HD Foam Cutting and shaping

Choosing the Right Foam - What type of foam should we use for our models?
  • We can use cores/sheets from several types of foams and densities, and if you are new to using foam construction, foam selection can be confusing.





The Basics.
  • Foam wings became popular in the 1960's when modelers quickly found out that the job of building a tapered wing a lot easier.
  • The first type of foam wings were constructed of EPS foam that was covered with 1/16" Balsa sheeting applied with Epoxy glue or contact adhesive.  This type of construction was popular for .40 and .60 sized glow engine R/C aircraft which ruled the sky of most R/C fields at the time.
  • The finish was typically Mono Cote or one of many heat shrink covering materials. 
  • Not surprisingly, this type of foam wing construction works well for many types of aircraft and is still very popular today.
Wing strength.
  • Foam type relates to overall wing strength, but not necessarily the way you might think.  In most cases, foam type is the least important of many factors as you will see as we get into the details.
  • Basic wing strength is all about tension and compression.
  • When a wing is placed under load, the lift that the wing produces tries to push the wing up, and the weight/mass of the airplane tries to bring the airplane down.
  • This results in increasing compression on the top surface of the wing, and increased tension on the bottom surface of the wing.
  • In a typical wing that is built from spars and ribs, the tension strength comes from the wing covering, and the compression strength is provided by the spars.
  • Primary failure occurs when the ribs that keep the spars in place crush, and the wing "kinks" like a piece of copper tubing that is bent too sharply.
  • The use of shear webs between the spars helps to prevent this. 
  • The most popular type of shear web is thin balsa pieces applied between the spars with the grain running perpendicular or normal to the spars.
  • The primary strength of a foam wing is provided by the sheeting and covering.
  • The foam gives the wing its shape, and keeps the sheeting in place, but most of the structural strength comes from the sheeting and covering.
  • When a foam wing is stressed to the point of failure, it will also "kink" when the balsa sheeting caves in from compression.
  • The failure is usually not a tension failure because film covering has an amazing amount of tension strength if it is intact and properly adhered to the balsa sheeting.
  • Another factor that controls the overall strength of a wing is its thickness.  Thicker wings are stronger because the sheeting and covering that provides the basic wing strength are further apart.
  • This is true for foam wings as well as built up wings. 
Thin wings
  • As wing thickness decreases, so does its inherent strength, as the forces that try to compress the sheeting have more leverage on a thin wing.
  • Higher density foam helps to keep the sheeting in place and helps prevent failure of the sheeting on the compression side of the wing.
  • "All right, I'll always use a high density foam," you say.  Let's not forget that high density foam adds weight to a wing, and the thicker the wing is, the more of a problem this is going to be. 
  • If weight is critical, and it is on most things that fly through the air, a light weight wing is important.
  • Put a spar in it!!  A popular solution is to add a tubular carbon spar in the center of the wing to increase it's strength.  This is a quick and simple way to add strength, but it also adds weight.
  • The weight increases not only by the weight of the carbon tube, but also from the glue used to adhere the spar to the foam.
  • A better solution is to use 2 smaller spars on the top and bottom of the wing.  Spars that are placed close to the sheeting have the most resistance to compression, and inherently greater strength.
  • The best solution is a full-depth shear web spar, sometimes known as a "D-Tube." This consists of a vertical grain shear web spar with some carbon tow, or thin carbon strips on the top and bottom of the shear web. 
  • The carbon tow/strips keep the shear web from breaking through the sheeting when the wing is under heavy stress by spreading out the loads on the top and bottom of the shear web.
Coverings
  • Most foams need some sort of covering as it provides the large portion of the wings strength and is a major consideration for foam selection.
  • Coverings include heat shrink plastics, shipping tape, balsa, plywood, obechi wood sheeting and various composites such as fiberglass, carbon fiber and aramid cloth. 
  • Combinations of all these coverings have been used. 
  • Heat shrink plastic coverings such as Monocote, Ultracote and many others have been around for years, and were originally intended to be used as a replacement for silk and dope!
  • Low heat versions of these coverings can be applied directly to most foams if the foam is covered with a light coat of spray adhesive.
  • These coverings provide a tremendous amount of tensile strength, but no compression strength and all but the lightest and smallest airplanes usually use carbon tube spars to provide the necessary compression strength.
  • This is a typical construction method that works well for Slope combat flying wings, but it does not scale up well for larger models.
  • Colored shipping tape is a cheap replacement for heat shrink coverings and will also provide high tensile strength if properly applied.
  • Wood coverings such as balsa, plywood, and obechi wood sheeting are common for many large and small models and provide high compression and some tension strength.  
  • These coverings are usually finished with Monocote-type plastic coverings or light layers of fiberglass or composite coverings and paint.
  • This results in extremely strong wings with unlimited design and color choices. 
  • Composite coverings of fiberglass, carbon fiber and Kevlar cloth and resin are common on high performance sailplanes, gliders and commercial UAVs.  They are also used on high-end scale and aerobatic aircraft. 
  • These coverings are used in a variety of ways, and are some of the most expensive material choices, but they provide the ultimate combination of strength and light weight.
  • All of the foams we stock and hot wire cut are compatible with Epoxy resins or Polyurethane glues such as Gorilla Glue or Elmer's ProBond.
  • Polyester resins will attack and degrade Polystyrene foam and should be avoided.
How do We choose the right foam?
  • Foam selection is dependent on the size of the model, type of covering, intended use and construction methods.
  • Foam is produced in many densities, colors and types.  
  • Only a few types and densities are used in the modeling world.
  • Density is defined as the weight of 1 cubic foot of the material.  In other words, a 12" x 12" x 12" cube of 1# density foam will weigh 1 pound (453 grams).
  • Higher density foams are harder to the touch and more rigid, but they weigh more. 
  • The higher density foams are typically used in high aspect ratio wings that are relatively thin, like high performance thermal gliders.
  • Powered pylon racers and even giant scale racers may also use 2# foam.
  • Large models with thicker airfoil cross sections will work fine with 1# foam.
  • The 1# foam is also used on most powered sport airplanes, trainers, and any airplane or glider where weight is a concern.
  • Here is a list of the foams , with their density, compression rating and relative cost level:
Material
Density
PSI
Cost
EPS (Expanded Polystyrene)
1#
10@10%
Low
EPS
1.5#
15@10%
Low
EPS
2.0#
20@10%
Med
EPP (Expanded Polypropylene)
1.3#
11@25%
Med
EPP
1.9#
23@25%
High
Owen F250 XP (Extruded Polystyrene)
1.8#
25@10%
Med
Dow Surfboard (Extruded Polystyrene)
2.2#
60+@10%
High+
  • In local market/factories you can ask for HD 20, 24, 28, 30, 34, 38, 40 grade of sheets or blocks. ( The price in Indian Rupees varies from Rs.6,000/ sq ft (HD 20) to Rs.15,000/sq ft (HD 40).
Wonder Material
  • Foam has revolutionized RC model airplane building.
  • It is inexpensive to buy, costing a lot less than an equivalent block of balsa.
  • It is of a consistently high quality and widely available.
  • It can be easily cut, sanded, and carved.
  • The stiffness of foam, when looked at by weight, is comparable to balsa.
  • Foams do not have balsa’s strength, and are generally lower in density than woods. 
  • All of these differences require a new set of building techniques.
  • But when used properly, foam is an amazing building material.
Making Foam from Plastic
  • Most plastics are made from petroleum. 
  • The foams that we use in our model airplanes are normally created through a two step process. First, tiny beads (about 1 mm diameter) are manufactured containing a small amount of a so-called blowing agent. 
  • This is a liquid that turns into a gas when heated. 
  • The second step is the actual expansion of the beads in high temperature molds. 
  • Depending on the plastic, an extrusion process may be preferable over an expansion process.
  • Besides creating foams from pure types of plastics, a popular trend nowadays is mixing two different plastics to try and create a foam that minimizes their disadvantages and enhances their strengths.
  • The density of foam is more a factor of the foaming process than the type of plastic that was used. 
  • Do not use it alone to identify the type of foam.
Polystyrenes (XPS and EPS)
  • Polystyrene foams have a long history of use in model airplanes. 
  • We prefer the extruded varieties (XPS) because of their superior strength. XPS foams include Depron, Midwest Cellfoam 88, Dow Blue Cor, foam board foam, and Owens Corning Foamular.
  • They have good stiffness, but are prone to snapping in a crash.
  • Expanded polystyrene (EPS) foams do not seem to be getting much use anymore in model airplanes.
  • Compared to the XPS foams, they are relatively heavy and weak.
  • Dow Styrofoam, foam coolers, and disposable foam cups are made out of EPS.
Expanded Polypropylene (EPP)
  • EPP foam is less stiff than XPS, but has great resilience (ability to spring back when deformed) and tear resistance.
  • In other words, EPP foams survive crashes much better than XPS foams.
  • Most foams that we use are rated up to 160 F (75 C).
  • This means that their physical properties are guaranteed to be consistent up to at least that temperature.
  • EPP, however, is good up to 212 F (100 C). You can imagine some RC applications where the difference might be important.
Expanded Polyethylene (EPE)
  • EPE foams are similar to EPP foams.
  • The best way to tell them apart is by feeling them. 
  • EPP foams feel rough to the touch compared to EPE foams.
  • EPE is almost indestructible in a crash, at the cost of some more stiffness (when compared to EPP).
  • Pool noodles are normally made out of EPP, but I have come across some made out of EPE.
Foam Blends
  • There are many proprietary foam blends on the market today.
  • Most are very similar to EPP.
  • One reason why the blends were created is because it is not easy to create accurate molded parts out of EPP.
  • You see, EPP has a tendency to expand a little bit after it is removed from the mold.
  • Marketing played a role in the creation of these foam blends, too. Would you rather say that your molded foam airplane is made out of EPP or of HyperXYZ Super Duper foam?
  • Sometimes you see the name expanded polyolefin (EPO) used. Well, guess what? EPP and EPE are both polyolefins. It is just sleight of hand naming from the manufacturers.
  • Some of the foams in this general category include Z-Foam, Aerocell, Elapor, EPO, and Arcel.
Hot wire foam cutter - The main tool used for HD Foam construction - http://www.instructables.com/id/Hot-wire-foam-cutter
  • Simple and and cheap hot wire foam cutter can be home made to suit different cutting needs with some basic components.
  • Here is a cheap  and easy to build hot wire foam cutter made from commonly available parts. 
  • Cuts foam for model plane wings, fuselage and other parts.
  • Read this and you will be able to build a cutter that is suited to your budget and the materials you have access to.
  • There are a lot of alternatives for the frame, the transformer, the wire, the enclosure, and the heat control. 
 Parts - The parts should be pretty easy to find.
  1. 12 foot, 16 guage extension cord.
  2.  2 wooden  sticks.
  3. 4 #10-24 x 1.25" machine screws with nuts.
  4. 10 #10 washers.
  5. About 12 feet of strong, low-stretch string. I used kite line.
  6.  A single-pole dimmer switch.(There are a lot of different kinds of dimmers and fan controls. First, you don't need a fan control. You can't use a light dimmer to control a fan because you will damage the motor but a hot wire foam cutter is most similar to a light bulb -- just a hot wire, but not hot enough to glow. Fan controls are more expensive because they have additional circuitry in them. Don't waste the extra money on a fan control.What you want is a 600 Watt, Single Pole, Rotary On/Off dimmer, with a  knob. )
  7.  A 25 volt, 2 amp transformer.
  8. Nichrome wire. (or  An electric guitar string, about .10 - .16 size. You can get these individually at a music store or you can use either of the 2 smallest strings from a packaged set.)
  9. You should keep a spare handy because they can burn out or break from too much tension. 
  10. A length of two conductor electrical wire with a regular plug on the end.  you can another extension cord if you like. 
  11.  A piece of wooden dowel or stiff plastic rod about one foot long (not shown). I used a bamboo skewer.
  12. A box at holds the transformer and dimmer switch, but you would be much better off with something like a "project box" from Radio Shack.
Tools and supplies
You might be able to do the whole project with just a knife, a drill and some tape, but it would be better to have the following:
  1. Utility knife 
  2. Small wood saw
  3. Drill with a bit slightly bigger than the #10 screws
  4. Screwdriver
  5. A couple of cable ties or twist ties
  6. Electrical tape
  7. Wide packaging tape
  8. Nibbler 
  9. Multitester (you don't NEED one but it's a good safety check)
  10. Wrench to match the nuts (I didn't have my SAE wrenches handy so I used a 9mm)
  11. Solder and soldering iron, if you like 
 Making the frame pieces
  • Cut one of the yardsticks in half. In each half, drill a hole in the middle and one about 1/2 inch from each end. One end of one half will already have a big hole in it, so you won't have to drill that end. 
  • One the remaining, uncut yardstick, drill a hole about 6 inches from the pre-existing hole (see photo), and another about 1 inch from the other end (not shown in this photo, but visible in later steps).
 Bolting the frame together
  • Make a sort of a big H shape out of your pieces by bolting them loosely together with the machine screws. Don't tighten the nuts down yet.
 Attaching the lead wires to the frame
  • Now we're going to attach the wires that carry the 25V current to the cutting wire. We'll use two machine screws as terminal posts. 
  • First, cut off each end of your 12 foot extension cord. Save the plug and outlets for future projects, if you like. Strip the insulation off the last inch of one end of one wire of the cord. 
  • Insert a machine screw in the top of the right leg of your "big H" as shown in the photo. This is the leg that is on the other side of the H from the handle. Use one washer on the screw head side and two on the nut side, as shown in the second picture below. Put the nut on, but don't tighten it down at all. You need room between the washers so you can put your wire in there. 
  • Bend the bare wire of the extension cord (the part you just strippeed) into a U shape, and hook it around the screw between the two washers. Now you can tighten down the nut. You can see what the final hook-up looks like close-up in the photos for step 7. 
  • Starting the the end you just hooked up, pull the two conductors of the cord apart so that it's split for about 3 1/2 feet. Cut the unmounted side of the split to about one foot. Strip the end of the one foot section and mount it to the other leg of the H the same way you hooked up the first wire. You'll see what I'm talking about here if you look at the photo. 
  • Use cable ties, twist ties or string to keep the wire close to the frame so it won't get in the way when you're using the tool.
 Making the tension to loop
  • Take a piece of string about 6 feet long and thread it through each hole on the other ends of the legs, as shown. Tie it into a loop so that when you pull the legs apart the string keeps the legs fairly parallel. So, the length of the loop when taut should be about the same as the distance between the bolts.

 Attaching the hot wire
  • While you are hooking up your wire, try to avoid making any kinks it. 
  • Your guitar string should have a sort of a head on one end. Make a loop by feeding the other end through this head. Hook the loop over one terminal and sinch it up. Keep the loop pretty close to the nut or there will be too much twisting force on the leg when the wire is tensioned. But also make sure it's not touching the wood. 
  • To hook up the other end of the wire, pull the frame legs towards each other so your string tensioning loop on the other end is taut. Wrap the end of the wire around the other terminal screw and twist it off. See the second photo. It might help to use pliers to keep good tension, but be careful not to pull too hard and break the wire. 
  • If the wire is not super tight at this point, don't worry. It can be kind of floppy when plucked but should be pretty much straight when at rest. We'll add more tension later.

 Cross strings
  • Now we need to install the mechanism that keeps the the legs square to the wire. Without this step your contraption will easily wobble into a parallelogram. 
  • Tie a loop in one end of a 3 foot piece of string and hook it under the washer of one of the middle screws. See the photos. Thread the other end of the string though the opposite leg's hole. Square up the frame and tie off the string. Repeat with another string for the other side, but when you tie off the string at the top of the leg this time, make sure you have some tension. 
  • Now both of the criss-cross strings should be pretty tight, and there should be some slight tension on the wire too. It's ok if the tensioning loop is a little floppy at this point. Now you can tighten down both of the middle screws with the wrench. No need to crush anything, just make them tight enough to hold everything together.
 Applying tension
  • Insert a ruler or dowel in the tesioning loop and twist it until it seems to be getting a little tight. Careful not to twist too much or you'll break the wire or the frame. Pluck the wire and listen for a musical tone. If it sounds like "fwubababa" it needs more tension. If it sort of hums it should be enough to start. You can always add more if it seems too floppy when you try to make your cuts. Once you are happy with the tension, slide the ruler or dowel down so that the yardstick keeps it from unwinding (see the photo). 
  • You'll have to readjust the tension later, after the wire gets hot for the first time. Or maybe every time.
 Wiring up the transformer and dimmer switch
  • This photo is your wiring diagram. The black two conductor wire on the left goes to the wall plug, and the brown one on the right goes to the hot wire. 
  • This photo is just to show what connects where. You should of course use the wire nuts that came with the dimmer switch (esp on the 120V connections) and/or tape to ensure that no bare wires touch each other, or you, or your pet. Be careful not to electrocute yourself or start a fire.
 The project box
  • This box was made from 4 cd cases and some packaging tape. Two CD cases are openned to right angles, then pushed together to make 4 walls. The corners join up nicely. Run a strip of packaging tape down each corner.
  • Now take the lid off another case and tape it to the bottom of the 4 walls. The bottom will touch two opposite walls and there will be gaps under the other walls. Don't tape over the gaps, they provide ventilation.
  • Take the lid and plastic tray off a 4th cd case and throw them out. Nibble or cut a hole for the dimmer switch and tape it in. Then connect up all the wiring according to the diagram in the previous step, tape in the transformer, and tape the top on. In the photo, the box has been turned on it's side. 
  • If you drop the box, the transformer will shatter it. So you can use a different design for your enclosure. 
 Turning it on
  • Set the rig up somewhere where it won't catch the house on fire or melt the carpet if something is wired wrong, or if the wire overheats and breaks. Take a moment to look over your creation and make sure all the wiring seems to make sense. 
  • Turn the dimmer all the way down (counterclockwise). 
  • With your fire extinguisher handy and your body away from the device, plug it in. Are the lights still on? Is the hot wire still whole? Sweet. 
  • You can use your multi meter to see if there's any current between the terminals. There shouldn't be yet. SLOWLY (like 5 degrees per second) turn the dimmer up (clockwise) til the wire starts to quietly hum. With the wire I used that's about 1/4 or 1/3 of the full rotation. If the wire doesn't hum or heat up by the time the dimmer is halfway up, turn the dimmer all the way down, push in til it clicks, and start again. 
  • If you turn up the dimmer too fast, your wire may burn out before you realize that it's even hot. 
  • Once you are sure everything's all set, grab the frame and try laying the wire on some scrap styrofoam. It should slice smoothly into the foam. You shouldn't have to push very hard. Try playing with the dimmer switch setting to get the best cut. Cutting slower and cooler makes a smoother cut. 
  • The wire heats up and cools down within a second or two. 
  • Next time you use the foam cutter, make sure nothing meltable or flammable is touching the wire when you plug it in, just in case the dimmer isn't off.
 Quick and dirty sample cuts
  • Here are some freehand and template-cut shapes to give you an idea of what the tool does. 
  • These are really basic one-cut shapes. You can of course bevel edges, etc, with a second pass.
  • Alternatives and expansion ideas
  • Some people set up their cutters in sort of a bandsaw configuration, so that small pieces can be manipulated against a table for precise cutting. Those are generally not useful for cutting wings or slicing chunks off larger pieces.
  • See this page for an Example:
  • http://www.hhhh.org/~joeboy/resources/hotwire_foam_cutter/hotwire_foam_cutter.html
  • The enclosure for the transformer and dimmer needs to be more rugged, and it should incorporate a fuse, maybe a power indicator, and maybe a modular connector to facilitate attachment of larger or smaller cutters.
  • Acknowledgements - I collected much of the raw info for this project from these two pages : 
  • http://members.fortunecity.co.uk/slmohr/rcinterest2.htm
  • http://www.vatsaas.org/rtv/construction/hotwirecutter.aspx
 
  • See bottom pictures for different varieties of foam cutter made as per users requirements.
 








  









  •  Hot wire cut parts.
   

  •  Wings made by hot wire cutting.


  •  Fuselage cutting


  •  Wing making with hot wire cutter from HD foam block.









  
  • Other tools used for cutting and shaping.







  •  Sanding blocks used for shaping.
  • Examples of HD foam shaping and cutting.
  • Bending a sheet.
  •  Wing construction with HD foam sheets.


  •  Cutting made for folding a sheet.


  •  Folded foam sheet wing.
  •  Wing made out of HD foam sheets.
Coming up Next : F-15 Eagle
- Twin Pusher Prop/ EDF Jet
- Full HD Foam Construction.