< RUNNERS
DIRECT (TOP FED) RUNNERS
There are two basic systems that are used by the art founder when setting out a runner network – the DIRECT SYSTEM and the INDIRECT SYSTEM. There are of course degrees of variation between extremes of the two types, but in general each system is quite distinctive in it’s working principals.
Direct running introduces the molten charge into the investment mould by cascading metal in a piston like motion from the head of the mould, down through the cavity until it’s flow is arrested at the base of the mould. In it’s simplest form, a direct system will consist of little more than a pouring cup mounted on a short runner placed at the highest point of the wax pattern.
The advantage of this system is that it offers simplicity of layout. Gravity is used to flush the CHARGE rapidly through the mould cavity, so relatively few runners are required for the system to function well. This in turn saves time (large wax patterns can be prepared for investment over very short period), and money (less metal is taken by the runners and less fettling and chasing is needed to finish the cast).
There are however, some definite disadvantages in using a direct running system. The flow of metal into the mould cavity is difficult to control and an excessive pressure of entering metal can easily burst an investment mould, moulds must therefore be adequately reinforced or supported. Direct running can also cause significant turbulence in the metal charge as it washes down through the mould cavity. Excessive turbulence can lead to a marked increase in surface faults, in particular metal porosity.
INDIRECT (BOTTOM FED) RUNNERS
Indirect running systems do not direct the flow of the charge into the investment at the head of the mould. Instead, the runners are designed to route the molten charge around the sculpture and down towards the base. The charge first enters at the bottom end of the sculpture, causing the mould cavity to fill from the bottom up in a ‘welling up’ motion, similar to the rising of a fresh water spring.
The main runners of an indirect system are always directed to the base of the wax pattern. The secondary runners which are attached to the pattern at higher levels are angles upwards, towards the cup, at an angle of about 30-45° or so above horizontal. Setting the secondary runners at an upwards facing angle means that the metal in the primary runner flows freely en-route to the base of the design without entering the mould cavity higher up. As the level of metal filling the mould rises and blocks off the lower end of the runners, the upper secondary runners begin to function, allowing the charge to enter the mould at increasingly higher points until arrested at the uppermost part of the mould as the cup fills.
The main advantage of the indirect method is that the founder has a great deal of control over the way a mould fills. Turbulence in minimised, and the casting process is an altogether more balanced procedure, less likely to rupture an investment mould through excessive metal pressure. The indirect system is the method traditionally used by lost wax founders, especially useful for casting complicated forms and detailed designs.
Disadvantages of using a indirect running system include the need for a more sophisticated runner system than usual for direct running, the founder may also need to deploy runners with a bigger cross section to maintain pressure and fluidity in the system (the mould is filling against gravity, relying on the pressure in the cup and runner system of force the charge upwards).
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