Making Sole Moulds-5: Mould making by Casting
Moulds prepared by CNC or conventional machines have certain limits. Like
- Difficult to create undercuts.
- Texture creation
- Reproduction of same sole in short period of time.
Casting process will overcome all these problems. The casting process used in footwear moulds is just a modified version of conventional casting system. The process of casting has the follows these steps.
- Creation of a model (i.e wood, pattern board) (called Positive)
- Creation of silicon negative from the model
- Then creates a chalk/Plaster of Paris (POP) cast from silicon (Positive).
- Material will be poured on POP model to create mould (Negative).
a) Creation of pattern: This is a replica of sole with top portion is extruded. In many cases, producing the sole replica is simple than creating mould. Once it is ready, make sure it is free from burrs and scratches as well as grease and oils, because stable silicones reproduce surface details and textures exactly. Typically, the model must be hand finished to the desired quality before making the mold. After this the model is fixed on plate (metal or wood) which looks like positive of mould cavity.
b) Creation of Silicon negative: A mold box or container is constructed around the sole replica, to hold the pattern in place. Later liquid silicone will be poured in it. Plastic, wood or aluminum are suitable materials for creating mold box. The Details that are need not to include in the first mold half, need to be masked with clay.
Liquid Silicon rubber is poured into the box and it is allows to set for some time. A general solidification time varies in between 3-4 hrs. Then all the wooden flats are removed and the silicon rubber is removed from the model. This is the first negative of the sole. Silicon rubber is having very good flowability, so it can go to every corner of the mold box. Very small design can also be produced with the silica rubber.
A correct partition surface maybe needed at this stage
3) Creation of POP Positive: Now a Positive is prepared from this Silicon rubber negative, so that metal can be poured on it. Again, a mold box or container needs to be constructed to hold the Silicon model. Liquid POP is poured in to this construction and it is allowed to set. Generally it takes 2-3 hrs to set POP. After this the POP model kept in the oven to remove the moisture. If moisture remain in the POP, it will evaporate during casting process due to high temperatures and creates air traps. Some times these may form on the surface of the mould and spoil the mould surface. So proper drying of POP model is very important. The oven temperature is increased gradually with intervals of 30 mins to reach temperature of 800oC in steps of 50 oC.
4) Metal pouring: Once the POP model is created and it is the time to prepare the actual metal mould. A furnace is used to melt the Aluminum ingots. A pneumatic press is used to hold the model and other supporting plates. Here again a mould box (metal) is prepared on the table of press which holds the POP model. The plates are tightened or hold tightly by means of pneumatic press to avoid the leakage of liquid metal. Once the side plates are tightened the assembly will be closed by a top plate, which contains a large circular entrance to pour the liquid metal. The opening is lined with a refractory material (Exothermic sleeve) to avoid quick solidification of liquid aluminum. This will also acts as a runner/riser. Once the material is poured in to the mould assembly the top of the opening is closed to avoid solidification of aluminum. Then the assembly is allowed to cool up to 1 hr. After this the mould supporting plates are removing and the mould is ready.
Defects in casting
Any irregularities in the moulding process causes defects in castings which may sometimes be tolerated, some times eliminated with proper moulding practice or repaired using methods such as welding and metallisation. The following are the major defects which are likely to occur in castings.
1.Gas defects: these are caused to a great extent by the lower gas passing tendency of the mould, higher moisture in model, etc.
- Air inclusions: The atmosphere and other gases absorbed by the molten metal in the furnace, in the ladle and during the flow in the mould, when not allowed to escape, they will trapped inside the casting and weaken it. The main reason for this defects are higher pouring temperature which increases the amount of gas absorbed. The remedy for this choose appropriate pouring temperature.
- Blow Hole and open holes: These are spherical cavities inside the casting, called blowholes. These are caused by the moisture left in the model. (i.e that is why proper drying of model is required in oven). Because of the heat in the molten metal, the moisture is converted into steam and entrapped in the castings and forms blow holes or open blows.
2. Shrinkage cavities: These are caused by the liquid shrinkage occurring during the solidification of the casting. To compensate this proper feeding of liquid metal is required.
Silicon rubber (RTV- Room temperature vulcanized rubbers):
Room temperature vulcanized silicon is fast, easy and are a relatively inexpensive way to create prototype and production tooling. New silicone products offering greater tear strength, reduced shrink and larger useful temperature ranges have helped this mold making technique
Silicone rubber is used most often for casting liquid plastic resins, it does not require release agent. Thus, silicones’ non-stick quality makes short run resin casting easy.
Contributor: A.V.Suresh, Technical consultant Romans CAD