login   |    register
Scratchbuilders!: Armor/AFV
This is a group for armor scratchbuilding questions, topics and projects.
Hosted by Mike Kirchoff
How do you make your resin molds?
HunterCottage
#116
Visit this Community
Stockholm, Sweden
Joined: December 19, 2001
KitMaker: 1,717 posts
Armorama: 629 posts
Posted: Sunday, February 24, 2002 - 04:14 AM UTC
Having dabbled a little with molding and sculpting, I'm curious as how you all go about making the molds to do the copying from??

I have things I would like to do, but I always destroy what I am making in the process. I want a mold that will last a while - they aren't all that fun to make I've tried using clay, but that has such a short life span, and a limited use. Rubber is soooo messy, stinks too
GeneralFailure
Visit this Community
European Union
Joined: February 15, 2002
KitMaker: 2,289 posts
Armorama: 1,231 posts
Posted: Sunday, February 24, 2002 - 06:25 AM UTC
I think we all tried clay (works quite well for easy pieces or a quick copy), plastiscine (melts from the heat of curing resin), candle wax (same - ooch ! ) latex (whew that stinks !)...

You should look for a chemist shop, specialising in modern stuff like polyester, PU, and other such products. Most (if not all) such products are two component polymers.
There must be over a hundred such products, all "specially made for making modeling molds'". In reality, one is much better than the other. I suggest you look for a silicone-based product.
When you go there the first time, have a thorough discussion with the shopkeeper before you choose, and maybe show him some of the parts you would like to copy. Having a good relationship with him (her) pays off : there's plenty of tricks to learn... !
These products don't come cheap : often in the range of 10 and 40 USD per kilo.
What you should look for is a product that is still very flexible and can be stretched once it's hardened. Some dry quickly, some take up to 48 hrs to cure. I used to have a product of Voss Chemie (German ?) that was very good. Now I use something labeled "TinSil 30-70", but I'm less happy with that. There are hundreds. Brand names are often very local. If international, often they are still bottled by local chemists for the local market and still get a local brand name).
I tried four or five such silicones. Though I found a good product quickly, I looked further to find something that hardened a bit quicker. But those often are less flexible and stretchy.
Maybe ask to see a piece of cured molding-product. When you squash it with your hand, it should feel like a soft rubber ball, not hard.
The reason for this "soft" and "flexible" aspect is obvious : complicated parts that have little 'arms' or 'tentacles' in several directions, cannot be removed from a stiff mold without breaking. From a soft mold, you can.

Then just a few warnings :

  • Always wear rubber gloves when you handle this stuff. Getting it on your fingers won't kill you, but it's a real misery to remove.
  • Make sure you don't get it on your clothes, carpet,...
  • When you smear it on any other object, don't clean.... just wait till its hard and it can be peeled off (when both parts are properly mixed).
  • Storage. If you don't use it all, make sure you store the rest airtight, and away from kids !!! If you let it freeze, it's dead.
  • This stuff must be mixed very thoroughly ! If you mix it poorly, parts will remain liquid. Believe me : you don't want that to happen to your model !!!
  • Avoid air bubbles. What is air during the molding process, becomes a bubble of resin when you cast the copy - ruining the detail ! For this reason, a brand that cures very slowly is better. During the many hours when it's still liquid, many air bubbles rise. There are several ways to avoid bubbles, but the easiest and cheapest (but not with the best result) is to tap on the recipient that holds the mold or use a vibrator (or anything that serves the purpose of releasing the bubbles).
  • DO NOT demold the original before it's fully hardened, all through.

All this is only on the product. Then of course there's a whole laundry list of instructions on HOW to make such mold. I'll go into that next time.

I know all this sounds a bit complicated, but don't let that scare you. It's really worth trying. Start with copying a small, easy object (of which you have many) like a wheel, etc...
Copying resin kits is often quite easy : they were constructed specially to be molded. Copying plastic kit parts often results in a broken original and a useless copy, unless you're really experienced.


Does anyone have other tips on the choice of product (criteria) or specific brand names ?
HunterCottage
#116
Visit this Community
Stockholm, Sweden
Joined: December 19, 2001
KitMaker: 1,717 posts
Armorama: 629 posts
Posted: Sunday, February 24, 2002 - 09:10 PM UTC
Jan,

What do you use a release agent?? I've used vaseline, but if that is put on too think it ruins the detail. I was wondering if there is anything in spray form??

I'm looking forward to the mold making process :-)
GeneralFailure
Visit this Community
European Union
Joined: February 15, 2002
KitMaker: 2,289 posts
Armorama: 1,231 posts
Posted: Sunday, February 24, 2002 - 09:13 PM UTC
Yes, there definitely ARE products in spray form.
But if you use the correct combination of resin for casting / silicone for the molds, you can do without release agent. Check with your local chemist supplier... !
GunTruck
Visit this Community
California, United States
Joined: December 01, 2001
KitMaker: 5,885 posts
Armorama: 3,799 posts
Posted: Monday, February 25, 2002 - 12:40 AM UTC

Quoted Text

Jan,

What do you use a release agent?? I've used vaseline, but if that is put on too think it ruins the detail. I was wondering if there is anything in spray form??

I'm looking forward to the mold making process :-)



Brian, I use Experts Choice red RTV compound for making molds. You can get it from Bare Metal Foil Co. I haven't had to use a release agent for molds I've made with this product - it holds up pretty well for about a dozen pourings. If you're making "knock-offs" (copies of other resin aftermarket parts) then this isn't too bad. As this is a RTV (room temperature vulcanizing) it generates heat as it sets. I've found that this can destroy some resin masters you might select to "knock-off" - they crack in two during the setting of the mold.

Gunnie
GeneralFailure
Visit this Community
European Union
Joined: February 15, 2002
KitMaker: 2,289 posts
Armorama: 1,231 posts
Posted: Wednesday, February 27, 2002 - 06:26 AM UTC

Brian,

Recently, I discovered the most wonderful web pages that show you how to make a resin model, step by step.
I just can't find it back (though I'm sure I had it bookmarked ?!).

These are two other links that may help you a bit on your way. I'm sure there are a lot more. I'll add them in the coming days as I discover more.


A general introdution on making molds.
http://www-2.cs.cmu.edu/~rapidproto/manufacturing/molds/silicone.html

A commercial website, but it gives you a good introduction on making molds.
These people even organize two day seminars to give you hands-on experience
!
http://www.smooth-on.com/howto1.htm

I hope this can help you on your way...

Jan

GeneralFailure
Visit this Community
European Union
Joined: February 15, 2002
KitMaker: 2,289 posts
Armorama: 1,231 posts
Posted: Wednesday, February 27, 2002 - 06:31 AM UTC
molding LOOPED parts . ( rings...)

Imagine you mold a wedding ring. Easy.

Only when you try to remove it from the mold, you will discover that the ring structure has an unexpected (well...) conscequence : there's mold material in the middle of the ring, that makes it impossible to remove the original without cutting the mold first.

This can be avoided by "filling" the hole by placing a piece of sticky tape on it. Trim the sticky tape to the correct size. In the mold, the sticky tape will separate the mold material from one side to the other, allowing you to remove it in one piece, without cutting.

This can be useful for copying all parts that have a sort of hole...

Jan
HunterCottage
#116
Visit this Community
Stockholm, Sweden
Joined: December 19, 2001
KitMaker: 1,717 posts
Armorama: 629 posts
Posted: Thursday, February 28, 2002 - 06:03 PM UTC
Thanks all for the tips and answers.

I've been a scout leader for years and one of the things we have done there I was thinking I do here with modelbuilding. To make a pretty good and quick mold, that holds the details well is ordinary plaster of paris. What we did there was to make a design, put a release agent on it pour the mold and let the mold set. After the mold set it could be used to make Lead and tin alloy pieces. What I was thinking of doing is using the above method for making a "master" in metal, thus being able to use it to make many molds. The metal alloy has a low melting temp, would the RTV melt the alloy? Of course the patterns would be basic things and not complex sprues etc... Do you think this would work??
HunterCottage
#116
Visit this Community
Stockholm, Sweden
Joined: December 19, 2001
KitMaker: 1,717 posts
Armorama: 629 posts
Posted: Thursday, February 28, 2002 - 06:14 PM UTC
Thanks for the links Jan. Those were great!! I'm feeling a renewed confidence!!! I might just be able to get going on some of the things I want to do!!!

Thanks again!!
GeneralFailure
Visit this Community
European Union
Joined: February 15, 2002
KitMaker: 2,289 posts
Armorama: 1,231 posts
Posted: Thursday, February 28, 2002 - 11:27 PM UTC

Quoted Text

The metal alloy has a low melting temp, would the RTV melt the alloy?



I can't really think of many metals that have a melting temperature (except mercury) that is lower than the curing temp of RTV. I have no figures on RTV's curing temperatures, but even at their peak temperature, you can still touch them without getting your fingers burned. And I would certainly not hold out my hand to let you poor any of your 'low melting temp" metal on them !!!

Please note you don't even need a metal master. Resin masters work fine. Even plasticard masters work fine - if you accept the risk of loosing your master. If you have a good mold, loosing the master is no problem. The detail quality of a successful copy should be the same as the master's. And once you have a first resin copy, that can be your master for future molds if you should need those.

From my own experience, I can hardly remember ever having to make new molds of the same original. One mold lasts at least a dozen (if not several dozens) of castings.