Recommendations for casting material?

[dropshipbob]

I'm looking to mold and cast copies of arrowheads I have made, and I'd like to cast them in a translucent black material to make them look like obsidian.

Is there an easy to use and cheap material to do this in?

What about molding material?

I looked at Smooth-on's site, but I didn't see what I was looking for AND I didn't find a way to actually order through them.

Also, I'd rather NOT have to invest in a degassing/pressure pot set-up presently. I'm not sure how many I will do and would rather not make that kind of investment yet.

 

[foxbatkllr]

Smooth-Cast 325 plus So-Strong color pigments will probably do the trick. You'd just have to play around with the amount of pigment to get the right amount of translucency. You can buy the products if you go the bottom of the page of the materials under technical and buying information

https://www.smooth-on.com/product-line/smooth-cast/
https://www.smooth-on.com/product-line/strong/

 

[E Williams]

Exactly what foxbatkllr said. You can find sample size kits of 325 on Amazon for $30 shipped, but you have to go to Smooth-on for the pigment.

Here are direct links to their shop:
https://shop.smooth-on.com/smooth-cast-325
https://shop.smooth-on.com/so-strong-tints

The pigment is so concentrated that you won't need more than a drop or two even for a large arrowhead (so just get the small bottle).

Before molding, make sure your master is polished up to be glassy-smooth and your castings will come out that way as well.

It will be tricky to get a completely bubble free cast without at least a vacuum chamber, but there are tricks to get close. Best tip would be after agitating the two parts and pouring them into separate measuring cups (and mixing the dye into part A), let them rest for a bit before combining. The dye should help hide the rest.


For rubber, try Mold Star 15 Slow. The sample kit is no more expensive than their "entry level" Oomoo, but the molds you make will last years rather than months (the Oomoo degrades very quickly). It has a long pot life and low viscosity, which are both good to let bubbles escape since you won't be using a vacuum chamber.

https://www.smooth-on.com/products/mold-star-15-slow/
https://shop.smooth-on.com/mold-star-15-slow
(the sample size is on Amazon w/ prime shipping for $2.70 more than the Smooth-on price, but if you're already buying from them it's balanced out)

 

[Zinger]

You can "long pour" to minimize bubbles. Put your mold x on the floor (on a piece of paper or plastic to protect against spills) and pour the resin from high up. The resin should narrow down to a very thin stream at the mold. Almost needle thin if you can get it.

That forces a lot of the trapped air out.

 

[foxbatkllr]

You can also run a vibrating massage chair or some other vibrating mechanism to bring bubbles to the surface.

 

[cavx]

If he wants bubble free parts that he can tint to be slightly translucent and does not have a degassing system, then he will need a long cure time resin as well being careful with the pour. I'd suggest he looks at epoxy. Most have a greater than 30min pot-life and if poured onto a thin stream, will pretty much degas themselves.

 

[JPH]

Alumitite is sold at Michaels crafts stores. Michaels regularly has 40-50% off coupons, it sells in store for $17-18, so $10 for 16 oz resin. When you mix this, tilt the container and I slowly sweep side to side like a pendulum to prevent bubbles.

Cast n Craft Mold release at Michaels.

Mold Builder for pain on mold.

 

[dropshipbob]

Thank you all so much for all the info!

cavx, I thought the problem with using epoxy in molds was that the material never fully cured? The last time I heard of someone using epoxy, when the opened the mold, the surface stayed sticky. Or is the epoxy you are talking about something specifically for molding?

 

[Matsuo]

DSB You're welcome to come up and use my shop, a lot has changed since your last visit.
We can degass and pressure cast all day long.

Most Clear formulas unless otherwise specified have pretty long cure times anyway. The experimenting is whats going to take a long time.

M

 

[cavx]

Thank you all so much for all the info!

cavx, I thought the problem with using epoxy in molds was that the material never fully cured? The last time I heard of someone using epoxy, when the opened the mold, the surface stayed sticky. Or is the epoxy you are talking about something specifically for molding?

Sometimes Polyester resins remain sticky on the surface, but I have never had that issue with epoxy. If the epoxy is not curing properly, then I will suggest it was not mixed properly and some people think they can mix this gently to reduce bubbles. No, you must mix this stuff up hard and fast and then degassing is the best way to have a bubble free pour. Pressure pot finishes it off if you have one.

The stuff I use is same epoxy that was sold as a 3D print smoother. They self level and fill in an small lines giving a glassy smooth finish without the need to sand. When you cast this stuff, you must be aware that it gets super hot and this can burn the molds! Then again, so can polyester resin if too much cat is added.

There are two different versions of epoxy now in Australia - original would yellow over time and was mixed at 100:50, but it cheap and good for making all sorts of stuff that you need a hard plastic for. It can be tinted and holds the true colour. The new stuff is UV stable and mixed at 100:100, costs a fair bit more (almost double), but much nicer product and stays water clear.

On the original pack, the instructions said 3 hour cure time, however you could have issues if demolding under 6 hours. I had a part that looked like it was made some form of expanding foam when I demolded too early as it swelled up. I found 6 hours to be really good for the parts I was making.

However, there is then a post cure time of 48 hours where the part can deform if stressed until it fully hardens. I found this useful because even though the part was cured, it was still flexible and could be shaped in a jig and left to set hard. After the 48 hours, it is hard plastic. One thing I like about epoxy is that I have dropped cast parts on concrete and they just bounce. Same parts made from Polyurethane parts tend to crack.

 

[dropshipbob]

Matt! Man it has been awhile since we talked. I've been reading about your shop on your FB page, glad to see you're still doing your thing. I'd love to see your new shop!

Cavx, sounds like I might be biting off more than I can chew? I was hoping to keep this pretty simple but I don't know if that's possible AND get the results I'm looking for.

 

[cavx]

Matt! Man it has been awhile since we talked. I've been reading about your shop on your FB page, glad to see you're still doing your thing. I'd love to see your new shop!

Cavx, sounds like I might be biting off more than I can chew? I was hoping to keep this pretty simple but I don't know if that's possible AND get the results I'm looking for.

If you have access to a work shop, then it is no brainer.

If not, you can do these casts in shallow layers. Takes more time, but epoxy tends to degass itself if the pour is thin enough. So really all you need to do is mix a small amount, pour, let it sit for the pot life time, then repeat. So long as you cover the mold for each layer with say cling wrap, you won't get bugs or dust contamination either.

In the end of the day, if you want perfection, you need the tools.

I've had discussions with the supplier where I get my stuff from and she has been in the film industry for 30 + years, so knows her stuff. She was asking about cheap vacuum kits (because they don't sell one) and I told her about the one on eBay for around $200~$250USD (3cmf pump, hoses, fitting and a 4.5 LTR chamber). This is a good little combo and cheap for a start up. For the average molder/caster, this is probably all you will ever need.

It really depends on how much of this stuff you want to do. If you are making less than 10 items, probably not worth it, but if you intend to keep making stuff and want good results, then this is a no brainer. Especially if you end up selling the parts you make.

 

[dropshipbob]

The epoxy you use, is it off the shelf? I think back to when I was last looking at epoxy at the local store and I don't remember seeing anything with a life longer than five minutes or so.

 

[cavx]

The epoxy you use, is it off the shelf? I think back to when I was last looking at epoxy at the local store and I don't remember seeing anything with a life longer than five minutes or so.

Both products are totally off the shelf items.
Pot life on the standard non UV product is about 40min at 25 degrees C, said sure time 3+ hours. Post cure after 24 hours. 1.5KG kit is about $75 and is mixed 1.0:0.5.
Pot life in the improved UV stable product is 50 min at 25 degrees C and said cure time for that is 6+ hours with up to 48 hours post cure. 2KG is about $130 and is mixed 1:1.