Latex casting (or something similar)

I have a hall pass which I made using a 3D printer. I like the shape of it, but I’ve found that it’s not as durable as I’d like: It sometimes gets dropped on hard tile floors, and (even though it shouldn’t) sometimes even gets thrown or kicked. So it’s chipping and wearing on the edges.

I think that it would be more durable if, instead of PLA plastic, it were made of some sort of rubber-like material. So my thought is to print a new one, and then use it to make a plaster mold. I’m pretty confident that I could make a two-piece mold that would pull away from the piece (both original and cast)… but then I’m stuck. I thought I had read somewhere that latex could be melted in a microwave and then poured into a mold, but looking around online, I’m seeing cites that it’d need to be re-vulcanized if I did that.

I’m also finding a lot of information about latex products which are liquid at room temperature, and which can be brushed or sprayed on and then solidify, but those seem to be designed for making casts of things (like human body parts) that can’t tolerate heat, and it also seems like they wouldn’t work well for filling in an entire mold.

What’s my best option here? What raw materials should I be using, and where does one get them? If I want it to be a particular color, should I get the material already colored, or mix some sort of pigment into it myself?

How about polyurethane?

Hall pass?
Are you in high school?

If I had a 3-D printer I certainly wouldn’t be wasting it on hall passes–even if I were still in high school.

Yes, but on the other side of the desk. Why would a student be making a hall pass?

Oh my, you have my sympathy.

I’ve only cast metal, and a bronze hall pass could be deadly. But with regard to casting rubber or foam, you’d do far better to get a set of chemicals that you mix together, and results in the material you want.

I imagine the flexible foam on this page might be what you’re looking for: (ewwww, bizarre flesh colored hall pass)

To forge yours, of course.

But really, I’m posting to mention that they have pigments at the bottom of the page I linked to. I’d try to figure out how to get a distinctive swirl effect, to combat counterfeiting. :smiley:

Honestly, if any student ever actually counterfeited my hall pass, I would consider myself an unqualified success as a STEM teacher. Especially since most teachers just use little green slips of paper. If I tried for a swirl effect, it’d be just because it’s cool (and that’s something I hadn’t thought of before; thanks!).

scabpicker, it sounds like you’ve had experience with something like this (or at least, with attempting it), and have run into difficulties. What difficulties did you have?

For what it’s worth, my plan would be to separate the halves of the mold, pour in a little of the whatever into both halves to fill in the fine details, then let that solidify and put them together and fill in the bulk in between.

Well, like I’ve said, I’ve only cast metal. There’s lots of things that can go wrong with that depending on the method you’re using to force the metal into the mold, and what detail you require in the piece. That is something I don’t think you’d have to worry about with rubber/plastic as long as you started with the binary chemicals that swell to fill the mold. They pour in, and then expand to fill the mold themselves. Plus, they don’t burn holes in protective gear.

The only one that would probably apply to casting in plastics from a mix is not putting enough vents in your mold. Make sure any fine detail has a vent so that an air bubble can’t get caught in that part of the mold. This video has a what I think is pretty good walk through on planning out a mold and casting in rubber. They made a vent for the trigger guard of their rubber gun.

As far as making two pour molds and attaching the two halves together, you might have to pick your material carefully, or just epoxy them together. The plastics usually have to cure together to make a solid mass.

Silicone is a better mold material for what you want than plaster, IMO. Here’s a nice guide to how to do a two-part mold for casting resin. Check out his other videos as well to get a good sense of what’s possible. If I were in your position, BTW, I’d go with *harder *plastic, not a softer latex.

The reason I’m thinking a flexible material is that the damage to the original has mostly been brittle chipping, not abrading.

And it had already occurred to me to make dimples in the mold halves for registration, but it hadn’t occurred to me to make the mold in two separate pours. That might be more difficult for my piece anyway, since the natural seam line is nonplanar. The way I was thinking of doing it was to attach plastic wrap to the edges of the piece, such that the plastic wrap would separate the mold halves.

On the other hand, of course, I could also do the molding digitally, and just 3d print the mold itself. Though there, of course, I’d be at the mercy of the fidelity of the printer for how well the pieces would match.

If you don’t mind the risk of slightly cutting your master (and since it’s just a print, I guess you won’t mind) you can just pour a solid block of silicone around and then cut it into two halves afterwards, as well as cutting a pouring gate and overflow vents into it. You don’t even have to cut it into two separate halves, if you can cut enough down the sides to get the master out. Molding silicone is very flexible.

I once spent some time trying to teach myself casting, wanting to make replicas of fossils. I got pretty good at making complete molds of 3D objects, then casting them in resin and a low-melt alloy. I was hoping to work my way up to silver (but a family health crisis interrupted me,and I never got back to it) but I started with the safe and simple stuff. What you will want for making the mold is a binary molding compound. This is one type that I used–it is a pair of viscus liquids (a little thicker than maple syrup, IIRC) that you mix together. (I realize that you won’t want 45 bucks worth for the same project, but you may find cheaper similar items such as this, and you might want to play around with it for more stuff, too.)

There are also binary putty options. ([+[Non-Brand]+[PLA]+[Shopping]&adgroup=[PLA]+[Shopping]+Category±+Tools&kwid=productads-adid^99554295364-device^c-plaid^89096652020-sku^H205252BS-adType^PLA"]Smaller amount]([ADL), bigger amount.) What I would do with a thin object like a hall pass is take some of the putty and flatten a piece on a piece of cellophane or similar and press the pass into that. Then paint a thin coat of petroleum jelly or similar over the surface of the putty and put on the top layer. You can also hand-mold in the registration marks. After you have your molds, you can then punch a fill hole in the top mold, and fill it with the substance you want to make pass out of. (You’ll have to close the two halves tightly, or course. Rubber bands and a few books on top will do.) Depending on what kind of material you use for your pass, it may come out easily and you can reuse the mold,or it may require you to crumble away the (pretty fragile) mold and redo the whole thing if you need more passes. Micromark is a good source for the various stuff.

I mentioned resin–this is what I used (along with a catalyst that you buy separately.) The resin is brittle, so isn’t suitable for the passes, but might be useful to you for something.

Too late to edit, but Here is one of the two-part putties that would be very useful for you. BUT hard molds only work when you have “simple” shapes–no bumps or grooves along the sides that would get stuck as you tried to lift the hard mold top and bottom off (complex objects require multi-part, not just two-part molds–nothing for beginners.)

My shape is “simple” to the extent that there are no “overhangs”, such that a frictionless mold should in theory be able to slide right off, but within that constraint, it has a lot of detail. Here’s the design (together with the belt clip I use to carry it), for reference. I realize that the real world isn’t frictionless, but I’m hoping that the flexibility of the cast material would make up for an inflexible mold.

The link to the design doesn’t work.

Oh, right, I forgot to make it public. It should work now.

Yeah, the pass should be no big difficulty to cast with the putty (but you might need to lose the thin scrollwork on the front.) The clip would be problematic, though. Have you considered using Shapeways? They have options all the way from “strong and flexible” plastic up to platinum. (I’ve never used them before, I just know that they exist, and that I’d love to have this trilobite in silver.)

The clip already exists, and works just fine as-is in PLA (though there are a few improvements I’d make if I ever needed to re-print it).

And it seems like it’d be more fun, and probably cheaper, to do it myself rather than using Shapeways (though they’re also cool).

Meanwhile, it’s also occurred to me that latex would not be a good choice of material, just because many people will be handling this thing, and so I’d want something hypoallergenic.

Another limitation is keeping the temperature within the limits for the molding material. Molten plastic is pretty hot (years before my experiments with molding at home, I spent a summer working in a plastics molding plant with presses that ranged from making small gears for electric drills up to single-piece dashboards for Mack trucks, so I’m double experienced) so I’d maybe conciser something putty-like that could be hand-packed into either half of a mold, them squish the two halves together, trimming off any excess later. (Something like a Play-Doh mold, now that I think about it.)