How to mold and cast a prop part

Awesome tut man, thanks. I'm going to check out harbor freight on some pressure tanks, I believe if you are going to be making props and stuff, then why not spend the little bit at first to get many great castings in return.

Do you know of any good stores to get the plasticine in bulk like that?

Thanks for the tut, and product info!
 
Excellent write up. Clear and easy to understand. Personally I use lots more keys than that to avoid mold distortion of any kind, but everybody has their way of doing things. Now who's going to do a mother mold/fiberglass shell/rotocasting write up? ;) BTW, do you ever use your pressure pot to de-gas your rubber?
 
Ruprekt said:
BTW, do you ever use your pressure pot to de-gas your rubber?
Click to expand...

No I haven't but I did buy a vacuum pump for a future vacuum former, I suppose I could hook the two up, would the tank be strong enough? It's built for internal pressure how about external pressure?

Also thanks for all the views and compliments on this post, I've been trying to help out with the "how to make a mold" questions that pop up now and then and thought I would just put this up for those who have questions.
Anyway really enjoying this forum and seeing all the wonderful projects that are being built. Keep it everyone!

ps, I do agree there could be a "how to" section but with so much going on here it might be more work. Perhaps an area that is non specific to a project and more to a skill base, like molding, vacuum forming, sculpting, etc.
 
Also, something I was thinking of, but assuming you didn't have to worry about making the mold fit into a pressure tank (either it's bit one or you just don't have one) would it work to use MDF or something instead of the cardboard and clamps to provide more even pressure across the mold?
 
Gattaca said:
No I haven't but I did buy a vacuum pump for a future vacuum former, I suppose I could hook the two up, would the tank be strong enough? It's built for internal pressure how about external pressure?...
Click to expand...

Internal or external shouldn't matter. if it's made for pressure it will handle vacuum okay, even full vacuum inside is only ambient room pressure outside.
 
I've never cast anything, but it's my understanding that if you are going to pressure cast resin, you should pressure cast the mold rubber too. Otherwise any air bubbles in the rubber will collapse under pressure and potentially distort the mold. But apparently that isn't as much of an issue as I assumed. Your pressure cast looks good to me. Nice visual tutorial.
 
Fawbish said:
If you dont mind, per prop, what kind of cost are you looking at? I realise it will vary per size etc.

ie. what are the costs of a can of mold release, the mold agent etc?

Great guide. Thanks
Click to expand...

The most expensive part of this process would have to be the silicon used to make the actual molds. The great part though is that once the molds are made (and you look after them) you will get many casts from them, so that initial cost is spread out over time. Cost per part then comes down to the type of casting material you use.

And I agree, this thread needs to be stuck.
 
Great post but one thing that was just hit on always seems to escape me. What is good after care of a mold. I would like someone to go into that a bit. I know to just store them in a cool dry place but is their anymore to that? Should you store them open, closed, vertical, horizontal, should they be be moistened periodically? How long can a mold really last? These are all the things that drive me bonkers.
 
Draphin said:
Great post but one thing that was just hit on always seems to escape me. What is good after care of a mold. I would like someone to go into that a bit. I know to just store them in a cool dry place but is their anymore to that? Should you store them open, closed, vertical, horizontal, should they be be moistened periodically? How long can a mold really last? These are all the things that drive me bonkers.
Click to expand...

Really good questions and ones I don't know if I can answer as I am still relatively new to all of this myself.

At this time, my molds are closed up in a cool dry place. I guess it would not hurt to seal them in a huge (I have 300mm x 300mm) zip lock bags with silicon pouches as well.

I made my main molds with thick walls, so they are pretty much self supporting, but yeah would think that if left bent or flexed, they might take on that shape and distort future castings.

AFAIK, you shouldn't have to keep the silicon moist, but if you did, you would need to clean it off before use.

One thing I did learn a while back is that if you used mold release during the casting process, residue may stay on or even impregnate the surface of the part making it difficult or impossible to adhere (glue) to other parts. I didn't use mold release or talc on my clear cast parts because -
1. the risk of not being able to adhere later and
2. that a substance like talc will cloud the surface.

I am thinking this is both an art and a science.
 
Draphin said:
Great post but one thing that was just hit on always seems to escape me. What is good after care of a mold. I would like someone to go into that a bit. I know to just store them in a cool dry place but is their anymore to that? Should you store them open, closed, vertical, horizontal, should they be be moistened periodically? How long can a mold really last? These are all the things that drive me bonkers.
Click to expand...

The short answer to your mold storage question is "it depends."

Some brands and types of silicone are more archival than others. Some degrade pretty quickly even under ideal storage (cool/dry). I've found that the less expensive silicones don't tend to last as long in general. But, I have had some expensive silicones crumble and fall apart on me after only a couple years and couple "cheap" silicone molds that have lasted me 20 years.
The resins that you cast in your molds will affect their lifespan too. Some casting materials are very rough on the molds, causing them to dry out and tear apart.

I have had some colleagues pour plaster or something "inert" into their molds if they are going to be storing them for a long time. The thought is that it will keep the mold from distorting while in storage, especially if the molds get stacked upon. I haven't tried this, but it seems to make sense.
 
Irken Tech said:
I've never cast anything, but it's my understanding that if you are going to pressure cast resin, you should pressure cast the mold rubber too. Otherwise any air bubbles in the rubber will collapse under pressure and potentially distort the mold. But apparently that isn't as much of an issue as I assumed. Your pressure cast looks good to me. Nice visual tutorial.
Click to expand...

Even molds I have made outside the pressure pot don't have bubbles big enough to make this a problem. One thing I have come across is when using the pressure pot to cast into a mold that was not made in a pressure pot I sometimes end up with a rough cast because the pressure forces the casting material into the micro bubbles in the surface of the mold. Giving it a rough texture.
 
CybranKNight said:
Also, something I was thinking of, but assuming you didn't have to worry about making the mold fit into a pressure tank (either it's bit one or you just don't have one) would it work to use MDF or something instead of the cardboard and clamps to provide more even pressure across the mold?
Click to expand...

Not the same kind of pressure, the cardboard and elastics is just to keep the two halves of the mold together. Clamping it will just squeeze the mold and probably distort it. The molds have to fit into the pressure pot with atmospheric pressure of 60lbs, that's what crushes the bubbles, not the clamps.
 
Irken Tech said:
... but it's my understanding that if you are going to pressure cast resin, you should pressure cast the mold rubber too. Otherwise any air bubbles in the rubber will collapse under pressure and potentially distort the mold. But apparently that isn't as much of an issue as I assumed...
Click to expand...

Results will vary. You can get really good results without specialized equipment, but using pressure and vacuum chambers is the best way I've found of getting consistent results.
I posted some pictures in another thread showing the difference between castings from molds made using vacuum de-aired RTV and straight pours. I expect molds made under pressure would yield pretty much the same results as vacuumed ones, but I haven't tried it. http://www.therpf.com/f11/vacuum-chambers-pressure-pots-need-some-advice-221053/
The pictures are near the bottom of the first page in the thread.
 
Last edited:
If I recall correctly you can also use talc brushed onto the molds prior to pouring resin to help fill in corners/fine details. Never had to do this myself, but might help to keep the bubbles down also?
 
Last edited:
Good point. When I first started casting I used talc and it helped. I still got some bubbles, but not as many. I find that pressure gives me better results than talc, but any improvement is an improvement, right? If pressure isn't an option (e.g, casting white metal, or a mold is too big for the pressure pot) I still use talc.
 
Great tut, surprised I didn't see this earlier!

As for using Smooth-on products and compatibility, you can always give them a call directly (sorry don't have the number in front of me, but it's on their site). Their customer service has been great for me when I've run into issues. If you do get on the phone with them (or you happen to be on their site), it's worth getting a catalog sent to you as well. It's free, and has a lot of information laid out concisely about most of their product line and applications. The web site is great too, but sometimes it's easier to grab the catalog when you want to check on something without being plugged in. :thumbsup

*Nope, I don't work for Smooth-on, but they've got enough of my money that I should be a shareholder :lol
 
Last edited:
I added some more info for anyone that's interested, just below the original post above.
Thanks for all the views.
 
This thread is more than 9 years old.

Your message may be considered spam for the following reasons:

  1. This thread hasn't been active in some time. A new post in this thread might not contribute constructively to this discussion after so long.
If you wish to reply despite these issues, check the box below before replying.
Be aware that malicious compliance may result in more severe penalties.

Similar threads

helix_3
Thoughts on this two-part composite mold workflow?
Replies
2
Views
390
helix_3
helix_3
dproc
Andromeda Strain (1971) Satellite Prop help... and ID'd part?
Replies
5
Views
408
smithjohnj
S
helix_3
How will expanding foam fill this hypothetical mold?
Replies
4
Views
644
helix_3
helix_3
PHArchivist
Jurassic Park - Props, find a way... Part Two
Replies
4
Views
607
Brendan
Brendan
clockworkp
District 9 Assault Rifle: My first prop posted here (and second prop ever)
Replies
3
Views
239
DamnYankee123
D
Back
Top