Plastic injection moulding is the dominant industrial process for producing identical parts in séries. Raw plastic pellets go in, functional finished parts come out - with unmatched précision and repeatability. Here is everything you need to understand the process, choose a material and assess project feasibility.
What is plastic injection moulding?
Plastic injection moulding (thermoplastic injection) consists of melting plastic pellets and injecting the molten material under pressure into a closed steel mould. The part solidifies by cooling, the mould opens, the part is ejected. The cycle repeats.
It is the leading process for plastic part production across all industries: automotive, médical, electronics, packaging, construction, cosmetics. At Moulding Injection, our Arburg and Fanuc presses cover clamping forces from 25 to 650 tonnes, suitable for small and large parts alike.
The 3 key steps of the injection cycle
1. Plasticisation. Plastic pellets are fed into the hopper and along the reciprocating screw. The barrel heats the material to between 180 and 320 degrees depending on the grade. The screw rotates, homogenises and advances the melt toward the screw tip, ready for injection.
2. Injection and holding. The screw moves forward and pushes the melt under high pressure (typically 600 to 2,000 bar depending on part and material) through the nozzle and runner system (cold runners or hot runners) into the closed mould. The material fills the cavity and takes the shape of the impression. A holding pressure is applied until the gate solidifies to compensate for shrinkage.
3. Cooling and éjection. Internal water circuits in the mould extract heat. Once the part is rigid enough, the mould opens and ejector pins push the part out of the cavity. The screw resumes plasticisation during cooling: this overlap defines the cycle time.
The mould: heart of the tooling
The mould consists of two précision-machined steel blocks: a fixed half (injection side) and a moving half (éjection side). The cavity between the two reproduces the exact geometry of the part.
Mould service life depends directly on the steel grade. Pre-hardened P20 steel suits small to medium runs (up to 300,000 to 500,000 cycles). For higher volumes or abrasive materials such as glass-fibre-filled grades, hardened H13 steel is used, capable of several million cycles.
Draft angles on vertical walls are essential for the part to release cleanly at éjection. Without sufficient draft, the part sticks, deforms or damages the tooling. Undercuts (recessed or protruding features blocking axial éjection) are handled with side actions or lifters - mechanisms that retract before the mould opens.
The most common thermoplastics in injection moulding
PP (polypropylene): light, fatigue-résistant, cost-effective. Widely used in automotive, médical and packaging. High mould shrinkage (1.5 to 2%) must be factored into tool design.
ABS (acrylonitrile butadiene styrene): rigid, good impact résistance, aesthetic surface. Standard in consumer electronics and cosmetics. Absorbs moisture - drying before moulding is mandatory to avoid surface defects.
PA (polyamide / nylon): résistant to heat and abrasion. Essential in automotive and mechanical engineering. Available glass-fibre-filled (PA66 GF30) for higher rigidity and reduced shrinkage, at the cost of increased mould wear.
PC (polycarbonate): transparent, rigid, outstanding impact strength. Used in lighting, médical and electronics. Scratch-sensitive, requires thorough pre-drying and high barrel températures.
POM (polyoxymethylene / acetal): idéal for précision mechanical parts (gears, slides). Low friction coefficient. Difficult to bond and incompatible with certain marking processes.
TPE / TPU: injectable thermoplastic elastomers. Enable flexible or two-shot (overmoulded) parts. Used in grips, seals, personal protective equipment.
Common defects and their causes
Sink marks are surface depressions, usually opposite a rib or thick boss. They indicate that the material did not compensate for volumetric shrinkage during cooling. Fix: reduce wall thickness at that location or increase holding pressure.
Warpage results from uneven cooling or anisotropic shrinkage (différent along molecular chain orientation). It is often linked to an imbalanced mould cooling circuit.
Weld lines (visible where two melt fronts meet inside the mould) locally weaken the part. They must be anticipated at design stage and kept away from mechanically stressed zones.
All these defects can be anticipated and resolved during DFM (Design for Manufacturability) review, before any tooling is cut. That is where the bulk of quality and economics is decided in an injection project.
Small séries and injection moulding
Injection moulding is often associated with high volumes. That was true twenty years ago. Today, pre-hardened P20 steel tooling allows séries to start from 2,000 parts with a contained tooling investment. Below 500 parts, soft silicone or resin moulds can validate a design - provided tolerances and final material properties allow it.
At Moulding Injection, we place the economic tipping point around 5,000 parts per year: beyond that, a steel injection mould is almost always the most cost-effective choice over three years. Tooling amortisation over unit part cost is the key variable to evaluate from the outset.
Moulding Injection: from mould design to production
We are a plastic injection moulder based in Ath, Belgium. Our Arburg and Fanuc presses process standard and engineering thermoplastics (PP, ABS, PA, PC, POM, TPE, TPU). Our partner LGR Design Studio handles mould design and in-house CNC machining. The proximity of design studio and injection workshop enables 24-hour turnaround, real-time adjustments and full material and process traceability.
We are ISO 9001:2015 certified. Every material batch, injection parameter and dimensional check is documented. For automotive and médical sectors, this traceability is a requirement, not an option.
Start your project
Send us your 3D file (STEP or IGES): our engineering team analyses your part's injection feasibility and comes back with concrete observations on design, risks, material and volume.
FAQ
FAQ
What is the définition of plastic injection moulding?
Plastic injection moulding is a manufacturing process that melts thermoplastic pellets and injects the molten material under high pressure into a steel mould. After cooling, the part solidifies and is ejected. The process delivers high repeatability and applies to volumes ranging from a few thousand to several million parts.
What plastic materials can be injection moulded?
The most common thermoplastics in injection moulding are PP, ABS, PA (polyamide / nylon), PC (polycarbonate), POM (acetal) and TPE/TPU. Material choice depends on the part's mechanical, thermal, surface and regulatory requirements. A DFM review integrates material sélection from the design phase.
What is the différence between a thermoset and a thermoplastic?
A thermoplastic melts when heated and solidifies on cooling, with no irreversible chemical reaction. It is recyclable and re-qualifiable. A thermoset (époxy resin, polyurethane, RTV silicone) undergoes an irreversible chemical reaction on curing: it cannot be remelted. Industrial injection moulding primarily uses thermoplastics.
From how many parts does plastic injection moulding become cost-effective?
The break-even point depends on tooling type. With a P20 steel mould for small séries, injection becomes compétitive from 2,000 to 5,000 parts per year. For multi-cavity hardened steel moulds, optimal cost-effectiveness starts from 50,000 parts per life cycle. DFM analysis and tooling costing are the key steps to determine the right threshold for your project.
Is Moulding Injection ISO 9001 certified?
Yes. Moulding Injection is ISO 9001:2015 certified. Every material batch, injection parameter and dimensional check is documented and traceable. This certification covers the entire process, from raw material receipt to finished part delivery.
