Investing is the process of making a "ceramic" mold of the waxes. Once you have done this, the waxes can be burned out, leaving a cavity in this ceramic mold so that the metal can be forced in to it. Remember, I am trying to teach this class with the idea of using the least amount of equipment and at the lowest cost.
Investment is the dry white powdered mold material that when mixed with water, dries to make a hard "ceramic" mold. This special investment is necessary to withstand the high temperature of burning out the wax. Plaster of Paris can not be used for investment. It will crack and fail during burn out. Follow the safety instructions that will come with the investment.
Step One: Prepare Flask For Investing.
Step Two: Measure The Water.
Fill a can the same size as the flask being used with water. I recommend that this water should be distilled bottled water or tap water that has been sitting for about a week. I just use an old milk jug for storing this water with the cap off. Pour this water into a small mixing bowl large enough to hold at least twice the volume of the flask.
Step Three: Measure the Investment.
Most investment companies recommend that you mix investment at the ratio of one part water and two parts investment by weight. To measure the investment without an accurate small scale use this method. Dry the can used to measure the water thoroughly and fill with investment, heaping the investment over the top of the can. I use Satin Cast 20, because it is available locally. I have used R&R Investment in the past, and many other casters have recommended using it. I believe that you will have less bubble problems with R&R. R&R's phone number is 419/865-9497 or fax at 419/865-9997. Satin Cast can be purchased in small quantities (and large) from the Rio Grande Company at 1-800-545-6566.
Step Four: Pour the Investment Into The Water.
I know this seems too simple to be a separate step, but it is how you pour it into the water that is very important. Pour the investment down one side of the bowl, tipping the bowl if necessary. The reason this is important is that, if you just dump the investment into the bowl, and the water covers the investment, it will trap air in the investment. This will not only cause bubbles, but also clumps in the investment (just like gravy clumps, yak!). By pouring it down the side of the bowl the water can seep into the investment from the bottom and force the air out through the dry powder. You will be able to watch the air come up.
Step Five: Repeat Step 3 and Step 4.
This will give you the approximate mixture for two parts investment to one part water.
Step Six: Mix The Investment.
I had a friend that would follow the above steps and never stir the investment at all. He would just pour it into the flasks. He would pour the investment into the water and just wait until it was completely absorbed. It worked great for him! I just cannot seem to be that patient! (It's a character flaw, I know, but one of my smaller character flaws!) I stir the investment with a plastic spoon to insure it is mixed well and to get a feel for the consistency. I used to do this with my hands, it really gave me feel for the consistency, but dried my hands terribly. It did give me a sense of being more of an "artist," though! Stir slowly so that you do not add bubbles to the investment. You just want to make sure it is mixed thoroughly.
Step Seven: Add more investment if needed.
The investment should be the consistancy of heavy cream or thick pancake batter. I find that with the great investments that are mentioned above, this is not as important as you may think. I have had students, both teenagers and adults, mix their investments with the constancy of 2% milk! They still got a good cast! I have had them also mix and pour it like molasses and still get a good cast. I have never had a flask blow out the bottom. I guess I may have been lucky over all these 25 plus years, but I think it is just the improvements in the investment. If you do mix it too thick, you are taking the chance of trapping large bubbles in and on the wax patterns that will cast as large "balls" on the finished casting. I add this extra investment by sprinkling it over the top and letting it soak in, then stir gently.
Note: A word or two about bubbles and debubblerizers. There is a debate about whether you should use debubblerizers or not. I do not recommend any one method over the other. Use debubblerizers if you wish, and just follow their directions. You can also make your own debubblerizers by mixing "green soap" with isopropyl alcohol (rubbing alcohol from the drug store, not denatured alcohol from the hardware store). Green soap can be ordered from most large drug store chains. I have mixed it at many different ratios and did not notice any differences in how it worked. Most casters and debubblerizers recommend that you spray or paint on the debubblerizer and let it dry before pouring the investment. I like to spray it on, because I have had students break waxes while painting it on. Again I have had students that let it dry, and I have students that forgot to spray it on in time to let it dry. So, they sprayed it on just before they poured. Both got good results.
Another method that you always hear about that I have never personally found to work, is painting investment on to the waxes before you pour. I have never been able to get the investment to easily "paint" on to the surface of slippery, smooth wax! It also took too much of my time, and you risk breaking the wax or breaking the wax off the sprue.
The new investments actually have debubblerizers contained within them. So, I just follow the steps below, without worrying about bubbles.
Step Eight: Pour investment into the flask.
I pour the investment down one side of the flask, while gently tapping the side of the flask with the plastic spoon, that I used to stir the investment. I pour down the one side for two reasons. If you pour directly onto the wax, you could break the wax pattern or break it off the sprue wire. I also pour down the side to eliminate the possibility of trapping air bubbles in and on the waxes. As you pour the investment down one side, and tap gently with the spoon, the investment flows to the bottom of the flask and flows up and around the waxes. As it slowly fills the flask from the bottom it pushes the air up and out, hopefully not trapping in air bubbles. Tapping the flask with the spoon helps "vibrate" the investment into all the detail of the wax pattern, and gently shakes air bubbles off the patterns. I had one student in her eighties who swore that after the investment was poured, she would tap the flask fairly hard three times. She just poured in the investment, then tapped it three times, and only three times. She told me she never had any bubbles. I have tried this and I never got any bubbles, either. I think what happens is that any bubbles that may be attached to the waxes are shaken off during this process. Even though they may only move off the waxes a fraction of and inch and are "frozen" into the investment, they do not cast. Even if they are still just barely touching the wax they will make a bubble that is easily cleaned off. Sometimes, they will just "twist" off the casting with pliers.
Important Note: I pour the investment just one quarter of an inch (1/4) over the top of the waxes. I feel that this will allow the air to escape faster out of the mold cavity. I have also filled the flask to the top to see if it made a difference and still got good castings. Again, these new investments are designed so that the air flows through them easily.
Step Nine: Let The Flask Set Up.
Do not move the flask at all after you have poured the investment. Leave it sit for about a half hour. Most investment will get hard (set up) with in seven to ten minutes depending on the temperature of the water used for mixing the investment. Then I take a t-pin and scratch my initials and a note of what is in this flask into the top of the investment.
Note: The process of the investment getting hard or "setting up" is a chemical reaction, not a drying action. This is important to know, because if you see the that investment has "set up" it still contains a lot of moisture. If not allowed to dry for a time, this moisture could turn to steam during the burn out process. The steam could expand and break and crack the investment, ruining the mold. This could be really bad if you spent hours carving the wax.
Step Ten: Let the flask dry.
How long should the flask dry? I don't know! Ask 12 casters and you will get at least 10 answers, and all of them are probably right. Again, these new investments are so good you can do almost anything you want to do without too much worry about how it is going to effect the casting. To prove this I offer the following examples. Once, I had a junior high student invest his wax pattern, let it set up and put it into a hot kiln (at least 1300 degrees). When I realized what he had done, I called the entire class over and explained again why this was just going to ruin his cast and his wax that he spent a week carving! Less than a hour later we cast it, and he got a good cast. There was a lot of flashing because the investment did crack, but it did not "explode" in the kiln as we have all heard. He cut and filed the flashing off, polished the ring, and is probably still wearing it. Every time he looks at that ring, he probably thinks to himself, "That Mr. Norris sure didn't know what he was talking about!"
I have also read that you should not wait too long to burn out the flask. There is something about it that makes it best to have a little moisture in the investment when beginning the burn out. It is supposed to turn to steam, push against the wax and help the wax pull cleanly away from the walls of the mold. I really do not know how it can do that. The investments are so porous anymore that air can easily pass through it. Steam always travels the path of least resistance through the investment. I do not believe that it can push against the wax. In any case, I always let my flasks dry over night, but not because of any concern for the investment. I just invest in the afternoon and cast the next morning. For students that come to class once a week, they invest one week and we cast the next week. We have no problems with burn out or casting! One school year ended, and I had several flasks left that we invested and did not cast. So, after three months of summer, I cast them to see what was in them and got good castings. I decided to experiment with the time a flask could sit before casting. I invested two flasks, dated them, and cast one about a year later and got a good cast. I forgot about the second flask for almost two and a half years (2 1/2 years). I burned it out as usual and cast it. I got good casting just as if I had invested it the day before.
So, I believe the drying time should be somewhere, between at least two hours (four would be better), and two years. I will not argue with anyone that has their perfect drying time! If it works for them, it is perfect!
Step 11: Take The Flask Off The Sprue Base.
We are now ready for the Burn Out. The next class will cover this process.