Silicone Mold Making Help

Hi all,

I’m looking to make some silicone molds from a few 3-D master prints. Does anyone have any experience with making 3-D models that are ready to be used as a master for pouring molds? I’m looking to make a mold of my first geometric planter as a test run. The only thing I’m missing is a base and a support shell around the planter to pour the mold. I’ve been looking into using melamine boards in place of a 3-D printed shell for it to be more cost effective. My problem is figuring out how to fasten the 3-D printed master down onto the melamine without it moving and building a mold box around the master. Ultimately I will be using the molds to cast resin, concrete, etc. A friend suggested designing the prints with holes on the bottom to allow for a screw to hold it in place on the melamine board. I would like to avoid glueing the master to the board in case I need to be able to take the master off. I know this all sounds a bit confusing but I’m hoping that some folks on here have a bit of experience in mold making will know what I’m talking about. Here are a few photos of my planter and examples of a mold box and support shell from other people. I’ve been struggling with getting my planter printed since I need to figure out these steps first. Please let me know if you can help. Thanks in advance!


Are you looking to make a one-piece mold like this?


I would go with the melamine and use superglue to hold down the master. It also needs to be smooth enough to prevent “bleeding” underneath. As your friend suggested, you can also leave holes for screws, but these details will show up in the parts as well. Maybe make the base a little thicker and drill and tap blind holes from the other side.

Yes, it will most likely be a square box made up of melamine with some blind holes from the other side like you mentioned. My friend said he used to screw them in and used clay to cover it up before actually pouring the mold. I will make the base a tiny bit thicker and will add a blind hole for a screw and see how that works. I’m just wondering what type of hole to make and what type of screws to use. I have zero knowledge of anything to do with carpentry. Thanks for your input!

How large is your planter?

It’s 4" across and 3" tall. The bottom thickness is about 1/4 of an inch but I might make it thicker if it needs to be for the blind hole on the bottom.

I’d get some Plasticine and stick it down. There are different ones that are compatible with different mold materials. The one that contains Sulfur for example is no good for Rubber Molds.

Ahh ok, I did think about using some type of clay but I want the bottom of the master to sit flush on the melamine board with no gaps or spaces as it will translate into the silicone mold. For example like in this photo here:

I would be worried about the clay at the very bottom of the planter being translated into the silicone mold so that’s why I was leaning towards the blind hole and screw approach.

Regardless of how you fix it in place, the bottom of the master needs to be smooth as possible. I would still opt for a couple of drops of super glue. If you need to remove it, a knife edge will pop it loose.

I don’t know your setup, but in a former life we used a vacuum bell jar to remove the trapped air bubbles from the silicone and urethane when they were mixed. We also over-filled the molds and placed them in a pressure chamber at 120 PSI to compress any bubbles in the urethane that was created when filling the molds.

It’s a lot of fun, actually :wink:

ahh, ok. Yeah my friend advised me to stay away from gluing since he said I may need to remove the master in the future. I’m worried it will be hard to remove even with a few drops of super glue. Is there a specific type that you have used for this application? Right now I don’t have access to any vacuums or pressure chambers so it would just be cured the normal way. I have heard of people using them but I didn’t think it would be necessary just yet. I’ll be using Moldstar 15 Slow which should yield great results.

You could drill a hole and counter-sink it on the inside. Make sure that the counter-sink is wider than the width of a flat-head wood screw so that it sits below the surface of the bottom. Fasten it down and then cover the screw-head with clay. Or just leave the detail as a feature in the bottom.

Yes I will most likely try this method or go with the blind hole and screw it from the other side of the melamine board. What method or type of screws would you use for the blind hole if you’re screwing from the bottom side go the melamine board? I’m wondering if I should design a pilot hole into the bottom go the planter or just drill them? I’m wondering if a 1/4" thick base would be thick enough to do this.

I’m a little confused - if you are going to cast resin, won’t you need to leave a hole in the silicone mould to pour the resin through? So the place where the hole will be is the place where you secure the master. I use Blue-tac or clay. I use lino for the shell, since it is easy to cut with scissors and join with glue or tape, and I secure it to the base (which can also be lino) with putty. Sometimes I secure the master with thread as well to give added resistance to movement (the thread can just poke through the lino with a needle and be secured on the outside, and it pulls straight out through the silicone afterwards leaving no hole).

Hi there,

The planter I have above will be fastened down to a melamine board with a screw to creative a positive mold. The bottom of the planter will sit flush against the bottom of the mold box hence the reason I won’t be able to use clay to secure it due to the fact that these details may transfer over to the mold. The hole or opening that I will pour the resin or concrete in will be at the bottom of the planter and mold box. So when the mold is done curing, I will take a part the melamine box and turn the silicone mold over to use for casting. Does this make a little more sense now? I’m not quite sure the method you’re using is right for this but I would love to see pictures of the process. Thanks for the input.

I guess I’m not quite following your plan. If you want the positive to sit flat on the floor of the mould box, then are you planning an open face casting afterwards (i.e. there will be no silicone underneath)? if so then I guess, if I’m understanding right, your problem is that clay will lift the positive off the floor a bit and silicone will leak underneath and have to be trimmed off, which would be a bit ugly.

If you are instead trying to make a fully enclosed mould, with silicone all around the model and pouring the resin through a small hole, then the place where the hole will be can be the place where your clay or putty goes. I’d probably make three holes (sprues) because then the positive can be supported on three legs of clay. and the three small defects on the underside would look neater.

I’ve attached (or tried to) a picture of my lino shell and plaster cast, and then the mould as it came out after with the blue-tac in the top (which was the bottom during casting). That was then pulled out to reveal the hole. I’ve also attached a photo of the thread securing method. Hope it helps!


Yes exactly, the mold will have an open face casting. I’ve seen this done so many different ways but I will give the screws a try and see how it goes. I could also have a 3-D printed base and shell or box designed specifically for pouring molds but I figured the melamine boards would be cheaper.

Sounds complex. if it’s open face I’d probs go with the advice to use glue. Good luck with whatever you decide!

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