The image given below shows a bolster with a series of riser and runner. What I really do not understand is which is the riser and which is the runner. Also a detailed explanation to why multiple risers and multiple runners are being used will be highly appreciated. In general what is the distinguishing feature between a riser and a runner? Out of the two rods in the periphery why is one relatively thick and the other one thin?
A riser is a static reserve volume of metal available to flow into the casting to compensate for shrinkage as the liquid freezes. Risers are relatively large because they should not freeze until after the casting has frozen. Risers may be "hot topped", exothermic material is put on to provide extra heat to keep the riser liquid as long as possible. A runner is a pathway in the mold for liquid metal to flow from the sprue to reach all parts of the casting . One reason for runners is that if all molten metal enters the mold at one location it will overheat the mold . Locations where the mold has been heated more than others can cause shrinkage porosity and cracks. There may be some small features to permit venting of air and steam from the mold.