Foamcore Adhesives

ADHESIVES

Ordinary silicone sealant (RTV) has proven to be an excellent adhesive for balloon payload packages.  Its adhesion and resilience appears to be unaffected by the extremely low temperatures encountered in flight.  Its only disadvantages are long curing time and out gassing of acetic acid vapor.  Where an ultra-reliable joint is a must, RTV is the adhesive of choice. 

Epoxy is very strong and can set up fairly fast, but it’s picky about the surfaces which it will bond to and embrittles in extreme cold. 

Cyanoacrylate (super-glue) is great for quick bonding non-porous surfaces which mate closely.  We use it to bond critical knots in the payload string just prior to launch.  But it’s expensive, bonds poorly to rough or porous surfaces and probably embrittles when chilled. 

Another excellent joining material is 3M Kapton tape, also known as “space tape.”  It’s very strong, and the acrylic adhesive bonds very well to nearly anything.  It’s also outrageously expensive; we have found industrial sources of recently outdated tape which is still quite sticky.  Watch out for bargain basement deals; the adhesive on long-outdated tape may not stick as well as needed.  Test a piece before you buy; you should need a knife to remove it from your fingernail!  We use a few patches of space tape to hold the covers closed on the package just prior to launch. 

We have had excellent results using low-temperature hot-melt glue on foamcore joints.  Hot-melt is resilient at room temperature, but more rigid than the same width bead of RTV.  So far, it has shown no signs of cold embrittlement.  Hot-glued joints are stronger than the foamcore.  The only joint failure we have encountered to date was attributable to delamination of the foamcore paper; this was corrected by enlarging the joint area. 

A freshly glued joint cools slowly on this material, providing a few seconds of free time for alignment.  In about a minute, the joint reaches full strength, so you needn’t plan your project around a series of overnight adhesive cures.  The high-temperature variety of hot-melt is usable with foamcore, but it tends to melt away the foam before it cools.  A high-temperature glue gun operated at about 50% line voltage from a variac or light dimmer works fine with low-temperature glue; if the gun is too hot, the glue discolors. 

FOAMCORE JOINERY:

Foamcore is a stiff, planar material which crimps when overstressed, so curved shapes can’t be formed.  But it can be cut by hand with near-machine precision, and strong, straight bends of practically any angle can be formed quite easily.  With a few simple tools, some patience and a fertile imagination, one can quickly fabricate some pretty elaborate shapes. 

Tools required:

• Modeling knife with a good supply of sharp blades.  Single-edged razor blades will also work in a pinch.
• Machinist’s square.
• Metal straightedge.  The scale on the machinist’s square works fine for most work.  Longer cuts and bends may need a metal yardstick clamped in place at one end.
• Hot-melt glue gun and glue.  The low temperature variety is preferred.
• A large piece of cardboard for a cutting surface.
• A flat work table.

Other common hand tools, such as needle-nose pliers, may be helpful for handling smaller pieces.  A sewing needle pressed into the end of dowel is handy for holding small pieces and for marking the centers of holes on both sides of a sheet.  Straight pins serve well to help align less manageable joints prior to gluing. 

SIMPLE CUTS:

The keys to making clean, precise cuts are a sharp blade and metal straightedge.  Mark the cut line with a pencil or pen directly on the paper surface.  Then place your workpiece on a cutting surface which extends past both ends of the cut.  Align the straightedge directly over the cut line and plan on holding it steadily in place until the cut is complete.  A C-clamp is handy for long cuts, but in most cases, the edge can be held in place fine with one hand while you cut with the other.  Be careful not to crush the foam; it’s especially susceptible to crushing at cut edges. 

The cut should be made in at least three end-to-end passes.  The first pass should just penetrate through the upper paper surface and only slightly into the core.  For accuracy, the blade should be aimed slightly into the straightedge so that it won’t drift away; this will also minimize the gap between the cutting edge and the straightedge. 

Start the cut by poking the blade point squarely into the surface through the top paper layer.  Then reduce the angle between the material surface and the blade edge to no more than about 30 degrees to avoid tearing the surface.  Using a steady motion, pull the blade through to the end of the cut.  Remember, the first pass should only cut the upper layer.  A dead end cut may be terminated precisely with a near-vertical poke of the blade. 

Keeping the straightedge in place, make the second pass like the first, except this time, cut through the foam and slightly into the surface of the bottom paper layer.  This pass establishes the angle of the cut edge.  If you want a simple square cut, then be careful to hold the blade perpendicular to the foamcore surface through this pass.  Square up any dead ends with vertical pokes completely through the lower paper. 

The third pass should cut completely through the lower paper.  You may dispense with the straightedge this time, but it’s still possible to let the blade drift off at this phase if you’re not careful.  Keep the blade angle steady from end to end, and use enough force to cut completely through the lower paper.  If the lower paper is not cut through end to end, turn the workpiece over.  Incomplete parts of the cut line should at least be visible as a distinct ridge; if so, insert the tip of the blade into a cut portion and carefully pull it through the ridge, allowing the blade to self-align on the opposite side.  If a ridge isn’t visible, then get a new blade and repeat the third pass from the first side. 

A sure sign that your blade is getting dull is ragged cut edges in the foam or tearing of the paper.  A fresh blade is good for about 3 - 5 lineal feet of cutting.  Blade life is definitely extended by use of a clean cardboard cutting surface.  I’ve had a little luck resharpening Exacto blades with a fine oilstone, but I’ve never gotten them any better than “half-dull”.  A brand-new blade is a pleasure to use; make sure you have enough on hand before you start your project.