How to make shaped carbon fiber rodsŽilvinas Lapačinskas
In our previous publications about shaped carbon fiber rod (CF rod) technology we have talked about advantages of it in indoor RC model building and step by step Rc model building by using it. In this article we will provide step by step tutorial how to make these shaped carbon fiber rods.
Some History behind
First of all we should say many thanks for our friends that inspired us in development of technology presented below:
Aleksey Lantsov – is the one who inspired us in making indoor Rc plane models from carbon fiber rods frame covered with thin plastic foil Mylar. He did a great job by developing technology and principles of building this type of models, and persuaded near all the F3P world to switch to carbon from foam.
Kimmo Kaukoranta – is the one who has showed advantages of shaped carbon rods in F3P Rc model building and we consider him as a Pioneer in this field.
Technology that You probably already know
General principle used by Kimmo is described in this post. As you can notice, shaped carbon rod is quite easy to produce by using carbon fiber filament, epoxy resin and silicone tube of desired diameter. After carbon filament is soaked with resin and pulled inside silicone tube it is fixed inside some kind of stencil or mold in order to keep desired shape until resin cures.
So why technology presented below is different and is it worth to read further?
In fact silicone tube technology has few essential downsides:
Relatively low fiber density in final product. Because carbon fiber filament soaked with resin must be pulled into tube, the more filament density the more force it is necessary to apply during pulling process. The more length it is necessary to pull, the less fiber density is possible to pull in at all. On the same higher carbon fiber density is essential for strength of final product especially if we talk about small diameters as 0.5 mm.
Very hard, if possible, to produce small diameters: 0.5 mm and smaller. The smaller diameter, the more difficult is to make joins between carbon filament and pulling string considering great force that is necessary to apply during pulling process.
Limited length. Again it is restricted by pulling force that grows exponentially with every next centimeter of length.
Objectives for new technology
Obviously objectives for new technology is to overcome downsides of old one, so they are:
- To achieve carbon fiber density inside final product comparable with best quality standard CF rods produced using pultrusion technology.
- To achieve virtually unlimited length of shaped rod. We practically make them up to 1 meter, but technology definitely allows to go much further in length.
Before diving into details it is necessary to say that from conceptual point of view shaped carbon rods with high mechanical properties is possible to produce only in a way when shape is given before resin cures. All efforts to give shape after resin cures through softening of resin with heat leads to essential loss of strength in bended parts. It is so because carbon fibers should slide freely against each other during bending process and this is possible only if resin is still not cured.
Next important thing is that carbon fibers should be well compressed together during curing process excluding any possibility of air bubbles or excess of resin between them.
So basically process is quite simple:
- Soak Carbon Fiber filament with resin
- Compress fibers
- Give shape and fix it until resin is cured. Fibers must stay compressed until resin cures.
Probably main question here is: How to compress fibers properly?
Among all possibilities, we have chosen heat shrink tube (one that is used for electric wire insulation) for compression of fibers.
Advantages here is that it is quite easy to pull required length and quantity of filament into it before it’s been shrunk, compression force is good enough to squeeze excess of resin and air bubbles out of material and as a result rods come with nice and smooth surface and round cross section.
Disadvantages comes from necessity to apply heat for compression of tube. So materials should be carefully selected and process should be designed in order to minimize heat impact to resin. In fact the essence of this technology is how to minimize heat impact to resin.
List of materials
Carbon fiber filament. Use highest quality Carbon Fiber filament you can get. Filaments usually are marked according consisting number of fibers: 1K, 3K, 6K, 12K … Use appropriate type of filament for chosen diameter. In case Your final product exceeds described nominal diameter it means that excess of resin is left inside material.
|Nominal diameter of CF rod mm||Quantity of carbon fibers|
Epoxy Resin. Because heat is applied during compression process and heat shortens pot life of resin significantly it is essential to select resin that has long enough pot life and tolerates heating without losing mechanical properties. Also resin should be low viscosity with very good laminating properties. We use resin L287 which has near 3.5 hours of room temperature pot life. Processing temperature is described as recommended up to 50 C on the other hand in Data sheet of this resin it is said that:
“Different temperatures during processing are not known to have significant impact on the mechanical properties of cured product, however a temperature increase of 10 C will halve a pot life”
So it is essential to have process when heat is applied to shrink tube at very short period while wall of shrink tube still acts as heat insulator.
Heat shrink tube. Because it is necessary to minimize heat amount and heat application time for shrinkage of tube it is essential to choose low temperature shrink tube with maximal shrink ratio you can get. Inner surface of heat shrink tube should be adhesive free. We recommend to use transparent tube in order to have ability to see material during shrinkage process especially for beginning. Selected tube should be able to shrink to little bit smaller diameter than required nominal diameter of finished rod.
Other materials that may be required:
- Kevlar thread – use as a tension string
- CA glue – use to connect filament with tension string
- Paper towel – use for removal of excess resin
- Cup and stick for epoxy mixing
Stencil. Stencil is some peace of board with groove where shrink tube with filament and resin inside should be placed after tube is compressed. We use Medium Density Board MDF for it. Groove on surface of the board should be equal in width with the diameter of compressed tube and little bit lower in depth than it. Rectangular profile of groove is good enough. In order to achieve maximal accuracy we suggest to use CNC routing miller for milling of groove on a board.
Assembling frame. Assembling frame is where filament is prepared and compressed for molding. It is simple board with the length just over double length of rod with hooks on both sides.
Heat gun. The most precise tool of all set. It is essential to apply heat in precise amount. Excess of heat leads to resin thickening before molding and thus preventing fibers from sliding against each other. Shortage of heat leads to poor and not even compression along rod. The best solution is to get heat gun with possibility to adjust temperature. Definitely You will need to do this for each type of shrink tube.
Other tools that may be required:
- Protective mask and gloves
- Sharp knife
- Peace of steel wire
1. Filament, string and tube assembly
2. Application of resin and compression. 3. Tube removal