Foam Ejection Plugs

by Andrew Tomasch

Here are a few of my findings regarding the production of foam plugs for ejection wadding and nose cone shoulders. I have produced nose cone bases which are cored out to rings for lightness, and ejection plugs with cored out rear skirts to reduce weight and minimize any tendency to jam in the tube.

Cutters: I needed good cutters for both 40 mm and 30 mm diameter, the latter for coring out the 40 mm parts when required. I laid up tubes from three layers 3.5 oz/yd^2glass cloth on my 40 mm and 30 mm mandrels. Lots of Crown release, both came off fine.

The resulting tubes are very robust. They can be cut on a band saw. Wrap masking tape around the tube where the cut is desired. Push the tubing into the band saw blade until it just breaks through the wall. Now rotate the tubing into the blade using the tape as a guide until it cuts through all around. I used 3″ lengths of both diameters for my cutting tubes. This will cut plugs up to 1 1/2″ thick. My plugs are 1″ thick blue Styrofoam insulation board.

To sharpen: sand the end of the tube with fresh 220 grit wet/dry paper placed on a flat, hard surface. Use a circular motion, just as you would sand the end of any body tube. You’ll find that the end surface of the tube is now a sharp abrasive surface, due to the glass fibers poking out of the epoxy. This actually helps to abrade the foam as the tool passes into the material, eliminating overheating and melting. Once the cutter is spinning in the drill press, a sanding block can be applied to the outside edge at an angle, to further sharpen. Easy does it. This is in fact not really necessary. Experiment.

Mounting in the drill press: purchase a rubber sanding drum for an electric drill, available at any hardware store. The 30 mm cutter uses a 1″ drum, the 40 mm usesa1 1/2″ drum. In each case, wrap black vinyl electrical tape around the drum until it will just slide into the tube. Now tighten the nut on the drum to swell it outward and trap the cutting tube. Drill a 1/4″ hole just past the end of the drum to allow air to be blown into the tool to remove plugs as necessary. I was able to do this with a sharp new machine drill. A step drill (“Unibit”) for sheet metal should also work. Whatever tool you use, it must be sharp!

To Use: Chuck the tool in the drill press, running on slow speed. If it wobbles, loosen the nut, push it slightly and try again. A couple of tries should yield a nice straight running tool. Cover the drill press table with manila folder stock to keep the tool from hitting metal. Plunge the tool smoothly and slowly into the foam. Most of the time the plug starts to fall out of my cutters. If not, a puff of breath in the hole will remove it. If a plug really sticks, push it up into the tube first, then blow.

Here’s a trick that really improves performance: The Crown wax, which coats the inside of the tool produces a very sticky, high friction surface. So put a little talc on your finger and rub the inside surface with it until it is all smooth and slippery inside. This makes a huge difference! The plugs will cut more easily with no tendency to melt, and they will fit the inside of your airframe tubes to perfection. Fresh talc is necessary every 20 or so plugs, or whenever you notice more friction (plugs not releasing easily for example). This really makes a difference when coring out a 40 mm plug with the30 mm cutter. Without the talc, the plug wants to twist out of your hand. With the talc, it glides right through! This also points out how important to coat the inside of your model with talc when prepping for flight. Also, rub some talc into the edge of the plug too. This goes for S6 too, not just S3.

Hollow nose cone shoulders and ejection plugs: To core out a 40 mm plug with the30 mm cutter to make a ring nose cone base, I just hold the plug by hand and eyeball the centering before plunging the tool. With a little practice, this does a nice job. Keep the tool lubricated with talc for best results.

Hollow skirt ejection plugs: Cut a 40 mm plug. Now cut a 1/4″ thick disk off the end with a hot wire cutter. I have a nice home made one with a table and adjustable fence, which functions just like a band saw. I published it in T Minus 5 a while back. I can post a copy on the web site if there is interest. Now core out the 3/4″ thick plug with the 30 mm cutter. Re attach the disk with a bead of UFO CA glue applied to the end of the ring. This sounds like a lot of work, but it isn’t bad at all. This goes very quickly, and I estimate I could build perhaps 50 of these in an hour.

I tested the hollow skirt plugs and the ring nose cone base this weekend, and both work fine. The 1/4″ thickness stands off the Delta ejection charges easily, in fact,without damage. No problem with melt-through whatsoever. The ring nose cone bases are fine too. They are rigged with a light Kevlar loop tied around the ring before gluing on the cone. The loop is knotted again just past the ring, trapping the ring in a Kevlar loop. I UFO CA’d the Kevlar to the inside of the ring as well.