Assigned Reading-McCreight Ch. 2 Sprues

Sprues
A rod that supports the model while making the mold; provides passageway for wax to leave; entrance for molten metal; positions the model
Round soft wax wire is preferred for the sprue.
Plastic sprues are not recommended.
Attach to the thickest section of the model; where they cannot damage surface texture and can be easily removed.

Orientation
Leave 3/8" clearance on all sides of the flask; for several models leave 1/8" between them; leave 1/2" between model and investment.
Try to get your model situated to flow completely downstream to avoid air bubbles and backflow.

Recrystallization
As metal cools in contracts.
To avoid pits make sure to attach the sprue at the thickest portion and make sure there is enough molten metal supplied.
Choose a flask just the right size--too small can cause it to burst and too large is wasteful.

Attaching Sprues
Use a sprue wax slightly heavier than the average section of the piece.
Heat the wire and lower to attachment site--hold there until it hardens; check the joints and trim the wire if needed.

Fillets
Enlarge mass and round off opening at joint for smoother flow.
To thicken, heat needle with wax and drop off at joint.
Electric wax pens work well for this.

Determining amount of metal needed
Weighing-calculate ratio between gravity and weight of the model and the metal being used.
*Multiply weight x gravity of metal*
Use pennyweight or gram scale.
Weigh model with sprues.
If scale is not available, use the water displacement method (not as accurate)
Submerge wax in jar filled with water; mark how high the water rises with wax; take out wax; add metal until reaching the same point and maybe add a little more for the button.