Metal Casting Materials, Processes and Selection: Why This Guide Exists

Context and scope

Every manufactured object you touch — every engine block, every turbine blade, every smartphone housing — began as molten metal poured, pressed, or sprayed into shape. Master these processes, and you master the foundation of modern industry.

Why This Guide Exists

Somewhere right now, an illustrative engineering practitioner is staring at a casting rejection report. Forty percent of her aluminum die castings came back porous. Her team is behind schedule, the tooling cost a small fortune, and nobody can agree on whether the problem is in the alloy, the die design, or the injection parameters.

Two floors up, an illustrative engineering practitioner is sketching a turbine component that must survive temperatures above 2,000°F. His boss wants it machined from billet stock. the practitioner knows investment casting could produce the part at a fraction of the cost — if only he could articulate why.

And across the ocean, an illustrative engineering practitioner is trying to decide between die casting and powder metallurgy for a complex gear mechanism. The wrong choice could burn through his entire seed round.

This guide is for all three of them — and for you.

Whether you are a beginner trying to understand the landscape of metal-forming processes, an expert looking for authoritative reference data, or a decision-maker evaluating manufacturing methods for your next project, what follows is a comprehensive, process-by-process breakdown that will serve you today and decades from now.

Selected Model: PREP (Point → Reason → Example → Point)

PATTERN MAKING — WHERE EVERY CASTING BEGINS

The Bold Claim: A Casting Is Only as Good as Its Pattern

If you get the pattern wrong, nothing downstream can save you.

Not the metallurgy. Not the pouring temperature. Not the finishing process. The pattern — that physical replica around which the mold is formed — is the genetic code of every casting. Get it right, and you produce parts with precision, repeatability, and minimal waste. Get it wrong, and you produce scrap.

The Reason: Patterns Dictate Geometry, Shrinkage, and Surface Quality

A pattern is not simply a "copy" of the desired part. It must account for:

• Shrinkage during cooling (every metal contracts differently)
• Draft angles so the pattern can be withdrawn from the mold
• Machining allowances for surfaces that require finishing
• Core prints that support internal hollow features
• Distortion compensation for complex geometries

Every one of these variables is baked into the pattern before a single drop of metal is poured.

Woods for Patterns

The traditional material for pattern-making is wood — and the choice of species is far from trivial.

Common Pattern Woods and Their Characteristics

The Expert Insight: For most applications, white pine remains the superior choice because it is easily worked, readily takes glue and varnish, and is fairly durable. But for medium- and small-sized patterns that will see extensive use, a harder wood like mahogany or cherry is preferable because it will hold its shape and dimensions over hundreds of mold cycles.

Selection of Wood: The Hidden Variable That Ruins Castings

Here is where many shops fail — and it costs them dearly.

It is critically important to select well-seasoned wood for patterns. The wood should either be kiln-dried or kept for one to two years before use, the time depending on the size of the lumber.

Why Seasoning Matters

During the seasoning or drying process, moisture leaves the wood cells and the wood shrinks. This shrinkage occurs almost entirely across the grain rather than lengthwise. Once this change has taken place, the wood is less liable to warp — although it will still absorb moisture in damp weather.

Patterns also tend to absorb moisture from the damp sand of molds, which is why they are protected with varnish.

Critical Wood Selection Rules

• Never use green or water-soaked lumber in a drying room — the ends will dry faster than the rest, causing cracks
• Avoid sap wood (the outer layers of a log) — it is not as firm as heart wood and is more likely to warp
• Never use young cherry timber — it lacks the dimensional stability of mature wood
• Always prefer heart wood for dimensional accuracy and durability

The Case Study: Consider a foundry that used unseasoned white pine for a series of medium-production patterns. Within three months, the patterns had absorbed moisture from repeated contact with green-sand molds, warping by enough to push castings outside tolerance. The cost of replacement patterns and scrapped castings exceeded ten times the savings from using cheap lumber. The lesson is universal: invest in properly seasoned wood, or pay the price in scrap.

Pattern Varnish: The Invisible Shield

Patterns intended for repeated use must be varnished to protect against moisture, especially when exposed to damp molding sand.

Varnish Specifications

• Standard varnish: Yellow shellac varnish — made by dissolving gum shellac in grain alcohol
• Core print varnish: Black shellac varnish — the color change allows pattern makers to instantly distinguish core prints from the body of the pattern
• Minimum coats: At least three coats of varnish should be applied
• Surface preparation: Rub surfaces down with sandpaper after each preliminary coat to build a smooth, sealed surface

Pro Tip: Wood alcohol is sometimes substituted for grain alcohol as the solvent, but it produces an inferior result. For patterns that will see high production use, always specify grain alcohol shellac varnish.

Shrinkage Allowances: The Numbers That Define Dimensional Accuracy

This is where beginners make their most expensive mistakes — and where experts differentiate themselves.

Every metal contracts as it cools from liquid to solid and then to room temperature. The pattern must be made larger than the desired casting dimension to compensate for this contraction. The amount of oversizing is called the shrinkage allowance.

Standard Shrinkage Allowances for Common Casting Metals

Why These Are Only Starting Points

These shrinkage allowances are approximate values only because the exact allowance depends upon:

• The size and shape of the casting — larger castings may shrink differently than small ones
• The resistance of the mold to normal contraction during cooling
• The molding method — which can vary so significantly from one foundry to another that different shrinkage allowances would be required for the same pattern

It is entirely possible that more than one shrinkage allowance will be required for different parts of the same pattern.

The Steel Bar Example: Proof That Geometry Changes Everything

The Steel Castings Handbook provides a powerful illustration of how casting shape affects shrinkage:

The same material, the same cross-section — but the shrinkage allowance varies by a factor of nearly three, simply because the end features resist contraction differently.

The Takeaway: The best practice in designing castings and making patterns is to obtain shrinkage values directly from the foundry that will produce the casting. There can be no universally fixed allowances. Any engineer who applies a single "rule-of-thumb" shrinkage value across all geometries is inviting dimensional failures.

Metal Patterns: When Wood Won't Do

Metal patterns are especially adapted to molding machine practice, owing to their durability and superiority in retaining the required shape over long production runs.

How Metal Patterns Are Made

The process follows a logical sequence:

1. A master pattern is made from wood (using all the principles described above)
2. A casting is obtained from the wood pattern
3. The casting is finished and refined to become the metal pattern

Metal Pattern Materials Compared

Expert Note: For metal patterns, the results of weight-estimation calculations are more accurate than for wood, because metal densities are more consistent than wood densities (which vary with moisture content, grain, and species).

Estimating Casting Weight from Pattern Weight

One of the most practical tools in a foundry engineer's arsenal is the ability to estimate the weight of a casting from the weight of its pattern.

The Formula

$$\text{Casting Weight} = \text{Pattern Weight} \times \text{Conversion Factor}$$

Conversion Factors: Pattern Material to Casting Metal

Worked Example

Problem: A white-pine pattern weighs 4 pounds. What is the weight of a solid cast-iron casting produced from that pattern?

$$\text{Casting Weight} = 4 \text{ lb} \times 16.00 = 64 \text{ lb}$$

For Cored (Hollow) Castings

If the casting has internal hollows formed by cores:

1. Fill the core-boxes with dry sand
2. Weigh the sand
3. Multiply the sand weight by the appropriate factor:
   • Cast iron: 4
   • Brass: 4.65
   • Aluminum: 1.4
4. Subtract this value from the solid casting weight

$$\text{Cored Casting Weight} = \text{Solid Casting Weight} - (\text{Sand Weight} \times \text{Core Factor})$$

Important Caveat: The weight of wood varies considerably, so results are only approximate. The factors are based on average weights of the listed woods. For metal patterns, the results may be more accurate.

Estimating Casting Weight Without a Pattern

When no pattern or die exists yet — for example, when preparing a cost quotation — the weight can be estimated through geometric calculation:

1. Calculate the volume of each casting feature (rectangular sections, cylindrical bosses, housings, ribs, etc.)
2. Sum all volumes together
3. Multiply by the unit weight (density) of the alloy to arrive at the weight of the finished casting

Cost Estimation Formula

$$\text{Casting Cost} = (\text{Metal Weight} \times \text{Cost per Unit Weight}) + \text{Melting Losses} + \text{Riser/Runner Metal} + \text{Melting Cost} + \text{Machining Cost} + \text{Pattern/Die Cost} + \text{Molding/Finishing Cost}$$

DIE CASTING — PRECISION AT PRODUCTION SPEED

The Bold Claim: Die Casting Is the Most Cost-Effective Way to Produce Accurate, Complex Metal Parts at Scale

When you need thousands — or millions — of identical metal components with tight tolerances, smooth surfaces, and minimal machining, die casting is unmatched.

The Reason

Die casting is a method of producing finished castings by forcing molten metal into a hardened steel die under extreme pressure. The die is arranged to open after the metal has solidified so that the casting can be removed.

The core advantages that make this process dominant:

• Dimensional accuracy: Castings can be accurate within ±0.001″, with tolerances of ±0.002″ to ±0.003″ per inch maintainable on many classes of work
• Surface finish: As-cast surfaces are often smooth enough to require no finishing
• Feature integration: Lugs, gear teeth, external and internal threads, figures, letters, and complex geometries can all be cast in place
• Near-net-shape production: Holes can be formed within ~0.001″ of size; bearings may require only a finish-reaming operation
• Machining elimination: The greatest advantage is that parts are accurately and often completely finished when taken from the die

The Example: What Die Casting Actually Produces

Die castings are used extensively in:

• Cash registers and meters
• Time-controlling devices
• Small housings and enclosures
• Washing machine components
• Counting wheels and printing device characters (cast sunken or in relief)
• Automotive components
• Consumer electronics housings

With ingenious die design, many shapes formerly believed too intricate for die casting are now produced successfully.

How Die Casting Works: The Physics of Pressure and Speed

Die casting uses hardened steel molds (dies) into which molten metal is injected at high speed, reaching pressures up to 10 tons per square inch.

The Force Calculation

Force is applied by a hydraulically actuated plunger moving in a cylindrical pressure chamber connected to the die cavity. The math is straightforward but the numbers are staggering:

$$\text{Clamping Force} = \text{Injection Pressure} \times \text{Projected Area of Casting + Runner System}$$

Example: If the projected area of the casting and its runner system covers 50 in², and the injection pressure is 10 tons/in²:

$$\text{Clamping Force} = 10 \text{ tons/in}^2 \times 50 \text{ in}^2 = 500 \text{ tons}$$

The die-casting machine must hold the die shut against this 500-ton force. Massive toggle mechanisms stretch heavy steel tie bars (typically ~6 inches in diameter) through approximately 0.045 inches on a typical 500-ton machine to generate this clamping force.

The Skin Effect: Why Die Castings Are Stronger Than You'd Expect

Although the die is hot, metal entering the die cavity is cooled quickly by the comparatively cold die surfaces. This produces layers of rapidly chilled, dense material approximately 0.015 inches thick on every surface in direct contact with the die cavity.

Because high injection forces allow castings to be made with thin walls, these dense layers form a large proportion of the total wall thickness, producing high casting strength.

Critical Design Insight: This phenomenon — known as the skin effect — means that the mechanical properties measured in standard (usually thicker) test bars may not accurately represent the properties of thin-walled die castings. The real-world part may actually be stronger than the test data suggests, because a higher percentage of its cross-section consists of the dense, rapidly chilled skin layer.

Hot-Chamber vs. Cold-Chamber Methods

There are two fundamental approaches to delivering molten metal into the die:

Hot-Chamber Method

┌─────────────────────────────────────────┐
│          MOLTEN METAL BATH              │
│                                         │
│    ┌──────────┐                         │
│    │ Pressure │ ←── Plunger (vertical)  │
│    │ Chamber  │                         │
│    │ (immersed│──────→  DIE CAVITY      │
│    │  in bath)│                         │
│    └──────────┘                         │
│         ↑                               │
│    Auto-refill hole                     │
│    (uncovered as plunger retracts)      │
└─────────────────────────────────────────┘

• The pressure chamber is permanently immersed in the molten metal
• Automatically refilled through a hole uncovered as the plunger retracts
• Best for: Alloys of low melting point and high fluidity — zinc, lead, tin, and magnesium
• Advantage: High production rates with simple automation

Cold-Chamber Method

┌──────────────────────────────────────────┐
│                                          │
│  MOLTEN METAL    ┌──────────────┐        │
│  (separate       │   Pressure   │        │
│   furnace) ──→   │   Chamber    │──→ DIE │
│  (ladled in)     │ (horizontal, │        │
│                  │  water-cooled)│        │
│                  └──────────────┘        │
│                     ↑                    │
│              Plunger (horizontal)        │
└──────────────────────────────────────────┘

• The pressure chamber is outside the molten metal and water-cooled
• Metal is ladled into the chamber for each shot
• Required for: Alloys needing higher pressure (like brass) or alloys that would attack and dissolve the ferrous pressure chamber (like aluminum)
• Trade-off: Slower cycle times than hot-chamber

Method Selection Guide

Porosity: The Enemy Within

Porosity is the single most common quality issue in die casting — and understanding it is essential to designing robust parts and processes.

What Causes Porosity

Molten metal injected into a die cavity displaces most of the air, but some air is inevitably trapped and mixed with the metal. The high injection pressure squeezes these air pockets to very small size, but they remain.

Why Porosity Matters

• Subsequent heating will soften the casting, allowing trapped air to expand and cause blisters
• Die castings are seldom solution heat treated or welded because of this blistering problem
• If machining cuts too deep, pores are exposed on the surface
• Pressure tightness cannot be achieved without special treatment

The Good News: The Skin Is Dense

The chilling effect of the comparatively cold die causes the outer layers of a die casting to be dense and relatively free of porosity. This is why:

• Machining depths must be limited to 0.020–0.035 inches to avoid exposing pores
• As-cast accuracy is usually good enough that only light finishing cuts are needed

Advanced Porosity Solutions

Designing Die Castings: The Rules That Separate Good Parts from Scrap

Fundamental Design Principles

1. Uniform Wall Thickness Design with uniform wall thicknesses to reduce cooling stresses. Uneven walls cool at different rates, creating internal stresses that can cause warping or cracking.

2. Simple Core Shapes Cores must be made of metal (not sand — the high injection pressures would destroy sand cores), so they must be designed for easy extraction from the die.

3. Avoid Heavy Sections Heavy sections should be avoided or cored out to reduce metal concentrations that attract trapped gases and cause porosity concentrations.

4. Metal Flow Path Design so that metal travels through thick sections to reach thin ones if possible. This ensures complete filling before solidification.

5. Avoid Small/Slender Cores Small and slender metal cores are easily bent or broken during injection. Prefer piercing or drilling operations on the finished casting instead.

6. Add Ribbing for Strength Ribbing adds significant strength to thin sections without adding problematic thick masses of metal.

7. Use Fillets on All Inside Corners Fillets (rounded transitions) eliminate high stress concentrations that cause cracking.

8. Avoid Sharp Outside Corners Sharp outside corners create stress risers and can cause die erosion.

9. Draft Allowances Draft allowances of 0.5 to 1.5 degrees per side are required to permit the casting to be pushed off cores or out of the die cavity.

Die Casting Design Decision Matrix

Alloys Used for Die Casting: A Complete Reference

The alloys used in modern die-casting practice are based on aluminum, zinc, and copper, with smaller numbers of castings made from magnesium, tin, and lead-based alloys.

Aluminum-Base Alloys

Aluminum-base die-casting alloys are used more extensively than any other base metal alloy because of their superior strength combined with ease of castability.

Key Properties:

• Linear shrinkage on cooling: ~12.9 to 15.5 × 10⁻⁶ in./in.-°F
• Casting temperatures: ~1,200°F
• Must use cold-chamber method (aluminum attacks ferrous chamber material)

Principal Aluminum Die-Casting Alloys

Why Silicon and Copper Matter: Silicon increases fluidity for complete die filling but reduces machinability. Copper adds hardness but reduces ductility. The alloy selection is always a trade-off driven by the specific application.

Zinc-Base Alloys

In the molten state, zinc is extremely fluid and can be cast into very intricate shapes. The metal is plentiful and has good mechanical properties.

Key Advantages Over Aluminum:

• Can be made to closer dimensional limits and with thinner walls
• Lower casting temperatures (750–800°F) enable use of the fast hot-chamber process
• Extremely smooth surfaces — ideal for plating and finishing
• Linear shrinkage on cooling: ~9 to 13 × 10⁻⁶ in./in.-°F

Principal Zinc Die-Casting Alloys

The Game-Changer: The newer zinc alloys with 8–27% aluminum achieve tensile strengths of 50,000–62,000 psi and hardness approaching cast iron. These can be used for gears, racks, and housings for shafts that run directly in reamed or bored holes — eliminating the need for separate bearing bushes.

Copper-Base Alloys

Brass alloys are used where corrosion resistance must be combined with strength and wear resistance — primarily in plumbing, electrical, and marine components.

Trade-off Alert: High silicon brass (tombasil) delivers 70,000 psi tensile strength and excellent wear resistance — but at the expense of machinability. If your part requires post-cast machining, silicon brass (58,000 psi) may be the better choice.

Magnesium-Base Alloys

Light weight combined with good mechanical properties and excellent damping characteristics are the principal reasons for choosing magnesium die castings.

Key Advantages:

• Magnesium has a low specific heat and does not dissolve iron → longer die life than aluminum
• More rapid solidification → production ~50% faster than aluminum
• Can be cast by either hot-chamber or cold-chamber methods
• To prevent oxidation: An atmosphere of CO₂ and air containing ~0.5% SF₆ gas excludes oxygen from the molten metal surface

Most Widely Used Alloy: AZ91D (ASTM B94; UNS 11916)

• Composition: Aluminum 9%, Zinc 0.7%
• Yield strength: 23,000 psi
• Corrosion rate similar to 380 aluminum

Tin-Base Alloys

Tin is alloyed with copper, antimony, and lead to create bearing metals and specialty components.

Additional applications: Milking machines, soda fountains, syrup pumps, and apparatus requiring resistance against acids, alkalies, and moisture.

Lead-Base Alloys

Lead-base alloys are employed where a cheap, non-corrosive metal is needed and strength is relatively unimportant.

Applications: Lead-acid battery parts, automobile wheel balancing weights, parts exposed to strong mineral acids, X-ray apparatus components.

Dies for Die-Casting Machines

Making die-casting dies requires considerable skill and experience — and the materials must be matched to the alloy being cast.

Die Materials by Application

Critical Die Design Parameters

• Vent channels: 0.003 to 0.005 inches deep, cut into the parting line of the die
• Gate and vent location: Must be positioned with reference to the particular shape being cast — improper placement leads to trapped air and porosity
• Shrinkage allowance: Usually varies from 0.002 to 0.007 inches per inch, but exact values for multi-element alloys are difficult to determine except by experiment

The Expert's Rule: Die design is where art meets engineering. The gates and vents must allow metal to rapidly flow to all parts of the impression while simultaneously allowing air to escape. This is not something that can be fully predicted by simulation alone — it requires experience, iterative refinement, and often physical prototyping.

Die-Casting Bearing Metals in Place

Practically all metals suitable for bearings can be die cast in place — a powerful manufacturing technique that eliminates separate bearing assembly steps.

How It Works

• The bearing bore is formed by a core pin in the die
• After casting, bearings are finished by boring or reaming
• The die-cast metal becomes harder upon seasoning for a few days

Recommended Bearing Alloy

For the highest-class bearing metal:

Cost Reduction Option: Some bearing metals use lead instead of tin to reduce costs. One alloy contains 95–98% lead. However, tin-based babbitts provide superior bearing properties.

Critical Process Detail

In die-casting bearings, the work is located from bolt holes drilled prior to die casting. It is critically important that the bolt holes be drilled accurately with relation to the machined surfaces — any positional error in the bolt holes translates directly into bearing misalignment.

Injection Molding of Metal: The Hybrid Process

The die casting and injection molding processes have been combined to enable the injection molding of many metal alloys.

How It Works

1. Powdered metal (5–10 µm particle size, or 0.0002–0.0004 inches) is mixed with thermoplastic binders
2. The mixture is injection molded at moderate pressures and temperatures
3. Parts harden as they cool and are removed as solids
4. Binder removal (may take several days to avoid distortion)
5. Sintering in a controlled atmosphere furnace at high temperatures to consolidate the metal

Key Specifications

The Limitation: The main restriction of metal injection molding is size — parts are currently restricted to approximately a 1.5-inch cube. For larger parts, conventional die casting or investment casting remain the better options.