A customer arrives with a 3D file and a straightforward question: 'Can this be injection moulded?' The answer is never simply yes or no. It depends on the mould, the material, the press, the expected tolerances. This guide lays out the fundamentals of what plastic injection moulding really involves - and helps you make the right décisions before committing to tooling.
What an injection press does
An injection press turns plastic pellets into a finished part in a matter of seconds. The principle is direct: pellets melt in a heated barrel, a screw pushes them under pressure into a closed mould, the plastic cools, the mould opens, the part is ejected.
The clamping force of the press is measured in tonnes. It must counteract the injection pressure exerted over the projected area of the part. A large part with high material pressure demands a more powerful press. This is a calculation, not a rough estimate.
Shot size, injection speed and températures are set for each mould and each material. The same tooling on two différent presses can yield différent results if parameters are not correctly transposed.
The mould: heart of the tooling
The mould defines the part shape. Two précision-machined steel blocks - a fixed half (injection side) and a moving half (éjection side). The cavity between them reproduces the exact desired geometry.
Steel grade determines mould service life. Pre-hardened P20 steel suits small to medium runs, typically up to 300,000 to 500,000 cycles depending on the injected material. For larger volumes or abrasive materials (glass-fibre-filled plastics), hardened H13 steel is used, capable of several million cycles.
Our moulds are designed by our partner LGR Design Studio, which operates an in-house design office and CNC workshop. This control over design and machining enables fast iterations between DFM analysis, design correction and tooling manufacture.
Cooling circuits and balancing
A poorly cooled mould extends cycle time and generates déformation. Cooling circuits must be sized and positioned to extract heat uniformly. Thermal imbalance typically results in warpage (part déformation after éjection), difficult to correct without modifying the tooling.
Draft angles, undercuts and side actions
For a part to release at éjection, vertical walls must carry a draft angle (slight taper allowing éjection without force). Without draft, the part sticks in the mould.
Undercuts (recessed or protruding features blocking axial éjection) are addressed with side actions or lifters: mechanisms that retract before the mould opens. They add complexity to the tooling and have a direct impact on tooling cost and lead time.
How to choose your thermoplastic material
A thermoplastic melts under heat and solidifies on cooling, with no irreversible chemical reaction. This is what enables recycling and material requalification. Not to be confused with thermosets (époxy resins, polyurethane), which require a différent process.
PP (polypropylene): light, fatigue-résistant, cost-effective. Widely used in automotive, médical and packaging. High mould shrinkage to monitor during design.
ABS (acrylonitrile butadiene styrene): rigid, good impact résistance, aesthetic surface. Common in electronics and cosmetics. Absorbs moisture: pre-drying before injection is mandatory.
PA (polyamide / nylon): heat and abrasion résistant. Widely used in automotive and industry. Available glass-fibre-filled for increased stiffness, but with greater mould wear.
PC (polycarbonate): transparent, rigid, impact résistant. Used in lighting and médical. Scratch-sensitive and requires thorough pre-drying before injection.
POM (polyoxymethylene / acetal): excellent for précision mechanical parts, low friction. Difficult to bond, incompatible with some marking processes.
Common defects and their causes
A sink mark (surface depression, usually opposite a rib or thick boss) indicates that the material did not compensate for volumetric shrinkage. Fix: reduce wall thickness at that point or increase holding pressure.
Warpage typically results from uneven cooling or anisotropic shrinkage (différent along molecular chain or fibre orientation). Weld lines (visible where two melt fronts meet) locally weaken the part and should be kept away from mechanically stressed zones.
All these defects can be anticipated at design stage, before the mould is machined. That is where DFM (Design for Manufacturability) work saves time and budget.
Small and medium séries: what it changes
Injection moulding is often associated with high volumes. But it remains relevant for lower volumes whenever repeatability and material quality matter. A médical component at 2,000 parts per year demands the same tooling rigour as a part at 100,000 units.
On small séries, tooling amortisation weighs more heavily on the unit cost. This parameter must be factored in from the start of the project, not discovered after the tooling has been ordered.
How to choose your plastic injection moulder
A good injection moulder is not selected on price alone. The key criteria are: press capacity (clamping force in tonnes matched to your part size), mastered materials (not all moulders process engineering grades like PA GF or POM), in-house DFM capability (analysing your part before tooling is real value), quality traceability and certification (ISO 9001:2015 for demanding sectors) and geographical proximity (real iterations happen during tooling tryout).
Moulding Injection is ISO 9001:2015 certified. Our Arburg and Fanuc presses cover a clamping force range suited to small and large parts. Our partner LGR Design Studio handles mould design and CNC machining. We are based in Ath, Belgium, on the French-Belgian border.
Start with a DFM analysis
Send us your 3D file (STEP or IGES): our engineering team analyses your part's injection feasibility within 48 working hours. You receive a structured feedback: identified risk areas, material recommendations, tooling estimate.
FAQ
What is DFM in plastic injection moulding?
DFM (Design for Manufacturability) is the analysis of a plastic part to identify design issues before the mould is built. A DFM review detects insufficient draft angles, predictable sink mark zones, non-uniform wall thicknesses and weld lines in stressed areas. It prevents costly tooling rework and additional lead time.
How to choose between P20 and H13 steel for an injection mould?
P20 steel (pre-hardened) suits small to medium séries: up to 300,000 to 500,000 cycles depending on the injected material. H13 steel (hardened) is needed for high volumes and abrasive materials (PA GF, POM). The choice depends directly on the planned annual volume and the chosen material. A P20 mould costs less to build but may require tooling rework if volume exceeds forecast.
Which plastic material to choose for a mechanical part?
For a mechanical part subject to température and abrasion stress, glass-fibre-filled PA (polyamide) is often the right choice. For a précision part with low friction coefficient, POM (acetal) is recommended. For parts with high impact requirements, PC or ABS are preferred. The choice must always be confirmed before mould design, as the material directly influences shrinkage and tolerances.
Can plastic injection moulding be done in small batches in Belgium?
Yes. Moulding Injection produces small-batch plastic injection parts in Ath, Belgium. We handle séries from a few hundred parts depending on geometry and material. Our partner LGR Design Studio handles mould design and machining. The proximity of design studio and workshop reduces tryout lead times and ensures full traceability.
What is a side action (slider) in an injection mould?
A side action (or slider) is a mechanical mechanism inside the injection mould that retracts laterally before the mould opens. It allows demoulding of undercuts, meaning recessed or protruding features on the part that block axial éjection. Side actions add complexity and tooling cost, but are often essential for functional géométries.
