Shinobi | A Home-Grown Moulded Fuselage
Part IV: We Begin the Mould Making Process with the Nose Cone
Readers who have not already done so may want to read the previous parts of this series before continuing with the article below. — JH
Now we attend to making the parting frame/plug seam. Here’s a short checklist to make sure everything is at the ready:
- The parting frame is waxed with mould release? Check!
- The plug is set at the correct height in the parting frame? Check!
- The mould fence is ready — waxed and beaded with modelling clay? Check!
Cutting the Modelling Clay Flush with the Parting Board
Cutting the modelling clay along the seam line to make the transition between the parting board and the plug can be easy if done right, and a nightmare if done wrongly. First, we need a cutting tool — and this has to be something thin and flat so that it can slide along the parting board surface and follow the plug profile without cutting it while slicing the modelling clay bead flush with the surface. There are many tools that can be used, but I have found that either a modified box cutter blade or a thin spatula works best.
Advice: Just to reiterate for emphasis: use a thin cutting tool — this is really important.
The objective is to cut the modelling clay neatly but without doing any damage in the process. The tool will have to be carefully rounded at the tip because the last thing we want to do is cut or score the plug surface. Finally — before attempting the cut, make sure your room is not too cold. The cooler modelling clay gets the harder it becomes, and while we don’t want it too soft, we also don’t want it to be hard either.
Let’s Cut the Seam
Place the cutter flat on the parting frame surface and push it gently into the modelling clay bead. With gentle pressure, you will feel the cutter touch the surface of the plug, but don’t push it — feel it. Now slowly draw the cutter along the seam, all around the entire profile. Once the tool has made its way around the plug, carefully remove the cut top of the bead to expose the newly cut and hopefully flush modelling clay surface, which — if you have done a good job will be nice and flat and flush with the parting board.
The seam between the plug, parting board and the finished mould surfaces is extremely important and spending a little time here will prevent you from having regrets later. If the cut is not exactly as you would like it, then don’t settle for a less than perfect joint — put in some more clay, check the alignment and repeat the process until it’s really nice and flush.
Now it’s time to place the mould fence, so just as we did with the fuselage plug, we’ll put a bead of clay around the cutout, and then press it lightly onto the plug surface until it seats on the parting frame board. A couple of pieces of double-sided tape under the block supports will anchor it in place for now. After that, use the cutting tool to cut the modelling clay bead parallel with the mould fence surface and the plug, in the same way as you did with the horizontal seam.
Now for the good bit: making the first part of the female mould set. Before proceeding, a few more checks:
- Are the parting board and plug surfaces all clean and pre-waxed with mould release? Check!
- Are the seams all nice and sharp? Check!
A Parting Shot: The Case For PVA
At this point, before you get your mixing pots out, 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. PVA, as it comes from the manufacturer is always rather thick but is easily diluted with water. It can be brushed on with a fine brush or better sprayed on with a normal paint spray gun or even a cheap plastic spray bottle set to a fine mist. My advice here is to seriously think about this option as it provides a virtual guarantee that the mould will release. But, if you do opt for this solution, please wait for the PVA to dry thoroughly before going to the sticky bits. If you are in a hurry, a waft with SWMBO’s hairdryer accelerates things.
Gelcoating the Plug and Parting Frame
There are choices here. If you will only want to make one, or at best a few parts with your mould set then there is no real point in using expensive tooling resins for the gelcoat. Yes, they are tougher and harder but they are also a bit costly, and depending on the size of your mould set could work out to be quite an investment. For small batch moulds, I use a mixture of epoxy, fumed silica (Cabosil) and often, though not always a paint colour dye. The reasons for using the paint colour dye are several:
A Coloured Gelcoat:
- will easily show up air trapped in the laminations during the layup
- will show up paint unevenness in use — unless the paint colour is the same as the mould
- will highlight the mould finish — a big asset during polishing
A Coloured Mould Surface:
- will easily show up any dirt or wax buildup
- will help with the final buffing of the mould release wax prior to the layup by showing up unbuffed wax
Applying the Gelcoat Mix
For a nose cone of this size, not much gelcoat is required — as I remember it was 60g. To do it the ‘homemade’ way, thoroughly mix up the epoxy and hardener then add two tablespoons of fumed silica (Cabosil) to give a nice double cream consistency. After mixing I usually leave it alone for five to ten minutes in order to allow the silica to develop a fully thixotropic property and also to allow some of the bubbles from the mixing to disperse. A rough recipe is about one tablespoon of Cabosil to 30g of mixed resin.
With the mixture mixed to a nice smoothness, use a small soft brush to paint it carefully into the seams first, then follow that by brushing the gelcoat over the surfaces of the plug, the mould fence, and the parting frame. Try to get the surface as smooth and as even as you can, and especially avoid ‘ruts’ on the surface. With a little care, it’s possible to get a smooth layer of about a millimetre or two (1/16~1/8”) in thickness all over the surfaces. If you have the right amount of Cabosil in the mix, the gelcoat will not run too much on the curved surfaces.
Advice: For each part of the mould, plan to get all of the glass laminating work done on the same day as the gelcoat is applied.
E-Glass or S-Glass: What’s the Difference?
E-Glass has the ‘E’ prefix because of its use in electrical applications such as printed circuit boards. It’s normally a lot cheaper than S-Glass and is alkali-free. Historically E-Glass was the first glass type used for continuous filament formation. It’s not as strong in tension as S-Glass but is normally much cheaper.
S-Glass has the prefix ‘S’ because of its higher dynamic strength than its ‘E’ counterpart. S-Glass is used when higher tensile strength (modulus) is required. For reference, S-Glass is approximately 30% stronger and 10% stiffer than E-Glass.
Advice: Always check when you buy fabric that what you are buying is actually S-Glass and not its lower tensile strength counterpart E-Glass.
Advice: S-Glass is superior in strength to E-Glass; however, we are making a mould here which will end up as a pretty thick and therefore strong laminate. You might want to save some money and use E-Glass, but remember that you may need more epoxy and that can be more expensive than the fabric!
Advice: If you buy from less-reputable companies sight unseen you may be duped, so make sure you are actually getting what you pay for. Sometimes E-Glass can be identified by thin black lines running through the weave.
The Epoxy/Fibre Matrix
Simply explained, what we are trying to achieve here is a composite consisting of glass or other fibres that are held rigid in an epoxy matrix and when bonded together will form a tough strong structure with both components contributing equally to the whole. But problems can arise if we are not careful and don’t do our planning well.
Two Kinds of Bonding: Chemical and Mechanical
The chemical bond is far stronger and more reliable than the mechanical type as the wet epoxy layers bonded actually unite on a molecular level to become an integrated mono-structure. But this can only happen when the first layer of epoxy is still ‘green’ and so is chemically filial to the new application. ‘Green’ is the term applied to the epoxy that has not yet hardened. If the first layer of epoxy has been allowed to harden for too long then it will have passed the stage where a chemical bond was possible.
As a simple test, in a good green state, the gelcoat should be easily marked by a fingernail but not sticky.
The mechanical bond relies on the adhesion of the wet, second layer of epoxy to a cured, or partially cured first layer, and ideally needs a roughened surface to work well. Hardened epoxy is not an ideal condition for actually bonding or glueing multi-layer composites.
That’s it for this session. Next time: glassing the first mould half — otherwise known as the The Big Stickup.
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. 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.
