Attempting a tricky cast of a hyperhemisphere using dragonskin- looking for alternate materials.

This is actually my 1st post, but I've been building models for many years- usually in the course of product design/engineering, but I don't have a forum quite like this in my repertoire so I this will be a nice addition!
Anyway- so what prompted this research is a particularly tricky mold that I have a few questions on.  It's tricky for two major reasons:  
1.  It is a shape sometimes described as a "hyperhemisphere" which is a hemisphere that continues past the centerline creating a big ole' undercut.  Think of a fishbowl, or lightbulb.  Usually something like this would be made in blown glass.  It's basically a hollow sphere with a hole in it (in my case around 30-50% of the diameter).
2. Both the inside and outside of the sphere need to be optically clear surfaces- this is why blown glass doesn't work, too much surface variance and too much distortion.  I'm looking to improve on that.

My solution so far has been to try to mold two rigid patterns in soft materials.  The patterns are accurate optical surfaces and I have been trying to avoid using any release on them for fear of impacting the clarity of the surface.  So far so good in that regard.  You just have wrestle and have some patience to remove the patterns and stretch the molds.  Each sphere pattern also contains a little flange which exists primarily to align them when they are combined.   This way you can locate the smaller (positive) sphere mold  concentrically with the larger sphere (female) sphere mold and when you pour the transparent rigid urethane into the void- voila you have a perfect part! (in theory).

The larger sphere is no problem- Just pour platinum silicone around my "lolipop" pattern and pop it out.  The interior sphere is harder, because it has to end up being a very soft and compliant material in order to pull out of the undercut, and the source pattern is rigid.  To accomplish this, I tried using a soft (40A) urethane for a mold- removed the pattern and then poured dragonskin (smooth-on) into it.  Unfortunately it turns out these aren't compatible evidently, and the part inhibited.  
Ok- so I will instead make a new mold with conventional platinum around my pattern, and this time pour dragonskin into it.
But now I'm going to need to pour the rigid clear urethane into a mold who's outer surface is platinum rubber and inner surface is dragonskin.  
Is that going to work?   
More specifically- I think the dragonskin will be ok with the undercut,
(BTW, it's kind of neither here nor there, but I installed a 3rd, less precise wax lolipop core as well so that I could melt/break it out and have a more collapsible interior mold, or apply vacuum to self-deflate)

but here are the things I'm specifically wondering about:
1. Will dragonskin be effective in preserving an optical surface?
2. Is it mandatory to use release in these processes and is my worry justified about using a release (re: optics)?
3. Will I be able to peel the dragonskin away from the rigid clear urethane?   I'm not trying to make an overmold surface, but this is probably how I would normally go about it if I were.
4. What, if any worries about inhibition or other issues should I have with this new scenario?
5. What other materials might be a better choice?   eg.  I could maybe make that interior core in solid wax also, but I rarely have had motivation to make a wax part.  I wonder if that process would be as effective or what tricks would be involved- I'd like to leave that interior surface as unscathed as possible and I worry about smears with wax.  What are my other material options?

If you've read this far, sorry for the long first post, but I hope I've made this clear (pun intended!)  and I hope someone can offer some advice.  Thanks so much for reading!
Here is a diagram:


  • I'll try and answer your questions as best I can.

    1:  Dragon skin, or any platinum silicone should work fine.  They will produce castings as smooth as the surface they were poured around.

    2:  You don't normally need to release the platinum silicones.  Though adding a release will extend the life of the mold.  But if you are going for optical clarity, I would avoid using a release.

    3: The dragon skin should peel away, as silicone only bonds to silicone.

    4:  The main thing to avoid is anything containing sulfur or latex.  You can a more complete list here:

    5:  I think your choice of using platinum silicone is a good one.  But because silicone is flexible you will want to back it with something solid.  For the outer mold that is easy since you can just make a shell.  For the inner mold I would build a series of plugs that lock together to form a solid core.  Magnets work great to lock the pieces together.  Here is an image that somewhat illustrates the concept in 2 dimensions.

    Though looking at your illustration the "neck" aperture would not provide access.

    The other option is to have a 2 part mold with a core, but that would leave a seam line on the outer surface.  You could polish that down to an optical surface though, it would just take a lot of work.


  • Hi Chris,
    I guess I don't get notifications set on this forum yet- sorry for the late reply!  Thanks for the response, I actually took some time to weld a window into my pressure/vacuum pot so just getting around to pouring rubber tonight and already have my molds set up, so going to go for it.
    One thing I'm still worried about somewhat is whether the dragon skin (which I've never used before) will stick to the rigid clear urethane (without release).  As I noted (see question 3 above) this is how I'd normally go about attempting an overmold part, but in this case I want to pour the soft over hard urethane and then pull it off.  Does anyone ever use dragonskin for overmolding?
    Fortunately I'll have a convenient way to try other materials if this doesn't work as I can just pour into my interim mold and cast a new mold interior.   What do you think of my wax idea?  This is a rare occasion that I'd consider using wax as a mold material, but assuming I can achieve comparable surface finish, I know it wouldn't inhibit and I know I wouldn't need to worry about release.  Also I'd probably first try to break it out, but if worse comes to worse I could melt it out too- just worry more about interfering with my precious interior optical sphere surface..

    Finally- I have some "composimold" thermal molding material that I've never tried before.  This seems like it could be a good candidate for an experiment as well.
    Thanks again!
  • I've never worked with wax as a mold material, so I'm not sure how it would turn out.  But you could always test it!  The platinum silicone should not bond to anything other than silicone, so you should be fine with demolding.  Especially since your parts are smooth and would prevent mechanical bonds.

    It's a pretty unique challenge, and I hope you can share your results with us!

  • I agree with the challenge part!  Part of my motivation I suppose.  Anyway, so far so good, I actually made a wax plug and with some air it blasted out entirely in one piece, so that was my first surprise!  (this is just the collapsible core- not a critical surface).  Anyway, even though my vac pump is on the fritz and not debubbling effectively things are looking pretty good.  I resisted the temptation to just pour my waterclear 220 in since OD and ID mold are ready and fit nicely.  I'm confused (or perhaps you are) by your silicone comments.  Isn't dragonskin a urethane?   As my final part will be made from Smooth-on crystal clear rigid urethane, and my interior core mold is dragonskin urethane- THAT is where I'm concerned that they will bond.  Not the silicone (although I will say that pulling that dragonskin out of the silicone was considerably more challenging that I expected- the surface doesn't appear to be quite as shiny as the silicone OD mold either, but that may be due to the translucent material and space at the center.  In any case, I did a test pour of rigid CrystalClear onto the surface of leftover rubber and dragonskin, so I will see tomorrow how/if they pull apart before risking my mold, but will try to keep you posted!
  • DragonSkin from Smooth-On is a platinum silicone:

  • Aha!  I didn't realize that!  Thanks for bringing it to my attention!  I was thinking it was a very soft urethane.   In any case, regrettably the concern I had was realized anyway- it took a herculean effort to rip the Crystal Clear free from the dragonskin and even so assorted chunks remained attached.  I did some post curing of the Crystal (as recommended at 210F for a few hours) before trying to separate, and this was an easy flat surface (I just poured some clear onto the remainder of dragonskin in the mixing cup) rather than pulling out of an undercut.  
    So now my internal debate is whether t next try that composimold or wax- either of which can be removed by heat and would be liquid at the post-cure temperature.  I'm reading up on surface finish now to see which might deliver a better optical result.  Any suggestions for other materials would be welcomed!
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