RESEARCH PROJECTCASTING

A how to guideBy

Christopher PooleBA TPA 2

Introduction

Casting has long been a passion of mine. It has aided me in my work as a model maker and miniature hobbyist for many years and am sure for many more. Casting is the process of creating duplicates of a single master in your chosen medium and creating an identical copy in a fraction of the time taken to manufacture the original or produce many copies. I write this report more as a how to in the hope others who read it can pick up on the techniques, tricks and nuances I have learned and am still learning.

Mold Making, Master

The first step in creating a cast is selecting or producing the item or items you wish to duplicate. Depending what your original (or master) is made from and its general shape will determine the best casting method used also you will have to decide what the duplicates will be cast from, will they need to be flexible or solid, hard wearing or short lived.During this report I will be using a few different things for examples.

Mold Making, Materials

Knowing what you can produce a mold from and its inherent properties can be the key to a successful cast. If you make a one-piece mold out of plaster and the master has an undercut then it’s a failure, where as if you make the same mold from the same material but use a two-part mold then it could be a success.

Molds can be made from a wide range of materials, the most commonly used is silicone due to its ability to capture fine detail, bend and stretch to allow undercuts to be released, the life of the mold and the range of thicknesses (Shore). It is not the cheapest medium out there but if you plan on making many high-end copies it’s the one to go for.

Plaster is perhaps the cheapest to get your hands on. Its easy to use and can catch some good detail, the molds will last for some time if looked after properly i.e. not dropped. The drawbacks to using plaster are that it can take some time to dry if a particularly wet batch has been made, it is rigid when set so planning your mold will be essential, it is also harder to remove trapped air from in the poring stage unlike silicone.

Resin can be a very good way to make a mold from if you are familiar with it. Gellcoat is a thicker type of resin is lightly brushed on to the master being careful not to leave air bubbles trapped. Once it starts to solidify (or cure) it is given a reinforcement of fiberglass matting. This method is good to produce a mold for many castings, as it is robust and easy to repair if something should happen to it. It takes very little looking after in terms of storage and will outlast most molds. The down side is it is a messy process and special safety gear is required throughout most of the process, it can be very time consuming and is only cheap if you buy everything in bulk or compare it with silicone prices.

Latex is a nice cheap easy way to start of a mold. It comes as a liquid but has the option of thickening and does not require excessive safety gear, but does tend to smell extremely bad. Set latex is very strong and can bend and stretch better than silicone but it has the unpleasant habit of shrinking. Unfortunately latex can’t be pored like plaster or silicone and left to set but has to be applied in layers and allowed to dry which adds on a lot of time over all.

Casting MaterialsA cast can be made from a great many different materials, from extremely solid concrete and metal to soft expanding foam. Each material has its own particular quirks that have to be understood and in some cases overcome to produce a good cast.We’ll start with resin as it is the most widely used.Resin is an exothermic material that requires a kicker to set it of allowing it to set (methol ethol ketone peroxide). Exothermic is the reaction where heat builds due to a chemical reaction, this is both a help and a hindrance as heat is required to allow the resin to solidify but too much heat and it could crack and fracture. During the setting as the resin cools it can shrink and distort, some cheaper resins do this more than the expensive ones. Even if all you can afford is the cheap stuff help is at hand, in the form of filler powders. Powders allow the excess heat to be redirected and dissipated preventing cracking, it also bulks out the batch saving money on resin and hinders distortion. Filler powders come in many different types metal, marble, chalk and even glass bubbles.

Making a cast in foam can be an interesting choice. Some foam solidifies hard when set and others end up very soft and pliable. Some of the slightly more expensive ones are self-skinning; meaning as the foam expands (being made of bubbles) it will press up against the mold walls and create a smooth surface. Foam that is not self skinning will need a thin brush of latex over the inside of the mold that is left to dry then foam pored in.

Metal is a totally different substance to work with so we shall go into further detail about that later.

MixingDuring the coming examples I will be telling you to mix or add certain materials, so you don’t get confused we will cover them here first.Casting materials come in many different forms and from many different places but will do the same job as their counterparts, with this in mind I cant tell you how to mix everything precisely. I will press the point of always read the bottle instructions first. Don’t assume the mix is the same as the last one you may have used. Always double check labels.As a general rule resin when mixed is a 100% to a 3% hardener. This is a simple way of working it out if you don’t have a calculator to hand. (Example. A cup is holding 600 grams of resin; we need 3%, which is 18 grams. 600 / 100 = 6 x 3 = 18) Gel-coat takes around 5% and silicone about 5% also. Some resins take a 50/50 mix, that’s why you should always read the label. Other materials such as thixotropic agents (for thickening latex, silicone etc.) are added to eye, just add small amounts until you reach the desired consistency (one a certain amount is added to will not thicken any more so don’t go nuts)

Making a Mold

There are a few way to make a mold, we will be covering single part and two part molds. As stated earlier choosing the correct type of mold can spell success or disaster for you’re casting.The first obstacle we shall look at is an undercut. These are parts of the master that will prevent the new cast from coming out the mold due to it being too large to exit the poring hole or hooking the mold stopping a smooth pull.Below are two molds with different shapes set inside. Each is a plaster mold and a resin cast.

Example 1 shows how the pyramid shape with its large poring base has no undercuts and so can be pulled without any trouble.

Example 2 shows how a cast can become stuck if not planed correctly.

There are three simple ways to amend the trapped cast problem. Firstly is to make the mold from a block of silicone, allowing the solid resin shape to be manipulated out. Secondly would be to make it a two part mold allowing it to be parted releasing the cast, this method mean’s we could still use plaster for the mold (example A). Thirdly we could change the position of the master if no detail will be lost (example B)

Example A Example B

Making a two part mold

In this section we will go step by step on constructing a two-part mold from silicon. There are two ways to make one, depending on the size and complexity of the master will determine which would be more economical and feasible to use.For this example we will be using a piece of a resin kit. I chose this piece because of its particularly troublesome shape. For this piece boxing up the master is the best option.

Step 1.Fabricate a container that is taller than the piece you are casting. Half fill it with oil-based clay, do not use air-drying, as it will dry out while the silicon sets, potentially trapping the master. I used plasticine because it was cheaper to get my hands on (clay is easier to work with). Now remove enough clay so that the master will sit snugly half way in. Make sure the mold surface is smooth and a good connection is made with the walls and the master.

Now we need to add the keys. Keys are locating points used then joining the mold back together to insure a good fit with no stepping. Keys can be made from almost anything that makes an imprint, the end of brushes is a common one to use, and as long it creates a hole that can be filled it should do. Once this is done we need to spray some mold release over everything the silicone will come in contact will, If you don’t have any mold release spray, petroleum jelly (Vaseline) will work just fine, only problem with it is its almost impossible to apply it totally smooth and silicon picks up ALL the detail. With our mold prepped were ready to pore the base.

Step 2

Step 3Anyway once the silicone is mixed ideally you should put it in a de-gassing chamber unfortunately not everyone has one. So simply bounce the cup lightly of a table top repeatedly and watch the trapped air rise to the surface. Don’t do this for to long as the silicone could start to set. Pore in from a single point allowing all air to be pushed out of the way by the wave of silicone. One the master has been covered de-gas it as we did previously when it was in the cup just to make sure there was no air in the mold. Wait for the silicone to fully solidify before turning it over.

Remove the remaining clay being careful not to disturb the seals. If there has not been any poring points added to the master before this stage they need to be fabricated now. Anything that will act as a channel to allow the resin to be pored through will suffice. Also if there are any areas where air can’t escape a smaller channel can be placed.

Once this is done apply more mold release and repeat step 3 to create the top.Having applied the mold release the two silicone part’s will not have bonded together and with a little teasing should split nicely.

This next method is used when dealing with larger items where it is not practical or possible to box up your master.

We will be using a person (life casting) for our example in this one, as it is the main way to cast a head. For simplicity sake we will imagine were casting a cast of a person (a resin head and shoulders). So we don’t have to emphasize on the health of our model, also things are done a little differently but we want to focus on the casting method for inanimate objects for now.

Step1.Again we will need oil based clay. Roll it out into long thin strips about 5-10 mm thick. Look at the head and imagine or draw a line from the base of the shoulder past the ear over the top of the head and down to the other shoulder making sure it is at the highest point. This will prevent any undercuts. This line is you’re guide line, starting with the back build a wall (flange) round the head making sure the clay is now covering the line (it will be on the face side). Smooth it out and make sure there are no gaps between the clay and the head, once this is done make keys all around.

Step2.Lye the head down so when the silicon is applied gravity wont take it to badly. Give a coat of mold release and make up a batch of silicone with some thixotropic agent through it to make it less runny. Paint on a thin layer to make sure it gets into all the creases and detail. Once this is done layer on the remaining silicone evenly.

Step3.Once the silicone has solidified it will need to be reinforced by a back layer of fiberglass or plaster-bandage it really doesn’t make to much difference except the rigidity in the long run. Make sure that the fiberglass or bandage creates a lip, this will be important later. Flip the head when it’s hard and remove all the clay. Repeat steps 2 making sure the reinforcement layer meets up with the first creating a seal.

Step4Drill securing holes around the flange big enough for bolts to go though. These will hold the two molds together whilst casting.

Tease the mold apart and remove the master, now were ready to cast into it with whatever you wish.