November 18, 2018

“New” Vacuum Unit and Mould Making

By Matthew Payor

So we’re hitting build season soon.  Coming off the back of constructing the IBIS airframe earlier this year, there were a bunch of aspects to our composites manufacturing that we wanted to revise and test.  Furthermore, the UNSW Redback and Sunswift car teams moved to a much nicer workshop, leaving behind their composites layup room that we previously relied on for laying on our moulds.  So Daniel Mann, Adam Temesvary, and I over the past month or so have built a portable vacuum unit for performing layups at our second home, the Michael Crouch Innovation Centre, and with this have performed some small scale tests, looking to improve our mould design and manufacture, train some more members in the process, and have some fun along the way.

Building the Vacuum Unit

The vacuum unit was essentially cobbled together by Dan and I from parts ever-generously donated by the dumpster elves.  The pump unit traveled to UNSW from a bin at CSIRO Lindfield, the chamber was thrown out from the UNSW Built Environment faculty.  The fittings, valves, and bolts were thrown out from the old UNSW car team workshop.  The acrylic panel was formerly an interior window from the Design Futures Lab.  The electrics hail from a random assortment of old disassembled machines.  We mixed all these ingredients together with some Chemical X at a 1:20 ratio, left them in an autoclave overnight, and then chipped off the scale the next morning to reveal our beautiful “new” portable vacuum unit.

Making the Control Panel

Yes, it is overdone and could’ve been much simpler.  Dan and I argued for over an hour about whether to have a fuse or breaker so we put in both.  But there’s something terribly satisfying about flicking one of those old, really strongly detented switches.  So we made it so you have to switch three of them to start the pump.  And then crank down some big ball valves.  More is better, right?

 

Doing some layups

From the last build, we mainly wanted to address  the amount of handwork involved in finishing our moulds.  The problem being that when sanding a 1.2m^2 mould, then recoating it, then sanding it again up to 4 or 5 times, the probability of making a mistake is quite high.  Mistakes come in the form of the edge of some sandpaper nicking the foam in a complex corner, residue picked up onto the sandpaper leaving scratch marks, or taking too much material off a feature.  The idea we came up with was to replace handwork with machine work.  With offcuts of carbon fibre and foam about 300mm^2, I put together some test moulds – an art palette I drew up, and some bicycle seats found on Grabcad.  Many thanks to internet heroes Max Morozov and Eric van Helmond for providing such models on a shared public platform.

Adam and I tested leaving a freshly machined mould in place in the mill, sealing it, and then re-finishing it the next day.  We compared the results from using PVA vs resin for sealing, and also tried thinning the PVA with water and alcohol at various ratios.  In the end we came to the conclusion that a proper resin coat was required to get the kindof finish quality we wanted – PVA sealing still left some small voids that would have to be filled by applying a thicker coat of mould release wax later on.  However the resin, in setting, heats up and soaks a fair amount into the foam we use, and results in a slight shrinkage of radial dimensions due to the heat effecting the foam.  We found that using thinned PVA first (using about 2:1 with water, alcohol proved worse) would harden up the surface of the foam to recieve the resin better.  In finishing the moulds, we tried finishing with -0.2mm radial and axial stock prior to resin coating, so that when we went to finish on the resin coat we wouldn’t machine most of it away, however it proved difficult to coax the dimensions out, probably a waste of time for the kindof work we do.  We’ll be buying some gel coat in ensuing weeks to retry this.  Another change we made was to use a spray gun to apply the polyvinyl alcohol to the moulds.  This allowed us to build up lots of thin coats to create a much stronger and more even coating than when we used to hand-apply it.

Dumb Stuff

So something I personally wanted to try was laying up with plant matter, just for the sake of interest.  I think palm leaf bowls are cool, and moulds that size were about the right size for our offcuts of foam.  Unfortunately we do not have food safe resin, so I thought the next best thing to a bowl would be a paint palette.  It took 3 attempts to figure out a good prep procedure for leaves.  You need to remove the volatiles and water from the plant matter before it makes friends with the resin, without taking it too far and turning them into dust.  The process I landed on was boiling leaves in soapy, salty water, and then repeatedly in plain water until the tea produced was weak and the leaves had thinned and lost some colour; then drying them between towels.  I used some maple leaves donated by a tree next to my apartment, the tea along the way was quite drinkable.

The technique when laying also needed to be different to when laying fibre weaves.  I made a lot of mistakes and Adam essentially guided us to success here.  Whilst the leaves are slightly permeable, the resin is much less able to move around under the vacuum pressure than with carbon fibre and fibreglass.  The successful product has some bubbles and a thick layer of resin on the mould side because we applied too much resin to the base of the mould, since normally that resin will be pulled up and through fibre weaves by the vacuum.

 

Mould Design

There’s a bunch of annoyances that I’ve come across during layups and finishing that we can retroactively work to mitigate through better mould design.  For example, its good to design a mould that has plenty of clearance from the geometry and a pooling area around the geometry to prevent resin from overflowing around the edges of the mould.  Sometimes these considerations for containing resin make the release more difficult.  If you inset geometry vertically to put a border around it, that vertical section then adds issues to the ease of release.  It’s a balancing act.  What is also useful is to work a lip into the first finishing of moulds prior to resin coating to keep that resin from spilling into the mill.  Its also very important to round the corners of the mould.  Fillets are a good thing to but square edges are a major strain on the vacuum bag. Making the mould thinner in the Z means the bag doesn’t have to work as hard, but the foam is more bendy.  Another balancing act.

Training

In the end this was a great opportunity to use some offcuts and leftovers to test and develop new processes, to dust off rusty hands and train up some new people.  The training lay had a few mistakes which impacted the results – a good learning experience for us all.  Due to a hurried sticking down of tape and cutting of the vacuum bag,  we didn’t give enough slack in the vac bag in one axis.  This lead to some voids and creases in the lay.  The small area in the negative seat mould had the polyvinyl torn off during handling prior to laying, and this left a noticable spot with matte surface finish.  Good fun in the end, a few days work resulting in some cute proof of concepts.  We walk out of this with some fancy kit, better informed mould design, and a far easier mould preparation process.