Injection moulding and 3D printing: making plastic parts

Most plastic parts around you, from bottle caps to car dashboards, were injection moulded. 3D printing builds parts without any tooling, one layer at a time. The two processes are often used on the same product: printed prototypes to prove the design, then a mould for production. Knowing how both work helps you design a part that can move smoothly from one to the other.

Injection moulding

An injection moulding machine melts plastic pellets in a heated barrel and forces the melt into a closed steel or aluminium mould under high pressure. The plastic cools against the cold mould walls, the mould opens and the finished part is pushed out. One cycle typically takes 10 to 60 seconds, and a mould can have several cavities, so a single machine can make thousands of identical parts a day.

The moulding cycle, step by step

1. Clamp. The two halves of the mould close and are held shut by the clamp force, often hundreds of tonnes.
2. Plasticise. The rotating screw carries pellets from the hopper along the heated barrel, where friction and the heater bands melt them.
3. Inject. The screw moves forward like a plunger and pushes the melt through the nozzle, runners and gate into the cavity.
4. Hold (pack). Pressure is kept on while the plastic cools, to push in extra material as it shrinks.
5. Cool. The part solidifies against the mould, which has water channels to take the heat away. Cooling is usually the longest part of the cycle.
6. Open and eject. The mould opens and ejector pins push the part out. Meanwhile the screw is already preparing the next shot.

Advantages

• Very low cost per part at high volume.
• Excellent repeatability and surface finish, including textures and logos moulded in.
• Wide choice of materials: polypropylene, ABS, polycarbonate, nylon, acetal and many filled grades.
• Fine detail such as clips, living hinges, threads and bosses in one shot.

Limitations

• The mould is expensive and takes weeks to make; changes to the design after the mould is cut are costly.
• Parts need near-uniform wall thickness and draft, and undercuts need extra mould mechanisms.
• Not economic for small quantities.
• Defects such as sink marks, warping, short shots and flash need good part and mould design.

Design rules for moulded parts

• Keep walls uniform, typically 1–4 mm depending on the material and part size. Thick sections sink and warp.
• Add draft of at least 0.5–1° on faces parallel to the mould opening, more on textured faces.
• Make ribs about 50–60 % of the wall they support, to add stiffness without sink marks.
• Use radii on corners: inside radius about half the wall thickness.
• Avoid undercuts where possible, or design them so a side action or a flexible snap can release.

3D printing (additive manufacturing)

3D printing builds a part layer by layer directly from the CAD model. There is no tooling, so the first part costs the same as the hundredth, and a change to the design means only a new file.

Main 3D printing processes

FDM printing, step by step

1. Model the part in CAD, with printing in mind (see the design rules below).
2. Export a mesh file (STL or 3MF).
3. Slice it: choose the orientation, layer height (typically 0.1–0.3 mm), wall count, infill and supports. The slicer writes the machine code.
4. Print. The nozzle draws each layer’s outline and infill, then the bed drops (or the head rises) by one layer.
5. Post-process: remove supports, sand or fill the surface, and drill or tap holes that need accuracy.

Advantages

• No tooling cost and very short lead time: a part can be in your hand the next day.
• Shapes that no mould or machine can make: internal channels, lattices, organic forms.
• Easy to change the design between prints, which speeds up development.
• Economic for one-offs, spare parts and small batches.

Limitations

• Slow and expensive per part at volume.
• FDM parts are weaker between layers than along them, so orientation matters.
• Visible layer lines and lower accuracy than moulding or machining (roughly ±0.2 mm on FDM).
• Fewer material grades, and properties that differ from the moulded version of the same plastic.

Injection moulding vs 3D printing

Where the break-even sits: for a small plastic part it is usually somewhere between a few hundred and a few thousand parts, depending on size, material and finish. Below that, print; above it, mould.

Designing one part for both

A good approach is to design the part for moulding from the start, with uniform walls, draft and ribs, and then print the prototypes from the same CAD model. The printed prototypes test the fit and the function, and the design goes to the mould maker without a redesign. Our guide to material selection for 3D printing explains which filament or resin to use for each kind of prototype.