Sean Cassidy's immaculate 1:19 scale Bristol F.2 (facing page) floats by at a very realistic speed. Molded Depron construction allows a very light airframe. This 24-inch model weighs just 73 grams. Dave Johnson’s Grumman F3F fuselage sits on its gear with initial detailing added. Note the thinness of the molded Depron shell (above left), as evidenced by the cockpit cutout area. Johnson used molded foam where the prototype Grumman (above right) had sheet-metal and replicated the fabric covered surfaces with traditional tissue-
covered balsa structures. Johnson extends the landing gear (below left) while passing in front of the judges. The intricate landing gear features approximately 200 pieces of carbon fiber rod, aluminum tube, music wire and laser-cut plywood. The static display table offers an excellent chance to examine Johnson’s Grumman F3F up close. The surface detail is a mixture of scribed panel lines and glue rivets. The landing gear (below right) not only retracts, but it also features a functional sprung suspension.
While I won’t go into depth here about how to form the shapes you’ll need, I will say that you should plan out your mold or form construction so it suits your needs. Typically I prepare a mold that has two faces which can be clamped together to sandwich wing parts inside, and one to stretch the foam over for fuselage shells. Plan out your molds so that they fit your oven too!
The wing molds shown in one of the pic- tures have two faces with washout and dihe- dral built-in. For larger models, you’ll need to make your wing panels in sections (left and right sides) and join them together lat- er. You can also make your wing molds with one curved face and stretch and clamp a 1⁄16 plywood outer face to it so it sandwiches your wing inside. The fuse form shown on page 46 was made using a template glued to one block of wood. The block was cut using a band saw and an- other block was tack-glued to it. The whole fuse was then shaped and then popped apart to make two mirrored fuselage halves. I glued a balsa flange all around the base so I could clamp the foam in position.
Setting up your oven
Start out the molding process by prepar- ing your oven. Set the rack somewhere in the middle of your oven. Depending on your oven, you may need to experiment with rack placement to avoid hot spots that can blister or unevenly heat your finished part. The heating needs to be as even as possible. Some conventional ovens have hot spots that are so severe they may burn through a sheet of foam in one section while waiting for other areas to come up to temp. Set the temperature so that you get a maximum temperature of 250°F. Your oven
FLYING MODELS
really wasn’t made to work at temperatures this low, so you’ll need to trust your hand- held temperature gauge. An infrared tem- perature gauge is very good for this purpose. Remember that your oven cycles on and off and that the temperature you set it at is not a maximum but an average temp. Shoot- ing for 250°F maximum will require you to set your oven fairly low. In my case, I had to set my oven dial to 200°F to get a maximum of 250°F. The forming temperature of De- pron foam is between 240°F and 260°F. If you get it hotter at any point, it will begin to distort and shrink too much. At any rate, you’ll need to take some time finding that sweet spot where you can set your oven. My procedure for finding the correct tempera- ture is as follows.
First, set a baseline temperature some- what low, start at about 210°F and leave your oven on with the door closed for an hour. After the timer goes off, come back to your oven and watch it until the oven kicks on by itself. Watch for the glowing heating element. This begins its cycle back to max temperature. If the element is not visible in your oven, consider using a digital baking thermometer with a remote probe to moni- tor when the heat begins to rise again. When most ovens get to their max temp you can usually hear when the thermostat kicks off. Now is the time to open the door and use your IR temp gauge to read the top and sides of the oven. If any of your readings are above 260°F, adjust the temperature of your oven down in small increments and re- peat the procedure.
When you’re ready to start molding parts, you’ll need to mold a few test scraps to fine tune your oven. I have very seldom ever been lucky enough to mold acceptable parts
on the first shot. Usually, they won’t com- pletely mold or will get too hot and melt. Your test pieces do not need to be full size. I often tape small pieces of foam to the mold and then place them in the oven to test the temperature. The indication that you have the right temperature is that the foam does not melt, but when you remove the mold and let it cool, the foam retains its newly formed shape without springing back at all. When you do get the right temperature and your oven is stable, I encourage you to leave your oven on and mold all of your parts in one sitting making a few extras in case you make a mistake while trimming them to fit. Having a few spare parts around will also help encourage your friends to give this building technique a try as well.
Molding wings and other single-direction curves To mold your wing, choose the right thick- ness of foam that will suit your needs. Usually for models up to a 14-inch span, 1mm Depron is suitable and strong. For larger models, you may want to choose 2mm or 3mm Depron. First, cut your wing to shape, but leave
the ailerons attached. You will trim them out later. Now align your part in the mold and clamp it up using spring clamps. Throw your part in the oven and set your timer. For small balsa faced molds, you’ll need to leave it in there for about five minutes. For larger molds, you may need 15–20 minutes. In any case, you’ll need to experiment on the time you need with a few test pieces before you get the results you want. When the timer sounds, pull the mold out of the oven and leave everything clamped up until the mold cools to less than 100°F. Again, the IR temp gauge is great here.
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