Pattern Making for Mold Making & Casting

What is pattern making?

Pattern making is the art of making an original pattern or form which will be used to make a mold. The mold will allow for mass production of the original pattern by poring some sort of casting material such as metal, resin, plaster, or other material. For our purposes (we will assume that) we are going to use a urethane resin such as Alumilite.

A pattern can be as simple as an object that has detail on only one side such as a plaque, or it could be very complex with detail on all sides such as a figurine, or possibly a machine part.

There are many things to consider when making a pattern. Durability, shrinkage, machine allowances, draft and undercuts to name just a few.

Durability is directly related to how often the pattern will be used, and the material that it is made of. The best compromise would be to make the original pattern of a material which allows for as much detail as possible and is easy to work with. You would then save the first few castings to use as patterns if the original were ever damaged.

Shrinkage is negligible when casting with urethane materials such as Alumilite, however it needs to be considered in relation to the material that will be used to make the finished casting. Steel for an example shrinks at the rate of 1/4 inch per foot as it cools from the molten state after it is pored into the mold.

If you are planning on casting with low melt metals or plaster then it would be wise to consider shrinkage when making your pattern. If you have large castings or thick pieces shrinkage may also be a factor especially if two pieces must fit together with any precision. It may be necessary to add a small amount of material to your pattern to compensate for this shrinkage.

The materials like Alumilite that we will be using for most of our modeling applications have very small shrinkage rates which should not be objectionable.

Shrinkage rate of castings

Pure aluminum	13/64 in. Per ft.
Nickel aluminum casting alloy	3/16 in. Per ft.
Cast iron	1/4 in. Per ft. *
Steel	1/4 in. Per ft.
Malleable iron	1/8 in. Per ft.
Tin	1/12 in. Per ft.
Britannia	1/32 in. Per ft.
Thin brass castings	11/64 in. Per ft.
Thick brass castings	5/32 in. Per ft.
Zinc	5/16 in. Per ft.
Lead	5/16 in. Per ft.
Copper	3/16 in. Per ft.
Bismuth	5/32 in. Per ft.
Hydrocal	3/16 in. Per ft. Or greater
Molding plaster	1/4 in. Per ft. Or greater
Alumilite	Negligible *

* note: the shrinkage for cast iron varies depending on the size and thickness of the castings.

* note: although the shrinkage for alumilite is negligible , it should be considered when making large or thick castings. It may also be a factor where two or more castings require a precision fit.

Machine allowance is a small amount of material which is added to the pattern in areas where it will be machined in order to meet exacting standards. It is unlikely that you will need to add machine allowance for most modeling applications.

Draft is a small amount of taper built into the pattern which will allow it to be more easily removed from the mold. It is not always possible to have draft on all surfaces, but it sure makes life easier if you can include draft where possible.

Undercuts are depressions or bulges which prevent the pattern from being removed from the mold. They usually require a more complex mold with two or more parts.

If the mold is going to be made of rtv rubber it is important that the pattern is sealed with lacquer or made of a material which will not inhibit the rubber and prevent it from curing

What is mold making?

Mold making is the art of making a mold which will be used to make a casting or finished part. The mold could be made several different ways, however the simplest and most cost effective way is to make an RTV "room temperature vulcanizing" silicon rubber mold from an original pattern.

Why use RTV silicon rubber to make the mold?

RTV rubber allows an almost exact replication of an original pattern without the tremendous cost of having an injection mold made in a machine shop. It is an excellent way for a model maker to make repeated copies of an original at a very reasonable cost.

When should you make an RTV rubber mold?

Any time you need repeated copies of a part or accessory that is not commercially available.

How do you make a mold?

For simple one piece molds you start with your original pattern. Glue your pattern down to a smooth flat surface. Build a mold box around it leaving at least 1/2 in. All the way around your pattern, and making sure that when you pour the RTV rubber in the mold box that it will also cover the highest part of your pattern with at least 1/2 inch of rubber.

The mold box can be made using styrene plastic for small molds, or any number of other materials including wood and metal. One easy way is to use small pieces of angle iron or aluminum. You would place them around your pattern leaving the proper clearance and hold them in place with klean klay.

Next apply a "mold release" to the mold box and the pattern. This will allow you to remove the pattern from the mold and the mold from the mold box with much less chance of damage to the mold.

Note: there are many "mold release" agents available commercially, however if you run out or want to make your own here is a simple formula. Melt five parts Vaseline petroleum jelly in an old soup can in the oven. Remove it from the oven and mix into ninety five parts of mek ( methyl ethyl keytone).

Caution note - mek is an extremely flammable liquid. It could also inhibit the RTV rubber from curing if you do not allow the mek to completely evaporate leaving only the Vaseline remaining in the mold.

Next you need to determine how much rubber is needed for the mold. Measure the cubic inches in your mold box - length times width times height. Next determine the cubic inches in the pattern and subtract that amount from the mold box to determine the amount needed to fill your mold.

There are approximately 22 cubic inches to the pound of RTV rubber, so you would divide the total cubic inches of the mold by 22 to give you the number of pounds of rubber to fill your mold. If you further divide the pounds by 16 that will be the number of oz. of rubber that you will need. Each type of RTV rubber has a slightly different weight, so data on the information sheet for the specific product you are using.

Before mixing the rubber, make certain that the mold box is on a flat level surface.

Mix the rubber following the instructions on the container. The ratio is 10 - 1 (ten parts rubber to one part catalyst).

Important: mix the rubber in a container that is five or more times larger than the volume of the rubber. This is necessary because when you de-air the mixture it will rise in the container three to four times its original volume as the air is removed.

Note: do not be alarmed when the rubber expands during this de-airing process it is quite normal. Much of this air was literally mixed into the rubber during the mixing process, and it must be removed.

Place the rubber in the vacuum chamber and close the release valve and turn on the pump. Continue to de-air the rubber until it has reached 29 inches and for at least two minutes after the rubber has stopped bubbling and it has returned to its original volume. Slowly open the relief valve and allow the chamber to gradually return to normal atmospheric pressure.

Remove the container from the vacuum chamber and slowly pour the rubber into your mold near a corner of the mold. Allow the rubber to flow across the mold slowly, this will help to prevent trapped air bubbles from forming that could ruin the mold. If you suspect an air bubble may be trapped you can carefully probe the area with a toothpick or dental tool to dislodge it before you continue poring the rubber in the mold.

Place a piece of glass that has "mold release" on it on top of the mold box allowing the excess rubber to flow out around the edges. This will give a nice smooth surface which is parallel with the bottom of the mold box. This will be important later on when casting with urethane in the mold.

After approximately 24 hours the pattern can be demolded. It is important not to be in a hurry during this process to keep from damaging the mold. A small amount of alcohol can be squirted into the joint between the mold and the pattern to help facilitate the patterns removal.

After the pattern has been removed from the mold you can trim all the corners of the mold with a razor knife, clean the mold with tolulene, and powder the entire mold with a coating of talcum powder to help protect the new mold.

Note: you can significantly increase the life of the mold by as much as 200% by using a barrier coat in the mold when you are casting with urethane materials.

Multiple piece molds

For patterns with undercuts, you will need to make a mold with two or more pieces.

First half

You will first need to decide where the parting line of the mold will be. You can draw a line on your pattern to help you determine how to position the pattern in the mold.

After you have built your mold box, place it on a flat level surface and position your pattern in the center of the mold box with the pattern embedded in the clay up to the predetermined parting line. The clay needs to be free of any sulfur content such as klean klay in order to prevent inhibiting the rubber from curing.

Using a pencil eraser or an Allen wrench or other such object press into the clay around your pattern to make keys or alignment pins for the mold. This will allow the two halves of the mold to line up perfectly when they are joined together for casting.

Coat the pattern and mold box with a release agent. Mix the rubber and pour in the same manner as a single piece mold.

Second half

When the rubber has cured demold and remove all the clay from the mold. Once again coat the pattern, mold box, and the surface of the first mold half with release agent. It is very important that the rubber is coated with release agent to prevent the two halves of the mold from sticking to each other.

Allow the mold to cure for 24 hours and then carefully remove mold from the mold box, separate the two halves of the mold and remove the pattern from the mold. Clean the mold with tolulene and coat both halves with talcum powder. You are now ready to begin making castings.

What are castings ?

Castings are objects which are made by pouring some sort of casting material in a mold. The mold can be used repeatedly to make castings that are identical to the original pattern.

Why use alumilite to make the castings?

Alumilite is a two part polyurethane plastic material which hardens in as short a time as three minutes. It is almost odorless, non toxic and non carcinogenic. Alumilite will allow an almost exact copy of an original pattern without the tremendous cost of having an injection mold made in a machine shop. After it hardens, it can be drilled, sanded, machined, and painted.

When should you make an alumilite casting?

Any time you need repeated copies of a part or accessory that is not commercially available.

How do you make a casting?

First prepare the mold by heating it in an oven or microwave until it is worm to the touch. This heat will help to speed up the curing process.

Next, coat the inside of the mold with a parting agent such as a commercially prepared release agent, or you can paint the inside of the mold with a lacquer base paint. The paint will add color to your casting and also act as a release agent. The paint will chemically bond to the plastic as it cures.

If you do not use a release agent you can use baby powder on the inside of the mold. This will help to release the surface tension in the mold that will cause small air bubbles on the surface of the casting. The powder will not however protect the mold like a barrier coat.

Mix the alumilite according to the instructions included in the package. Make sure to shake the "b" side of the mixture before combining it with the "a" side.

Measure equal amounts of part "a" & "b" and pour them into a separate mixing container. Stir until thoroughly mixed (about 30 seconds) and pour into your mold.

If you want a flat surface on the back of the mold, you can over pour the mold and put a piece of glass on the mold. This will give you a nice flat surface on the back of your casting.

The casting will begin to harden within three minutes, and will be ready to remove from the mold in around six minutes. Within twenty-four hours, the casting will reach its full hardness and can be machined, drilled, or tapped.

If you plan on painting the castings they will need to be washed with soap and water prior to painting to remove any oil residue that is left on the surface of the casting.

Reference materials

1) Alumilite instruction manual (included with the 28 oz. kit.)

2) Making and Casting RTV Rubber Molds (M.S.Productions)

3) "Tips for Successful Mold Making" from Dow Corning

4) Product information sheets from Dow Corning

Materials, tools and other equipment

1) stirring sticks and mixing cups

2) scale

3) styrene plastic or other material to make a mold box

4) super glue or other glue to glue the mold box together

5) clean clay or a sulfur-free modeling clay

6) spray lacquer

7) baby powder

8) oven or microwave

9) vacuum pump and chamber

10) latex rubber gloves

11) dow corning rtv (room temperature vulcanizing rubber)

12) alumilite

13) cleaning solvents such as toluene or xylene, mineral spirits, acetone, mek or denatured alcohol.

Instructions for mixing hydrocal

Notes before you mix:

1) mix only small amounts which you can use within three to five minutes.

2) always add hydrocal to the water, this will help to reduce lumps in the hydrocal. The exact amount of water and plaster is not critical, but it will work best if the consistency is similar to a milkshake. As a starting point you can try two parts hydrocal, to one part water.

3) if you plan to add color to the hydrocal add it to the water before you mix in the hydrocal.

4) if you want to slow the hardening time of the hydrocal, you can add a tablespoon of vinegar to the water before mixing in the hydrocal.

5) it would be best to use a flexible plastic bowl to mix the hydrocal in.

6) you can use paper towels as a foundation to strengthen the hydrocal. Tear them in small pieces to make them convenient to dip in the hydrocal after it is mixed.

7) be sure and dampen the area where the hydrocal is to be applied.

Now you are ready to mix.

1) pour a cup of cold water in your mixing bowl.

2) sprinkle the two cups of hydrocal into the water.

3) allow the water to be absorbed into the plaster before you start to stir.

4) stir the hydrocal into the water until you have a smooth mixture the consistency of a milkshake. You can add more hydrocal if the mixture is too thin, and you can add more water if the mixture is too thick.

5) working quickly dip the paper towels in the hydrocal covering both sides.

6) place the plaster covered towels on your layout, overlapping each piece about half way.

7) as soon as the hydrocal starts to harden, you need to work quickly to clean your bowl and mixing utensil. Note: as the hydrocal cures it will become warm, this is normal.

Instructions for mixing molding plaster

Notes before you mix:

1) mix only small amounts which you can use within in three to five minutes.

2) always add plaster to the water, this will help to reduce lumps in the plaster. The exact amount of water and plaster is not critical, but it will work best if the consistency is similar to a mayonnaise. As a starting point you can try two parts plaster, to one part water.

3) if you plan to add color to the plaster add it to the water before you mix in the plaster

4) if you want to slow the hardening time of the plaster, you can add a tablespoon of vinegar to the water before mixing in the plaster.

5) it would be best to use a flexible plastic bowl to mix the plaster in.

6) be sure to dampen the area where the plaster is to be applied.

Now you are ready to mix.

1) pour a cup of cold water in your mixing bowl.

2) sprinkle the two cups of plaster into the water.

3) allow the water to be absorbed into the plaster before you start to stir.

4) stir the plaster into the water until you have a smooth mixture the consistency of mayonnaise. You can add more plaster if the mixture is too thin, and you can add more water if the mixture is too thick.

5) working quickly apply the plaster to the layout, or the mold as appropriate.

6) if you are using a mold, it is ready to apply to the layout when the surface of the plaster shows signs of small cracks if you try to bend it in the middle.

7) as soon as the plaster starts to harden, you need to work quickly to clean your bowl and mixing utensil.

Note: as the plaster cures it will become warm, this is normal.