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1.2 Injection moulding

This section is the process crash-course: what actually happens when an injection mould turns plastic pellets into a finished part.

1.2.1 The process: fill, pack, cool, eject

Injection moulding makes a part by melting plastic pellets and forcing the melt into a shaped steel cavity under high pressure. One part (or one set of parts) is produced per cycle; cycles typically run for tens of seconds. Each cycle has four phases:

  1. Fill. A screw injects molten plastic into the mould cavity in roughly one to a few seconds. The melt front races through the cavity, cooling against the steel walls as it goes. Where it slows too much it freezes early; where two fronts meet, they knit together imperfectly.
  2. Pack. Plastic shrinks as it cools — several percent by volume. To compensate, the machine keeps pushing more material in under pressure ("packing") while the part is still partly molten. Too little packing leaves sink marks and internal voids; too much over-stresses both the part and the mould.
  3. Cool. The part sits in the closed mould while heat conducts out into water channels drilled through the steel. Cooling is usually the longest phase — often more than half the cycle — so cooling-channel design directly sets the economics of the part.
  4. Eject. The mould opens and pins push the part out. The part must slide off the steel cleanly — one of the main constraints on how a mouldable part may be shaped.

1.2.2 The mould

The mould (the "tool") is a precision steel assembly, often costing tens to hundreds of thousands of euros and taking weeks to months to build:

  • Cavity and core — the two halves that shape the part. The cavity forms the outer (visible) surface; the core forms the inner surface. The part shrinks onto the core as it cools, which is why ejection needs force.
  • Sprue, runners and gates — the delivery plumbing. The sprue is the entry channel; runners distribute the melt to each cavity; the gate is the small opening where the melt actually enters the part. Gate position is one of the most consequential design decisions: it sets the flow pattern, and the flow pattern sets where weld lines and air traps land.
  • Cooling channels — drilled water lines that carry heat away. Uneven cooling means uneven shrinkage, and uneven shrinkage means warpage.
  • Ejector pins — push the finished part off the core.
  • Side actions and lifters — extra moving mechanisms needed whenever the part has features that would otherwise trap it in the mould (undercuts). Each one adds cost, maintenance and failure modes.

A mould is a product in its own right, designed and built by a toolmaker — usually a different company from both the part designer and the moulder.

1.2.3 The defects

Four defects dominate the field, and they recur throughout any study of moulded parts:

  • Warpage — the part comes out bent or twisted because different regions shrank by different amounts. It is the single hardest thing to predict and the most expensive to fix, because the fix usually means re-cutting the steel mould.
  • Sink marks — shallow depressions on the surface opposite a thick section (such as a rib or a boss), where the extra material shrank inward. A cosmetic killer on visible interior parts.
  • Weld lines — visible seams (and weak zones) where two melt fronts met and knitted imperfectly. Controlled mainly by gate placement.
  • Short shots — the cavity did not fill completely; the melt froze before reaching the far corners. Caused by thin walls, long flow paths or low pressure.

1.2.4 Which variant this study concerns

Everything above describes thermoplastic moulding: the plastic melts, freezes, and could in principle be re-melted. This is the process the rest of the study is about — injection moulding of thermoplastics — and the design and DFM that follow all concern this case.

It is worth knowing the second family as a contrast. Thermosets cure — a one-way chemical reaction hardens the resin permanently, like an epoxy glue setting — and are shaped by processes such as the transfer moulding introduced among the branches earlier. The engineering difference that matters: in thermoplastic moulding you wait for the part to cool; in thermoset moulding you wait for it to cure, and cure progress is chemical, varies from batch to batch, and is invisible from outside the mould. From here on, "moulding" means thermoplastic injection moulding unless stated otherwise.

1.2.5 Designing the part

Before any steel is cut, the part exists only as a design. An engineer builds it as a three-dimensional model in CAD (computer-aided design) software — a precise digital solid that fixes every wall, rib, boss, hole and rounded corner, together with the plastic the part will be moulded from. This CAD model does two jobs at once. It lets the whole team see the part — turn it, section it, check how it fits its neighbours — long before a physical one exists. And it is the master description from which the mould, the simulation and ultimately the finished tool are all derived.

Getting that geometry right matters more here than in many kinds of design, because the shape itself decides both how well the part moulds and how much the mould will cost. Nearly every defect described above is set at this stage: a wall left too thick, a rib in the wrong proportion, a gate in a poor position, a face with no draft — each is a design choice that later surfaces as a sink mark, a warp, a badly placed weld line, or a part that jams in the mould. Geometry also drives tooling cost directly: a feature such as an undercut cannot be released by a simple two-part mould, so it forces the extra side actions and lifters described earlier — moving mechanisms that make the tool more expensive to build and more prone to failure. Whether the mould needs them is decided in the CAD model, not on the shop floor.

The cost of fixing a mistake climbs steeply the later it is caught: changing a dimension in the CAD model takes minutes, while changing it after the steel is cut can mean re-machining or re-making the tool — weeks of work and a large slice of the tool budget. The whole discipline is to get the geometry right while it is still just a model.

1.2.6 Why simulation exists

All four defects share a property: they are invisible in the CAD model and expensive to discover in steel. By the time a physical mould reveals a warpage problem, the tool budget and months of lead time are already spent. Mould-flow simulation exists to move that discovery forward. It numerically solves the filling, packing, cooling and warpage physics on the part geometry before the mould is cut, so problems can be found and fixed while they are still just changes to a CAD file.

Mould-flow solvers are physics engines: geometry, material properties and process settings go in; predicted flow, pressure, temperature and deformation fields come out. Their accuracy rests on validated numerical methods paired with measured material databases — libraries of thousands of lab-characterised plastic grades. The design rules of the field (the subject of §1.3) are largely public and stable; the hard-won, proprietary part of simulation is the solver plus material data that turns a geometry into a trustworthy prediction.