Mold Making and Resin Casting
by: Steve Sherman

Making your Molds
There has been quite a lot of recent interest in this subject so I figured I would help out and contribute a step by step article on how to go through the process of mold making and reproducing parts in resin.

First of all, let's go through the materials you will need to get started.
  • Clean mixing bowls - Plastic rubbermaid type works the best
  • Spatulas - The flexible kind that you would frost a cake with
  • Mixing cups ( I use the clear plastic drinking cups you can buy at a discount store or supermarket. They come in different sizes and usually have lines molded into the sides of them which makes 1/1 mixing easier)
  • Wooden stir sticks
  • Rubbing alcohol
  • Vaseline (petroleum jelly)
  • Different sized small paint brushes (cheap ones, not your Winsor and Newtons!)
  • Some clean rags - for wiping spills and your hands!
  • Sharp Hobby Knife
  • Plasticine clay (Any non sulfurous clay such as Kleen Klay)

There are other materials that will be discussed as we go along, so you may want to make note of them
The Basic Single over the top mold

This is the easiest mold to make. We use this type of mold for simple parts that have a front and sides, but no exposed detail on the back or bottom, such as turret hatches for instance; or cylindrical parts with no undercuts such as a figure torso that can stand upright in a mold box. An undercut is detail on a part that would prevent it from being removed easily from a single part mold. For example, a torso with arms attached could never be done with a single part mold because the arms would prevent the piece from being removed. You would literally have to cut the mold open to remove the piece being molded, thus, in most cases, ruining your mold.

For this tutorial, we will make molds for multiple small parts at once. (Fig 1) This allows flexibility in that when you are ready to cast parts, you can cast all of them in one pour, or just the ones you need. Here we have an ammo box and wood crate.

For this mold, we can make a mold box (see 2 part mold section) but because of the small size of these parts, we will use a piece of acrylic for the base, and hot glue a small cut-off section of PVC tubing for our walls (Fig 2). With this method, you have a mold box created in about 10 seconds. Small food storage containers also work well for makeshift mold boxes.
You want to make sure that your subject sits relatively flat in relation to the bottom of the container or base you are using. . You want about a half an inch of clear space on each side of the part for stability (Fig 3).

Now, depending on the material your subject is made from, it might have a tendency to float when you pour the silicone into the mold box. . If you think it may, you can secure it to the bottom of the container with a drop of super glue.

Now that our subject(s) is in the mold box, we are ready to mix and pour some silicone. For simplicities sake, we will use a 1/1 ratio RTV (Room Temperature Vulcanizing) Silicone (Fig 4). Although this type is more expensive by volume than it's 10 to 1 cousin, it is far easier to use and you are less likely to make a mistake with it. The 10 to 1 mix requires a digital scale and has a 24-hour set time. The type we will use here sets in 4 hours. You can start making parts after the set time expires!

Here is a tip that will help you in knowing how much silicone to mix without getting into complex math problems or guessing.
mixing tipWith your part(s) in the mold box, pour enough water into the box to cover the parts being molded by about a half an inch. Pour this amount of water back into an empty mixing cup. Now, in another cup of the same volume, pour half of this water into it until both cups have an even amount in them. This is the amount of each part of your Silicone you will need. Easy huh? In other words, volume of water in mold box equals total volume of silicone for mold. Mark a line on each cup at the water level and empty the water and dry them out really good. Do the same with your mold box. Use a hairdryer to make sure that it is completely dry. This is important.

Now, using the cups that we marked earlier, pour an equal amount of parts A and B silicone into each cup, right up to your marked line (Fig 5). The silicone we are using has one white part (A) and one blue part (B) and when mixed will by a light powder blue in color.

In your separate clean mixing bowl, pour both of these in. Make sure you scrape the sides of the cups really good to get all of the mix. Use your spatula to mix these together in the bowl really well. You want to mix, not whip these as whipping causes air bubbles to form in your silicone.

Scrape the sides repeatedly during this process. When it is thoroughly mixed, you will not see any white streaks (Fig 6).

We are now ready to pour. Holding your mixing bowl 8 - 12 inches above the mold box, begin pouring a thin stream into a corner away from your part(s). Let the silicone flow over the part (Fig 7). This is also important in insuring that no air bubbles get trapped against the part(s) and that there are no voids in the mold. Once poured, you will see the silicone start to de-air as bubbles will rise to the surface of the mold.

Once all of the mix is poured, let it sit for 4 hours. (I usually mix before I go to bed or leave the shop at night and then de-mold my parts in the morning, but 4 hours is all you need for the 1/1 type rapid cure RTV) Your part should be gently coaxed from the mold, but should come out rather easily (Fig 8 and 8a).

That is it for your Single one-piece mold! We will make parts with this mold later. Now, lets make a more complex 2-part mold.
The 2 Part Mold
The 2 part mold requires a bit more planning than the single discussed previously. This mold is for any part that has detail on both sides, such as road wheels, figure parts or even whole figures and parts with heavy undercuts that would get locked into a one-part mold.

For this example, we will use a head from a figure that I have been meaning to make a duplicate of for practicing painting techniques, so no better time than now. You can see why a one-part mold would not work on this piece, as the chin would lock it into the mold. Sometimes you can get away with a single piece mold for a part like this if, for example, you only need a couple of pieces reproduced. If you pulled hard enough, it will come out! Forcing a part from a single mold like this would certainly result in tearing of the mold, maybe not the first time, but eventually.

Here is where we will need to construct a mold box. I use a few different materials for my boxes, such as particleboard, Plexiglas, styrene and even Legos!! (plastic building blocks)

We will use particleboard for this one.

The first thing you need to do when making a 2 part mold, is to decide on where to split your part. In other words, you have to use common sense here because you don't want a seam line running across some important detail like a facial feature. This is why on molded figures, you see the seam line running up the sides of the head as it is less conspicuous there and easier to remove. That is where we will put this one too.
Lets use a piece of acrylic for our base. When duplicating a hollow part, be sure to fill any open cavities that are not part of the detail to be molded (Fig 9). To start, we need to make a clay bed under our subject for support. You want these supports, or "pillars" to be 1/2 to 1 inch high, depending on the size of the piece to be molded. (larger pieces require more clay for support as more silicone, thus more weight, will be on top) (Fig 10) The last thing you want is for your clay bed to collapse once your silicone is poured) By suspending your prototype, you are also allowing air bubbles to rise away from it more easily to what will eventually be the outside of the mold.

Next, use similar sized pieces of clay to build your bed outwards (Fig 11). Remember that we want about a half an inch space between the sides of our box walls and the prototype.

Now that your part is suspended from your base, use fairly flat pieces of clay and start sealing your part at a right angle to the clay. Work a side at a time, sealing as you go. I just use my finger here to "rough in " the prototype (Fig 11a).

Use a flat scraping tool (I use a bent, flat file, sanded smooth and with rounded corners) to be sure that your clay bed is sealed against your prototype (Fig 12). Try to be sure that you seal the clay at right angles. In other words, try not to have your clay at an up or down slope.

Once your clay is sealed all the way around to the prototype, it is time to clean the part of any bits of clay that will affect the mold. Use your scraping tool to remove what you can, and then use a brush dipped in rubbing alcohol to clean the rest (Fig 13). Rubbing alcohol will dissolve the clay residue left on your part to insure that none of the detail is lost. Remember, your silicone will pick up every bit of detail including bits of clay!

Use a hacksaw blade to cut each side square as shown in Fig 14. Once this is done put up one wall of your box. Seal it to the base using hot glue. Repeat this all the way around the subject until you have your completed mold box, using hot glue at the corners and at the base (Fig 15). Then seal the clay where it meets the walls. I have found that the rounded end of a paintbrush works best. If you have any gaps here, break off small chunks of clay and press them in using the brush or your flat scraping tool.

Many people build their mold boxes before they embed their subject but I build them a side at a time. The reason I do this? I have found that it is easier to work the clay bed and gives you better access to the sides of your prototype to make sure you have a good clay to prototype seal.

The next step is to make dimples all the way around your clay bed. Use two different size brush ends to make these, making sure not to get too close to your prototype. Make the dimples about 1/4" deep (Fig 16). These dimples will become keys after the silicone is poured and will help lock your mold halves together when making a resin pour (Fig 17).

I usually run a bead of hot glue up each corner of the mold box from the clay bed to seal this area.

That is it. You are now ready to pour your silicone! You can use the same method to figure out how much silicone to use as earlier. Once you have done this for a while, you will start developing a feel for how much silicone to mix. If you are a little short, don't panic. Silicone bonds to itself so it is a simple matter of mixing some more and pouring right over the top of your cured or uncured silicone. I have run completely out of RTV and have had a part sitting half molded for a week and it comes out fine once you finish the pour.

Again, you can use the same process discussed in the single part mold section to measure out your silicone.

Mix in the same manner described earlier and pour (Fig 18 and Fig 19) Once this has set for 4 hours, you can now break down the mold box to begin the process of pouring the other side. Once the sides are removed, put them to the side, as we will re-use them for the second pour. Turn your half poured piece over and set it back on the same base (Fig 20). Start removing the clay bed. The clay bed will come off fairly easy.

You will find that you still have some clay bits adhering to your proto type. Use your flat scraping tool to gently remove the bigger pieces and a toothpick to remove bits from any detail (Fig 21).

Now, as before, use your brush dipped in alcohol to dissolve the remaining clay bits and residue right down to the silicone seal at the prototype. Be careful not to pull on the prototype itself, thus breaking the seal or you will have silicone from the second pour run down the seam where we don't want it.

Now rebuild your mold box in the same manner as before. Next, use a brush dipped in petroleum jelly and liberally apply it to the surface of the cured silicone (Fig 22). This is very important. Silicone bonds to itself and without this barrier, your part will be locked into a cube of silicone.

Try to be careful about getting Vaseline on your prototype. It won't hurt anything but can fill in fine detail so wipe any excess off with a clean brush.

We are now ready for the second pour. Mix silicone as described before and pour. In 4 more hours, we can begin making duplicates of our master.

After the second pour has cured, break down your mold box. Gently separate the 2 halves of the mold (Fig 23). They should come apart pretty easy here but you may have to coax it a bit.

Now remove your prototype. We now have to create a pour spout and a vent hole. There are a few ways of doing this. You can embed a piece of tubing in your clay bed before you pour the silicone on both halves, then when you are finished, remove the tubing and you have a pour spout pre-formed and ready. I used to do it this way, but it seems like more work so here is the way I do it now.

Study the mold and decide the logical place for the pour spout and vent(s) to go. Obviously, it needs to be at the top of the mold, but remember. Wherever your pour spout and vent are located, you will end up with a "sprue" at that location so an inconspicuous spot is best. For this head, the most logical place for the pour spout is at the neck where it will not be seen. Where your pour spout goes, your vent needs to be located in the general vicinity. The idea is to allow the air bubbles in the resin to escape through the vent, so it logically needs to be at what you determine the top of the mold to be. Some air bubbles will escape through the pour spout also, but with a vent, you get less bubbly mess coming back up through the pour spout.

I have made mistakes before and put the vent hole lower than the top of the pour spout. Guess what happens? Your resin runs out the vent hole before the mold is filled. So again, plan this step carefully.

Figure 24 is an example of pour spout and vent locations, and a description of how the vent works.

The way I create these is simple. I put the 2 mold halves together and mark both sides with a marker at the locations where the spout and vent will be. Then I separate them and using a sharp hobby knife, simply cut away these areas of the mold (Fig 25). You need to make sure that your pour spout is wide enough where it meets the detail part of the mold (the part we keep) to allow a good flow of resin. Note that on this piece, I did not have room to cut in a vent. So our pour spout will have to act as the vent on this one and that is OK. Whenever possible though, put in at least one vent. For very complex, heavily detailed pieces, multiple vents are very helpful in assuring that you end up with minimal air bubbles on your casting. Lets get the mold ready to pour some resin. For a 2 part mold like this, we need to band it together tight enough to where no resin will escape from the seam. I simply take a couple of particle board blocks from the mold box, and rubber band them to the front and back of the mold, fairly tight but not tight enough to where it will deform the mold (Fig 26).

For our single mold, all we have to do is mix up some resin and pour.

We will fill our molds in the next section, making resin casts.

Cleanup: What do you do with your mixing bowls, spatulas and any containers with cured silicone? Toss them right? No, re-use them! Since RTV only bonds to itself, you can easily peel away all cured silicone from your tools and containers and they will be as good as they were before you started.

That covers our mold making section. Silicone is really easy to work with and I hope that I have been able to help you on your way to making nice molds of your work. My way isn't the only way. But I have found that these are the methods that work well for me. Give them a try!
Mixing Resin and Making Cast Parts
We now have our two molds ready for some resin. But before we start, let's discuss resin, the pre-cautions you should take when working with it, and the materials you need before you start.

First of all, there are many resins that you can use, but the most common for making the plastic like parts that we use in modeling is a 1/1 ratio polyurethane. There are a few variables when dealing with this type, such as working time, de-mold time, shore hardness and so on. For our purposes, we want a resin with a shore hardness of 70-75D as that sets very hard but not brittle. Most basic casting resins have this rating. There are a few variables within this range also. The brand that I work with, has a 1 1/2 minute working time (mixing), a 3 minute pot-life and gels in 5 minutes with a de-mold time of 20 minutes. You can produce multiple parts very quickly with this type of resin. Believe me, 1 1/2 minutes doesn't sound like very much time but once you start mixing, you will find that it is more than enough. I usually let my parts sit in the mold for a half an hour as 20 minutes, depending on the temperature and humidity in the pouring location, can sometimes leave the parts a bit soft, thus deforming them while removing. Resin generates heat, so you will know if the part is ready to remove or not. If your mold is very warm to the touch, let it sit a bit longer. A general rule of thumb is, the larger the part, the more heat generated, thus the quicker the part will set.

Another point about resin; Moisture is it's enemy. Always keep your resin stored in the airtight containers that it comes in and always try to use up your stock within 30 days of opening.

Figure 27 is a picture of the resin that I use and stock

The materials you need to mix the resin are pretty basic. Some clear plastic mixing cups (the same ones we used for the RTV are fine) stir sticks, some disposable latex gloves and some wiping rags to address spills. When deciding on the amount of resin to use, you can rely on the same water principle that was discussed in the silicone section. However, as mentioned before, water and resin do not mix! Any moisture left in your mold will ruin your casting. So I use a different approach. Because I do a lot of casting of many different products, I have a multitude of molds on hand. So I usually try to mix up enough resin to fill multiple molds, or at least keep another large mold near by so if you have extra resin from the part you are making, you can just pour it into the "reserve" mold so it doesn't go to waste. I make many parts this way in multiple pours. That is, if I have any excess resin from the primary part I am making, it goes right into a reserve mold of another part, even over the top of cured resin. It is possible that new resin over cured resin may not form as strong a bond, but I have only had this problem once, where a part separated at two different pour lines. Unless you are dealing with a structural part, you don't need to worry about this. I stress though, if the part needs to be a structural member, do it in one pour.

OK, we have our molds ready to go from earlier. Let's mix some resin. Earlier I suggested the use of latex gloves because that is what the manufacturers recommend. I personally do not use them. I have had my hands covered (accidentally of course) in resin and have had no ill effects from it. I caution you though. If you do get resin on your bare skin, use the wiping rag you have handy to take care of it, finish the pour, then get to a sink and wash the affected skin immediately with soap and water. I should also mention that the resin I use has no odor or noxious fumes while setting so no mask system is needed. Most urethane resins are this way. Polyester and epoxy resins are another subject and we will not cover the hazards of such here.

Decide on the amount of resin needed to fill your molds and pour half that amount of part A and half of part B into separate mixing cups. It is VERY important that these amounts are identical (Fig 28).

Then combine these parts into one cup and mix thoroughly with a mixing stick. You should see no swirls of either part when mixed properly, that is, it will be a nice amber color when mixed (Fig 29). The cups will all be thrown away as once the resin cures, they are unusable. I do use both sides of a mixing stick though, so I get 2 mixes out of one stick. Now we will pour resin into both of the molds that we made earlier. Form a pour spout in the plastic cup that you will pour from and carefully fill the mold(s) with resin (Fig 30). In certain situations, for example, if you have a part that runs long horizontally, you may want to make 2 pour spouts instead of one to insure that the mold gets completely filled. A turkey baster works really well for pumping resin into hard to reach areas of a closed mold. You will know they are filled when you see resin come back up through both the vent and the pour spout.

If you have a pressure pot, put the molds in now and pressurize to 60 psi. A pressure pot works by "squeezing" the air bubbles so tight, they are microscopic, and thus you get cleaner castings.

Warning: Never try to make a pressure pot out of homemade items such as a pressure cooker. Only a pot rated for this psi should be used and it needs to have a locking lid.

A home made pot with an improper psi rating can explode, sending shrapnel in every direction, causing serious injury and yes, even death!

I have found that if you plan your pouring and venting system carefully, you will only need a pressure pot for the most complex, detailed parts, and even then, it is not necessary, but expect to do a little filling of air bubbles. Here is a tip for filling air bubbles and pinholes. When I get air bubbles that need filled, I usually put that part aside until I make another pour. After you do this, you will have some resin still in the bottom of the pouring cup. Wait until this gels and using a stirring stick as a spreader, fill the holes in the affected piece.

Once poured, you will see, in about 5 minutes, what we call the "bloom". That is the catalytic reaction between the 2 parts, which causes the resin to harden. You will know that you mixed your resin properly when you see this (Fig 31).

After 30 minutes, you can remove the parts. Remove the rubber bands and support pieces and separate the 2 halves on the 2 part mold. Gently flex the piece and it will pop right out. On the single part mold, I usually apply a little upward pressure with my fingers from the bottom of the mold to loosen the parts up (Fig 32). Some have asked me about mold release. I personally do not use it for a couple of reasons. First, it is not necessary. Your parts should come cleanly out of the molds on their own without the aid of a release. Secondly, some resins and mold releases can react to each other and ruin your castings. You should be able to get 75 - 100 casts out of a single mold. After awhile, the oils in the silicone will breakdown and your castings will start to stick a bit. There is a solution to this that will net you many more castings and revitalize your molds. When your parts start to stick (usually after 20 or so castings) place your mold in a 250 degree F oven for a couple of hours. This redistributes the oils thus reconditioning the molds.
A note about re-casting:
Whenever you make a mold, unless it is your own work such as a sculpt or scratch-built part, you have to take into consideration the ethics of what you are about to do. Remember, if it is not your work, it is somebody else's, and copying that work, however innocent it may be, is still probably a copyright violation to the licensee of some sort. While I doubt that any of the companies involved would get too upset about you making a few copies for yourself, distributing those copies is another story and this practice can lead to big problems for you and whoever is involved.

Another even more frowned upon by your peers example of re-casting would be to copy a forum members hard work without their express permission. Even if it is just making copies for yourself, it is an unethical practice that has garnered many a violator banned from certain other forums. While I don't think we would have that problem here on Armorama, it is still an important point to make and one not to be taken lightly. So use discretion whenever you copy a piece that did not originate from you. Show the artist the respect they deserve and harmony will abound!

That about covers it. I hope this article has helped you in gaining a basic understanding of molding and casting. If you have any questions about anything to do with this subject, please feel free to contact me on the forum. I am there quite often!! Happy modeling.

This article comes from Armorama