Motorsport Composite Fundamentals: Compression Moulding & Forged Carbon
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Compression Moulding & Forged Carbon
04.12
| 00:00 | The construction techniques we've discussed so far use some form of fabric reinforcement laid up in an open mold and often vacuum bagged. |
| 00:08 | Compression molding techniques differ in that they make use of a closed mold and don't always use a fabric for the reinforcement. |
| 00:16 | In a standard arrangement, a closed mold match tooling will have two sides. |
| 00:21 | One is the female side and the other is the male which is slightly smaller, so when brought together they form a A good example of a compression molded composite is commonly referred to as forged carbon. |
| 00:34 | This term is used to describe randomly orientated short strand carbon fiber in a resin matrix that has been compression molded. |
| 00:42 | This can be achieved using chopped carbon fiber tow in a resin that cures under ambient temperatures. |
| 00:48 | The surface of the cavity is first covered in resin to help prevent any voids on the surface of the part. |
| 00:54 | Then the resin and fiber are loaded into the cavity which is usually done with a similar method to lamination, where the reinforcement is placed in the mold and then wetted through with resin, as opposed to being pre-mixed. |
| 01:08 | After this the tooling is clamped together transferring this compression to the material in the cavity while it cures. |
| 01:15 | An important feature of the mold is the extension of a very small gap between the tools around the perimeter of the cavity. |
| 01:22 | This gap is near parallel to the direction of the movement of the tools as they're brought together, but it needs a slight angle for a normal two-piece mold to help with the separation of the mold after cure. |
| 01:35 | As the mold is clamped this acts like a piston compressing the material into the cavity of the mold. |
| 01:41 | The gap is too small for the reinforcement however the excess resin is forced out through the gap in the extension as a way of improving the reinforcement to resin ratio. |
| 01:51 | This clamping needs to take place gradually so the excess resin has time to flow out of the mold otherwise it can get caught in the mold and cause hydraulic locking meaning that the mold won't be able to fully close. |
| 02:03 | Forged carbon parts can also be made with chopped unidirectional prepreg and cured under compression and also elevated temperature which of course contains close to ideal resin for the weight of reinforcement anyway. |
| 02:17 | Following the curing process the tool can be opened and the part ejected from the mold and any finishing process is complete. |
| 02:24 | The key benefit of this compression molding process over the lamination techniques is primarily based on the geometries and forms of the parts we are able to make. |
| 02:33 | In other words we're able to create solid pieces that would usually be reserved for parts machined from metals or injection molded or 3d printed plastics. |
| 02:42 | This is also advantageous in that we're able to accurately control the wall thickness of the parts and have a high quality mold surface on all sides of the part. |
| 02:52 | Open molds even when combined with vacuum bagging techniques will still have a rougher finish on the back side of the part that isn't in contact with the mold. |
| 03:00 | Due to the random orientation of the fiber reinforcement control over the mechanical properties isn't optimized in any particular direction. |
| 03:08 | However, we still end up with reasonable strength and stiffness that's relatively uniform in its distribution which we refer to as isotropic. |
| 03:16 | With all this focus on forged carbon using chopped tow, it would be easy to assume that this process can't be done with other forms of reinforcement but it absolutely can. |
| 03:26 | Although not nearly as common, continuous tow and fabrics can be used in compression molds to achieve specific mechanical properties and aesthetics. |
| 03:35 | In summary, compression molding involves loading resin and reinforcement into a closed mold which is then clamped, transferring pressure to the material in the cavity while it cures. |
| 03:46 | The excess resin is squeezed out from the mold to achieve a good fiber to resin ratio. |
| 03:52 | Forged carbon parts are common application of this technique using chopped tow to produce an alternative aesthetic with good isotropic properties. |
| 04:00 | The key advantage of this process is the solid geometries it allows for, giving the ability to construct high quality surfaces on all sides of the parts. |
