Mold Making?

Hi guys,

I'm having some difficulty figuring out the best way to mold a two sided part. I should add that I'm completely new to this!

What I plan to do is machine a master on the CNC out of 100mm Polyurethane board and then make a mold box, placing the master in this, and filling with Silicone rubber. I should add that this is a large part measuring 950x180x100mm. I'm not sure what shore rating silicone to use for this. I'm guessing the 15 Shore A is far too soft for such dimensions? I'll be needing around 20kg for the mold so want to get it right since it'll be an expensive mistake if not.


The main problem though is figuring out how to get the rear side detail into the casting without trapping lots of air or distorting the flat rear side portion of the cast part. The way I've tackled it at the moment is to level fill the mold with the casting resin and that will form a guide for the rear and be perfectly flat. I guess I could leave out the rear details but that'll mean throwing this on the CNC to do that and wasting resin.


I've attached the front and rear views of the part along with the first half of the mold but I would be grateful for suggestions on how to tackle the second half of the mold.

Rear of Master:
Attachment 8295
Front of Master:
Attachment 8294
First half of the mold:
Attachment 8296

Speaker enclosure eh :-) nice! Just out of curiosity what drivers are you intending to use! ( i know its not advice on your question :-) ) ive been planing on making a pair or ATC100's with a solid milled front baffle out of aluminium, sculpted and then anodised black.

Do they need to be made in one piece or could you manufacture them in 2 halves & then bond the 2 halves together. Would probably make things easier especially as far as expelling air goes.

Sounds like a nice project you have planned there. Can't go far wrong with ATC.

These will use scanspeak tweeter, accuton midrange and peerless xls sub.

Very nice, do you use wilmslow audio for components?

Quote:

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Originally Posted by martin54

Do they need to be made in one piece or could you manufacture them in 2 halves & then bond the 2 halves together. Would probably make things easier especially as far as expelling air goes.

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Depends on how invisible and well matched the joint is which is like asking do I have the experience and skill with molding to ensure that... hmmm I have my doubts!

I have thought about taking the existing mold design I have and suspending the rear parts over that leaving the rest open to avoid trapping air as much as possible.

Quote:

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Originally Posted by AdCNC

Very nice, do you use wilmslow audio for components?

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I used to but they are expensive on most things. I now order from Europe or some cases the US. Even with shipping/taxes it works out cheaper. A few driver manufacturers such as Raal and Audiotechnology will deal direct too.

Information and Practical Advice for Advanced Composites - Easy Composites have a butchers here, ive done abit of fibreglass moulding in the past and used the same method at these vids to very impressive results. That might be sufficient for your mould master.

Hmmm i might try giving ATC a call see if they will deal direct.

I have no idea how easy or difficult it is to expel any air as I have never used silicon for casting but the only place I can really see air getting trapped is the 2 shapes in the rear half of the mold. Small hole drilled in each would allow air to escape, it looks from the drawing to be about the highest point. Would then be easy to trim off the excess once the silicon had cured.

Well at 950x 180x100 mm it wont fit in my vacuum chamber, I'd be tempted to make a mother mould out of fibre glass and pour the RTV into this, making a 1 piece skin mould which you then make a jewellers cut into to release the master. to produce the castings and whilst maiking the glove i'd be tempted to Vacuum bag it.
Its not a process thats easy to explain, in text, and its a pain in the arse if you don't know what your doing.

There are plenty of different methods of achieving the same objective, and many many different opinions on the right way, ( A bit like on here)
But , a quick look on You tube "how to make a mother mould"
Found this, Its not definitive by any means , but it shows the principle.

Mold Making Techniques: Resin Mothermold Process - YouTube

Regards
Rik
If you need any advice drop me a PM