Block Mold - Injection Type | Page 5
The arrow in Figure 29 points to the tip of the hollow tube that has been bored through the tip of the inner silicone rubber tube.
The arrows are pointing to the other holes that have been bored through from the top of the inner rubber mold to its back side. Using an Exacto knife the mold maker is widening each hole to make certain there are no ragged edges obscuring them.
Examining the top side of the inner rubber mold you can see that the mold maker bored holes all along the top edge of the mold (the mold will be flipped upside down so then it will be the top). That is because air tends to flow to the top in liquids and it needs a place to escape or it will create gaps in the casting material when it has cured. The sprue holes a space about one inch apart.
Not only are sprue holes added all the way around the perimeter, but they are also added to the top surfaces of the inner mold where air will tend to accumulate. The holes will allow the air to escape. Remember, this is an injection mold which means the casting material is forced into it under pressure. If the air that is in the mold has no place to go, the cast will cure with hollow spaces where the trapped and compressed air has prevented the liquid casting material (in or example it will be resin) to fill the mold completely.
Once all the sprue holes have been created and the entrance holes have been cleared, it is time for casting. At this point the mold is reassembled and held together with a number of rubber bands.
KastEZ resin which is a two-part polyurethane resin with a one-to-one mix ratio is prepared for use. One part (Part A) is poured into a 16-Oz. Cup and then Part B is poured into a separate cup, filled to the same level as Part A. Before mixing the mold maker adds red polyurethane dye to Part A and nixes well. The dye is highly concentrated so less than 1/2-Oz. is sufficient to create a bright red color.
Once the dye is thoroughly mixed into Part A, the Part A and Part B are combined in a third cup and vigorously mixed.