Composites join the jet set!

Cutting composite material is a perfect application for abrasive waterjets. A precision waterjet with a taper compensating head will machine 6mm thick carbon fibre as fast as 4.5m/minute at 4,100bar with taper of less than 25µm per side. And 20mm thick G10 can be machined as fast a 1m/minute at 4,100bar with the same precision. While speed and accuracy are two big reasons that manufacturers machine composites with abrasive waterjets, easy set-up and minimal tooling are also key factors. Since composite parts often consist of small runs, minimal set-up time is the key to profitability. One example is Kevlar fibre used for vehicle armour. Each piece has a unique geometry, resulting in dozens of cuts unique to each vehicle. The ability to quickly set-up and cut armour panels means faster turnaround time. In addition, the ability to nest geometries within 1mm of each other minimises material waste and makes the process even more cost effective. Another consideration is that in some high precision applications, raw composite sheets can be relatively small, sometimes only 1m x 1m and only 6mm thick. In these applications the small waterjet platform along with a low horsepower pump will significantly reduce initial capital outlay. The need for speed Depending on the speed of the cut and the grit of the abrasive, composites machined with an abrasive waterjet can have a surface finish of 5µm, and may not require further processing. The abrasive does not impinge the material, and even consumer grade carbon fibre (which naturally has voids between the material layers) can be cut on an abrasive waterjet without being damaged. Machining Dibond for signage is not a high precision application. However, superior surface finish with zero surface frosting is critical for the finished product and it is the reason increasingly more signage companies turn to waterjet for this application. When machining composites, it’s important to support the material in a way that doesn’t allow the jet to deflect onto the bottom of the composite. It’s also important to choose the correct abrasive grit. For example, most composites will have a better surface finish if they are cut with 120grit abrasive. While cutting composites is pretty standard, piercing composites is a different story. In the past, piercing a wide range of composite materials from phenolics, carbon fibre and G10 to Kevlar and even Dibond would cause material delamination. Delamination is the result of the high velocity water and abrasive following the path of least resistance, and leaching into the epoxy that is binding layers of material. When piercing results in delamination, the geometry of the hole has to be large enough to remove the entire delaminated area, otherwise the part has to be scrapped. Even delamination of support material like paper phenolic, will compromise the structure. To avoid delamination, smaller holes are often drilled by hand. A piercing glance One option for automating composites piercing is to use a process that automatically reduces the water’s velocity during piercing, while still drawing a strong enough vacuum - known as the Venturi Effect - to pull abrasive into the jet stream. This option is not absolutely 100% effective, but with careful set-up the process is very effective. This process does require test cuts to optimise the water velocity for different materials and thickness. An option that ensures 100% delamination free piercing is to use a pneumatic drill mounted next to the nozzle. The waterjet operating software automatically moves the drill into position and a 3mm bit drills a start hole. The software then automatically positions the nozzle over the pre-drilled hole and starts cutting. While the drill option completely eliminates delamination, there are limits on the diameter of the hole that can be pierced and it will add to the overall cycle time of the part. Despite increased cycle times, the drill option is inexpensive and very easy to adapt to different materials and different material thicknesses. Abrasive waterjet offers an affordable process for machining all types of composite materials quickly and reliably. www.omax.com