13. Molding and Casting¶

group assignment:

  • review the safety data sheets for each of your molding and casting materials,
     then make and compare test casts with each of them
  • compare mold making processes

Group Assingment

individual assignment:

  • design a mold around the process you'll be using,
     produce it with a smooth surface finish that
     does not show the production process toolpath,
     and use it to cast parts
  • extra credit: use more then two mold parts
  • extra credit: make your own materials

Molding and Casting

Image source: Molding, Casting

Molding and Casting¶

Molding and casting are formative manufacturing processes that shape materials within a cavity to create parts.

In simple terms:

Molding creates the negative cavity or tool.
Casting is the act of filling that cavity with a liquid or pliable material that solidifies into the final part.
The mold holds a negative impression.
The cast is the positive part that comes out of it.

Why It Matters¶

Each process has different strengths for materials, volumes, tolerances, and costs.

Foundries often report casting rejection rates around 5 to 7 percent due to defects like gas bubbles and inclusions, which underscores the importance of process selection and quality control.

What Is Molding?¶

Molding is the process of forming a part by shaping a material inside a prepared mold cavity.

In industry, molding commonly refers to plastic forming processes, where a molten or softened polymer is injected or pressed into a mold, then cooled or cured to a solid shape.

Conceptually, the mold is the tool. The molded part is the output.

Common Molding Processes¶

Injection Molding¶

Molten thermoplastic or thermoset is injected into a steel or aluminum mold under pressure, then cooled or cured. Ideal for high-volume, tight-tolerance plastic parts.

Compression Molding¶

Pre-measured charges of thermoset or composite material are placed in a heated cavity, then pressed to shape. Good for structural parts and fiber-reinforced components.

Blow Molding¶

Plastic is extruded or injection molded as a parison, then inflated against a mold to form hollow parts like bottles and tanks.

Thermoforming¶

Heated plastic sheet is drawn over a mold by vacuum or pressure. Suited to packaging and large enclosures with moderate detail.

Rotational Molding¶

Powdered plastic rotates in a heated mold to form large hollow parts with uniform walls.

Silicone Rubber Molding¶

RTV silicone molds capture fine details for prototyping and short-run replicas.

This is the one I will be using.

Silicone Rubber

Molding Materials and Properties¶

Material	Properties
Thermoplastics	ABS, PC, PP, PE, Nylon. They soften with heat and can be remelted. Good for fast cycles and recyclability.
Thermosets	Epoxy, phenolics, melamine. They crosslink during cure and do not remelt. Good for heat resistance and dimensional stability.
Elastomers	Silicone, TPU, TPE. Flexible, impact resistant, and suitable for seals and soft-touch components.

Key property considerations include viscosity and flow length, shrinkage control, fiber orientation in filled plastics, heat deflection temperature, and chemical resistance.

Design choices like wall thickness, ribs, and gate location strongly influence warpage, sink marks, and weld lines.

Tooling and Equipment for Molding¶

High-Precision Molds¶

Typically steel or aluminum with cooling channels, runners, gates, ejectors, and surface finishes matched to part aesthetics.

Injection Machines¶

Clamp tonnage, shot size, and screw design must match the part and material.

Auxiliary Systems¶

Hot runners, mold temperature control, dryers, and automation for repeatability.

Tooling investment is meaningful for injection molding. The upside is low cost per part at volume and excellent repeatability.

Typical Applications and Volumes for Molding¶

Consumer goods housings, clips, and fasteners
Medical disposables and device enclosures
Automotive interior and under-the-hood plastic components
Electronics enclosures and connectors

Injection molding shines at medium to very high volumes, often from tens of thousands to millions of parts, due to fast cycle times and multi-cavity tooling.

What Is Casting?¶

Casting

Image source: Image source

Casting is the process of producing a part by pouring or injecting a liquid material into a mold and allowing it to solidify.

In metalworking, casting is essential for creating complex geometries in aluminum, zinc, magnesium, and steel alternatives.

In polymers and resins, urethane and epoxy casting can produce short-run parts with excellent surface finish.

Conceptually, the mold is still the negative cavity. The cast is the final solid part.

As Eko Industries and Smooth-On both note, the distinction often confuses newcomers because the terms are sometimes used interchangeably, but casting is the act of producing the part from the mold.

Common Casting Processes¶

Sand Casting¶

Flexible, economical patterns form sand molds that are destroyed after each pour. Excellent for large parts and low to medium volumes. Finish is rougher and tolerances are broader.

Die Casting¶

Molten metal is injected into a metal die at high pressure. Suited to aluminum, zinc, and magnesium for high-volume, near-net-shape parts with fine details.

Investment Casting¶

Also called lost-wax. Wax patterns are coated to form a ceramic shell, then metal is poured to capture intricate geometry and fine surface finish. Ideal for complex metal parts with tight dimensional needs.

Permanent Mold Casting¶

Reusable metal molds provide better finishes than sand casting and more consistency, at moderate tooling cost.

Centrifugal Casting¶

Metal is spun in a rotating mold to form cylindrical components like bushings and tubes with good material density.

Urethane and Vacuum Casting¶

Used for prototype and bridge production of plastic-like parts using silicone molds under vacuum to reduce bubbles.

Casting Materials and Properties¶

Metals¶

Aluminum, zinc, and magnesium are common in die casting.

Iron and steel alternatives often use sand and investment casting.

Alloy choice drives strength, corrosion resistance, and temperature performance.

Non-metals¶

Urethanes, epoxies, concrete, and plaster are cast for prototyping, architecture, and art.

Core metallurgy topics include solidification sequence, grain structure, porosity, shrinkage, and heat treatment.

Feed paths, risers, and cooling control microstructure and reduce defects.

Tooling and Equipment for Casting¶

Patterns and Cores¶

Create cavities and internal passages. Cores form internal voids in sand and investment casting.

Gating and Riser Systems¶

Manage metal flow and feeding to prevent cold shuts and shrinkage cavities.

Furnaces and Handling¶

Crucibles, ladles, and temperature control for melt quality.

Die casting uses hot-chamber or cold-chamber machines selected by alloy and melting point.

Tooling cost ranges widely. Sand patterns are relatively low cost. Permanent molds and die casting tools represent higher investments but offer long tool life and fast cycles.

Typical Applications and Volumes for Casting¶

Automotive engine blocks and housings, transmission cases, brackets, and heat sinks
Industrial pump bodies, valve components, and structural fittings
Aerospace and medical components that combine complex geometry with metal performance
Short-run urethane parts for pre-production testing

Molding vs Casting: Core Differences at a Glance¶

While both use a mold cavity, molding often refers to pressure-based forming of plastics with rapid cycles, and casting often refers to pouring or injecting metals or resins that solidify by cooling or curing.

Molding excels at high-volume plastic production with tight repeatability.

Casting excels at metal parts, complex shapes, and a wide range of sizes and volumes.

Comparison Table: Molding vs Casting¶

Comparison Area	Molding	Casting
Process mechanics	Injection or pressing of softened material into a closed mold under pressure	Pouring or injecting liquid material into a mold followed by solidification
Typical materials	Thermoplastics, thermosets, elastomers	Metals like aluminum, zinc, magnesium, steel alternatives, plus urethanes and epoxies
Tooling cost and lead time	Higher for injection molds, detailed machining and cooling required	Low to high depending on method, from sand patterns to hardened dies
Volume sweet spot	Medium to very high volumes with low cost per part	Low to high volumes, flexible with process choice
Tolerances and finish	Tight repeatability and smooth surfaces with proper tool design	Broad to tight depending on process, investment and die casting deliver finer detail
Design complexity	Thin walls achievable, but flow and knit lines must be managed	Excellent for complex internal geometries using cores and investment shells
Typical defects	Warpage, sink, short shots, flash, weld lines	Porosity, gas bubbles, sand inclusions, cold shuts, shrinkage cavities
Part size range	Small to medium parts, large parts possible with processes like rotational molding	Very small precision parts to very large industrial castings
Cycle time	Seconds to minutes per shot	Tens of seconds to hours, depending on melt, mold, and section thickness
Secondary operations	Minimal, often limited to trimming and basic finishing	Often includes trimming, machining, heat treating, and surface finishing

Technical Deep Dive: Subtypes, Mechanisms, and Suitability¶

Choosing between molding and casting hinges on the mechanics of each subtype, material behavior during flow or solidification, and your requirements for geometry, tolerances, finish, and cost.