METAL CASTING PROCESS
1. Permanent Pattern
a. Sand casting
b. Shell molding
2. Expandable pattern.
a. Investment Casting
b. Expanded polystyrene process (full-mold)
c. Permanent mold casting process
3. Variation of permanent mold casting
Figure 13.2
Steps in the production
sequence in sand casting.
The steps include not
only the casting
operation but also pattern
making and mold making.
Figure 13.3
Types of Pattern used in
sand casting
a) Solid Pattern
b) Split Pattern
c) Macth-Plate Pattern
d) Cope and Drag
Pattern
Figure 13.4
a) Core held in place in
the mold cavity by
chaplets.
b) Possible chaplet
design
c) Casting with internal
cavity
Figure 14.20
V8 engine block (Bottom
Center) and the five drysand cores that are used
in its construction.
(Courtesy of Central
Motors Corporation)
Figure 14.21
Four Methods of making a
hole in a cast pulley
Figure 14.24
(Top) Typical Chaplets.
(Bottom) Method of
supporting cores by use
of chaplets (relative size
of the chaplets is
exaggerated)
Figure 13.5
Steps in shell molding:
1) A match-plate or cope
and drag metal pattern
is heated and placed
over a box containing
sand mixed with
thermosetting resin;
2) Box is inverted so that
sand and resin fall
onto the hot pattern,
causing a layer of the
mixture to partially
cure on the surface to
form a hard shell;
3)
4)
5)
6)
Box is repositioned so that loose uncured particles drop away;
Sand shell is heated in oven for several minutes to complete curing;
Shell mold is stripped from the pattern;
Two halves of the shell molds are assembled, supported by sand or metal shot in a box, and pouring is
accomplished.
7) The finished casting with sprue removed is shown in 7
Figure 14.18
(Top)
Two halves of a shell-mold pattern.
(Bottom)
The two shells before clamping and
the final shell-mold casting.
Figure 13.6
Steps in vacuum molding:
1) A thin sheet of
preheated plastic is
drawn over a matchplate or cope and drag
pattern by vacuum; the
pattern has small vent
holes to facilitate
vacuum forming;
2) A specially designed
flask is placed over the
pattern plate and filled
with sand and sprue
and pouring cup are
formed in the sand.
3) Another thin plastics sheet placed over the flask and vacuum is drawn, which causes the sand grains to be held
together forming a rigid mold
4) The vacuum on the mold pattern is released to permit to pattern to be stripped from the mold;
5) This mold is assembled with its matching half the form the cope and drag and with vacuum maintained a both
halves, pouring is accomplished. The plastic sheet quickly burns away on contacting the molten metal. After
solidification nearly all the sand can be recovered for reuse
Figure 13.7
Expanded polystyrene casting process:
1) Pattern of polystyrene is coated with refractory compound;
2) Foam pattern is placed in mold box and sand is compacted around the pattern;
3) Mold metal is poured in the portion of the pattern that forms the poring cup and sprue. As the metal enters the
mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus allowing the resulting mold cavity
to be filled
Figure 13.6
Steps in investment casting:
1) Wax pattern are produced
2) Several pattern are attached
to a sprue to form a pattern
tree;
3) The pattern tree is coated
with a thin layer of
refractory material;
4) The full mold is formed by
covering the coated tree
with sufficient refractory
material;
5) The mold is held in an
inverted position and
heated to met the wax and
permit it to drip out of the
cavity;
6) The mold is preheated to
high temperature, which
Ensures that all contaminants are eliminated from the mold; it also permits the liquid metal to flow more easily
into the detailed cavity; the molten metal is poured; it solidifies and
7) The mold is broken away from the finished casting. Parts are separated from the sprue
Figure 13.6
Steps in Permanent Mold Casting:
1) Mold is preheated and
coated;
2) Core (if used) are inserted
and mold is closed;
3) Molten metal is poured into
the mold;
4) Mold is opened.
Finished parts is shown in 5
Figure 13.13
Cycle in Hot Chamber Casting:
1) With die closed and
plunger withdrawn,
molten metals flows into
the chamber;
2) Plunger forces metal in
chamber to flow into die,
maintaining pressure
during cooling and
solidification; and
3) Plunger is withdrawn, die
is opened and solidified
part is ejected.
Finished part is shown in 4.
Figure 13.12
General configuration of a (cold-chamber) die-casting machine.
Figure 13.11
Low Pressure Casting. The Diagram shows how air pressure is used to force the
molten metal in the ladle upward into the mold cavity. Pressure is maintained until
Casting has solidifies.
Figure 13.13
Cycle in Cold Chamber
Casting:
1) With die closed and
ram withdrawn,
molten metal is
poured into
chamber;
2) Ram forces to metal
to flow into die,
maintaining pressure
during and
solidification;
3) Ram is withdrawn,
die is opened and
part is ejected.
(Gating system is
simplified)
STING QUALITY
There are numerous opportunities for things to go wrong in a casting operation, resulting in quality
defects in the casting. In this Section we compile a list of the common defects that occur in casting
and we indicate the inspection procedures to detect them.
Casting Defects:
Some defects are common to any and all process. These defects are illustrated in figure 13.22 and
briefly described in the following:
a) Misruns: A Misruns is a casting that has solidified before completely filling the mold cavity.
Typical causes include
1) Fluidity of the molten metal is insufficient,
2) Pouring Temperature is too low,
3) Pouring is done too slowly and/or
4) Cross section of the mold cavity is too thin.
b) Cold Shut: A cold shut occurs when two portion of the metal flow together, but there is lack of
fusion between them due to premature freezing, Its causes are similar to those of a Misruns.
Figure 13.22
Some common defects in castings:
a) Misruns b) Cold Shut c) Cold Shot d) Shrinkage Cavity e) Microporosity f) Hot Tearing
c) Cold Shots: When splattering occurs during pouring, solid globules of the metal are formed that
become entrapped in the casting. Poring procedures and gating system designs that avoid
splattering can prevent these defects.
d) Shrinkage Cavity: This defects is a depression in the surface or an internal void in the casting
caused by solidification shrinkage that restricts the amount of the molten metal available in the
last region to freeze. It often occurs near the top of the casting in which case it is referred to as
a pipe (Figure 12.7). The problem can often be solved by proper riser design.
e) Microporosity: This refers to a network of a small voids distributed throughout the casting
caused by localized solidification shrinkage of the final molten metal in the dendritic structure.
The defect is usually associated with alloys, because of the protracted manner in which freezing
occurs in these metals.
f)
Hot Tearing: This defect, also called hot cracking, occurs when the casting is restrained or early
stages of cooling after solidification. The defect is manifested as a separation of the metal
(hence the terms tearing or cracking) at a point of high tensile stress caused by metal’s
inability to shrink naturally. In sand casting and other expandable mold processes,
compounding the mold to be collapsible prevents it. In permanent mold processes, removing
the part from the mold immediately after freezing reduces hot tearing.
Some defects are related to the use of sand molds and therefore they occur only in sand castings.
To a lesser degree, other expandable mold processes are also susceptible to these problems.
Defects found primarily in sand castings are shown in figure13.23 and describe here:
a) Sand Blow: This defect consists of a balloon-shaped gas cavity caused by release of mold
gases during pouring. It occurs at or below the casting surface near the top of the casting.
Low permeability, poor venting and high moisture content of the sand mold are the usual
causes.
b) Pinholes: A defect similar to a sand blow involves the formation of many small gas cavities
at or slightly below the surface of the casting.
c) Sand Wash: A wash is an irregularity in the surface of the casting that results from erosion
of the sand mold during pouring. The contour of the erosion is imprinted into surface of
the final cast part.
d) Scabs: This is a rough area of the casting due to encrustations of sand and metal. It is
caused by portions of the mold surface flaking off during solidification and becoming
embedded in the casting surface.
e) Penetration: When the fluidity of the liquid metal is high, it may penetrate into the sand
mold or sand core after freezing, the surface of the casting consists of a mixture of sand
grins and metal. Harder packing of the sand molds helps to alleviate this condition.
f) Mold Shift: This is manifested as a step in the cast product at the parting line caused by
sidewise displacement of the cope with respect to the drag.
g) Core Shift: A similar movement can happen with the core but the displacement is usually
vertical. Core shift and mold shift are caused by buoyancy of the molten metal. (Figure
13.1.3)
h) Mold Crack: If mold strength is insufficient a crack may develop in to which liquid metal can
seep to form a fin on the final casting.
Inspection Methods:
Foundry inspection procedures include;
a. Visual Inspection to detect obvious defects, such as Misruns, cold shut and severe
surface flaws;
b. Dimensional measurements to ensure that tolerances have been met;
c. Metallurgical, chemical, physical and other tests concerned with the inherent quality of
the cast metal. Tests in category 3 include
1) Pressure testing to locate leaks in the casting
2) Radiographic methods, magnetic particle tests, the use of fluorescent
penetrants and supersonic testing to detect either surface or internal defects
in the casting;
3) Mechanical testing to determine properties such as tensile strength and
hardness. If defects are discovered but are not too serious, it is often
possible to save the casting by welding, grinding or other salvage methods
to which the customer has agreed.