Injection Moulding Process – Structure, Workflow and Advantages of Modern Plastic Manufacturing
How is a precise plastic component created by injection moulding, and from when is the process economically viable? Here you will find all essential information on the process, materials and cost-effectiveness. Whether small or large series: assemblean delivers injection moulded parts quickly, reliably and ISO-certified.
Injection moulding, also known as injection moulding, is one of the core manufacturing techniques of modern plastic processing. Wherever complex components are needed in high volumes with consistent quality, injection moulding is employed – from automotive and medical technology to consumer goods production.
The principle is as simple as it is efficient: plastic granulate is melted, injected under pressure into a mould, cooled, and then demoulded. This results in precise components with defined geometry, high dimensional accuracy, and repeatable quality.
The image shows an injection moulding machine
Basics of Injection Moulding
Injection moulding is a primary shaping process for plastics, meaning the material—here plastic—is liquefied by heat and then formed in a mould where it solidifies.
The injection moulding technique was developed in the early 20th century and evolved into one of the most important industrial production methods. Millions of parts used daily—from toothbrushes to connectors to precision automotive components—are produced this way.
Material selection and the specific moulding process depend on desired properties.
Overview of Injection Moulding Processes
Beyond standard processes, numerous specialised methods are used depending on component requirements:
Materials and polymers for injection moulding
Material choice majorly influences function, lifespan, appearance, and cost. Injection moulding materials fall into three main groups: thermoplastics, thermosets, and elastomers.
All are processable by injection moulding but differ significantly in behaviour and applications. Thermoplastic moulding is the economically dominant method.
Typical thermoplastic properties:
Wide range from soft/flexible to stiff/high-strength
Good processability and short cycle times
Depending on type: transparent, coloured, chemical- or heat-resistant
Examples and applications:
ABS: impact resistant, easy to process – ideal for housings, consumer goods, interiors.
Tool types in injection moulding
The mould is central to every project, dictating cycle time, surface quality, dimensional accuracy, and ultimately cost. Different mould concepts apply depending on quantity, geometry, and material.
Main mould types include:
Sprue Types in Injection Moulding
Simple, multi-cavity, and family moulds define how many and which parts are made per cycle. Any mould type can employ cold or hot runner systems, i.e., how melt flows into cavities. Multi-cavity and family moulds often use hot runners to minimise sprue waste.
Cold Runner Moulds
In cold runner moulds, the plastic solidifies in the sprue along with the part. The sprue is then separated and either recycled or discarded, depending on material. Cold runner systems are robust, relatively simple, and suitable for almost all plastics.
They fit small to medium quantities or cases where material cost is less critical. Downsides: higher material use and more visible sprue marks.
Hot Runner Moulds
Hot runner systems keep melt in the distribution channels heated by electrical elements, so only the part solidifies on mould opening, not the sprue. This reduces waste, often shortens cycle times, and allows precise pressure and temperature control, vital for expensive or complex plastics and mass production.
Trade-offs are increased investment, maintenance and complexity.
Guide
Design guidelines for injection moulded parts
Good component design is fundamental for a robust, economical injection moulding project. Design decisions shape quality, cycle time and cost from the start.
We support clients in applying design guidelines early, including feasibility analyses to identify and optimise risks like undercuts or uneven wall thickness.
Typical Defect Patterns in Injection Moulding
Even with careful design and precise tooling, common injection moulding defects can occur. Knowing their causes and how to avoid them is crucial.
Flow Lines
Flow lines appear when the melt front cools unevenly or intersects with varying flow speeds, visible as lines or dull patches. Uniform wall thickness, increased injection speed and optimised mould temperature help even material distribution.
Proper gate placement also ensures uniform filling and prevents premature solidification.
Weld Lines
Weld lines form where two melt fronts meet but do not fully fuse, often caused by low temperature or injection speed. Raising melt and mould temperature and pressure improves fusion. Gate design should avoid opposing flow directions.
Air Traps and Bubbles
Pros and cons of injection moulding
Injection moulding combines highest precision with economical serial production. Few processes allow complex plastic parts so efficiently, reproducibly and attractively. Yet consider both advantages and limits to select the right approach for each project.
Advantages of Injection Moulding
When does injection moulding make sense, and what are alternatives?
Injection moulding is particularly cost-effective from medium to high volumes.
Key cost drivers are:
Tooling: one-time investment depending on complexity and cavity count.
Material costs: resin, additives, colourants.
Machine time: cycle and finishing.
Setup and maintenance time.
Economic advantages come from scalability: higher volumes lower per-part fixed costs. From several hundred pieces, injection moulding can undercut additive methods or machining.
Break-even example:
A mould costing €8,000 means €0.80 tooling cost per piece at 10,000 pieces. Adding material and machine costs often makes injection moulding clearly better from this quantity onward.
Industries and application examples
Injection moulding is one of the most versatile industrial manufacturing processes, benefiting nearly every sector:
Injection moulding offers optimal scalability for production. Especially suitable for mass production, you can tailor the moulding process by material choice to your manufacturing needs. Thus, [...]
Multi-component injection moulding (2K, 3K) for parts with different materials or colours,
Gas-assisted injection moulding (GID) to reduce material use and weight,
Micro-injection moulding for extremely small, precise components, e.g. in medical devices,
Insert and outsert moulding, where inserts (e.g. metal bushings) are overmoulded.
Comparison with Other Methods
While 3D printing (additive manufacturing) excels in prototyping and small batches, injection moulding offers short cycle times and low unit costs for medium to large volumes. Compared to vacuum casting, injection moulding provides significantly better reproducibility and dimensional stability.
1. Tooling
The core of every injection moulding project is the mould, comprising two halves—the cavity (negative) and the core (positive). The sprue channel directs molten material into the mould. Depending on the part, sliders, ejectors, or hot runner systems may be incorporated.
2. Melting and Metering
In the injection barrel, plastic granulate is heated and melted by a rotating screw that transports, homogenises, and meters material for the next shot.
3. Injection and Filling
Molten material is injected under high pressure (typically 500 to 2,000 bar) into the closed mould cavity. The aim is to completely fill the cavity without air entrapment or excess pressure.
4. Holding Pressure Phase
After filling, additional pressure is maintained briefly to compensate for volume shrinkage during cooling. This prevents sink marks and ensures dimensional accuracy.
5. Cooling and Solidification
The mould is actively temperature-controlled, usually via water channels, to optimise cooling time. Proper temperature management is crucial for cycle time and part quality.
6. Demoulding
After solidification, the mould opens and ejectors push out the part. Then the cycle repeats.
7. Process Monitoring and Quality Control
Modern injection moulding machines are fully digitally networked, with sensors capturing temperature, pressure, and cycle time data in real time.
Start your injection moulding project with us
Thermoplastic Injection Moulding
Thermoplastics are the most commonly used variant. They can be reheated and reshaped multiple times, facilitating recycling and post-processing.
Thermoset Injection Moulding
Thermoset injection moulding is applied when form-stable, heat-resistant parts are required, e.g. electrical components. Thermosets cure chemically and cannot be remelted afterwards.
Elastomer Injection Moulding
Elastomer injection moulding is used for flexible, rubber-like materials, such as in seals or vibration dampers.
Injection moulding process – step by step
The injection moulding process consists of precisely coordinated steps, each influencing the final part's quality and dimensional accuracy:
PP: chemical resistant, cost-effective, flexible – used in packaging, automotive, medical fields.
PA (e.g. PA6, PA66): high strength, good heat resistance – suited for technical parts.
PC: transparent, impact-resistant, temperature stable – used for optical parts and housings.
POM: excellent dimensional stability and sliding properties – preferred for gears, bearings, precision elements.
PEEK: high-performance polymer with exceptional heat and chemical resistance – e.g. aerospace, medical technology.
Thermosets – maximum form stability and heat resistance
Thermosets behave differently: they harden chemically in the mould. Once cured, they cannot be remelted, only mechanically processed.
They are used where thermoplastics reach thermal or dimensional limits, such as hot, mechanically stressed parts like housings for electric/electronic parts or engine bay components.
Their downside: longer cycle times and higher demands on mould temperature control and process management.
The uncured material is injected at relatively low temperature into the mould.
The mould itself is heated to a higher temperature where curing occurs.
This reaction produces highly form-stable, temperature- and dimensionally stable parts, often with thick walls.
Typical thermoset properties:
Very high form stability and stiffness
Good thermal resistance, often above 150 °C
Excellent electrical insulating properties
Not meltable, limited recyclability (mostly as regrind)
Elastomers – sealing, damping, flexibility
Elastomers are rubber-like plastics that can stretch significantly under load and return to their original shape. Injection moulding of elastomers resembles thermosets: crosslinking occurs under heat in the hot mould, often via vulcanisation.
Elastomers are useful when parts must remain flexible, absorb movement, or isolate vibrations—e.g. in drive technology, machinery, or sealing systems.
The material is fed as powder or strips into a specially designed screw.
The barrel is moderately heated (around 80 °C) to avoid premature crosslinking.
The mould is significantly hotter; vulcanisation happens there.
Vulcanisation time is often much longer than screw preparation time; machines with multiple moulding stations may be used to maintain cycle efficiency.
Typical elastomer properties:
Soft to medium-hard, rubbery
High resilience under dynamic loads
Good damping of vibrations and shocks
Often good resistance to media and weathering
Additives and Fillers – fine-tuning material properties
Independent of base polymer, many materials are enhanced by additives and fillers, allowing targeted adjustment of mechanical, thermal, electrical and optical properties.
Glass- or carbon-fibre reinforced types differ in flow, shrinkage and surface formation compared to unfilled materials. Material selection should consider component geometry and tooling design.
Reinforcing fibres (glass, carbon) for strength, stiffness, thermal resistance
Mineral fillers (talc, chalk) to reduce warpage and influence shrinkage
UV stabilisers, flame retardants, antistatics for outdoor use, fire safety, or ESD protection
Dyes and masterbatches for defined colours, effects (transparent, translucent, metallic) and surface impressions
At assemblean, we support customers early in design phases with optimal material selection based on temperature requirements, mechanical load, and surface quality. Below are popular injection moulding materials.
If your desired material is not listed, please contact us. We manufacture parts with your preferred material in consultation.
Post-processing - Surface finishes for injection moulded parts
SPI Standard
Application
Method
Surface Roughness (Ra µm)
A-1
High-gloss polished parts
Grade #3, 6000 grit diamond
0.012 - 0.025
A-2
High-gloss polished parts
Grade #6, 3000 grit diamond
0.025 - 0.05
A-3
Highly polished parts
Grade #15, 1200 grit diamond
0.05 - 0.1
B-1
Moderately polished parts
600 grit sandpaper
0.05 - 0.1
B-2
Moderately polished parts
400 grit sandpaper
0.1 - 0.15
B-3
Low to moderately polished parts
320 grit sandpaper
0.28 - 0.32
C-1
Lightly polished parts
600 stone
0.35 - 0.4
C-2
Lightly polished parts
400 stone
0.45 - 0.55
C-3
Lightly polished parts
320 stone
0.63 - 0.7
D-1
Fine matte surface
Glass bead blasting
0.8 - 1.0
D-2
Matte surface
Dry blast #240 oxide
1.0 - 2.8
D-3
Matte surface
Dry blast #24 oxide
3.2 - 18
-
Machine finish
-
3.2 with visible machining marks
Thermoplastics – the injection moulding standard
Thermoplastics are by far the most utilized materials in injection moulding. They are fed as granules, melted, injected into the mould, and solidify on cooling. This process is reversible: thermoplastics can be remelted repeatedly, facilitating recycling and regranulation.
Simple Moulds (Single-Cavity)
Single-cavity moulds produce one part per cycle, suited for prototypes, small series, complex geometry, or tight tolerances. They have lower tooling effort and allow quicker modifications but higher piece costs.
Multi-Cavity Moulds
Multi-cavity moulds have several identical cavities producing multiple parts per cycle, increasing output and lowering costs for medium to large production runs. Channel and cooling design is more complex to ensure balanced filling and avoid defects. This type is cost-effective for large series and durable projects.
Family Moulds
Family moulds combine several different but related parts (e.g. housing top and bottom, lid and base) in one mould, producing all parts in a single cycle. This saves tooling as separate moulds are not needed.
Parts should have similar volume and wall thickness to ensure uniform filling and shrinkage; otherwise scrap rates rise.
Trapped air or bubbles arise from insufficient venting or excessive injection speed. Air that cannot escape forms voids. Remedies include precise vent grooves, tuned fill speeds, thorough drying of hygroscopic plastics and consistent holding pressure to prevent void growth.
Short Shots
Short shots are incompletely filled parts from inadequate injection pressure or temperature. Stabilising flow by increasing pressure and monitoring temperature, optimising venting, and mould flow channels helps. Multi-point injection may assist complex geometries.
Sink Marks
Sink marks typically show over ribs or thick sections due to uneven shrinkage. Maintain uniform wall thickness, fine-tune holding pressure and ensure homogenous mould temperature. If possible, reduce rib height or blend rib roots into thinner walls.
Flash
Flash occurs when injection pressure is too high, mould parting surfaces are worn, or clamp force insufficient. Regular mould maintenance, adjusted injection parameters and correct machine sizing prevent material escaping at parting lines.
Warpage
Warpage stems from uneven cooling or internal stresses, manifesting as distortions after demoulding. Symmetrical wall thickness, uniform temperature control and sufficient cooling time reduce warpage. In some cases, conformal cooling or stress relaxation treatments post-production help ensure shape stability.
Start your injection moulding project with us
Wall Thickness
Uniform wall thickness avoids sink marks and warpage; ideal thickness 1–4 mm depending on material.
Draft Angles
Draft angles of at least 1° to 2° per side facilitate demoulding.
Ribs and Reinforcements
Ribs and stiffeners enhance stiffness without increasing wall thickness.
Radii
Internal radii should be 0.5–1× wall thickness to avoid stress concentrations.
Undercuts
Undercuts must be created with slides or ejectors.
Gate Types
Point, film, tunnel or hot runner gates selected per part geometry.
1. Cost efficiency in series production After tooling, injection moulding offers very low unit costs. The highly automated process delivers consistent quality, ideal for medium to large runs. Moulds can last tens of thousands to millions of cycles—an investment paying off long-term.
022. Precision and repeatability Injection moulding excels in dimensional accuracy. Parts match mould geometry exactly; tolerances within a few tenths of a millimetre are standard. This suits components requiring precise assembly or automated processing.
033. Broad material spectrum From standard plastics like ABS or PP to engineering polymers like PA or POM and high-performance plastics like PEEK or PEI— injection moulding covers nearly every requirement. Additives and fillers enable tailored strength, surface hardness or chemical resistance.
044. Design freedom and functional integration Complex geometries, snap-fits, ribs or living hinges are implemented directly in the mould. This allows multiple functions in a single part, reducing assembly steps and variety.
055. High surface quality Precision machining of moulds creates surfaces from glossy to finely textured without rework. Even fine structures are reliably reproduced.
066. Short cycle times and scalability A full cycle often takes only a few seconds. Multi-cavity moulds manufacture several parts simultaneously, lowering unit cost and enabling high production rates with consistent quality.
Limitations and considerations of injection moulding
011. High initial tooling cost The largest cost is mould production, potentially thousands of euros depending on complexity. Injection moulding is thus mainly viable for medium to large quantities.
022. Longer lead times Tooling design runs take time; design changes usually require new or adjusted moulds, which must be planned.
033. Design requirements Injection moulding needs manufacturable design: uniform walls, drafts, minimal undercuts. assemblean assists in design to avoid costly later corrections.
044. Mould maintenance and wear Especially with glass fibre-reinforced plastics, moulds wear faster. Regular cleaning and maintenance ensure part quality throughout mould life.
055. Limited economic efficiency in small series For orders below roughly 500 to 1,000 parts, additive manufacturing or vacuum casting may be more economical. assemblean helps select the best approach from 3D printing, rapid tooling to full-scale injection moulding.
The decision hinges on fixed costs (tooling + setup) vs. variable part costs. Larger runs spread fixed costs over more units. Alternative methods like 3D printing or CNC machining may be better for small runs or frequent design updates.
Prototype / Small series (approx. 1–500 pieces)
In early phases, short iteration cycles and low upfront cost matter more than minimal unit cost. Additive manufacturing (3D printing) is often the choice: SLS and MJF produce robust functional prototypes and small runs without tooling; SLA offers fine surface and precision.
This facilitates fast design validation without expensive steel moulds. Vacuum casting (silicone moulding) is another proven small series option: silicone moulds from a master (e.g., 3D printed) enable dozens to hundreds of identical parts with good surface finish and shorter lead times than injection moulding.
Pilot / Validation series (a few hundred to few thousand)
Rapid tooling then gains focus—e.g., 3D printed or softer metal mould inserts—allowing processing of real injection materials with realistic cycle parameters at lower tooling cost and limited mould life.
Vacuum casting can bridge the gap when surface appearance and material choice fit. This hybrid phase reduces risk by validating design, material, and process windows under near-series conditions before investing in durable multi-cavity moulds.
Series production (thousands to hundreds of thousands+)
Injection moulding excels at this stage. Multi-cavity (often hot runner) moulds reduce unit costs via short cycles and high output, while maintaining dimensional stability and reproducibility.
Ramp-up paths like: rapid tooling → pilot → multi-cavity mould are typical. assemblean scales seamlessly—centralised digital control and documented process data cover all phases.
Special cases: thermoforming with 3D printed moulds can cost-effectively produce thin-walled covers or blister packaging in low volumes. Soft/medical parts may be economically cast over 3D printed mould inserts, useful for beta testing or niche small series.
These bridging methods fill the gap between prototype and full series without upfront investment in durable steel tools.
In automotive, injection moulding produces technical interior plastic components such as ventilation grilles, trims, and brackets. These parts require both aesthetic appeal and resistance to temperature and stress.
Glass fibre-reinforced plastics make them lightweight, stiff and vibration-resistant. Injection moulding integrates multiple functions—clip connections, sealing lips, guide elements—reducing assembly effort and part diversity.
High dimensional repeatability allows consistent assembly across production batches—a key factor in modern automotive production. Ready to start your injection moulding project? Injection moulding is key for economical, precise, and reproducible plastic parts.
Its modular process—from material selection through tooling and quality control—provides flexibility for almost any application. assemblean combines this proven technology with modern digitally controlled production.
Customers benefit from: making injection moulding efficient, predictable, transparent and future-proof.
ISO-certified manufacturing,
single point of contact for all project phases,
transparent process data, and
flexible delivery times—from prototype to mass production
Injection moulding and die casting are common part production methods. To select the right technique for your products, you should understand differences between injection moulding and die casting. [...]
Vacuum casting is suitable for fast, easy production of high-quality cast parts. The casting occurs under vacuum and offers an excellent alternative to injection moulding and 3D printing. How [...]
