Silicone Rubber Bonding With Fine Detail Plastic

Discussion in 'Materials' started by freedy, Dec 10, 2018.

  1. freedy
    freedy Member
    Has anyone else had problems with fine detail plastic (both smooth and smoothest) sticking to silicone rubber? A search of the forums turns up a couple of posts where people have experienced this and have believed the problem was caused by the model not being properly cured but I don't think this is the problem. There seems to be some sort of chemical reaction going on between the two materials.

    First let me say I have been creating silicone rubber moulds for 25 years. I've used many different materials for the master model (wood, metal, resin, plastic, glass, etc) but as long as the surface wasn't porous, I've never had adhesion problems.

    I wanted a system which produces many moulds reliably and accurately so I had several pieces printed which fit together to make a negative of the mould. I prepared the pieces by sanding and polishing the faces to remove the fine ridging cause by the printing process. Then I assembled the pieces and filled the cavity with silicone rubber. This worked very well for the first 4 or 5 moulds, then the rubber began to stick in places. To cut a long story short, the problem got worse with every attempted mould, until now I have to coat the entire surface with release agent otherwise the rubber will bond so strongly I have to tear it off and scrape the surface clean again!

    Release agent spoils the intended mirror finish on the mould surface. I don't want to coat the surfaces with sealer because that would upset some very fine tolerances, but I suppose I could try to include a compensation factor for this in my model design if there is no other solution.

    If the problem was caused by patches of uncured plastic, I would have expected adhesion problems from the first attempt. But actually it was okay at first then got progressively worse - from small patches to the entire surface. It's as though something is changing in the chemistry of the outer surface layer to encourage the silicone rubber to bond with the plastic. It doesn't seem like the surface is becoming porous - I can see no difference under strong magnification between a problem surface and another part of the piece which hasn't ever contacted silicone rubber.

    Can anyone throw any light on this problem, please? If I can overcome it I will have a very useful technique which I can use for many future moulding needs.
     
  2. Spitfire2865
    Spitfire2865 Well-Known Member
    Do the FUD and FXD parts exhibit signs of a crystalline growth coming through the outer layers?
     
  3. freedy
    freedy Member
    Hi Spitfire2865,

    No. I presume you mean the white cloudy patches I sometimes see in prints. There are some patches of that, but the adhesion problem isn't localised to these patches. Something is progressively changing in the surface of the printed pieces every time it is in contact with silicone rubber. Parts of the print which bear no signs of the cloudy crystalline growth, and which at first DIDN'T stick to the mould material now DO, after several periods of contact.
     
  4. Roolz
    Roolz Well-Known Member
    Disclaimer :
    I have no firsthand experience with silicone + Fine Detail parts (I have already used other stuff to copy/mold Fine Detail, such as epoxy putty and heat-softening putty, but that's quite different).
    Though, colleagues at my real job regularly use SLA-printed cavities to mold silicone parts. Without any major issue (the SLA printer uses a photopolymer too, but not the exactly same as Shapeways' MJM). No release agent whatsoever.

    Suggestion:
    If the culprit is some kind of chemical reaction, you might try :
    1- post-cure (UV lamp, or under sunlight) to make sure there is no remaining uncured photopolymer
    2- wash thoroughly (Simple green or Denaturated alcohol) to remove any trace of support wax
    3- sand/polish the parts, where needed
    4- wash again (Soapy water), in an ultrasonic bath (if you have one)

    If you still have the problem, try a 5th step (if the geometry/shape of the cavity allows it) : a thin coat of gloss spray varnish on the cavity. With a bit of "spraying practice", the coat can be very even and very thin (<25µm).
     
  5. freedy
    freedy Member
    Thanks, Roolz, for your helpful and knowledgeable response.

    The way the problem has worsened with time (and exposure to silicone rubber) makes me think that further UV cure isn't the answer, but I'll give it a go with some spare pieces I have. I'm prepared to throw a lot of time and effort at this problem because if I can find a way to make the technique work, it will repay the trouble in the long run.

    Steps 2 - 4 pretty much describe my existing preparations.

    I think I will try out your varnish suggestion. 25µm is better than I dared hope, I could cope with 3 or 4 layers of that. Again, I'll test on some spare models.

    Can you recommend a spray varnish, ideally available in the UK, or offer some general guidelines? It's not a subject I know much about. A quick scan of eBay tells me they can be polyurethane-, acrylic- or water-based. I guess my priorities are : (1) effective with the thinnest possible layer, and (2) tough. Colour is not important. Neither is expense, within reason.
     
  6. Roolz
    Roolz Well-Known Member
    I'll just tell what I know about, i.e.glossy spray varnish for miniatures:
    - Water-based : Liquitex Professional gloss varnish
    - Solvent-based : Army Painter satin varnish

    Liquitex and Army painter are tough and good quality, IMHO.
    But there are many other (good) brands though (Krylon, Citadel, etc).

    Water-based and solvent based products have advantages and drawbacks, mainly related to drying time, smell, and nozzle clogging. In a nutshell, solvent-based dry faster, smell much more, and clog a bit less ; water based are the opposite. Once dryed, no noticeable difference.
    In any case, practice is he key to get a thin and even coat (along with reading the label/instructions...).

    PS: Note that my varnish suggestion is definitely not a cast-iron guarantee to solve your problem :). It will only create a more or less efficient chemical barrier between the rubber and the photopolymer.
     
  7. freedy
    freedy Member
    OK, I think I'll try a solvent based varnish first.

    I take your point about varnish being 'a more or less efficient chemical barrier between the rubber and the photopolymer'. I would have tried it before but I didn't think varnish could be applied thinly and evenly enough to be an option in my circumstances. You have given me encouragement that it might meet my requirements. It's now up to me to practice and master the techniques involved.

    I'll come back to this thread and report on the results in due course.

    Thanks very much for your help.
     
  8. JoeRalat
    JoeRalat Member
    Has anyone from Shapeways or a member of the forum found an answer to this issue. Thanks
     
  9. Spitfire2865
    Spitfire2865 Well-Known Member
    Ive recently had a similar issue with someone I designed a master for. The part stuck to the rubber and ruined the mold.
    You will have to seal the part unfortunately. Just a thin spray of an aerosol varnish should do the trick and not fill in any detail.
     
  10. JoeRalat
    JoeRalat Member
    Have you done this? I know that I may have to do a post cure cleaning..someone says acetone to clean it..(nail polish?) Doesn’t acetone melt plastic?
     
  11. barkingdigger
    barkingdigger Well-Known Member
    Since 3D-printed plastics are porous by nature, I wonder if your problem is the rubber creeping into the pores? If you prepped the original mould by sanding, this would have clogged the pores, maybe enough to avoid infiltration. But each pour would pull out some of this dust until eventually there were enough open pores to get a grip. Worth having a good look under a magnifying lens...
     
  12. freedy
    freedy Member
    @ barkingdigger

    This is an excellent suggestion which fits the facts as I described them in the original question. I DID sand the printed pieces before making a mould from them, and the moulds WERE initially OK, before the sticking problem gradually developed. However exactly the same problem occurs if I use a printed piece straight from Shapeways which HASN'T been sanded. The first few moulds are OK, then the silicone begins sticking to the printed part until eventually they have to be torn apart, destroying the mould.

    I have been unable to solve this problem. I tried various varnishes but couldn't apply it thinly and evenly enough for my purposes. (I am making tight tolerance engineering pieces rather than, say, figurines).

    In the end I compromised with a workaround. I selected the best 2-piece mould I had managed to make before the adhesion problem built up, then cast a negative of each piece using polyurethane resin. I can now "cast" as many moulds as I like in these resin negatives. Polyurethane resin has a pretty low shrinkage, and I found that both halves shrunk uniformly and consistently so that although my final moulds are about 0.5% smaller than originally intended, they are still a sufficiently good match.

    I appreciate this workaround won't be appropriate for all situations.
     
  13. Silicone rubber products manufacturer use high precision silicone in the manufacturing of silicone 3d model, animatronics, toys, masks, models, otoplasty, soft robotics, adult toys, and medical silicone products.
     

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  14. Anne_
    Anne_ Member
    In the past my prints in fine detail plastic were OK. But I have recently had a similar issue with someone I designed a master for (2x). The model sticks to the rubber. A thin coat of gloss spray varnish does not solve the problem. Has the composition of the Fine Detail Plastic been changed ?
    I also discovered some uncured plastic in these last two models. If that material is released (under pressure, by heat), it will probably also easily attach to the rubber (but that is not the intention!). How can this be solved ?
     
  15. barkingdigger
    barkingdigger Well-Known Member
    If the plastic is not fully cured, try exposing it to sunlight - the curing process uses UV light. Uncured resin mingling with the liquid RTV certainly won't be good...