College of Engineering

Mechanical Eng. Dept.

Riser Design

Ch5

The solidification of most alloys is accompanied by appreciable volume contraction. The magnitude of this contraction which gives values for several metals forming alloys of commercial importance.

Riser Design

The purpose of a feeder head, therefore, is to provide a reservoir of liquid metal under a pressure head sufficient to maintain flow into the casting. Without feeding, the final casting will be subject to shrinkage defects in the form of major internal cavities, centre line or filamentary voids

· Liquid shrinkage: The liquid metal loses volume as it gives up superheat and cools to its solidification temperature · Solidification shrinkage: The metal freezes, changing from a liquid to a higher-density solid. For pure metals, this contraction will occur at a single temperature, but for alloys it will take place over some temperature range or freezing interval · Solid shrinkage: The solid casting cools from its solidification temperature to room temperature

Riser Design

The contribution of each one of the three distinct stages of volume contraction is shown: liquid shrinkage, solidification shrinkage, and solid contraction

Schematic of the shrinkage of low-carbon steel.

(a) Initial liquid metal. (b) Solid skin and formation of shrinkage void. (c) Internal shrinkage. (d) Internal shrinkage plus dishing. (e) Surface puncture

(a) Initial liquid metal.

sequence of solidification shrinkage in an iron cube.

(a) Open-top riser. (b) Open-top riser plus chill. (c) Small open-top riser plus chill. (d) Insulated riser. (e) Insulated riser plus chill

Methods of controlling shrinkage in an iron cube to reduce riser size

Optimum Riser Design

The role of the methods engineer in designing risers can be stated simply as making sure that risers will provide the feed metal: · In the right amount · At the right place · At the right time To this list can be added several other considerations: · The riser/casting junction should be designed to minimize riser removal costs · The number and size of risers should be minimized to increase mold yield and to reduce production costs · Riser placement must be chosen so as not to exaggerate potential problems in a particular casting design (for example, tendencies toward hot tearing or distortion)

Shrinkage (volume contraction)

Riser Location

To determine the correct riser location, the methods engineer must make use of the concept of directional solidification.

The ability to achieve such directional solidification will depend on:

· The alloy and its mode of solidification · The mold medium · The casting design

Directional and progressive solidification in a casting equipped with a riser

(section width greater than 3T, where T = thickness)

Feeding distance relationships in steel plates

Feeding distance relationships in steel bars (section width equal to thickness, T

Feeding distance relationships in steel plates

Effect of chills on feeding distance relationships in steel bars.

Riser necks used

in iron casting applications (side view and top view, respectively). (a)General type of side

riser. (b) (b) Side riser for plate

casting. (c) (c) Top round riser.

The modulus method is based on the concept that the freezing time of a casting or a casting section can be approximated by using Chvorinov’s rule

Riser Design

The modulus method

where t is the freezing time of the casting, Vc is the volume of the casting, Ac is the surface area of the casting, and k is the constant governed by metal and mold properties.

The volume-to-area ratio of the casting is termed the casting modulus, Mc

The freezing times of risers and castings are proportional to their respective moduli, and if the modulus of the riser, Mr, is sufficiently greater than the modulus of the casting, Mc, good feeding will be obtained. In steel, if Mr = 1.2 % Mc, feeding will be satisfactory.

Mr = 1.2 % Mc

Derivation of cooling modulus values. (a)Simple shapes,

Derivation of cooling modulus values. Simple shapes,

Derivation of cooling modulus values. irregular shapes,

Riser configurations and their characteristic values

Riser configurations and their characteristic values

Riser configurations and their characteristic values

Riser configurations and their characteristic values