advice for duplicating NASA Mercury helmet visor without risk of damage...

dogstar

Active Member
Hi...attached are two pix of a US Navy Mark IV helmet visor which I'd like to duplicate without risking damage...some thoughts I had was
trying to cast a vacu forming buck by putting the visor in a plastic bag kind of enclosure and then casting plaster into its curve and starting with that...I"m sure someone knows how this might be done and would love any ideas...I just don't want to risk screwing up this real Mark IV visor...
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You are gonna need BIG vacuformer. If you vacuform, I'd have the angle slightly off the surface and have the back and the front ends off the surface of the vacuformer. DO NOT USE THE ORIGINAL! Use something like plaster to make the puck.

or!

Make a mold using rebound 25, then cast with Epoxacast or any clear resin.Be sure to degas.
 

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Hi JPH...thanks for that...I don't have any vacuforming stuff, so am looking for things I can do on
my workbench...(I assume your drawing was of the suggested seating on a vacuform buck and suction surface?)
what I can't figure out is how to cast such a large thin object...would it be a two part mold in rebound 25? If so,
how would the Epoxacast get in there between the two parts with such a narrow space to fill...

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Make a mold using rebound 25, then cast with Epoxacast or any clear resin.Be sure to degas.
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When you make a mold using Rebound, you apply layers of rebound and embed registration points on the outer surface. I use cut up pieces of foam. So, after applying several layers of Rebound, apply a layer or two more with foam. After you coat with rebound and foam registrations points, you then can use plaster of Paris or Free form air to make a hard mold that fits on and holds the Rebound in place.

The hard outer mold is usually in two pieces that come off easily from the Rebound.

The drawing was a vacuformer box on bottom, with the prop you showed on top. You can make a vauformer in a day with some 2x4s and the brown peg boards. When I get home, I will take some pics for you.
 
So, the goggles are made from the orange rebound mold. The yellow lumps are pieces of foam embedded in the Rebound and the grey stuff that is on both sides of the rebound mold (interlocking with the foam) is FreeForm Air.

SmoothOn has great video tutorials for this.

The vacuformer is just a box woth a top made from peg board, and a hole on the side for a shopvac. Then you need a frame to hold the PetG that fits snug on the pegboard.
 

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Hey JPH...thanks so much for taking the time and effort to get snaps...I will do a deep dive into
this and report back...thnx
George
 
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Why don't you make a cast from the original with silicone, then make the visor from plaster using the silicone mold. Isolate the plaster visor and make an epoxy mold from it. You have to vacuform it from the inside. Therefore you have to build a new vacuformer for it... It's not easy, but feel free to ask, I have 30 years experience in mold making and vacuforming....I'm making a mercury space helmet from scratch at the moment, therefore I'm familiar with the visor problems. You are very lucky to have a original visor, I have to sculpt it completely from scratch...
 
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You can safely make a mold using silicone. The silicone will not stick to the acrylic of the visor or the FRP reinforce. However, If you want to duplicate the visor, you would need to remove the FRP reinforce from you tool.
I would not try to "cast" this in resin, the chances of getting a usable part are pretty slim. Vacuum forming or blow-forming are your best options.

Vacuum forming can be done with a gypsum tool. You will not avoid distortion or "mark-off" no matter how smooth the tool is, but smoother is still better. I would make the v-form pattern in Hydra-stone, as it is very hard and can be preheated to give you a better part. If you vacuum form it, an have enough suction, I would recommend Acrylic over PETG, as it is harder, and easier to polish out imperfections.

Vacuum forming will give you "mark off" because you are contacting a cooler tool, with hot plastic. As soon as the plastic touches the tool, it will cool, and other areas, which are still hot, will continue to stretch, and thus change the material thickness in tiny uneven amounts. As soon as you have a thickness change, you make a "lens" that bends light, and gives you this distortion. If it's minor, it can be sanded and polished out. This is a tough part to v-form, as the sides (pivot points) are parallel and vertical. When you mold it, I'd brace the pivot points outward a bit, to help your v-form tool shape, making it easier to remove your part.

What is your intended use of the copy? If it is for a mercury helmet visor, then this is too large. This is the sun visor of a MK IV helmet, which is about a 1/2" larger radius than the clear visor. If you mold the outside of this visor, and pour you tool into the mold, you will add another 1/8" or so to the radius when you vacuum form it. This will make a visor that is 11/4" wider than a mercury visor.

If you are in need of a mercury visor, we may have some for sale in the next couple of weeks. These are surplus visors with minor damage or imperfections that we do not use. They typically sell pretty cheap, likely less than the materials you would purchase to make a copy of this visor.
 
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