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Old 28-01-2016, 01:31   #1
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Casting Porthole Latches

hi all, i have plastic porthole latches on my 20 year old beneteau 53. several are broken and impossible to find so i am trying to cast some.

i just bought some silicon rubber to make a mold and wondering what to use to cast the part.

i'm thinking either epoxy resin (very strong but maybe hard to get out of the mold) or polyester resin (maybe not strong enough) or maybe my old favorite, car body filler (but maybe too viscus to pour into a small mold.

any ideas form anyone who has tried?

i'm in Thailand and specialty products available in the US and the civilized world are not often available here so looking for generic products than i can find in chandlers or hardware store.

thx steve
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Old 28-01-2016, 02:06   #2
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Re: casting porthole latches

From experience many years ago, I'd suggest splitting the mold for easy release, and using epoxy resins and some glass tows laid in the mold to add strength to the cast epoxy casting. The glass doesn't need to be very scientifically placed to be useful, but does need to be wet out pretty well.

Good luck with whatever method you choose.

jim
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Old 28-01-2016, 02:12   #3
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Re: casting porthole latches

Haven't done it (although I have fabricated a replacement bronze latch that broke on one of my portlights, so I do feel your pain), but forget about body filler - it's not strong enough. I guess the best stuff would be casting epoxy resin (if possible reinforced with some CSM strands in the mold before pouring) although something like JB Weld would work too if you could get away with it's toothpaste-esque viscosity. In any case you'd need to paint the finished product.

Now I don't really know enough about it to comment with authority on the suitability re mechanical properties, but thinking outside the box, have you considered finding someone who could 3D print your parts?
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Old 28-01-2016, 02:30   #4
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Re: casting porthole latches

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Originally Posted by Reefmagnet View Post
Haven't done it (although I have fabricated a replacement bronze latch that broke on one of my portlights, so I do feel your pain), but forget about body filler - it's not strong enough. I guess the best stuff would be casting epoxy resin (if possible reinforced with some CSM strands in the mold before pouring) although something like JB Weld would work too if you could get away with it's toothpaste-esque viscosity. In any case you'd need to paint the finished product.

Now I don't really know enough about it to comment with authority on the suitability re mechanical properties, but thinking outside the box, have you considered finding someone who could 3D print your parts?
i doubt 3d printed would be strong enough and hard to do here. will try epoxy with a big of glass then, thx.
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Old 28-01-2016, 02:34   #5
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Re: casting porthole latches

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Originally Posted by Jim Cate View Post
From experience many years ago, I'd suggest splitting the mold for easy release, and using epoxy resins and some glass tows laid in the mold to add strength to the cast epoxy casting. The glass doesn't need to be very scientifically placed to be useful, but does need to be wet out pretty well.

Good luck with whatever method you choose.

jim
so i'm inferring from this you would pull some strands out of a glass fiber mat then mix them in with the epoxy resin?

i'm not sure i will need to split the mold, its a small part and no hidden inside shapes or anything. the main mounting hole in the center i was planning to drill out after casting.

UPDATE

on second thoughts, i could cast in the mounting hole using a 2 part mold. thx for the idea.
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Old 28-01-2016, 02:42   #6
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Re: casting porthole latches

I am in Thailand and machine work is cheap, so I would go to a CNC shop and simply have them copied in the material of choice. Or if not too complicated a manual shop could do it. You would be surprised what can be done here with a lot of patience

Or even sand cast them in aluminium. Again cheap here in Chiangmai
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Old 28-01-2016, 02:50   #7
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Re: casting porthole latches

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I am in Thailand and machine work is cheap, so I would go to a CNC shop and simply have them copied in the material of choice. Or if not too complicated a manual shop could do it. You would be surprised what can be done here with a lot of patience

Or even sand cast them in aluminium. Again cheap here in Chiangmai
there is a machine shop just up the road from me, he has lathes, milling machines, everything you can think of in there. i asked him to copy one on aluminium, he said he couldn't do it.

if casting in epoxy works, its quick and easy, if not, i guess i'll have to find another machine shop.
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Old 28-01-2016, 03:53   #8
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Re: casting porthole latches

Here's an illustration of a die-cast part that shows what a parting line is. Also a wiki article on draft here-

https://en.m.wikipedia.org/wiki/Draft_(engineering)

If the original part was similarly cast, trying to identify the parting line will help you make your mold. If it is a simple shape without undercuts you can make a mold out of plaster. Here's an article that explains how -

Make a two part reusable mold using plaster

Better to drill the hole post casting than to try and incorporate it into the mold. Use some kind of wax, petroleum jelly, or mold release so your part doesn't stick to the mold.

Epoxy generates heat as it cures so hopefully your part doesn't have too much mass. Could try to put it in your cooler if it gets hot or else use polyester.

I would skip the glass and opt for epoxy slightly thickened with high density filler because you will need to inject it into the mold with a syringe while providing a way for the air to escape if you don't want voids in your finished part.

Can you post a picture of the latch? Could better advise you with one.
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Old 10-02-2016, 19:43   #9
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Re: casting porthole latches

i thought i would update my little project which seems to work quite well.

after some experimentation, i've made some nice looking and very strong latches.

first off, i made the original about 5% bigger by adding tape all around it. the idea is to make the copies a bit bigger, so i can sand them back down to the real size.

i made the silicon rubber mold which is easy enough, even with the bolt hole down the middle, i have no problem removing the original or the cast products. i use Vaseline as a mold release helper. a one piece mold is fine, silicon rubber is very flexible and i've made 15 parts now and the mold is still good as new.

i cut a strip of fiber glass, about 12" long and 1cm wide. i loop that around the mounting nut hole then up and down the handle and around the latch part a couple of times.

then i add the epoxy resin/hardener mix, but i find i need a little more hardener than the 3:1 ratio recommended on the bottle. i find about 2.5:1 gets a harder final product.

then i remove the cast part from the mold after about 3 hours, let dry overnight, then clean out the center mounting hole with a drill, then sand it to look nicer and take the paint.

then i spray paint it. i considered mixing color into the resin but read that can reduce the strength, so didn't try it.

anyhow, they seem to work fine and look ok.
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Old 11-02-2016, 02:18   #10
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Re: Casting Porthole Latches

In Thailand you should find many shops that work in brass or aluminum. By the time you screw around with epoxy and have the latches break again later, you'd be better off with metal.
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