[bradlar90]

Hello all,

I need some advice for a small run of parts I need to cast.

The part is essentially a hollow hemisphere with a flange and o-ring groove and a filling port.
For scale the sphere diameter is 170mm (~7") and the wall thickness is 5mm (~1/4").
I need to make around 10 of these parts and a key factor is that they are as dimensionally accurate as possible. They need to be rigid and transparent, so I'm thinking of using a resin.
Attachment 352816Attachment 352818



My concern is how best to produce the moulds for casting. I think it needs to be a two part design, but if I use silicone I worry that the large mass and cavities of the moulds would not remain dimensionally accurate.
(This is just a concept section view to illustrate the point, I haven't designed the mould yet)
Attachment 352820


Does the mould need to be flexible? Could I just get the two mould halves CNC machined / 3D printed from a rigid material, or would removing a rigid cast from a rigid mould be too difficult?

[awerby]

Assuming that all the holes and edges in the model are drafted sufficiently, then a rigid mold should work. A fiberglass-reinforced polyester or epoxy layup would be lighter, cheaper, and more dimensionally stable than the silicone block you've drawn. A 3D printed mold would not be as strong, since they tend to come apart at the lamination lines. Use several layers of a good mold release and you should be fine.

[bradlar90]

OK, I'm new to this. When you say polyester or epoxy layup, would that involve creating a master pattern and then forming the moulds over that by hand, including features like keys for alignment?

Cost isn't a major issue, but I need to ensure very high quality parts and as this is my first time casting I'd like to simplify the process somewhat.

I would need to get the master part machined anyway, could I instead design the two moulds and get them machined? They are around 100mm deep so not sure what material choices I would have. Would something like Renshape offer a quality surface finish for transparent resin, and hold up to 10+ castings? Overkill I know but how about solid aluminium moulds?

[awerby]

OK, I'm new to this. When you say polyester or epoxy layup, would that involve creating a master pattern and then forming the moulds over that by hand, including features like keys for alignment?

[Right.]

Cost isn't a major issue, but I need to ensure very high quality parts and as this is my first time casting I'd like to simplify the process somewhat.

I would need to get the master part machined anyway, could I instead design the two moulds and get them machined?

[The master part could either be machined or printed. I suppose you could machine the molds from big blocks of plastic instead of laying them up, but the amount of material and the cost would be a lot higher, while the strength would be lower.]

They are around 100mm deep so not sure what material choices I would have. Would something like Renshape offer a quality surface finish for transparent resin, and hold up to 10+ castings?

[Renshape would be acceptable as a material for the model, but not for the molds.]

Overkill I know but how about solid aluminium moulds?

[They would probably work, but be sitting down when you read the quotes for that...]

[dpryor]

I agree with Andrew. A CNC machined Renhape or Aluminum mold would work fine but cost may be an issue.
If you take the time and expense to machine a master pattern you might as well machine a mold.