Because they are small I started with a small plastic container:
I made walls of clay to avoid wasting mold making rubber for no good reason.
Because the trashcan have a flat bottom with no details to worry about (making casting easy) I placed them on a small platform of clay This will create a barrier that will allow a hole to later pour resin into.
I use Mirco Mark 2 part silicone rubber mold making compound. Mix one to one and pour into the molding area.
After you flip it over after the compound has set up ( 4 hours) you have a mold to pour resin into as shown in this photo. The resin I use is also Micro Mark CR-300 casting resin. Again it’s a one to one mix, sets fast , so you mix it and pour into the molds. This particular mold is so small I actually drip it in off a popsicle stick. One the resin gets hard , just pop them out and you got a perfect copy of what you molded.
Is that mold rubber pretty flexible? Is it difficult to get the originals or the new casts out of the mold? No type of release agent needed? Have you done any more complex shapes than a garbage can?
Sorry for all the questions. I’m just really interested in this.
The mold is very flexible; you can bend it to get the trash can out. It can be reused almost indefinately. For flat objects like molds of brick walls or a girder, no release agent is needed, but I am not sure about for something almost entirely enclosed like the trash can is. Others might know.
So far I have never needed a mold release agent. Although they recommend it. If you look at this page of my photobucket , this is page two also look at page three. Page two has a building that is all cast resin from molds , page three is a GE prime mover I did from a pieced together master then molded and I made resin copies. The molds are completely re-usable. The sides flex enough to get most parts out with no problem. The prime mover mold was made in two parts because of the details. The block was suspended in a cup 1/2 the cup was filled after it set up I poured the upper half. I did use a little silicone spray the ensure the halves would seperate cleanly. Also set locating pin in to make sure everything would line up later. It’s kinda fun but time consuming.
A word of caution for you. I know in the past you mentioned some health issues. Although the mold rubber material is fairly safe, the casting resins can be a concern. They have been known to cause sensitivity problems. Once you become sensitized to these there is no going back. some research into this might be in order for you before jumping in. On Yahoo groups there is a casting group and If I recall there was a lengthy discussion on this a couple of years ago.
Since I am doing a little more complicated castings, I am a little more investment heavy than Gappleg. As he has pointed out, its good for those things you want several of.
I have a triple beam balance scale (bought on e-Bay), a painter’s pressure tank (used to pressurize the castings while curing to minimize bubbles), a post curing warmer (made from 2 lasagna pans and a floodlight, and an air compressor (which I had already, to charge the pressure tank). You don’t need all this to cast but it helps for complicated castings. I do the car shells as “squish” molds. There is a female mold, resin is put in there and then a male mold is inserted to squish the resin between the to molds. Gives a very thin, “hollow” casting. In order to keep the male mold in the right location, I build several mold frames from scrap plastic pieces ($15 a pound a a plastic supply house) that keep everything aligned.
I have cast replacement underframes to upgrade Bachmann HO 34’ cars, replacement underframes to replace the MDC cast metal ones (in boxcars after I use the metl ones in gons), underframes for my 34’ XMe cars, shells for the XMe cars and shells for the gons. In addition I have cast some car parts.
Averaging the cost of all that it has cost me about the same as a commercial resin kit for the XMe boxcars I have. But as I produce more cars and more types of cars the cost per car will plummet. Expect your first several tries to be a little rough, there is a learning curve and thereis a difference in the materials. The best RTV I have used is Dow HSII from Alumilite. The best resin I have used is from Hobby Silicones. The Hobby Silicones RTV I bought was the “firm” grade and has been a little more firm than I would like for some of my castings. I will try some of their “soft” next. Since I use a rigid mold box to hold the molds I’
Mold release is almost never necessary in the case of of a one-piece mold. When pouring a two-piece mold for more complex parts, mold release is absolutely necessary to form a barrier, so that the rubber doesn’t adhere to itself.
Dan, what sort of sensitivities are associated with urethane? Are you talking about skin sensitivity, like rashes?
To supply cattle to my meat packer I am building a cattle feed yard. I need many, many cows, with four legs I invision the mold to be nigh to impossible.
You would have to make a 2 part mold with a parting line down the cows back and then the on the underside touching all four legs. There would have to be a vent on each leg and on its muzzle. I would use a large sprue on attatched at the tail and pour the resin in there until it started up the vents. It could be done, but considering it will cost about $60 to get resin and RTV, you can buy a whole bunch of cows for $60.
Not impossible but tricky in the least. A two part mold somehow would be the best bet. I would see filling the legs with resin as being the hard part. Possibly molding upside down and filling the mold through the legs. Those would be a tiny access though.
Coupla thoughts on molding resin. If all your’e gonna make is a coupla small detail parts molding from resin is too expensive. Costs about $70 to get started. If your gonna do car frames and bodies then go for it . You can make a fleet for less, and use the leftovers for detail stuff.
If you wanna do details for much cheaper used the latex rubber and plaster. Once painted no one will know,even you if you tend to forget things like I do.
As for the resin. If its epoxy you are using, there certainly are fair chances for developing a sensitivity that can be pretty serious.Use a respirator for organic vapor and rubber gloves. The gloves are cheap,use new ones for each session.
All in all the tutorial was good,always great to see how folks do stuff.
From what I remember reading the sensitivity issue is from contact with the uncured resin. They contain cyanides that can cause alergic reactions either thru inhalation of the almost oderless fumes or thru direct skin contact. My casting table is a downdraft vent system and I always wear gloves when working with the stuff. I have been told that respirators will not filter out the harmfull stuff. I am trying to find the information I have on it. This is not meant to scare anyone off from casting parts. It is just meant as information to take the proper precautions. I don’t think the casual casting of a couple of parts now and then will be any cause for alarm unless you have a pre-existing condition that might become aggrivated which is why I cautioned Jeff
