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Note:: Please follow all safety precautions when molding and casting. When using casting resins, make sure to work in a properly ventilated area (or wear a respirator), and wear latex of vinyl gloves to avoid contact with the liquid components. Keep all molding and casting materials away from children. About recasting:: Obviously, the instructions below can be used to recast existing kits. I do not condone or advocate this activity. But the fact is, recasters will (and do) get the info somewhere. I offer this information to those who wish to make castings of their own prototype sculptures (or want to cast small parts to duplicate them for dioramas, etc). Part III -- Casting in Polyurethane Resin: Getting Started First you will need to gather your materials for resin casting. Molds
made from your sculpture For this demonstration I will make a casting from my Slimer Mk II molds. The most important ingredient in this process is, of course, the resin. There are many different resins and many companies out there, so I will tell you what I have used to get the best results. This is also the resin used by a number of professional model makers.
Model-Cast 36-XXX Off White Medium Cure Resin This resin is available from Vagabond Corporation, P.O. Box 39, Warner Springs, California 92086-0039. Their telephone number is 760-782-3136. Model Cast is a two part polyurethane resin that is mixed in equal portions. You can get it in three curing times. The medium that I use sets up to the gel stage in about 2.5 minutes. That's generally enough time to fill a mold and have it set up fine. Note: I've seen a lot of people try to cut corners when it comes to resin, using cheaper polyester- or epoxy-based resins, and I've heard horror stories as a result. The question you want to ask yourself is: after making some expensive molds for my project, do I really want to cut corners and use a cheaper resin that might ruin them? I've been getting great results (and long mold life) by using Vagabond's products for years now. The extra expense is worth it. Another note: Polyurethane resin is extremely sensitive to moisture (water vapor) in the air. Once you open your resin, you will need to use it up as soon as possible. When finished with resin for the day, reseal it tightly. I actually reseal my resin after pouring it into my measuring cups. Resin that has absorbed too much moisture takes longer to set up (not a real problem) and tends to foam up and expand, which can create problems with your casting. The shelf life for opened resin is probably about three months, but your mileage may vary. The next most important product is a release agent for your molds. This helps the casting come out of the mold easier and helps protect your molds from the chemicals in the resin.
Parfilm Ultra II Mold Release This spray-on release agent is terrific stuff. It's available from Price-Driscoll Corp., 17 Industrial Drive, Waterford, CT, 06385. Their telephone number is 800-442-3575. You spray Parfilm directly into your molds. The thing to remember with this stuff is that a little goes a long way. You just need a light puff into the mold to coat it. More about this in a bit. Okay, let's get started. Work in a well-ventilated area, and wear a respirator if you're going to be pouring a lot of resin. Some people are sensitive to the chemicals in resin, so wear vinyl gloves while working with it. Spread plenty of layers of newspapers down to catch spills and drips. Pouring resin can be messy work, so wear old clothing. Keep paper towels and rubbing alcohol to help with your cleanup. While I do believe in buying good resin, I get cheaper when it comes to my basic supplies. Since you mix the exact amounts of resin together, you can use graduated measuring cups, but I use a fast-and-dirty substitute. First I use my bonsai clippers to clip off the end of a craft stick (popsicle stick) and create a makeshift measuring stick.
Trimming a craft stick Next I mark on the stick with a Sharpie marker to show the various amounts of resin I want to use (you have to do some trial and error to arrive at these amounts beforehand). Then I hold the measuring stick up to a disposable cup and mark of the measurements.
Preparing to mark the cup I align the line marked Big with the bottom of the cup and then use a Sharpie marker to mark the very top of the stick. I repeat this with another container and then mark the first container with an A (for the A component of the resin) and the second cup with a B. Note that I have two marks on the stick. This is for something called a two-stage pour. Since I don't have the facilities for pressure casting (which helps reduce the problem of air bubbles in the resin as it sets up), I use two-stage pours for some parts to help reduce air bubbles and voids in my castings. We'll cover this process as we go along.
The cups are now marked and ready These translucent 8-oz. cups are great for measuring larger amounts of resin, but for smaller amounts I like to use the Dixie brand 3-oz. plastic bathroom cups.
3 ounce bathroom cups marked to pour Slimer's arms Note: Resin will eat through these cups given enough time (like overnight). So pitch them when you are done casting for the day and mark up a fresh set the next time. Okay, let's prepare our molds. Note that you won't need any Parfilm mold release for the first five or six castings. Silcone rubber has a lubricant property that will see you through these first castings. After that, however, you will need to spray some Parfilm into your molds after every two or three castings (also called "pulls"). Remember, just a quick puff of the spray will coat the mold--you don't need to go heavy with the release.
A quick puff of Parfilm every three or four pulls is needed Above you see half of the Slimer body getting the Parfilm treatment. Because I was taking the picture at the same time I was spraying, I sprayed too much into the mold. Remember, just a light puff into the mold will do. Okay, your mold is almost ready to go. Now you need to close it up and band it. Below you will see the various molds banded up.
Note the side braces (foamcore and sheet styrene) on the molds These molds have been rubber banded and arranged for a resin pour. Use foamcore braces on the sides for larger molds and sheet styrene braces for smaller pieces. Sometimes you can get away with just banding the mold itself, as I did at lower right in the picture above. Notice that I have a turkey baster handy. This is used to pump resin through the thin channels of the arm molds and the smaller parts.
Using a turkey baster to pump resin into the thin arm molds Mixing resin is easy. Just mix equal amounts of A and B components, stir for a few seconds and pour. Always mix in a separate container. I use an 8-oz cup for my mixing container with resin measured in the Dixie cups and I use a half-gallon pitcher (see picture above) for mixing resin measured in the 8-oz cups. I stir small amounts with craft sticks and larger amounts with a wooden paint stirrer. Stir five to ten strokes in one direction and then switch directions for five or ten strokes. Then pour. You may feel the container heating up--this is the catalytic reaction that transforms the resin from a liquid to a solid. Resin goes from a liquid stage to a thick syrupy gel stage, and then you'll see what is called "the bloom" as it catalyzes into a solid.
Resin going from the gel stage into solid stage Note in the photo above that I poured (or pumped) resin until it came up through the air vents, too. This means that air has escaped and the resin has completely filled your mold. Clean up spills of unhardened resin with paper towels and rubbing alcohol. If you mis-mix your resin and it doesn't set up, empty it out and clean your mold with paper towels soaked in alcohol and with alcohol-soaked toothpicks. Make sure the molds are dry before you try another pour. Resin takes about ten minutes to solidify completely. The bloom will spread until the entirety of the resin is an off-white color. Small amounts of resin cure slower than big amounts. When the resin drips left over from your pour (see the tops of the molds above) are completely cured, it's safe to remove your castings. You will always need to do some trial and error when casting and you will sometimes see bubbles collecting in certain areas. If, like me, you don't have a pressure casting setup, you can try a two-stage pour. When doing the Slimer main body, I noticed air bubbles in his eyelids and eyebrows. See the problem areas marked below:
Air bubbles often cling to small detail areas like these I decided to adopt a two stage pour to solve this problem. Basically, resin will stick to itself, so you can pour some into your molds while they are open, let it bloom, and then before it is completely set up, close the molds and do a second pour to finish up the casting. This gives you a chance to chase out some of the air bubbles from the problem areas.
Resin measured for the first half of the two-stage pour
After mixing I pour the resin into the two mold halves
Then I use a toothpick to tease the bubbles out of the problem areas
The resin has bloomed, so it's time to close up the mold halves At this point I gently invert the rear part of the mold and place it on the front part. Then I add the foamcore braces, band it together, mix the resin for the second pour and fill the remaining space with until the mold is full. Don't wait for the resin to completely cure before doing the second pour because it may not adhere to the first pour as well. Here is the result of the two-stage pour:
This is the result of the two-stage pour I used two different colors of resins here, so you can see the results of the two-stage pour. Air bubbles are minimal or nonexistent on the eyes nose and eyebrows. The teeth and lower jaw have been added here, but they are separate castings. Opening your molds should be done gently. Smaller parts will still be flexible, and so care should be taken when unmolding them. They will fully harden in about twenty minutes.
Opened mold for upper teeth and lower jaw casting This is what parts look like coming out of the molds. You trim off the sprues (filled pour spouts and air vents) and clean off the paperlike flash (resin that seeps between the mold halves) and the parts are ready to go. The lower jaw has already had the sprues and flash removed. Air bubbles sometimes creep in despite your best efforts. These are easily filled with epoxy putty. Note: After pulling about twenty or thirty castings, you will want to recondition your molds. You do this by baking them at a low temperature. Place some foil on a cookie sheet and then place your molds on the foil. Bake them for about four hours at 250 degrees. This will help redistribute the lubricants in the rubber to help retain its flexibility. Eventually the interior of your molds will become stiffer and less flexible, and bits will begin to tear off, adhering to your castings. This is a sign that the molds are wearing out and it is time to retire them. You can cut up old molds and recycle them into any new molds you pour (see the first two parts of this tutorial). That concludes what
has become a massive tutorial. I hope this helps you, and you get some
great castings of your sculptures or prototypes. Good luck! |
kay,
we've made our molds. Let's pour some resin!
