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How to choose a foundry partner for your ferrous metal casting projects


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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