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Making a missing part
lawzer
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Scotland, United Kingdom
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Posted: Tuesday, March 07, 2017 - 03:57 PM UTC
Hi folks,

I recently got 2 1/48 Revell JU88's off ebay (both in same box).

One half of one of the wingtips is missing (seller told me about it so I knew). Now I have the part from the other kit so what would be the easiest way to copy it? The part itself isn;t complicated so I thought perhaps some plasticard to make the rough shape then sand as best I can? I have seen a few articles where people have used plasticard as a sandwich filler with some thin balsa on top and bottom (although in my case only the top).

Cheers

Ian

RobinNilsson
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Posted: Tuesday, March 07, 2017 - 10:29 PM UTC
If there is surface detail on the missing part I would make a silicone mold and cast a copy with some 2 component resin.
If the part is smooth I would use the other half as a building surface, glue formers/frames to the other half and then shape thin plasticard on these. Replicating the way a real wing is built ...
Trying to sand down a thick slab usually leads to a lot of work ...

/ Robin
lawzer
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Posted: Wednesday, March 08, 2017 - 12:24 AM UTC
Thanks Robin.

Do I need anything special if I go down the resin roadc(tools wise)?
RobinNilsson
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Posted: Wednesday, March 08, 2017 - 01:51 AM UTC
If you haven't cast resin in silicone molds before you will be learning many things

It will be a simple open mold so it shouldn't cause problems but the silicone and casting resin will probably cost you more than getting a complete new kit.

Another option since silicone molds is overkill:
Look around and see if you can find latex mold making material, it's used to make molds for casting plaster ("white" powder, mix with water).
It is "painted" onto the original part in several layers and then you pour plaster over the outside of the mold to make a stabilising cast before removing the original part. You can use epoxy as resin to cast a copy of the missing part.
Illustrated instructions:
http://www.wikihow.com/Make-a-Latex-Mold
You will not have any issues with undercuts and tricky release angles.

https://www.artmolds.com/molding-materials/liquid-latex-rubber.html

http://www.tiranti.co.uk/EdgeImpactShop/subcatdivision.php?Division=187&Content=Latex
It's a "arts & crafts" type of material so that is the kind of shop or vendor that you need search for.

/ Robin

lawzer
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Posted: Wednesday, March 08, 2017 - 02:04 AM UTC
Cheers!
RobinNilsson
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Posted: Wednesday, March 08, 2017 - 04:01 AM UTC
One more thing:
Glue a strip of styrene, say 1x1 millimetre, to the reverse side of the original part, 1 millimetre in from the edge. This will create an undercut to the mold and the molded part will hopefully get a well defined thickness around the edges and the removal of the excess lump in the middle will be easier since you have a one millimetre edge which has already got the correct thickness. If the strip is too far in from the edge you might get problems with air bubbles caught in the undercut when casting the resin and it might get tricky to get the mold latex into the undercut (badly formed mold).
/ Robin
bat-213
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Posted: Wednesday, September 27, 2017 - 04:49 AM UTC
hi Robin, have you ever tried composi mold? if so did you have a mold relese agent? I will try to use fastcast its a 2 part urethane casting resin,to make some spare tiers for a kit ,I would your input or anyone that can shed some light on this for me .all anewres would be great .thaks guys .
barkingdigger
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ARMORAMA
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Posted: Wednesday, September 27, 2017 - 03:19 PM UTC
If you really want to experiment with casting and money isn't a problem, go for one of the Alumilite starter kits because it's fairly simple. But you will need both the rubber moulding kit and the resin casting kit (unless they sell them together these days?) and I doubt it would be cost effective just to replace the part. However, if it is the start of a new facet of the hobby for you it might be worth the cost spread over a bunch of other projects...

Instead, I'd look at plastic strip - is it possible to get one that's bigger than the part in all directions? Then you'd just need to whittle it down until all that was left was your replacement wingtip. I've used tons of Evergreen plastic over the years, from replacing kit parts to complete scratch-builds, and the main cost has been elbow-grease.
bat-213
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Posted: Sunday, October 01, 2017 - 04:55 AM UTC
hi Tom ,you skills are not good ennfeto a scratch build ,but I have the rubber to make the mold and the resine to pour ,so for me all I wanted to know did I need a relase agant to help the part? thanks for your input .
barkingdigger
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Posted: Monday, October 02, 2017 - 01:30 PM UTC
I only used release agent (vaseline petrolium jelly) when I made two-part rubber moulds, to keep the rubber from welding itself together. If your rubber mould is made thin enough to flex, it can be peeled off of plastic easily without release agent. Others might have different opinions...
bat-213
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Posted: Thursday, October 05, 2017 - 03:29 AM UTC
thanks Tom I will try that.
sgtreef
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Posted: Thursday, October 05, 2017 - 05:39 AM UTC
For a one off regular silicon in tube can be used, but still need resin.
Pave-Hawk
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Posted: Thursday, October 05, 2017 - 05:57 AM UTC
If using normal hardware store tube silicone I would also use some form of release agent on the part, since normal tube silicone is designed to stick to things.

Or alternately you can use it to make your own silicone putty by mixing with corn flour.

https://www.youtube.com/watch?v=eyOGSQpdjJI

Silicone putty is good for making impression moulds or doing one time moulds of small parts.
Can be used to do two part moulds with a bit of care and planning, and the commercial putty tends to be a bit stiffer than normal casting silicone, so doesn't have to be as large to maintain good shape when casting. Not sure how the diy stuff is in that regard, but as a disposable one shot mould it's a viable option.
RobinNilsson
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Posted: Monday, October 16, 2017 - 01:44 AM UTC

Quoted Text

hi Robin, have you ever tried composi mold? if so did you have a mold relese agent? I will try to use fastcast its a 2 part urethane casting resin,to make some spare tiers for a kit ,I would your input or anyone that can shed some light on this for me .all anewres would be great .thaks guys .



Please excuse me for not checking this thread so often.
No, I haven't used 'composi mold', never heard of it before since products of this type tends to be "local" to the market they are sold on.
I have tried to make tires/tyres a few times, I used silicone molds and polyester resin (think glass fibre reinforced plastic, GRP, boats) without any release agent whatsoever. No problems getting the parts out of the molds but due to the polyester being thick like syrup I had problems with air bubbles in the finished parts. It would probably have worked better with a more "easy flowing" casting resin.
I did manage to cast copies of the upper shell for the Polish tankettes by RPM

using polyester resin but this time I used an open mold so I could "paint" the resin on the inside of the mold and "reinforced" it with tissue paper to soak up the resin and stop it from flowing down to the deepest point of the mold.

If I were to try making tires/tyres again I would use a better resin and maybe also prepare the insides of the mold, "paint" the tire/tyre pattern to eliminate air bubbles before closing the mold and pouring the rest. If it is only the tires/tyres without the rim (i.e. not a complete wheel) then casting two halves (just like the tires/tyres in many plastic kits) in open molds might be a solution. It could work for tires/tyres with a well defined centerline like these:

or this


This type would probably be easier in a closed mold:


/ Robin
bat-213
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Posted: Tuesday, October 24, 2017 - 03:08 AM UTC
thanks Robin ,the tires that you showed at the top of the page are the same type ,jeep tires in 2 parts .and I think you have anwerd all my questions .thanks very much.