A finished Shinobi nose cone fresh out of the completed nose cone mould.

Shinobi | A Home-Grown Moulded Fuselage

Part VI: Here’s Where We Become Pin/Ball Wizards

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 Bit of Joining Information

Before we get into alignment I will say one thing about joining the laid-up parts in your new moulds because it has a direct effect on the end results: I do it wet with the layups still soft, or at least green and preferably immediately after finishing glassing the two halves. I have never tried to do this operation dry, on cured mouldings — mainly as I know a chemically receptive join will give a far superior result. A wet join has the added benefit of an overlap where the layers of glass form a reinforced ridge along the seam. Wet joins are therefore mechanically and chemically far stronger than dry joined parts. Also, there can be difficulties and possible damage to the mould that comes from trying to trim ragged, hardened glass edges back to the seams by cutting or grinding. Some people do this regularly and swear by it, but I don’t.

Notes for Techies Only

Here, I’ll explain just a little more about the physics of epoxy binding for those who are curious. If not — skip this part!

The ability for epoxy resins to chemically (ionically) link diminishes as the epoxy cures and then the joint becomes a secondary ¹ or mechanical bond. Whereas, epoxy applied to or over partially cured epoxy will chemically link with it and is a primary bond.²

¹Secondary bonding relies on the mechanical linking of the epoxy/glass to a different material or to a cured epoxy surface.

Here is a simple adhesion diagram in this case joining epoxy to concrete but in fact it does not matter too much what the substrates are the effects are the same. (image: Jovan Tatar et al)

²Primary bonds involve sharing or donating electrons between atoms to form a more stable electron configuration. Primary bonding occurs when electrons are lost or gained so that the outer shell is filled.

Here is a diagram of the crosslinking that occurs when epoxy substrates are chemically joined for a primary bond. (image: Jinjun Zhang et al)

Meanwhile, Back at The Ranch

Having separated the first half of the nose cone from the parting board, we now need to decide how to make sure that the two halves of the finished mould will actually locate in exactly the same place each time so that we get those lovely wispy thin seams.

There are two main choices: pins and balls. Each has its merits, plus there is also the case for a ‘hybrid’ pins/ball alignment method which should work well although I have never tried it.

A Shinobi nose cone fresh out of the mould — note the crisp seams.

The Case for Ball Alignment

I use ball bearings, 12 to 15mm in diameter embedded and epoxied into the mould lands for alignment. The spherical bearings will provide alignment that, if not as crisp as Pins, is nevertheless pretty darn good. But in fact, the reason I use this is that it literally gives a little ‘wiggle room’. Sometimes in order to get the glass overlap to align nicely with its mating area on the other mould half, it’s nice to be able to skew the moulds a little bit sideways as they are being joined. With smaller moulds or odd shapes, this can be a distinct advantage.

The Case for Pin Alignment

Vertically orientated 6mm to 12mm pins arranged around the outside lands of the mould are a good way to align the mould halves. After a layup, with the wet glass as yet not bedded down, the two halves of the mould can be loosely positioned and the pins can then be pushed through the pre-drilled holes to accurately locate the mould halves. After inspection to make sure none of the glass is trapped in the seam, the joint can then be bedded down with a roller or similar, then the mould halves clamped for curing.

Advice: Hardness: Make sure that the pins you will use are hard — at least steel or stainless steel and preferably hardened carbon steel. The reason for this is that if by some chance a pin becomes stuck in the mould, it can be gripped tightly to twist it out.

Advice: Pin length: make sure that the pins are long enough to go completely through both sides of the mould and then some so that they can be gripped to remove them if needed.

The Case for Hybrid: Balls + Pins

I have never tried a combination of pins and balls but on thinking about it, this could be a very good way to go. This new method would comprise of aligning the mould halves using the balls to allow a bit of skew, then when all is aligned and the seams bedded down push the Pins into place. I have never tried this approach but I think I will on my next mould as it would likely provide the best of all worlds.

Here the alignment balls have been inserted into the holes and the surrounding gaps are filled with epoxy.

Selecting Ball Alignment for Shinobi

I like to space my alignment aids (pins or balls) at a distance that will allow them to do their jobs well but not be so numerous as to be a chore to clean at the end of a moulding job — but even for a small part, I would still use at least four. 8” (200mm) to 9” (230mm) spacing is about right and try to make the distances as even as you can — but remember they don’t have to be exactly spaced the same.

First, we need to mark the positions of the balls or pins on the mould lands, and then drill the appropriately sized holes. Here we need to make sure that the pins positions will allow the pins to go clear through the mould, and not interfere with any cross bracing. In the case of pins, it’s a good idea to use a drill press to bore the holes if humanly possible so as to get a ninety-degree vertical alignment. Also, it’s best to trial drill a couple of holes first in a spare piece of timber to make sure that the pins are not too loose. If necessary buy a new drill bit but if the hole sizes do turn out to be a bit too large, and the Pins are loose, then don’t despair- we have a fix to cure that problem later.

For ball alignment, the holes are purposely drilled a little larger than the ball bearing diameter and are only drilled to just over the ball centre depth, but it’s a good idea to test drill a couple of holes in a piece of scrap wood first to get the depth correct. I simply use a piece of tape wrapped around the drill bit to set the depth by hand, but a drill press with a depth setting feature is always preferable. Thus:

I simply use a piece of tape wrapped around the drill bit to set

Shallow holes are drilled into the mould surface to allow the balls to settle just below the centre line. Then epoxy is applied to secure them.

Setting the Locators into the Mould Half

For the ball method, the next step is to set the balls into the mould half. First, check that the holes are drilled to the correct depth by dropping them one by one into the holes. Check each one individually. They should all sit with the centers at a depth of just over 1/16” to 3/32” (1mm~2mm) below the lands and no more. If they are a little shallow then tickle them a bit deeper. If too deep then insert small scraps of wood to raise the ball up a little. That done, and when you are satisfied with the ball height mix a small amount of epoxy and using a thin piece of wood (I use chopsticks — a valuable tool in any workshop) drop some epoxy into the hole — not too much as you will almost certainly have to add more later — and then drop in the ball.

Repeat this process with all of the balls, adding a little more epoxy to any that do not have a nice meniscus, and wiping the epoxy away from any that spill over. When the epoxy has begun to harden then leave the assembly for at least 12 hours to allow the epoxy to cure.

Advice: Check the epoxy levels around the balls: leave the balls for 10 or 20 minutes and then check again — you’ll probably have to add a little more epoxy here and there as the wood reinforcement absorbs some of the resin.

Advice: Waxing: for the pins method, first wax all the pins with mould release wax following the instructions in wax drying times and process — do this a few times. If the pins are a good fit in the land holes, then push them all down into the mould half with half of the pins protruding.

Advice: Loose pins: if the pins are a little loose then wrap a little thin tape around them where they go into the bottom of the first mould half to secure them in the holes. If this fix has to be used, try to get the pins as vertical as possible.

Advice: Balls: if you intend to use clamping bolts through the mould then mark the positions of the pins or balls on the outside of the mould reinforcements so that you have a reference when both halves of the mould are completed. It’s not good to drill the bolt holes into the balls. In the case of pins, the positions can easily be seen.

The first trial finished Shiniobi nose cone. All is looking good.

Making the Second Nose Cone Mould Half

Preparation By now we have done quite a lot of work so the first mould half has had quite a lot of handling. So unless you did it in an operating theatre and used surgical gloves throughout, then there may be microscopic traces of dirt, oil or grease. It’s a good idea to clean the mould lands and the exposed part of the plug thoroughly with alcohol. After that, apply release wax according to the instructions — several times. I like to use at least four applications and preferably more — always waiting between coats to allow the wax to harden in the air.

Wow…the nose cone mould is complete now and separated from the plug.

PVA Release Agent Here, I’ll reiterate what I have mentioned before: you might like to think about an insurance policy in the form of a layer of PVA release agent. A light layer of PVA will almost certainly guarantee that the plug will release nicely from the mould when that exciting time comes.

Advice: Loose pins: If your pins are loose, then it’s best to take them out and wax them separately before putting them back again, secured with tape prior to applying the second mould half gelcoat layer.

OK, let’s clear the decks first. Ready to roll? Then mix and apply the gelcoat epoxy to the waxed mould/plug surface and as usual, leave it severely alone until its begun to set. The waiting time can be used to prepare the light and heavier glass strips to be used for the glassing over the gelcoat. Just to remind you, in a good green state, the gelcoat should be easily marked by a fingernail but not sticky. After that, it’s on with the glassing and then the wood reinforcements just as we have done before.

Here the outside ragged glass edges have been trimmed and rounded and the clamping crews inserted.

For the next part, since I have no reason to reiterate what I have already written, you can use the same methods we have used on the first half to make the other side of the nose cone mould.

In the next part of this series, we separate the nose cone mould halves, start to clean and polish them and then begin preparation for making the first fuselage half shell.

Here the moulds have just been separated from the plug. Note the PVA release agent residue around the edges.

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

Resources

• Durability Evaluation of Florida’s FRP Composite Reinforcement for Concrete Structures by Jovan Tatar, Mark Lisek, Natassia Brenkus and H. R. Hamilton.
• An optimized cross-linked network model to simulate the linear elastic material response of a smart polymer by Jinjun Zhang, Bonsung Koo, Nithya Subramanian and Yingtao Liu

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.