Metal Casting Guide [Metal Modling]

Last Updated on January 2021

Introduction

Metal casting refers to a manufacturing process in which liquefied metal is poured into the desired mold and allowed to solidify. Then, the solid is removed from the mold to produce a fabricated casing, part, or object. Metal casting when done with only molten metal is known as founding.

It’s ideal for creating metal shapes that aren’t possible with conventional methods. It’s also used to create copies of the original artwork or do it yourself welding projects.

The most common cast metals are zinc, magnesium, steel, copper, and aluminum. Casting can also be used to form thermoplastics (meltable plastics), epoxy (after mixing several cold setting materials), plaster/concrete (after mixing water setting materials), and clay objects.

Metal castings are widely applicable in heavy industries, such as architecture, automobiles, oil industries, home appliances, mining, water-processing, farming, forestry, and gas extraction industries. For instance, metal casting is used in the automobile industry to create engine blocks and cylinder heads.

Metal castings are durable and strong; however, it takes a combined effort between the manufacturer and suppliers to achieve a quality, economical product.

Besides casting, the manufacturer can try a number of substitution methods, such as:

  • Use of highly active metals
  • Cold bending
  • Extrusion
  • Stamping out on a punch press or deep-drawn

Categories of Metal Casting

The metal casting process is categorized into: expandable and non-expandable mold casting.

Expendable mold casting

Expendable mold casting includes plastic, shell, sand, and investment (or lost-wax-technique) moldings. This casting process utilizes temporary and non-reusable molds. As such, the mold is reformed after each production cycle. It requires gravity to force molten material into the casting cavities.

Non-expandable mold casting

Non-expandable mold casting makes use of reusable folds. It includes different casting methods, including die, centrifugal, permanent, and continuous casting. See their description in a later section.

Recent Metal Casting History

Metal casting has evolved. With technology, it’s now possible to mass-produce molds – accurately. These molds are thin to minimize material wastage. Superficially, they resemble paper mache – similar to those used in egg cartons.

These molds are refractory in nature and are supported by dry sand surrounded in a box – or other means – during the metal casting process.

Given the accuracy that comes with the evolving technology, it’s possible to make even thinner (and as such lighter) casting molds. Moreover, it’s possible to create different variations in the molds without the addition of extra metal.

The use of these thin-mold metal casting began in the 1960s, in the production of cast iron cylinder heads and engine blocks for automotive use.

Nowadays, most automotive mechanical components like the parts at Tesla are made of aluminum, through sand or die casting methods. The latter is more accurate, and as such, reduce machining and finishing costs.

While aluminum and the processing setup is pricier than the use of iron, it sure does reduce weight in automobiles. That, in turn, reduces the amount of fuel required while accelerating the vehicle’s performance.

Lost foam process

In the 1980s, the lost form process, a sand casting technique was discovered and used to manufacture metal castings, like automotive engine blocks.

In the lost foam process, the pattern is made using polystyrene foam. The sand is packed around to keep the foam in place. When metal is poured into the mold, the heat in the molten metal vaporizes the foam a slight distance from the metal surface. As such, a cavity is created into which the metal flows.

This process supports the sand better than conventional sand casting methods. As such, the lost-foam method allows greater flexibility in the design of fabricated parts, while reducing the cost associated with machining and finishing.

The lost-foam process was developed for clay mold castings used in the production of abstract art pieces. It was adopted for commercial use by the Saturn company, and its use surged from there.

On the downside, the lost-foam process burns plastics to produce lots of smoke, thus having proper venolation such as a welding air purifier and a respitory mask are required.

Metal Casting (DIY) and Supplies

In the current market, there exist numerous kits and metal casting supplies for easy casting at home. As a hobbyist, you can search for these kits and supplies on e-commerce stores – especially eBay and Amazon. The latter has a limited offering but worthwhile.

The following are the common DIY metal casting processes.

#1: Lost wax casting process (also known as investment casting)

Despite being an ancient practice, this process is very common today. The lost wax casting process varies from one production to the next. For instance, the production of bronze sculptures follows the following process:

Sculpting

Here, a hobbyist creates an original piece of artwork from clay, wax, or another material. Note, oil-based clays and wax are preferred as they can retain their softness.

Mold making

A casting mold is created of the original artwork. Often, the mold is a two-piece assembly, with a shim of keys placed in between the pieces. A shim with keys helps to put the mold pieces back together accurately – during construction.

Often, molds are made from plaster or fiberglass. An inner mold/lining made of vinyl or latex is added to preserve the fine lines of the artwork. The inner mold is usually supported by the outer mold.

Usually, the sculpted artwork is destroyed in the construction and initial deconstruction of the outer mold. The reason is, wax or oiled-clay (used to sculpt the original artwork) is solid and doesn’t bend easily.

If the artwork is too long or too large, different parts are cut and cast differently.

Waxing

With the outer and inner molds complete, molten wax/clay is poured into it and swished to achieve an even coating. The molten material should cover the entire surface area of the inner mold, and achieve a thickness of about ¼ inches.

Removal of wax

Once it has solidified, the new, hollow copy of the original artwork is removed from the mold. Then, the hobbyist can reuse the mold to create similar copies of the original artwork. However, you should not reuse the mold if it’s worn or torn.

Often, a well-designed mold can produce up to 25 copies of fabricated objects before it’s remodeled.

Chasing

The hollow wax or clay copy is then chased. Here, a heated metal tool is used to even out marks showing flashing or parting lines.

If parts of the artwork were cast differently, they can be heated and re-attached accordingly.

To ease the re-attachment process, registration marks should be used to indicate where each piece (that’s cast differently) should go.

Spruing

Once the wax copy resembles the original sculpture, it’s sprued. Spruing makes use of a tree-like structure made of wax to provide paths for the molten metal of choice to flow.

Spruing starts at the top with a wax cup attached to the wax copy suing wax cylinders.

Slurrying

With the spruing complete, the hobbyist can then dip the wax-copy into a ceramic slurry, which is then followed by a mixture of powdered sand and clay. Next, the copy is allowed to dry, and the process is repeated until a ½ inch or thicker cover is achieved.

Burnout

The ceramic-coated copy is placed in a kiln to harden the coating into a shell. In the process, the wax is heated; and as such, flows out.

Heating also serves to melt the wax, which, in turn, runs out. While the melted wax can be collected and re-used, it’s usually combusted during the process. That, in turn, leaves a space (formerly occupied by the wax) and a hardened ceramic shell. The vent tubes, cup, and feeder will be left hollow too.

Testing

The hardened ceramic shell is allowed to cool, after which, it’s tested to determine whether the water will flow via the vent tubes and feeder as necessary.

In case of leaks or cracks, they are patched with a thick ceramic slurry/paste.

To test the resulting thickness, the hobbyist can drill a hole into the shell, and patch it once done.

Pouring

Reheat the shell in the kiln to harden the patches as well as heat it in preparation for the metal pouring process.

Next, place the shell into a tub filled with sand.

Melt bronze (all the metal of choice) in a crucible and pour it into the shell. Then, allow the bronze filled shell to cool.

Releasing

Hammer or sand-blast the shell away to release the rough bronze. Here, cut the spruing created in the process and reuse them in another production cycle.

Metal-chasing

Work the bronze copy to remove any telltale signs of casting. Fill the pits left by air bubbles and file and polish the stubs left after cutting the spruing.

Patinating

Color the artwork to your preference, using chemicals applicable to cold or hot metals. This coloring is known as patina and can be black, white, green, or brownish to resemble the surfaces of ancient bronze artworks.

However, you can also paint your sculpture in bright colors.

Being less opaque than paint, patinas allow the luster of the metal to show through.

Finally, apply a layer of ice on the surface to protect against oxidation.

Note, the lost-wax metal casting process applies to any material that can melt, evaporate, or burn to leave a mold cavity.

For example, some automotive brands use the lost-foam process to manufacture headlights and engine blocks.

#2: Sand casting process

Sand-casting often uses aluminum metal to cast flat, relief-like artwork.

Here’s an overview of the sand casting process.

First, the tub (to be used) is filled with sand. Then, the sand is wetted and the hobbyist uses his hand or an object to draw the desired original artwork on the sand. The setup is then left to dry. Next, molten aluminum is poured into the depressions, and allowed to solidify. Finally, you can choose to chase the fabricated object or leave it as it is.

To refine the casting, you can use one of the following methods:

  • Metal Plating
  • Machine grinding
  • Rough grinding
  • Machining
  • Forging
  • Hammer peening
  • Shot peening
  • Polishing

Sand castings that aren’t refined through polishing or peening are recognized by their sand-like texture.

Nonetheless, grinding or machining is necessary to remove any extra material – resulting from imperfections in the mold.

#3: Cuttlefish casting

This process makes use of a cuttlebone as its mold. It’s common among silversmiths and jewelers to fabricate small objects. It’s particularly beneficial in replicating copies from a metal original.

#4: Plaster casting

The plaster casting process is similar to sand molding only that plaster is used instead of sand. The plaster compound used here is 70-80% gypsum, and 20-30% strengthener & water.

Generally, the mold takes up to 7 days to prepare, after which items ranging from 30g to 45Kg can be produced with high fine tolerance and surface resolution.

Plaster casting is expendable as the mold is not reusable. It’s ideal for casting nonferrous metals, such as aluminum, zinc, and copper-based alloys. On the contrary, it cannot cast ferrous materials as the sulfur contained in gypsum can react with iron.

The artwork should be sprayed with a thin layer/film of a parting compound to ease its removal once the casting process is complete.

Once the pouring is done, the setup should be shaken to achieve a uniform distribution of the material.

Plaster casting requires skills and precision; however, some basics and automatic functions can be handed to robots.

#5: Shell molding

Shell molding is similar to sand molding only that sand is replaced with a mixture of sand and a 3-6% of resins. The latter serves to hold the sand grains together.

With this process, you can cast objects weighing up to 90Kg. The shell can be reused by burning the resin at high temperatures.

#6: Permanent mold casting

Permanent mold casting is often used to cast ferrous metals. It makes use of a steel tool that takes weeks to prepare. After that, fabricated objects weighing 0.1Kg to 9Kg can be cast at a rate of 5-50 pieces/hour.

To ease the removal process, the steel mold is first coated with a reflective wash of acetylene soot. The coating also serves to prolong the life of the workpiece.

The durability of the permanent molds is dependent on maintenance, after which; the mold can either be refinished or replaced.

Permanent mold castings have a 20% higher tensile strength and 30% more elongation as compared to those resulting from the sand casting process.

The process is highly automated, with the only input being the acetylene soot coating.

#7: Metal die-casting

Here, the molten metal is injected into a fond at high temperatures. The die molds take up to 2 hours to set up, after which, fabricated objects weighing from 30g to respective maximums per metal are produced – at a rate of 20-200 pieces/hour.

The die injection equipment is large and operates at pressures of 100+ megapascals. However, aluminum can be cast at relatively lower pressure.

A well-designed metal die-casting can yield over 500, 000 workpieces in its production lifetime. The high number of castings helps offset the high cost of the dies.

Note, die-construction requires skills and attention to detail. With it, one can easily cast non-ferrous metals, such as aluminum, zinc, copper, and magnesium.

#8: Centrifugal casting

Centrifugal metal casting is pressure and gravity independent as it creates its own (centrifugal) force – using a temporary sand cavity held in a spinning chamber at 900 m/s².

The lead time depends on the application. Semi and true-centrifugal casting allow a production rate of 30-50 pieces/hour. When it comes to weight limitation, an individual item should weigh 2.3 – 4.5Kg. The practical limit for the total weight in batch processing is around 9000Kg.

The method was first adopted for industrial use by Krupp, a German industrial company; to produce railway wheels. Currently, the method is widely used to fabricate small art jewelry.

#9: Continuous casting

In this process, molten metal is poured into a water-cooled, open-ended copper mold. The mold allows a “skin” of the solidified metal to form over the still-water.

The resulting “skin” better known as the strand, is then removed and passed into a chamber containing rollers and water sprays. The roller serves as a support to the metal strand while the water provides a cooling effect – that solidifies the strand from the outside layer.

After solidification, predetermined lengths of the metal are cut off by traveling oxyacetylene torches or mechanical shears and transferred to a stockpile or other forming process.

Cast sizes range from trips a few millimeters thick and 5m wide to billets 90-160mm square and slabs 230mm thick & 1.25m wide.

Note, the strand may be hot-rolled before cutting if need be.

Continuous casting produces quality objects due to one, the inherent advantage associated with continuous casting, and two, the possible finer control over the metal casting process.

The process is ideal for casting steel, copper, and aluminum.

Design Requirements

The artwork to be fabricated and its pattern are designed by taking into consideration each stage of the casting process.

For instance, in sand casting, the design should consider the possibility of removing the pattern without affecting the molding sand. Or the possibility of using the cores and chills; as such, allow for their proper locations.

Also, the design should allow for an easy removal process of the artwork. For instance, some processes may call for the application of coating around the artwork to prevent sticking. Others call for a slight taper on surfaces perpendicular to the parting line.

Also, if the design requires cores, measures should be put in place to facilitate their easy removal.

While you should be careful when pouring the metal into the mold, the design should allow for risers and sprue – specially arranged to facilitate a proper flow of the molten metal. Otherwise, you might end up with an incomplete casting.

The design should also factor in the possibility of gas pockets; and as such, provide measures to eliminate and correct their effects.

Finally, the design should account for the testing methods. For instance, if the cost of the effort put towards casting is a factor, non-destructive testing techniques may be applied to test the workpiece in every stage (if needed). For more information on where to access free metal for welding art [read our full Guide]

Patterns

The pattern of an artwork depends on its use, as well as the preferences of the artist, designer, engineer, or client. The pattern maker works to produce a master of the object in the desired pattern.

The master must be relatively larger-sized than the finished product to allow room for expanded molten metal.

To simplify the pattern making process, the maker can use a scaled oversize ruler (commonly known as a shrink rule) that’s specific to the metal in question.

Pattern making should also consider additional paths of the molten metal such as the riser and sprue.

Key Factors to Note in Metal Casting

Cooling rate

The cooling rate affects the quality, properties, and microstructure of the resulting casting.

For instance, products from the slurry-mold and sand casting process have thicker walls; and as such, cool slowly. As a result, such castings have larger grains – a coarse microstructure that reduces the strength of the fabricated object.

Nonetheless, a sow cooling rate is beneficial; in that, it creates more time for waste metal and gases to escape – that, in turn, reduces the inclusions and voids that can weaken the casting.

Conversely, fabricated objects resulting from metal-mold and die-casting processes have a faster cooling rate. They have finer microstructure, small grains, less alloy segregation, but more inclusions and trapped gases.

Overall, a faster cooling rate results in higher quality products as de-gassing techniques help remove any trapped gas that may otherwise weaken the casting. Besides, the Pilings Bedworth Ratio dictates that: the strength of a metal is inversely proportional to the

Chills are added to the mold to accelerate the cooling process.

A chill is a material that conducts heat away from the casting at a higher rate than the mold itself. For instance, if the mold is made of silica, then the chill can be made of iron, copper, graphite, chromium, or aluminum.

Shrinkage

Metal is less in liquid form than it is in a solid-state. As such, molten metal shrinks as it solidifies. The solid metal further shrinks as the temperature drops. Shrinkage can either be volumetric or linear as discussed below.

Volumetric shrinkage

Volumetric shrinkage is the reduction in the overall volume of the metal.

Unfortunately, volumetric shrinkage can leave cavities that can weaken the casting.

Risers/feeders are placed into the pattern to correct the volumetric shrinkage. These feeders are specially designed to solidify at a slower rate compared to the actual casting. The liquid metal in the feeder flows into the solidifying metal and feeds it until the casting is fully-solid.

Risers add the cost of materials; nonetheless, they help eliminate internal shrinkage voids.

Linear shrinkage

Linear shrinkage is corrected with an oversized pattern, in which; the master should be 2-6% oversize depending on the metal.

As such, when the molten metal solidifies, it will shrink to achieve the desired size.

Additional Considerations in Metal casting

When casting, you should take into account the following:

The client’s specific needs

Sometimes the clients may have different requirements from the industry’s specifications. As such, consider their specifications while maintaining international standards.

You should also conduct tests to determine the thickness and mechanical properties of the casting. As such, see if they match those of the client.

Generally, the casting walls should be thicker than ¼ inches.

Making the “core” and “draft”

A core is a hollow space within the casting. It’s required to create space that is otherwise impossible with conventional methods. Its diameter is dependent on the casting thickness, casting method, and core length.

The draft is the tapering portion of the vertical surfaces of a pattern. It helps remove the “pattern” from the mold – easily. The amount of “draft” used depends on the casting size, manufacturing process, and whether the molding is by hand or machine.

Casting design

You should create the casting design with the simplest possible method, cost-effective products, needed net shape, and cosmetic appearances.

The quote should state detailed drawings of the dimensions, size tolerances, and the number of castings. It should also anticipate any defects and the recommended solutions.

The design should also state any anticipated changes.

Cost

The cost of metal costing is dependent on many factors, including the quantity to be produced, the casting process, and delivery logistics.

In conclusion, metal casting requires skills and precision. For commercial purposes, the process calls for coordination between manufacturers and suppliers for optimum results.

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