Advanced Mold Making Suggestions


New Member

I've been piecing together my first full build, an Ubersaw from TF2.

I've gotten over a lot of the hurdles, but after filling, priming, sanding in more iterations than I can count, I realize I don't want to have to do that again for when I return its twin (red + blue teams).

My question is do you have a suggestion on the viability of making a silicone mold of the main body? Current state seen:
20141221_180647.jpg 20141221_180805.jpg

I've made block and simple 2 part molds before, but the syringe cap (cylindrical piece molded to the blade) proves to be a problem since it is hollow inside and sealed on the back. it is hollow so that it will accept acylic pipe that will serve as the syringe's body.
IMG_20150721_204303904.jpg IMG_20150721_204324596.jpg

I see that a 2 part mold with the blade lying flat on it's side would be the best option, but I fear that attempting to fill the syringe cap will just create a tight vacuum seal and wreak havoc upon removal.
The best options I see before me:
1) Still a 2 part mold, but allow one side of the mold to fill the inside of this syringe cap. Think a big thumb of mold rubber reaching from below the blade into the syringe cap to plug the inside of the mold. Reasoning: one piece of thicker silicone will be stronger, more resistant to tearing. Likely strongest mold, but also strongest vacuum

2) 2 part mold right down the middle, including the inside of the syringe cap. reasoning: if the silicone is already divided in half, the vacuum between silicone to part won't be perfect, and can be worked around. Weaker mold, weaker vacuum.

3) Attempt to remove the syringe cap, and mold it on it's own. the blade becomes safer to mold, but the issue of molding the syringe cap still suffers from the vacuum problem. Attempting to match up from syringe cap to blade will be imprecise.

If you've read this far, thank you.
Sages of the silicone, do you have any experience you can share about molding a shape like this?


Well-Known Member
Best bet may be to create a plug for molding and casting purposes. Something about the size of the mod podge container you've got in the photos.

Need to find/build something solid that fits snugly inside your cap with half sticking out and possibly a gentle raised lip around the exposed section so it can 'lock' into the silicone and not slide out during casting.

You mold the piece with this plug inserted to make it an easily moldable shape. Demold and remove the plug, wax the hell out of it and reinsert the plug into the mold on each casting. It needs to be made of something sturdy and smooth so you can wrench it out of the castings - which may be the only snag to this process based on the geometry - it looks like there will be a lot of surface area 'grabbing' the plug. There are surely other good ways to do it too, this is just one option.


Master Member
As @Logan74 has suggested and possibly consider making that tube as a separate part.
Last edited by a moderator:


Sr Member
Hi There Tacocaserole, and welcome to the forum.

Before I go typing away, I want to make sure that I understand the problem.

1ACapture.PNG Is my circle the problem? Is this the Syringe Cap?

As I understand your issue, you would like this to remain concave, and accept an acrylic cylinder after casting?

My solution to this would be to mold the gun with the acrylic tube in place. Once the model is de-molded, and you have two silicone halves, use an identically sized acrylic tube, and place it into the mold cavity and close the mold. The acrylic tube will stay in place by friction, and when the material is cast into the mold, it will flow into all the cavities NOT occupied by the cylinder,

Once the part is removed from the mold, the orange acrylic cylinder is in place, and there may be some flashing around the surface of the cylinder, but it is easily removed. (even easier if you add some release to the outside to the cylinder.

This would be similar to the "Plug" method described above, but eliminated that step.

- - - Updated - - -

Oh, BTW, why is this part not in the model? Will it be cast separately? 1a2Capture.PNG


New Member
Thanks for your help! I hadn't considered those possibilities before. There will have to be some testing.
@logan74k, I think this would also allow for me to put a heavy wire in the plug, removing which could allow for a easier release of the vacuum. I'll need to go back and sand down the inside of the syringe cap. it is currently very rough from it's 3-D printed origins.
@clonesix You understand completely. The circle you drew is the open end of the syringe cap. I think your suggested process would prove simplest, If I were to mold it with the acrylic tube in place, the molde rubber would provide a solid resting place for the tube, so it would be easy to replace for castings.

You are right again in your assumption that the syringe's needle end is currently separate from the rest of the piece. I have a separate block mold for the plastic base of they needle, and then the needle itself is a cleaned up wooden dowel. With your suggestion, this part could also be cast at the same time, as a whole set. Although, then the acrylic tube would have little more than the surface grip on its circumference to hold it in place, and it would require the acrylic tube to be capped at both ends. Not difficult, just an extra step. I would also be concerned about the resin flowing into the needle end of the syringe properly. It does make contact with the blade, but not much. I had originally though to orient the mold for pouring so the thick handle is up with the blade point straight down. (think sticking a knife into a table). In this orientation, would you expect an airpocket to form at the farthest corner?:

I had originally planned to cast them separate thinking gluing the parts together would hold the acrylic tube. I'll mull that over as I finish cleaning up the parts.... intriguing....


Sr Member
1aaneedbleed.PNG Please excuse the bad lines drawn with a track-ball mouse, but I think this shows pouring orientation.

Air gets trapped in high spots and tight spots. Adding bleed lines in the areas shown will reduce any trapped air and yield a cleaner casting. The sprue clean up should be minimal.


New Member
Thanks again for the help,
Cleanup is going slowly but, well.

I'm curious about how harshly the resin will adhere to various materials. Obviously the rubber mold(with adequate release agent) will not stick to the resin. If I were to cap the bright red tube (made of extruded Acrylic) and cast the rest of the body around it as clonesix mentioned in post #4, would mold release be adequate to stop the resin from adhering to the acrylic, that I could then remove the acrylic tube for final cleanup without destroying the resin casting?
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