pengecoran logam 1

PELEBURAN & PENGECORAN LOGAM

(Ir. Abrianto Akuan, MT.)(Ir. Abrianto Akuan, MT.)

1. Casting Fundamentals for Molten Material

• Factors affecting solidification characteristics from the molten state– Fluidity

• Flow of molten material into the cavity

– Heat transfer effects• During solidification and cooling

– Solidification effects– The type of mold material

3.2. Thermal behavior of fluids in the cavity

Chvorinov’s rule: the temperature needed for the molten metal to completely solidify in the cavity

nm A

VCTST )(

In the cavity, the fluid flows and solidifies.

At this period, no further energy is fed to the fluid.

Time needed to fill the cavity fully ?

nm A

VCTST )(

TST: total solidification time, min

V: volume of the casting, in.3 (cm3)

A: surface area of the casting, in.2 (cm2)

N: exponent, 2

Cm: mold constant, min.in.2 (min/cm2)

Cm is a function of

- mold material

- thermal properties of the cast metal

- pouring temperature relative to the melting point of the metal.

Determination of Cm can be done through experiment.

The principle of such an experiment is to have a scenario that is the same as the casting and a known cavity geometry, to operate the process, and to record the Time TST.

Implication of Chvorinov’s Rule:

A casting with a higher volume-to-surface area ratio will cool and solidify more slowly than one with a lower ratio.

Question: How to design a casting, i.e., V, A, materials, so that there will be a guarantee that the molten metal will not completely solidify before the cavity is completely filled ?

Complete relying on the casting design will create too much constraint on a casting design!

The solution is to have a so-called riser

CastingRiser

Cup and Sprue to pour the molten metal

Riser = Reservoir

4. Riser Design

4.1 Design principle

1. TST of riser should be Greater Than TST of casting

2. Riser and Casting are in the same process conditions

4.2 Design content and optimal design

To determine the geometry of the riser. The riser could be a cylinder, cone, etc.

To design a riser with the minimum of materials used

Example 1. Riser Design

(1) Casting geometry: rectangular plate

length = 200 mm, width=100 mm, thickness=18

(2) TST of the casting itself: 3.5 min

Design a riser:

TST of the riser take 25% longer for it to solidify

(3) The riser is a sphere

Given:

Casting

Make a cavity

Basic Terms