Week 17 - Composites (Wildcard)

Week 17 - Composites (Wildcard)

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Individual assignment

design and fabricate a 3D mold (~ft2), and produce a fiber composite part in it, with resin infusion and compaction.

Assignment summary

what I achieved/learned this week:

As part of our teaching product design, we have an ongoing theme where we apply various fabrication techniques to make a little duck. This is the original duck that is covered as part of the Rhinoceros 3D modelling tutorial, it all flowed from there. We now have a lovely arrangement of ducks sat on our office shelf. There are cast rubber ducks, sectioned MDF ducks, cast transparent resin ducks, milled ureal foam ducks, cast chocolate ducks, and now vacuum moulded carbon fibre ducks.

We wanted to setup a small, clean composites lab to make small parts using pre-preg rolls for student projects in mechanical engineering and product design. Here is a reallly good guide to creating composite parts, and here's the setup:

Initially we wanted to make some flat samples (plaques), with varying number of plies (A+B= 9 plies: 200x100x2mm, C=18 plies: 300x200x4mm). These could then be used as part of our materials labs to measure the mechanical properties of some carbon fibre samples and compare their behaviour to other materials that we test in the lab (mostly metals and plastics). The first step is to put down six coats of release agent onto the aluminium plate.

Then cut the plies and lay them down on top of each other in the arrangement that's needed. For larger numbers of plies, place a perforated layer on top so they can be 'debulked' before final curing. Place the gum sealant tape around the perimeter to bind and seal the top impermeable vacuum bag layer.

A breathable membrane is used to help apply a nice even pressure onto the surface of the top ply.

Once the top layer is adhered with a good seal, cut out a small (10mm diameter) hole to attach the vacuum connector. Attach the vacuum pump and carry out a consolidation process to check the seal and the surface consistency across the vacuum bag.

Place the vacuum bag in the oven and using the PID controller programme the oven according to the desired curing cycle.

We then went through the same process but for the duck. The difference was that we used an aluminium mould that we had Adam in the machine shop mill out for us on his 5-axis CNC machine - thanks Adam! We also made another one out of wax to try to cure it using the low-temperature curing cycle. The carbon sheets were laid up inside the tool around a rubber bladder (we used a rubber glove!). The glove was slightly inflated to give some initial volume to lay up the carbon around. The entire tool was then placed inside the vacuum bag. The thinking was that the rubber bladder would then be sucked up against the internal wall of the duck, pressing the carbon fibre outwards against the internal surfaces of the mould tool, taking up the desired shape.

Of course the duck got stuck! and was clearly not coming out. We then put it back in the oven at a lower temperature (5 degrees below the specified glass transition temperature for the curing cycle we used). Then the duck came out easily and we could clean the mould easier too with the gel-like resin.

We experimented with both the recommended curing cycle, and a lower temperature curing cycle. This was the 9-hour recommended cycle.

and this was the longer, 14-hour low-temperature curing cycle, which lowered the glass transition temperature of the resin in the final cured part.

we also experimented with not using a cycle at all, by simply putting the duck straight in and ramping the temperature up to 80 degrees. This really isn't recommended, the resin oozes out of the carbon fibre sheet and you're left with a part with almost no resin left in it - it's dry and you can feel the fibres, although it did hold its shape - mostly. Here's a section view of the uncured part from the 80 degree straight in test.

Some good results me thinks, and now some fun can be had. Here's the final result.

here is the original Rhinoceros duck.3dm file to download

here is the Rhinoceros file for the mould tool to download

Other composite projects I now want to tuck into include a new wardrobe hanger, a new bicycle stem, and a nice label/panel for my final project.

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