but it's also quite heavy so don't add too much. It has been known to deteriorate with UV light so you might consider covering with one of the following.
Packing tape is available in a number of colours, not just 'parcel brown' and will stick well with a layer of spray adhesive, but not too well on it's own. It's hard to apply without wrinkles, and overlaps show when you hold the model up to the light. It looks OK in the air though and stripes are very easy. More complex shapes can be made by applying the tape to a sheet of wax paper (e.g. the bit left over after peeling the stickers off a sheet of labels) and then cutting with scissors or a craft knife; the tape can then be peeled off and applied like a shaped sticker, over a contrasting colour.
Standard modeling heat shrink film coverings can be used, although the low-temp versions (UltraCote) are best to avoid melting the foam. These give a much better finish than tape. Use an Iron temp between 250-270 degrees F, to activate the glue with very little shrinking of the covering.
