Before you can even consider making a cast, the weather (or casting environment) has a fairly limited range of conditions. Those conditions have to do with temperature and humidity. The temperature must be between 70 and 80 degrees. If the temperature is below 70 the cast could be brittle a break easily. If the temperature is above 80 the casting mixture sets up too rapidly (I will go into that more, later). If the humidity is above 15% you could wind up with blisters on the finished surface of the cast. If you cast in a controlled environment, then you have venting and other issues, especially cost. That will increase the cost of the end product. In an effort to keep costs (and final prices) down I cast with one side of the space open to the outside. The problem here is that I now have to worry about weather, humidity and changing temperatures. Once conditions appear to be favorable, you can attempt the casting process.
The first step, obviously, is to choose a mold that you want to create a cast with. Next, the mold must be prepared so that the cast will come out easily, by spraying with mold release. The mold release is worked in with a soft brush to insure that there are no blobs or areas which were missed with the spray.
I like to let the mold release sit for a minute or two. While it is sitting I measure out part A. Then the mold is sprayed with barrier coat. This is a layer of material that will combine with the casting mixture, at the surface, and help with the painting process. The barrier coat is allowed to sit, as with the mold release, while I add part B and sand to the mixture. The sand is added to give the cast a little heft.
Now that all the components have been combined, it is time to blend the mixture. This is also where the clock starts ticking. From the time you start mixing until the mold is in the press you have 30 seconds. The warmer the temperature is the less time you have. At 80 degrees you lose more than one second, of those 30 seconds, for each degree you increase. So, here is how the process goes. Blend the mixture for 10 seconds (20 seconds left). Clean mixer (19 seconds left). Pour mixture into mold, cleaning mixture off of sides and bottom of bucket (8 seconds, 11 seconds left). Tilt the mold so that all areas have been coated with mixture (8 seconds, 3 seconds left). Place backing paper over open back of mold (2 seconds left). Place board over back of cast (1 second left). Get the mold in the press NOW! You made it, barely. You can imagine how the pressure increases as the temperature rises and the time shortens. You can cut off a couple of seconds of mixing time or choose molds that are smaller and easier to work with, but on the whole it is this way with all molds. The larger ones, like our big ammonite and egg nests, are almost impossible to manage in less than 30 seconds. The Archaeopteryx mold that you see here is fairly average in size and representative of the time constraint issues. Oh, don't forget to set the timer!
The material we use to create our casts with is a high density polyurethane foam. Once you start mixing it, it will expand to what ever space you have allowed for it to fill. Or until it can expand no further, which is considerable. We use a press, to restrict the expansion to the mold itself. If you exceed your 30 seconds, the material will expand beyond the top of the mold. It is probably a good idea to still try to get it in the press to limit the mess and try to save the cast. See the last photo in this series. I will discuss that one later.
All you have to do now is wait 30 minutes to see if the stress you just put yourself through has paid off. What better way to spend your time waiting, than cleaning the bucket. My favorite part of the whole process! First you clean off the spatula. Next you scrape the dried mixture off the sides and dump, then scrape off the bottom of the bucket. If you look at the photo closely, you can see pieces flying out of the bucket. This is a vigorous process, but is necessary. Again, in order to keep costs down, we can't just throw the bucket away.
Usually I have at least three casts going at the same time. So, the time really gets away quickly. it is now time to remove the mold from the press and start to remove the cast from the mold. We start by removing the frame. The frame is around the mold to prevent the press from squashing the mold while it is in the press. You then peel the mold from the cast, usually starting at a corner and working your way around until all edges are free. Then lift up the mold off the surface of the cast until it is free. If you are lucky there will be no broken edges or corners and no blisters on the surface of the cast. If there are, they will need to be repaired before it can be painted.
If you are unfortunate enough to have a bad day and you don't get your mold into the pressure bed on time, it will start to flow over the top of the mold. If the mold is a tight fit in the press to begin with, then the press will not lower itself fully onto the mold and frame, due to the overflow of material. What that means is that when it is time to take the mold out of the press, the press cannot be raised to release the mold. Now the mold and frame must be DRIVEN out of the press by what ever means necessary. This is very hard on the mold frame. If the mold frame is damaged or destroyed, the mold is also worthless. The frame is made first, then the mold. You can't make a new frame to fit an old mold. The molds cost between $200 and $1,600 to make. After you have driven the mold and frame from the pressure bed, you still have to remove the foam from the frame. You didn't spray the frame with mold release and the wood is porous so the mixture doesn't want to come off and it is nearly rock hard (or so it seems). Again if you destroy the frame the mold is also no good. It takes about two hours to clean up one mistake. Based on that you can see by the last photo, I was having a really bad day. Fortunately, this has only happened to me about four times and the double you see at the end, only once.
This discourse does give you an idea of some of what we go through to bring you our "Museum quality fossil art replicas."
To see some of the other steps in the creation process, please view other pages in this web site. Also enjoy the products and information on this web site. New additions, and updates, are made to the site monthly.
Thank you for visiting.
Spray on mold release.
Measure out part A.
Measure out part B .
Stir cast mixture .
Pour cast mixture into mold.
Entire mold must be coated..
Board over back to create a seal.
Clean sides of bucket.
Clean bottom of bucket & dump.
Separate cast from mold.
Removing Archaeopteryx cast from mold.
Work in mold release.
Spray on barrier coat.
White sand to add weight to cast.
Clean mixer .
Coat mold with casting mixture.
Backing paper over back of mold.
Place mold into press.
Remove mold from frame.
Cast removed from mold.
S_it happeds . . . repeatedly!!