Mold making and casting are relatively simple processes. Make a mold of the model you want to reproduce, and then pour the casting material into the mold. Wait for it to cure properly before removing it and viola that your work of art is done!
However, certain technicalities also creep into the picture. For example, you need to choose the right mold making and foundry products that are right for the job. Sometimes release agents, fillers, and other items are also required.
Another important point is that as the casting material is poured through the sprue it displaces the air in the mold, some air can easily get trapped inside and the casting material will not be able to completely fill the mold. These air pockets create hollow spaces that can result in an unsightly, misshapen cast.
So how do you get rid of these air traps and pockets when molding silicone rubber or polyurethane resin?
The answer is simple – make ventilation holes! These are holes that are created in the sides of the mold, usually at the high points of the mold where air tends to migrate first. This allows air to escape from the mold when the casting material is poured inside. Now the casting material can reach all corners and points of the mold without gaps.
The location and number of vents is important. The holes are ideally placed along the top surface of the mold as air tends to rise to the top of the mold. It is best to create vents along the top perimeter of the pan while making sure to cover all high spots. Keep them an inch or so apart. If you are making sprues on the sides of the mold, make sure they taper or the casting material will easily flow out of the mold.
As?
Vents are made by drilling holes in the surface of the finished mold. Alternatively, you can even insert hollow tubes during the mold making process. When removed later, the tunnel will serve as a drinker.
The same vents can serve a dual purpose. In addition to functioning as a vent for air to escape from the mold, the hole can also be used as an alternate sprue for adding liquid casting material to the mold. Think injection molding!
If the same vent is used as the sprue and vent hole, it should be located at the highest point of the mold and be large enough to allow air to escape even while material is being poured.
Carefully pour the silicone rubber or polyurethane resin into the sprue hole and continue until it overflows from all vent holes. This shows that the material has displaced all the air inside. The material will also cure in the vents and can be cut after demoulding.
Please note that only closed molds require ventilation. If the mold is open at the top (it will become the flat bottom of the plaster), air can easily escape and the material will fill without the need for vents.
