Mold thickness and mold supports

[Hiflt4@juno.com]

Hey guys I just wanted to see if i am makeing the best use of my maerals.

Currently I am useing 2 part molds that are about 3-4 inches in thickness on each side *6-8 inches total* I have been doing this so that the mold is rigid enough to be taped together without a wood support frame however it is fairly expensive material wise and had been considering halfing the thickness and adding a perminate support such as a plaster mother mold shell.

Was wondering how thick are your molds and if anyone has a methid they use to minimize the amount of rtv they use.

[Lathanm]

It really depends on te shape of what I am making a mold of. I try and keep the recommended 1/2" wall minimum for my projects. To conserve materials I build custom boxes instead of always using square and rectangle shapes. This causes most of my boxes to come out octagonal but I have some bolds that are triangular and L shaped. For the boxes I am using a mix pf 1/2" thick plexiglass strips and countertop remnants.

Other things I have done is to use rigid foam, the pink or blue stuff to fill voids at the corners. It is easy to work with and it is always readily available. Lastly I grind up my old worn out molds using a food processor and use the pieces as filler. I am not pressure casting so I normally paint on my first layer of rtv to insure I don't get air bubbles. Once it sets up I fill the rest of the mold with the grinds then add additional rtv to bind int all together.

I have tried the plaster/fiberglass over mold thing a couple of times with limited success. The big thing is making sure your over mold is strong enough and has sufficient attachment to the rubber mold so it doesn't move around. My experiment with this type ended up just shifting my rtv cost to plaster as I ended up needing an extra thick over mold.

[Lane]

Three inches wall thickness is a waste of material , even if the parts are large.

Many people use foamcore to make custom mold boxes.

If the parts are large enough to need 1" or more to prevent sagging then use some type of support or mother mold.You could also use a board with keys inside the mold box. Just make sure you can remove the board so the mold will flex for part removal.

[mangozac]

The mould thickness depends on what is being cast, but I agree with Lane it sounds like you're wasting a lot of silicone. Large parts need a lot thicker mould in order to ensure rigidity and minimise warping but smaller, individual parts not so much. My moulds I generally make double the thickness of the average thickness of the master, or at least 5mm from the thickest part of the master (whichever is smaller). For casting I then sandwich the mould with 3mm MDF boards and rubber bands.

MDF is FAR cheaper than silicone for making moulds rigid!

[paulson games]

Small parts I leave maybe half an inch of mold wall, as most of may parts are fairly thin I rarely have any molds over 1.5 inches thick. Like Zac mentioned, I sandwhich them between MDF board with rubber bands, that way there's no need to mess aroudn with supports in the mold itself.

A few of my thcker molds for large parts might be 3-4 inches wide and don't seem to need anything beyond the MDF boards. I haven't cast anything large like complete tanks, cars, etc so I don't know firsthand if I'd need supports for larger pieces, but the silicone seems to be failry rigid on it's own.

[blind pig]

I have one mould that is 9Kg A Condensation cure RTV2) and casts up a part that is 3KG.
The walls are 1 1/2" thick and are self supporting, even when I roatation cast it.

You are wasting silicone. Think of the whales!!!!!

[Anvils Hammer]

I leave mold walls in places as thin as 2-3 mm at the narrowest point.. and it works fine. No point using more than the minimum ammount of silcone.

I use wood supports at teh moment, but Im about to change to support strips made out of Acetal Sheet.

Wood supports tend to bend in time, and also resin sticks somthing aweful so my wooden supports have to be replaced fairly often.

Acetal is more rigid, less prone to warping, and also, resin just will not stick to it!

If you have access to a band/circular saw, I reccomend getting a sheet of acetal, maby 8mm thick.. and cutting yourself some vastly improved mold supports!!

AH

[mangozac]

[Anvils Hammer]