Introduction
A gray iron foundry or ductile iron foundry manufactures metal
castings through the process of melting metals into a liquified
substance in order to pour it in a mold that contains a hollow
cavity of the required product. After the metal has been
solidified by the coolling process, the mold material gets
removed and a casted product is finished. The solidified product
is called a "casting". Casting is an economical technique to
produce complex shapes and products compared to other methods.
Our steel foundry works with aluminium and cast iron mostly, but
we can also work with metals such as zinc, bronze, magnesium,
brass, steel and ductile iron. Our cast iron foundry and
workshop manufactures mostly low tech products in medium to high
quantities.
Melting
The melting process is done with a furnace, which is charged
with: virgin materials, internal scrap, external scrap and
alloying elements. Virgin materials are pure forms of the
primary metal that we use to form an alloy. External scrap is
material from other forming processes such as machining, forging
or punching whereas internal scrap refers to defective castings,
risers, gates and other metal surplusses produced within the
casting foundry. The charge is melted, refined, the melt
chemistry is adjusted and then tapped into a transport vessel.
To avoid defective castings, deleterious gasses and elements is
removed by refining. Materials are added during the melting
process to reach a specific product requirement specified by
industry standards. Sometimes, certain fluxes are used to
seperate metal from slag and to remove dissolved gas from metals
that dissolve certain gasses, we use degassers.
Furnaces
A casting manufacturer melts the metal with special furnaces.
These are large vessels to hold the materials and provide energy
for the melting process. Modern furnaces types are: crucible,
reverberatory, cupolas, induction and electric arc furnaces.
Crucible and reverberatory furnaces are common for the
production of aluminium, bronze and brass castings, while
electric arc furnaces, cupolas and induction furnaces are often
used for ferrous materials. Designing and setting up a furnace
is a complex process and can be any size dependant on your
requirements. Furnaces of casting manufacturers range from small
ones to melt precious metals to large furnaces weighing several
tons, capable of melting hundres of pounds of scrap at one time.
The design of the furnace is according to the type of metals
that need to be melted.
Furnaces used by iron foundries or metal foundries also need to
be designed based on the fuel that is used to produce the
desired temperature. Zinc or tin for example have a low melting
point and in this case melting furnaces reach around 500° C. To
reach 500° C, electricity, propane or natural gas is used. Steel
or nickel based alloys that have high melting points even
require furnaces that reach temperatures over 1600° C. To
achieve high temperatures, such as these, electricity or coke is
used by casting foundries. So depending on the type of metals a
foundry is working with or specializing in, different types of
furnaces setups are used. A grey iron foundry for cast iron may
use: a cupola, induction furnace or an electric arc furnace. A
steel foundry will use an electric arc furnace or induction
furnace. Induction furnaces or crucible furnaces are often used
by brass or bronze foundries. Gas heated crucible furnaces,
electric resistance or reverberatory furnaces are commonly used
by aluminium foundries.
Degassing
Degassing processes in an iron casting foundry are necessary to
reduce the amount of hydrogen that is dissolved in the liquid
metal substance, especially when working with aluminium alloys.
A casting will end up with porous if the hydrogen concentration
is too high, as the hydrogen comes out of the solution when the
aluminium cools down and solidifies. Porosity has serious
impacts on the quality of the product. By bubbling argon or
nitrogen through the melt, it is possible to remove hydrogen
from the melt. When bubbles in the melt move upwards, the
dissolved hydrogen are brought to the top surface. The amount of
hydrogen can be measured with special equipment. It is also
possible to check the amount of hydrogen dissolved by
calculating the density of the aluminium sample. If porosity
still is a problem after degassing, a process called metal
impregnating is used to accomplish porosity sealing.
Mold Making
Foundry companies work with patterns made in the required shape
of the product. If the design is simple, we can use a single
piece pattern or sold pattern. Complex designs are made in two
parts, also called split patterns. Split patterns have an upper
section (cope) and a bottom section (drag). Split and solid
patterns both can have cores inserted. The function of these
cores is to create hollow areas in the mould. The parting line
is where the cope and the drag separate. A good practice when
making a pattern is to taper the edges, in order to remove the
pattern without breaking the mould. Materials that are used to
make a pattern are: wax, wood, plastic or metal. Molds are
constructed by several different processes depending on: the
type of foundry, the type of metal that is poured, quantity of
parts that must be produced, the size of the casting and the
complexity of the casting. The mold processes are: sand casting,
lost-foam casting, investment casting, ceramic mold casting,
V-process casting, die-casting, billet (ingot) casting.
Pouring, Shakeout and Degating
The molten metal is then poured into the mold, either by hand
using ladles or by automatic pouring machines. When the product
has solidified, it is removed from the mold. Sand based molds
are removed by shaking or tumbling. At this point the foundry
castings are still attached to the runners and gates. These are
the channels in which the metal travels to reach inside the
mold. After that, the runners, heads, gates, and risers are
removed by the steel casting foundry from the casting using
bandsaws, cutting torches or ceramic cutoff blades. This removal
process is also called degating. Depending on the metal type and
design of the gating system, the sprue, runners and gates can be
easily removed by breaking them away with a sledge hammer or
special knockout machinery. Risers are usually removed with a
cutting method but modern methods use knockoff machinery that
allows the riser to break off at the right place. The gating
system yields leftover material, including risers, head and
sprue, which are sometimes called sprue all together, that could
even exceed more than half of the metal required to pour a full
mold.
Grinding or Blasting
After the casting is degated, there may still be sand or other
molding material attached to the casting. As such, a blasting
process is used to clean the surface. Normally, in this blasting
process, tiny granular media or grinding balls are propelled
against the surface to blast away the sand and other unwanted
materials. These grinding balls are either blown with compressed
air or are hurled by a shot wheel. Materials that could be used
as grinding media are: iron, steel, aluminium oxides, glass
beads, other metal alloys, walnut shells or baking powder.
Blasting media is selected to develop the reflectance and the
color of the cast surface. This blasting process is also
referred to as: cleaning, bead blasting or sand blasting.
Afterwards a process called shot peening could be used to finish
the surface.
Finishing
Final steps in the casting process are: grinding, sanding or
machining the products in order to achieve the required
dimensions, shapes or surface finishing. Products that require
tight dimensional control are machined in CNC milling centers.
It is, however, not uncommon for many components to be used
without machining processes. Our foundry also offers other
services such as painting the products to prevent corrosion and
to improve the visual appeal. We could also weld wrought metals
together to form a completed product.
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