First cast ever: thoughts?

[u/C22_H28_N2_O]

I want to make a custom tray to hold two pieces of equipment together. My current plan is to cut thin PVC board to match the bases of the two pieces in their desired position as my pattern. Using a pattern means I can go deeper than the thickness of the two steel bases.

The image is a cross section of my plan. I intend to use 30A silicone, and an inverted silicone mold to make the resin tray. I purchased a two part resin epoxy that doesn't need UV curing.

What do you think of this plan as a first time cast? What are some things you'd adjust or change? What are some pitfalls I can avoid as an inexperienced user?

I've read to pour high and slow to help prevent bubbles, as well as vibration to help bubbles rise out of the liquid resin. I'm also planning to hot glue the PVC both to the steel and to a craft cutting board to get a better cut.

Edit: the tray will be asymmetrical, but I've already taken that into account and the resin should be the correct orientation so long as I put the pattern in as if it was the steel plates.

[u/BTheKid2]

It a pretty convoluted way of creating something. And you will not have luck with the first step of your process.

If you want to make a mold of a pattern, you absolutely don't want the pattern upside down and be level. That will catch every bubble that tries to rise to the surface. Instead flip the whole thing upside down. So basically make the entire first step upside down from what you are showing. Then you would have a chance of it working out.

The other thing you seem to be assuming is that silicone will cure flat and level on it's exposed side, and the vertical sides of your thing will be even and flat. Both these things are not the case. The way to make sure the surfaces that should be flat and smooth, will also be so in the final mold, is to make them that way from the beginning. That usually means creating a master pattern (so the model you make the mold of), that is exactly similar in all surfaces, to what you want your end product to be. That is by far the simplest method.

Designing on a computer or sketching in hand, it can be easy to think that making things flat and joints and corners exactly flush or 90° is possible. In reality that is never the case. Well not unless you are working with extreme precision CNC machines. But if you were, then making a mold would not be a hard task.

[u/C22_H28_N2_O → u/BTheKid2]

Thanks for the advice. So instead of letting a layer of silicone cure, and pouring the second layer around the PVC, I should lay the PVC against the acrylic tray and pour the silicone on top of it; flipping for asymmetry.

I was worried it would leak under the PVC because it's light weight.

The vertical walls should cure upside down for the positive mold.

As for a master pattern, that's what the PVC is for. I can't cast the metal plates themselves since they have screw holes I don't want to capture and they're too thin. I figured a PVC plate would be thicker, giving more depth for the mold, but smoother.

[u/BTheKid2 → u/C22_H28_N2_O]

I am not sure I follow your explanation, or exactly what your end product is supposed to be.

What I am saying is, if you want your end product to be what you have labeled as "Resin Epoxy" in step 4, then you should use a master pattern that have the same size and shape as that end product.

So if your master is a thin PVC board, but you want to have the end product look like a thick PVC (or whatever), then make the PVC board thick to begin with. That could be done by bulking out the underside of the PVC with clay or some cut block or something.

And yes, silicone would leak under the PVC, and also make it float, if it is lighter than silicone. That is why you would normally fix a master to the base plate. With something thin, you would probably need to glue it down. And yes, that will most likely mean that the master can't be removed from the base again without destroying it. That is just one of the costs of making a mold. But trust me, that to float the master, or somehow lowering it down under the surface of the silicone, is not something any experienced mold maker would do. Because it just wouldn't work well.

[u/C22_H28_N2_O → u/BTheKid2]

You're not understanding the vision, and that's okay. I laid it out in a 2D slice of an upside down end product.

https://movemaster.tech/cdn/shop/files/Anthrazit_BC_R5.jpg?v=1780221794&width=1800

This is more or less two keyboards. The larger keyboard can be pushed forward ever so slightly over time. And because they're joined at an angle the front keyboard slide forward and to the side when this happens. This means when you blindly extend a finger to hit a key, you hit the wrong key.

I didn't explain any of this because it's all a fuck-ton to explain and people lose interest when they have to read a novel to help you. I'm trying to solve this problem with a resin tray with a depression the exact footprint of the two combined to act as a frame for the devices. I'm trying to solve it this way instead of permanently affixing both plates because occasionally you need access between each keyboard to adjust the keys.

I can't use the steel plates you see them mounted to because that would leave them with the line between them, or the grooves from the screws in the mold. So... The PVC plate acts as the master of the footprint of the two steel plates as one single plate. The reason things look convoluted is because I don't know what I'm doing and I'm coming at this with the mentality that the crafting materials can or should be preserved.

My plan is/was to make one silicone mold, in the original tray, with the depression in the middle (you taught me to place the master on the bottom). Then to trim the entire boarder of that mold, place it back in the same tray (looks like I need a second tray instead) and to pour silicone into that to make a second mold with a release agent to keep the two molds from bonding together. Because the boarder of the first mold was removed, the second mold will have a self contained lip around it, allowing me to use it without having to have the resin cure while touching the acrylic tray. I wouldn't even have to use the acrylic tray from that point on.

The end result would be a resin tray, with the depression of the master, sitting upside down in the second silicone mold to act as a frame for the metal plates of the two devices. Does this help you understand the vision? Does your advice change at all now understanding the full 3D end goal?

[u/BTheKid2 → u/C22_H28_N2_O]

I see. In that case, I would take some clay. Roll it out flat as you can. Then press the keyboards down into the clay just like you want them seated. Then remove the keyboards and make a mold of the clay master. This mold will now produce copies of that clay. It will however look a bit like clay depending on your clay skills, but you could make it oversized so you can sand it flat and neater.

An alternative cleaner process and product would be:

• Use some nice board stock (maybe PVC if you like that stuff).
• Cut out pieces that fit around the keyboard as well as under the keyboard, thereby constructing a prototype of the tray you want to end up with. Glue it all together of course.
• Then make a mold of that and cast copies in a resin.

A third cheaper option would be:

• Don't make a mold, just sculpt it from an epoxy sculpting material.

Just like you would with clay, you can make it from epoxy clay. 2 component clay that when mixed together gives you a working time of about 30 minutes, and cure to a very hard product in 24 hours. The clay is called "Apoxie Sculpt". Comes in different colors and other variants. There is also similar cheaper products you can search for.

The clay would be hard to get looking super good as sculpted, but it sands well and paints well to, if you don't mind shaping it after it has cured.

[u/C22_H28_N2_O → u/BTheKid2]

Your alternative is pretty much what I'm doing. I'm very interested in PVC because thin PVC board is easy to cut but thicker than the steel (4mm vs 2mm). If you have a better board option to trace and cut a master with, I'm interested in hearing it.

This is the second 2D slice outline. https://i.imgur.com/BtmNtCO.png

In that example, the master fitting in the resin tray would be the keyboard steel plate fitting into the resin tray/frame.

[u/BTheKid2 → u/C22_H28_N2_O]

Yeah the thing you are illustrating is however making the master (the thing that produces a the final mold) from silicone (picture 1-3). This is not a good nor easy option. I suggest making the master directly from board or clay. That way you skip step 1-3.

PVC board is fine, if you feel like working with that. I have never used PVC for this, so I can't speak to it. I can suggest better materials (like PU tooling board), but they will not be easy or cheap to source, so it doesn't really matter for a beginner. It also depends on what tools you have available to shape with. But a master can be made of any material, as long as it is compatible with the silicone.

You might also consider, not making the master completely tight with no tolerances (which it would be in the example you are showing). Because silicone shrinks an extremely tiny bit, but resin shrinks a little bit more. Meaning that if your cutout is made complete tight to the master, it will be ever so slightly too small once you have cast it in resin. So if you go ahead with the method you have shown, you probably want to at least apply some tape along the thin edge of the steel. The tape would act to make the cutout, be about 0.1-0.2 mm larger, meaning you would have a tiny bit of tolerance.

[u/C22_H28_N2_O → u/BTheKid2]

I'll take your advice under consideration. Just need to think about how I'll make a master of the entire end product. Or at least a master of the intended frame. Not a fan of the idea of hand sculpting a tray. 😮‍💨

[u/annyshell]

I don’t know about your mold making process, but I can’t say that you need to do some practice pores with epoxy before planning your final pour because epoxy never cooperates exactly like you want it to and there is a learning curve

[u/C22_H28_N2_O → u/annyshell]

I was going to do that for the silicone layers, but that's great advice for the epoxy too! Thank you.

[u/xkayaking]

Silicone guy here, I don't do resin honestly but do silicone to no end.

Recommend for 1st you do the reverse, pattern at the bottom. Make sure to glue the heck out of it with hot glue otherwise anything floats, and do one single pour, make sure the mold has the form you want, don't "cut it".

Second step silicone release spray will also work fine, that one should be done exactly like shown, but you also need to glue the silicone, you see stuff you are not being told?... everyone telling you yadayada about pressure pots, when to make a mold better, you should use vaccuum degassing; and remember you also need to glue the silicone down, but hot glue wont work this time, now you can use, taratara, screws!... just regular woodscrews to fasten the first mold should work with 30A so it won't float, then seal any potential gap just in case with the hot glue, if you dont want to use screws then you can use fishtank caulk, but that might be more annoying to peel.

Then put some good release agent, and make sure to leave the damn 30A at least 24h to dry, because those 30As tend to be slow cure, specially since you should use enough silicone thinner to capture detail.

So other than the first one being upside down, and securing the thing so that it doesn't float in the 1st step and the 3rd step, your plan is fine and sound.

I can assure you it will work provided you turn the 1st process upside down, and secure in both 1st and 3rd cases so things don't float.

Flotation is the biggest issue, trust me; I have done hundreds of molds and casts, silicone, on silicone, all types of silicon.

So things in order of importance.

1. Flip the 1st.

2. Secure everything.

3. Use a vaccuum chamber if possible if not you ought to be generous with that thinner; using a vaccuum chamber will also ensure the material remains mostly uncompressible during your pressure resin cast. I always use a vaccuum chamber even when I don't resin cast, bubbles are a real dealbreaker.

I am sure it will go fine :) seems simple enough, I have had molds with dozens of pieces out of different materials, and often I one shot it, I one shot even my first casts ever; trust the procedure, and it will be fine.

[u/C22_H28_N2_O → u/xkayaking]

Thanks. I'm thinking about using hot glue to act as a sealant around the first mold base in an acrylic tray so that the second silicone doesn't leak under it. I realized the very top of the second mold, which will be at the very bottom of the first mold during curing, doesn't need to have a perfect border since resin will never be poured to the very top.

I'm guessing "taratara" is more like "voila" proceeding your solution to use screws and not an actual adhesive 😂. Can you recommend something other than screws to hold the silicone to an acrylic base? Is there a particular reason hot glue underneath the silicone wouldn't work? In addition to using it as kind of a caulk around the base.

[u/xkayaking → u/C22_H28_N2_O]

When you are first gluing the og part in the inverted step #1, which I assume is not silicon, hot glue works to glue it to the PVC bottom, because you don't want it to float and you can peel it off later easily, hot glue will most certainly work with PVC and your part.

Yeah it's like voila, but spanish thing sorry, it's the drums after the curtains open. 😂

However hot glue doesn't work on a silicone surface reliably, you risk it moving anyway, the join needs to be mechanical, woodscrews will keep it in place nicely a 30A silicone.

But if not. https://cdn.shoplightspeed.com/shops/642439/files/62693882/1600x2048x2/aqueon-aqueon-3-oz-silicone-clear.jpg fishtank silicone will do, or any 100% acetoxy cure silicone with no extra aditives, note this is to bond silicone, the issue is that the bond might be "too strong" to the PVC and hard to undo, whereas woodscrews will simply unscrew.

If you use fishtank, wait for it to cure!... otherwise it will cause cure inhibition.

Another thing you can use is magnets, but you need to somehow keep pieces of iron on top of the 1st mold as it cures.

The point is no matter how you do it, in that 3rd step, the silicone at the bottom needs to be held to the base of the PVC, so as long as you find a creative solution to hold it that will work, do not rely on simple weight, silicone is awfully dense most metals will float or be moved around, it needs to be held securely.

[u/C22_H28_N2_O → u/xkayaking]

A little confusion. Check this image for an updated approach.

https://imgur.com/BtmNtCO

The PVC plate is a master impression for the two devices. Those devices are intended to rest in the end product resin tray. Once the first silicone mold is cast, that acrylic tray and PVC will not be used again.

However, a new acrylic tray will be used to house the first silicone mold. I can't use screws or nails to hold down the silicone to the acrylic unless I drill holes into the acrylic. Do you recommend drilling into the acrylic, or using an adhesive to hold the silicone #1 to the acrylic?

[u/xkayaking → u/C22_H28_N2_O]

The only good adhesive is that fishtank for holding a silicone base, otherwise you need mechanical bond.

Sometimes superglue works temporarily for this kind of load, but it's risky because the type of load.

I don't recommend cutting silicone, make is so that you don't have to cut it.

Nails won't be as good, screws would be better, but again, you can use magnets or get creative. Clips, etc... anything to keep it down and prevent it from floating

https://imgur.com/a/oJylk8F

Just an example of where you could in theory place magnets while casting so they are perfectly embbeded, of course it is a 2d view you could place 6, 8 rows of magnets, long magnets because remember you reverse the thing, and very strong, so they hold well.

But yes you can drill holes in the acrylic, that's the point.

The only thing you need to make sure is that it doesn't float, because that's the biggest issue.

[u/Dependent_Occasion65]

Without a pressure pot, there will be bubbles. A vacuum pot can get bubbles out of the resin, but more will form as it cures as a byproduct of the chemical reaction. A pressure pot stops the bebbles from ever forming if cured within a pot of about 35psi. Another byproduct is Heat. Depending on the resin and the amount being poured at once, the reaction can get very hot, even to the point of smoking, burning the mold, and even igniting. Work in a ventilated area. Have a plan for thermal runaway(the burning described). Get lots of nitrile gloves. Exposure on skin and breathing the vapors is harmful and can lead to becoming severely allergic.

[u/Dependent_Occasion65 → u/Dependent_Occasion65]

Also, I hear a lot of mixed results with release agents for silicone. I have always just used a thin layer of Vaseline or any petroleum jelly. Super cheap and works perfectly. It is only needed for releasing silicone from silicone. The epoxy should release from the silicone mold without an agent.

[u/C22_H28_N2_O → u/Dependent_Occasion65]

Thanks. I bought a release spray specifically for this, but I'll test Vaseline too. I'll also test a small scale model to evaluate reaction, bubbles, and technique.

[u/Staaaaation → u/Dependent_Occasion65]

Just to be the pedantic asshole, pressure pots don't take bubbles out of resin. They compress them so small so we can't see them with the naked eye.

[u/Dependent_Occasion65 → u/Staaaaation]

That's what I said "stops bubbles from forming"

[u/Staaaaation → u/Dependent_Occasion65]

Right, and that's why I said it doesn't. They're very much still there. They just compress instead of expand.

[u/Dependent_Occasion65 → u/Staaaaation]

Whatever dude, try saying something useful next time.

[u/C22_H28_N2_O → u/Dependent_Occasion65]

The kind of resin I bought says low vapors and beginner friendly, but I bought an organic vapor respirator just to be safe. I have mechanics gloves and I'll wear long sleeves too. 🙂‍↕️

Edit: and I have chemistry eye protection PPE.

Edit²: noticed I'm downvoted and thought about it. To clarify, nitrile mechanics gloves. Thicker than hospital gloves, but disposable.

[u/Claerwen94 → u/C22_H28_N2_O]

Perfect, never ever believe these "low vapor" claims, they're 99,9% bullshit and misleading, they're just using a legal loophole so they can claim low VOC contents.

[u/Shalfeus_megalocase]

Is there a reason you're doing a two-stage silicone process (silicone into silicone)?

Casting silicone into silicone is risky — even with release spray, they can fuse together, and the soft bottom mold can easily warp under the weight of the second pour, messing up your dimensions.

It’s much easier and cleaner to go straight to Step 2:
Build your positive master pattern first (with the PVC pieces glued to the base where you want the recesses), construct walls around it, and pour ONE silicone mold. You’ll save half the silicone, avoid deformation, and get a much crisper resin tray!

[u/C22_H28_N2_O → u/Shalfeus_megalocase]

Check out some replies that go into a bit more detail as to what I'm building and why. But long story short, I'm building a tray to hold something specifically in place. That means the silicone mold needs convex portion to allow for the resin to have a concave middle.

I could have my master be a frame that creates a concave impression around the intended convex portion, but that would be a lot less perfect as I'd have to make either a single frame around the device, or I'd have to literally construct a frame which would reflect all of the imperfections of the hobbled together nature in the mold.

[u/13thmurder]

I find mold release spray is kinda useless for silicone/resin.

Resin demolds from silicone just fine without spray.

If you want to make a 2 piece silicone mold, release spray isn't enough to keep it from bonding. Vaseline that's been warmed to a liquid state and brushed on works way better.

[u/C22_H28_N2_O → u/13thmurder]

Thank you. I am planning a silicone to silicone mold to achieve a negative to positive mold, then resin. You're the second person to recommend Vaseline, and the first to recommend heating it to liquid state. I'll definitely tests that.

[u/burtsdog]

One issue I see is that the top flat area of your epoxy resin is not going to be flat. When you leave an area like that exposed you assume the poured resin will self level, but often it will not. The edges will wickup onto the silicone and the top surface may end up uneven. You'll have to sand it flat. If you don't want to sand then you'll need a two piece mold.