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'HOW TO MAKE STUFF' by Jessie the Jeep


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Lost Plug - Lost plug mouldings are ideal for one off’s and perhaps odd shapes with undercuts, and are more usually know by the material they are made from, i.e. lost foam mould, lost wax mould etc. The lost plug method involves making a copy of the object to be moulded slightly under size from a material that can easily be carved or dissolved out once the moulding is complete. Wax or clay can be used as they are. Foam plugs must be coated in something to protect it from the resin.

 

Which ever material is used, once the final shape is complete, a ‘gel’ coat followed by fibre glass and resin is laid over the plug (‘Gel’ coats are usually a slightly thickened form of, often coloured resin, which will stay on the item without running or sagging due to gravity ). Once the resin is cured, the plug material is carved out leaving the hollow fibreglass copy which just needs the outside sanding and filling to a smooth finish. While the undercut in the example below would prevent a normal single piece mould, because the plug can be removed in pieces, it is not an issue.

moulding_lost_foam.jpg

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Simple Shapes - Simple shapes can often be formed in a similar way to the ‘Lost Plug’ method. Lynne’s lightweight glass fibre Nurses helmet was made in this way. Because it was a rounded tapering shape, the steel helmet could be waxed and glass laid up directly on top of it.

 

Once cured it could be popped off the helmet, usually by forcing strips of tough plastic down between the item being moulded and the glass ( like the type of plastic gallon bottles of oil come in ). The outside of the helmet was then rubbed down to remove high spots and low areas filled. The first picture shows the original steel helmet, next comes the moulding after release, with some carbon fibre 'tows' wrapped around the rim for strength. Last is the helmet sanded and filled, ready for primer.

moulding_helmet1.jpg

moulding_helmet2.jpg

moulding_helmet3.jpg

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A rim was added with filler pasted on against a strip of masking tape, and then rubbed down until flush with the surface of the tape. Once the tape is removed, a lip is left. After that it was painted, cork sprinkled into the wet paint and painted over again. Repro straps were ordered and wire bales made to fit them to. The pictures below show the filler rim being added, the rim complete and the helmet complete.

moulding_helmet4.jpg

moulding_helmet5.jpg

moulding_helmet6.jpg

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Female Mould ( Single Piece ) - Sometimes you may want to produce several copies of an item in fibreglass, in which case you need to make a female mould, from which, male copies can be formed. If you have an original item to copy, that can form the master pattern for the mould, otherwise you will have to make a copy to form the ‘plug’.

 

When making a ridged fibreglass mould, this plug must have a slightly tapered shape in order that the finished mould will release. This is polished with a release wax to prevent the resin from sticking to the plug, as both polyester or epoxy resin will stick like glue. The pictures below show a diagram of a good shape for moulding, tapered to allow easy withdrawal of the mould from the plug. The wooden pattern for my B-17 engine cowling has parallel sides, and while the front is rounded, it will be much harder to withdraw the mould from the plug.

moulding_mould.jpg

moulding_cowl1.jpg

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A thin ‘Gel’ coat of resin is painted over the waxed plug. Before the ‘gel’ coat has fully cured, at what is often called the ‘green’ stage, the laminating begins. This involves covering the gel coat with resin and fibreglass cloth or chopped mat. The resin is worked into the glass with a brush to ensure it is completely wetted out and there are no air bubbles between the gel coat and resin/glass layer.

 

Depending on the size of the mould, several layers of glass/resin may be needed, usually only two layers at a time until cured as too many layers at once can cause rapid heat build up as the resin cures and this can damage the plug. While the original cowl mould was made complete, an accident laying up a cowl, locked it into the mould.

 

As a result, the mould was cut in half and further cowlings made in halves and joined. The reason for the cowling gluing itself into the mould was that I had washed out the brush in thinners after its last use, and it wasn’t fully dry when I started gel coating the mould. The slight trace of thinners in the brush attacked the release wax causing the cowl to stick fast. Here’s the surviving half of the mould showing it to be about ¼ inch thick to give a ridged shape that won’t distort when moulding or trying to remove the moulded object.

moulding_cowl2.jpg

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Once the mould is completed, and released from the plug, it is waxed several coats, and the gel coat/glassing process is repeated inside the mould. The first picture shows the cowling laid up in the mould, and again after release. Once this is cured, it can be removed from the mould to leave you with a glass copy of the original object. The engine cowling on A-35B Vengeance in the third picture, is a glass moulding taken from a one piece mould. While the cowling is about 10 inches front to rear, and so quite a deep mould, due to its tapered and smoothly curved shape, it released easily from the mould.

moulding_cowl3.jpg

moulding_cowl4.jpg

glass_cowl.jpg

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For an object that has large flat areas, some reinforcing of the mould or final object may be needed. After two or three layers of glass, a rolled up newspaper or strip of wood can be taped to the surface, and another couple of glass added over the top. This ridge will stiffen up the mould a great deal.

 

The ½ scale B-17 nose art shown below is all fibreglass, but to keep it light, it is only two layers of glass thick. To keep it stiff, a balsa structure was added to the rear and it was glassed over. All the rivet and panel line detail were manufactured on the original plug so that they would be reproduced in the gel coat of the final item. Glazing was clear acrylic held in with small self tapping screws, while the gun barrel was a plastic tube drilled to resemble the cooling jacket.

moulding_noseart1.jpg

moulding_noseart2.jpg

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Female Mould ( Multi-Piece ) - Some more complex shapes need a two or multi-part mould in order that the pieces of the mould are free to release from the plug. I usually do this by making a plastercine or clay wall along a centreline on the object. For example, if I were moulding a ball, the wall would be created half way around the ball to make two equal halves of the mould which would release from each side of the ball.

 

I recently made some scale three blade propellers for my B-17. These used a two part mould for the blade and a separate silicone mould for the hub ( more of that shortly ). The master pattern for the blade was made from wood, then release waxed, and laid onto a bed of plasticine, so that a flat area extended all around the outline of the blade. The exposed front of the blade was then covered in gel coat and fibreglass as normal. Once cured, the half mould complete with blade was removed from the plastercine, and it was cleaned and waxed. The picture below shows it at this stage.

moulding_prop1.jpg

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The now waxed second half of the mould was now laid up with gel coat and glass as before until that had cured. The waxing would prevent the resin from bonding not only to the blade, but also the exposed glass lip around the blade ( the mould split line ). Before splitting, at several points around the lip, holes are drilled so that the two halves can be bolted together. The next picture shows the two parts of the mould separated and with the blade pattern removed.

moulding_prop2.jpg

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With the mould now in two pieces, each half is waxed. Then as before, gel coated and laid up with resin/glass, but with an excess slurry of resin and filler painted around the edge of the blade to join both parts. The two parts of the mould are then bolted together and left to cure.

 

Once cured, the mould is split and out comes the copy of the blade in glass, ready to be trimmed. This process was repeated eleven more times to make enough blades to make four propellers. I usually re-wax the mould before each re-use to ensure it will release correctly. The last thing you want is the plug or final item to be bonded into the mould.

 

This was just a two piece mould, but let’s say you had a cylindrical object with a handle on the side like a mug. A three piece mould could be made, using two pieces formed each side of the handle, split down the handle centre line, and extending one quarter of the way around the cylinder. The last piece would be around the remaining half cylinder. When cured, each of the three parts would pull away from the object without being trapped by its shape.

moulding_prop3.jpg

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Silicone Moulds – Silicone rubber is a good material to make moulds of smallish, highly detailed parts. As previously mentioned, the hub of the B-17 prop was made using silicone rubber. Some objects with slight undercuts can also be made using a single piece silicone rubber mould. This mould has the flexibility to stretch a little to allow the plug and final moulded object to be freed from the mould.

 

In addition, it can also be partly split with a sharp knife to aid release. On small items, this split can be held together by an elastic band around the mould during the casting process. The pattern for the prop hub was made from wood, and as can be seen, there is a small lip running around the piece, along the centre line, where each blade would fit. Because of its small size and undercut shape, a conventional glass mould was ruled out in favour of a rubber mould.

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There are a variety of proper moulding rubbers available to suit different applications, some as thin as water so that they will easily flow around the object being moulded and pick up every surface detail. It is also quite expensive. For small parts, I often use household sealing silicone, ‘though because it is air dry and not catalyst activated, I have to build it up in several thin layers, and the pattern must be well waxed to prevent it from sticking. ( Moulding rubber usually only sticks to itself, where as household sealing silicone will stick to just about anything ).

 

The two pictures below show the hub pattern and the silicone mould. Once the mould is cured and the pattern removed, some form of resin is poured into the mould, being careful not to trap air bubbles, and allowed to cure ( ideally a slow cure as heat caused from a fast cure can cause the moulded item to crack during the cure ). This method produces a solid ( heavy ) item.

moulding_prop4.jpg

moulding_prop5.jpg

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Another example of an object made from a silicone mould was this bronze tigers head. The original tiger was carved from clay, with all the surface hairs scratched into the clay prior to moulding. For this I used proper moulding silicone to capture the subtle surface detail. Once the silicone mould had cured, it was time to lay up the bronze tiger. The first gel goat of resin had a fine powder of bronze dust mixed into it to create a bronze effect. Equally, I could have used iron, marble, aluminium powders, or sand etc., to create a different final finish.

moulding_tiger.jpg

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Vacuum Forming - The last subject in moulding is vacuum forming. As with a simple ‘glass’ mould, the object being formed needs to be tapered. A strong plug is made of the object to be formed as with a ‘glass’ mould. This plug is made slightly taller than needed, as the plastic never pulls right down to the bed of the vacuum forming machine and always leaves a slight radius.

 

The plug is placed in a vacuum forming machine which is a sealed chamber, open only at the top where a sheet of thermo-plastic is clamped. This plastic is heated until soft, the air from the chamber is then withdrawn via a vacuum pump, and the outside air pressure forces the plastic down over the plug.

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Once cool, the plastic can be removed from the plug and retains its shape. The clear canopy below was made by vacuum forming. Firstly an orange plastic version was pulled over the plug. After it cooled, it was rubbed down and polished to a smooth finish. Then the clear version was pulled over the orange version supported by the wood version inside. This reduced preparation time on the plug having to fill and sand the wood grain.

 

Where the pine of the plug meets the MDF of the base, a small line was formed in the moulding which was the line to trim down to. Vac–forming machines can be home made using old electric fire heating elements and an old vacuum cleaner. A search of the net will probably reveal a ‘how to’ of making your own.

vacform.jpg

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Well, that's the lot! I hope I haven't put everyone to sleep, and perhaps you may have picked up some hints and tips that could be useful. If you do want to give anything a go, do a bit more searching and reading on the net before diving in. There's plenty of info out there. As I said at the start, there's no way I can pass on 30+ years of modelling experience, but if you are really interested in taking this further, there are even university courses available on modelmaking!

 

Joris, perhaps you can now compile it into a multi part article for Jack, as that's what he originally wanted.

 

Steve

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  • 6 years later...

Regarding your comments about scaling from a photo, I have had problems doing that with a photo enlarged by using a photocopier only to find that the x and y axis do not enlarge at the same rate.

 

Do you have any comments on that and how to combat it?

 

Thanks

 

R

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