Picking a Mandrel
Mandrels are used to suspend positive casts in vices and mandrel holders. It is crucial to select the correct length of mandrel to ensure that the positive cast is strong and that there is a sufficient length of mandrel exposed for fabrication. In our shop the mandrel should typically extend from 1” proximal to the bottom of the negative to 7” above the brim.
This length of mandrel can be clamped in the vacuum tube and the cast is very close to the edge of the tube so we can maintain good vacuum pressure.
There are multiple ways of suspending a mandrel inside a cast. During the orientation week you will be using a mandrel holder.
A mandrel holder, mandrel and TT negative.
The mandrel is inserted into the mandrel holder and then suspended on top of the negative cast. The mandrel holder is placed on the brims of the negative in order to suspend the mandrel.
The mandrel holder installed on the mandrel.
Negative Cast Placement In Sandbox
Negative casts should be partially buried into the sandbox to prevent tipping when filling with plaster.
Burying the AFO like this will prevent it from tipping over when filled. Ensure that it is vertical in the coronal plane and that the foot plate is parallel to the ground in the sagittal plane.
The mandrel should sit in the center of the shank section, not getting too close to any of the walls, and it should extend distally to at least the level of the malleoli.
A TT should be set up in 5 degrees of preflexion. This can be done either by tilting the mandrel or the socket.
In this case the mandrel is kept perpendicular to the ground and the socket is tilted anteriorly. This is method is easy to do visually but tilting the negative means you cannot fill it up all the way (plaster will spill out the front). This can artificially lower the trimlines on your final device. Consult a professor to make sure your cast will extend proximal enough to fabricate your device.
In this instance, the brims of the negative are kept parallel to the ground and the mandrel is tilted 5 degrees posteriorly. This avoids the brim height issue but it can be difficult to visually estimate the mandrel angle correctly.