Powder Metallurgy & Polymer Processing

What is powder metallurgy?

A process that produces parts by shaping metal powders and bonding the particles into a useful solid product.

Why doesn't PM melt the entire part?

PM bonds particles through compaction + sintering, avoiding full melting to improve efficiency and enable difficult materials.

Common PM products?

Gears, bushings, filters, cutting-tool materials, porous bearings, structural parts, small precision components.

Key advantages of powder metallurgy?

High material utilization, little machining, ability to make difficult-to-process metals, controlled porosity, high production rates.

Key limitations of powder metallurgy?

Powder cost, tooling cost, density gradients, size limits, shape restrictions, residual porosity reducing ductility/toughness/fatigue.

What are the major PM processing steps?

Powder production → mixing/blending → compaction → sintering → secondary operations.

Why does powder-production method matter?

It affects particle size, shape, surface condition, cost, and processing behavior.

How does particle size affect PM?

Influences packing, flow, surface area, sintering behavior, and final density.

How does particle shape affect PM?

Affects flowability, compressibility, die filling, and green strength.

What is green strength?

The mechanical strength of compacted powder before sintering.

Purpose of mixing/blending?

To distribute powders, alloying additions, lubricants, and binders uniformly.

Why are lubricants used?

To reduce friction between powder and die walls during pressing/ejection.

Why is segregation a risk?

Powders with different size, density, or shape can separate during mixing.

What is a green compact?

A pressed powder shape with enough strength to be handled but not fully bonded.

What causes density gradients?

Wall friction resisting powder movement; pressure loss near die walls; uneven compaction.

How to reduce density gradients?

Use double-action pressing instead of one-sided pressing.

What is sintering?

Heating compact below melting point so atoms diffuse and form bonds; necks grow, porosity changes, and strength increases.

What does furnace atmosphere do?

Protects from oxidation; reducing atmospheres can remove oxides.

How do time & temperature affect sintering?

Higher temperature/time → stronger bonding & densification; too much → distortion or unwanted reactions.

What is coining/sizing?

Post-sintering pressing to improve dimensional accuracy.

What is infiltration?

Filling pores with lower-melting metal to improve density/properties.

What is impregnation?

Filling pores with oil or polymer for functional purposes.

What is the goal of high-density PM?

Minimize porosity to achieve properties close to wrought metals.

What is HIP?

Hot isostatic pressing — high temperature + high gas pressure → uniform high density.

Why avoid undercuts and reentrant angles?

They complicate die filling and ejection.

Why use uniform wall thickness in PM?

Improves powder flow, compaction, and dimensional accuracy.

Difference between thermoplastics and thermosets?

Thermoplastics soften when heated and remelt; thermosets chemically cure and cannot remelt.

Why is polymer processing important?

Allows high production rates, complex shapes, low weight, and part consolidation.

What does extrusion produce?

Continuous products with constant cross-section (pipe, tubing, film, sheet, profiles).

What is injection molding?

Melting polymer and injecting it into a closed mold cavity.

Why is uniform wall thickness important in polymers?

Reduces sink marks, warpage, residual stress, uneven cooling.

What does blow molding make?

Hollow parts like bottles, containers, tanks.

How does rotational molding work?

Powder coats inside of a rotating heated mold to form hollow parts.

What is thermoforming?

Heating sheet → forming over/into mold → trimming. Used for packaging, trays, panels.

What is compression molding used for?

Thermosets, rubber, composites.

What is transfer molding?

Material forced through runners into closed mold; handles more intricate shapes than compression molding.

What is polymer casting?

Pouring liquid polymer into mold to cure/solidify.

What does foam molding create?

Cellular structures for insulation/cushioning.

How are elastomers processed?

Molding, extrusion, calendering, curing/vulcanization.

Why does fiber orientation matter in composites?

Strongly affects strength, stiffness, and failure behavior.

When to use extrusion?

Long continuous products with constant cross-section.

When to use injection molding?

Complex discrete parts at medium-to-high volume.

When to use blow molding?

Hollow containers.

When to use thermoforming?

Thin-wall sheet products.