Injection Moulding

Injection moulding is the technique that produces most of the plastic objects you use every day, from car components to electronics housings to caps and toys. It is so widespread because, once the mould is ready, it turns out the same part thousands of times with repeatable dimensions and at a cost per part that falls as volumes rise.

On this page we explain how the process really works: the stages of the cycle, the parameters that count, the materials and the typical defects. We do so from a slightly unusual standpoint. At FPM we don’t just talk about injection moulding, we have practised it for over 40 years, and before that we build the mould the parts come out of. When a defect originates in the tooling and not in the press, we notice it immediately, because we have our hands on both.

Injection moulding is a manufacturing process in which a thermoplastic polymer is melted and injected at high pressure into a mould shaped like the desired component. The material fills the cavity, cools and solidifies taking that shape, then is ejected as a finished part. At that point the cycle starts again, identical, dozens or hundreds of times an hour.

The idea is simple, but the result depends on many details: the shape and number of cavities, the point where the material enters, the way the mould dissipates the heat and the parameters the press works with. It is the combination of these choices that separates a compliant part from one full of defects. This is why injection moulding is a precision technique, not a simple pour: the mould and its fine-tuning matter as much as the machine.

The raw material arrives in granules, often already coloured or with additives. Almost exclusively thermoplastic plastics are worked, those that soften with heat and resolidify on cooling, a cycle that can be repeated. Thermosets and elastomers have their own processes.

The cycle always repeats the same way, stage after stage. Knowing them helps to understand where defects arise and where time is recovered.

Behind a well-moulded part there is a balance of parameters kept under control. Changing one shifts the others, which is why fine-tuning is a craft and not a fixed recipe.

• Melt temperature. It decides how well the polymer flows. Too low, and the material does not fill; too high, and it degrades. Every material has its own window.
• Mould temperature. It affects cooling, surface finish, shrinkage and internal stresses. Engineering polymers need a hot mould, even over 100 degrees.
• Injection pressure and speed. They govern how and how quickly the cavity fills. The wrong speed leaves streaks, jetting or flow lines.
• Holding pressure and time. They compensate for shrinkage in the solidification phase. They are the first remedy against sink marks.
• Cooling time. Often the longest part of the cycle: it determines how many parts you produce in an hour and how stable they are dimensionally.
• Back pressure and screw speed. They control the homogeneity of the melt during dosing.

Finding the right combination is the work of fine-tuning, and it is what separates a consistent production run from one full of scrap. The values change with every material, every geometry and every mould: the experience built up on many different parts is what shortens this phase.

Almost the entire thermoplastic family is injection moulded, from the most common ones to high-performance engineering polymers, including glass-fibre filled grades for the most demanding applications. Every polymer has a temperature window within which the melt must be brought and a shrinkage with which it contracts as it solidifies, on which the dimensions to be cut into the mould depend. Some materials, called hygroscopic (polyamide, polycarbonate, ABS, PBT, PET), absorb moisture and must be dried before moulding, otherwise the water in the granules causes bubbles and streaks.

Good moulding shows in the parts that don’t come back. Almost all defects arise from an imbalance between part geometry, mould and press parameters. Something we often say: a defect frequently originates in the mould, not in the press, and trying to fix it with parameters alone is pointless. Here are the most frequent ones, where they come from and how they are kept under control.

• Sink marks. Surface depressions where the part is thicker, for example above a rib. They come from non-uniform wall thickness, holding pressure that is too low or too short, or uneven cooling. They are prevented by evening out wall thickness in the design and tuning the holding phase properly.
• Weld lines. They form where two material fronts meet again, for example around a hole, and do not fully weld. They depend on gate position and temperatures that are too low. They are reduced by moving the gate, adding vents and raising temperatures.
• Flash. Excess material along the mould parting line. It comes from excessive pressure, insufficient clamping force or a worn mould. It is solved with the right parameters, a press of adequate tonnage and mould maintenance.
• Warping and bowing. The part comes out twisted. The most frequent cause is uneven cooling combined with the differential shrinkage of semi-crystalline materials. It is prevented with balanced cooling in the mould and a geometry that shrinks evenly.
• Jetting. The material enters as a jet and leaves a snaking mark on the surface. It depends on an injection speed that is too high and a poorly positioned gate. It is corrected by reducing the initial speed and reviewing the gate.
• Short shot (incomplete filling). The cavity does not fill completely. It arises from insufficient shot size or pressure, clogged vents or material that is too cold. It is solved by increasing shot size and pressure and improving the vents.
• Burn marks. Dark spots caused by trapped air which, as it is compressed, burns the material, the so-called diesel effect. They are eliminated by improving the vents and reducing the speed at the critical point.

Many of these defects are closed out before reaching production, already at the mould design and testing stage. It is the reason why for us it matters to have mould and moulding under the same roof.

Injection moulding has become the standard for producing plastic components for good reasons. Once the mould is amortised, the cost of the individual part is low and falls further with volume. It makes complex geometries possible, with ribs, holes and details that are hard to achieve otherwise. It offers very high repeatability, because every cycle is the same as the last, and leaves great freedom of materials. What’s more, in many cases the scrap is reground and reintroduced.

It is not the right answer for every case. The initial investment in the mould is high, so for very few parts it is rarely justified: below certain quantities 3D printing or machining from solid is preferable. For hollow bodies such as bottles and tanks, blow moulding or rotational moulding pay off more, and for large thin shells, thermoforming. But when you need to produce many identical technical components, it is hard to find anything better.

A frequent question is the difference between injection moulding and die casting. The logic is the same, a molten material injected into a mould, but the raw material changes: injection works plastics, die casting works metals such as aluminium and Zamak, at much higher temperatures and pressures. At FPM we build moulds for both processes, so we know their similarities and differences at first hand.

Compared with the other plastic processes, injection stands out for precision and repeatability. Blow moulding is the choice for hollow bodies such as bottles; thermoforming starts from a sheet and suits shells and trays; rotational moulding is used for large hollow parts made in a few units. For precision technical components in series, however, injection remains the reference.

The basic technique has several evolutions, decided already at the mould design stage.

• Insert moulding. It integrates a component into the part, often metal (a bushing, a threaded pin), placed in the mould before injection.
• Two-shot moulding (2K). It combines two different plastics in a single part, in one press with two injection units.
• Overmoulding. It moulds one material over a ready substrate, such as the soft TPE part of a grip on a rigid body. We cover it in depth on the page about injection overmoulding.
• Gas-assisted injection (GAIM). An inert gas creates hollow channels inside the part, useful for thick sections and ribbed parts.

These are solutions that, in a single moulding, deliver what would otherwise require several parts and an assembly step. The choice depends on the function of the component and the volumes.

Knowing the technique is one thing, having a partner that masters it is another. At FPM injection moulding is part of a complete cycle: we design and build the plastic injection mould, produce your components with contract moulding and, when needed, complete the part with finishing and assembly. A single point of contact from drawing to supply, at our site in Lodrino, in the province of Brescia, with ISO 9001:2015 verified quality.