Ordering steel castings and qualifying a supplier foundry can be a
time-consuming process. Because a high level of interaction between
the supplier foundry and the customer will be necessary, qualifying
the correct foundry partner will be of the utmost importance. The
supplier foundry should be involved from the early stages of design
through to manufacturing and product delivery. A cooperative effort
is essential to the successful outsourcing of a casting project.
The purpose of requesting a quotation for a casting is to determine
the lowest purchased casting cost. The customer must weigh all of
the provisions of the quotation; including exceptions taken to
drawings, specifications, and processing requirements, as well as
supplier foundry experience, tooling requirements, tolerances,
finish allowances, and delivery. Other factors, such as reduced
machine work, better tolerances, improved delivery schedules, and
reliability, are all particularly important to determine the
lowest-end cost of the casting. Cast steel should be suitable for
the intended application. As such, cast steel grades conforming to
the relevant standards should also be identified upfront, as the
needed steel grade affects the overall cost.
To avoid misunderstandings, to reduce costs, and to expedite the
processing of quotations, the following information should be
included in a request for a quotation:
Design – Provide drawings with the exact dimensions of the parts
required. See DESIGN below.
Quantity - What is the anticipated or required volume; both present
and future?
Material and inspection requirements - What should the part be made
of? (What is the cast steel grade?) How should the part be tested
before delivery? ASTM or other nationally recognized specifications
should be used whenever possible to identify the material and
inspection requirements. See MATERIAL SPECIFICATIONS and SOUNDNESS
below.
Actual or estimated casting weight - Actual weight information is
preferred. Estimates can be provided by the supplier foundry in the
absence of actual weight information, but this may require offering
prices that are subject to changes based on the actual weight of the
casting(s) in question at the time of production.
Drawing - Machine drawings are preferred over casting drawings.
Drawings or sketches are mandatory if samples or patterns are not
available. The drawing should include dimensional tolerances,
indications of critical areas, and surfaces to be machined. See
MACHINING below.
Pattern - If patterns and core boxes are available, the request for
a quotation should indicate the type, condition, and set up of the
equipment. See PATTERNS below.
Production/delivery schedules - Present and anticipated need should
be included in quotation requests.
Beyond these basics, there are levels of customer requirements,
which affect the casting cost drastically. These could include the
supplier foundry receiving inspection acceptance and their back
charge policy, casting return policy, expediting procedures, and
sophisticated controls not normally associated with the standard
inquiry. At Reliance Foundry, we find that a complete understanding
of these areas is best developed through open communication between
the customer and the foundry representative.
Design
A cast metal part should be designed to take full advantage of the
casting process. This will allow for a more efficient and cost-senestive
production of the casting. A pattern or drawing is necessary and the
number of castings to be produced (length of run) must be clearly
stated.
Castings are generally furnished with un-machined, as-cast surfaces,
unless otherwise specified. To take advantage of the casting
process, the supplier foundry should also know which surfaces are to
be machined and where datum points are located. The acceptable
dimensional tolerances must be indicated when a drawing is provided.
Tolerances are normally decided by agreement between the supplier
foundry and customer. Close cooperation between the customer's
design engineers and the supplier foundry is essential to optimize
the casting design.
Material Specifications
Industry standard specifications provide the casting customer with
the tools necessary to establish criteria for almost any casting
application. These specifications do not preclude special
requirements that the customer's technical staff members may
require. Variations from standard specifications can result in
misunderstandings, higher costs, and disqualification of potential
supplier foundries. If exception is taken to a provision in the main
body of a specification requirement (as opposed to taking exception
to a supplemental requirement of a specification), the resulting
casting cannot be held to compliance with those specifications.
Mechanical properties may be verified by the use of test bars cast
either separately or attached to the castings. The mechanical
properties obtained represent the quality of the steel, but do not
necessarily represent the properties of the castings themselves,
which are affected by solidification conditions and rate of cooling
during heat treatment, which in turn are influenced by casting
thickness, size, and shape. In particular, the hardening ability of
some grades may restrict the maximum size at which the required
mechanical properties are obtainable. Short of destructive testing
of an actual casting sample, the use of a test bar is the best
measure of the steel quality.
Soundness
Soundness of metal components refers to the level of freedom from
impurities and/or discontinuities such as sand inclusions, slag
inclusions, macro porosity, and shrinkage.
Steel castings are formed within a mold. They can solidify quite
quickly and the volumetric contraction must be matched by the feed
of the liquefied metal. Liquid is fed to the heavier section of the
mold through a riser that serves a reservoir. If feed of the
liquefied metal is blocked or not enough feed metal is delivered to
compensate for the volumetric contraction during solidification,
shrinkage cavities will develop in the casting.
After production, all castings should be tested to ensure that they
meet the requirements of the specification. In many applications,
testing is mandatory and additional tests may be required to ensure
that material specifications or other general requirements are met.
Testing and acceptance criteria must be clearly communicated and
agreed upon prior to production. Generally, projects with more rigid
the terms of testing will be more expensive. Therefore, the terms of
testing and acceptance should only be based on the requirements of
each application.
It is impossible to produce a defect-free casting, only castings
with defects of varying degrees of acceptability. The acceptance
and/or rejection of such castings can only be determined by
examination and analysis of parts (in accordance with
internationally recognized standards such as ASTM) based on
customers' formal engineering requirements. A defect in one
application may not be a defect in another application. The size of
flaw(s) can vary significantly. What is acceptable and what is
defined as a REJECTED defect depends on agreement between the
supplier foundry and the client prior to production. Large cavities
often exist in thick-section castings and can be perfectly
acceptable depending on the application and the location within the
casting. On the other hand, some applications are very critical and
tiny flaws (or even micro-porosity - as defined by a specific NDT
process and acceptance/rejection level) may be considered as defects
that may be detrimental to the intended use of the product.
Acceptance and rejection criteria for castings production must be
determined at the time of quotation and certainly at the time of
order, as such criteria affect the price of castings, as well as the
production procedures and processes used to produce the castings.
Patterns
Pattern equipment design and the resultant costs can constitute a
major source of misunderstanding between the customer and supplier
foundry. The need to construct new pattern equipment when existing
equipment is not available, a requirement for a full split core box
in place of a half core box, pattern material, and mounted or loose
patterns are but a few of the many areas of discussion that affect
the cost of the equipment. Invariably, the lowest casting cost and
highest casting quality evolve from the more sophisticated pattern
equipment, which generates the highest pattern cost.
Minimum Section Thickness
Every casting has a minimum thickness that is affected by its
rigidity. Foundry professionals use strength and rigidity
calculations to work out how thin a casting can be. Designing
castings thinner than the specifications may make the project un-castable.
To be successful, a castings design must allow liquefied metal to
fill the mold in the thinner sections.
Liquefied (molten) metal cools at an exceptionally fast rate. It may
cool too quickly to enter thin sections that are far away from the
mold’s gate. As a general rule, designs should not have areas that
are thinner than of 0.25 in (6 mm), when conventional processes are
used. Investment casting allows for a greater level of freedom and
wall thickness can drop to as low as to 0.030 in (0.76 mm).
Draft
The term “draft” refers to the degree of angle that must be added to
vertical faces of a pattern. Ignoring the limitations of these
angles will cause the mold to tear when it is removed from the
casting. Draft should be added to the angles so that minimum metal
thickness is maintained.
It is necessary to consider draft in all casting projects no matter
what process is employed. The use of cores may eliminate the need
for draft but it will increase the cost of the project. If the draft
will affect the use of the casting in its intended application, the
amount to be added or subtracted should be specified in the drawing.
Several factors will affect how much draft is required for each
casting. The manufacturing process, the casting’s size and whether
the moulding is performed by hand or machine will all change the
level of draft that is needed. Less draft is required for castings
that are machine moulded. Castings produced with green sand molds,
however, will require more draft than usual. In normal
circumstances, a general rule of thumb is to allow 3/16 in. of draft
per ft. (approximately 1.5 degrees).
Parting Line
Casting often requires two pieces to be put together, resulting in
parting lines. To ensure a successful casting, it is best to design
in the parting in one plane. Symmetrical designs allow this to be
done most easily and are therefore encouraged by Foundry
professionals. Straight parting lines will create more
cost-sensitive castings than castings with irregular parting lines.
Straight parting lines will also make it easier to add cores and
will help during the moulding process. Split patterns (separate cope
and drag) simplify the process and reduce the required handwork and
cost while improving the finish of the casting.
Cores
A core is a hollow space created within a casting by a piece of
moulding sand. Cores are necessary when the spaces or cavities
cannot be created with the pattern alone. As they add to the cost of
the casting, their use should be limited.
Three factors affect the minimum diameter of a core that can be
placed in a casting:
The thickness or depth of the area in which the core will be placed.
How long the core is
The casting method that the supplier foundry will employ
Thermal conditions may also affect the core as extreme heat can
increase the metal thickness surrounding the core and place an
adverse amount of heat and pressure on the core. Heat should be
dispensed evenly throughout the core. As the heavy sections of the
casting will heat up more than the lighter ones, they will transfer
more heat into the core. With the increased levels of heat, the core
will become more difficult to remove and thus increase the cost of
the casting.
Adding Cores can cause bending stress within the casting and this
will be amplified by buoyancy forces and the thickness of the metal.
The increased stress may make it difficult for the foundry
professionals to obtain the necessary tolerances. Rods can be used
to strengthen larger cores but as thickness and core length increase
so does need for reinforcement to counter the bending stress. Thus
increasing the reinforcing will require the diameter of the core to
be increased as well.
Areas in the casting that are less accessible make it more difficult
to remove cores and affect the economic feasibility of the projects.
Castings should always be designed with openings large enough to
allow for the removal of cores.
Machining
Tolerance refers to the dimensional accuracy achievable for a given
production method. Mold expansion, solidification shrinkage, and
thermal contraction all influence the tolerance of the finished part
in the green sand casting process. Consequently, there are limits
for tolerances in an as-cast part. Subsequent machining is commonly
employed when a tighter tolerance is required.
Supplier foundries are responsible for supplying cast products that
can be machined to meet the exact requirements of the specification.
To facilitate this process, cooperation between the casting producer
and the customers design or purchasing staff is necessary. The
following points should be discussed and agreed upon:
The pros and cons of the casting system that will be employed
The machining stock allowance
The products design and how it may be affecting by the casting and
machining equipment
The material and heat treatment that will be used
The quantity or length of run
All casting designs should be checked by a foundry professional
prior to production, in order to determine the feasibility of the
casting project. Often, it is beneficial to have a complete layout
of the casting produced to ensure a proper stock allowance on all
surfaces that will require machining. Layouts are not required for
basic designs that can simply be measured with a ruler. For castings
with more intricate machining dimensions, it is often beneficial to
indicate all machined surfaces with target points and scribing
lines.
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