Manufacturing techniques

Additive, subtractive (wasting), forming, joining and finishing.

Ask what happens to the amount of material: more is additive, less is subtractive, the same in a new shape is forming, pieces becoming one is joining, and only a changed surface is finishing.

Moulding needs a material that can be melted or softened and set in a new shape. Timber burns rather than melting, and its fibres along a grain cannot be reformed.

A pairing of the technique with a reason — a material property, the volume required, a tolerance, a cost, or the environment the product lives in.

Protect the material from its environment, improve how the product looks and feels, and by doing both extend the product's working life.

It is not a coating. The oxide layer is grown out of the aluminium itself, so there is nothing on top of the metal to lift — though it is thin and brittle.

Zinc in a galvanised coating corrodes in preference to the steel, so it goes on protecting even where a scratch has exposed the steel beneath.

A finish that doubles a product's life halves the impact of everything that went into making it — usually a larger saving than any change of material.

In order: how the basic shape was made, what brought it to size, how the parts were joined, and what was done to the surface — with a reason at every stage.

A material property, the volume required, a tolerance, a cost at that volume, or the environment the product lives in.

At high volume, moulding is far faster and cheaper per part. The printed stage is where the shape was settled before the tooling was cut.

Deep-drawn body, trimmed and machined base, printed handle prototypes, riveted handle, anodised inside and polished outside.

Only in how each layer is made. LOM glues down sheets and cuts each outline, FDM extrudes melted thermoplastic, and SLA cures liquid photopolymer with light.

The bond between layers is weaker than the material within a layer, so the part breaks along a layer. Build orientation decides which direction it can be loaded in.

Nothing holds up material printed over air, so overhangs steeper than about 45° need support printed beneath and removed afterwards — costing material, time and clean-up.

Internal channels, lattices and shapes that would need a mould split into many pieces, because no tool ever has to be withdrawn from inside the part.

Not the machine but the specification. A base model answers one question and is thrown away; a production part must meet a specification, survive its service life and be repeatable.

There is no tooling to pay for, so the first part costs what the thousandth costs. Below a few thousand parts that beats moulding.

Making every unit different at no extra cost. A printer works from a file, so a hearing-aid shell or dental aligner matched to one person costs the same as a standard part.

At high volume, where moulding makes a part in seconds; on large simple shapes; and anywhere the layer-to-layer weakness or surface finish cannot be accepted.

A thin layer of powder is spread across a bed and a laser or electron beam fuses the cross-section. The bed drops, another layer is spread, and unfused powder supports the part.

The surrounding unfused powder holds the part up, so overhangs and internal cavities need nothing printed beneath them. The loose powder is recovered and reused.

No tooling, so the first part costs what the thousandth costs. Below a few thousand parts it beats moulding, and it removes the lead time and minimum order too.

Build volume caps part size, cycle times are hours, metal powders are costly and need careful handling, and safety-critical parts need qualification of powder, parameters and post-processing.

3D printing with a material whose physical or chemical state changes over time in response to an external stimulus — pH, temperature, water or light.

Printed in its permanent shape, warmed and deformed into a temporary one, then cooled so the chains lock. A stimulus lets them move again and it returns to the printed shape.

A vascular stent that opens at body temperature; a drug capsule that opens at intestinal pH; flat-pack that folds when wetted; shading that closes in direct light.

Few materials, high cost, poor recyclability, changes that are usually one-way or repeatable only a few times, and products that are hard to test, certify and trust.

Rotation of the print bed and rotation of the extruder head, which together let material be laid along a curved path rather than in flat horizontal slices.

A printed part is weakest between layers. When the layers follow the curve of the part, a load along that surface runs along the layers rather than peeling them apart.

The surface, because a curve is printed as a curve rather than a staircase, and the support material, because an overhang can be rotated until it is no longer an overhang.

Curved, loaded and made in small numbers — implants, prosthetic sockets, automotive brackets, aerospace ducts. Flat, lightly loaded or mass-produced parts gain nothing worth the cost.

In turning the work rotates and the tool is fed along it, so it makes round shapes only. In milling the tool rotates and the work is moved past it, so it makes almost any reachable shape.

It removes very little material at a time with many small hard particles, holding tolerances and producing surfaces a cutting edge cannot — grinding, honing and polishing.

Everything removed is waste that was bought, transported and often processed first. A machined aerospace bracket can start as a block ten times its finished mass.

Accuracy and surface finish no other category reaches, in any material, with no tooling to make first — which is why prototypes, one-offs and mould tools are machined.

Nothing is added or removed — the same material takes a new shape with almost no waste. All of them trade an expensive tool made once for a very low cost per part.

Blow moulding makes thin-walled parts with a narrow neck, such as bottles. Rotational moulding makes large seamless hollow parts, such as tanks. Vacuum forming makes open shapes with one good face.

The precision steel tool costs thousands and must be spread over the run. At 400 parts the tooling dominates; at 400,000 it is a fraction of a penny each.

A constant cross-section along the whole length — window frames, pipe, guttering, curtain track and aluminium sections.

Does it ever need to come apart? Temporary joints allow repair, upgrade and recycling; permanent ones are stronger, lighter, sealed and cheaper to assemble.

For dissimilar materials that cannot be welded, thin sheets a rivet would tear, and joints that must be sealed — because load is spread over the bonded area rather than at points.

Compatible materials only, heat that distorts and weakens the area around the joint, skill or a robot, and no way to reverse it.

It is one object that can only be shredded. A screwed product is a pile of materials that can be separated and sorted.