The boom mould just after opening — note this time I DID use coloured gelcoat.

Shinobi | A Home-Grown Moulded Fuselage

Part VII: We Finish Both Sides of the Fuselage and Nosecone Moulds

Readers who have not already done so may want to read the previous parts of this series before continuing with the article below. — JH

First, a Few Notes on Finishing off the Moulds before Opening

It’s time to trim all those nasty needle-sharp glass edges off the new mould. There are several ways to do this, ranging from a simple hacksaw, through filing to using a hand angle grinder. Use the way that is best for the place and the equipment you have, but please do be very careful — those super sharp glass shards projecting from the edges of your mould can lacerate your skin in a heartbeat. Please do wear stout working gloves!

Finally! After all that hard and careful work, it’s time to see the fruits of our labours, as shown above.

Opening the Moulds

First, put the mould on its edge, seam up, and take a good look at the mating edges of the lands. If you look carefully you should be able to see a microscopic line where the two halves of the mould mate. Take time to try to find it if it’s not obvious at first. After locating the joint, take a box cutter knife and gently work the blade down into the line between the two halves of the mould. Don’t push too hard but try to wiggle it in a little way. If the mould does not separate immediately, have patience and try the same thing at several points around the circumference. By now — especially if you opted to use PVA release as an insurance — it’s likely that one half of the mould has separated and the plug is still embedded in the other half. Tug the plug out of the other half of the mould — it should come out quite easily — and then sit back to admire your work.

Mould Not Cooperating?

If the mould still refuses to separate, then a little more ‘persuasion’ might be in order. Using a mallet (not a hammer!) tap the tip of a screwdriver into the mould seam. A couple of light taps should be enough to part the mould. But if it’s still being difficult you can gently tap several screwdrivers (or thick bladed table knives — but I didn’t tell you that) into the seam to provide more pressure. If that does not do the trick then you have a stuck plug and only agricultural methods can be used to recover the mould, but it’s likely the plug will be damaged or destroyed in the process.

Time to make more progress on our fuselage moulding project. Assuming you have been diligently beavering away and now have the first sides of the nose cone finished, you should be seeing something like this:

Nosecone mould, untrimmed — note the needle-sharp glass fibres.

I use an electric hand grinder to remove my sharp mould edges, but I do it outside in the open air, well away from anybody and always wearing a good quality mask and Kevlar gloves. Saw, file, or grind the exposed glass edges back to the wood reinforcement and please do make sure there are absolutely no sharp edges.

Left: Fuselage mould parted and trimmed with the plug still in place. | Right: Here is the nose cone mould — trimmed and with screws/captive nuts for clamping added. This is a bit overkill and might not be the best method. Using simple G-clamps around the mould might be better.

Mould Clamping

The mould needs to be clamped shut for joining after laying up the part; and as usual there are several ways to do this. When I made the Shinobi mould set, I opted to use captive nuts and countersunk screws — why? Because I had a box of them left over from another project, and I also have an air screwdriver and no other reason! In fact, using G-clamps, C-clamps or other types of linear clamping systems are adequate and might be better than a bunch of fiddly screws. As I found to my dismay, sometimes epoxy gets on the screws and if you have forgotten to wax them, you can end up with a troublesome bind up.

Here is a part freshly pulled from the mould. Note the finish on the moulded part transferred from the mould.

Cleaning and Polishing

First, it’s always a good idea to wash the inside of the mould with hot soapy water — especially if you have used PVA release agent which is water soluble. After that, dry the mould and then you can begin polishing.

To polish or not to polish? If you have used a proprietary gelcoat then the mould surface will be hard and non-porous and so it won’t need, or may not even accept much polishing. But if you are like me and have made your own colloidal silica/epoxy gelcoat, then the mould would be better with a good polish for two reasons:

1. If we could look at the mould surface under an electron microscope — which incidentally I have done several times — the surface would appear to be quite uneven, maybe like a long series of peaky hills and valleys, but the problem is, those hills and valleys don’t hold wax that well. What we need is more like the surface of a ploughed field, where the furrows hold the wax and the peaks provide minimum surface area in contact with the moulded part.
2. Rubbing the inside of the mould exposes any slight defects that you may have missed when you made the plug, as the surfaces are now all negative and in relief, rather than positive.

The other extreme: Redshift F3f model nose cone moulds CNC machined from Alcoa cast bar aluminium. But still hand-polished.

Using a soft block to wrap the paper around — I use a common rubber pencil eraser as these can be bought in small convenient sizes — and soapy water — I use a squirt of washing up liquid — rub the inside surfaces with 400 or 600 grit wet-and-dry evenly to a nice matt finish. Follow that with finer and finer grit paper 400–600–800–1000 until a nice low polish is achieved. Use ‘tools’ — I mean borrowed household items — if needed to get into those harder to reach places. Follow that with a nice rub down and then a buff with car paint restoring cream as the medium — I use ‘T-cut’ to give a high polish. A rotary linen mop in a low speed electric drill works well for this, or just good, old fashioned elbow grease. It’s amazing what you can do if you stick at it.

Here is a commercially available flexible sanding block. (credit: Konrad Dudek)

Note that there is no real point in trying for a glass-like mirror finish as the mould will have to be re-polished from time to time with a lot of use, also the actual mould surface finish will not transfer exactly to the surface of the part — but it will be close.

Preparing the Moulds for Use

There are a number of mould sealants on the market and some people swear by them providing a really hard glossy surface, however they do tend to be expensive and of dubious use in home projects where just a few parts are likely to be moulded. I prefer rubbing the mould surface down with fine abrasive paper to a semi-polish and then a thorough polish and buffing to a good shine with cutting compounds. Incidentally metal polish often makes a really good cutting compound. Thorough cleaning with denatured alcohol and a few hours of drying will provide a microscopically scored surface that is highly receptive to mould release wax and will give the part to be made a really good finish. For the mould release wax, please do follow the manufacturer’s simple instructions for use, and also do give the new moulds a few coats.

Here is the entire mould set finished or almost finished with some of the plugs and a couple of the moulded parts.

Next Time

In the eighth and final part of this series, we make the inner nosecone plug from the original nosecone — how it’s done, along with a lot of useful tips and tricks.

Between now and then, if you have any questions please don’t hesitate to post them to the Responses section below and I will do my best to answer them. Putting them here also means others will benefit from both your question and my answer.

Thanks for reading and good luck with your project!

©2022 James Hammond

All images by the author unless otherwise noted. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.