Using Two-Part Silicone to make a Repro-One (Cast Plastic) Pattern

Discussion in 'Sand Casting' started by Melterskelter, Feb 12, 2023.

  1. Melterskelter

    Melterskelter Gold Banner Member

    One of the patterns that I use over and over for casting I feel should be reproduced in plastic to save wear and tear on the pattern. The pattern itself represents many many hours of initial fabrication and many more hours of tweaking so that I hate to think of having to remake it should it come to harm.

    I decided to make a silicone negative of the pattern and then, using the negative, pour a Repro-One positive.

    Kelly advised me a long time ago to be cautious when using RTV-2 platinum silicones watching out for inhibition due to mold coatings and compositions. He was very right. Here is a partial list of what can cause cure inhibition:
    • Chlorinated solvents
    • Acetone, MEK
    • Adhesive tape (e.g., duct tape)
    • Coatings, paints, solvent carriers
    • Clays containing sulfur (ask the manufacturer for sulfur-free clay)
    • Sulfur-cure organic rubber (e.g., neoprene or natural rubber)
    • Latex gloves (use latex-free gloves)
    • Amines, epoxy, TDI urethanes
    • Gel coats
    • Bondo
    • Polyester paints
    • Condensation-cure RTV silicones, silicone or urethane caulking
    • Composite pre-preg
    I will add to the list some common 3-D Printer Resins---I found severe inhibition with a couple that I tried. A coat of XTC-3D solved that and maybe other epoxies would also work well given the fact that they are relatively inert

    I did run into cure inhibition with various coatings and did find that Smooth-On makes two products that do seem to really help prevent cure inhibition. Both their Inhibit X (a thin liquid) surface prep and XTC-3D epoxy both seem to prevent cure inhibition. So, I did carefully test my mold setup before mixing up 15 bucks worth of resin. Not only did I not want to waste the time and money on casting the silicone negative, I also did not want ot spent an hour cleaning off partially-cured gooey silicone from my pattern.

    My pattern was made from baltic birch (nat an inhibitor) painted with shellac and lacquer (both inhibitors) and resin printed lettering (an inhibitor). I found that first coating the resin lettering with XTC effectively prevented inhibition on the letters and applying Inhibit X and generic spayed silicone mold release [prevented any trouble with inhibition. To hasten curing of the 6-hour cure silicone I placed it in a 90 degree F warmer.

    The result was an excellent negative that I will soon use to cast Repro-One. The whole thing was cured well at 4 hours. Separating the negative from my pattern took some judicious prying and a burst of Dust-Off through a .090" hole in the pattern. The pressurized gas did initiate separation. After some puffs of gas I was just able to get the negative out of the pattern. (My nightmare was getting stuck with a "permanently" attached neg in my pattern.)

    On thing I had to figure out was how to seal the apcae between the pattern and a frame that holds it. Any of the masking and duct tapes I tried were potent cure inhibitors. But, I did have a roll of HVAC aluminum tape---basicly heavy aluminum foil with a strong adhesive backing. That tape worked well as plain aluminum does not inhibit silicone cure.

    Here are a few pics. You can see the seam between the white pattern and the greyish core box that I use to make cores for this pattern. I used the core box to make the negative casting by taping up the seam. Mold and Silicone negative.JPG

    The back of the negative: Negative Back.JPG

    Front view Overview.JPG

    Side view. It looks like there is bare wood showing on the sides. But that is an illusion as the silicone is nearly transparent.: Side View.JPG

    The silicone renders excellent detail: Redition of silicone.JPG

    This is the silicone I used---not the cheapest and by no means the most expewnsive out there. It did work better than another bran I purchased on eBay. I did try some MagicFly brand silicone and found it to not cure to nearly as strong a rubber as the Sartso did.

    I did try another brand that I did not think cured to nearly as strong a rubber. That was MagicFly brand. I cost 30% less. But I would not recommend it.

    I hope to cast the positive in this mold in the next few days. I have found the Repro-One very easy to work with in the past. My expectation is that it will make a very rugged and durable positive that I can use without worry about damage.

    Last edited: Feb 12, 2023
  2. Melterskelter

    Melterskelter Gold Banner Member

    Just for the general knowledge of the group I will post an inquiry I made to Smooth-On concerning Inhibit X:
    Denis Foster 02-12-2023 1:10pm

    I see that your website cautions tha Inhibit X has a short shelf life. Having purchased a can recently and havingfound it quite helpful for Platinum RTV-2 molds, I am wondering how to mitigate the shelf life problem. I suspect the issue may be evaporation or it may be some sort of in-the-can chemical reaction similar to that seen with urethane products. If the latter, would topping off the can with inert gas prior to sealing it improve shelf life? If it is an evaporation problem, are their ways to reduce that issue?

    Thansk for your attnetion,


    [​IMG] Rick Higgins, Material Specialist 02-13-2023 9:32am
    It's a multiple tiered issue, yes evaporation is one issue as well as half life of some of the raw materials. I usually seal it in a plastic bag after I tighten the lid good.
    I followed up:
    "Thank you.

    How long should expect the shelf life to be in a well-sealed can at 72F?

    Could topping off with Dust-Off or argan harm the product and could it help by eliminating O2?

    [​IMG] Rick Higgins, Material Specialist 02-13-2023 10:02am
    As long as you decant what you need, quickly seal it up and store correctly there is no reason you couldn't get several months out of it... as far as the topper stuff, nothing will affect it positively"

    So ,unanswere d is whether topping off with Dust-Off would be helpful. I guess the tech is suggesting bagging the closed container---that seems like a relatively weak way to address the evaporation issue. Maybe taping a lid-to-can "seal could help. I recall no internal pop-out cap that they had placed under the screw-on cap. Seems like I often see such a pry-out on volatile paints and similar. Anyway, that is the response I got and I thought I'd pass oit along FWIW.

    Last edited: Feb 13, 2023
  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    We all have scars. Always wise to first try a small sample on your combination of materials. I'm an impatient sort but the combination Silicone expensive and the labor to recover a pattern full of uncured Silicone is enough to activate what little patience I have.

    Last time I was bit was making a very small mold for a wax embossment. Sampling and molding were about the same, so off I went. That was a 3D resin printed pattern. I didn't have UV source to try additional curing of the print. After several tries I was never able to seal it well enough to use Platinum/Pt/Addition cured. Ended up with Tin/Sn/Condensation cured.

  4. Melterskelter

    Melterskelter Gold Banner Member


    I do not know what methods you used to seal the resin print. I also found even a well-cured resin print to be badly inhibiting of platinum silicone linking. But, I also found that a single coat of XTC-3D solved that issue completely. I did not try Inhibit X on a cured resin print by itself. I am suspicious it MIGHT be enough. It sure did solve my lacquer inhibition isssue and the wax inhibition issue. I'd sure like to know what is in it. Proprietary info unfortunately.

  5. rocco

    rocco Silver

  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I've never had any problem with either lacquer or wax,as long as the lacquer is fully cured and the wax is buffed with no residual carrier.

    Never used it but may if the need arises in the future.

    Routinely with resins in non-self releasing molds but not when pulling a silicone molds because even sprayed, it degrades the pattern finish and silicone is definitely capable of replicating the degredation, and one of the beauties of silicone, it's self releasing on most surfaces......but dont use shellac!

  7. Melterskelter

    Melterskelter Gold Banner Member

    There is lacquer and then there is lacquer. As a car guy (I wanted to say NUT!) you may be referring to acrylic lacquer commonly used in automotive finishes as "lacquer." In theory they are supposed to be good with platinum silicones. But, I use a "true" volatile-based evaporative lacquer on my patterns. The brand is Rustoleum Specialty Lacquer. I found it to be highly inhibitory even though it had been months since its application. I bought some acrylic lacquer but may not have allowed it ewnough time to fully cure/off-gas as I had significant inhibition with it too.

    I would not use on PVA on a high-rersolution mold for the reasons you listed.

    I am not sure how Johnson's Paste wax fits into the picture. I may try applying some to a piece of aluminum and doing a small test batch.

    Last edited: Feb 13, 2023
  8. mytwhyt

    mytwhyt Silver

    How long is it , and how thick do you think the cast resin should be?

  9. Melterskelter

    Melterskelter Gold Banner Member

    About 18" long.

    I plan to build a Baltic bich plywoood shell and pour the resin into it expecting the resin and shell to bond. I am going to approximate maybe 3/8" resin and 3/8" p[lywood for a total of 3/4" thickness. I really do not know if this is adequate. But it seems like it should work. The Repro One is pretty robust material on its own. I think the combination of plywood and Repro should hold up to the abuse it will be subject to in making cores. Time will tell. If someone has some experience to suggest that these dimensions are not appropriate, I would like to hear about it. I would guess that the Repro on its own without a backing would do Okay at 1/2" wall thickness. So, puting them together should work----I hope!

  10. Melterskelter

    Melterskelter Gold Banner Member

    I did a bit of patch testing concerning the platinum silicone I am using and inhibition. I tested plain aluminum which I knew from prior experience was OK as well as patches of aluminum covered with a coat and buffed Johnson's Wax, Minwax, and PETG filament. All were OK. FWIW.

    Chazza likes this.
  11. Melterskelter

    Melterskelter Gold Banner Member

    And to add to the list of non-inhibiting materials that may be useful:
    Plastilina Modeling Clay by Sargent
    I plan to make a split pattern out of my followboard/pattern for my 36" camel back by making silicone negatives and then casting each half in Repro One. I will use this stuff to fill crevices between the follow board and the pattern. More on that later.


    Attached Files:

  12. Rocketman

    Rocketman Silver Banner Member

    Can I ask, why not use Repro for both the negatives and positives?
  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I do, but it can depend on the size of the mold, and doing so requires a very good pattern finish and more mold prep, but that usually means the repops can be used as patterns essentially as cast/molded so especially worth it for multicavity molds. When molds get big, silicone gets very expensive. Even with urethane/Repro, I make a wooden backer to minize the thickness/volume of urethane to conserve on cost of material........wood/mdf is much cheaper than urethane.

    The other factor, there's eseentially zero risk of damage or pattern adhesion (or worse) with silicone so if it's a precious pattern.......

  14. Melterskelter

    Melterskelter Gold Banner Member

    The short and most honest answer is that I am chicken. What you are suggesting is very doable in theory and I am sure also in practice. BUT, the pattern I am reproducing is one that I have been using since 2019 and was not originally made with Repro copying and casting in mind. And, to me, it is very valuable. I would hate to have to remake it from scratch.

    The risk in using repro to make a negative mold is that Repro is extremely adherent should you have any defect in your release agent application or should there be any area where draft is imperfect or a surface defect might allow Repro to insinuate itself and harden. If any of those conditions occur, removal of the very strong and rigid Repro would likely result in significant damage to the original and might be catastrophic.

    Two part silicones, on the other hand, are inherently non-adherent (I did do a confirmatory patch test on the silicone prior to pouring this mold, though) and the one I am using is only gel-like in the rigidity. So, they can be easily removed even if the original has very poor surface prep and has a lot of areas of inadequate or even negative draft. For that reason using silicone to make the negative is quite safe and relatively simple. Both materials reproduce detail exquisitely.

    Finally today I had gotten the wood backing/box for the repro made and sealed with simple peroxide one-part silicone to the silicone negative mold I made the other day, After spending about a half hour with a drill and agitator getting the settled Repro One back into a suspension (it had been in the closet for more than 6 months and settled out badly), I mixed up about 6 ounces of Repro and poured the mold. One hour later it was ready for demolding. And the results were very good. Demolding was not very difficult as the silicone not only releases easily but it also flexes in a way that greatly aids demolding.

    Here are a couple pics:
    The interior of the mold. The wall thickness of the Repro is about 3/16 to 1/4"---I'm cheap and pretty stingy with the Repro.
    Repro Mold1.JPG
    A detail of the interior. You can see less than perfect ridges around the 3-D printed date plate. I was aware of those previously but they were too fine to reproduce in the sand/silicate cores. But, I am sure that they would have made separation of a Repro negative mold a nightmare. I will probably smooth them a bit as the Repro is quite easy to sand and tool.
    Repro Mold2.JPG

    The flip side of the mold. Baltic birch makes a very stable and pretty strong backing. I plan to reinforce the 3/8 ply with another layer of ply and I will need to build up around the top surface of the mold to complete the core box. I anticipate this box will be subject to plenty of abuse over the next few years. I will be so glad to no longer risk damage to the original pattern with each core I make from now on. I think I will make a second core box from the silicone neg I have. That will allow me to make two cores together rather than making one, allowing it to cure, and making another as is my current routine.
    Repro Mold3.JPG

    I will post some more later as I finish this mold into a functional core box.

    I do feel some relief that this step did go well.

    Chazza and Rocketman like this.
  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That's for the birds. Get yourself a paint shaker (or befriend someone at a paint or big box store that has one). I bought one of the pnuematic Hula Girl models 20 years ago. It can handle up to gallon cans and you simply mount the can (upside down is best) turn it on and walk away for 15 minutes. Even a fully settled Repro can will be mixed better than ever. Faster if you pry up a few chunks with a screw driver first.

  16. Melterskelter

    Melterskelter Gold Banner Member


    The original “Hula Girl” shaker is no longer in production I gather. It appears there are many current models derived from the original. Do you favor one or two of them? There is also a wide price range. Mine would likely only be used a few times per year. Are they really noisy?

  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Dunno, haven't looked in years. Links? In general I'd get the most powerful you can afford, and power to shake loose settled repro versus just mix paint.

    Mine sounds sort of like smacking a large plastic handled screw driver on a full gallon can a couple times per second. Not loud like a router but you probably couldn't talk on a phone if you were in close proximity. Need to make sure you have the compressed air capacity if pnuematic, but they aren't as big of air pigs as most other pnuematic tools.

    Melterskelter likes this.
  18. Melterskelter

    Melterskelter Gold Banner Member

    I finally got around to finishing up my core box. I was not totally happy with the date and nameplates. So, I routed them out and glued in new ones that I resin printed with a bit more relief in the letters. And there was some hand sanding to make sure all the surfaces were free of any irregularities that might make release troublesome. So, tonight I packed it up with silicate-bound sand and let the propanyl carbonate kick off the silicate. Happily and surprisingly, frankly, it drew cleanly. I really did expect that there would be some ditzel somewhere that I would have overlooked despite going over it and over it with my fingers to feel for trouble spots. This time it worked out. I'm happy happy!

    Tuesday looks like there may be a short dry spell. So, i plan to give this and another core a whirl then. I'll take a cople pics of the box and core tomorrow. I really like not having to use the original pattern to make cores as I worry about the wear and tear on that pattern. Next step is to make a copy of the other non-core side of the pattern. Then the original can be retired and used only if something happens to the copies.

    Tops likes this.
  19. Billy Elmore

    Billy Elmore Silver

    We used repro quite often in the past. The best thing I have ever used to release it was just plain old SC Johnsons wax. Apply a coat, let it sit for a few minutes then wipe it it three times to make sure everything is coated nicely. Never had any issue with wax. It makes super negatives and positives!
    Tops likes this.
  20. Melterskelter

    Melterskelter Gold Banner Member

    I know I should be able to do that Billy and it is good to hear that it really does work when folks know what they are doing. I just was afraid I'd screw up my pattern. Repro is for sure great stuff. Unfrotunately, Johnson's wax is no longer made. But I think Minwax is pretty close to equivalent, at least in performance. I have a can of each in my shop. And I use them interchangeably.

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