Stage 2 -
Moulding & Casting

Moulding parts of this size would need a lot of silicone rubber and some forward planning, particularly the upper secondary hull with its complex shape. A two- or three-part silicone jacket with multiple-part mother mould would be needed for that, whereas the others would need more simple moulds, so I started with those. I didn’t see the need to rethink how these parts were originally moulded and cast, those decisions had already been made by Greg decades ago, so I just followed his lead and let the parts inform how they were originally moulded.

The finished moulds

(I forgot to take a photo of the upper saucer mould...!)

The saucer halves and keel plate were a simple matter of fixing them to a flat moulding board, so a flexible silicone mould jacket could be made using the “brush-up” method. Since these parts were going to be preserved in the museum’s archive, the method of fixing them to their boards needed to be temporary and, most importantly, non-destructive when removing them again later. A common method is to lay parts on a layer of clay and seal any gaps around the edges with more clay, but moulds of this size would need a LOT of clay, and the process can be quite time consuming. Instead, I decided to use self-adhesive aluminium foil tape - the stuff used to seal ducting, or joining insulation boards etc. The tape has a high level of tack and conforms and sticks well to uneven surfaces, and it also doesn’t need any additional clean-up after removal like clay does, so it’s ideal for this kind of application. Small pieces were cut and applied to the inner surface of the outer edges of the parts, creating a narrow flange with the sticky side facing up when the parts were laid on their boards. This flange was then attached to the board using more pieces of foil tape, making sure no gaps were left between the edges of the parts and the board surface. Not a perfect method by any means, but it’s simple, fast, and very effective!

Mouth of the deflector dish housing walled-off

Foil tape flange around edge of the keel plate

Foil flange taped down to moulding board

The pylon wing shape required something a little different, so some basic formers which matched the general inner contour of the part were cut from foam board and placed underneath. These acted as stand-offs which held the part off the surface of the moulding board. The foil flange was then applied to the outer edge of the part, again with the sticky side facing outward, and some thin card (an old Star Destroyer model kit box!) was then cut up and used to create a flat plane which followed the contour of the part. The part was then positioned on the mould board and fixed into position using more tape.

Lower saucer cast, ready for moulding

Upper saucer cast ready for moulding

Lower pylon wing ready for casting

The secondary hull would be more complex than the other moulds. Firstly, the front opening for the deflector housing was walled off using card covered with foil tape. Then, after marking out some guidelines on the moulding board, some card was used to match the bends of the pylon “elbows” where they swept up to vertical, and a couple of pieces of chipboard were fixed behind the card for support. Due to the dorsal being part of the hull and the top of the dorsal being open-faced rather than a closed-off and detailed surface, this created a situation where the mould would need to be inverted during the casting process to allow access to the dorsal area – in effect, the mould would consist of two open faced moulds mounted back to back, with a small opening connecting them where the dorsal meets the body below. So, the foil flange method was employed again to attach the part to the moulding board, and a sheet of foam board was used to temporarily close up the top of the dorsal area and create the surface area needed for the flange to run around the edge of the “cobra head” …hard to describe, hopefully you’ll get the idea from the photos! And with that done, the parts were ready for silicone!

First layer of silicone being applied to lower

saucer cast

Upper saucer cast with two or three coats

of silicone

"Keys" being added to

lower saucer jacket mould

Brush-up jacket moulds are pretty straight forward to make, and basically the process is exactly as it sounds – you brush, or otherwise manually apply the mixed silicone to the surfaces of the parts being moulded. Now, silicone (being a liquid after all, albeit a very thick one) wants to run off things until it reaches a level surface it can pool onto, so you have to keep chasing it around and brushing it back over the part until it starts to cure and stops moving. This is obviously quite labour intensive as you need to keep a close eye on everything and keep moving to avoid losing or wasting a lot of material. Having a fast-curing rubber makes this process less time consuming, but it also increases the risk of trapping air pockets or not having enough time to actually coat the entire part before it starts to cure and stiffen up on you. It also gives you less time to de-gas the rubber in a vacuum chamber after mixing, which is a very important step when trying to avoid a lot of bubbles getting trapped in the cured mould. Silicone is expensive, and I didn’t want to risk having to do this all again due to any of those issues, so I went with a relatively slow-curing material that would stay workable for about 30 minutes before curing. This would mean a lot more elbow grease to keep the silicone where it needed to be while it was still liquid, but it also bought me plenty of time to vacuum de-gas the rubber and chase out any voids and bubbles during the process. Once the initial surface coat was cured, further coats could then be applied using the faster curing catalyst to help speed up the whole process. Any voids or bubbles in subsequent coats wouldn’t really matter, as the details would all be captured by the first layer of rubber. Any deep pockets or significant protrusions would be backed up with thickened rubber, almost like peanut butter or cake icing consistency, which would add a lot of thickness and support in specific spots without needing to build up the thickness over the entire mould. Lastly, cured “keys” made of the same rubber would be added to the outer layer. These keys are very important when it comes to making the mother moulds (outer supporting shells), but I’ll come back to those later.

Tackling the saucer halves, keel plate, and lower pylon parts was simple. Pour, spread over part, chase it all around, wait for it to stop moving. Rinse and repeat for three or four layers, add keys, leave to cure. The same steps applied to the secondary hull, only things were a lot more cumbersome and messy due to the dorsal “cobra head” surface being upside down, which meant the rubber needed to be applied in thinner layers to minimise dripping. Several layers were done, with some spots reinforced with the thickened “icing”, keys applied and left to cure. Once all the moulds had fully cured, the keys were trimmed and tidied up where necessary, and it was on to making the fibreglass mother moulds.

First layer of silicone being applied to

secondary hull cast

Deep undercuts and sharp corners are filleted

with thickened silicone

First layer of silicone being applied to

secondary hull cast

"Keys" applied and curing...it's not

pretty, but it did the job!

Fibreglass Mother Moulds

The mother moulds are critical in supporting and maintaining the correct shape of the rubber jacket moulds once the pattern masters have been removed - without them the rubber jackets would just collapse and be unusable for casting. The “keys” on the surfaces of the moulds would lock into their mother moulds, ensuring the flexible jackets didn’t move around during the casting process. The mother moulds need to be strong and resilient, and preferably as light as possible, as casting and de-moulding parts can be a hard-going, and they need to withstand the process multiple times. You’d be hard-pressed to find a material that satisfies all those criteria better than fibreglass, and it’s also quite inexpensive and a lot faster to work with if using polyester resin rather than epoxy, so that was a no-brainer for moulds of this size. Once again, the saucer halves, keel plate, and lower pylon wing were all a pretty straight forward matter of laying-up a few layers of fibreglass over the rubber jackets and leaving to cure. The moulds would need to be stable when it came to casting, so timber frames were built and attached to the mother moulds after trimming away the rough outer edges of the cured fibreglass.

Lower saucer fibreglass mother mould shell,

curing over the silicone jacket mould

Timber support frame, to stabilise the

mould during the casting process

Finished lower saucer mother mould

The secondary hull would need a multi-part mother mould which could be disassembled and allow the silicone jacket mould to be removed from the eventual cast, so I decided to have it split into three sections – one covering the pylon wing, rear half of the body, and back of the dorsal, one covering the front left quarter of the body and left side of the dorsal, and one mirroring that on the right side, with the parting line of those two running up the leading edge of the dorsal. So, some thin partitions were made using card and foil tape and placed along the three parting lines, held in place with a bit of clay where necessary. The same routine of laying up a few layers of fibreglass was done and left to cure. Once cured, the excess fibreglass at the parting line flanges was trimmed, bolt holes drilled through the flanges to ensure they could re-assembled and fastened accurately, and the sections carefully removed. Another timber frame was built and attached to the rear mother mould section, with allowances made for the mould to be inverted and allow access into the open topped dorsal section when it came time to cast the part.

Mother mould partitions

Rear section mother mould curing

Front left section curing

Front right section curing

Finished rear section, mounted to a

timber frame

The mould needs to be inverted for full

access during casting

The silicone jacket was then carefully cut along the same parting line that the original had been – down the right edge of the dorsal and down the body of the hull. I chose to move the 2nd parting line on my jacket mould though, rather than follow Greg’s original, and decided to put it in the mirrored position to the 1st instead of down the leading edge of the dorsal. This meant the entire front half and both sides of the dorsal would be captured by the front mould jacket, making it easier to keep an eye on alignment of the jackets during casting. Time to cast some parts and see if these moulds would do their jobs….

Cutting the jacket mould, following the line

body the body

Rear jacket mould section removed. The front section envelops the entire front half, eliminating any seam along the leading edge of the dorsal

Casting New Parts

Casting parts in fibreglass is relatively simple, and this is where the extra preparation work and forward planning done during the mould making process starts to pay off. To make lighting all the eventual windows later down the line, it was decided to cast the working parts in translucent fibreglass. That way, any windows which would appear lit could be masked off before any primer or paint was applied. An initial layer of gelcoat (a thicker type of resin, like treacle) was brushed over the surfaces of the moulds, to capture all the fine details. This “beauty coat” would be the visible outer surface of the final casts, so extra care needs to be taken here to make sure there are no bubbles or voids in the gelcoat layer. Any deep under-cuts or sharp corners were filleted with further applications of gelcoat in those specific areas, to help the subsequent layers of glass cloth conform to the moulds. The gel coat layer was then allowed to cure enough to tack-off, before being backed with a couple of layers of very fine 80g cloth wetted-out with laminating resin. Once this “veil” layer had tacked-off, a couple more layers of heavier 195g woven roving cloth was laid down and wetted-out. The larger surface areas could be done with larger sheets and/or pieces of cloth, but due to its complex inner contours the secondary hull needed to be laminated using smaller patches of material, overlapped slightly and built up over a couple of layers. The casts were allowed to fully cure before being carefully demoulded. The secondary hull was by far the hardest part to mould and cast, but now the hard work was all done and would pay off greatly during the actual construction of the model…and that was finally about to begin, at last!