TUTORIALS - 5
PRINCIPLES
OF
RESIN CASTING
by Charles J. Parker
Casting parts is another way to make new parts or replace damaged parts. The problem with casting parts is that it is complicated process, very expensive and to get quality parts will require special equipment. That said, if you want to try to cast parts here are some tips.
The first thing you have to have is a master, the part that is going to be duplicated by casting. Masters must be clean and sharp, as any flaw or mark on a master will be duplicated and stand out in the resin copy. You need a glass finish to get a good part. Additionally, masters must be well built. A master will have to be immersed in RTV, stand the pressure of vacuuming and then be pried or cut out of the RTV mold. If it is not well made, it will break and become embedded in the RTV, destroying the master and the RTV mold.
The first thing you will need to do is determine if you can make your master part in one piece, or if you will have to make it in several pieces. Something else to watch out for when making your master is what is called undercuts or lips. These are parts of the master that will become hooked in the RTV and prevent it from popping out of the mold. You will also have to consider how your part will be molded and where and how you want to establish your pore vents.
The first thing you will have to do after building your master is make a RTV mold. Making a case for a mold is a simple process, basically all you have to do is construct a box out of plastic sheet or LEGO bricks. Some things you have to consider when making the box are:
After you have solved all of these problem you will be ready to mix your RTV to pour your mold.
First you need to decide what type of RTV and resin you will need. Dow Corning manufactures a number of types of RTVs for casting both resin and metals. Additionally there are a number of casting kits available on the market containing both suitable RTV's and resins. Things to consider when making your selection are the flexibility required to remove the part from the mold the strength required to support the part and the number of times you will need to use the mold to determine the type of RTV you will need.
To make a good RTV mold you will need to vacuum the RTV to remove the air bubbles from the RTV that will form during the mixing process and curing of the RTV. To do this you will need a vacuum chamber. Your vacuum chamber must be able to pull a vacuum and sustain it for several minutes to adequately vacuum resin and RTV. Mix the RTV according to the instructions of the manufacturer. Next you will need to vacuum the RTV just after you mix it. When vacuumed RTV will do what is referred to as boiling. The air in the RTV will bubble rapidly to the surface causing a froth on the top of the RTV. This froth will need to be sustained for several minutes to fully evacuate air from the RTV.
After this first vacuuming of the RTV it can be poured into the mold case around the part to be duplicated. Now you need to vacuum the mold case after it has been filled with RTV. This will ensure that the RTV has fully filled the mold and no air pockets are trapped around the master part. The RTV must now be allowed to cure and set up. Don't rush this process give the RTV plenty of time to set. After the mold has set up remove the mold from the mold case and then remove the part from the mold. (SEE DRAWINGS.) You now have the mold to make your parts.
Next you will have to determine what type of resin you will need to make the part out of. Things you will need to consider before deciding what type of resin to use is:
After you have decided on what type of resin you will use you are ready to start casting. Well almost now you will need a mold release agent. Another decision! You will also have several choices here. Liquid, spray, or powder, how do you want the agent to react to the part and the resin and what will the clean up on the duplicated part be. Now you are ready to cast well almost. How do you want to cast free pour, vacuum, or pressure. A free pour casting usually gives you your poorest results, air bubbles and under filled spaces. Vacuuming and pressure casting usually gives you your best results. To Vacuum you will need the vacuum chamber that you needed to mix your RTV. For pressure molding you will need a chamber that can be pressurize. Your pressure chamber should be capable of holding a 45lbs. per sq inch pressure. The process from here is to place your mold on a disposable surface that will collect any overflow. You will have spillage from boiling during vacuuming and it is messy.
Next mix (following the instructions) and pour your resin into the mold. You may need to physically work the resin into hard to fill areas of your mold even if you are vacuuming or pressure casting. Then place the filled mold into your chamber and pull a vacuum or apply pressure. If you have a clear chamber you can watch the process and adjust the pressure to minimize the spillage. If not you will just have to experiment on how much extra resin will be required in the reservoir to give you a fully filled part. After the mold has been vacuumed place it on a flat level surface to cure. After the part has been allowed to cure (again be sure to give it plenty of time) you can remove the part from the mold.
Now you will most likely ask yourself, what did I do wrong??? Casting high quality parts even with the best of equipment is difficult and will take a lot of experimenting and experience. In most cases however a suitable part can be reproduced from simple casting methods using all or only part of the process described. You may have more clean up on the part, and it may require additional sanding and filling but for reproducing just a few simple parts it maybe satisfactory. Don't expect to get manufactures quality cast parts with home casting methods.
