Material choice was difficult as there are so many on the market. I had to decide what would be the most effective and produce the results I wanted. However the first thing to understand is the process of molding and casting.
What is the difference between casting and molding?
Casting is material that uses gravity to find its own form within a hollowed mold which is then broken out of the mold, whereas molding involves pressure to form a mould
The process of molding uses a solid frame (mold) where it is filled with a liquid or pliable material. The material then sets taking the shape of the mold.
Usually bifold molding is used, which has two separate parts.
Piece molding – large complicated pieces, usually in a variety of shapes to create the end product.
Types of molding
First the hard exterior shell is made and then fluid materials introduced.
This is where heat is applied to the soft material over a preformed mold creating a hard structure. When working in a dental laboratory I used this process on Shellac trays, that were laid over a plaster mold of a patients teeth. This was then heated gently (otherwise overheating would cause holes) and slowly it took the shape of the teeth.
Impression of teeth and shellac mould
After the shellac impression has being drilled out, this is ready be filled with alginate, which takes a final impression of teeth for dentures.
The same technique is used for making gun shield, the only difference being the material is softer.
Thermoplastic polymers are injected into a mold for precision casting for plastic objects, for example toys and is similar to casting. Heat is applied and the molten liquid is forced by pressure into the mold. This sometimes gives a split line where mold is incorrectly joined. It is a fast process making a cost effective process but expensive to set up.
A rubber compound is a traditional method for compression molding, where it takes the shape of the end product. Heat and pressure is applied which takes the form of the mold. The rubber goes through vulcanization, where polymer chains are formed, which forms the shape. The cost is relatively cheap as less machinery (usually aluminium) is used than some other methods and is good for large scale projects. Disadvantages of this is there is a lot of waste due to forcing the material in and spillage from this. Also it takes longer to cure, therefore simpler designs are better as limiting the flow can eliminate air bubbles. However contamination can occur due to compounds used, which is where a loss can happen.
Blow molding is used to make bottles. The method used involves air that is blown into the mold forcing the plastic to fill the void giving thin walls. It is a quick method for production but has limitations with big objects.
Things to consider when choosing casting materials:-
How would it look, whether colour or the appearance of glass, metallic or natural forms.
Dyes can alter the colour, but needs to be clear Poly-optic plastics and can be tinted. A Stone effect can achieved by adding marble dust for example. Whereas bronzing powders produce a metallic finish. Wood effect would require a suitable mold with a textural grain and dyed to represent the colour.
Weight should be considered, whether light or heavy. Should it be hollow or solid?
Heavier castings that look like bronze metal are usually filled like polyurethane plastic inside to give it weight like metal. Sand, marble dust and other materials achieve the same effect.
Lighter casting could use Polyfil reducing cost. This doesn’t affect the density of the casting. Blocking out an area, using wood can create voids and reduces weight. Distortion in cast can occur where air bubbles are trapped. Foam makes a lighter casting and is inexpensive.
What would it be used for? This raises the question should it be heat resistance, or give a certain appearance. Would it be for internal or external use? External use would mean it would need to be sealed to prevent damage.
Plastics and rubber can be used for reducing impact, however may not be heat resistant, but sprays can combat this.
Durability of a casting depends on the material used. Rubber for example can be discoloured or breakdown when exposed to UV rays. Which is protected by a poly additive.
Also the cost of production depends on the time scales in producing a prototype. This can be alleviated by using fillers and block outs.
Foams allow irregular items to be filled quickly and more efficiently. Pouring is quicker and slush casting can be quickened by a rotation machine.
When using materials, its important to check the manufacturers recommendations. HEALTH AND SAFETY – AMEND
HEALTH AND SAFETY
Health and safety recommendations are on all products as recommended by the Health and Safety executive which is the governing body for welfare.
This is a quick process, more flexible and stronger than plastic. And allows for delicate and hollow castings. It is less brittle allowing the material to be sanded or drilled.
When set it is opaque (either white or beige). There is little odour. The viscosity of it is low and allows for thinner models to be produced.
Detail is easily reproduced and it mixes well with powders and dust. Because of the nature of this, bubbles can be seen before the end product is made.
The quick curing time is a disadvantage.EXPLAIN Also mixing needs to be done carefully or the end product could be ruined.
Rubber gloves, eye protection and respiratory equipment must be used. As it is highly flammable, there should be no naked flame. Wash hand before eating, smoking or using the toilet.
Disposal of the residue needs
This is inexpensive and versatile. Discolouration is avoided by UV blockers.
It is strong, allows pigments and has longer shelf life. However, the fumes created are hazardous and should be done outdoors. It is prone to cracking in large volumes also can be brittle and affected by water.
Plaster is cheap to buy, easily mixed and allows the material to be poured. There is no odour. But dust particles can be an irritant.
There are different ones available, differing in setting times, viscosity etc. Colour can be added with pigments, but would be muted. Disadvantages are air bubbles can occur. It is less robust than the materials already discussed and difficult to repair.
Latex is flexible, and be used to form body molds. It is inexpensive and widely available. It is good at replicating fine detail and long shelf life.
Producing on mass is difficult as moulds such as silicon are unable to set due to excessive water. However, if the mold is absorbent this does not occur. This can only be layered allowing the latex to dry and will shrink by 10%. Painting of the latex is difficult but can be achieved with acrylic paint. Talc is needed when casting to prevent it sticking together not petroleum (Vaseline).
Silicone is a pliable soft material. It is opaque, but paint in small amounts can be added and this can be expensive.
This can be useful in created bendable puppets. However, is not a cheap material. Also application of it can’t be poured only spread.
This is relatively cheap to use and is pliable. It is easy and quick to produce prototypes, and allows alterations to be made. However if dried too quickly results in cracking, so care is needed. When using clay it needs to be slightly wet otherwise in breaks down.
Although pliable to work with, when fired it becomes hard and makes it more fragile, than unfired clay. Also hands and clothing can be stained by mineral clay.
Unlike the mineral clay is made of synthetic plastic and instead of air or kiln drying require low heat to ‘cure’ it. The texture is soft and silky, whilst some can be stickier than others. There are a range of colours available. It needs to be stored in an airtight container otherwise it dries out. Examples of this are Playdoh and paperclay.
This type of clay allows for more precision work, such as Sculpey. These are polymer based and clean to work with. When air cured they are almost unbreakable with a smooth finish that allows drilling for example. Some polymer clay needs to be fired in an oven, which isn’t practical for home use due to fumes it emits.