Casting in plastic


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Casting using
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Casting using plastic or resin will reduce the life of the mold!
If you cast using plastic or resin, this will greatly shorten the life of the mold. However, there are many items that simply must be cast in resin or you will not be able to remove them from the mold (such as the graveyard fence), or there are pieces that will not hold up to constant handling unless you cast them in resin.

Try not to let this bother you too much. Keep in mind that molds are tools that are meant to be used. As long as you get the pieces you need out of the molds, then they will have served their purpose.

The pros and cons of spray mold release.
Preparing a mold for resin.
Mixing and pouring resin.
Removing and cleaning the pieces.
Painting resin cast pieces.
Basic information about casting resin.
Types of resin I have used.


The Pros and Cons of Spray Mold Release
Spray mold release has it's good and bad points. The mold release you see here is Smooth-on Universal Mold Release

The good point is that spray mold release will extend the life of the mold greatly if you are casting in plastic. So far, I've gotten 50 casts from a mold with no apparent wear on it using casting resin. However, results may vary depending on how well you spray the mold, what brand of mold release you use and what type of casting resin you use.

The bad point is that pieces are extremely difficult to paint if you use the spray mold release. Even if you wash the pieces you will have a very difficult time getting regular hobby paint to stick to the plastic. The mold release forms sort of a greasy coating which protects the mold but doesn't help much when you're trying to paint your pieces.

I have added some painting instructions further down on this page. This will help paint stick even if you do use mold release but the paint will stick better if you do not use it. I still use mold release to pop air bubbles in the resin and keep the resin from sticking to the plastic or glass when casting so you'll probably want to pick some up anyway.

You may want to do one casting using the mold release and see if it causes you any problems when painting. If so, then you might not use the mold release (only use talcum powder) and just let the molds get damaged over time. I have not found any good solution to this problem yet.

Preparing a Mold for Resin
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1. For this demonstration, I will be using a product called Smooth-Cast 300 from www.smooth-on.com. However, this doesn't mean that this is the best product to use! It just happens to be the one I've tried out and found that it works well. There are simply too many products out there to review them all.

The mold release agent I'm using is called Universal Mold Release and you can find it at www.smooth-on.com.

At the end of this article, I will review a couple of products and tell you where you can find them. I also intend to add a section on casting resin to the "Customer reviews of US products" page.

2. Tape a piece of plastic or trash bag down onto your work surface. The liquid plastic will spill over, and it's much easier to get it off of a trash bag than to scrape it off of your table. Be sure your work surface is level!

Spray the plastic with mold release. The kind of mold release I'm using is Smooth-on's Universal Mold Release.

3. You'll need a piece of glass or plastic large enough to cover the face of the mold. The easiest way to get a piece of glass is to buy a small picture frame at Wal-mart (it only costs a couple of dollars).

Spray the glass or plastic well with mold release. This will keep the casting resin from sticking to it.

You can use a CD jewel case cover instead of a piece of glass.

Several people have had trouble with the liquid plastic sticking to the glass. A piece of hard plastic or plexiglass is much safer than using glass and it's much easier to remove from the casting.

4. Spray the mold with mold release. Be sure to cover the entire face of the mold. Mold release will protect and add silicone back into the mold.

Dust the surface of the mold with talcum powder. Talcum powder will help the liquid plastic flow into all of the small crevices and details of the mold.

5. Hold the mold over a trash can. Tilt the mold and tap the sides to spread the talc around. Turn the mold a 1/4 turn and tap the next side. Continue all the way around until the talc covers all of the pieces in the mold.

Turn the mold upside-down and smack it face down onto your hand (over a trash can). This will knock the excess powder out of the mold.

Mixing and Pouring Resin
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1. Measure equal amounts of part A and part B. I'm using 3 ounce cups. You only need about 1/2 ounce in each cup to fill this mold (#46 gothic graveyard).

Pour them together and stir with a stick. I'm using craft sticks I purchased at Wal-mart (in the craft department).

2. Pour the resin into the mold. You'll want to cover the entire face of the mold with resin.

You'll notice small air bubbles coming to the surface. Spray mold release on top of the mold to break these bubbles! They could cause problems later on if we don't get rid of them.

3. Now for the piece of glass. Be sure to put the sprayed side of the glass facing down! If not, plastic will stick to the glass making it difficult to remove the pieces.

Place a piece of glass over the mold, setting down one end first. As you hinge the other side down, the excess resin will squeeze out from under the glass, pouring onto your work surface.

4. Press down hard on the glass and shove the glass around in a circular motion to remove the excess casting plastic.

You want as thin a layer as possible of plastic between the glass and the mold. If too much plastic is left between the glass and the mold, then trimming the fence pieces will be more difficult.

The excess plastic will spill over onto your work surface. Once it hardens it can be removed easily. Let this set for 15 minutes.

Removing and Cleaning the Pieces
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1. Once the mold has set for 15 minutes, the plastic will have hardened but it will still have a little flex to it. You want this flexibility because it will keep the fence pieces from breaking as you remove them from the mold. If you are using a different casting resin than I am, the setting time will be different.

Since we sprayed the work surface (trash bag) with mold release, the mold and excess plastic will pull off of the surface easily. Remove the ring of excess plastic that formed around the mold when it squeezed out from under the glass.

2. Remove the glass from the mold. If you sprayed enough mold release on the glass, it should pop off easily.

Sometimes plastic will stick to the glass anyway. To remove this excess plastic, use a straight edge razor or glass scraper. You can find these at any hardware store in the paint department. Do not scrape toward your hand or fingers!

3. Remove the excess plastic around the side of the mold.

Start removing the tombstones that set around the outside of the fence pieces. With these pieces out of the way, it will be easier to remove the fence pieces.

4. Break the two fence sections apart. To remove the fence and post as one piece, start at the bottom corner of the fence post.

Flex the mold away from the bottom of the post. Try not to bend the fence or post. Remember, the molds are meant to flex, not the plastic pieces you cast in them.

5. Remove the other fence section and post in the same manner.

If you pressed hard enough on the glass when casting, the excess flash (left over plastic) should be very thin and easy to break off.

6. To remove the excess flash from the inside of the fence sections, use an x-acto or hobby knife.

Poke the knife up through the thin plastic and trace around the inside of the fence with the knife.

The flash should be so thin that the knife will easily trim off the excess with little or no pressure.

7. This finished fence section only took me 90 seconds to trim out from start to finish.

Here is a sample of the plastic cast pieces from mold #46. The skeleton in the coffin is a Warhammer(TM) figure from Games Workshop.

Painting Resin Cast Pieces
1. Paint will not stick to your pieces if you do not do these steps!

First, you should wash the pieces with soap and water to remove any greasy residue. I use dish soap and a toothbrush.

If you did not spray mold release in the mold and only dusted it with talcum powder instead, then you may not need to wash the pieces.

2. Once the pieces are dry, give them a quick coat of Krylon Fusion for Plastic spray paint. If you cannot find this kind of paint then a good quality auto body primer may work also.

You can use black or white depending on how you usually paint your pieces. Be sure to follow the instruction on the paint can and let it dry the recommended time.

The Fusion paint will bond to the resin and the your acrylic craft paint will bond to the Fusion fairly well. This information was supplied by Steven Trotter from our forums.

Basic Information about Casting Resin

Here are a few terms you might want to know. They will help you decide on what type of plastic to use. I've only listed four that are the most important to me. If you go digging for information you'll find a bunch of other factors that I haven't listed here, but for what we do they really aren't as important.

  • Mix Ratio - Most are equal part of A and B (mix ratio of 1:1).
  • Pot Life - The amount of time you have to pour it into the mold after you mix it up.
  • Demold Time - How long you have to wait before you can remove the piece from the mold. The thinner the piece is that you cast, the longer you have to wait.
  • Viscosity - How thin the mixture is when you pour it. Viscosity is measured in Centipoise (cps). The higher the number, the thicker the mixture is. Water is from 1-50cps. Corn oil is from 50-100cps and honey is from 2000-3000cps. The thinner the casting resin, the more easily it will pour into the details.
  • Tensile Strength - How much you can bend a piece before it breaks. The higher the number, the more force it can resist before breaking.

Types of Resin I've Used

These are a few types that I have tried out. It doesn't mean that they're the best, it just means that I haven't had time to try out any others.

Smooth-cast 300 from Smooth-on - My favorite casting resin so far.

Mix Ratio 1:1
Pot Life 3 minutes
Demold Time 10 minutes
Viscosity 80 cps (almost water thin)
Tensile Strength 3000 psi
Cost 2 pint kit is $21.00, 2 gallon kit is $62.60

Smooth-on's web site is at www.smooth-on.com or you can call them at 1 (800) 762-0744. You can order a trial sample directly from them, but you'll have to find one of their distributors to order anything else (which is easy to locate on their web site).

Advantages: It's water thin, which helps the plastic flow into all the details. Bubbles also pop easily because the mixture is so thin (especially with a quick spray of mold release).

It sets up quickly. The pot life says 3 minutes, but don't believe it. If you don't get it poured in 1 minute then you might as well forget it. Once you pour it in the mold, it sets up and can be removed from the mold in about 15 minutes. If you wait longer than that, the pieces slowly turn more brittle and are harder to remove from the mold without breaking.

Disadvantages: It's a little more brittle than other plastics I've used when fully cured. It's also fairly expensive.



Hardcast 300 from Silpak - My second favorite casting resin.

Mix Ratio 1:1
Pot Life 3 minutes
Demold Time 30 minutes
Viscosity 150 cps (consistency of milk)
Tensile Strength 2389 psi
Cost 2 pound kit is $5.78, 2 gallon kit is $63.04

Silpak's web site is at www.silpak.com or you can call them at 1 (909) 625-0056. You can order the product directly from them, but they prefer to sell to distributors.

Advantages: It's not quite as thin as the smooth-cast, but pretty good. The bubbles don't pop quite as easily, but spraying mold release will get rid of most of them.

This product is very durable (pieces will flex instead of breaking) so they hold up a little better than the smoothcast. The price for the 2 pound kit is the cheapest I've seen for casting resin.

Disadvantages: The company will fill the order for you directly, but prefers you go through a distributor. But since they don't have an organized distributor list, that makes things a bit awkward.



Alumilite Super Light Liquid Casting Plastic

Mix Ratio 1:1
Pot Life 90 seconds
Demold Time 5 minutes
Viscosity 190 cps (consistency of thin honey)
Tensile Strength 6000 psi
Cost 28oz kit is $30.25, 2 gallon kit is $94.00

To order, call the Alumilite company at (800) 447-9344 and order it directly from them. They have a web site at www.alumilite.com, but you can't order online.

Advantages: It sets up quickly. Once you pour it in the mold, it sets up and can be removed from the mold in about 5 minutes so you can make lots of copies quickly. Thin areas and small sharp edges take longer to cure. It's also the strongest material I've used so far.

Disadvantages: It's thicker than the other casting plastics I've used, so you might have a problem casting fine details or undercuts. Bubbles are also difficult to get rid of, because they don't pop easily. Combine that with a very short cure time and you don't have much time to get the material into where you need it. It's also one of the more expensive plastics I've used.

However, they do have a slower setting white version that is very close to the Smooth-cast 300 listed above. It does not set up as hard as their tan product shown here but it also has the advantage of not being as sensitive to moisture as the smooth-cast products.



There are hundreds of other products available. To find them on the internet, type in the words "casting resin" on your favorite search engine. I'll try to update my customer reviews page in the future to include different types of casting resin.

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