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Most iron and steel castings are produced by casting into sand molds. Sand cores are needed primarily to form hollow cavities in castings for collapsibility and ease of cleaning. This chapter begins with an overview of the classification of molding and core-making systems. This is followed by a section discussing the process involved in shell molding, along with its applications. A brief description of the special casting processes is then presented. Next, the chapter discusses the processes involved in core making. Further, it provides an overview of casting manufacturing. Finally, the chapter provides information on the factors that influence a casting facility layout.

MOST IRON AND STEEL CASTINGS are produced by casting into sand molds. Hollow sections of castings need sand cores for collapsibility and ease of cleaning. Sand molds and cores are inexpensive, and their refractoriness is adequate even for steels, which need higher temperatures compared with cast irons. Most of the sand for molds is recycled because it is reusable with some replenishment of the bond (which is burned out due to the heat of the molten metal). Much of the core sand gets mixed with the recycled molding sand.

Molding and core-making processes are broadly classified into two major groups:

  • Green sand systems

  • Chemically bonded sand-core molds

Green sand systems are predominantly used for molding but rarely for cores. Green sand cores are used only in the very few cases where core collapsibility is critical for free casting contraction to avoid any casting cracks. In the green sand system (the word green indicates that moisture is used for bonding with clay and sand), the refractory sand is mulled (in a muller with two rollers and plows) with a clay called bentonite along with water and other additives. The sands that are used include silica sand, chromite, olivine, and zircon. Typically, silica sand is used for cast iron and smaller steel castings. The other mixes (of higher refractoriness) are used for medium- and large-sized steel castings. Sand mix additives for cast iron include sea coal (finely ground coal), wood flour, and dextrin, depending on the surface finish and the green strength needed to withstand the metal pressure. Figure 3.1 shows green sand and the chemically bonded systems for molds and cores. Only the more popular systems for both molds and cores are included in the figure.

Fig. 3.1

Molding and core making systems

Fig. 3.1

Molding and core making systems

Close modal

Green sand molding has come a long way with the introduction of high-pressure squeeze molding and highly automated high-production molding machines. Human intervention is minimum; labor is required only to set chemically bonded cores or to assist core setting into core fixtures or into core masks. The direction of sand compaction is either vertical or horizontal depending upon the molding machine manufacturer.

Several practical examples of cast irons and some steel castings illustrate the scope of the green sand molding application. In general, small- to medium-sized castings that are not heavily cored are good candidates for green sand molding.

Medium- to large-sized castings may need more rigid molds to withstand the weight of the metal poured and also the weight of the cores. Furan resin–based self-setting or air-setting systems that use a catalyst are good choices in such cases. The ability to mold-wash air-set molds with a refractory coating is a significant advantage for steel castings.

Large-sized castings are also produced using either air-set sands or sodium silicate–bonded molds cured using carbon dioxide gas. Some examples of large-sized steel castings illustrate such applications (See Chapter 12, “Engineering Carbon and Alloy Steel Castings,” in this book.).

Medium- to large-sized cores are produced with air-set sand systems with furan resin–bonded sands. Small- to medium-sized cores are produced at high production rates using sulfur dioxide gas for curing furan-based sand systems. Hollow shell cores are produced using the thermosetting property of resins by blowing phenolic resin–coated sands into heated core boxes to form a shell. The uncured sand is drained back into the investment magazine for reuse. These cores have a very long shelf life, and they produce castings to a very fine finish. The hollow shell cores are more expensive compared with the furan systems, but their application is justified if long shelf life, fine finish, and absence of breakage in transportation (for bought-out cores) are major requirements. Shells clamped together as clamshells to form molds are used for casting in some special applications.

Molds are usually produced in two parts by compacting prepared sand over patterns mounted on plates. The molds are stripped from the patterns and assembled for pouring. The sprue and gates for the metal entry as well as the feeders to feed the solidification shrinkage are built onto the patterns over pattern plates as fixed pieces or loose (removable) pieces. Mold assemblies are poured through the sprue. Solidified castings are separated from the molds by shaking or punching out or by feeding molds into a revolving drum with baffle plates. The mold sand is recycled. Castings are cleaned by shot-blasting followed by additional operations, such as de-gating or feeder removal and flash-grinding.

Match plates are aluminum pattern plates where the pattern halves for the two-part molds are mounted on the same plate on opposite faces. Sand is sequentially compacted with drag first and then the cope. Figure 3.2 is an illustration of match plate molding.

Fig. 3.2

Match plate molding

Fig. 3.2

Match plate molding

Close modal

The ejector mechanism enables the mold halves to be separated from the flask. A steel frame is fitted to guard against any metal spills at the parting plane, as shown in the bottom right-hand side of Fig. 3.2. The molds are loaded with weights to prevent the rising of the cope against the force of buoyancy. Match plate molding machines that compact the cope and the drag simultaneously are popular because they offer faster cycle times.

Cope and drag plates are two separate pattern plates where each of the two parts is mounted on separate pattern plates for simultaneous molding. Cope is the top mold half, and drag is the bottom mold half.

Figure 3.3 illustrates an example of cope and drag molding with flasks. Examples of pattern plate applications to ductile iron knuckles and brake calipers, cast steel draft gears, and axle housings are discussed in Chapters 10, “Engineering Austempered Ductile Iron Castings,” and 12, “Engineering Carbon and Alloy Steel Castings,” of this book. Molding the cope and the drag simultaneously is advantageous for cycle time reduction.

Fig. 3.3

Cope and drag molding

Fig. 3.3

Cope and drag molding

Close modal

Patterns are made of wood, plastic, aluminum, cast iron, or stainless steel, depending on the number of molds needed. Table 3.1 provides a general guide to the choice of pattern material based on the number of molds. Because the pattern life depends on several factors, such as the molding media, tooling design, and hardness of the pattern material, Table 3.1 is for general guidance only.

Table 3.1
General guide for pattern material choice
Pattern materialTotal number of molds
Hard wood500 – 5000
Epoxy resin reinforced with fiberglass5000 – 20,000
Aluminum alloy20,000 – 100,000
Gray iron100,000 – 300,000
Steel (stainless steel or chrome plated)300,000 – 500,000
Pattern materialTotal number of molds
Hard wood500 – 5000
Epoxy resin reinforced with fiberglass5000 – 20,000
Aluminum alloy20,000 – 100,000
Gray iron100,000 – 300,000
Steel (stainless steel or chrome plated)300,000 – 500,000

Flasks are rectangular, square, or circular boxes laid over the pattern plates to hold the sand that will be compacted. Smaller flasks are made of cast iron (sometimes of aluminum for ease of handling in hand molding), and larger flasks are made of fabricated steel, sometimes with cast steel sides bolted together. The flasks are reused by returning to the molding station from the mold knock-out station. Flasks are designed with hardened steel bushings for accurate location on the pattern plates, with the help of locator pins.

The two mold halves are assembled together (cope over drag) with accurate use of removable guide pins. Flasks are expensive to build and maintain. Automation is needed to return the flasks from the knock-out station back to the molding station. The sand that sticks to the inner walls needs to be cleaned off. However, molding processes such as high-pressure molding generally require them for producing well-compacted molds. Larger cope molds, which need reversing for stripping, core setting, and other manipulation needs, require well-designed sturdy flasks and trunnions.

Smaller green sand molds can be produced using a two-part tapered flask mounted on the machine. The molds are separated from the mounted flask halves, assembled, and moved to the pouring stations. The tapered flask halves stay on the machine.

Medium-sized high-pressure squeezed molds are also produced as flaskless molds both in horizontal and vertical squeeze modes.

Molds made from air-set sands or sodium silicate–bonded sands are rigid enough for handling. They are formed in core boxes and are stripped after curing. Unless there is a need for turning over, these do not need a flask. Simpler cope molds can be engineered for turning over without flasks.

Molds have been parted horizontally from the time of hand molding. About 50 years ago, the innovative technology of vertically parted molds was introduced. Many machines that use high-pressure, vertically parted, horizontally squeezed match plate molding have been in use over the years. Millions of castings have been produced and are still being successfully produced today. The system is highly automated and includes core-setting (using core masks) and automated pouring, which needs very little human intervention.

Figure 3.4 (Ref 1) is an example of vertically parted, vertically filled flaskless molding. The left-hand side of the pattern plate is attached to a hydraulic cylinder. The right-hand side of the pattern plate is hinged so that it can be swung out, making room for the mold to move forward. Prepared green sand that flows from the overhead hopper is pressurized (or blown) to fill the space between the left and right pattern plates (view A). The hydraulic cylinder advances compacting the sand (view B). The right pattern plate is swung out (view C). The hydraulic cylinder pushes the mold forward (view D). Cores are loaded onto a core mask and held in position by vacuum. The automated core setter advances and sets the core into the mold (releasing the vacuum) (view D). The mold advances further to close on the previously formed mold. After the molds move together, the auto-pour furnace pours the molds in sequence (view E). The right-side pattern swings down to the position shown in view F to prepare for the next mold.

Fig. 3.4

Vertically parted flaskless molding. Source: Ref 1 

Fig. 3.4

Vertically parted flaskless molding. Source: Ref 1 

Close modal

Vertically parted molds are ideally suited for work with simple cores because the cores must be held in a vertical position against gravity. Some examples of the production of cast iron brake rotors and malleable iron fittings are covered in Chapter 7, “Engineering of Gray Iron Castings,” and Chapter 8, Engineering of Malleable Iron Castings,” in this book.

Complex core assemblies and deeper jobs benefit from horizontally parted molding. More recently, a hybrid process of horizontally parted, horizontally squeezed but rotated for horizontal core-setting has become very popular. Also, the production rates are higher compared with the process illustrated in Fig. 3.4, as the two mold halves are created simultaneously.

Figure 3.5 (Ref 1) illustrates the development of the machine, producing both mold halves in vertical position and manipulating them to the vertical position for closing, core setting, and pouring. The match plate is located between the flasks (view A). The flasks with the pattern plate (and with gating) are turned over to the vertical position (view B). Sand fills the space between the pattern plate and the flasks using air pressure over the sand in the hopper. The mold is squeezed from both sides simultaneously (view C). The mold assembly is turned over to the horizontal position (view D). The mold halves are separated for the pattern plate to be ejected out. The core is set (view E), and the mold-core assembly is lowered from the flasks (view F). The molds move on the conveyor to the auto-pour station for pouring.

Fig. 3.5

Vertically parted, horizontally closed system concept. Source: Ref 1 

Fig. 3.5

Vertically parted, horizontally closed system concept. Source: Ref 1 

Close modal

Chemically bonded sand-core molding is a versatile process used for medium- to large-sized castings in cast irons and steels. The green sand process is suited for large-volume jobs, which are typical of the automotive, air-conditioning, and pipe-fitting industries, with less complex shapes and less intricate coring. Larger molds with deeper pockets and larger cores need higher mold strength to withstand the weight of cores and metal. Chemically bonded sand molding offers these advantages.

The processes are slower than green sand processes but they are a good fit for a variety of industries, such as the railroad, and for construction machinery where the production volumes are much less than those for the automotive, air-conditioning, and pipe fitting industries.

Figure 3.6 (Ref 2) is a schematic layout of a chemically bonded, no-bake sand molding line. A continuous sand mixer (bottom left-hand side) mixes the sand and the binder. Wooden core boxes (or combinations of aluminum, epoxy resin, and wood) are used instead of the flasks. The cope (top half) and drag (bottom half) are molded like the cope and drag pattern plates described in Section 3.1.1.2 in this chapter. The core boxes carry the pattern profile at the bottom. Hardening systems vary depending on the bonder system. Either the hardener or the catalyst is mixed with the sand for air hardening or the molds are hardened using a gas after compaction.

Fig. 3.6

Layout of a chemically bonded sand molding line. Source: Ref 2 

Fig. 3.6

Layout of a chemically bonded sand molding line. Source: Ref 2 

Close modal

The sand in the core boxes is compacted on a vibrator assisted by tucking and light ramming with pneumatic hammers. The core boxes move to a roll-and-draw machine where a pallet is laid over the compacted core boxes and the core mold is rolled over or reversed and the core box is lifted to unload or strip the core mold onto the pallet. The core boxes are conveyed back to the core mold-making station.

Cores are set in the bottom core mold. The top is flipped over, and the core mold is closed in a closing unit. An auto-pour pours the core molds. An infrared sensor controls the auto-pour to stop each pour as the sensor senses the metal in the feeders or risers reaching an acceptable (preset) level.

Poured core molds move to a cooling tunnel and onto a shake-out unit. The castings get separated from the core mold and move to the cleaning room. The sand lumps pass through a magnetic separator unit to catch any iron or steel flash and then through a lump crusher. Screened sand is conveyed to a sand reclamation unit where the residual bond is burned. Sand passes through a cooler, and the cooled sand is made available for the mixer, closing the loop.

The shell-molding process is a precision molding process with costs and dimensional capabilities that are higher than those of chemically bonded sand molds.

Figure 3.7 is a schematic of a basic dump box machine that produces shell molds. There are many designs, including two or more station machines for increased productivity. The main machine consists of a circular framework that can house the pattern plate and the shell sand container. The machine is designed for a 180-degree rotation of the pattern plate along with the sand container. Other means of engineering the 180-degree reversal are available in newer machines. The two match plate pattern halves are usually made of cast iron or steel because they should be able to be heated up to 175 to 370 oC (350 to 700 oF). Each match plate is fitted with a gas burner rack at the back (view A). A thermocouple embedded in the pattern plates sends a signal to the programmable controller for close-loop temperature control of the pattern plate within close preset temperature limits. The sand container can be clamped to the pattern plate. The container can be moved away from the pattern plate (view C).

Fig. 3.7

Shell molding process

Fig. 3.7

Shell molding process

Close modal

The machine operation starts heating the pattern plate to the preset temperature. Once the thermocouple registers the preset temperature, the machine turns by 180 degrees to position the container above and the pattern plate below (view B). Sand dumped on the hot pattern plate cures the resin to build up a shell to a thickness of about 8 to 10 mm (0.3 to 0.4 in.). The machine reverses back by 180 degrees to position the pattern plate above the container (view C). Uncured sand falls back into the container ready for the next core. The sand container is unclamped and moved away from the pattern plate.

The shell is ejected on the catch pan (view D). The other half shell is produced in a similar manner. The shells are heated in an oven to ensure that any leftover uncured resin is fully cured (view E). Care is needed to ensure that the cured shell is not totally burned, which would result in loss of the resin bonding. The two shells are pasted or clamped and positioned vertically, backed by sand in boxes (view F). After the casting has solidified and cooled to a safe temperature, the shells are knocked out to separate the castings. The castings then move to the cleaning operation.

The shell-molding process is chosen for products where deep pockets cannot be formed by the conventional green sand or no-bake molds, where conventional core-making processes cannot form deep pockets, or where the drafts needed would be larger than what the design would allow. Drafts as little as 0.25 to 0.50 degrees are feasible with shell molding. Close tolerances of 0.005 mm/mm are common.

Figure 3.8 shows an example of an air-cooled cylinder head where the narrow distance between the cooling fins is very difficult to form, making shell molding one of the best options. Some casting engineers use a shell core for the fins, set in a green sand mold.

Fig. 3.8

Air cooled engine cylinder head

Fig. 3.8

Air cooled engine cylinder head

Close modal

Both the refrigeration and air-conditioning industries use scroll compressors in which an orbital or moving scroll revolves against a fixed scroll, providing the compression needed, with very little noise and vibration. Iron scrolls are popular in stationary units, while aluminum alloy scrolls are popular in automotive air-conditioning units. Figure 3.9 is a schematic of a stationary rotary compressor scroll. The low draft and the close tolerances achievable in shell molding allow for precision manufacturing with minimum finish stock.

Fig. 3.9

Compressor scroll cast in shell molding

Fig. 3.9

Compressor scroll cast in shell molding

Close modal

Other applications of shell molding include components in alloyed cast irons (Ni-hard) for abrasive wear-resistance applications. This group of alloys is very hard to machine, and the near net shape or the net shape capability of shell molding eliminates the need to machine or at least minimizes the expensive machining. (See Chapter 7, “Engineering of Gray Iron Castings,” in this book for discussion about this group of alloys and their applications.)

Shell cores are suitable for use in several types of molds. They can be set in shell molds, air-set molds, or green sand molds because of the advantages of net shape and close tolerances that they offer. The shells’ light weight and their long shelf life are advantages to casting manufacturers who buy cores from outside. Also, the light cores make manual core setting in the mold much easier (e.g., shell core for a differential carrier).

Figure 3.10 shows the special casting processes detailed in this book. Other processes such as evaporative foam or lost foam are used primarily for special applications.

Fig. 3.10

Special processes

Fig. 3.10

Special processes

Close modal

The investment casting process (also called the lost wax process) is a precision casting process with the greatest design flexibility for complex shapes with very thin wall capability, providing exceptional surface finish and dimensional accuracy. Firearms, defense applications (in precipitation hardened steels), stainless steel components for the food industry and hospital equipment, and precision hand tools are some of the typical applications of the investment casting process. Figure 3.11 (Ref 2) summarize the investment casting process.

Fig. 3.11

Investment casting process. Source: Ref 2 

Fig. 3.11

Investment casting process. Source: Ref 2 

Close modal

Large-sized castings (e.g., 1200 to 1500 mm [4 to 5 ft] long or mid-sized castings with as-cast details) usually run in smaller annual volumes (about 10,000 to 15,000 pieces per year). Such volumes need wood tooling that is not expensive. The V process is a viable process that uses unbonded sand for molding. The process has the capability for very low drafts on the vertical surfaces, which is an additional advantage. The refractory coating sprayed over the plastic sheet at the mold–metal interface results in a fine surface finish that may be attractive, especially for steel castings. The cost of recycling the sand is minimal, with no need for expensive mechanical or thermal reclamation, unlike chemically bonded sand molds. The special flasks for vacuum applications require higher capital costs upfront. Figure 3.12 (Ref 2) is a schematic of the V process.

Fig. 3.12

Schematic of the V process. Source: Ref 2 

Fig. 3.12

Schematic of the V process. Source: Ref 2 

Close modal

A brief description of the process follows. View 1 shows the wooden pattern mounted on a hollow pattern plate with a connection to the vacuum pump. Fine holes 1.0 to 1.5 mm in diameter are drilled through the pattern to communicate with the vacuum chamber.

View 2 shows a plastic sheet spread over the pattern and softened by heating with an electric coil. In view 3, as vacuum is applied to the pattern plate, the plastic sheet is sucked tight onto the pattern. A refractory suspension is sprayed over the plastic sheet over the pattern. View 4 shows a flask that is surrounded by a vacuum chamber that is placed over the pattern plate. Dried, unbonded sand is poured over it up to the top level of the flask. In view 5, another plastic sheet is laid over the top of the sand, and vacuum is applied to the flask. The sand is held rigidly in the flask between the two plastic sheets. The vacuum on the pattern plate is released. View 6 shows the mold stripped off of the pattern plate. In view 7, the bottom mold is prepared similar to the top half. Cores are set in the bottom half or the drag. View 8 shows the mold that is closed and poured. While holding the vacuum in the mold, it is moved to the shake-out unit along with the poured casting. In view 9, after releasing the vacuum, the sand falls into the hopper below; the casting can be retrieved from the grating on the hopper. The plastic sheets on the top and bottom flasks as well as the partially burned plastic sheets are recovered for recycling. The casting moves to the trim station for sawing of the gates, risers, and feeders.

Advantages of this process include:

  • Potential for zero draft or minimal draft

  • High surface finish of about 125 to150 μm RMS value

  • Low finishing costs

  • Close tolerances of ±0.25 mm/mm up to 25 mm and 0.05 mm/mm beyond

  • No moisture-related defects

  • No toxic fumes of burning binders

  • No pattern wear

  • Ability to cast fine detail and as-cast features

  • Being very suitable for medium-sized steel castings, offering advantages of superior finish with the ability to avoid hot tears by timing the vacuum release

Limitations of this process include:

  • Production rates that are slower than for chemically bonded sand systems for the same or comparable sizes.

  • A vertical sprue that increases the velocity and turbulence with the potential for sucking slag into the metal stream.

The technology of three-dimensional (3D) printing has been a breakthrough in rapid sand casting. Cores and molds are created directly from the 3D component geometry, skipping the need to create tooling. This saves the time and cost of developing conventional tooling. This method is particularly attractive in rapid development of sand-cast prototypes for proof-of-design concept, prototype testing, and assembly verification. The method lends itself to making design changes rapidly during new product development. Low-volume production quantities can also be met using this method.

Figure 3.13 (Ref 2) outlines the sequence of steps in producing rapid sand-cast prototypes. Starting with a 3D component model, a casting 3D core assembly model is developed with finish stock, drafts, parting planes, coring, gating, and feeders. The assembly of such cores manufactured using 3D-layered printing enables the development of castings for bench testing and assembly verification. Castings are static-poured with the engineered alloy to closely replicate the prototypes needed.

Fig. 3.13

Rapid sand casting. Source: Ref 2 

Fig. 3.13

Rapid sand casting. Source: Ref 2 

Close modal

Figure 3.14 (Ref 2) is a representation of the 3D layering machine for sand-core manufacturing to produce a ductile iron differential carrier. The furan binder jet printer head is mounted on slides capable of traversing in X and Y directions, as illustrated. A thin layer of sand is spread, and the furan binder jet head traverses to harden the profile in the chosen cross section. Once this layer is hardened, a second layer is evenly laid out for the jet head to traverse. The entire core mold is built in successive layers.

Fig. 3.14

Three-dimensional rapid core-making system. Source: Ref 2 

Fig. 3.14

Three-dimensional rapid core-making system. Source: Ref 2 

Close modal

The reproducibility is excellent, and the tolerances are very close. Complex cores with little drafts can be produced using this method. Uncured sand is removed by vacuum and brushing in the next work station while the next core is being cured on the machine. Sand that is not needed to be cured is reused.

The side cores are assembled on a base core. A central core is set to form the cavity. The core mold assembly can be backed by loose sand in a box or a flask as a precaution against leaks. The assembly is clamped and poured. The castings move to the next station where they are separated from sand and shot-blasted to clean. The gates and feeders are removed. The casting is shipped for machining.

A small percentage of iron castings (brake cylinders and pipe fittings) are cast in permanent molds with the mold cavity covered with acetylene soot. The heat-treatment cost makes permanent molded castings less competitive, although the fine graphite shape of permanent molded cast iron is very conducive for wear resistance. The process of covering with acetylene soot is not environmentally preferred.

Cores are needed primarily to form hollow cavities in castings. Cores need to be strong enough to handle and to be set into the molds, core-setting fixtures, and core masks. Collapsibility of cores after pouring is essential for easy shake-out. The binder type and content in the core sand mix are chosen to ensure that the bond breaks down at the casting temperature.

Figure 3.1 indicates the main core-making processes. There are some variants in the binders and in the hardening gases or catalysts.

Small- and medium-sized cores are mass produced by shooting or blowing the sand mix into a core box; the core box is made of resin-lined wood, aluminum, or cast iron. Vents are engineered into the core box for the air to exit at high velocities as the sand is blown in. The location and number of core box vents are critical for uniform filling and density. Medium- and large-sized cores are vibrated or sand is tucked in and lightly rammed with pneumatic rammers.

Engineering of the core boxes includes:

  • Designing the parting plane for ejecting the core from the core box and drafts;

  • Engineering of core prints, clearances, and sand traps for loose sand during core setting;

  • Designing loose pieces, pull-backs, or inserts to form complex shapes;

  • Designing the direction of core blowing and the location of the blow tubes;

  • Positioning the core vents for filling the core cavity to ensure uniform sand density;

  • Choosing ejector pins for uniform core ejection;

  • Selecting core gassing locations for uniform core hardening;

  • Designing core gas venting;

  • Engineering the needed core assemblies and fixtures;

  • Checking to ensure that flash formed by the patching (or mudding) with graphite paste can be eliminated, removed, or left without any problem inside the cavity;

  • Engineering any reinforcements and handling hooks as needed;

  • Designing the core gripping method for automated core setting; and

  • Engineering for core assembly when multiple cores need to be put together for core setting.

Figure 3.15 is an example of a typical alloy ductile iron exhaust manifold. The core box features are shown for engineering. (See Chapter 9, “Engineering Ductile Iron Castings,” in this book for details about the material and processes of the exhaust manifolds.)

Fig. 3.15

Core box engineering features

Fig. 3.15

Core box engineering features

Close modal

The selection of the parting plane is driven by the geometry and the ease of separation (or ejection) of cores from the core box. In many cases, the parting plane may have to be offset or profiled. The ejector pins (E in Fig. 3.15) need to be designed for even stripping of the cores without damage. The casting function should not be affected by the potential impression (or digging in) of the ejector pins into the core. The location of vents (V in Fig. 3.15) is important because they control the uniformity of the core sand filling. The vents (slotted or screen vents) need to be properly maintained during routine core box maintenance.

The passage of gases to harden the core is designed into the core boxes. Gases, such as sulfur dioxide used for hardening, need to be collected and conveyed to a scrubber per environmental regulations for operator safety. The core room supervisor’s experience is useful in the design of these features.

Core boxes for complex cores are expensive to build, and modifications by trial and error are prohibitive. Computer simulation offers the potential to do it right the first time. At a minimum, simulation reduces the time and cost of changes. Core boxes need to be engineered to match the capabilities of the core-making machines (such as core blowers and core shooters) available in the core room. Tooling designers have to accommodate the maximum number of core cavities to suit the machines and ensure high productivity. The geometry-driven parting plane may pose some challenges when engineering the core box clamping, core ejection, handling, and storage.

Several computer software programs have been developed based on fluid dynamic principles to simulate and verify the blow pressure needed, position of vents for air exhaust, and to confirm the adequacy of the vent area and ensure complete filling of the core box cavity within the targeted time. Core blowing or shooting simulation requires computations of the coupled flow analysis of sand and air. Additionally, the flow of gases in gas curing can be simulated for gassing the cores for curing (Ref 3). Many foundries use core sand simulation software.

Variables encountered in engineering the core box design include (Ref 3 and 4):

  • Maximum number of cores to be nested to suit the core shooter

  • Orientation of the cores and the parting plane

  • Number of nozzles and operating pressure range

  • Location and sizes of core vents

  • Vents in a parting plane, if any

The number of variables requires multivariant DOE to be set for the inputs. The output for evaluation by actual trials would normally be the core weight or visual assessment of core surface quality.

Reference 4 provides an example of an engine intake manifold core where the vents had to be optimized to fill the core cavity, resulting in maximum density of compaction. Computer simulations were performed with a number of blow tubes, blow pressure, and venting to optimize the core quality. Figures 3.16 and 3.17 illustrate the simulation of blowing a cold box phenolic urethane core cured with amine gas for an intake manifold cast in aluminum. Figure 3.16 shows the core sand filling at nearly 30% of the cycle time, and Fig. 3.17 illustrates the fill at nearly 90% of the cycle time. Simulation was effectively applied to optimize gassing for hardening and purging of a good-quality core (Ref 5). A vertically parted core, blown and gassed vertically, produced unacceptable core quality due to poor curing. The numerous variables in the core-making process required a DOE to establish the process and solve the problem of poor curing. The curing simulations of phenolic urethane core sand systems required analysis of the flow of hardening gases, such as amines, through the permeability of blown cores. Computer simulation was used to analyze the amount of gas that reached different areas of the core. The analysis showed that the amine gas did not reach the bottom of the core; this resulted in poor curing and core crumbling. The analysis also showed the optimum positioning of the vents and the number of nozzles needed to achieve the highest density of the cores. The blowing pressure was found to have only a marginal effect on core filling. It should be noted that excessive blowing pressure would increase the core box wear. Simulation was effectively applied to simulate core box temperature distributions in the case of hot-box processes. The analysis helped to establish a close-loop temperature control of the tooling based on output.

Fig. 3.16

Core sand filling simulation, 30% cycle time. Source: Ref 4, Courtesy of FLOW-3D CAST

Fig. 3.16

Core sand filling simulation, 30% cycle time. Source: Ref 4, Courtesy of FLOW-3D CAST

Close modal
Fig. 3.17

Simulation of core sand filling, 90% cycle time. Source: Ref 4, Courtesy of FLOW-3D CAST

Fig. 3.17

Simulation of core sand filling, 90% cycle time. Source: Ref 4, Courtesy of FLOW-3D CAST

Close modal

Casting manufacturing operations include different processes integrated for continuous product creation with a streamlined flow. The basic elements of melting, molding, and core-making create the casting configuration. Several support and secondary operations, such as the supply of charge material, molten metal, patterns, raw and prepared sand, and core production and supply, all need to be coordinated before casting is poured. A few critical process elements for recycling flasks, sand, and returns (gates and feeders) need to be synchronized in real time. Post casting operations, such as shot-blasting for cast cleaning, de-gating, flash removal, and grinding, need to be coordinated and sequenced for continuous product flow.

Figure 3.18 shows the main connected activities in a green sand-casting facility. Charge materials are stored in hoppers. An overhead crane moves the charge materials to the furnace. A monorail carries molten metal to the pouring station. The patterns from the pattern storage are delivered to the molding machine. Molds are made using various methods (described in Sections 3.1.1.1 to 3.1.1.5 in this chapter). Cores from the core shop are transported to the core-setting station. Poured molds move to a shake-out unit to knock out the castings. Gates and feeders are removed. Return sand moves to the sand preparation center. The flasks return to the molding station. The castings move to the shot-blasting unit for cleaning the sand that is stuck to the castings. The castings are transported to the grinding stations where the parting plane and core print flash are ground. The castings are now ready to be transported for machining operations.

Fig. 3.18

Basic casting operation overview

Fig. 3.18

Basic casting operation overview

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Competition demands that the movement of material through the process is properly engineered for optimum efficiency. A properly planned layout is critical for achieving effective product flow and productivity at a competitive cost.

Factors that influence the development of a well-engineered layout are:

  • Product mix

  • Extent of automation

  • Plant capacity

  • Future expansion

Castings for different industries have individual engineering characteristics that influence the process. The annual product volumes also depend on the characteristics of specific industries. Table 3.2 lists several industries whose products are categorized by size and annual production volumes. The table shows a relationship between casting size and annual volumes. Figure 3.19 is a schematic representation of the trend. Layouts of large-volume, small-sized castings in capital foundries can afford significant automation and robotics in handling. The payback for capital investment is normally very attractive. The automation also adds to process consistency and product quality in addition to the reduction of labor. Huge capital for automation cannot be justified for large-sized castings of small volume. Medium-sized castings with medium annual volumes would benefit from engineering of some mechanized handling. Captive foundries may have multiple molding bays of different groups nested together and fed by one or two melting sources. Table 3.3 provides guidelines for the weight ranges and volume classifications shown in Table 3.2 and Fig. 3.19.

Table 3.2
Product categories in various industries
MetalIndustryCasting sizeAnnual volumes
Gray ironAutomotiveSmallLarge
TractorsSmall to mediumLarge
LocomotivesMediumMedium
Heavy diesel engines, power generation, and marine enginesLargeSmall
Machine toolsLargeSmall
Steel plant equipmentLargeSmall
Turbines, power generationLargeSmall
Boiler fittingsSmallSmall
Low-pressure valves and fittingsSmallLarge
Rolling mill equipmentLargeSmall
Fertilizer and chemical industryMediumSmall
Paper and pulp machineryMediumSmall
Electrical machinerySmall to mediumLarge
Crane equipmentMediumSmall
Presses and hammersLargeSmall
Textile machinerySmall to mediumLarge
Agricultural machineryMediumMedium to large
SteelLocomotivesMediumMedium
TurbinesLargeSmall
Steel plant equipmentLargeSmall
Rolling mill equipmentLargeSmall
Presses and hammersLargeSmall
Cement mill machineryLargeSmall
Sugar mill machineryMediumSmall
Excavators and earth-movingMediumSmall
High-pressure fittingsSmallLarge
TractorsMediumMedium to large
Ship buildingLargeSmall
Malleable ironAutomotiveSmallLarge
Valves and fittingsSmallLarge
TractorsMediumMedium to large
Agricultural machineryMediumMedium
Ductile ironAutomotiveSmallLarge
Vans and sport utility vehiclesMediumMedium
Valves and fittingsSmallLarge
Off-highway vehiclesMediumMedium
Compacted graphite ironAutomotiveMediumMedium
Diesel enginesMediumMedium
MetalIndustryCasting sizeAnnual volumes
Gray ironAutomotiveSmallLarge
TractorsSmall to mediumLarge
LocomotivesMediumMedium
Heavy diesel engines, power generation, and marine enginesLargeSmall
Machine toolsLargeSmall
Steel plant equipmentLargeSmall
Turbines, power generationLargeSmall
Boiler fittingsSmallSmall
Low-pressure valves and fittingsSmallLarge
Rolling mill equipmentLargeSmall
Fertilizer and chemical industryMediumSmall
Paper and pulp machineryMediumSmall
Electrical machinerySmall to mediumLarge
Crane equipmentMediumSmall
Presses and hammersLargeSmall
Textile machinerySmall to mediumLarge
Agricultural machineryMediumMedium to large
SteelLocomotivesMediumMedium
TurbinesLargeSmall
Steel plant equipmentLargeSmall
Rolling mill equipmentLargeSmall
Presses and hammersLargeSmall
Cement mill machineryLargeSmall
Sugar mill machineryMediumSmall
Excavators and earth-movingMediumSmall
High-pressure fittingsSmallLarge
TractorsMediumMedium to large
Ship buildingLargeSmall
Malleable ironAutomotiveSmallLarge
Valves and fittingsSmallLarge
TractorsMediumMedium to large
Agricultural machineryMediumMedium
Ductile ironAutomotiveSmallLarge
Vans and sport utility vehiclesMediumMedium
Valves and fittingsSmallLarge
Off-highway vehiclesMediumMedium
Compacted graphite ironAutomotiveMediumMedium
Diesel enginesMediumMedium
Fig. 3.19

Casting size and annual volume relationship trend

Fig. 3.19

Casting size and annual volume relationship trend

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Table 3.3
Casting weights and annual volume of products by foundries
GroupCasting weights, kgAnnual volume piecesAutomationExamples of industry/products
AUp to 100>100,000HighAutomotive, tractor, textile machinery, air compressors, pipe fittings, high-pressure fittings, valves, pumps, electrical motors, small engines
BUp to 1000>10,000MediumDiesel engines, locomotive castings, off-highway trucks, armored vehicles, machine tools, and wood-working machines
C>5000>5000Medium to lowHeavy machine tools, steel plant equipment, forging equipment
GroupCasting weights, kgAnnual volume piecesAutomationExamples of industry/products
AUp to 100>100,000HighAutomotive, tractor, textile machinery, air compressors, pipe fittings, high-pressure fittings, valves, pumps, electrical motors, small engines
BUp to 1000>10,000MediumDiesel engines, locomotive castings, off-highway trucks, armored vehicles, machine tools, and wood-working machines
C>5000>5000Medium to lowHeavy machine tools, steel plant equipment, forging equipment

Plant capacity planning is based on:

  • Number of shifts

  • Number of working days in a year

  • Estimated uptime of the equipment

  • Cycle times and productivity of the machinery

Future expansion planning is based on:

  • Assurance of a continued market

  • Scope for product standardization and monopoly

  • Need for diversification for balancing annual market cycles

  • Potential for obsolescence due to new technology

  • Market fluctuations

  • Increase in productivity

  • Analysis of competition

  • Labor pool availability

  • Environmental regulations

  • Political climate changes

The availability of raw materials of the specified quality and quantity and the proximity to the market for the products that are produced are primary considerations. Labor availability and wage rates, transport facilities for raw material and finished goods, and availability of power and water are important considerations. Soil-bearing capacities, wind direction, and waste disposal are vital considerations to address early on. Different state tax incentives also play a vital role because they give relief for the first few years, accommodating an anticipated slow ramp-up and potential launch issues.

Plant layout is an arrangement of workplaces, machinery, equipment, and areas for shaping the castings most effectively. The initial concept layout should target objectives such as:

  • Overall integration of all related activities

  • Material movement with each process step

  • Smooth workflow without interruption or backtracking

  • Effective space utilization both horizontally and vertically

  • Flexibility of arrangement for addressing alternatives for equipment maintenance or breakdown

  • Safety of workers with an acceptable work environment

Proper layout of the foundry is critical to the efficiency of material processing, throughput, and cost of manufacturing. The supply and storage of raw materials, timely coordination of the connected production activities, control of all work in process, and production schedules are critical issues to be addressed. The access to delivery of raw materials, disposal of waste products, and shipment of finished goods need to be planned in detail to avoid bottlenecks.

The melting facility is the heart of the activity. The ferrous charge materials are the heaviest for receiving and charge makeup. The hot metal from the furnace demands care and safety in distribution and pouring. The location of the melting bay with the molding lines needs to be established upfront, with the analysis of backups for continuous metal supply if the designed equipment breaks down or needs maintenance. The layout is influenced significantly if the metal supply is needed for a single molding bay or multiple bays.

The supporting activities, such as pattern storage and delivery, core room and core storage and delivery, sand storage preparation, delivery, and return sand systems, are built around the molding shop and conveyor lines. The placement of the shake-out, de-gating stations, and cleaning room can be engineered around the end of the molding lines.

Properly engineered gates and runners of gray iron castings usually break either during the rotary or vibratory knockout. Heavier gates and runners are manually knocked out at the discharge end of the shake-out units. Feeders of ductile iron castings are relatively heavier, and hydraulic wedges assist in their removal. The toughness of steel prevents the removal of runners and feeders of steel castings by hammering or by hydraulic wedges. The runners and feeders of steel castings are removed using oxyacetylene blow torches after shot-blasting in the cleaning room. (See Chapter 12, “Engineering Carbon and Alloy Steel Castings,” in this book for discussion about knocking off neck-down feeders.) Cleaned, de-gated, ground, and approved castings are moved to the machine shop.

Handling methods for charge materials, molten metal, patterns, sand (raw and prepared), molds, and castings need to be engineered properly. Both material-handling methods and automation influence the type of layouts.

  • Charge materials: Furnace charge ferrous materials that are magnetic offer an advantage in transport and handling with the use of electromagnets operated from overhead cranes. The coke and limestone needed for cupola operations are handled using grab buckets operated by the cranes. The dosing from the storage bins is facilitated by vibrating conveyors that discharge into the charge buckets. All charge materials are stored in well-marked bins or hoppers covered under a roof and serviced by a crane. The crane operator controls the weight of the individual charge materials that go into the charge bucket, assisted by a weighing scale. All casting facilities keep records of the charges, preferably by automated recording and with printouts.

  • Molten metal: The molten metal that is tapped from the furnaces into the main ladle (if used) is usually distributed using the overhead crane. Many foundries tap metal into the pouring ladle directly. Smaller foundries use forklifts for metal distribution. Automated molding lines are poured by ladles conveyed on pouring monorails. Larger foundries use a pouring furnace or an auto-pour unit that is replenished periodically by tapping from the main furnace or by regular replenishing with a ladle suspended on a monorail.

  • Patterns: Patterns are stored in the pattern storage room that is isolated from the foundry, providing a better environment and reducing the fire risk. Patterns are properly numbered, and logs of daily incoming and outgoing patterns are maintained. Patterns are handled from storage to the foundry by battery-operated trucks.

  • Raw and prepared sand: Sand is delivered by trucks and unloaded down into a hopper and elevated into overhead hoppers through a bucket elevator. If wet sand is procured the sand passes through a heated rotary drum to remove the moisture. Heated sand is cooled in a cooling drum before storage. Sand should be stored at room temperature and with controlled moisture content.

  • Molds: Molds move on conveyors, usually on pallets. The automation is designed for the cleaning of the pallets and the return to the molding station from the shake out station (this can be coupled with the flask cleaning and return). Modern flaskless molding units use a middle raising bar system for mold movement, eliminating the need for roller conveyors and pallets (Ref 1).

  • Castings: Castings from the shake-out units to the cleaning room are either moved by an overhead conveyor or by using baskets manually loaded. The automation of this activity is difficult especially in a job shop or where the castings produced vary in size and shape.

Figure 3.20 is a schematic of a layout of one high volume production molding line. The location of the meting shop and the sand conditioning plant in relation to the molding shop has some flexibility. Two alternative locations for sand conditioning plants are shown.

Fig. 3.20

Layout of a large-volume foundry

Fig. 3.20

Layout of a large-volume foundry

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There is a second generic layout that positions the melting shop in line with a single high volume molding line. Figure 3.21 is a schematic of this concept. This type of layout extends the foundry in one direction.

Fig. 3.21

Melt shop alternative layout

Fig. 3.21

Melt shop alternative layout

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This design of the layout is also beneficial when more parallel molding bays are planned. Casting facilities meeting the needs of diverse sizes of castings plan two or three sizes of molding machines laid out usually parallel to each other. The overhead cranes in the bays with heavier machines handle the pouring. The cross distribution of metal across the bays is handled by the overhead crane in the melting bay. Figure 3.22 is a representation of such a concept.

Fig. 3.22

Cross-bay metal transfer car with multiple bays

Fig. 3.22

Cross-bay metal transfer car with multiple bays

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Figure 3.22 is a layout of multiple molding bays, with a transfer car for cross bay metal delivery. A monorail delivers the metal to the high-volume molding line. A transfer car that runs from the melt shop across multiple bays enables the ladle transfer across bays as illustrated.

Figure 3.23 illustrates another concept of metal handling. The metal needs for the medium and large sized casting bays are met by the combination of two cranes working together. The melt shop crane places the ladle on a platform. The molding shop cranes pick it up for manual pouring. This system avoids the cross-bay metal transportation that uses a transfer car or a forklift.

Fig. 3.23

Dual crane metal handling system

Fig. 3.23

Dual crane metal handling system

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The distribution of areas for production depends on the coring complexity of the product and the cleaning as well as grinding needs. Table 3.4 provides a general guide.

Table 3.4
Guidelines for distribution of production areas
ActivitySpace, %
Melting8–10
Molding35–45
Core room9–11
Cleaning and grinding10–12
Storage and other areas38–22
Total100
ActivitySpace, %
Melting8–10
Molding35–45
Core room9–11
Cleaning and grinding10–12
Storage and other areas38–22
Total100

The length of the molding conveyors needed is decided by the cooling time needed for the casting before shakeout. The solidification must be complete and the casting must have cooled long enough to have the strength to withstand the impact at the shake out unit, the forces on the vibratory shakeout or the impact in the rotary cooling-drum. Designing the conveyors for excessive cooling time makes the conveyors too long increasing the capital. Table 3.5 (Ref 6) provides some general guidelines for the iron castings shake out time.

Table 3.5
General guidelines for iron castings shake-out time
Casting weight, kgDominant wall thickness, mmShake-out time, min (below about 600 °C and cool in air)
Up to 55 – 810
5 – 1010 – 1220
10 – 3010 – 1530
30 – 5012 – 2055
50 – 10015 – 30120
100 – 25020 – 40200
Casting weight, kgDominant wall thickness, mmShake-out time, min (below about 600 °C and cool in air)
Up to 55 – 810
5 – 1010 – 1220
10 – 3010 – 1530
30 – 5012 – 2055
50 – 10015 – 30120
100 – 25020 – 40200

The table is for general guidance only. Variations in wall thickness and complexity of designs for a variety of castings require changes based on individual cases.

Heavier and thicker castings need more time to cool in the mold before stripping. Castings that are cleaned by hydro-blasting require temperature limits to avoid cracking due to sudden cooling by the water jets. Molds waiting to be stripped need to occupy production areas, and cooling areas should be provided for while engineering the layout. Many provide a cooling yard with heavy duty cranes where poured molds await cooling, making the active molding area available for the next molding.

The cooling time is also a function of the effective wall thickness or the modulus (ratio of the volume to the surface area of the casting) and the pouring temperature and the mold thickness. A diagram to establish the conveyor length in vertical molding is shown in Figure 3.24 (Ref 7).

Fig. 3.24

Diagram for establishing the length of cooling zone. Source: Ref 5, Courtesy of DISAMATIC®

Fig. 3.24

Diagram for establishing the length of cooling zone. Source: Ref 5, Courtesy of DISAMATIC®

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Figures 3.25, 3.26, and 3.27 offer guidance for stripping times of different grades of steel castings for planning and providing for mold cooling without affecting the productivity from the active molding and pouring areas.

Fig. 3.25

Mold stripping times for carbon steel castings

Fig. 3.25

Mold stripping times for carbon steel castings

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Fig. 3.26

Mold stripping times for medium alloyed and manganese steel castings

Fig. 3.26

Mold stripping times for medium alloyed and manganese steel castings

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Fig. 3.27

Mold stripping times for high alloy steel castings

Fig. 3.27

Mold stripping times for high alloy steel castings

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FLOW 3D-CAST, Flow Science Inc.
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M.V.
 
Blandino
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Green Sand Castings (Formovani odlitku na syrovo) – SNTL
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DISAMATIC Manual – Application
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