This is a brief write up on designing sprues. Please come offline if
you need any further assistance.
Gating system refers to all passage ways through which the molten
metal passes to enter the mold cavity.
BASICS OF GATING:
The flow of metal in to a mould is very much affected by the type of
gating system employed, for example the design of a system having
several ingates will influence the distribution of the flow between
the ingates. A good design will have even distribution between the
ingates both initially and during the subsequent period in which the
mold is being filled.
THE GATING SYSTEM IS COMPOSED OF:
Tree button or pouring basin
Main sprue (or) Runner
Feed sprue (or) gate
Pattern (or) pieces
RUNNER (OR) MAIN SPRUE:
The main runner which also acts as a feeder is usually truncated
cone in shape. A runner originates at the main sprue and has several
feed sprues to multiple pattern or multiple points on a single
pattern. Runners generally fill to the end and then back pressure
starts the metal to flow in to the sprues. The runners form hot spots
in the mould which create torsional stresses with consequent possible
distortion of the casting. Much of the above applies when gravity
casting methods are employed. The most significant point; however is
that owing to the slower velocity of the metal, it is usual to
increases the size of runners, gates and feeders over those normally
employed for pressure casting techniques.
TREE BUTTON (OR) POURING CUP:
The size of the gate depends upon the rate of solidification. The
gate basin or button to act as a reservoir or store for molten metal.
It prevents turbulence
FUNCTIONS OF GATING SYSTEM:
GATING SYSTEM SHOULD:
Fill the mold cavity completely before freezing
Introduce the liquid metal into the mold cavity with low velocity
and little turbulence
To promote temperature gradients favorable for proper directional
Consume least metal
PURPOSE OF THE SPRUE:
To permit the wax to escape during the burnout.
To provide a passage for the molten metal to reach the pattern
To provide a reservoir of molten metal for the casting, as they
shrink upon solidifying there must be more metal fed into the space
To properly position the wax pattern so that it receives the molten
metal in the smoothest & most efficient way.
MAIN POINTS TO BE CONSIDERED FOR SPRUE ARE :
It should provide sufficient space for the molten metal to flow into
the mould so that all cavities are filled completely.
Main feed should be the last segment of the casting aggregate to
Sprue should have smooth radius & taper so as to avoid turbulence,
when liquid metal enters.
The sprue should have a parabolic taper generally a straight taper
The sprue entrance should be generously filleted.
Taper sprue helps establishing proper flow characteristics rapidly.
IMPORTANT FACTORS AFFECTING THE MOLTEN METAL:
Type and size of sprue
Type and size of runner
Type of main sprue
Type of feed sprue
Calculating the volume of metal & the properties of metal.
Thermodynamics of the system (Flask temperature, pouring temperature,
& rate of solidification) Designing Sprues.
CAD is extensively used for carrying out the designing of sprues
once the CAD files are ready. In many complicated designs we print
the designs along with the model being printed. CAD has made the
process of designing spures easier.
If you need any further assistance please come offline or get in